Compression Stroke

     When the piston reaches bottom dead center at the end of the intake stroke，the intakevalve closes and seals the upper end of the cylinder. As the crankshaft continues to rotate，it pushes the connecting rod up against the piston. The piston then moves upward andcompresses the combustible mixture in the cylinder. This action is known as thecompression stroke (Fig. 3. 4). In gasoline engines，the mixture is compressed to aboutone-eighth of its original volume. (In a diesel engine the mixture may be compressed to aslittle as one-sixteenth of its original volume.）This compression of the air-fuel mixtureincreases the pressure within the cylinder. Compressing the mixture in this way makes‘－more combustible；not only does the pressure in the cylinder go up，but the temperaturethe mixture also increases.