Volumetric Efficiency

There are several standard ways to improve volumetric efficiency. A common approach for manufacturers is to use a larger number of Valves , see Multi-valve , which cover a greater area of the cylinder head. Carefully streamlining the ports increases flow capability. This is done with the aid of an Air Flow Bench for testing. Today, automobile engines typically have four valves per cylinder for this reason. Many high performance cars in the 1970s used carefully arranged air intakes and "tuned" exhaust systems to "push" air into and out of the cylinders through the intrinsic resonance of the system. Two-stroke Engine s take this concept even further with Expansion Chamber s that returns the escaping air-fuel mixture back to the cylinder. A more modern technique, Variable Valve Timing , attempts to address changes in volumetric efficiency with changes in RPM of the engine -- at higher RPM the engine needs the valves open for a greater percentage of the cycle time to move the charge in and out of the engine.

More "radical" solutions include the Sleeve Valve design, in which the valves are replaced outright with a rotating sleeve around the piston, or alternately a rotating sleeve under the cylinder head. In this system the ports can be as large as necessary, up to that of the entire cyclinder wall. However there is a practical upper limit due to the strength of the sleeve, at larger sizes the pressure inside the cylinder can "pop" the sleeve if the port is too large.

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Volumetric Efficiency