Differences of the timing of the air inlet and exhaust valves differ between naturally aspirated and pressure charged four stroke engines. These differences depend to some extent on the speed of the engine and also on the degree of supercharging.
FIG: VALVE TIMING DIAGRAM FOR NATURALLY ASPIRATED ENGINE 4-STROKE ENGINE
FIG: VALVE TIMING DIAGRAM FOR NATURALLY TURBOCHARGEDED ENGINE 4-STROKE ENGINE
What is overlap period?
The overlap period is when both the exhaust and air inlet valves are opened together. It is sometimes referred to as the scavenge period in pressure charged four stroke engines.
Overlap period in naturally aspirated and supercharged engine:
In the naturally aspirated engine, the overlap is 36°; in the pressure charged engine, it is 120°
Advantage of large overlap period:
The large overlap period in the pressure charged engine allows complete expulsion of exhaust gases from the cylinder. During this period, a large amount of air passes through the inlet valve, cylinder, exhaust valve and cooling the parts in the process. This cooling helps to keep the surface temperatures of these parts at a lower value, which reduces thermal stresses.
After the exhaust valve closes, less heat is passed into the air that follows into the cylinder and a greater mass of air is therefore present when compression begins.