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8 phase motor how 4 strokes really work part two 4 compression with the intake fully closed the upward moving piston compresses the air fuel mixture causing the molecules to pack together more closely which allows for more effective combustion the compression phase extends all the way to TDC Top Dead Center overlapping into the ignition phase 5 pre power burning phase the spark plug ignites the fuel air mixture BTDC Before Top Dead Center the fingers of the flame spread slowly at first in a 3-D fractal pattern throughout the combustion chamber since the speed at which the flame front propagates is constant but the speed of the engine varies ignition must be started earlier as the engine goes faster for the flame expansion to keep up with the speed of the piston ignition occurs around 5 to 10 degrees BTDC at idle and as advanced as 35 BTDC at maximum rpm each engine is different the piston continues to TDC Top Dead Center as the ignition begins to spread and create pressure 6 power production stroke the fuel air mixture is now completely combusting and the resulting expanding gases create intense pressure which drives down the piston with considerable force this is the phase where the chemical energy in the fuel is turned into mechanical energy efficiency usually 35-40 percent.