The present invention is a linear internal combustion free-piston engine with three acting pistons per combustion chamber, that has the ability to store energy until needed. This engine utilizes a spring to store potential energy after combustion. The present invention also introduces additional air into the combustion chamber during the combustion cycle providing a more complete burn of the fuel, has the ability to self-start, and does not idle. Multiple engines can be married together and run as a single unit.