In an engine cylinder, the gas molecules are moving about at considerable speed in the space occupied by the gas, colliding with other molecules and the boundary surfaces of the cylinder head, the cylinder walls, and the piston crown. The rapid succession of impacts of many millions of molecules on the boundary walls produces a steady continuous force per unit surface which is known as pressure.
When the gas is compressed into a much smaller space, the molecules are brought closer to one another. This raises the temperature and greatly increases the speed of the molecules and hence their kinetic energy, so more violent impulses will impinge on the piston crown. This increased activity of the molecules is experienced as increased opposition to movement of the piston towards the cylinder head.
The process of compressing a constant mass of gas into a much smaller space enables many more molecules to impinge per unit area on to the piston. When burning of the gas occurs, the chemical energy of combustion is rapidly transformed into heat energy which considerably increases the kinetic energy of the closely packed gas molecules. Therefore the extremely large number of molecules squeezed together will thus bombard the piston crown at much higher speeds. This then means that a very large number of repeated blows of considerable magnitude will strike the piston and so push it towards ODC.
This description of compression, burning, and expansion of the gas charge shows the importance of utilising a high degree of compression before burning takes place, to improve the efficiency of combustion. The amount of compression employed in the cylinder is measured by the reduction in volume when the piston moves from BDC to TDC, the actual proportional change in volume being expressed as the compression-ratio.
The compression-ratio may be defined as the ratio of the maximum cylinder volume when the piston is at its outermost position (BDC) to the minimum cylinder volume (the clearance volume) with the piston at its innermost position (TDC) – that is, the sum of the swept and clearance volumes divided by the clearance volume,
Petrol engines have compression-ratios of the order of 7:1 to 10:1; but, to produce self-ignition of the charge, diesel engines usually double these figures and may have values of between 14:1 and 24:1 for naturally aspirated (depression-induced filling) types, depending on the design.
