In a 4-stroke engine, there are 4 strokes, or stages, rather. The first stage is intake, where air is drawn in and mixed with fuel. The second stage is compression, where the air-fuel mixture is compressed to increase power output and the 'explosive' energy. The third stage is combustion, where a spark plug (or in diesel engines, just extremely high pressures) ignite the air-fuel mixture to create an explosion (or chemical energy) which in turn, pushes the pistons and turns the crankshaft which results in mechanical energy. The fourth and final stage, is the exhaust, where the air-fuel mixture is now expended through the exhaust hole and in a different state of matter than the original.
But it is still in a state of matter with usable energy, right? So, according to the laws of thermodynamics, if energy cannot be created nor destroyed, only transformed, than could you create a sort of 'recycling' stroke in an engine, to 'recycle' and 'reuse' the expended exhause energy (the chemical energy of the fuel/air mixture being combusted)? Perhaps, taking the exhaust smoke, cooling and compressing it again, adding more fuel to it and combusting it once more? Or would this be too inefficient and not really even do anything in the end result of the whole matter?