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 twenty-five to thirty-five miles per gallon (MPG); Purpose-built gas hybrids often get 45 MPG city, and often lower mileage on the freeway. Dodge 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 and was removed from the market for "lack of interest" — hardly surprising at that price.
A hybrid vehicle is one which has multiple power systems. The typical example today is the gasoline-electric parallel hybrid, but the most familiar example (even if we don't know it is one) is the diesel locomotive. While some diesel locomotives use a gearbox, this is usually restricted to low-speed yard vehicles and those engines which will not be pulling heavy loads. The reason is simple: trains have to channel immense amounts of torque between the engines and the track. The solution was to replace the gearbox or transmission with a generator, and then connect electric motors to the wheels. This type of series hybrid comes at a cost in efficiency, but made the use of the powerful diesel locomotives of today possible.
In a parallel hybrid system, both the fuel engine and the electric motor system are capable of exerting force on the wheels of the car. Most hybrid systems today converge the electric and fuel motors' systems together at the gearbox, but it is possible instead to connect them only via the road, for example by having a small gasoline engine hooked up to the rear wheels, with an electric power system connected to the front. Hybrid vehicles of this sort have multiple power storage mechanisms, for example a traditional fuel tank full of gasoline to run a petrol engine, and a bank of dry cell batteries to operate the electric motors.
While there is a certain inherent overhead involved in a hybrid vehicle, which has multiple power systems, there are certain benefits which while not necessarily inherent to the technology, become very easy.
The explanation for both the phenomenon of better city than highway mileage and the overwhelming general potential efficiency of hybrid vehicles 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.
There are reasons to use a hybrid power system other than the increased efficiency. One such reason is that electric motors and small internal combustion engines are good at different things. The engine in your car is extremely inefficient until it gets into what we call the power band, the engine speed (typically measured in revolutions per minute or RPM) and load at which it produces peak or near-peak power. While even a particularly anemic typical modern gasoline engine is capable of producing at least 100 horsepower flat-out, while cruising on the highway the car may use only 25 horsepower, or even less. This additional power is necessary to carry the vehicle up steep hills, and to provide acceleration, but it represents wasted efficiency when it is not being used.
An electric hybrid vehicle solves this problem by simply not using the gasoline engine for low-end acceleration, or indeed typically using it for acceleration at all. Instead, this task falls to electric motors, which make their peak torque output at 0 RPM! Once the vehicle has been accelerated to speed, the electric motor can be started (without the need for a starter motor, since the vehicle is now moving) and the gasoline engine can be used to help propel the car down the road. This dramatically reduces emissions and increases overall efficiency.
In general, purpose-built parallel 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. Most 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. There is of course an exception, which is known as the "Plug-In Hybrid". Generally speaking, this type of vehicle has more battery capacity than other hybrids. Also onboard the vehicle is a charging system which can take input from household current, or even higher-voltage sources.
Parallel vs. Series Hybrids
As mentioned before, a diesel locomotive is an example of a series hybrid. The same principle can be exploited in automobiles, where it can provide an even greater advantage: regenerative braking. While regenerative braking has been attempted in diesel locomotives, in practice the overall power consumption has so far proven to be too great. It may one day turn out to be possible to replace the passive trucks beneath every rail car with a driven truck, thus distributing power distribution and collection, but so far this has not proven to be practical. In the mean time, the technology is highly applicable to smaller land vehicles, specifically cars.
One major advantage of the series hybrid is that it eliminates the drivetrain more or less completely. If the generator is integrated into the fuel motor, then significant reductions in weight are possible with nearly any type of engine. One of my pet ideas is to perform this integration with a turbine engine. Chrysler built prototype turbine-driven cars in the 1960s and even drove one coast to coast with excellent mileage for the day, but the high-RPM transmissions necessary with the technology of the day proved to limit longevity too seriously for even U.S. automakers to put the system into production. Eliminating the drivetrain eliminates the infeasibility of the project; turbine engines can theoretically be tuned such that a single engine is capable of burning nearly the widest possible range of fuels (short of using an external combustion engine, as in the case of a steam locomotive.
The internal combustion 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, yet still drive the vehicle at a variety of speeds. 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.
Nearly all hybrids to date have used a gasoline engine, although there have been some concept cars from major manufacturers (including a sport-oriented Subaru Impreza) which utilized diesel engines. Most of them use the familiar otto cycle, but some use the more efficient (but only at high RPMs) miller cycle. In theory, any type of engine can be used; for example, MDI uses an air-driven motor (like the Tomy toys with which I grew up) and a high-pressure tank instead of a combustion engine. The use of a gas turbine is equally possible, although it lends itself primarily to a series hybrid.
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.
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 can 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.
While one of the primary ideas behind the hybrid vehicle is efficiency and thus reduced ecological footprint, the lifetime energy consumption of most hybrid vehicles is not as favorable as one might like. While alternative alternative vehicles like the MDI air car deliver on their promise of low impact, vehicles like the Prius actually have a dramatically higher footprint than a simple vehicle like a Volkswagen Rabbit Diesel, Honda CRX HF, or a Geo Metro (all of which provide comparable or even superior fuel mileage, if not performance) because of their batteries. Not only do they contain heavy metals and other toxics, but during their production their parts are shipped needlessly back and forth across the ocean. While the batteries can be recycled, there is a significant energy cost involved in doing so, and as hybrid vehicles continue to gain acceptance it becomes more likely that they will fall into the hands of a class of owner which can or will not dispose of them correctly.
The owner of a car which needs a couple thousand dollars in repairs will have to drive a hybrid for a decade or more (even at today's fuel prices) to realize a cost savings, and probably far longer to ever see any reduction in environmental impact.
Hybrid vehicles would seem to be a perfect marriage of technologies. The electric system has better starting torque and higher efficiency than the gasoline, and allows for regenerative braking, while the gasoline engine provides extended range (greater than any purely electric vehicles) due to the energy density of gasoline. However, in reality, they are a gigantic boondoggle. Volkswagen's small four-cylinder TDI (turbo direct injection) diesel engines propel their Golf and Jetta models quite nicely, while getting real-world mileage over 50 MPG on the freeway. In addition, they are capable of running on biofuels, while hybrids are currently not even being designed to use alternative fossil fuels like CNG, let alone biofuels like E85, biodiesel or vegetable oil. The TDI engine can potentially even run on E95 (95% ethanol, 5% gasoline), waste vegetable oil (WVO) or a number of other fuels with only minor modifications. In addition, the most efficient possible hybrids (e.g. turbine series-electric) are not even being produced, and are hardly being researched.