stop it from reaching starting position 44 exhaust valve 28 is then closed the mass of the piston and the time of the closing of valve 28 will determine the compression ratio and therefore the size of the combustion chamber prior to piston assembly 13 returning to starting position 44 fuel is injected by fuel injector 34 and as piston assembly 13 reaches starting position 44 the fuel is ignited forcing piston assembly 13 again to apex position 46 during this ascent of piston assembly 13 the spring biased valves 22 and 24 will direct the column of air above piston assembly 13 to enter through tube 26 into air reservoir 14 further compressing the air therein so long as valve 31 continues to be held in a closed position however as piston assembly 13 ascends past inlet port 29 spring biased valve 31 is allowed to open and admit a charge of compressed air below the piston which assists in pushing the piston upward and expelling the contaminated gases beneath piston assembly 13 downward through exhaust port 27 and supplies a fresh charge of compressed air beneath the piston assembly 13 after a sufficient charge of compressed air is passed through inlet port 29 spring biased valve 31 is again held closed causing the column of air remaining above piston assembly 13.