As many people already know, an engine is a system which transforms chemical energy into mechanical energy: an engine receives the energy from one or more types of fuel (for example, gasoline, in most motorcycles); this energy is transformed into mechanical energy (mechanical work at the engine shaft) and heat by mean of combustion.
In order to have combustion, however, fuel is not enough: it must be mixed with air (oxygen, O2). In case of gasoline, the combustion reaction, which is a chemical reaction between fuel and oxygen, is initiated by sparks provided by the spark plug, which receives a specific command from the ECU/CDI. Before starting the ignition, it is necessary to mix air and fuel according to a specific stoichiometric ratio. This ratio depends on the fuel, and in case of gasoline, the ideal stoichiometric ratio (mass of air divided by mass of fuel) is 14.7:1. Thinking about "mass", it means that, for each "part" of fuel, you need to have 14.7 "parts" of air (it means much more air than fuel), if you want to obtain the best combustion, in the ideal case. In case you put more air than what is needed by the stoichiometric ratio, some air will not react with the fuel (air is in excess); on opposite, if you put too much fuel, some will not react (fuel is in excess) and it will be wasted, flowing out from the exhaust pipe into the air.
The ideal stoichiometric ratio, however, is just a theoretical value. In a real combustion process, due to the differences with the ideal combustion, the best performances can be obtained with an air-fuel mixture containing a bit more fuel than what is theoretically needed: in this case, the mixture is said to be "rich". In the example, the maximum power can be obtained with a lambda value of 0.86 (air-fuel ratio is 12.6:1). On the other side, if the target is to obtain the best fuel economy, the mixture should be "lean", that means that the combustion should happen with an excess of air (as an example, lambda 1.05, which means air-fuel ratio 15.4:1).
Except for the most recent Direct Injection Engines, usually the air is mixed with the fuel before entering the cylinder, in the intake manifold. In the past, and in few recent motorcycles carburetors were used to mix air and fuel: this system is using the "Venturi effect" to increase the speed of the air, creating a loss of pressure, so that the fuel flows into it. It can be considered some kind of fluid-dynamics injection. In contrast to that, actual engines (100% of the cars and about 90% of the motorcycles sold nowadays) are using Electronic Fuel Injection. In case of EFI, the fuel is injected into the intake manifold using the so called "injectors", which can be imagined as a valve electronically controlled (open or closed).
The injector has a solenoid which is controlled by the ECU. When the ECU closes the circuit, the injector coil is turned ON, it opens, and it lets the fuel flow into it, directed into the the intake manifold. It is simple to imagine that the amount of fuel that flows towards the intake manifold is proportional to the time that the injector is left open. Usually, this time (called "injection time") is hundreds of microseconds, or few milliseconds. On opposite, when the ECU opens the circuit, the current decreases and drops to zero, causing the injector valve to close, so no fuel flows to the injector. The ECU controls the injection time in order to maintain the lambda (air-fuel ratio) value close to the best ratio. In order to do this, the ECU needs to estimate the quantity of air which flows into the intake manifold. This quantity depends on many engine and vehicle variables, but mainly:
- Throttle position (%): it depends on how much you open the "gas". The more you open the throttle, more easier will be for the air to flow into the cylinder.
- Engine rotation speed (rpm): depending on the rotation speed, the engine capability of "sucking" air into the cylinder changes. At low rpm and at high rpm, the engine is not able to suck much air (low volumetric efficiency); there is a particular rotation speed which maximizes the quantity of air which can flow into the cylinder. This phenomenon is caused by resonances, which depend on the size ans shape of engine components (cam shape and timing, intake manifold, air filter and air-box, exhaust, and so on...).
- Air pressure (Pa) and Air temperature (K), which determine the density (mass-volume ratio) of the air flowing into the cylinder, according to the physical law of ideal gas.