Figure 2-55. Valve Timing Diagram Showing Valve Overlap
Because the final 15 to 20 degrees of the power
duration and overlap, an engine can be made to
stroke have almost no piston movement, there is
produce more power in the higher speed ranges.
no problem with exhaust being drawn in by the
This is because the exiting exhaust gas will have
more inertia, making its scavenging effect last
longer. This same engine will run poorly at low
As stated earlier, engines can be designed to
speed due to the piston having a tendency to pull
produce power in a specific speed range by
exhaust back into the cylinder and blow it back up
altering valve timing. By increasing the valve
into the intake port.
Figure 2-56. Crankshaft Rotational Periods of Imperceptible Movement.
2-36 Change 1