Hybrid Vehicle Technology: An Overview

A hybrid vehicle is a vehicle with multiple distinct energy sources that can be separately

or simultaneously used to propel the vehicle. The energy can come from a number of

different sources, including batteries, fuel, solar energy, or flywheels. Different energy

converters are also used. Generally, electric motors are used with electrical energy from

batteries, solar cells, or generators driven by flywheels or heat engines. Fuel energy is

converted by a number of different heat engines, including internal combustion engines

and gas turbines. The most promising hybrid vehicle today is the hybrid electric vehicle

using batteries and an internal combustion engine. This vehicle design makes the best

use of existing technology by providing the benefits of both electric and conventional

vehicles, while minimizing the shortcomings of each.

Commercial hybrid vehicles are becoming available for purchase to the public. Toyota

has been in production of a hybrid 4-door sedan in Japan for over a year. Using a 1.5

Liter 4 cylinder engine, the Prius achieves about 80 MPG with emissions levels at about

10% the legal limit. Honda has also introduced a hybrid vehicle. The Honda Insight is a

parallel hybrid two-seater with a 1.0 Liter 3 cylinder engine and weighs about 1,800

pounds. It is recently becoming available in the United States, and is claimed to get 75

MPG in city driving and 70 MPG on the highway.

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Configurations

The two common configurations of hybrid vehicles are the series and parallel designs,

which are shown schematically in Figure 1. Series and parallel refer to the orientation of

the two power plants in the propulsion system.

In the series hybrid, the engine powers a generator that either supplies power to charge the battery pack, or power the electric drive motor.

In the parallel hybrid, the engine supplies mechanical power directly to the propulsion system, while the electric motor is also coupled directly to the propulsion system. The parallel hybrid vehicle can be run on the engine alone, the electric motor alone, or on both engine and electric motor simultaneously, depending on how control is set up.

Series hybrid vehicles are similar to purely electric vehicles except the series hybrid

vehicle has an on-board generator system. The internal combustion engine is used to

power a generator to generate electricity, which is then used to power the electric drive

motor or charge the batteries. The internal combustion engine is typically sized for the

vehicle’s high-speed cruise loads. These loads are typically small in comparison to

acceleration and hill-climbing loads, so the result is a smaller engine than would be used

if the vehicle were conventionally powered. The electric drive motor is then sized to

handle the acceleration and hill climbing loads. The series configuration hybrid vehicle

results in a relatively simple connection of the electric drive motor to the drive wheels. In

most cases, a multiple speed transmission is not required due to the favorable torque and

speed properties of electric motors. The vehicle can also be operated for a finite amount

of time as a zero emissions vehicle (ZEV) by running off the batteries only. Full vehicle

power is also available while running as a ZEV if the battery pack is sized for full vehicle

power.