Why do engines feature a valve overlap? The early opening of the
intake valve and the late closing of the
exhaust valve? In order to improve the filling of the cylinder with pure air (
diesel engine) or with
fuel-air mixture (petrol engine) and to improve the removal of the old gases or to reduce them consciously.
The valve overlap is the sum of the angles of opening the intake valve before TDC and the closing of the exhaust valve after TDC. If the piston is positioned between the 4th stroke (exhaust) and the 1st stroke (intake) at TDC, both valves are open. Exhaust gas and fresh gas can exchange their kinetic energies. In this process, the old gas leaves the
cylinder more completely, and the fresh gas takes up its space. Because we are talking in this case of an open exchange of gases for a short time period, it only functions optimally in the speed range of the highest torque. The higher the nominal speed of an engine, the larger the valve overlap. It can amount (taking the as
crankshaft reference point) up to 180° for racing engines.
If the engine features adjustable camshafts, it is possible to adapt the valve overlap to the single speed ranges. In addition,
valve overlap is used as "internal" exhaust gas recirculation. 06/08
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