A four-stroke engine turns the crankshaft through intake, compression, power, and exhaust. Only the power stroke contributes horsepower to the crankshaft. The other three strokes consume that horsepower to keep the cycle going. That is the core reason parasitic losses matter. When you reduce the power required for the non-power strokes, you free up more of the engine’s existing output so it can reach the wheels instead of being spent internally.
The discussion returns to a fundamental engine concept: the four-stroke process used by most modern engines in trucks and cars. These engines are either spark-ignition or compression-ignition designs, but both follow the same basic cycle.
In a four-stroke engine, the crankshaft rotates twice to complete one full operating cycle. During that cycle, the piston moves through four distinct strokes in sequence: intake, compression, power, and exhaust.
On the intake stroke, the piston drops and draws air, or an air-fuel mixture, into the cylinder. On the compression stroke, the piston rises and compresses that charge. On the power stroke, the piston drops again as combustion forces it downward. On the exhaust stroke, the piston rises once more to push the spent gases out of the cylinder.
Of those four strokes, only the power stroke adds horsepower to the crankshaft. That is the stroke in which combustion produces useful work and delivers torque into the rotating assembly.
The other three strokes do not contribute power directly. Instead, they consume horsepower from the crankshaft in order to carry out intake, compression, and exhaust. In that sense, they represent parasitic losses within the engine cycle.
The engineering objective is therefore to reduce how much power the crankshaft must give up to support those non-power-producing strokes. Minimizing those losses leaves more net power available at the crankshaft.
The core takeaway is simple: engine efficiency improves when the parasitic demands of intake, compression, and exhaust are reduced, while the useful output of the power stroke is preserved or increased. The goal is to maximize the horsepower that remains resident in the crankshaft.
