a method for starting a free piston , internal combustion engine includes supplying an air charge to a space in a combustion cylinder , and reciprocating the piston in the cylinder so that the maximum pressure of the air charge in the space cyclically increases . air and fuel are cyclically admitting to the cylinder to produce an air - fuel mixture , and spark ignition is used to produce cyclic combustion of the air - fuel mixture . the air - fuel ratio of the mixture is increasing when a maximum pressure in the cylinder occurs within a predetermined period following a tdc position of the piston . spark ignition is discontinued , and cyclic compression ignition of the air - fuel mixture occurs .