A hybrid vehicle is one which uses a combination of electric motors and internal combustion engines for motive force in order to improve efficiency and decrease emissions. Hybrid versions of preexisting vehicles tend to achieve gas mileage in the range of thirty to forty miles per gallon (MPG); Purpose-built gas hybrids often get 50-60 MPG city, and often lower mileage on the freeway. Dodge has even made a hybrid Dodge Durango which got approximately 27 MPG, quite a coup for a full-sized SUV. Unfortunately, the vehicle had an US$85,000 price tag.
The explanation for both the phenomenon of better city mileage and the overwhelming general efficiency is simple; regenerative braking. Put simply, the electric motors (which drive the front wheels) are used as brakes through their employment as generators to charge the batteries. When a generator is put under load, its resistance to being turned increases; this provides the braking force. Varying the number of batteries or cells connected to the motors will change the load, which can be used to provide anti-lock braking. If the voltage coming back from the motor decreases too sharply, the load is reduced. Conventional brakes are used when speed drops too low for this effect to provide deceleration, or in the event of extremely hard braking.
However, on the freeway one (hopefully) does not do nearly as much braking, so the benefit is greatly reduced. Electric motors used in hybrid autos are quite reliable, often as good at 80% and frequently nearly as efficient while operating as a generator as well. Of course additional losses can be had in the limited drivetrain and in power regulation systems during acceleration, and in the charging system during braking, but in general these systems are very efficient, which means that the vast majority of power spent on acceleration can be recouped during deceleration.
In general, purpose-built hybrids are front wheel drive for both the electric and gasoline (again, or diesel) engine, while other vehicles may sometimes use the gasoline engine to power the rear wheels (as the hybrid durango does) in order to gain an all wheel drive effect.
Hybrid vehicles are never "plugged in". Their electrical systems are charged only by the vehicle themself, which may take the place of engaging the motors as generators while cruising using the internal combustion powerplant, or by an additional more efficient generator.
The gasoline engines are generally of very small displacement, and are often connected to an electronically controlled continuously variable transmission, or CVT. This, coupled with the use of the electric motors for initial acceleration, allows them to always run in their power band, the place at which they are most efficient. These engines are typically fairly high-compression, small-displacement, and run at reasonably high RPMs. Their small size lets them run very quietly as you can tune the intake and exhaust to a very small range of RPMs.
One might think that a car with an engine as small as 1,500 ccs would accelerate very slowly, but to make that assumption would be to discount the electric motors used for acceleration. While these motors generally have a peak horsepower rating of around 30-50 HP, they often put out as much as 200-300 foot-pounds (ft-lb) of peak torque. One of the great advantages of electric motors over internal combustion engines is that they develop their peak torque at 0 RPM, making them ideal for acceleration. As speed increases, torque drops somewhat sharply, at which point the gasoline engine is activated.
Even so, these are hardly speed demons. The first Hybrid marketed in the US, the Honda Insight, has only 71 peak horsepower in the 2003 model. The Toyota Prius, its primary competition, does somewhat better with a peak net power of 98 hp. The insight and the prius both take about 12 seconds to hit 60 MPH; clearly they won't be winning any drag races any time soon.
Hybrid cars tend to have an extremely streamlined appearance, with stubby noses, and flat sides. This is of course to reduce aerodynamic drag in order to improve, you guessed it, efficiency. The honda insight has a .22 CD (coefficient of drag) which greatly contributes to the high gas mileage.
- Specifications: Honda Insight. Honda Motors, 2003. (http://www.hondacars.com/models/specifications.asp?ModelName=Insight)
- Prius Specifications. Toyota, 2003. (http://www.toyota.com/html/shop/vehicles/prius/specs/prius_specs.html)
- 2003 Toyota Prius. Consumer Guide, 2002. (http://auto.consumerguide.com/auto/new/reviews/full/index.cfm/id/23342.htm)