Combustion is more sophisticated than it seems. It is more than the mere burning of fuel. Combustion is a precise process that requires proper timing and coordination. It involves many components that either work in synchrony or in succession. When you turn the ignition key of your automobile, you are initiating combustion. High-voltage current from the ignition coil flows to the spark plugs. The metallic gaps of the spark plugs discharge electrical sparks that are very similar to lightning. However, this is not as simple as switching-on the spark plug circuits. The distribution of current to the spar plugs are regulated by the rotary switch known as the distributor.
The rotation rate of the distributor rotor is determined by the rotation rate of the crankshaft. Since the crankshaft merely translates the linear reciprocating motions of the pistons, the firing timing of the spark plugs corresponds to the motions of the pistons. Specific spark plugs only fire when the corresponding pistons are in certain position. They only fire when the pistons are in power stroke position. Hence, the firing sequence of the spark plugs mimics the reciprocating or alternating motions of the pistons. If the spark plug timing is premature or delayed, the engine might breakdown.
Aside from the spark plug timing, combustion timing is also regulated by the closing and opening of the valves. In a typical gasoline engine, the process of combustion undergoes four stages. These stages are also collectively known as the four-stroke cycle or Otto cycle. This cycle involves the opening and closing of the air intake valves and exhaust valves. The four stages of the cycle are the intake stroke, compression stroke, power stroke, and the exhaust stroke.
Intake stroke is the stage wherein air and fuel are drawn into the combustion chambers. The intake valves open while the injectors spray fuel into the combustion chambers. The fuel is sprayed as very fine droplets to maximize the surface area exposed to the air. This allows the fuel to be optimally combusted. During this stage, the fuel is mixed with the air. This stage is followed by the compression stroke. It is during this stage that the fuel/air mixture is thoroughly homogenized by compressing it. The pistons exert pressure on the mixture. It is during this stage that the fuel is being prepared for burning. During this stage, the valves are closed to maximized compression. The third stage of the combustion cycle is the power stroke. It is the most important stage of the cycle. It is during this stage that the chemical energy of the fuel is released. It is then converted into mechanical output. It is during this stage that the molecular bonds of the fuel molecules are broken as they combine with the oxygen in air. New molecular bonds are formed, namely, water molecule bonds and carbon dioxide bonds. Finally, the fourth stage of the combustion cycle is the exhaust stroke. It is during this stroke that the byproduct gases of combustion are released. Although these gases are predominantly carbon dioxide and water vapor, some residual traces of metals and pollutants are also present.
As the molecular bonds of the fuel molecules are broken, thermal energy is released. The release is almost instantaneous. This explosive release of heat creates a sudden buildup of pressure inside the combustion chambers. As the exhaust gases tries to escape, the pistons are pushed. However, not all pistons are simultaneously pushed. Some pistons are undergoing other stages of combustion. Hence, the pistons move in alternating or reciprocating manner. Collectively, these linear motions are converted to rotational motion by the crankshaft. Thus, the cycle begins again.
