I understand a spark advance can give an engine more time to burn its fuel to get more power, But I don’t see how adjusting the moment at which the valves open or close can also boost performance.
I understand a spark advance can give an engine more time to burn its fuel to get more power, But I don’t see how adjusting the moment at which the valves open or close can also boost performance.
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I’m considering replacing my old engine’s valve timing gear with a VVT gear when I replace the water pump and timing chain, so I’m getting some extra information here
A normal engine without VVT had the valve timings determined by a mechanical connection to the crankshaft. This has the problem that the amount of air going into the engine is determined by the amount of time the valves are open, and the duration the valves are open is determined by how fast the engine is turning because of the mechanical connection.
So let’s say that at lower RPMs, the valve is open for just long enough for the perfect amount of air for a given amount of fuel. At higher RPMs, the valves aren’t open long enough for the correct amount of air, leading to incomplete combustion. Meanwhile if the valve timing is optimized for higher speeds, then the valves would need to be open for quite long, potentially being opened while exhaust gas is still in the process of being expelled, and that would be real bad for exhaust to be pushed back into the intake.
VVT solved this by having different timings depending on the RPM of the engine, so that the valves are open for a shorter amount of the crankshaft’s rotation while at lower RPMs and it’s open longer when the RPMs are high.