Hybrid-electric vehicles (HEVs) are cars and trucks that use two sources of power to increase fuel efficiency and provide a high level of performance.  HEVs can be diesel-electric, gasoline-electric, ethanol-electric, etc.

The two sources of power can be found in hybrid vehicles in different ways.  One way, known as a parallel hybrid, has a fuel tank that supplies gasoline to the engine and a set of batteries that supplies power to the electric motor.  Both the engine and the electric motor can turn the transmission at the same time, and the transmission then turns the wheels.  The fuel tank and gas engine connect to the transmission.  The batteries and electric motor also connect to the transmission independently.  As a result, in a parallel hybrid, both the electric motor and the gas engine can provide propulsion power to the vehicle.

                                            

By contrast, in a series hybrid, the gasoline engine turns a generator, and the generator can either charge the batteries or power an electric motor that drives the transmission. Thus, the gasoline engine never directly powers the vehicle.

Gasoline-electric hybrid cars contain the following parts:

Internal Combustion Engine - The hybrid car has a gasoline engine much like the one you will find on most cars.  However, the engine on a hybrid is smaller and uses advanced technologies to reduce emissions and increase efficiency.

Fuel Tank - The fuel tank in a hybrid is the energy storage device for the gasoline engine. Gasoline has a much higher energy density than batteries do.  For example, it takes about 1,000 pounds of batteries to store as much energy as 1 gallon (7 pounds) of gasoline.

Electric Motor - The electric motor on a hybrid car is very sophisticated.  Advanced electronics allow it to act as a motor as well as a generator.  For example, when it needs to, it can draw energy from the batteries to accelerate the car.  But acting as a generator, it can slow the car down and return energy to the batteries.

Generator - The generator is similar to an electric motor, but it acts only to produce electrical power.

Batteries - The batteries in a hybrid car are the energy storage device for the electric motor.  Unlike
the gasoline in the fuel tank, which can only power the gasoline engine, the electric motor on a hybrid car can put energy into the batteries as well as draw energy from them.

Transmission - The transmission on a hybrid car performs the same basic function as the transmission on a conventional car.
 

Some of the advanced technologies used by most hybrids include the following:

Regenerative Braking  The electric motor applies resistance to the drive train causing the wheels to slow down.  In return, the energy from the wheels turns the motor, which functions as a generator, converting energy normally wasted during coasting and braking into electricity, which is stored in a battery and used later. 

Electric Motor Drive/Assist  The electric motor provides additional power to assist the engine in accelerating, passing, or hill climbing.  This allows a smaller, more efficient engine to be used.  In some vehicles, the motor alone provides power for low-speed driving conditions where internal combustion engines are least efficient.

Automatic Start/Shutoff  Automatically shuts off the engine when the vehicle comes to a stop and restarts it when the accelerator is pressed.  This prevents wasted energy from idling.
 

Hybrid-electric gasoline vehicle performance

The key to a hybrid-electric vehicle is that the gasoline engine can be much smaller than the one in a conventional car and therefore more efficient.  Most cars require a relatively big engine to produce enough power to accelerate the car quickly.  In a small engine, however, the efficiency can be improved by using smaller, lighter parts, by reducing the number of cylinders and by operating the engine closer to its maximum load.

There are several reasons why smaller engines are more efficient than bigger ones:

• The big engine is heavier than the small engine, so the car uses extra energy every time it accelerates or drives up a hill.
• The pistons and other internal components are heavier, requiring more energy each time they go up and down in the cylinder.
• The displacement of the cylinders is larger, so more fuel is required by each cylinder.
• Bigger engines usually have more cylinders, and each cylinder uses fuel every time the engine fires, even if the car isn't moving.

                                                              

 

 

Source:  www.fueleconomy.gov and www.howstuffworks.com