In order to increase the power of an engine there has to be an increase in pressure, and hence force exerted on the piston, during the power stroke. The amount of work that the power stroke delivers is basically determined by the air-fuel mixture in the combustion chamber. The combustion that occurs during the end of the compression stroke and throughout the power stroke is determined by how much air is mixed with the fuel. When air is compressed its density increases but volume decreases. Hence compressing air at the beginning of an engine cycle increases the power output by increasing the amount of air that is mixed with fuel. Since the total volume of occupied space within the cylinder is decreased when compressing air, more air can be used to combust with the fuel. Therefore lower values of compression ratios are preferred for turbocharged engines.