Home - Background: Drivetrains
In the vast majority of cars and trucks on the road today, a single fuel, either gasoline or diesel, is burned in a piston or reciprocating engine (or, in a few cases, a rotary or Wankel engine) to produce power, which is transferred to the wheels by a set of shafts and gears (or, in a few cases, belts in a continuously-variable transmission). There are many other ways to convert fuel energy into energy of motion; I'll describe here a few variations on the theme, with schematic diagrams. Above is one for a conventional vehicle, with a single fuel tank for gasoline or diesel fuel and an internal-combustion engine providing power to the drive axle. (I'll omit details of transmissions, control systems, front vs. rear wheel drive, etc.)
First of all, a vehicle need not be restricted to a single fuel. If a vehicle is to be used in an area where an alternative fuel is not readily available, the ability to run on gasoline or diesel as well as some alternative fuel is a safety net against getting stranded without enough fuel to drive to the nearest alternative-fuel refueling station. Vehicles that run on an alternative fuel without the ability to run on gasoline, like my van, are referred to as dedicated alternative-fuel vehicles; in that sense, most cars and trucks are "dedicated" gasoline or diesel vehicles.
The engine of a dedicated vehicle can be optimized to run on its one fuel; for example, the Honda Civic GX is a dedicated natural-gas vehicle, and because natural gas has an octane rating of 130, its engine has an extremely high compression ratio (12.5:1!), which enables it to make good power despite being tuned to produce extremely low emissions. (In its earliest model years, it had more power than any of the concurrent gasoline versions of the Civic sedan, despite being the cleanest internal-combustion engine vehicle ever made!) If it had to be able to run on gasoline as well, it couldn't use this high compression ratio without seriously compromising fuel economy, emissions, and low-end torque by using a "looser" cam and other modifications.
There are two common ways to allow a vehicle to run on an alternative fuel as well as gasoline, depending on whether gasoline and the alternative fuel can be mixed in one fuel tank. If the two fuels cannot be mixed, like natural gas and gasoline, then one needs two complete separate fuel systems--that is, two sets of fuel tanks and injectors or carburetors, with the ability to switch back and forth between them. For example, a natural gas carburetor (or "mixer") can be installed as a "hat" over the throttle body of a fuel-injected gasoline engine, and fuel flow to the gasoline injectors is turned off when natural gas is going to the carburetor. Such an arrangement is called a bi-fuel system. It gives the ability to run on alternative fuel where it is available, with gasoline as a backup; however, it requires the vehicle to bear the weight and cost of two complete fuel systems.
If the alternative fuel can be mixed with gasoline, like methanol or ethanol, then both fuels can be put into the same fuel tank. A sensor in the fuel line tells the engine computer what the mixing fraction of the alternative fuel with gasoline is, so that it can adjust ignition and injector timing accordingly. The driver doesn't have to make any adjustments, so such a flex-fuel system is the simplest way to give consumers the ability to use alternative fuels. In fact, starting in the 1999 model year, some major automakers began building all vehicles in certain model lines with ethanol flex-fuel capability, at no extra charge.
In addition, a dual-fuel system is sometimes used for heavy-duty vehicle engines (buses and large trucks). I have heard this term used interchangeably with "bi-fuel", but my understanding is that it properly refers to a system that uses both a conventional and an alternative fuel in a fixed ratio all the time, as opposed to one at a time (bi-fuel) or in variable amounts (flex-fuel). The example with which I'm most familiar is a conversion of a diesel engine to burn mostly natural gas, with just enough diesel fuel injected to initiate combustion. This has the advantage of not requiring as much change to the engine as a dedicated natural-gas conversion (which requires spark plugs for ignition, for example), but it does not confer the flexibility of a dual-fuel or flex-fuel conversion because it cannot run on conventional (diesel) fuel alone.
The high efficiency of electric generators and motors makes electricity attractive for reducing transportation fuel use; even accounting for transmission-line losses and battery-recharging losses, it is more efficient to burn a fuel in a power plant and use the resulting electricity (stored in a battery) to power the car with an electric motor than to use the fuel in an internal combusion engine to drive the car directly. If the electricity comes from a renewable source like water, wind or solar power, then no fuel is used at all--pretty efficient, eh? The electric motor can also act as a generator to recycle energy of motion by putting it back into the battery when the vehicle slows down, instead of wasting it as heat in the brake linings; this is called regenerative braking.
However, because present batteries are heavy and bulky, and take a long time to recharge (though quick chargers have been tested), a lot of development work has taken place to allow the electricity to be generated on board the vehicle. That way one gets some of the efficiency advantages of the electric drivetrain, but the energy to power the electric motor is stored onboard not in a large battery pack but in a tank of some combustible fuel (gasoline, diesel, or some alternative fuel), along with an engine and generator to turn it into electricity. Such an arrangement is called a hybrid electric vehicle (HEV), by contrast with the simpler battery electric vehicle. Actually, a hybrid will include a (relatively small) battery pack, or another form of electrical energy storage such as an "ultracapacitor," in order to store energy recovered with regenerative braking, and also to put out extra power beyond what the gasoline engine can produce on its own when fast acceleration is needed. The electric motor also acts as a generator to recharge the batteries using power from the combustion engine, instead of from a wall plug; and it can also act as an extra-powerful starter motor for that engine, enabling it to shut down instead of idling (to save fuel), but to be restarted instantly when needed.
The idea is that the combustion engine can be set up to feed the average power demand of the vehicle, with the batteries available for "surge" power; this is in contrast to a conventional vehicle, whose engine must be big enough to handle the peak power demand, and which as a result is much less efficient under average driving conditions. Thus a hybrid electric vehicle can be thought of as a very efficient gasoline (or diesel, or natural-gas, or whatever) vehicle, rather than as an electric vehicle that runs on the juice coming out of a wall socket. An HEV with an electric motor powerful enough to propel the vehicle around town can be given a somewhat larger battery pack, plus the ability to recharge from the utility grid, which enables it to drive some distance without using any gasoline at all, while still having the ability to use gasoline for long road trips. This can be thought of as a bi-fuel or flex-fuel vehicle for which the alternative fuel is electricity; as of about 2006 conversions of existing hybrids to add this capability began to appear, and several major automakers are promising to start building such grid-connected or plug-in hybrids starting about the 2010 model year.
There are two typical ways to arrange the flow of power in a hybrid electric vehicle. If the combustion engine is capable of turning the drive wheels as well as the generator, then the vehicle is referred to as a parallel hybrid, because both the electric motor and the combustion engine can push the vehicle in tandem. The HEVs sold today are of the parallel variety; there's a further distinction between "mild" parallel hybrids and "full" parallel hybrids, depending on whether the electric motor is powerful enough to propel the vehicle on its own (a "full" hybrid like the Toyota Prius or the Ford Escape HEV), or just large enough to provide regenerative braking, instant engine startup, and a boost to the combustion engine (a "mild" hybrid like the Honda Insight and Civic HEV).
Parallel hybrids can have other configurations; the Chevrolet Triax concept vehicle had an ordinary combustion engine powering one axle, and an electric motor powering the other...
...while the all-wheel-drive Lexus RX 400h hybrid has a combination of these, with a gasoline engine coupled with an electric motor powering the front axle, and another electric motor on the rear axle!
If, on the other hand, the combustion engine is set up only to turn the generator, with power only reaching the drive wheels through the electric motor, then the vehicle is called a series hybrid.
Actually, the term "combustion engine" above is used loosely. In a hybrid electric vehicle, the fuel-burning engine can be a small piston (or reciprocating) engine or a rotary (or Wankel) engine; these would be of fairly conventional design, though as noted they would be smaller and more efficient than ones that had to power the vehicle entirely on their own. There have also been experimental series hybrid vehicles with gas turbine engines driving very high-speed generators; gas turbines are essentially small jet engines where the "thrust" is entirely used to spin an output shaft, rather than being blasted out a nozzle for propulsion. Gas turbines have efficiency and emissions advantages compared to ordinary internal combustion engines, but they are still expensive and difficult to make using automotive-style high-volume production techniques.
A form of "engine" that is now attracting a lot of interest is the fuel cell, which "burns" the fuel at relatively low temperature and extracts the electrical energy directly rather than through a driveshaft turning a generator. A fuel cell is sort of like an electrochemical battery that is "recharged" continuously by the fuel and air being fed into it. Fuel cells are very efficient at extracting fuel energy in the form of electricity, and because they operate at only a few hundred degrees Fahrenheit (or less) they do not produce oxides of nitrogen (an important smog-forming pollutant caused by high-temperature combustion). As for the kinds of fuels they can use, recently fuel cells that directly use methanol and natural gas have been announced; however, the best-developed designs are fueled by hydrogen. You'll note the similarity of this fuel-cell drivetrain diagram to that of the series HEV above it, with the fuel cell replacing the combustion engine and generator; for that reason, some automakers (notably Toyota) refer to such a vehicle as a "fuel cell hybrid," in that it has both fuel cells and a battery.
There are also non-hybrid fuel-cell vehicles among the prototypes out there, which run power directly from the fuel cell to the motor without a battery pack as a buffer; the first fuel-cell vehicle I ever drove was of this non-hybrid type. As with a conventional gasoline vehicle compared to a gasoline hybrid-electric vehicle, this means the fuel-cell "engine" needs to be larger than it would if there was a battery (or ultracapacitor, or other energy storage system) to handle "surge" power demand; for this reason, non-hybrid fuel-cell vehicles are rarer than hybrids, and in fact I didn't even know they existed until I drove one!
The hydrogen can be generated off-board by using electricity to split water into hydrogen and oxygen, or by extracting it from natural gas, with the hydrogen then being stored aboard the vehicle. In the late 1990s and early 2000s, there were some research projects into ways of generating the hydrogen onboard the vehicle from methanol, natural gas, or even gasoline, since these fuels are easier to obtain and store than hydrogen. The process of "re-forming" these fuels into hydrogen is well understood; so-called "partial-combustion reactors" are 19th-century chemical engineering technology. However, making them run clean enough to provide environmental benefits (and also to enable the fuel cell to survive--carbon monoxide "poisons" fuel cell catalysts, as well as people!) involved some decidedly 21st-century techniques, such as advanced catalyst materials and computer controls of the combustion process. As far as I am aware, this line of research has been abandoned by the major automakers, whose efforts now focus on fuel-cell vehicles with externally generated hydrogen stored onboard.
new 3 July 1998, revised 4 December 2008