Starting a compression ignition free piston internal combustion engine having multiple cylinders

A method for starting a free piston internal combustion engine that includes a first pair of mutually connected pistons, a second pair of mutually connected pistons, a first piston of each pair moving in a first cylinder, a second piston of each pair moving in a second cylinder, the method comprising the steps of:

• supplying an air charge in a space located between the pistons in each cylinder;
• closing the spaces against passage of fluid while reciprocating the pistons in the cylinders and cyclically increasing a maximum pressure of the air charges in the spaces;
• cyclically admitting air and fuel to the cylinders to produce an air-fuel mixture;
• using spark ignition to produce cyclic combustion of the air-fuel mixture;
• reducing an air-fuel ratio of the mixture when a maximum pressure in the cylinder occurs within a predetermined period following movement of a piston pair past a TDC position;
• discontinuing spark ignition;
• and
• producing cyclic combustion ignition of the air-fuel mixture.