Hybrid Electric Vehicle Power Units and Transmissions
Several types of engines can be used to power a hybrid electric vehicle (HEV). There is also another power unit—fuel cells—that could eventually be used to power HEVs. Learn more about the following Hybrid Electric vehicle components:
- Spark Ignition Engines
- Compression-Ignition Direct-Injection Engines
- Gas Turbine Engines
- Fuel Cells
- Transmissions
Spark Ignition Engines
A spark ignition (SI) engine runs on an Otto cycle—most gasoline engines run on a modified Otto cycle. This cycle uses a stoichiometric air-fuel mixture, which is mixed prior to entering the combustion chamber. Once in the combustion chamber, the mixture is compressed and then ignited using a spark plug (spark ignition). The SI engine is controlled by limiting the amount of air allowed into the engine. This is accomplished through the use of a throttling valve placed on the air intake (carburetor or throttle body).
The SI engine can easily meet emissions and fuel economy standards, and it is the lowest-cost engine because of the huge volume of SI engines currently produced. As new emissions standards must be met, SI engines may begin to have difficulty meeting the standards while keeping costs down.
Compression-Ignition Direct-Injection Engines
Technical advancements continue to be made on the compression-ignition direct-injection (CIDI) engine, which is more commonly called the diesel engine. This engine has the highest thermal efficiency of any internal combustion engine. Challenges to improvements include a lower specific power than the gasoline engine; significant particulate matter and nitrogen oxides in the exhaust; and the noise, vibration, and smell of the engine when operating.
Recent advancements in automotive diesel engines, which address some of these shortcomings, have made them nearly ideal candidates for HEV applications. These advancements include high-pressure direct fuel injection, low nitrogen oxides catalysts, and sophisticated electronic controls. With a thermal efficiency upwards of 40 percent and well-understood maintenance, reliability, manufacturing, and operating characteristics, the CIDI engine shows great promise as a near-term HEV power unit.
Gas Turbine Engines
The gas turbine engine runs on a Brayton cycle using a continuous combustion process. In this cycle, a compressor 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.
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.
The turbine is light and simple; the only moving part of a simple turbine is the rotor. This engine can also produce low levels of emissions and can run using various types of fuel. Because of this multi-fuel capability, a fuel that burns completely and cleanly can be used to reduce emissions.
The turbine engine has a few drawbacks, which have prevented its widespread use in automotive applications. These include high manufacturing costs, slow response (relative to a reciprocating engine) to changes in throttle request, less suitability for low-power applications, and requirements of intercoolers, regenerators, and/or reheaters to reach efficiencies comparable to current gasoline engines.
Fuel Cells
Fuel cells generate electricity through an electrochemical reaction that combines hydrogen with oxygen in ambient air. Pure hydrogen is required to make fuel cells work, but a wide variety of fuels can be "reformed" to extract hydrogen for fuel cells. For the most part, the fuel cell's only emission is water vapor, giving it potential as the cleanest HEV power unit alternative. Efficient, quiet, and reliable, fuel cells are predicted to demonstrate energy conversion efficiencies up to 50 percent, compared with the 20 to 25 percent efficiency of standard SI gasoline engines.
The choice of fuel for a fuel-cell-powered HEV has important implications for required infrastructure, system accessories, efficiency, cost, and design. Although the viability of fuel cells has been well proven in the space program, as well as in prototype vehicles developed by the U.S. Department of Energy (DOE) and industry partners, very high capital costs, large size, long start-up times, and immature technologies make it a longer-term enabling technology for HEVs.
DOE initiated the FreedomCAR & Vehicle Technologies Program (now called the Vehicle Technologies Program) in 2002 to advance high-efficiency vehicles. This program works closely with DOE's Hydrogen, Fuel Cells & Infrastructure Technologies Program to apply hydrogen technologies to vehicles.
Transmissions
Hybrid electric vehicles can use a variety of conventional and advanced transmissions based on the system design of the vehicle, including continuously variable transmission, automated shifted manual transmission, manual transmission, and traditional automatic transmission with torque converter.