Journal of KONES Internal Combustion Engines 2002 No. 3‐4 ISSN 1231 ‐ 4005 ADVANCED PROPULSION SYSTEMS
FOR HYBRID ELECTRIC VEHICLES
Zdzisław Juda
Zakład Mechatroniki Samochodowej, Politechnika Krakowska
Ul. Warszawska 24, 31-155 Kraków
Tel.:4812/636-79-79, e-mail:zjuda@usk.pk.edu.pl
Abstract
One of the directions of the spread of motor vehicle use is hybridization of powertrains, which should result in
essential ecological and economic effects. A hybrid vehicle has on board at least two energy sources of which at
least one must be a secondary energy source. A secondary energy source converts energy in two directions and has
the ability to store energy. Additionally, a fully Electric Hybrid Vehicle (HEV) must have an electric motor as a
main drive unit. Basic hybrid vehicle configuration are: parallel, series and mix. Beside defining vehicle
configuration and chosing primary and secondary energy sources, it is essential to choose control strategy. What is
meant is a choice of the manner of vehicle control in its different states from the point of view of energy flow and
conversion. One of the possible control strategies for series HEV is a thermostatic strategy consisting in turning the
Internal Combustion Engine (ICE) on when the parameter State of Charge (SOC) of storage battery decreases
below a defined limit and turning it off after reaching SOC’s upper limit. In the area of advanced solutions in the
group of primary energy sources the main development directions concern Spark Ignition ICE with Direct Gasoline
Injection (SIDI), Compression Ignition ICE with Direct Diesel Injection (CIDI), Gas Turbine, and Fuel Cells (FC).
The paper presents current state of development and trends in the field of primary energy sources in HEV’s.
1. Introduction
Hybrid Electric Vehicles are most useful as city cars. City cars cover daily up to 25 km. Total
mileage of this type of vehicles makes up 9% of daily course of all vehicles. However, emission
of this group of vehicles with conventional powertrain solution is high and achieves up to 34%
HC and CO of total quantity of emission of all cars. This is an important reason for the search for
better solutions. Hybrid configuration of city vehicle uses both small internal combustion
engines or stacks of fuel cells. Solution with fuel cells operating on hydrogen – at expected
decreased price – is the most profitable one, considering lack of harmful emission. The nature of
city vehicle movement causes frequent stopping while the car idles. This causes increased
emission with lack of transport effect while energy loss ensuing from idling reaches 11%. In the
case of the second or third vehicle in the household, shortening of average distances the cars
cover appears. Norms of permitted emission are being constantly tightened in such a way that
conventional solutions of propulsion systems are no longer able to fulfil the requirements. Rapid
progress in alternative energy sources R&D projects is being observed. It brings about more and
more mature technical solutions accompanied by successive decrease in cost of such advanced
propulsion systems. Typical HEV uses the conventional ICE and an energy storage device,
mostly battery. There are different kinds of energy storage devices: classic batteries, advanced
technology batteries, flywheels and ultracapacitors. Demands for increase in effectivity of fuel
usage are being formulated by goverments and other public organizations – 80 mpg (34 km/l) as
an objective of Partnership for a New Generation of Vehicles (PNGV). This may be a stimulus
for using new, unconventional energy sources.
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2. Partnership for a new generation of vehicle (pngv)
Partnership for a New Generation of Vehicle was initiated by US President B. Clinton on
September 29th 1993. This program connect federal goverment and United States Council for
Automotive Research (USCAR) in common research & development. Tree goals of PNGV
program:
1. Significantly improve national competitiveness in manufacturing for future generations of
vehicles.
2. Implement commectially viable innovation from ongoing research on conventional vehicles.
3. Develop vehicles to achieve up to three times the fuel efficiency of comparable 1994 family
sedan.
To achieve the goal 3, fuel economy target, the efficiency of the primary energy source, obtained
during standard driving cycle will have to reach minimum 40% thermal efficiency.
3. Sources of energy
A hybrid vehicle needs to have on board at least two energy sources and it is a necessary
condition that one of them is a secondary energy source. The secondary energy source can
transform energy from one kind into another in a reversible mode and is able to store energy.
Both combustion engine and a stack of fuel cells are primary energy sources because they
convert energy one way: the combustion engine converts the chemical energy of the fuel into
mechanical energy of rotational torque and the fuel cells convert the chemical energy of the fuel
directly into electric energy. Both combustion engines and fuel cells are unable to store energy.
The secondary source in solutions of series HEV with SI engine and a hybrid vehicle with fuel
cells are storage batteries which convert both ways the chemical energy into electric one and the
electric one into the chemical one and store energy in the chemical form. Combination of
primary and secondary energy sources enables different configurations of hybrid vehicles
(Fig.1).
Fig. 1. Energy sources emissions vs. fuel economy
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Tabele 1
Emission for various primary energy sources
Primary energy source
CO [g/km]
NOx [g/km]
VOC [g/km]
Hydrogen (from natural gas) Fuel Cell
0,17
0,48
0,17
Hydrogen (from methanol) Fuel Cell
0,32
0,64
0,17
Hydrogen (from gasoline) Fuel Cell
0,32
0,32
0,32
SIDI engine
16,0
1,6
0,96
Hybrid vehicle with SIDI engine
9,65
0,96
0,64
4. Use of energy
Each of powertrain subsystems has to be consistent with demands for minimal essential power.
The minimal essential power from source of energy depends on many factors and is described
with general formula [4]:
⎛ 1
P = ∫⎜
⎜η
⎝ p
⎞
dv v
⎡
⎤
2
⎟
⎢mv C r g cos(α ) + dt + g sin (α ) + ρ a C d Av (vv − v a ) + Br ⎥ vv + Pa ⎟dt
⎣
⎦
⎠
Internal combustion engines have been applied for over 100 years, but the most dynamic
progress has been achieved only lately mainly due to the introduction of direct injection
technology for both gasoline and diesel. Requirements for this engines are: decrease in emission,
higher performances and increase in safety. However further development of these technologies
is required to fulfil increasingly stricter exhaust gases toxicity standards. Last changes were
introduced in Europe in 2000, more are planned for 2005. Level of HC, NOx, and particulates
emission cosistent with EURO IV norm must be decreased by another 20% in comparison with
current requirements during tests based on driving cycle including engine warming period.
American requirements are even stricter and introduce norms ULEV. From 2003 in California at
least 10% of new cars must be zero-emission vehicles [2].
5. Spark ignition direct injection engines
Spark Ignition Direct Injection (SIDI) engines with stratified charge (Fig.2) adopt the features of
advanced Diesel engines and improve their efficiency up to CIDI engines level. The
conventional Spark Ignition Engines efficiency is limited by throttling losses. In the SIDI
engines rotational velocity is reduced by decreasing the amount the fuel injected per cycle. SIDI
engine can run with lean air-fuel mixture. Besides reducing pumping losses, lean mixture
combustion gives some efficiency advantages such as ability to increase compression ratio. Also
fuel economy is up to 15-20% higher than conventional SI engines due to higher compression
ratio, lean mixture and advanced valve control. The SIDI engines technology is an example of
the strategy of operating engines under lean air-fuel ratio conditions for improved fuel
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efficiency. The ICE as a primary energy source of a hybrid electric vehicle is the least efficient
component in the powertrain. The HEV powertrain itself may offer ways to overcome the
limitations of spark-ignition, direct injection (SIDI) engines and unlock their potential even with
current emissions-control technology. The ability to load-level engines with electric power in an
HEV can be used to overcome the emissions liabilities of SIDI engines. Because acceleration can
be moderated and high-speed, high-load operation avoided, much of the operation that produces
the highest emissions can be eliminated, greatly easing the burden of the still-immature leanNOx control systems. Vehicles with SIDI engines have a 10% improvement in torque and as
much as 20% fuel economy benefit compared to current PFI (Port Fuel Injection) engines.
Compared to CIDI engine technology, the SIDI engine technology has the following
advantageous features:
- Well known technology
- Low cost of manufacturing
- Compatibility with known emissions control technologies
- Ability to use certain alternative fuels
- Better power-to-weight ratio
- Lower noise
This technology has also some disadvantages:
- Lower efficiency due to throttling
- Large mechanical friction losses
- Limited compression ratio – low efficiency
Some of the potential benefits of spark-ignition direct-injection (SIDI) engines include improved
thermal efficiency (Fig. 3), better transient response, and greater displacement-specific power.
The barriers include meeting nitrogen oxide (NOx) and hydrocarbon emission standards and the
high cost of fuel injection hardware.
Fig. 2. Fuel injection in SIDI engine
η [%]
Fiat 900 - 29 kW
40
30
20
10
0
0
5
10
15
20
25
30
35
P [kW]
Fig. 3. Efficiency vs. specific power for ICE
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The main activities in SIDI engines development are focused on techniques and tools to better
quantify and characterize the fuel-air mixing and combustion process, tools to observe and
measure the interaction of fuel spray with air charge, wall wetting effects, flame propagation,
and identification of those conditions that generate soot, unburned hydrocarbons, and NOx
emissions. New exhaust gas sensors that are robust, fast responding, and highly selective are
developed.
6. Compression ignition direct injection engines
More commonly known as the diesel engine, the Compression Ignition Direct Injection (CIDI)
engine has the highest thermal efficiency of any internal combustion engine and therefore
produces the least "greenhouse" carbon dioxide (CO2) from its exhaust. It has, however, a
number of disadvantages, including a lower specific power than the gasoline engine; significant
amounts of particulate matter (PM) and nitrogen oxides (NOx) in the exhaust, noise and
vibration, but this is a most promising technology for meeting PNGV fuel economy goals. In the
R&D phase currently there is a new catalyst technology with potential to remove 95% NOx from
exaust gases. High-speed CIDI engines are now ideal candidates for hybrid electric vehicle
(HEV) applications. Advances of such engines include high-pressure direct fuel injection, lean
NOx catalysts, and sophisticated electronic controls. With a thermal efficiency greater than 40%
(better than their spark ignition counterpart efficiency), reliability, manufacturing, and operating
characteristics, the high-speed CIDI engine shows great promise as a near-term hybrid
propulsion unit. In an advanced CIDI diesel engine, fuel is injected directly into the cylinder
near the top of the compression stroke using Common Rail System. This sophisticated fuel
injection equipment is more costly than injection system of conventional engine. Higher
operating pressures and temperatures also increase the cost of the engine structure.
Compression Ignition Direct Injection (CIDI) engines have advantages:
- No throttling losses
- Higher compression ratio – increase of efficiency
- High torque at low rotational velocity
and some disadvantages:
- High mass
- High noise
- High emission of NOx
7. Gas turbine engine
The gas turbine engine runs on a Brayton cycle using a continuous combustion process. In this
cycle, a compressor (usually radial flow for automotive applications) raises the pressure and
temperature of the inlet air. The air is then moved into the burner, where fuel is injected and
combusted to raise the temperature of the air. Power is produced when the heated, high pressure
mixture is expanded and cooled through a turbine. When a turbine engine is directly coupled to a
generator, it is often called a turbo generator or turbo alternator (Fig.4). The power output of a
turbine is controlled through the amount of fuel injected into the burner. Many turbines have
adjustable vanes and/or gearing to decrease fuel consumption during partial load conditions and
to improve acceleration [6].
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Fig. 4.Auxiliary Power Unit – Gas Turbine Engine
Gas Turbine Engine have the following advantages:
- The gas turbine is light and simple; a turbine has no reciprocating motion and the only
moving part of a simple turbine is the rotor.
- A turbine runs smoother than a ICE.
- Can operate on various fuels
- Low emissions; fuel burns completely.
- Suited for a wide range of transport applications.
and some disadvantages:
- High manufacturing costs.
- A turbine engine can not change speed fast; gas turbine is slow to respond (relative to a
conventional ICE) to changes in throttle request.
- Less suitable for low-power applications. At partial throttle conditions, the efficiency of
the gas turbine decreases.
- A turbine requires intercoolers, regenerators and/or reheaters to reach efficiencies
comparable to current gasoline internal combustion engines.
8. Fuel cells
Fuel cells are electrochemical devices and are by nature more effective. There is a short chain of
energy conversion: chemical energy of the fuel is directly converted into electrical energy
without combustion. The chemical reactions of hydrogen and oxygen taking place in fuel cells in
the presence of a catalyser lead to production of electric energy. Water is only a by-product of
this action. Maximum efficiency of hydrogen fuel cells reaches 70% (Fig.5) and is available at
low power (0.15-0.30 of maximum power) [3]. High efficiency is achieved, because the fuel cell
is not limited by temperature. This is a very advantageous characteristic enabling high
exploitation of energy at small and medium loads of propulsion system – which is often the case
in city traffic. The advantages of fuel cells are also their size, similar to that of combustion
engines, and the possibility to function on different fuels (gasoline, hydrogen, methanol).
Assuming that the current fuel distribution infrastructure should be maintained, it would be
desirable to use petroleum-derived, liquid hydrocarbon fuels as a source of hydrogen for fuel
cells.
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η [%]
Fuel Cell IFC
50
60
55
50
45
40
35
30
0
10
20
30
40
50
60
P [kW]
Fig. 5. Efficiency of 50 kW Fuel Cell Stack
Gasoline or other hydrocarbon fuel could be used in an on-board reformer to obtain hydrogen.
The hydrocarbon fuel must have a high hydrogen-to-carbon ratio to maximize the efficiency of
the system. Fuel reformers are catalytic systems that are also adversely affected by the presence
of sulfur in the fuel. Sulfur from the fuel is also disadvantageous for catalytic system of fuel
reformer. The fuel reformer is sensitive to the formation of deposits. The petroleum fuel for fuel
cells is different in composition and properties from conventional gasoline. It is possible to sell
such fuel cell gasoline by existing petrol station net and this fuel could be well suited for
advanced CIDI engine technology.
Table 2
Characteristics of various Fuel Cell
Fuel Cell
type
PEMFC
DMFC
PAFC
MCFC
AFC
SOFC
Electrolyte
PEM
PEM
phosphoric
acid
molten
carbonate
aqueous
alcaline
solid oxide
Operating
Temp.
70-80oC
70-80oC
160-210oC
650oC
70-100oC
700-1000oC
H2 ions
H2 ions
H2 ions
carbonate
ions
H2 ions
O ions
H2
H2
H2
H2, CO
H2
H2, CO, CH4
O2 from air
O2 from air
O2 from air
O2 + CO2
from air
O2
O2 from air
Reformer
external
internal
external
external or
internal
external
external or
internal
Catalyst
Pt
Pt
Pt
Ni
Pt
Ni
40-60
30-40
40-50
over 60
Current
density
Fuel
Oxidant
Efficiency
(%)
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up to 70%
over 60
9. Ultracapacitors
Difficulties of simultaneously obtaining high values of battery specific energy, specific power
and cycle life leads to incorporation of additional secondary energy source to the HEVs design.
Battery size is calculated for the range of vehicle while the second energy source for gradeability
and acceleration improving. Ultracapacitor can be used as the second energy source. Maximum
stored energy is given by [1]:
Wcap =
(0.5m v
2
v
= mgh
)
η
According to US DOE goal, the specific energy should be better than 5 Wh/kg and specific
power should be better than 500 W/kg. For today available ultracapacitors, for instance PC7223
from Maxwell Technologies, the specific energy is 2.48 Wh/kg and specific power is 732 W/kg.
10. Flywheels
Flywheels are very often used in conventional vehicles, but for HEV’s they have to be different.
As a secondary energy source, advanced flywheel can store mechanical energy and deliver it to
vehicle axle without converting energy from one kind to another one. The simpliest flywheels
have a round, rotary form. More sophisticated flywheel has a form of rotary cylinder. The
titanium hubs are connected to high durability composite cylinder, strength with carbon fibre.
Such flywheel can rotate 60000 rev/min and more. Cylinder is placed in vacuum for friction
decrease. Flywheel is mounted with magnetic bearings for keeping constant clearance between
rotary and immobile components of system [1].
11. Batteries for electric vehicles [7]
Table 3
Various batteries parameters
Battery parameter
Pb
NiCd
NiMH
Li-polymer
Li-ion
Specific energy [Wh/kg]
35
50
70
150
120
Power density [W/l]
80
85
125
100
200
Specific power [W/kg]
150
175
200
100
200
Life cycle
600-800
500-600
500-600
Operating temperature
ambient
ambient
ambient
ambient
ambient
Cell voltage [V]
2
1,2
1,2
3,0
3,0
Charging time [h]
8
2–8
2–8
10-12
10 – 12
1000-2000 800-2000
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Nickel – Metal – Hydride Battery (NiMH)
Nickel – Metal – Hydride Batteries are known since 1970. Due to environment protection trnds
laboratories are looking for more environment friendly batteries then those based on Nickel and
Cadmium. The main difference between Ni-MH and Ni-Cd batteries is the use of hydrogen,
absorbed in a metal hydride, for the active negative electrode material in place of the Cadmium.
Such battery is free from Cadmium toxicity and carcinogenicity.
Lithium-Ion Batteries (Li-ion)
Lithium-ion technology builds on a large base of commercial experience in small cells for the
consumer electronics market such as mobile phones or notebook computers. Lithium-ion cells
are based on insertion electrodes, in which lithium ions move from the positive terminal to the
negative one on charge and in the opposite direction on discharge. Both electrodes are made of
insertion compounds, which accept lithium ions interstitially with little structural change to the
host material. Ambient-temperature lithium-ion batteries with a liquid electrolyte promise to
fulfill the energy-storage requirements for traction applications in the near future.
Lithium-Polymer Batteries (Li-polymer)
Lithium polymer battery technology has been under development for nearly 20 years. Lithium is
the lightest metal with the highest electrochemical potential. Using lithium metal as an anode
provides exceptional specific energy and energy density. The use of polymer electrolytes adds
packaging opportunities unavailable in liquid electrolytes. Because the intrinsic conductivity of
the polymer electrolyte material is low, individual cells are constructed of thin films about 100
micrometers thick. Many such cells must be combined for use in an EV battery. The technology
relies on generating high surface areas to build the high energy and power capacities required of
advanced technology batteries.
12. Conclusion
Very low or non-existent harmful emissions, low fuel consumption (counted as equivalent of
gasoline), good traction parameters indicate the direction, in which the development of advanced
automotive propulsion systems will certainly go. The Fuel Cells are most promising primary
energy sources for HEV’s. But in a series HEV a SIDI/CIDI engine or gas turbine can be used as
the APU. There are purely electrical vehicle constructions using fuel cells as a prime mover. It
seems however that a configuration of hybrid vehicles with fuel cells as a series HEV is sensible
because of the possibility of regenerative braking, which is especially significant in city traffic.
Such a vehicle combines the advantage of fuel cells and battery.
13. References
1. Chan, C.C; Chau, K.T: Modern Electric Vehicle Technology, Oxford University Press, 2001,
2. Walzer, P: Future Power Plants For Cars, SAE Paper 2001-01-3192, ATTCE 2001, Barcelona 2001,
Proceedings Volume 2: Powertrain and Heat Transfer/Exchange,
3. Juda, Z: Simulation of Energy Conversion in Advanced Automotive Vehicles, SAE Paper 2001-013341, ATTCE 2001, Barcelona 2001, Proceedings Volume 5: Electronics,
134
4. Plotkin, S; Santini, D; Vyas, A; Anderson, J; Wang, M: Hybrid Electric Vehicle Technology
Assessment: Methodology, Analytical Issues, and Interim Results, Center for Transportation Research,
Argonne National Laboratory, Argonne 2001,
5. Thomas, S; Zalbowitz, M: Fuel Cells – Green Power, Los Alamos National Laboratory, Los Alamos
2000
6. Capstone MicroTurbine, Catalogue Data, 2002,
7. Anderman, M; Kalhammer, F; MacArthur, D: Advanced Batteries for Electric Vehicles: An
Assessment of Performance, Cost and Availability, Battery Technology Advisory Panel 2000.
SYMBOLS AND ABBREVIATIONS
P
η
mv
Cr
g
α
ρ
Cd
Av
va
vv
Br
Pa
PEMFC
SOFC
DMFC
PAFC
MCFC
AFC
ICE
SI
HEV
h
v
η
powertrain power demand
powertrain efficiency
vehicle mass
wheels rolling resistance
gravity (9.8 m/s2)
road angle
air density
vehicle drag coefficient
vehicle cross-sectional area
air velocity
vehicle speed
resistance due to braking
accessory loads
Proton Exchange Membrane Fuel Cell
Solid Oxide Fuel Cell
Direct Methanol Fuel Cell
Phosphoric Acid Fuel Cell
Molten Carbonate Fuel Cell
Alcaline Fuel Cell
Internal Combustion Engine
Spark Ignition
Hybrid Electric Vehicle
- high difference
- maximum vehicle speed
- ultracapacitor use energy efficiency
135