ADVANCED PROPULSION SYSTEMS FOR HYBRID ELECTRIC VEHICLES

Transcription

1 Journal of KONES Internal Combustion Engines 2002 No. 3 4 ISSN ADVANCED PROPULSION SYSTEMS FOR HYBRID ELECTRIC VEHICLES Zdzisław Juda Zakład Mechatroniki Samochodowej, Politechnika Krakowska Ul. Warszawska 24, Kraków Tel.:4812/ , 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. 126

2 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 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 127

3 Emission for various primary energy sources Tabele 1 Primary energy source CO [g/km] NO x [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]: P = 1 η p mvcr g cos dv dt v 2 ( α ) + + g sin( α ) + ρ C A ( v v ) + B v + P dt a d v v a r v a 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 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 128

4 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 lean- NOx 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 kw P [kw] Fig. 3. Efficiency vs. specific power for ICE 129

5 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]. 130

6 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 ( 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. 131

7 η [%] 60 Fuel Cell IFC 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. Characteristics of various Fuel Cell Table 2 Fuel Cell type PEMFC DMFC PAFC MCFC AFC SOFC Electrolyte PEM PEM phosphoric acid molten carbonate aqueous alcaline solid oxide Operating Temp o C o C o C 650 o C o C o C Current density H 2 ions H 2 ions H 2 ions carbonate ions H 2 ions O ions Fuel H 2 H 2 H 2 H 2, CO H 2 H 2, CO, CH 4 Oxidant O 2 from air O 2 from air O 2 from air O 2 + CO 2 from air O 2 O 2 from air Reformer external internal external external or internal external external or internal Catalyst Pt Pt Pt Ni Pt Ni Efficiency (%) over 60 up to 70% over

8 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]: W cap = 2 ( 0.5m v = mgh) v 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 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] Various batteries parameters Table 3 Battery parameter Pb NiCd NiMH Li-polymer Li-ion Specific energy [Wh/kg] Power density [W/l] Specific power [W/kg] Life cycle Operating temperature ambient ambient ambient ambient ambient Cell voltage [V] 2 1,2 1,2 3,0 3,0 Charging time [h]

9 Nickel Metal Hydride Battery (NiMH) Nickel Metal Hydride Batteries are known since 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 , 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 , ATTCE 2001, Barcelona 2001, Proceedings Volume 5: Electronics, 134

10 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 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 SYMBOLS AND ABBREVIATIONS P powertrain power demand η powertrain efficiency m v vehicle mass C r wheels rolling resistance g gravity (9.8 m/s 2 ) α road angle ρ air density C d vehicle drag coefficient A v vehicle cross-sectional area v a air velocity v v vehicle speed B r resistance due to braking P a accessory loads PEMFC Proton Exchange Membrane Fuel Cell SOFC Solid Oxide Fuel Cell DMFC Direct Methanol Fuel Cell PAFC Phosphoric Acid Fuel Cell MCFC Molten Carbonate Fuel Cell AFC Alcaline Fuel Cell ICE Internal Combustion Engine SI Spark Ignition HEV Hybrid Electric Vehicle h - high difference v - maximum vehicle speed η - ultracapacitor use energy efficiency 135