Here's why:
* Valve Overlap: There is a brief period called "valve overlap" during the transition from the exhaust stroke to the intake stroke, where both the exhaust and intake valves are open *simultaneously*. This is intentionally designed for a few reasons:
* Scavenging: The escaping exhaust gases help to push out the remaining burnt gases from the cylinder.
* Improved Volumetric Efficiency: This helps draw more fresh air into the cylinder.
* Valve Timing: The exact timing of valve opening and closing is critical for engine performance. The valves are not designed to be open at the same time for a prolonged period.
Key Points:
* Valve overlap is a very short duration (usually just a few degrees of crankshaft rotation).
* The valves are not open "slightly." During overlap, they are both fully open.
* Valve timing is carefully controlled by the camshaft.
Let me know if you have any other questions!
