ENGINE BASICS

To All:

Recent posts about the Pobjoy radial engine have generated a number of
private messages which indicate some subscribers to this Newsgroup
lack a basic understanding in how the Internal Combustion engine
actually works. For example ALL engines have some degree of overlap
in their cam timing. If you have a text book that says otherwise, it
is in error.

The reason for this overlap is fairly simple: The incoming fuel/air
charge has mass and all mass has inertia. If the cam did not open the
intake valve until TDC there would be no in-flow of the fuel-air
charge until several degrees of crankshaft rotation AFTER the valve
opens because it takes that long for the difference in PRESSURE
between the combustion chamber and the inlet manifold to overcome the
inertia inherent in the fuel/air charge.

By the same token, the exhaust valve must remain OPEN for several
degrees past TDC for the pressure in the combustion chamber to fall to
a value equal to or less than that of the inlet manifold. Without
that difference in pressure there can be no flow.

The amount of overlap determines the engine's maximum rpm. If you
want an engine that turns 6,000 rpm, it would have to have an
appreciable amount of overlap.

The opening and closing of a valve takes the SAME amount of time,
regardless of the speed of the engine. Factors that effect the RATE
at which the valves open or close is the MASS or weight of the
components in the system, meaning the tappet, push-rod and valve, as
well as the strength of the SPRING, the tension of which must be
overcome in order to OPEN either valve. And of course, if you want
the valve to CLOSE quickly, you will need a stronger spring, but you
must pay for that greater strength upon opening the valve.

The thought that OVERLAP will cause some of the fuel/air charge to be
drawn out the exhaust port is largely nonsense except for engines
having a great deal of overlap -- too much to allow the engine to be
installed in a normal car.

This matter becomes significant with small aircraft engines, or any
aircraft engine that drives the propeller directly. For such engines
the AVERAGE OVERLAP is 33 crankshaft degrees (14 on the INTAKE, 19 on
the EXHAUST) but for some geared engines the overlap may be as much as
95 degrees.

The R-Type Pobjoy used cam-rings giving an overlap of 15 degrees (4 on
the intake, 11 on the exhaust).

-R.S.Hoover