The carburetion system supplies a controlled mixture of fuel and air to the engine, optimized for the current engine temperature, load and speed, all the while trying to keep fuel economy high and emissions low. This is not a simple task. Carburetion is achieved in one of two basic methods: fuel injector or carburetor. These methods can be assisted through complex engineering designs, computer control and turbocharging to make the engine more efficient.
Fuel injection is the current rage, although it has been around for years. This system delivers fuel to the fuel injectors via a pressurized fuel line. The injector is an electromechanical device that sprays and atomizes the fuel into the intake air. The more it is able to atomize the fuel, the better combustion becomes, allowing a full burn of the fuel. The injector is actually, nothing more than a solenoid through which fuel is delivered to the air stream.
Modern fuel injection systems are computer controlled and called EFI or Electronic Fuel Injection, in contrast to older systems which were mechanically controlled. Using a series of sensors, the engine, exhaust and environmental conditions are monitored. This information, accompanied with throttle position information, is used by the computer to determine the optimal fuel mixture. When determined, the computer instructs the injector system to provide a certain quantity of atomized fuel.
A carburetor is a mechanical fuel atomizing device. Carbureted systems can also be computer controlled, using the same types of sensors for controlling the fuel mixture as noted above. The major difference is, instead of electronic devices controlling the amount of fuel delivered to the air stream, mechanical devices are used. Inside a carburetor, fuel is pulled into the air stream by creating lower pressure (i.e., high vacuum) areas around a venturi cluster supplied with higher pressure fuel. The higher pressure fuel will flow to the lower pressure air stream in the intake manifold. This concept, known as Bernoulli's Principle, is the basis for all mechanical atomizing devices.
Added complexity comes from the fact that the engine does not draw a smooth steady air flow. Instead, the air flow is a series of high-demand/no-demand droughts compounded with reverse valve shock waves and exhaust suction. Coupled with the need to change the fuel mixture based on engine speed, engine temperature, air pressure, etc., the role of the carburetor is very complex.
The advantages of fuel injection are obvious. A mechanical device cannot control the countervailing forces and demands as well as an electromechanical device. Fuel injection is a more precise method of measuring fuel. Fuel injection can increase horsepower, increase torque, improve fuel economy, improve cold starting and reduce emissions. The disadvantage has been cost, although computer controlled carbureted systems can be as expensive as computer controlled fuel injection systems.
How the carburetion system channels air to the intake valve is extremely important. The engineering of the air intake path plays a significant role in engine power. Designs that provide a lower resistance path for the air to flow will be much more powerful than designs that do not. The reason is simple. An engine is an air pump. For any given engine, if you reduce the energy it must expend pumping air, you increase the energy available for other uses. The ideal path for air flow would be a straight line from the air filter through to the piston.
An alternative to designing a low resistance air path is to assist the engine's efforts to pump air via turbocharging or supercharging. Both devices help to overcome the friction losses of the intake systems by blowing air into the cylinder. Superchargers and turbo-chargers perform the same function. They use a turbine or impeller to compress the intake air. Superchargers derive their power from the engine using belts or gears to power the compressor. Turbochargers use a turbine in the exhaust systems to power the compressor. Both systems have found little acceptance in the mass produced motorcycle market. Forced air induction via velocity (Ram effect) has found its way into the main stream with many manufactures highly touting it. It is much harder to prove how effective it is as most dynamometers may not simulate real driving conditions with the amount of air they provide to the RAM-air ducts.