20076559 (JSAE)
2007-32-0059 (SAE)
The Potential of a New Type of Carburettor to Assist SORE in Meeting
EPA / CARB Phase 3 Legislation
Stephen Glover 1)
Roy Douglas 2)
Kristjan Omarsson 3)
Copyright © 2007 Society of Automotive Engineers of Japan, Inc. and Copyright © 2007 SAE International
Small off-road engines (SORE) have been recognised as a major source of air pollution. It is estimated that non handheld
SORE annually produce over 1 million tonnes of HC+NOx and over 50 million tonnes of CO2. The fuel system design
and its operating AFR are of key importance with regard to engine operation and engine out emissions. The conventional
low-cost float carburettors used in these engines are relatively ineffective at atomising and preparing the fuel for
combustion requiring a rich setting for acceptable functional performance. EPA and CARB have confirmed that Phase 3
limits are achievable for a “durable” engine fitted with a conventional well calibrated and manufactured “stock rich
setting” float carburettor together with catalytic oxidation after-treatment and passive secondary air injection. The EPA
and CARB strategy for meeting Phase 3 only considers the use of conventional float carburettors that operate at rich
AFR’s over their entire engine operating range as no other cost effective alternative fuel system is yet available on the
market. A cost effective alternative to the conventional carburettor that enabled leaner or optimised AFR operation with
load and improved combustion performance would open the door to alternative strategies to meeting the phase 3 limits.
This paper presents a completely new form of mechanical carburettor that gives AFR control with load, improved mixture
preparation for improved combustion performance and has a lower production cost than conventional carburettors. The
conventional and new fuel system designs and operation are discussed in detail and their technical merits demonstrated in
the form of engine test data. The performance of different after-treatment systems is also simulated for different AFR
profiles with load for a conventional or unmodified SORE engine. With optimised leaner operation and improved
combustion characteristics, this new carburettor technology can provide significant engine out CO and HC+NOx
reductions on the J1088 test cycle without loss of functional performance. Depending on the chosen emissions control
strategy, minimum engine out emissions or optimum engine AFR for oxidation or three-way after-treatment or another,
this new carburettor technology can be easily calibrated to provide the desired engine operating AFR profile on the J1088
cycle.
Key Words: Utility Engine, Intake System, Exhaust Emissions
Since then the gradual implementation of several stages
of steadily increasingly more severe legislation has
resulted in a decade of intensive emissions control
development for utility engines.
Introduction
Small Off Road Engines (SORE) are found in a very
wide and diverse range of products (Figure 1.) that can
be separated into two main product groups, handheld
and non handheld. These products are generally
classified as utility engines with a power output of less
than or equal to 19 kW. Common examples of handheld
products are chainsaws, trimmers, concrete saws etc. and
common examples of non handheld products are
lawnmowers, pumps, generators, small tractors, etc.
The handheld market is around 30 million units per
annum and is virtually completely dominated by the 2stroke engine. New carburetted stratified charge 2-stroke
engines and catalytic after-treatment are being
developed for the handheld products where weight and
multi-orientation operation are key requirements. There
have been many SETC technical papers presented on
these new stratified charge 2-stroke technologies by
companies such as Stihl, Electrolux and Komatsu (8 to
12). The 2-stroke engine is currently not the focus of the
new fuel system technology discussed in this paper
although the technology is applicable to 2-stroke
engines. For the non-handheld 4-stroke dominated
market, manufacturers are looking at improved fuel
system design, improved engine design and the use of
after-treatment to meet current and future legislative
requirements.
With annual worldwide production of over 100 million
units per annum, small off-road engines (SORE) have
been recognised as a major source of air pollution. It is
estimated that non handheld SORE products in
circulation annually produce over 1 million tonnes of
HC+NOx and over 50 million tonnes of CO2. These
SORE did not have to meet any emissions control
legislation until its introduction in the USA in 1995.
1), 3) Fjölblendir ehf
2) Queen’s University, Belfast
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A cost effective alternative to the conventional
carburettor, as detailed in this paper, that enabled leaner
or optimised AFR operation with load with improved
combustion performance would open the door to
alternative strategies to meeting the phase 3 limits. Such
a fuel system fitted to a standard Phase 2 SORE would
allow leaner operation at part load whilst maintaining
rich operation at full load for engine thermal durability.
The strategy for meeting Phase 3 could then be to
optimise the part load AFR for minimum engine out
emissions of HC+NOx and use catalytic oxidation aftertreatment with or without SAI to reduce tail pipe
emissions. The AFR could be further optimised to
balance the engine out emissions to the after treatment
system (SAI and catalyst) with regard to muffler
temperature and tail pipe emissions. Alternatively the
AFR could be set to operate at Lambda 1 at all part load
conditions and a three way catalyst used to oxidise HC
and CO and reduce NOx emissions. For more durable
SORE developed to give low deterioration factors and
lower engine out emissions it is possible that such
engines may not require after-treatment to meet Phase 3
limits and would only require the new fuel system on its
own with an optimised AFR/ load curve.
The bulk of the total 4-stroke SORE engine market, at
around 65% (Douglas and Glover ref. 7), is taken up by
single cylinder 4-stroke gasoline engines of under 6hp or
under around 160cc. In this, the largest segment of the
market, manufacturers must look to more conventional
or new technologies that can be applied without
significant add on cost or more preferable with a cost
reduction.
The fuel system design and its operating AFR are of key
importance with regard to engine operation and engine
out emissions. The conventional float carburettors used
in these engines are relatively ineffective at atomising
and preparing the fuel for combustion requiring a rich
setting for acceptable functional performance. With this
inherent disadvantage and an inability to directly control
operating AFR at part load, SORE legislation has been
set with “rich operation” CO legislative limits of over
ten times the HC+NOx limits. Operating the engines at
“rich” conditions makes the job of any after treatment
system much more difficult in terms of the heat
produced and the levels of tail pipe emissions that can be
reached. EPA (2,4) and CARB (1, 3) have tested many
examples of Phase 2 compliant SORE adapted to meet
Phase 3 with after-treatment and SAI. Many of these
adapted engines have been found not to meet the Phase 3
limits after the required operating life due to poor engine
durability, poor fuel system calibration / durability and
generally high base level engine out emissions. EPA and
CARB have confirmed that Phase 3 limits are achievable
for a “durable” engine fitted with a conventional well
calibrated and manufactured “stock rich setting” float
carburettor together with catalytic oxidation aftertreatment and passive secondary air injection. The EPA
and CARB strategy for meeting Phase 3 only considers
the use of conventional float carburettors that operate at
rich AFR’s over their entire engine operating range as no
other cost effective alternative fuel system is yet
available on the market. Given this, the EPA and CARB
Phase 3 strategy is effectively limited to dealing with
high engine out HC and CO emissions by using
oxidation catalytic after-treatment with SAI.
Some manufactures are also looking at improved engine
design to modify the engine out emissions. A reduction
in crevice volumes and improvements in bore roundness
and valve sealing etc. would reduce HC emissions.
Engine out NOx emissions would remain relatively
unchanged and relatively low due to the required rich
operation of the engine as the conventional carburettor is
still being used and no improvements have been made to
the combustion robustness. With rich operation HC
emissions would still be high but the combined engine
out HC+NOx would potentially be lower than the non
modified engine. The new fuel system discussed in this
paper could be used in this instance to optimise the
engine operating AFR for minimum HC+NOx or
potentially for operation at Lambda 1 for three way
catalytic conversion.
Figure 1: Examples of Some Handheld and Non Handheld SORE
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for service adjustments. The new fuel system described in this
paper is not yet in mass production so there is currently no
data on production variations or its ability to hold a lambda 1
window of operation. The paper does present data that shows
the new fuel system has a much smaller cyclic variation (cycle
to cycle) in delivered AFR compared to a conventional
carburettor which is clearly advantageous for three way
catalytic conversion. The paper has been written assuming that
the same if not better production variation can be attained with
this new fuel system compared to the conventional fuel system
(EPA defined Phase 3 fuel systems with improved
manufacturing quality) and also that any “in use” deviation
from a Lambda 1 calibration can be re-set in service by a
service dealer.
Improvements to the base engine for minimum HC emissions
as described above and improved combustion robustness by
improved air motion could significantly raise the engine out
NOx emissions. Operation with the conventional fuel system
would not allow the AFR to be optimised for minimum engine
out HC+NOx emissions due to the lack of AFR control with
load and the need to run rich at full load. Similarly, use of the
conventional fuel system would not allow part load operation
at Lambda 1 for effective three way catalytic conversion as no
AFR control with load is possible and also the convention fuel
system will not allow the engine operate at lambda 1 due to
poor fuel / air mixture preparation. Use of the new fuel system
described in this paper on this developed engine would most
likely have the AFR set for Lambda 1 operation at part load to
catalytically reduce the high engine out NOx and catalytically
oxidise HC and CO in a three way catalyst. If EGR was used
to reduce engine out NOx emissions, the new fuel system
could be used to optimise the AFR for minimum engine out
HC+NOx and operate with oxidation after-treatment with or
potentially without SAI. Alternatively, lambda 1 operation at
part load could be selected even with EGR and three way
catalytic conversion used.
Other factors such as ambient air temperature, altitude and
fuel type / quality will clearly effect a given or fixed
calibration of the new fuel system described in this paper in
the same way as they affect the conventional carburettor. As
for the conventional carburettor, re-jetting the fuel system or a
variable manual control jet as is provided in the new fuel
system can be used to account for differences in altitude and
fuel quality. OEM’s are known to sell products with a range of
jets with instructions on setting up the product for different
altitudes and fuel type. Products like chain saws, trimmers etc.
are sold with variable low and high speed adjustable jets to
allow the user to make any required adjustments to the
running calibration. The new fuel system described in this
paper can employ the same calibration adjustment techniques
as a conventional carburettor.
The ability of this new fuel system to cost effectively control
AFR with load and improve the robustness of the combustion
system by improved atomisation and reduced cyclic variation
in AFR, opens the door to many more possibilities for meeting
Phase 3 limits with different emissions control strategies. The
actual strategy used in production is very engine dependent in
terms of its engine out emissions and the deterioration factor
over the useful life. With leaner operation (optimised AFR at
each J1088 mode point) and improved combustion
characteristics, this new carburettor technology can provide
significant engine out CO and HC+NOx reductions on the
J1088 test cycle without loss of functional performance.
Depending on the chosen emissions control strategy,
minimum engine out emissions or optimum engine AFR for
oxidation or three-way after-treatment or another, this new
carburettor technology can be easily calibrated to provide the
desired engine operating AFR profile on the J1088 cycle.
Clearly, as for any new fuel system or indeed the conventional
fuel systems, their ability to hold a given AFR setting and a
given AFR characteristic or curve with load in mass
production and over the life of the product (with service
adjustment) is a key requirement. For any fuel system, the key
to operating the engine at lambda 1 is holding the Lambda 1
window for efficient 3 way catalytic conversion over the
operating life of the engine and fuel system. The wider the
“Lambda 1 window” of operation in mass production or the
higher the in-service drift from Lambda 1 between service
adjustments the less efficient the catalytic conversion. Clearly
a Lambda sensor in the exhaust and a feed back control system
provides an interactive solution but at higher add on cost. In a
similar way to the Lambda 1 scenario, the ability to optimise
the part load AFR of an engine for lowest engine out
emissions is clearly dependent on the ability of the fuel system
to maintain this optimised AFR curve in mass production (fuel
system and engine) and over the life of the product allowing
Cost-effective, self regulation at lambda 1 by means of feed
back control is clearly a key goal for the new fuel system
described in this paper. This paper mainly describes a new fuel
system that has an open-loop Lambda 1 calibration that is
factory set and set in-service. As described above there are
questions that need answered as to the ability of the fuel
system and engine to retain a lambda 1 calibration in real life
usage and if the calibration varies from lambda 1 what level of
variation is allowable before three way catalytic conversion is
no longer effective. These questions can only be answered on
completion of the development of the fuel system and on
starting of the production process with field trials.
This paper also introduces an electronic version of the new
fuel system that has an ECU and feed back control. Although
more expensive that a conventional open loop carburettor, the
electronic version of the new fuel system is seen as being
comparable to EFI in terms of performance but at a reduced
cost (no fuel pump, no injector, potentially less sensors and
potentially a smaller lower processing power ECU).
Conventional Fuel Systems and Their Characteristics
All non hand-held SORE are currently fitted with
conventional float carburettors as shown in Figure 2. Hand
held units are fitted with diaphragm carburettors to allow multi
orientation use.
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Progression Jets
Idle Jets
Figure 2: Conventional Non Handheld SORE Multi Point Metering Float Carburettor
reach further and further back or upstream, activating more
and more progressions holes and eventually the main jet.
All non hand-held SORE are currently fitted with
conventional float carburettors as shown in Figure 2. Hand
held units are fitted with diaphragm carburettors to allow multi
orientation use.
All the metering holes or jets are fed by both air and fuel. The
air helps to atomise the fuel but it is mainly used to decrease
the sensitivity of the metering jets. If more air is bled into the
fuel stream upstream of the jet metering orifice or within the
metering system, then less fuel is delivered by the jet for a
given vacuum. If air bleeding was not used it would make the
metering holes more sensitive to production tolerances. One
important point about bleeding air upstream of a jet or within
the jet (main jet, emulsion tube etc.) is that it generally leads
to irregular fuel delivery from the jet due to the time varying
The conventional non hand-held float carburettors are multi
point metering in that they have idle, progression and main
fuelling orifices or jets. They have an aluminium die cast body
and have a butterfly throttle valve to control the flow of air.
The butterfly valve, when closed, is down stream from the
progression and main metering orifices. As the butterfly is
progressively opened the depression in the intake manifold,
which decreases with throttle opening, is effectively able to
Class I Phase 3 Emissions
• A 40-50% HC+NOx reduction from Phase 2 is technologically feasible using:
o
A small 3-way catalyst
o
Catalyst volume at ~25% of engine displacement (125-hr. useful live)
o
Passive (venture) secondary air entrainment
o
5:1 Pt:Rh, 30-40 g/ft3
•
Improvements in carburetor calibration production tolerances
o
Improvements to carburetor casting and machining processes and assembly to reduce air-tofuel ratio calibration variability
o
Not a major issue for Briggs and Stratton
•
Reduction of vacuum leaks in the intake manifold and carburetor body (intake manifold gasket)
Figure 3. EPA Phase 3 Strategy for Class 1 Phase 2 Engines (ref 2, 5)
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The same characteristic shape or profile can be found for
several different engines with similar type multi point
metering float carburettors in reference 3. These multi point
metering float carburettor fuel systems are generally not fitted
with accelerator pumps as the generally rich setting of the
engine does not require the use of accelerator pumps for good
transient response.
density of the air / fuel fluid. This in turn leads to a high cyclic
variation (engine cycle to engine cycle) in the delivered AFR
to the engine.
The position and size of the metering holes needs to be very
accurately maintained in production. Although the author does
not have any direct supporting production data, it is believed
that there can be considerable production variation in the fuel
delivery characteristics of these fuel systems as indicated by
EPA in Figure 3 below (ref 2, 5). It is one reason why EPA
and CARB have specified that the production variations of
these fuel system needs to be reduced for their Phase 3
strategy. It is also one reason why motorcycles, in light of
steadily more severe emissions legislation, have seen the move
from multi point carburettors like these to single point slide
carburettors and finally to single point CV carburettors. These
carburettors are not truly single point metering as there is a
separate idle circuit. Each upgrade of carburettor represents an
improvement in metering accuracy but increased cost.
Machining four to five progression holes and an idle hole in
precise size and position is more difficult to keep to tolerance
than for a single metering hole with a profiled needle.
With the engine necessarily set rich at full load, the part
throttle fuelling is therefore also rich. As discussed by
Douglas and Glover (7), it is one reason why the EPA/CARB
legislative CO limits are set very high. It may be possible to
machine the progression holes such that leaner operation could
be obtained at part load and an accelerator pump fitted for
good transient response. However, as will be shown in the
combustion analysis section, these fuel systems do not provide
a relatively stable and well mixed air fuel mixture to the
engine making the combustion unstable at leaner operating
conditions. Perhaps it would be possible to develop the
conventional float carburettor to provide AFR control with
load but this would likely add cost and complexity which are
undesirable from a market acceptance and manufacturing
stand point.
The general fuelling characteristic of these multi point float
carburettors is such that the AFR gets richer as the load is
decreased at a constant engine speed. Figure 4 shows this
characteristic for a 160 cc 4-stroke SORE generator engine (6).
The calibration of one float carburettor for production for a
specific engine can take a relatively long time, anywhere
between six months to a year. The calibration process is
Figure 4 : Constant Speed (3000 rpm) AFR Characteristic of Conventional Multi Point SORE Float
Carburettor on Honda GX160 Generator Engine (ref 3 and 6)
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to the stock carburettor. Both TCT AFR profiles maintain the
rich setting at full load for power and for engine durability. As
Douglas and Glover (7) describe there are a number of
different possible emissions control strategies involving
operating AFR, engine design improvements and aftertreatment strategies.
generally experience based and requires several hundred
prototype castings that are machined on a trial and error basis
until the right calibration is achieved.
Douglas and Glover (7) have demonstrated that rich AFR
settings at all part load conditions is undesirable for lowest
engine out emissions or for optimising the efficiency of the
after treatment system. They have also demonstrated that AFR
control with load, improved combustion robustness (air fuel
mixture preparation) and reduced cyclic variations in AFR
could reduce engine out emissions and improve the efficiency
of the after treatment system. This general strategy could give
a more effective solution for Phase 3 limits than the CARB
and EPA Phase 3 strategies.
Once the fuel has been metered it is drawn into the fuel nozzle,
as shown in figure 7, where it is atomised with air. Figure 7
also shows a CFD simulation of the atomisation process. One
other key difference between the TCT and the conventional
fuel system is that no air is added to the fuel before it is
metered. This, the authors believe, leads to lower cyclic
variations in the quantity of fuel delivered to the engine as will
be demonstrated in the combustion analysis section. In the
TCT fuel system it is only after the fuel is metered that the
fuel is atomised by air in the nozzle. Figure 6 also shows a one
way valve positioned at the base of the fuel tower/ column.
This valve, which can be either a ball or disc valve, allows
fuel to enter the fuel tower from the float chamber but
prevents fuel from returning to the float chamber. It is
believed by the authors that this valve also assists in reducing
cyclic variations in AFR by maintaining, to a greater extent,
the fuel supply reservoir volume. Any intake manifold
pressure pulses that are able to enter the fuel column via the
nozzle are unable to force fuel out of the column back to the
float chamber.
There is clearly a requirement for a cost competitive or lower
cost single point metering fuel system that not only has AFR
control with load but improves the air fuel atomisation and
reduces cyclic AFR variations. The authors believe that the
new fuel system detailed in this paper fulfils this requirement.
The New TCT Fuel System and How it Works
This paper and Douglas and Glover (7) introduce a new type
of fuel system known as TCT (Total Combustion Technology).
Figure 5 shows several pictorial representations of how the
TCT fuel system works. The fuel system is a single point
metering fuel system but can have a separate idle circuit either
working in conjunction with the main metering orifice at idle
or independently with the main metering circuit switched off.
This optional independent idle circuit, shown in Figure 5, is a
conventional idle circuit as found in other fuel systems
currently on the market today.
With the throttle now where the choke is traditionally
positioned in a conventional fuel system, there is clearly more
vacuum over the main jet at all part throttle conditions. Clearly
at part throttle the vacuum sensed in the carburettor bore at the
progressions holes in a conventional carburettor and at the
TCT nozzle are much the same. Figure 8 shows the flow
bench test results of a conventional SORE carburettor and a
TCT carburettor under the same flow test conditions. The air
bleeding in the standard SORE carburettor and the TCT air
nozzle are both active as per normal operation. The graphs
show the vacuum as measured in the main fuel circuit where
the fuel is picked up in mm H20 with throttle angle. As would
be expected the graph shows that for any given throttle angle
the vacuum over the main metering jet is higher for TCT, even
at wide open throttle. Apart from the throttle being upstream
of the jet and therefore providing some pressure loss at WOT,
the main reason for this difference at WOT is due to air
bleeding in the standard carburettor and the nozzle design in
the TCT fuel system.
There are several key differences between the new TCT and
conventional fuel systems. The first main difference is that the
throttle valve is upstream of the metering jet. Secondly, the
metering of fuel is dependent on the position of the needle
controlled by a cam (MLC or Mechanical Lambda Cam) and
linked to the throttle. The fuel is metered by controlling the
flow area around the needle in the region around the side
metering tube or delivery tube which connects the fuel column
to the nozzle. Clearly the variable vacuum over the metering
nozzle also controls the fuel flow. The movement of the cam
can be linear or non linear with throttle movement depending
on the actual design of the linkage between the two
components. The cam can have any profile and as such the
engine can have any AFR profile or curve with load at a
constant engine speed. It is for example possible to operate an
engine at 12 to 1 AFR at full throttle and say at 14.5 to 1 AFR
at all other mode points on the J1088 cycle. As Douglas and
Glover (7) describe, the idea of the TCT fuel system is to
allow the part load AFR to be optimised for what ever strategy
is being used, be it for minimum engine out HC+NOx
emissions or to operate at Lambda 1 for three way catalytic
conversion. Figure 6 shows the ideal AFR profiles for a
particular 160 cc 4-stroke SORE engine for minimum engine
out HC+NOx emissions and for part load Lambda 1 compared
As mentioned, no air is bled into the fuel stream before it is
metered in the TCT. Air bleeding, in for example an emulsion
tube in a conventional carburettor, reduces effective signal or
de-sensitises the fuel system. The TCT nozzle achieves the
same de-sensitising effect by drawing air across the nozzle
exit orifice as shown in Figure 7. By changing the design of
the nozzle or the diameter/length of the fuel delivery tube, it is
possible to reduce the pressure signal as measured where the
fuel is picked up in the TCT fuel system. In standard
carburettors the emulsion tube bleeds in air and reduces the
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Needle follows cam
profile, increasing or
decreasing fuel flow
as calibrated
Cam
The cam moves forward
as the throttle opens. The
shape of the cam adjusts
the A/F ratio as required.
Needle Carrier Roller
Sectional View
View of Needle, Needle Case
and Atomization Nozzle
Figure 5 : Representations of How TCT works
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Ideal for Minimum Engine Out HC+NOx
Honda GX 160 4-Stroke
G
t
18
Standard Carburettor
Ideal for Three Way Conversion
AFR
16
14
12
10
8
0%
25%
50%
75%
100%
Engine load / J1088 Load Mode Point
Figure 6: Optimised TCT AFR Profiles with Load for Minimum Engine Out HC+NOx Emissions and
for Three Way Catalytic Conversion Compared to the Conventional Stock / Standard Carburettor
Atomization Nozzle
Needle
Metering Tube
Fuel Tower
Fuel Metering
One way valve
Figure 7 : TCT Atomisation Nozzle, Fuel Column and One Way Valve; and a CFD Image of the
Atomisation Process
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Figure 8 : Steady State Flow Bench Tests of Pressure Measured at the End of the Main Fuel
Circuit where the Fuel is Picked Up Versus Throttle Angle for the TCT Fuel System and a
Conventional Honda GX160 Fuel System. Also Shown is an Estimated Progression Hole Pressure
Profile.
Primer
Normal Fuel Level
Cold Start Level
Normal Running Operation
Cold Start Operation
Figure 9 : TCT Carburettor with Side Float Bowl and Cold Start Primer Mechanism
vacuum signal at the main jet. Air bleeding between the
progressions holes and from additional air bleeds reduces the
fuel pick up signal for the progression holes. If the fuel pickup pressure was also measured at the progressions holes then
clearly the pressure profile would be closer to that of the TCT
fuel system. The only difference would be the air bleeding in
the conventional carburettor reducing the effective signal
compared to the effectiveness of the TCT nozzle design at
reducing signal. With higher vacuum, the TCT will respond
quicker to engine demand changes in fuelling. As mentioned
above, the design of the TCT nozzle and the fuel delivery tube
can be changed to reduce the fuel pick up signal to the same as
measure in the standard fuel system at the progression holes.
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Figure 10 : Original TCT Tapered Needle with Corresponding Sinusoidal Fuel Flow Characteristic
with Adjustment and New Solid Ground Profile Needle
The new needle is a solid rod with an accurately ground
profile on one or more sides. This needle does not revolve
when adjusted in production, it simply moves up and down by
using a V groove or rotary joint in the needle to translate
rotational and vertical movement of the needle adjuster into
only vertical movement of the needle. Rotation of the needle is
prevented by a groove or a flat on the needle working against
a stopper. The idea of having several ground profiles on one
needle would allow the fuel system to have several
calibrations for one type of fuel or potentially different
calibrations for different fuels.
However, this would likely be at the cost of response which
the current development phase wishes to improve upon.
With the tested TCT design showing higher vacuum than the
standard carburettor there is clearly a question of how
sensitive does this make the TCT over the standard carburettor.
This is discussed below in the discussion on nozzle and needle
design. The other question regarding the position of the
throttle is how is the engine enriched for cold starting. The
answer is that a simple primer mechanism or a needle lift
system can be used. For the primer system, as shown in Figure
9, the float chamber is flooded by depressing the float and
allowing fuel to enter the bore of the carburettor. For the
primer system, the volume of fuel in the float chamber
between the cold start and normal run conditions clearly
effects the duration of the cold start enrichment. The needle
lift method (not shown) is to have a simple mechanism that
lifts the needle (or needle and cam plate) to increase the fuel
flow on pull starting by increasing the metering area between
the needle and delivery tube.
This new ground needle was found to reduce sensitivity as the
fuel flow from around the needle in the tapered design
(leakage fuel) to the metering tube is greatly reduced for any
given vacuum. Figure 11 gives an indication of the sensitivity
of the new TCT needle design compared to a motorcycle slide
carburettor. The authors understand the limitations of this
sensitivity comparison in that the angle or profile on the
motorcycle carburettor needle is generally variable along the
length of the needle and that it is effectively the entire length
of the needle that is within the main jet assembly that is
controlling metering. Nevertheless the test still serves to give
an indication of sensitivity between the two fuel systems. The
graph shows the change in engine AFR or fuel flow In terms
of CO emissions at the same starting point test load for a
change in lift for both carburettors. The engine was initially
set at 2 Nm and 6.5 % CO and the needle lift then varied
without changing the throttle position. Clearly the angle of the
profile on the needle is key here as a change in the angle will
change the results. Also a change in motorcycle carburettor air
bleeding or TCT nozzle design can also change the results.
The motorcycle carburettor has a taper angle of 1.8 degrees (as
indicated on Figure 11) over the needle height used for 2 Nm
and the TCT needle shown an angle of 1.5 degrees. It should
be noted that the overall lift of the TCT needle from idle to
WOT is about 12 mm, whereas for the motorcycle carburettor
it is 18 mm. In the case of the standard motorcycle needle the
The fuel nozzle and needle design are critical for the TCT fuel
system. Figure 10 shows the original TCT needle design was
tapered. This design was investigated and found to be too
sensitive, the machining accuracy being very tight. Also, when
adjusting the fuelling as is necessary for all production
carburettors by turning the needle, it was found that the
resulting AFR variation was cyclic or sinusoidal. This
indicated that the taper did not revolve exactly in the centre of
the needle case or that the shaft of the needle was not perfectly
straight. This sinusoidal variation is depicted in Figure 10. In
terms of sensitivity the tapered needle was metering fuel as
designed from below using the taper but also from the sides
giving less control over actual metering for a given vacuum
and needle position. Figure 10 depicts this situation and shows
the new type of needle.
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Figure 11: Indication of Sensitivity of TCT Fuel System Compared to Honda 125 cc Motorcycle
Carburettor in Terms of AFR or CO Variation at Constant Load Versus Needle Lift.
sensitivity of the fuel system. Cleary air could be bled in
before the metering orifice to reduce tolerances, increase
clearance and reduce friction, but this would likely be at the
expense of the improved or reduced cyclic AFR variations and
could lead to a slower response to changes in fuelling. With
tight tolerances between two critical components the question
of wear or durability is necessarily raised. Clearly, material
choice is key in reducing friction and minimising wear. Cost is
also important, with tighter tolerances the manufacturing cost
increases.
varying taper or profile is effectively like the TCT cam, the
variable taper or profile varying the required fuel flow
characteristic as the needle is lifted. So, effectively the varying
taper or profile on the motorcycle carburettor needle provides
the same function as the cam on the TCT. Clearly the ground
profile on the needle on the new fuel system can also be used
to vary AFR in conjunction with the cam.
With more vacuum over the main nozzle the TCT should
have greater response to changes in engine demand than the
conventional slide motorcycle carburettor or conventional
multi point SORE carburettor. Although not yet measured in
terms of a “governor droop test” the engine response has been
subjectively observed as being faster than when fitted with
motorcycle carburettor or a standard SORE carburettor from
or at any given engine operating AFR. Most OEM’s have their
own specific “governor droop test”. It is a test that effectively
measures the change in governed engine speed following a
change in engine load or demand. As the load is applied or
removed the engine speed either falls or rises accordingly until
the centrifugal governor is able to bring the speed back to the
governed speed.
These frictional and manufacturing issues, together with the
desire to further de-sensitise the fuel system, lead to the
development of another new needle metering system. It is still
based on the principle of a solid non rotating needle with a
ground profile or flat on one side. Figure 12 shows this
innovation. The needle case has a floating seal or saddle that
magnetically attaches itself to the needle. The floating seal sits
on the main needle or rod diameter so that it effectively seals
the leakage paths previously described. The metering as before
is therefore determined by the difference in geometric area
between the main needle external diameter and the ground flat
on the needle. Springs could also be used rather than magnets
but it adds complexity and potentially cost to the design. The
floating seal does not have to be magnetic itself as shown in
Figure 12. Here the floating seal is potentially metal or
injection moulded plastic with a low friction (Teflon) coating.
The magnet itself can also be injection moulded plastic with
magnetised metal powder mixed in with the plastic. The use of
this innovation will reduce friction, reduce leakage, improve
sensitivity, reduce key component manufacturing costs and
The governor droop test is not only about the fuel system
“fuelling” response it is also about friction in the fuel system.
The tight tolerances between the needle and the needle case in
the case of TCT and the movement of the cam clearly mean
that the TCT has potentially higher friction than the
conventional SORE carburettor. The friction was found to be
highest between the needle and the needle case. The tolerances
here are tight to reduce leakage of fuel and reduce the
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Needle
Floating Seal with Metering Hole / Delivery Tube
Atomization Nozzle
Figure 12 New Floating Seal Needle / Needle Case Design.
will remove any wear or durability issues from these key
components. It will also allow the main fuel circuit to be
switched off by moving the metering hole or delivery tube on
to the base needle diameter above the ground metering groove
effectively blocking the metering hole or delivery tube. This
could be used in conjunction with an independent idle circuit
if required allowing the main and idle circuits to be
independent.
The above section described the main working principles of
the TCT fuel system and some of the areas under development.
It can be stated that in a worst case scenario, the TCT fuel
system would be the same as a standard SORE fuel system in
terms of fuel delivery or atomisation performance with the
one-way fuel valve removed, the nozzle designed for the same
sensitivity and with air bleeding in the fuel metering section,
except it would have the additional benefit of AFR control
with load.
Combustion Analysis
All the test data presented in this section is from the earlier
TCT fuel system with a tapered needle and no floating seal.
The authors consider that the recent and proposed innovations
described above to the needle design and the introduction of a
floating seal would not cause any decrease to the combustion
improvements described below but would be more likely to
enhance the combustion improvements due to more precise
metering control and its effect on cyclic AFR variation.
Early testing of the TCT fuel system subjectively showed
improved engine operation at leaner AFR’s over the
convention SORE carburettor. The engine was seen to have
improved response and ran smoother, even at leaner AFR’s
than the stock carburettor. A prototype TCT (with the tapered
needle) was adapted for used on a Honda GX 160 generator
engine and combustion analysis tests carried out on the TCT
and standard carburettor builds. The test work was carried out
at a medium load of 4 Nm and at and an engine speed of 3000
rpm. The main tests consisted of constant torque AFR swings
at different ignition timings.
The electronic concept or version of the TCT fuel system, as
mentioned in the introduction, and how it is intended to work
is described at the end of the paper.
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Figure 13: Graph of COV versus AFR at 4 Nm Constant Torque with Different Ignition
Timings for the TCT Fuel System and the Standard Honda GX160 Multi Point Float
Carburettor
Figure 14: Graph of LNV versus AFR at 4 Nm Constant Torque with Different Ignition
Timings for the TCT Fuel System and the Standard Honda GX160 Multi Point Float
Carburettor
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Figure 15: Graph of 10 to 90% Burn Angle versus AFR at 4 Nm Constant Torque with
Different Ignition Timings for the TCT Fuel System and the Standard Honda GX160 Multi Point
Float Carburettor
Figure 16: Graph of CO2 (%) versus AFR at 4 Nm Constant Torque with Different Ignition
Timings for the TCT Fuel System and the Standard Honda GX160 Multi Point Float
Carburettor
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Figure 17: Graph of CO (%) versus AFR at 4 Nm Constant Torque with Different Ignition
Timings for the TCT Fuel System and the Standard Honda GX160 Multi Point Float
Carburettor
Figure 18: Graph of NOx (ppm) versus AFR at 4 Nm Constant Torque with Different
Ignition Timings for the TCT Fuel System and the Standard Honda GX160 Multi Point
Float Carburettor
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Figure 19: Graph of NOx (ppm) versus Average Delivered AFR for Different Imposed
AFR Variations or Distributions (0, 1 and 2) on the Mean / Average Delivered AFR
The combustion data shows that the COV (coefficient of
variation) and LNV (lowest normalised value) of combustion
are better for the TCT fuel system than the standard fuel
system, especially at leaner operating AFR’s on the rich side
of stoichiometric. The combustion of the standard fuel system
becomes increasingly more unstable as the AFR is made
leaner, especially at more retarded ignition timings. The TCT
fuel system has a flatter characteristic showing its combustion
is more stable and able to better tolerate leaner operation. At
all operating AFR’s tested, the TCT fuel system has a better
(lower) COV and a better (higher) LNV than the conventional
fuel system, Figures 13 & 14.
The NOx curve versus AFR in Figure 18 is particularly
interesting. It shows that the NOx is lower at richer AFR’s and
higher at leaner AFR’s on the rich side of stoichiometric for
the TCT fuel system compared to the standard fuel system.
This trend is explained in Figure 19 which shows the effect of
imposed cyclic variations on delivered AFR to the engine. The
imposed variations are 0, 1 and 2 AFR’s on the mean AFR.
The trend shows that with a smaller AFR distribution, the
AFR-NOx trend matches that of the test data showing that the
TCT fuel system is able to deliver fuel in a more cyclically
stable manner compared to the standard fuel system. The data
indicates that the standard fuel system has an AFR distribution
of around two whereas the TCT fuel system has cyclic AFR
distribution of less than one. With a lower cyclic variation in
AFR the combustion is clearly more stable for the TCT fuel
system and clearly the additional benefit is the improved
efficiency of any catalytic after-treatment system. The CO2
and CO trends versus AFR in Figures 16 and 17 for the TCT
and conventional fuel systems also show that the TCT
provides a lower cyclic AFR variation.
The burn rate test data shows the TCT fuel system provided a
faster burn than the conventional fuel system, between 5 to
10% faster depending on the AFR.
The question of why the combustion is better with the TCT
fuel system has been discussed in the section of “How TCT
Works” and is though to be due to better atomisation, better
mixing of fuel and air and lower cyclic variations in the
delivered AFR to the engine. The graphs of CO and CO2
versus AFR show that at leaner AFR’s the TCT fuel system
has higher CO2 and lower CO for the same operating AFR
compared to the standard fuel system. This shows the mixture
combusted is more homogeneous, with the TCT providing a
more stable and better mixed air fuel mixture to the engine.
Figures 13, 14, 15, 16, 17, 18 and 19 show the test results of
the combustion analysis.
With reduced metering leakage with the floating seal needle
design, the authors believe the TCT will potentially give even
lower AFR cyclic variations and importantly make the
production variation from carburettor to carburettor lower
while at the same time reducing production tolerances. It is
hoped that the results from tests with the new needle design
will be presented in a future SETC technical paper.
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Starting
Startability is a key issue with all SORE and in general the
engine must start within two pulls. A fully detailed
comparison of the startability of the TCT fuel system against a
standard fuel system has not yet been carried out. Subjectively
the startability is improved with the TCT fuel system. The
results shown in Table 1 are from a startability analysis carried
out on a Honda GX160 fitted with a TCT fuel system. The
tests where carried out at a constant 10 degrees C room
temperature and taken over a 4 day period. Once the engine
fired it was immediately shut down and allowed to cool for ten
minutes. Tests where carried out in batches of ten with one
hour wait between batches.
TCT Pull Start Tests on Honda GX160 Generator at 10
degrees C
%
StartOK
Fail 1 Pull 2 Pulls 3 Pulls 4 Pulls ability
205
5
199 6
Pass Pass
4
Fail
1
Fail
97.62%
Table 1: Results of Startability Tests on Honda
GX160 with TCT Fuel System
The results show that out of 210 tests that 205 started within 2
pulls, 199 of them in 1 pull. Only 5 starts required more than 2
pulls.
Cost Impact of TCT Strategy
Figure 20: J1088 Test Cycle Results for a
Class 1 Tecumseh Side Valve Engine Fitted
with Standard / Stock and TCT Carburettors.
Manufacturing cost is of paramount importance in the SORE
market. To meet Phase 3 limits will generally require
improvements to the base engine / fuel system design and
manufacturing quality and will also generally require the use
of after-treatment. Any additional cost in meeting Phase 3 is
highly undesirable, especially for entry level products which
are more cost sensitive.
at being 25% less than the conventional fuel system at the
same annual volume.
Engine Out Emissions Cycle Results
The benefits of the TCT fuel system is terms of reducing
engine out emissions, improving combustion performance and
improving the efficiency of an after-treatment system have
been presented in this paper and by Douglas and Glover (7).
All of this must be done at as low an add-on cost as possible
or at reduced cost.
The development of the TCT fuel system is continuing on the
dynamometer. J1088 cycle tests where carried out on an early
tapered needle version of the TCT on a Tecumseh engine and
gave engine out cycle emissions reduction of 34% for
HC+NOx and 40% for CO over the stock carburettor. These
tests results are shown in Figure 20. As discussed by Douglas
and Glover (7), the actual J1088 cycle engine out emissions
reductions are dependent on the optimised AFR curve with
load.
When counting all components (gaskets, “O” rings, fixing
screws, springs, seals, etc.) the TCT fuel system (new floating
seal design) has around 20 % fewer components compared to
the standard fuel system. Most of these components like the
main body can be made from plastic. There is some debate in
the industry as to whether or not a carburettor body should be
made from plastic or not due to heat retention or fuel
evaporation issues. If there is an issue here then the TCT body
can still be made from aluminium. The manufacturing cost of
a TCT fuel system, without the floating seal, has been quoted
More recent tests using the TCT fuel system on a Honda
GX160 show that a 27% reduction in engine out HC+NOx
emissions and a 30% reduction of CO emissions can be made
by using the TCT fuel system and optimising the part load
AFR at each mode point. The results of these tests are shown
for the main J1088 mode points in Figure 21 and 22.
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Figure 21: Weighted J1088 Mode Point
CO Test Results for a Honda GX 160
Simulation Build
1
2
3
4
5
6
7
Figure 22: Weighted J1088 Mode Point
HC+NOx Test Results for a Honda GX 160
Specific Simulation Characteristics
Baseline Engine Out (No catalyst)
Baseline - Rich 12 to 1 AFR all Modes with Passive SAI
12 to 1 AFR at WOT and Stoich. At all Part Loads. High +80% Conversion of HC, CO and NOx. No SAI
12 to 1 AFR at WOT and Stoich. At all Part Loads. Low - 40% Conversion of HC, CO and NOx. No SAI
Slightly Rich Part Load AFR at 14 to 1 AFR, High Conversion of Nox, Low Conversion of HC and CO. No
As Build 5 but Richer at 75% Torque Mode Point of 13 to 1 AFR. No SAI
As Build 5 but High Excess SAI Flow ( Pulse Air System)
Figure 23: Simulated After Treatment Performance on Honda GX160 SORE Fitted With a TCT
Fuel System With Various Engine Operating AFR Profiles Over the J1088 Cycle and Various
Catalyst Conversion Efficiencies.
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catlayst design and estimates temperature icreases from these
conversion rates.
Reducing engine out emissions is clearly important, but
depending on the general emissions control strategy adopted
(Douglas and Glover (7)) it may be that the J1088 engine out
HC+NOx emissions are actually higher with an optimised
AFR curve with load than for the stock carburettor in order to
improve the efficiency of the after treatment system for over
all lower tail pipe emissions (ie. Lambda 1 operation and no
EGR with three way after-treatment).
Compared to the current EPA/CARB aftertreatment strategy
build, all the new fuel system builds show:
•
•
The New Fuel System and After-treatment
•
To date no test work has yet been carried out using after
treatment on an engine fitted with the TCT fuel system.
However, an after treatment simulation has been carried out
using real engine out test data from an unmodified 160 cc
Phase 2 certified engine as feed data for a detailed catalyst
simulation. The results of this simulation together with the
various simulation builds in tabular form are as shown in
Figure 23.
The use of the new fuel system to operate at lambda 1 and
allow 3-way conversion in a larger catalyst produces the
lowest HC+NOx emissions but relatively high exhaust gas
temperatures (adiabatic) of 830°C. The strategy of controlling
NOx at the engine by AFR and using Pulse Air SAI is very
effective at reducing emissions and temperatures
Given that the new fuel system can “allow” the engine operate
lean at part load, what is the emissions control strategy for a
unmodified or developed Phase 2 certified SORE fitted with
the new fuel system? Will muffler temperature be an issue?
The scenarios considered in the simulation were as follows:
•
•
•
•
Significantly reduced tail pipe CO emissions of 31 to
93%
Except for the stoichiometric build with a smaller
catalyst (Build 4), significantly reduced tail pipe
HC+NOx emissions of 37 to 60%
Except for the stoich. with a larger cat build, similar or
reduced exhaust gas temperatures
Ultimately we need to burn less fuel and facilitate effective
aftertreatment emissions reductions to globally reduce all
pollutant emissions from SORE
Clearly, the simulation has its limitations without making it
much too complex:
Part load lambda 1 operation with 3-way catalyst
conversion.
Part load 13/14-to-1 AFR operation to reduce engine out
NOx with relatively low catalytic oxidation of HC and
CO.
Part load 13/14-to-1 AFR operation to reduce engine out
NOx and passive SAI for catalytic oxidation of HC and
CO.
Part load 13/14-to-1 AFR operation to reduce engine out
NOx and pulse air SAI for catalytic oxidation of HC and
CO and for exhaust cooling.
•
•
Peak temperatures of 830°C do not allow for any cooling
due to the engine cooling circuit
EPA/CARB have shown that muffler skin temperature
reductions of 150°C are possible by re-designing the air
cooling circuit and the muffer. Given this it is
theoretically possible that the stoichiometric three 3-way
build (Build 3) could operate at less than 700°C muffler
temperature whilst achieving over 60% HC+NOx and
CO tail pipe emissions reductions compared to the
current EPA/CARB after-treatment strategy build
How should the engine part load AFR actually be set for best
tail pipe emissions and regulated catalytic muffler
temperatures? Experimentation is needed to answer this
questions as improved cooling effects with revised cooling
circuits and larger muffler surface areas are not easily
simulated. However, the simulation has shown that the use of
the new fuel system to control or optimise part load AFR
provides lower tail pipe emissions of HC+NOx and CO than
the proposed EPA/CARB stratagies for Phase 3.
Operating leaner produces less heat for full conversion of
engine out pollutants, so ultimately for the same tail pipe
emissions, produces less heat in the catalyst. This lowers tail
pipe or muffler temperatures, even when accounting for the
increased engine out temperatures (leaner operation results in
higher engine temperatures but lower catalyst temperatures so
overall the tail pipe temperature is lower with leaner
operation). Temperatures above 800ºC are unacceptable for
SORE so the engine out emissions, the catalyst strategy and
the air cooling need to be designed to limit temperature whilst
obtaining the lowest possible emissions
The Electronic TCT
At the moment, the electronic version of TCT is only a B
concept. The idea, as shown in Figure 24, is to use a solenoid
or a piezoelectric stack to adjust the fuel quantity delivered
The solenoid or stack/ring does not close off completely the
fuel metering hole or delivery tube, but merely make fine
adjusts to its size. The adjustment afforded by the solenoid or
piezoelectric stack or ring is to maintain the AFR at Lambda 1
for three way catalytic conversion and to account for engine to
The simulation applied here uses real engine out J1088 mode
test data for one particular engine, but since it is a catalyst
simulation, comparisons can really only be made between
builds in the simulation and actual values not compared to any
actual engine test data with a catalyst. The catalyst simulation
takes into account the feed gas concentrations and operating
conditions to predict the likely conversion for a particular
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EFI. It is thought that more cost savings can be made with
regard to the number of sensors and the size and power of the
micro processor bring the cost down to 50 to 60% compared to
EFI. Clearly as it is only a concept accurate costing cannot yet
be made and the technical details of the system cannot fully be
defined.
engine and fuel-system to fuel-system differences in
production. The solenoid or piezoelectric stack/ring cyclically
pulses at a set frequency and adjusts the fuel quantity
delivered by increasing the pulse width or “on” time or
alternatively by changing the pulse frequency. The ideal
position, as shown in Figure 24, is thought to be in the
metering hole or delivery tube but could also potentially be in
the area of the one way valve at the base of the fuel column.
By changing the effective diameter of the metering hole or
delivery tube by say 10% (0.7 to 1 mm) the flow delivered can
be changed in the order of 20 %. Tests carried out on the TCT
with regard to metering hole diameter on a conventional TCT
fuel system have shown this to be the order of change in fuel
delivery for a change in metering hole diameter. It is though
that the solenoid or piezoelectric device by pulsing would give
a smaller change in fuelling of the order of between 5 to 10%.
Clearly adjusting the pulse frequency, pulse width, clearance
and adjusted hole size when electronically activated will allow
these estimated effects on fuel delivery to be varied.
It is hope that the electronic version of TCT will be a
technology than fits between EFI and mechanical carburettors
in terms of cost but providing similar if not better performance
(combustion quality) to EFI.
Conclusions
The conclusions of this paper are as follows:
•
•
As discussed in detail the mechanical version of TCT is able
to adjust AFR with load at a fixed speed. The electronic
version will allow that AFR control to be carried over to other
engine speeds.
The use of the TCT with a piezoelectric device or solenoid
does away with need for a fuel pump and an injector (if the
piezoelectric device is used rather that a solenoid). This would
give a cost saving or around 10% compared to single point
•
•
•
•
•
•
•
Figure 24. Electronic Version of TCT Showing
Two Versions Using Either a Solenoid or
Piezoelectric Stack / Ring Either in the Fuel
Delivery Tube or at the Base of the Fuel Column
20
SORE are recognised as a major source of air pollution
The EPA / CARB Phase 3 legislation has very high CO
limits due to the general inability of the conventional fuel
system and undeveloped engine to operate lean at part
load.
Developing the engine with regard to crevice volumes,
bore roundness, valve sealing, etc but without developing
the combustion system or increasing specific output will
reduce engine out HC emissions whilst maintaining the
NOx emissions and CO emissions at much the same
level.
Developing the engine’s combustion system by
improving its combustion robustness or increasing its
specific output will increase NOx emissions and
therefore increase HC+NOx emissions. At the same time
developing the engine to reduce HC will still lead to
higher HC+NOx emissions due to the higher NOx
offsetting a decrease in HC. EGR could potentially be
used to limit or control the engine out NOx.
The new fuel system has distinct combustion and
functional operating advantages over the conventional
fuel system and at a lower manufacturing cost
The new fuel system has the ability to allow the engine
to operate with any desired AFR curve or profile with
load at a given engine speed.
The new fuel system opens the door to new emissions
control strategies for unmodified or undeveloped SORE
and for modified or well-developed SORE.
It is possible that the TCT fuel system fitted to a
improved SORE engine with low engine out emissions
and a low deterioration factor could meet Phase 3 limits
without after-treatment
The result of the after-treatment simulation for the new
fuel system fitted on an unmodified or undeveloped
Phase 2 SORE show that much lower J1088 cycle
HC+NOx and CO emissions with similar or lower
exhaust gas temperatures are potentially achievable
compared to the current EPA/CARB strategy using the
conventional carburettor.
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Acknowledgments
The authors gratefully acknowledge Ricardo Consulting
Engineers for their professional consultancy services and
Queens University, Belfast for use of testbed facilities.
References
(1) CARB WEB Site : www.arb.ca.gov
(2) EPA WEB Site : www.epa.gov
(3) Durability of Low Emissions Small Off Road Engines
SwRI 08.05734
(4) EPA Technical Study on the Safety of Emissions
Controls for Nonroad Spark Ignition Engines less than
50hp – EPA420-R-06-006, March 2006
(5) EPA-OTAQ-ASD November 22, 2004 / Fall 2004
(6) Ricardo UK Ltd. Fjolblendir TCT Assessment Report
Dec 06
(7) R Douglas and S Glover. The Feasibility of Meeting
CARB / EPA 3 Emissions Regulations for Small Engines,
SAE Paper, 2007-32-0059, SETC 2007.
(8) Moesner et al, Emissions and Performance Potential of a
Small Stratified Charge Two-Stroke Engine using Reed
Valves. SAE Paper No, 2006-32-0058, SETC 2006.
(9) M Bergman and J Berneklev, A Novel Method of Tuning
a Stratified Scavenged Two-Stroke Engine. SAE Paper
No 2006-32-0055, SETC 2006.
(10) R Gustafsson, A Practical Application to Reduce Exhaust
Emissions on a Two-Stroke Engine with Tuned Exhaust
Pipe. SAE Paper No 2006-32-0054, SETC 2006.
(11) M Bergman and R Gustafsson, Emissions and
Performance Evaluation of a 25cc Stratified Scavenged
Two-Stroke Engine. SAE Paper No 2003-32-0047,
SETC 2003.
(12) R Douglas and S Glover, The Feasibility of Meeting
CARB / EPA 3 Emission Regulations for Small Engines.
SAE Paper 2007-32-0059, SETC 2007.
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