Petroleum-electric hybrid vehicles combine the desirable properties of electric propulsion with the portability and convenient refueling of petroleum fuels. Most of the locomotives and many of the ships built during the past 50 years are diesel-electric hybrids. In such vehicles, a large diesel engine turns a generator that recharges batteries, which in turn power small electric motors connected directly to the wheels or propellers. This arrangement operates the diesel engines under the uniform loads and low revolutions per minute that optimize their fuel efficiency. It also eliminates the need for complex transmission systems between an engine and several wheels or propellers. Finally, isolating the diesel engine’s vibrations from the frame of a locomotive or hull of a ship leads to much smoother and quieter operation.
Today’s hybrid cars take a different approach: They connect both a gasoline-powered engine and electric motors to the wheels. In some vehicles, the electric motors assist the gasoline engine, whereas in others the gasoline engine assists the electric motors; in most, the gasoline engine shuts down when the vehicle stops. The vehicles currently on the market use nickel-metal hydride batteries and recharge the batteries with the gasoline-powered engine and regenerative braking. Unfortunately, these batteries have small capacities (e.g., a 2008 Toyota Prius can travel only 8 miles on the electric motors without a recharge), are difficult to recycle, and are guaranteed for 8 years or 100,000 miles. A replacement battery for a 2005 Toyota Prius costs $3000 retail. 
In city driving, a hybrid vehicle takes advantage of engine shutdown, electric motors, and regenerative braking to achieve fuel efficiencies of 30% greater than the same vehicle equipped with a gasoline powered engine alone. Efficiency gains of hybrids are far more modest under highway driving, where they depend heavily on their gasoline engines. The next generation of hybrid cars should be able to travel substantial distances (40 miles) at high speeds on electric power alone when lithium batteries, which have higher energy-storage capacities per size and weight, become more reliable and affordable.
Most automobile manufacturers have developed prototypes of diesel-electric hybrid vehicles that achieve substantial improvements in fuel efficiency. A number of them have attained 70 mpg (30 km L–1) or better. The main impediment to such a propulsion system is cost: A diesel engine plus electric drive system adds over $5000 to the purchase price of the same vehicle equipped with a gasoline engine. At current gasoline prices, an owner would not recover the additional capital costs over the life of the vehicle.
 O'Dell, J. (2005) Prius Keeps Car Dealers—but Not Repairmen—Busy. L.A. Times, Los Angeles, June 1, 2005.
This is an excerpt from the book Global Climate Change: Convergence of Disciplines by Dr. Arnold J. Bloom and taken from UCVerse of the University of California.
©2010 Sinauer Associates and UC Regents
CitationBloom, A. (2012). Hybrid Cars. Retrieved from http://www.eoearth.org/view/article/51cbf0387896bb431f6a0bce
I own a hybrid with over 100,000 miles. average mileage over its lifespan is 57 miles/gal. I now must replace the batteries at a cost of over 5,000.00. There is a cost to these cars, even though I have have saved the 5,000.00 over those miles with reduce gasoline usage. The all electric cars would have greater savings on fuel costs but those batteries also must be replaced and recharging stations are not abundant at present.