addition, the volumetric energy density of a lithium ion battery is currently 0.5–1 MJ/L (150–300 Wh/L), with energy densities of the order of 5 MJ/L under development, significantly lower than gasoline or diesel fuels with energy densities of the order of 35 MJ/L. In cold weather, there is a degradation of battery performance of the order of 15‐30%. A nationwide network of high voltage (240 to 950 V) charging stations is needed to compensate for the limited range of electric vehicles. The charging time for electric car batteries is at least two hours, depending on the charging station voltage, in comparison to a liquid fuel refueling time of the order of a few minutes.
Hybrid electric vehicles (HEV), which incorporate a small internal combustion engine with an electric motor and storage batteries, have reached the production stage, primarily due to their low fuel consumption and emission levels. A hybrid electric vehicle has an internal combustion engine to provide the energy to meet vehicle range requirements. The battery then provides the additional power needed for acceleration and climbing hills. The battery in an HEV vehicle typically has a capacity of about 50 MJ (14 kWh). Hybrid electric vehicles have a long history, as the first HEV, the Woods Dual Power automobile, was introduced in 1916. A similar engine‐motor‐battery combination has been used on diesel‐electric submarines since 1900 to allow both surface and underwater operation.
As shown in Figure 1.25, two elementary configurations for an HEV are series or parallel configurations. In a series configuration, only the electric motor with power from the battery or generator is used to drive the wheels. The internal combustion engine is maintained at its most efficient and lowest emission operating points to run the generator and charge the storage batteries. With the parallel configuration, the engine and electric motor can be used separately or together to power the vehicle. Some hybrid vehicles use an internal combustion engine to power the front wheels and an electric motor to power the rear wheels, and they synchronize them to provide all‐wheel drive capability. The motors are used as generators during braking to increase vehicle efficiency.
Figure 1.25 Hybrid electric vehicle powertrain configurations.
Fuel Cells
A fuel cell converts the chemical energy in a fuel directly to electricity through electrochemical reactions. The first fuel cell was invented by W. Grove, an English scientist, in 1838. For vehicular applications, hydrogen is used as the fuel, and oxygen is the oxidizing agent. Fuel cell technology competes well in applications requiring reduced emissions, as recent developments in polymer‐electrolyte membrane (PEM) technology indicate that a PEM fuel cell produces much lower
Both the anode and cathode are composed of platinum particles embedded in a substrate surface of porous carbon. At the anode, the hydrogen is split into protons
Current PEM fuel cell stacks are small enough to fit beneath a vehicle's floor next to the storage batteries and currently can deliver up to 125 kW to an electric motor. Studies indicate that the best opportunities for fuel cell adoption are in the commercial vehicle market, i.e., trucks and off‐highway applications. Since there is presently no hydrogen fuel storage infrastructure, one option is on‐board reforming of methanol fuel to hydrogen and
Gas Turbines
Gas turbine engines compete with internal combustion engines on the other end of the power spectrum, at powers greater than about 500 kW. The advantages offered depend on the application. Factors to consider are the efficiency and power per unit weight. A gas turbine consists basically of a compressor‐burner‐turbine combination that provides a supply of hot, high‐pressure gas. This may then be expanded through a nozzle (turbojet), through a turbine, to drive a fan, and then through a nozzle (turbofan), through a turbine, to drive a propeller (turboprop), or through a turbine to spin a shaft in a stationary or vehicular application.
One advantage a gas turbine engine offers to the designer is that the hardware responsible for compression, combustion, and expansion are three different devices, whereas in a piston engine all these processes are done within the cylinder. The hardware for each process in a gas turbine engine can then be optimized separately; whereas in a piston engine compromises must be made with any given process, since the hardware is expected to do three tasks. However, it should be pointed out that turbochargers give the designer of conventional internal combustion engines some new degrees of freedom toward optimization.
With temperature limits imposed by materials, the reciprocating engine can have a greater peak cycle temperature than the gas turbine engine. In an internal combustion engine, the gases at any position within the engine vary periodically from hot to cold. Thus the average temperature during the heat transfer to the walls is neither very hot nor cold. On the other hand, the gas temperature at any position in the gas turbine is steady, and the turbine inlet temperature is always very hot, thus tending to heat material at this point to a greater temperature than anywhere in a piston engine.
The thermal efficiency of a gas turbine engine is highly dependent on the adiabatic efficiency of its components, which in turn is highly dependent on their size and their operating conditions. Large gas turbines tend to be more efficient than small gas turbines. That airliners are larger than automobiles is one reason gas turbines have displaced piston engines in airliners, but not in automobiles. Likewise gas turbines are beginning to penetrate the marine industry, though not as rapidly, as power per unit weight is not as important with ships as with airplanes.
Another factor favoring the use of gas turbines in airliners (and ships) is that the time the engine spends operating at part or full load is small compared to the time the engine spends cruising, therefore the engine can be optimized for maximum efficiency at cruise. It is a minor concern that at part load or at take‐off conditions the engine's efficiency is compromised. Automobiles, on the other hand, are operated over a wide range of load and speed so a good efficiency at all conditions is better than a slightly better efficiency at the most probable operating condition and a poorer efficiency at all the rest.
Steam‐ or vapor‐cycle engines are much less efficient than internal combustion engines, since their peak temperatures are about 800 K, much lower than the peak temperatures (
