The power stroke is the third part of the four-cycle engine and is the stroke which produces power. Included in the typical four-cycle engine, which is common in automobiles, are: an intake stroke with the piston moving downward; the compression stroke with the piston moving upward;, the power stroke or ignition stroke with the piston moving downward, and the exhaust stroke with the piston moving upward. In any engine, from diesel to steam, the power stroke is the stroke which produces the energy to propel the vehicle or cause the engine to operate.
The secret to making horsepower with an internal combustion engine lies in getting the most power or energy possible from the engine's power stroke. Many factors affect the power stroke. Valve timing, ignition timing and fuel delivery are some of the most notable. One process of tuning a power stroke that is often overlooked is the exhaust flow. The exhaust directly impacts the power stroke and must be factored in when building a high-performance engine.
The camshaft profile is the heart of the power stroke. Keeping the intake valve open to ensure maximum fuel gathering, then slamming the valve closed to get the proper compression without blowing fuel out into the exhaust, can be tricky. Once the fuel combusts and the power is unleashed, the exhaust valve must be opened to allow the spent gasses to vacate the cylinder. Pushing all of the exhaust out of the cylinder prior to opening the intake valve is the secret to high horsepower.
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Many innovations have been made in an attempt to increase the stroke power produced from an engine. They include fuel injection, forced induction in the form of turbo chargers and super chargers as well as roller cams and improved rocker arm ratios and geometry. On the exhaust side of the cycle is header design, head porting and location.
Whether it be racing, boating or just working harder, the designers of the modern power plant have struggled to top each other in the production of more horsepower. Racing series such as Formula 1™ and NASCAR and off-shore boating series are responsible for design improvements which find their way into street-driven automobiles. The lineage between racing and everyday use is found in the improvement of the power stroke.