The compression stroke occurs when the trapped air-fuel mixture is compressed inside the cylinder. The combustion chamber is sealed to produce the charge. The charge is the quantity of compressed air-fuel mixture kept inside the combustion chamber that is ready for ignition. Compressing the air-fuel mixture makes for more release of energy when the charge is ignited. Intake and exhaust valves needs to be closed to ensure that the cylinder is sealed to provide compression. Compression is the simply the process of reducing or squeezing a charge from a large volume to a smaller volume in the combustion chamber. The flywheel assist to maintain the momentum needed to compress the charge.
When the piston of an engine compresses the charge, an increase in compressive force produced by work being done by the piston makes the heat to be generated. The compression and heating of the air-fuel vapor in the charge leads to an increase in charge temperature and increases fuel vaporization. The increase in the charge temperature happens uniformly throughout the combustion chamber to give out faster combustion (fuel oxidation) after ignition.
The increase in fuel vaporization occurs as small droplets of fuel get vaporized more completely from the heat created. The increased droplet surface area exposed to the ignition flame permits more complete burning of the charge in the combustion chamber. Only gasoline vapor ignites. An increase in droplet surface area makes gasoline to release more vapors instead of remaining a liquid.
The more the charged vapor molecules are compressed, the more energy obtained from the process. The energy required to compress the charge is substantially less than the gain in force produced in the combustion process.
The compression ratio of an engine is basically the comparison of the volume of the combustion chamber with the piston at BDC to the volume of the combustion chamber with the piston at TDC. This area, combined with the design and pattern of combustion chamber, determines the compression ratio. Gasoline engines normally have a compression ratio ranging from 6:1 – 10:1. The higher the compression ratio, the more fuel-efficient the engine. A higher compression ratio normally gives a substantial gain in combustion pressure or force on the piston. However, higher compression ratios increases operator effort required to start the engine. Some small engines have a system to relieve pressure during the compression stroke to reduce operator effort required when starting the engine