Q: With all the talk about rising gas prices and global warming, I'm interested in finding transportation that is both economical and environmentally friendly. What can you tell me about cars with the new "hybrid" engines?

A: Two companies, Honda and Toyota, have been marketing hybrid electric vehicles in California for a couple of years. These vehicles have small gasoline internal combustion (IC) engines and electric drive systems with small battery packs. These vehicles are not intended to be driven in the pure electric mode, but rather use the electric motor to enhance the performance of the small internal combustion engine.

The need for high power levels on IC engines is intermittent. High power is needed mainly for accelerating and hill climbing. Small efficient cars running at constant freeway speeds only need 20 to 50 horsepower, which is much smaller than most automotive gasoline IC engines.

What happens when most cars drive in stop and go traffic? The vehicle accelerates to some speed and then you have to stop. Most of the stopping is done by applying hydraulic brakes, which brings fixed brake pads into contact with a rotating metal disc. Friction between the brake pads and disc slows the vehicle down by converting the kinetic energy of the vehicle into heat in the brake pads and the rotor. (Notice what happens when you drive down a mountain road for a while-you start to smell the heated up brake system). Hybrid vehicles are designed to recover the energy of braking and use this energy to move the vehicle rather than waste the energy by creating heat and wearing out the braking system.

The Toyota Prius (5 passenger), the Honda Insight (2 passenger) and the Honda Civic Hybrid (5 passenger) use their electric motors as generators to help stop the vehicles and convert the kinetic energy of the vehicle into electrical energy and then store that energy in a battery system. (They still have hydraulic brakes but the electrical brakes do recover a large portion of the vehicle's energy, which is lost in regular vehicles.) The electric motor provides extra torque when needed for acceleration and/or hill climbing. Because of this, the IC engine can be small and more efficient. In addition, when coasting or stopped, the IC engine is turned off. You can actually start the vehicle moving on pure electric, but any hard acceleration will automatically restart the IC engine running.

The Toyota Prius was introduced in Japan approximately three years before coming to America. With a starting price just over $20,000, it is a bit heavy, but has been qualified as a Super Ultra Low Emission Vehicle (SULEV) by the California Air Resources Board. It has a 1.5-liter engine, which produces 70 hp at 4500 RPM and a 33KW (44hp) electric motor. Because of the new technology involved, the hybrid components have an eight-year, 100,000-mile warranty. The 2002 Environmental Protection Agency (EPA) estimate for fuel efficiency is 52 mpg city and 45 mpg highway. Notice that the city fuel economy is better than the highway number due to the capture of braking energy.

Honda introduced the Insight as the first production hybrid in this country in late 1999. It is a 2-passenger sports car made with lightweight construction. It has a 1.0-liter, 3 cylinder gasoline IC engine, with a starting price tag of slightly over $20,000. Because of its lighter weight and better aerodynamic characteristics compared to the Prius, the Insight has a 61 mpg city and 68 mpg highway rating by the EPA.

Honda has also introduced the Civic Hybrid to compete with the Prius. It is a 5-passenger vehicle with a 1.3-liter, 4 cylinder engine. Both Honda vehicles utilize a 10-KW electric motor. The Civic Hybrid is rated at 46 mpg city and 51mpg highway by the EPA.

Hybrids are even more economical at present because the government currently offers tax incentives to encourage their sale. Additional information is available at www.irs.gov. -Michael T. "Doc" Shelton Professor Shelton is chair of Cal Poly Pomona's mechanical engineering department and advisor in student solar car competitions.