Hybrid Electric Vehicles: Pros and Cons

Simpler and somewhat less expensive “mild” parallel hybrids, such as Honda’s Integrated Motor Assist (IMA) and GM’s Chevrolet and Saturn hybrids, use an electric motor to automatically restart the engine after it shuts down (to save fuel at rest) and boost its performance on demand, but not to drive the wheels.

Series Hybrid Examples
The best-known example of a series hybrid is a diesel/electric locomotive, which uses diesel-driven generators to continuously pump electricity to the massive batteries that power its motor-driven wheels. Start-up automaker Fisker Automotive promises to have small quantities of its series hybrid Karma luxury sedan on the market by the end of 2009, while the first volume example should be GM’s now-famous Chevrolet Volt about a year later.


GM—which experimented with a variety of hybrids during its 1990s EV program, including one series design that used a high-efficiency gas turbine to drive its generator—says Volt’s “Voltec” powertrain system will provide roughly 40 miles of range on its (lithium-ion) batteries alone before a small gas engine starts to keep it going. Future variations will use alternate-fuel and diesel ICEs, maybe someday even fuel cells, as range extenders.

Hybrid Propulsion Efficiency
Because electric propulsion is three times as efficient (in terms of how much stored energy gets to the wheels) as the typical heat-wasteful ICE, the more a hybrid’s electric motor propels it, the more fuel it saves and the fewer emissions it generates. A hybrid propulsion system can be tuned to maximize fuel economy, enhance performance using the motor to “boost” the ICE’s output (while retaining good fuel economy), or achieve the desired balance of both. Hybrids also save fuel by recapturing energy through regenerative braking: during coasting and light to moderate braking, the vehicle’s kinetic energy drives the motor, turning it into a generator that charges the battery, slowing the vehicle while saving the friction brakes.


So hybrids enjoy the major advantage of consuming less fuel and generating fewer emissions as a result. Most also qualify for federal income tax credits (check http://www.fueleconomy.gov/). But their major disadvantage is toting around two propulsion systems to do it, nearly doubling the powertrain weight, cost (including the necessary high-tech controls) and the space required to package all that hardware in the vehicle.