What is the effect on engine performance if valve overlap is significantly increased in an internal combustion engine?

Significantly increasing valve overlap, the period when both intake and exhaust valves are open simultaneously, affects engine performance by influencing cylinder scavenging and charge dilution. Cylinder scavenging refers to the process of removing exhaust gases from the cylinder. Increased valve overlap enhances scavenging, allowing more exhaust gases to exit the cylinder. This reduces the amount of residual exhaust gas remaining, which can dilute the incoming air-fuel mixture. However, excessive overlap can lead to charge dilution, where some of the fresh air-fuel mixture escapes out the exhaust valve before it can be combusted. At low engine speeds, increased valve overlap typically results in poor idle quality, reduced low-end torque, and increased emissions due to unburnt fuel escaping. This is because the exhaust pressure is lower, and the intake charge has more time to escape. At higher engine speeds, the benefits of improved scavenging often outweigh the losses from charge dilution, leading to increased power output and improved fuel efficiency. The higher exhaust velocity at high RPM helps to create a stronger vacuum, effectively drawing more intake charge into the cylinder and reducing residual exhaust gases. This effect is optimized for a specific RPM range; exceeding that range can again lead to excessive charge dilution and a decrease in performance. Therefore, the ideal amount of valve overlap is a compromise based on the intended engine operating range. An example would be a racing engine designed for high RPM operation, which would utilize a larger valve overlap compared to a tractor engine which requires high torque at low RPM.