Paper Number 2008-01-0138
Project Sabre: A Close-Spaced Direct Injection 3-Cylinder
Engine with Synergistic Technologies to achieve Low CO2
Output
D. Coltman, J.W.G. Turner, R. Curtis, D. Blake, B. Holland, R.J. Pearson and A. Arden
Lotus Engineering, Norwich, Norfolk, UK
H. Nuglisch
Continental Automotive France S.A.S., Toulouse, France
Copyright © 2008 SAE International
ABSTRACT
INTRODUCTION
The paper describes the design and development of
‘Sabre’, a 3-cylinder engine encompassing a combination
of technologies to realise low CO2 in a practical
automotive application while retaining driving pleasure
(vehicle acceleration performance). This project is a
partnership with Continental Automotive, in which Lotus
Engineering is responsible for the base engine and
combustion system.
The Sabre project was formed to create an advanced
engine to act as a baseline for future spark-ignition
engine technologies. This is reflected in the acronym
Sabre, short for ‘Spark-ignition Advanced Baseline
Research Engine’. The Project Sabre engine employs a
novel and synergistic combination of engine downsizing,
gasoline direct-injection and variable valve train
technology to realize low CO2 and improved driving
pleasure in a cost effective, affordable manner.
The decision process that led to a close-spaced direct
injection combustion system that does not target high
BMEP as the chief route to low fuel consumption is
described. Instead of pursuing an approach in which
specific power is maximized in order to reduce throttling
losses at part load, mild downsizing coupled with
throttling loss reduction and turbulence manipulation
enabled by a switching valve train is employed. This
approach, together with the use of an integrated exhaust
manifold, enables cylinder head packaging benefits
together with improved driveability and reduced high-load
fuel consumption as a result of a lower boost
requirement. The interaction of spray and air motion in
the resulting homogeneous combustion system is briefly
discussed.
The rationale behind the choice of a 3-cylinder layout is
also discussed which, in addition to reducing friction and
engine mass, also contributes towards knock control by
removing blowdown pulse interaction between the
cylinders. Friction reduction methods are discussed
which centre around the use of laser-deposition bores
and roller bearings on the balance shaft. The balance
shaft helps to address the NVH issues expected from the
layout, together with the very low CoV of IMEP afforded
by the combustion process. The technology underpinning the integrated exhaust manifold is also
discussed. This simultaneously improves thermal
management and reduces mass and bill-of-material
(BOM) costs in this integrated engine concept.
The challenges facing the automotive industry include a
requirement to reduce CO2 emissions from the vehicle
fleet whilst retaining driving pleasure (vehicle
acceleration performance) in a cost effective manner.
CO2 emissions are directly related to fuel consumption
and so many new technologies have been proposed,
developed and productionised with the aim of improving
fuel economy [1]. Of those related to engine efficiency,
engine downsizing, direct injection and variable valve
trains have all been introduced with varying degrees of
success [2, 3, 4].
Extreme downsizing is one route to greater fuel economy
that, in 2005, saw the series introduction of an engine
combining turbocharger, supercharger and gasoline
direct-injection system (GDI) in the Volkswagen TSI
engine [5, 6]. This increased level of technology has
been shown to significantly improve vehicle fuel
consumption and so the trend is expected to continue,
though the combination of roots blower and turbocharger
has previously been employed by Lancia in their rally
cars [7].
Despite this current developmental direction, there is still
some desirability in not reducing swept volume
excessively for a downsized engine. This is due to the
fact that throttle response – so-called ‘turbo lag’ –
becomes less of a challenge, and with it the engine
knock challenges of operating a spark-ignition
combustion process at high brake mean effective
pressure (BMEP). Indeed, the challenges faced by the
engine designer are so great as the degree of
downsizing of an engine increases that variable
compression ratio (VCR) has been proposed [8], though
there are significant challenges in productionising this
concept in current engine architectures.
All of these approaches and problems stem from the fact
that the automotive industry remains committed to the 4stroke, or Otto, cycle and reducing the amount of
throttling loss it suffers is the primary aim of most of
these concepts.
That vehicle driveability starts to become a major issue is
amply illustrated by the adoption of a supercharger in the
Volkswagen TSI concept.
This device is used to
maximum effect because as well as providing ‘tip-in’
throttle response it has also allowed the turbocharger
turbine to be enlarged to the benefit of full-load fuel
consumption – the vehicle it has been introduced in is
reported not to need fuel enrichment for component
protection until a speed of 200 km/h (124 mph) is
reached [6].
However, the supercharger, its drive
system and extra air ducting (including bypass valve) all
combine to add cost to a turbocharged GDI engine, as
well as control challenges. Any approach which would
permit the excellent fuel economy to be retained in a
simpler, cheaper package while not requiring an overly
small turbine to be used (to the benefit of high load fuel
economy) should be of interest.
Additionally, the higher the degree of supercharge
applied to an engine the worse the knock problem may
be expected to become, particularly at low engine speed.
Knock, resulting from uncontrolled autoignition of the end
gas of a spark ignition engine, has been one of the chief
barriers to increased efficiency throughout the history of
the spark ignition engine [9]. New technologies such as
turboexpansion [10] and cooled EGR [11] are being
developed to decrease the octane appetite of engines,
but the adoption of GDI in high specific output engines
has already allowed their compression ratio to be
increased due to the beneficial effect of the evaporation
of the fuel in the cylinder lowering the temperature of the
charge (and with it end gas temperature). The advanced
ignition angle also made possible by this effect together
with the higher expansion ratio have both permitted lower
exhaust gas temperatures, so helping to reduce preturbine temperature and with it the need for economyreducing component protection fuelling at high road
speeds. The introduction of fuel into the charge air only
after the exhaust valve has shut permits greater amounts
of valve overlap to be used. This results in an attendant
shift of operating point on the compressor map,
increasing the surge margin, and greater enthalpy in the
exhaust due to secondary oxidation of CO in the
manifold, both to the benefit of driveability. Because of
all these advantages and others, the turbo-GDI synergy
is one of the strongest technologies for general fuel
consumption reduction.
The combination of variable valvetrain and gasoline
direct-injection unlocks the potential for true downsizing
through reduction in throttling losses during low–load
operation [1] while pressure charging maintains engine
power. Thus the Sabre engine makes an excellent
alternative powertrain for a modern, clean C-class
vehicle – retaining the torque delivery of a 2.0 / 2.2 litre
engine and thus retaining driving pleasure.
DESIGN RATIONALE
The Project Sabre engine set out to achieve three key
objectives:
Demonstrate low CO2 emissions;
Improved driving pleasure;
Use of cost effective, affordable technology.
The decision process that led to the engine design
concept that meets these objectives is set out below.
Although appearing as discrete decisions in the text
below, they are in fact all interlinked and the overall
decision process was an iterative one.
Low CO2 emissions can be achieved by downsizing, this
being a clear industry direction with increasing degrees
of downsizing being implemented. However, extreme
downsizing requires operation at relatively high BMEP to
be realised, and this brings about issues with driveability,
combustion variability and fuel enrichment for component
protection. Additionally, if the cylinder count is not to be
reduced, the architecture potentially forms a type of
barrier to the engine (since the bore size and thus the
valve and injector packaging may become constraints).
Some of these issues can be addressed by advanced
charging strategies, e.g. sequential turbocharging,
variable geometry turbochargers [12] or electrically
driven compressors [13] but these all add to the cost and
complexity of the overall powertrain system. Against this,
a fixed geometry turbocharger offers significant
advantages in terms of BOM and control issues.
There are also related mechanical advantages from
limiting the BMEP levels used. The maximum cylinder
pressure is lower and thus the structure of the engine
can be sized appropriately, which in turn has cost and
weight benefits as well as potentially reducing the
engineering required to ensure durability and reliability.
Decision 1 – Mild downsizing: Downsize from 2.2 litre 4cylinder with natural aspiration to a 1.5 litre turbocharged
engine, thus maintaining performance. Maximum BMEP
required: 20 bar.
The synergies between direct injection and turbocharging
are well known and result in cold start emissions
improvements and high load fuel consumption
reductions. Further reductions in CO2 emissions, along
with performance and exhaust emissions benefits
brought about by in-cylinder air and trapped exhaust gas
management, can be obtained by using direct fuel
injection and twin continuously variable cam phasing
(CVCP) devices on turbocharged engines [14].
Centrally-mounted “vertical” or close-spaced fuel
injectors exhibit less potential for cylinder bore washing
with fuel and allow for alternative combustion strategies,
e.g. homogeneous or stratified, compared to an underport injector location. Such a close-spaced configuration
does not compromise the intake port in the same way
that an under-port injector location does.
Close-spaced spray-guided systems are relatively
expensive, with current stratified production systems
adopting high-pressure piezo-injector technology [15].
Lean stratified operation also requires the use of an
expensive de-NOx catalyst giving cost and fuel
consumption
penalties.
Since
a
cost-effective
combination of technologies was an objective for the
project, a homogeneous system was chosen due to the
reduced demands for exhaust gas aftertreatment.
However, while outward-opening piezo injectors offer
charge preparation advantages over traditional swirlinjector-based solenoid systems, new developments
have resulted in ‘multi-stream’ injectors offering better
spray shape and fuel atomization than pressure-swirltype components.
Decision 2 – Adopt direct injection in a centrally-mounted
close-spaced configuration: Use new multi-stream
solenoid injectors rather than a pressure-swirl type.
Adopt twin cam phasers for improved control of gas
exchange.
Choosing a close-spaced combustion system gives
advantages in freedom of intake port design but clearly
represents a challenge in terms of packaging in the
combustion chamber. While 10 mm spark plugs are
being productionized and were adopted for the project, it
was considered that at 1.5 litres, a 4-cylinder engine (i.e.
375 cc per cylinder) would have had too restrictive a bore
to package a central injector, spark plug and four
adequately sized valves, given the targets and objectives
for this particular project. A brief early study showed that
an 83 mm bore would to all intents and purposes be too
small to package all of the necessary components in this
case.
Compared to a 4-cylinder layout, for any given swept
volume a 3-cylinder configuration clearly allows a bigger
bore size. While 83 mm was considered too small
regardless of cylinder number, the same study showed
that with a bore of 86 mm uncompromised packaging
space is available for a centrally-mounted direct injection
system. With the 500 cc cylinder made possible by the
3-cylinder layout, a bore diameter of 86-88 mm is typical
for a light-duty engine. The choice of a larger bore size
clearly made packaging easier; packaging for a smaller
bore size is possible with a different set of design targets
and objectives.
Another advantage of adopting a 3-cylinder configuration
is that in such even-firing cylinder groupings there is the
eradication of inter-cylinder exhaust pulse interaction in
the valve overlap period which may occur when the firing
interval is less than 240°. This is of significant benefit in
turbocharged engines. (Note that adoption of such a
strategy on a 4-cylinder engine without a pulse-divided
exhaust manifold [14] results in a very short exhaust
valve event [16], though strategies such as Divided
Exhaust Period can help offset the need for a 180° valve
event. [17]). Removing pressure wave interference in
the valve overlap period improves the scavenging
process, improves enthalpy transport to the turbine and
extends the knock limit further through reducing the
mass of trapped residuals in the combustion chamber.
While removing a cylinder reduces friction, a
disadvantage of the 3-cylinder configuration is the
creation of a primary couple, but this is of conical nature
and can be addressed by using a counter-rotating
balance shaft. Fitment of roller bearings to this shaft will
minimize its friction.
Decision 3 – Adopt a 3-cylinder configuration for
adequate packaging, lower BOM and knock limit
extension: Adopt a roller-bearing balance shaft for NVH
purposes.
Further reductions in CO2 emissions can be obtained by
reducing throttling losses. Stratified charge, sprayguided combustion systems enable direct reduction of
throttling losses during part load operation because for
any road load they permit a wider-open throttle [15].
There are also thermodynamic benefits through lean
operation. However, they require relatively expensive
piezo-injector technology and expensive exhaust
aftertreatment systems capable of removing NOx from
the exhaust stream in an oxygen-rich environment. Such
de-NOx systems also require periodic regeneration with
a rich/stoichiometric pulse which, while it can in many
cases be synchronized with driving style, contributes to a
reduction in the potential overall fuel consumption
improvement.
Instead of stratified operation, other means of reducing
throttling loss exist. Among other researchers, Tuttle
[18, 19] showed that both early and late intake valve
closing (EIVC or LIVC) could improve part load
efficiency. This improvement resulted in the introduction
of Valvetronic, a variable EIVC system, by BMW [4]. The
advantages in this system stem from direct dethrottling
of the intake phase, because gas exchange is performed
at lower manifold depressions. Also, while the effective
compression ratio reduces with advanced intake valve
closing (EIVC), the expansion ratio does not change,
giving a thermodynamic benefit.
Tuttle [18, 19] reported that, particularly in the case of
EIVC, the combustion duration starts to increase, an
effect believed to be due to the reduction in charge
motion in the cylinder. To counter this, deactivating one
intake valve at part load has the effect of increasing
turbulence at reduced air flow. Such a technique has
been employed in production engines with conventional
valve events by GM. Honda have also used this
approach with asymmetric valve profiles in their VTEC-E
engines, and similar valve strategies were investigated
by Wilson and co-workers [20] in a fully variable valve
train research engine.
Similar research work undertaken by the HOTFIRE
consortium (funded by the Engineering and Physical
Sciences Research Council in the UK) using the same
TM
electrohydraulic Active Valve Train (AVT ) system was
targeted at realizing the benefits of homogeneous DI with
variable valve actuation. This research project provided
results that substantiated the adoption of this approach
[21, 22] and helped to develop the strategy of using the
valve profiles necessary for Miller-cycle operation to also
manipulate the air motion in the cylinder which promotes
swirl and improves mixture preparation.
Extensive
experience of the development and productionization of
switching valve train systems (especially the so-called
cam profile switching or CPS tappet) ensured that the
technology could be implemented in Sabre in a cost
effective manner.
Hence, by combining valve deactivation with cam profile
switching, a solution could be realised that reduces
throttling losses whilst simultaneously enhancing incylinder air motion (swirl) - this was a significant factor in
developing a close-spaced DI combustion system. The
adoption of so-called ‘swirl-DI’ allowed the extension of
the combustion period noted by Tuttle [18, 19] to be
minimized.
Note that similar synergies between
asymmetric valve events and DI combustion systems
have been noted by other workers. Audi have just
productionized an engine with a combination of an
under-port direct injection system and switching valve
train to reduce throttling loss [23].
In concert with the adoption of this strategy, less extreme
downsizing to mitigate throttling loss is permitted
because a proportion of this parasitic load is removed by
the valve train. As a consequence, the need for everhigher BMEPs is avoided, and with it the attendant
challenge of controlling knock and irregular combustion.
It is also suggested that since good thermal efficiency
can be arranged in what is effectively a larger
displacement engine, the Sabre engine may be more
attractive to the marketplace.
Decision 4 – Adopt switchable valvetrain system to afford
Miller cycle operation: Deactivate one intake valve,
switch the other to a low lift / short duration profile to
generate increased turbulence.
As a consequence of employing direct throttling loss
reduction through the functionality of the valve train the
use of a larger swept volume engine is supported.
Improving driving pleasure through mild downsizing and
turbocharging avoids the issues of driveability associated
with extreme downsizing but the concept can still be
vulnerable to “turbo-lag”. To help address this, mild
hybrid systems have been studied in conjunction with
turbocharged engines through their ability to enhance
transient torque while the turbocharger is spooling up
[12]. They have also been shown to contribute to a
reduction in CO2 emissions through stop-start operation
and idle stop functionality. As a contribution from the
engine side, the reduction in turbo-lag by the requirement
for only modest downsizing means that the requirements
placed on the hybridization system are less severe and
hence its costs can be better supported. In the Sabre
engine, the use of mild hybridization also indirectly
supports the use of an exhaust manifold integrated into
the cylinder head (IEM) because the transient torque
increase of the hybrid system offsets any effect that the
reduction in enthalpy has as a result of cooling the preturbine exhaust gas in the aluminium of the manifold.
This will be returned to later.
Decision 5 – Use a mild hybrid system: Crankshaft
mounted starter / alternator for start / stop, regeneration
and torque assist functions.
The considerations discussed above led to the decision
to design a relatively large displacement 3-cylinder
engine in which the use of direct injection and dual cam
phasers, in concert with the removal of pulse interaction
through the 240° period of the firing impulses, would all
contribute to a minimization of the requirement for
overfuelling for knock suppression. However, even if
overfuelling to extend the knock limit was avoided, it is
often necessary to over-fuel at high load for component
protection of either the piston crown, exhaust valves,
turbine wheel or catalyst. This results in a reduction in
combustion efficiency and fuel economy which is not
required to suppress knock. Methods to remove this
requirement altogether were thus considered desirable.
Correct valve and seat material specification can mitigate
the problem of exhaust valve temperature, but turbine
and catalyst limitations are more difficult to mitigate. For
this reason, an integrated exhaust manifold (IEM) was
specified, a technology area in which Lotus had built up
considerable expertise [24].
In a cylinder-head IEM, the water jacket cools the
manifold directly as part of the engine cooling system.
The system adopted for Sabre was of this nature and not
of the more complicated water-cooled separate exhaust
manifold type. As a consequence, the number of parts is
reduced (and with it the assembly process made simpler)
and the volume and surface area of the exhaust manifold
before the turbine minimized. From this comes the
potential advantages that engine warm-up is accelerated
(since less heat is wasted to atmosphere, being instead
absorbed by the coolant) and that the catalyst light-off
time is reduced because the sum of the products of
surface area and heat transfer coefficient for all
components in the gas path before the catalyst is lower.
The engine mass is lower too, and the package size is
reduced.
The reduction in pre-turbine volume also contributes to
better throttle response, though at low speeds there is a
reduction in exhaust gas thermal energy to drive the
turbine. This is addressed by the mild hybrid system
already discussed (but can also be addressed by
exhaust manifold dimensioning). At full load, however,
the cooling system removes heat before the turbine and
therefore helps towards achievement of stochiometric
operation throughout the engine map.
Again, the
avoidance of severe downsizing helps in this situation,
since a lower turbine expansion ratio results in less heat
rejection to the structure of the engine.
A 3-cylinder configuration also helps the adoption of an
integrated exhaust manifold. Fitting such an integrated
manifold to a 4-cylinder engine effectively removes the
possibility to easily specify a pulse-divided manifold
arrangement.
The benefit or otherwise of such
manifolds on turbocharged DI engines has been
extensively discussed [14, 25], but as already mentioned,
a 3-cylinder arrangement with its 240° firing interval
effectively gives the best of both worlds: no reduction in
available energy together with no pulse interaction. As a
consequence, this advantage is used in a production
turbocharged DI engine by BMW [26].
The use of an electric water pump can also aid warm up
(as well as reducing power consumption) and, together
with a split cooling system which preferentially warms the
cylinder block through restricting water flow at part load,
can optimize the warm-up process to the benefit of fuel
consumption.
The IEM is synergistic with both
strategies, permitting increased rates of heat transfer to
the coolant during the warm-up phase. The use of a
water-oil heat exchanger in the coolant circuit ensures
that some of the heat ordinarily lost during the heating of
a conventional cast-iron manifold is instead used to heat
the oil to the benefit of friction reduction.
Decision 6 – Adopt a water-cooled exhaust manifold
integrated into the cylinder head (IEM): Use an advanced
cooling system to tailor the characteristics of the heat
rejection from the engine to vehicle operation. Specify a
water-oil heat exchanger to assist in friction reduction.
All of the preceding reasoning meant that the design
process converged on the configuration discussed in
detail in the later sections of this paper. All of the
principal technologies discussed above are synergistic
and led to a basic engine design which would be capable
of low fuel consumption while leaving potential for future
technology upgrades to improve efficiency further. It is
hoped that future versions will include such technologies
as spray-guided direct injection (because the degree of
downsizing means that dethrottling is still attractive) or
two-stage charging systems (because of the benefits of
3-cylinder banks in turbocharged engines).
DETAIL ENGINE DESIGN
From the above, the basic specification of the Sabre
engine was decided as a 3-cylinder, 1.5 litre
turbocharged engine with four valves per cylinder and
twin overhead camshafts with dual CVCP. Since a bore
size greater than 86 mm was required for cylinder head
packaging reasons (see above), the base engine
dimensions were taken from an existing engine design to
save time during the initial layout phase. This base
engine design was the same as the engines used in the
HOTFIRE research project [21, 22], and was itself based
on an automotive V6 engine. The bore and stroke of the
Sabre engine are therefore 88 mm and 82.1 mm
respectively. Adoption of this geometry also permitted
direct carryover of knowledge gained within the
HOTFIRE project to the Sabre engine.
Adoption of existing geometry allowed the design team to
focus their efforts on including the novel combination of
engine technologies. These included:
Variable valve actuation based on switching tappets
and twin cam phasers.
Homogeneous air / fuel mixture with direct injection
using multi-stream solenoid injectors.
Single-stage,
turbocharger.
Cylinder head with integrated exhaust manifold
(IEM).
Mild hybridization.
Friction reduction concepts.
single-entry
fixed
geometry
In this first phase of engine evolution, all of these
technologies were chosen to be consistent with an
“affordable technology” concept. Switching tappets, cam
phasers, solenoid injectors and the fixed geometry
turbocharger are all known, current production
technologies that are available at reasonable cost.
Once the basic packaging of these engine technologies
was complete, and the engine loads better understood
through detailed simulation, the design was re-optimized
to reduce friction through the use of smaller bearings
throughout the rotating assembly.
SWITCHABLE VALVETRAIN SYSTEM
A variable valve train system employing switching
tappets was used at low load to deactivate one of the
intake valves in a conventional 4-valve combustion
chamber while switching the other to a low-lift, short
duration profile. This arrangement is shown in Figure 1.
Fig. 2c: Unthrottled case – low lift on
one intake valve (valve near
minimum lift (closing)), other
intake valve closed
Fig. 1: Intake camshaft and switching tappets
This strategy has been shown, from extensive work
using single-cylinder optical and thermodynamic engines
in conjunction with advanced measuring techniques on
the HOTFIRE research project, to simultaneously reduce
throttling losses while increasing charge motion through
swirl to the benefit of combustion efficiency [21, 22].
Figure 2 shows an example, taken from work on the
HOTFIRE project [21, 22], of the in-cylinder airflow
structures quantified by particle image velocimetry for
various valvetrain strategies.
Fig. 2a: Throttled case – full lift on
both intake valves, valves near
maximum lift
Fig.2: In-cylinder airflow structures (swirl plane, 454.8°
crank angle ATDC compression)
In conjunction with homogeneous gasoline direct
injection and the positioning of the injector in relation to
the one open intake valve, the airflow structure combines
to give good air-fuel mixing.
GASOLINE DI COMBUSTION SYSTEM
The high cost within the after-treatment and fuel systems
for a spray-guided stratified combustion system lead to
the decision to realize within this engine a more
affordable homogeneous combustion system using
solenoid driven fuel injectors and a classical three-way
catalyst.
A basic decision for the design of the cylinder head was
the positioning of the injector within the DI combustion
chamber. In general the current ‘conventional’ position
for an injector is to mount it under the intake ports. An
under-port solution has the advantages of more easy
integration of the injector into an existing head design,
better accessibility of the injector, easier integration of
the fuel rail and lower injector tip temperatures.
Disadvantages include the design impact on the intake
port.
In this project the decision was made to integrate the
injector in a central position similar to that successfully
used in the HOTFIRE research project [21, 22], which
was feasible for this completely new head design.
Injector and spark plug are both located in a closespaced central position in the combustion chamber.
This head architecture has several advantages:
Fig. 2b: Unthrottled case – low lift
on both intake valves, valves
near minimum lift (closing)
Better geometric conditions for a spray giving
maximum homogenisation and minimal wall wetting.
Compared to an under-port mounted injector a better
potential for catalyst light off strategies due to higher
tolerance to very late ignition with multiple injection
strategies [27, 28].
Better potential for cold start strategies.
Protects the head architecture for a future sprayguided stratified combustion system.
Another special feature of the head design is the
integration of the high-pressure fuel pump drive into the
head casting. The single piston, 200 bar fuel pump is
directly driven by the camshaft using a three-lobe cam
with roller follower, see Figure 3. The pump is volume
flow controlled on the inlet side. Direct drive with roller
follower and flow control minimises the torque demand
for the high-pressure supply.
The tool was validated in cooperation with Loughborough
University using measurements taken with the single
cylinder optical research engine (SCORE) used for the
HOTFIRE programme [22]. Figure 5 shows clearly for a
given intake valve lift and injection timing the fuel impact
on the intake valve in the optical engine (viewed through
the transparent piston) and the corresponding results of
the geometric design tool under the same conditions.
Exhaust valves
Intake valves
Optical engine - SCORE
Matlab simulation - IMPACT
Fig. 5: Comparison of fuel spray results: SCORE optical
engine and IMPACT program
Fig. 3: Integration of high-pressure pump into cylinder
head
The spray geometry of the multi-stream injectors was
optimized using a 3-D geometric mapping tool developed
by Continental Automotive for pre-design of the spray
geometry. This tool, IMPACT, estimates the fuel quantity
impacting on piston, cylinder wall and valves as a
function of valve timing, start of injection, load point and
spray geometry. Figure 4 shows the user interface for
this program.
Injection pressures between 40 bar and 200 bar are
calibrated as a function of the operating point as well as
the start of injection. In addition to fuel economy, criteria
for the optimization of these injection parameters were
HC, CO, NOx and soot emissions to guarantee low
engine-out emissions to achieve EURO V vehicle
emissions performance.
BASE ENGINE ARCHITECTURE
Cylinder Head Assembly – The cylinder head is an
aluminum alloy casting (LM25TF specification) which
houses the complete valvetrain as well as the actuators
for the twin cam phasing and intake cam switching in a
single component. The head is also protected for
exhaust-side CPS, so a position on the rear of the
casting is also available for a switching solenoid. The
exhaust camshaft-driven DI fuel pump is also mounted
directly to the cylinder head as discussed above. A
secondary casting which houses the thermostats for the
split head and block cooling systems is bolted to the rear
of the cylinder head.
The combustion chamber arrangement is carried over
from the original HOTFIRE engine design. As discussed
in [22] the exhaust valves were reduced by 1mm in
diameter in comparison to the original V6 engine and a
10mm centrally mounted spark plug was used. Relative
to the V6, the exhaust valve angle was also reduced by
1°. These changes made it possible to mount the direct
injector centrally in the combustion chamber,
immediately adjacent to the spark plug.
Fig. 4: IMPACT fuel spray visualization tool developed by
Continental Automotive
Increased swirl in the combustion chamber is induced at
low loads by means of deactivating one of the intake
valves using the switching valvetrain. The other intake
valve is simultaneously switched to a profile having a low
lift and short duration. This high swirl ensures a
homogeneous fuel / air mixture even when operating on
the low-lift cam profile which reduces pumping losses
through unthrottled operation.
Because of the centrally mounted injectors, the upper
cylinder head structure is divided into two sections
containing the intake and exhaust camshafts. These
sections are sealed from the atmosphere, providing free
space in between to house the fuel feeds to the DI
injectors and the coil-on-plug ignition system. Oil is
allowed to drain from the intake side to the exhaust side
of the cylinder head through three channels, and the
whole engine is installed at a 5° angle, with the exhaust
side lower, specifically to facilitate this.
Valvetrain Assembly - Although higher in friction than
roller-finger followers, direct acting tappets were retained
to allow the use of the Lotus CPS compact switching
tappet (manufactured under license from Lotus by INA
Schaeffler KG). In order to reduce valvetrain drive
torque, the hydraulic lash adjuster elements from these
tappets were removed and replaced with solid shims.
This also had the desirable secondary effect of
simplifying the lubrication feed to the cylinder head since
normally these tappets require two feeds: one for lash
adjustment and one for switching. Figure 6 shows the
valve train of the engine, and in this illustration the ‘trilobe’ cams for each of the CPS tappets can be seen on
the intake side. The exhaust valves are actuated by
solid lifters with valve-top shims.
Valve springs were optimized using Lotus’ Concept
Valvetrain software. The resulting spring and tappet
stack was slightly longer than in the original base engine,
and as a result the spring seat was lowered, requiring a
small redesign to the upper surface of the intake ports.
Although fitted with fixed geometry valvetrain, the
exhaust side of the cylinder head was package-protected
for the future installation of switching tappets. This
leaves the possibility for a gasoline HCCI version of the
engine in the future.
The camshafts are supported in the cylinder head using
four bearings. The front bearing journal is larger both in
diameter and length to react the camshaft drive bending
loads and also to provide room for the annular grooves
that feed the variable timing pulleys. The camshafts are
hollow and the second journal has two cross-drillings 90°
apart which feeds lubricating oil to the centre. The rear
two journals are lubricated via small holes in the
camshaft which allows the lubricating oil to escape. In
this way, the oil feed to lubricate the camshaft is
minimized through only having to have a single feed,
thus minimizing oil leakage in the cylinder head and
allowing for further oil pump downsizing.
The camshafts are driven via a single-stage chain and
two variable timing sprockets, again supplied by INA
Schaeffler. These provide up to 50° of timing variation for
the DCVCP system. The exhaust sprocket has a “helper
spring” to return the camshaft to the fully-advanced
position for startup. Because the DI fuel pump is driven
from the rear of the exhaust camshaft, a detailed study
of the interaction of the hydraulic forces required to
control the phaser, the valvetrain drive loads and the fuel
pump drive loads was undertaken to ensure that full
phaser authority was retained at all times despite the
phaser having to transmit high pressure pump drive
torque.
Fig. 6: Valvetrain of the Sabre engine. Note ‘tri-lobe
cams’ above Cam Profile Switching tappets on intake
camshaft, and three-lobe driving cam for DI highpressure pump on exhaust camshaft
Cylinder Block Assembly – The cylinder block is split
along the crankshaft centreline into upper and lower
halves. The general arrangement is shown in Figure 7.
Both castings
are aluminium
alloy (LM25TF
specification) with cast iron inserts in the lower
crankcase. Joining the upper and lower crankcases
there is one pair of plastic-region tightened fasteners per
main bearing adjacent to the crankshaft, plus a second
pair of elastic-region fasteners outboard of these. On the
front- and rearmost panels, these outboard fasteners
also bolt through the bearing supports for the balance
shaft. Further stability and sealing is provided by
fasteners arranged around the perimeter of the joint. The
lower crankcase also incorporates an integral crankshaft
windage tray and oil scraper.
intervals facilitating the engine firing order of 1-3-2. It is
machined from a solid billet of EN40B alloy steel and due
to the small crankcase package it is dynamically
balanced using tungsten inserts in the crank webs. The
state of balance is such that a uniform first-order rotating
couple is generated which is then balanced by the
counter-rotating balance shaft (see below). In order to
reduce rotating mass further, the connecting rods are
guided axially at the small end using thrust faces on the
piston pin bosses.
Piston-guiding of the connecting rod also allows the
thrust faces to be removed from the crankpins and the
connecting rod big ends, which has a further benefit in
friction. In concert with this, all of the other bearings
were optimized for friction reduction.
Fig. 7: Twin-walled cylinder block and bedplate. Note
double fixing arrangement for main bearings, cast iron
bearing panel inserts and twin-wall construction of upper
crankcase, with breather volume between the walls
The upper crankcase houses the pistons in aluminum
bores for reduced mass and improved heat transfer.
Prior to final machining, the surfaces of the cylinder
bores are re-melted using a laser while a silicone powder
is introduced into the metal. Thus the silicone becomes
incorporated into the cylinder bore surface giving it
excellent wear and friction characteristics.
The crankcase has a double walled construction in which
the main engine loads are transmitted into the bearing
panels via the cylinder walls and inner wall, and a
second, outer wall provides additional structure between
the water jacket, flame face and lower crankcase
peripheral fasteners. The outer wall has a contoured
profile to increase stiffness and reduce radiated noise.
The space between the walls is utilized as an oil
separator volume for the crankcase breather system.
This negates the requirement for any inter-bay breathing
passages in the lower main bearing panels, which can
weaken the overall structure. This twin-walled design is
visible in Figure 7 and has its roots in the Lotus 3.5 litre
twin turbo V8 engine used in the Esprit sports car. Interbay breathing in 3-cylinder engines is an important
consideration due to the movement of approximately
one-half of a cylinder’s displacement volume from one
end of the engine to the other as the cranktrain rotates.
Within the crankcase assembly, the crankshaft is
supported in four bearing panels, with a half thrust
washer housed in the upper crank half of bearing panel
number 2 (number 1 being at the front).
Cranktrain – The general arrangement of the cranktrain
is shown in Figure 8. The crankshaft has throws at 240°
As a result of FEA analysis, a “full round” skirted forged
piston was selected for this application over a lower
friction “slipper” design. For the same piston size the full
round skirt had significant strength and durability
advantages over the slipper design albeit at the expense
of a small friction and mass penalty. The gudgeon (or
wrist) pin is of the full-floating type.
The piston has a 7cc regular dish-shaped bowl in the
crown to permit some containment of the spray when
necessary for start-up and to achieve the 10.2:1
compression ratio with the valve cutouts necessary to
allow for the cam timing variations facilitated by the twin
cam phasing system. Because of the high specific power
output and large bore size, the pistons are cooled from
beneath by oil jets fed from the main lubrication gallery.
These become operational above a certain gallery
pressure to permit flow optimization of the oil pump.
Balance Shaft System - The balance shaft system
cancels the first-order free moment of inertia inherent in
even-firing 3-cylinder engines by rotating in the opposite
direction to the crankshaft. The conical couple created
by the crankshaft counterweights is cancelled by two
masses placed 180° apart at opposite ends of the
balance shaft. To achieve the necessary timing and
counter-rotation to achieve this, the balance shaft itself is
driven at engine speed via a narrow helical gear at the
rear of the crankshaft. These were adopted instead of
spur gears to improve the overall NVH of the system
without the increased friction of a split scissor-type gear.
The balance shaft is supported in two single-row ball
bearings placed in separate housings at the front and
rear of the engine. The use of ball bearings allows for
any slight misalignment in assembly as well as reacting
the small thrust loads produced by the helical drive
gears. In addition, friction is greatly reduced over plain
bearings and the requirement for a pressurized oil feed is
removed. Bending stiffness between the two supports is
provided without adding excessive weight by enclosing
the whole shaft in a hollow, thin walled steel tube. This
has the additional effect of minimizing the oil aeration
normally produced by placing eccentric rotating balance
weights in the oil pan, through direct shielding of the
balance shaft.
The engine cooling system is fed by a 12 volt electric
water pump supplied by Continental Automotive,
illustrated in Figure 9. This pump has a flow capacity of
up to 151 litres per minute.
Feed from the pump is split 31% to the cylinder block
and 63% to the cylinder head, flowing longitudinally from
front to back along the crank axis in both. The remaining
6% flows directly to the oil cooler and turbocharger. The
block and head each have separate thermostats. This
arrangement allows the block to heat up quickly to
reduce friction, while the cylinder head is kept cool to
increase charge density and reduce the tendency to
knock. A schematic of the cooling system is shown in
Figure 10.
Fig. 8: Cranktrain and Balance Shaft System
Oil Pan - The oil pan is an aluminium casting which, as
well as acting as a reservoir for the lubricating oil, also
provides a mounting for the oil filter and oil cooler
connection. By incorporating most of the oil galleries into
the upper and lower crankcases and oil pan, the use of
external pipe work is minimized. The oil pan includes a
baffle to control the oil level at accelerations of up to 1.0
g in the fore/aft direction (lateral acceleration in the
vehicle). Additional elements on the baffle shield the
crankcase breather inlet tracts from ingesting any oil
shed directly from the semi-immersed balance shaft.
The crankcase is joined to the cylinder head by eight
plastic-region tightened bolts. Block-to-head sealing is by
a two-layer metal gasket.
Lubrication and Cooling Systems - The lubrication circuit
was extensively modeled using a 1D simulation code to
ensure adequate pressurized oil delivery to the rotating
assembly as well as sufficient hydraulic pressure for the
camshaft phasing and cam profile switching systems.
The oil pump is a gerotor design mounted concentrically
on the crankshaft nose. As stated above, in order to
maximize the opportunities for pump downsizing,
hydraulic lash adjusters were rejected in favour of solid
lifters with graded shims on both intake and exhaust
camshafts. As a further development, a package study
was undertaken for a variable displacement oil pump
mounted in the oil pan. This would allow the parasitic
loss to be reduced during periods of low oil demand, or
during cold start when the oil viscosity is highest, and,
since the engine was protected for it as a result of this
study, the pump may form part of a future development
programme.
Fig. 9: Electric Water Pump
The thermal management system includes the water
pump control, the electrically controlled thermostats and
the radiator fan control. Passive components are the oilwater heat exchanger, the flow-optimized cylinder head
with the IEM and the split cooling architecture (allowing
for independent control of head and block temperatures).
Both thermostats have integral heating elements, which
allow them to be artificially opened to provide additional
control. The full utilization of the heated thermostats will
form a future development of the engine. A control valve
in the oil cooler feed is also being considered as a future
enhancement.
The coolant that flows to the cylinder head passes
simultaneously through the main part of the head and the
IEM. Therefore, when both thermostats are open the
coolant has three parallel paths through the engine which
results in much lower backpressure than is normal for a
longitudinal flow system. This reduced backpressure is
ideal for the use of an electrical water pump. At the
same time, the integration of an electric water pump
allowed the development of the intelligent thermal
management system for the engine.
point in the speed / load range. In order to minimize
direct mechanical parasitic losses this vacuum is
generated by a vehicle-mounted 12 volt electrical pump
which charges a reservoir. A PWM signal is used to
control wastegate position. Because of the vehicle’s
hybrid functionality, this vacuum pump was also required
to service the brake booster.
The position of the turbocharger and its mounting elbow
is shown in Figure 11.
Fig. 10: Schematic of Cooling Circuit
The entire thermal system and its control were modelled
using 1D software, allowing model-based control
structure selection and pre-calibration of the controller.
Crankcase Ventilation System - The breather system is
integrated directly into the major castings and the intake
manifold. It is designed to keep oil carry over to an
absolute minimum in order to minimize hydrocarbon
emissions and increase combustion stability by reducing
the tendency for carried-over oil to induce knock. The
prime means of achieving this is to minimize entrained oil
in the blowby gases through large inter-bay breather
passages, the integral windage tray / oil scraper and
effective baffling in the oil pan. Crankcase gases exit the
cylinder block through large cross-section galleries and
enter a volume between the inner and outer structural
walls shown in Figure 7, where the velocity is reduced to
the point where any entrained oil will naturally fall back to
the oil pan. For part throttle operation, this chamber is
evacuated directly into the intake manifold via a restrictor
and one-way valve (fitted to protect the crankcase from
positive plenum pressure under boosted conditions).
Fresh air is simultaneously drawn into the chamber by
the ventilation system.
Fig. 11: Turbocharger mounted on engine
The Integrated Exhaust Manifold (IEM) - The engine has
been designed with an IEM whereby the exhaust runners
and collector are incorporated into the aluminum cylinder
head casting in order to realise substantial weight
savings and therefore fuel consumption benefits. This is
shown in Figure 12.
At high load, the breather gases pass into a secondary
large cross-section chamber in the cam cover. The cam
cover is arranged so that the gases can take two
separate routes through it, reducing the gas velocity and
increasing the potential for oil separation. The cam
cover is connected to the intake system upstream of the
turbocharger compressor by a short pipe, providing
depression under full-load conditions. Immediately
before this connection there is a labyrinth type oil
separator in the cam cover. During testing this system
carried over as little as 0.24g of oil per hour at part
throttle operation.
Turbocharger - A turbocharger provides up to 100 kPa
boost, and is mounted to the cylinder head via a short
cast iron elbow. This elbow serves only to ease engine
bay packaging constraints with mounting the
turbocharger directly onto the cylinder head, while also
serving to thermally decouple the turbine housing from
the IEM. The turbocharger has a normally-open
wastegate which is controlled by a vacuum capsule so
that the engine can be taken off-boost as required at any
Fig. 12: Cylinder Head with Integrated Exhaust Manifold
The IEM changes the heat paths in the engine, which
offers advantages in terms of heat management as well
as providing a more durable product for the customer. As
discussed above, for a turbocharged DI engine, removal
of exhaust gas heat at high engine loads and speeds can
permit operation over greater proportions of the speedload map without full load enrichment for component
protection, thus contributing to improved fuel economy
and CO2 reduction. However, at high engine loads and
low engine speeds this heat loss is potentially
detrimental to the turbine work function (or enthalpy
available in the turbine to produce boost pressure). The
choice of a 3-cylinder engine with good pulse separation
minimizes this effect through preserving the pressure
wave amplitudes. In addition, the incorporation of the
mild hybrid system (a crankshaft mounted integrated
motor-generator) provides an additional source of torque
to compensate for any loss of boost pressure.
Additionally for a 3-cylinder engine the blow-down pulse
interaction is removed and so is not detrimentally
influenced by the compact design of runners and
collector. Any gases that re-enter the cylinder during the
valve overlap phase have been resident in a cooled
chamber and so are at a lower temperature than is the
case for a conventional uncooled steel or cast iron
manifold. All of this assists with good control of residuals
and therefore extends the knock limit, removing the
amount of over-fuelling necessary to control this.
The IEM concept is not without challenges but with the
use of analytical tools to simulate engine cooling water
flows, thermal loads and thermal stresses the design can
be optimized to achieve many benefits. Chief amongst
these benefits is a reduction in engine weight, however
cost savings (both BOM and assembly), lower emissions
(particularly at start-up), improved fuel economy and
better heat management all feature in this innovative
concept.
Intake System – The intake manifolds for the prototype
engines were manufactured from plastic, to ensure low
mass, using rapid prototyping methods. The fixed
geometry design was optimised for airflow using CFD
analysis. A plastic throttle body was used for similar
reasons. An illustration of the intake system is shown in
Figure 13.
MILD HYBRIDISATION SYSTEM
In the test vehicle, the base engine was combined with a
mild hybrid system and a six speed manual gearbox with
optimized long gear ratios. The general arrangement is
shown in Figure 14. The mild hybrid system includes a
12 kW peak power electrical motor mounted directly on
the crankshaft. There is a single clutch that is located
between the motor and gearbox. Electrical power is
supplied by a double layer capacitor pack designed to fit
in the spare wheel space in the boot. Water-cooled
power electronics are used. The complete powertrain
controller was implemented by fast prototyping using a
dSpace Microautobox.
Fig. 14: General arrangement of mild hybrid system
The functionalities integrated in the powertrain controller
include:
Stop / start.
Energy recuperation and intelligent generator mode
control.
Torque boost during low speed transients.
Launch assistance.
The complete controller is simulated under Matlab /
Simulink and a co-simulation using a 1D-based vehicle
model allows simulation based parameter optimisation to
speed up the calibration process.
A fuller description of the design and benefits of the
vehicle-specific components such as the hybridization
system is planned for another publication; this brief
description is provided here because of the previously
mentioned synergy between the mild hybrid system and
the integrated exhaust manifold.
RESULTS
Fig. 13 – Intake system
Prototype engines have been built and are undergoing
testing. Preliminary results from dynamometer-based
test work are presented in this paper. Further results
along with those from vehicle testing will be provided in
another publication.
FUEL CONSUMPTION
A fuel consumption map is shown in Figure 15 which
was produced from simulation studies conducted at the
start of the project with real test data superimposed for
comparison. The test data comprises results with the
intake valvetrain in both unswitched (low lift /
deactivated) and switched (high lift) modes.
Fig. 17: Part load BSFC with low lift intake cams
Measured fuel consumption at the 2000 rev/min., 2 bar
BMEP mapping point is shown in Figure 18 in relation to
typical upper and lower boundaries. Data for some
specific examples of other engines has been highlighted.
Fig. 15: Simulated BSFC map with selected test results
The switching point from the low lift profile to the high lift
profile was calibrated to occur in the part-load area of the
map at engine speeds and loads defined by a boundary.
More detailed measured fuel consumption maps in the
part-load region are shown in Figures 16 and 17.
Fig. 18: Fuel consumption at 2000 rev/min, 2 bar BMEP
It should be noted that the result for the Sabre engine
does not include electrical loads since the crankshaft
mounted starter / alternator was not fitted to the engine
during dynamometer testing and a brake-energy
recuperation system will charge the battery in the vehicle.
FULL-LOAD PERFORMANCE
Fig. 16: Part load BSFC with high lift intake cams
Full-load performance testing is currently being
conducted. The target torque curve is shown in Figure 19
in comparison to a representative 2.2 litre naturally
aspirated DI engine [29]. Preliminary full load
performance and fuel consumption results are also
shown.
FULL-LOAD PERFORMANCE
Preliminary full-load results given in Figure 19 show that
the target torque curve is achievable. The engine gives a
peak torque in excess of 240 Nm (20.1 bar BMEP) and
power of 117 kW (157 bhp) at 5000 rev/min. (18.6 bar
BMEP) using 95 RON gasoline. Note that these test
results were obtained against a turbine inlet temperature
limit of 980 deg.C which enabled fuelling at Lambda = 1
up to an engine speed approaching 5000 rev/min. In
order to protect the turbine, this only had to be enriched
to Lambda = 0.9 at 5000 rev/min. and above, indicating
that the IEM was performing a role of thermal energy
removal at these speeds. The exhaust system
backpressure was 36 kPa at maximum power.
Fig. 19: Engine performance
DISCUSSION
FUEL CONSUMPTION
When the fuel consumption in the part load region is
compared for the two valvetrain conditions, high lift
intake cams versus low lift / deactivated intake cams,
fuel economy benefits of up to 6% have been realized,
see Figure 20. The decision to adopt a switchable
valvetrain system to enable Miller cycle operation
(deactivate one intake valve, switch the other to a low lift
/ short duration profile) has successfully enabled
throttling losses to be reduced whilst simultaneously
generating increased in-cylinder air motion.
Fig. 20: Fuel consumption benefit with switchable
valvetrain system
Although the test results do not include electrical loads
this is expected to have a minimal effect since, for
example, the water pump flow rate in the part load region
is low and thus the parasitic load will be low.
Two different turbocharger configurations have been
tested and the test results shown in Figure 19 relate to
the second configuration. This second turbocharger
however, was a series production unit not specifically
designed for this engine. It was tested due to excessive
pre-turbine pressure with the original turbocharger
configuration, but nevertheless illustrates that the target
torque curve could be achieved with further optimization
of the turbocharger to the engine.
The 3-cylinder, 1.5 litre turbocharged Sabre engine
produces similar performance to typical 2.2 litre naturally
aspirated engines. This is also shown in Figure 19. It
achieves this through mild downsizing and BMEP values
that are around 20 bar maximum. The incorporation of
direct injection, twin cam phasers and an integrated
exhaust manifold allows operation across most of the
speed range at Lambda = 1. This has a significant effect
on full-load fuelling and is within the range of naturally
aspirated engines.
Consequently, the preliminary full load fuel consumption
results are believed to be very good. Throughout much
of the speed range the full load BSFC is 250 g/kWh or
less, increasing to 272 g/kWh at 5000 rev/min. and 299
g/kWh at 5500 rev/min. It is believed that with further
aerodynamic optimization of the turbocharger and an
increase in permissible turbine inlet temperature from
980 deg.C (where these results were recorded) to 1050
deg.C, Lambda = 1 operation may be achievable. Given
that this is the case, technologies which have hitherto
been suggested to support this at the brake mean
effective pressures of above 20 bar that the Sabre
engine operates at, such as cooled EGR or separate
water-cooled exhaust manifolds, would not be required
[11, 30]. Much of this possibility is due to the design of a
properly-functioning IEM. Of course, for higher BMEPs,
cooled EGR could still be applied to this engine, the
advantage with this configuration of IEM being that less
mass of cooled EGR need be added for any given output
level, with benefits in terms of combustion stability and
charging system matching [11].
CO2 EMISSIONS AND DRIVING PLEASURE
Demonstrating low CO2 emissions whilst improving
driving pleasure (vehicle acceleration performance) were
two of the key objectives of the Sabre project. The
engine testing conducted so far has provided results that
help plot a path to realizing these two key objectives in a
C-class vehicle. In this instance CO2 emissions relate to
the vehicle when driven on the NEDC cycle and driving
pleasure is quantified as 0 – 100 km/hr acceleration.
The C-class vehicle chosen as the demonstrator vehicle
for this project was a GM Opel Astra, originally fitted with
a 1.8 litre naturally aspirated engine.
The path to realizing low CO2 emissions and improved
driving pleasure in this demonstrator vehicle is shown in
Figure 21.
The engines have not yet been tested in the vehicles so
the vehicle-dependent aspects are based on simulations.
The full set of vehicle results are planned for another
publication.
Cylinder head assembly + 4 studs + 4 nuts + 1
gasket = 10 parts.
This is an 18-part reduction in BOM (or 35% reduction in
exhaust system part count). This yields an associated
component cost (and weight) saving and also reduces
the cost and complexity of assembly in the
manufacturing environment.
A stratified-charge combustion system to achieve the
targeted fuel economy would require an expensive NOx
trap and corresponding regeneration strategies. This is
avoided in the Sabre engine, which utilizes relatively low
cost switching tappets in the valvetrain that are
integrated in the same packaging space as a
conventional tappet. A restrictor and an actuator in the oil
circuit are the only additional items in the parts count to
complete the system. This again keeps manufacturing
assembly cost and complexity to a minimum compared
to other variable valvetrain actuation systems.
The new single-cylinder fuel pump with a simple cam
drive taken off the rear of the exhaust camshaft was
designed as a low cost component compared to multicylinder high pressure pumps capable of delivering fuel
at 200bar.
A general view of the Project Sabre 3-cylinder engine is
shown in Figure 22.
Fig. 21: The path to low CO2 and improved driving
pleasure (vehicle acceleration)
COST EFFECTIVENESS
The Sabre engine has been designed with a philosophy
of cost effective, affordable technology but includes all
elements required to deliver the most effective technical
solution to achieve low CO2 output. Sabre being a 3cylinder engine gives obvious parts count and associated
cost savings over a similar capacity 4-cylinder.
The base engine design includes the IEM which along
with its thermodynamic advantages has a reduced parts
count in comparison to a standard 3-cylinder assembly,
which would comprise:
Cylinder head assembly + manifold + 12 studs + 12
nuts + 2 gaskets = 28 parts.
Fig. 22: General view of the engine
CONCLUSIONS
The Project Sabre engine employs a novel and
synergistic combination of engine downsizing, gasoline
direct injection and variable valve train to achieve three
key objectives: to demonstrate low CO2 emissions, whilst
improving driving pleasure, using cost effective,
affordable technology.
Whereas the IEM design comprises:
The 3-cylinder, 1.5 litre turbocharged engine has been
designed to produce similar performance to typical 2.2
litre naturally aspirated engines. The engine produces in
excess of 240 Nm peak torque (20.1 bar BMEP) and 117
kW at 5000 rev/min. (157 bhp, 18.6 bar BMEP). Lambda
= 1 operation up to an engine speed approaching 5000
rev/min. is possible, with only moderate enrichment
necessary above this with a turbine inlet temperature
limit of 980 deg.C. Part-load fuel economy benefits of up
to 6% have been realized with the switchable valvetrain
from the testing to date.
dethrottling is still attractive) or two-stage charging
systems (because of the benefits of 3-cylinder banks in
turbocharged engines).
When fitted in place of a 1.8 litre naturally aspirated
engine in a C-class vehicle, a GM Opel Astra, the CO2
emissions are predicted to reduce by 25% (from 175 to
140 g/km CO2) in conjunction with a 1.1 second
reduction in 0 – 100 km/hr acceleration time. Vehicle
testing results are planned for another publication.
Some of the initial development of the combustion
system adopted in Sabre was carried out as part of the
HOTFIRE consortium project, comprising Loughborough
University, Lotus Engineering, Continental Automotive
and University College, London, which was funded by the
UK Engineering and Physical Sciences Research
Council. Although no direct funding came from the
EPSRC to the multi-cylinder engine design project
reported here, the authors wish to acknowledge the work
of all members of the HOTFIRE team and the funding
that this project received from the EPSRC.
This has been achieved using a novel and synergistic
combination of engine technologies, including:
Variable valve actuation based on switching tappets
and twin cam phasers.
Homogeneous air / fuel mixture with direct injection
using close-spaced, multi-stream solenoid injectors.
Single-stage,
turbocharger.
Cylinder head with integrated exhaust manifold
(IEM).
single-entry
Mild hybridization.
Friction reduction concepts.
fixed
geometry
The well documented synergies between direct injection,
turbocharging and cam phasing have been combined
with the packaging, firing interval and friction benefits of
a 3-cylinder configuration. The adoption of a switchable
valvetrain, an integrated exhaust manifold and a mild
hybrid system compliment the engine design concept in
a manner that enables low fuel consumption to be
achieved from mild downsizing with a relatively large
swept volume.
In this first phase of engine evolution, the choice of
engine technologies was consistent with an “affordable
technology” concept. The key items are all known,
current production technologies that are available at
reasonable cost and the resulting homogeneous charge
combustion system enables the use of conventional
three-way catalysts.
Engine testing to date has produced results that are
consistent with the decision process and the engine
design concept. The planned vehicle testing will verify
the achievement of the three key objectives.
It is hoped that future versions of the engine will include
such technologies as spray-guided direct injection
(because the degree of downsizing means that
ACKNOWLEDGMENTS
This engine programme was partly funded by the UK
Energy Savings Trust and the partners extend their
thanks to that organization.
The work of all members of the Sabre team within Lotus
and Continental Automotive is gratefully acknowledged.
Finally, the authors wish to thank the directors of Group
Lotus and Continental Automotive for permission to
publish this paper and their continuous support during
the project.
CONTACTS
Dennis Coltman
Chief Engineer, Powertrain Development
Lotus Engineering
Norwich
Norfolk NR14 8EZ
United Kingdom
Email: dcoltman@lotuscars.co.uk
James Turner
Chief Engineer, Powertrain Research
Lotus Engineering
Norwich
Norfolk NR14 8EZ
United Kingdom
Email: jturner@lotuscars.co.uk
Hans-Joachim Nuglisch
Senior Manager, Advanced Development, Gasoline
Systems
Continental Automotive France S.A.S
1, avenue Paul Ourliac
B.P.1149
31036 Toulouse cedex 1
France
Email: hans.nuglisch@continental-corporation.com
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ABBREVIATIONS
TM
AVT
ATDC
BMEP
BOM
BSFC
CFD
CO
CO2
COV
CPS
CVCP
EIVC
(G)DI
HC
IEM
IMEP
LIVC
NOx
NVH
PM
PWM
VCR
Active valvetrain
After top dead centre
Brake mean effective pressure
Bill of materials
Brake specific fuel consumption
Computational fluid dynamics
Carbon monoxide
Carbon Dioxide
Coefficient of variation
Cam profile switching
Continuously-variable cam phaser
Early intake valve closing
(Gasoline) Direct injection
Hydrocarbons
Integrated exhaust manifold
Indicated mean effective pressure
Late intake valve closing
Oxides of nitrogen
Noise, vibration and harshness
Particulate matter
Pulse-width modulated
Variable compression ratio