Valve overlap

from Wikipedia, the free encyclopedia

The valve overlap occurs in the four-stroke engine towards the end of the 4th  stroke when the exhaust gas is expelled and continues into the 1st stroke of the intake. The inlet valve opens even before the outlet valve is completely closed. As a result, exhaust gases are sucked back into the combustion chamber by the accelerated exhaust column in the exhaust manifold and burned again during the next work cycle. This creates an internal exhaust gas recirculation to reduce pollutants. It is used especially for 4-stroke naturally aspirated engines.

The valve overlap is a constructive result of the design of the camshaft (s); it depends on the valve geometry, the space available in the cylinder head , the valve clearance and the type of valve train control .

In high-performance engines, the exhaust valve opens before the bottom dead center is reached , which means that the gas column is relaxed at an early stage, so that the subsequent upward movement of the piston is not braked as much. During this upward movement, the inlet valve also opens so that both are now open at the same time. However, since the gas mixture has already been accelerated strongly in the direction of the outlet valve, no air is pushed back out of the inlet valve, but a purging process takes place. This is promoted by a large acceleration of the valve, which also entrains air. The outlet valve ideally closes when the air column comes to a standstill. The inlet valve also remains open until the column of air drawn in by the downward movement of the piston comes to a standstill. The column of air compresses itself through its kinetic energy when it hits the piston, which is already moving upwards again. This additional compression results in better filling and thus an increase in performance of up to 50% compared to engines without valve overlap. The valve overlap also cools the exhaust valve somewhat because the exhaust gases have a temperature of around 900 ° C.

literature

  • Richard van Basshuysen, Fred Schäfer: Handbook Internal Combustion Engine Basics, Components, Systems, Perspectives. 3rd edition, Friedrich Vieweg & Sohn Verlag / GWV Fachverlage GmbH, Wiesbaden, 2005, ISBN 3-528-23933-6 .