Environmental Engineering Reference
batteries are unsuitable for automobiles (see Table 4.2). Adding battery mass to the vehicle will
not bring about a commensurate increase in range, which is better improved by increasing the
efficiency with which stored energy is utilized in propelling the vehicle.
Hybrid vehicles are those that combine conventional power sources (SI or CI engines) with electric
motors to power the vehicle. In one extreme, the SI or CI engine can power a generator that
delivers electric power to an electric traction motor; this system is used in railway engines. The
more attractive system for automobiles is to attach an electric motor/generator to the combustion
engine shaft, between the engine and the transmission. The motor/generator can store energy in a
battery bank when excess power is available, during deceleration or when the power need is less
than what the combustion engine can deliver, and can deliver extra power to the wheels when it is
temporarily needed for acceleration or hill climbing. Such a system can level the peaks and valleys
of power demand in the driving cycle, enabling the combustion engine to run closer to average
driving cycle power and potentially better average engine efficiency.
The characteristics of two hybrid vehicles of this type are listed in Table 8.4. In the Honda
vehicle, the electric motor/generator maximum power is only 10 kW compared to the SI engine
maximum power of 47 kW. Nevertheless, this is sufficient to provide extra power for acceleration
and absorb braking power in the urban driving cycle. The ratio of electrical energy storage to
maximum electrical power is greater than that for the battery-powered vehicles (see Table 8.3).
The vehicle fuel economy is impressive, a weighted average of 27.5 km/L (64.7 miles/gallon).
While this is still short of the PNGV goal of 35.1 km/L (82.5 miles/gallon), and the vehicle is not
mid-size, it is a promising development.
2000 Model Year Hybrid Electric Vehicle Characteristics
Number of cylinders
Maximum engine power (kW)
Maximum bmep (bar)
Maximum electric power (kW)
Vehicle mass (t)
Frontal area (m 2 )
Engine power/displacement (kW/L)
Engine power/vehicle mass (kW/t)
Battery mass (kg)
Battery energy storage (kWh)
Urban/highway fuel efficiency (km/L)
Battery energy/maximum electric power (h)