Compression pulse starting of a free piston internal combustion engine

Abstract

A method for starting an engine uses an actuator, such as a hydraulic or pneumatic pump-motor or an electric linear alternator-starter to move the pistons to a position where the inlet ports are opened. This ensures that air is present in the cylinder in a space where fuel will be admitted and combustion will occur. This strategy compresses, with a minimum actuator capacity, such air to a state that the pressure and temperature satisfy the ignition requirements. The air stores kinetic energy of the moving pistons that partially form the air spring force of the opposite cylinder and partially from the actuator. Accumulation, cycle by cycle, of this stored energy accelerates the piston motion, increases the piston displacement, and increases the compression ratio. The cylinder pressure and temperature increas cycle by cycle until the fuel ignition conditions are satisfied. The actuator force is a periodic force preferably having a frequency that is the same or nearly the same as the natural frequency of the system that includes the inertia of the pistons and other masses reciprocating with the pistons and the variable spring, represented by the compressible air spring in the combustion chamber. When piston displacement reaches a sufficient magnitude, fuel is admitted to the air charge, preferably by injection. The actuator continues to increase piston displacement and the compression pressure of the air-fuel mixture in the cylinder until combustion of that mixture in the first cylinder occurs. Fuel is then admitted to the second cylinder while continuing cyclic displacement of the pistons, and combustion of the fuel-air mixture in the second cylinder occurs.