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

Veeduber wrote:
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
You might edit the line between the ***'s to say 'same degrees of crank
rotation'. (Time obviously varies with rpm.)

Charlie

On Jun 9, 4:16*pm, Charlie wrote:

You might edit the line between the ***'s to say 'same degrees of crank
rotation'. (Time obviously varies with rpm.)
---------------------------------------------------------------------------------------

Dear Charlie,

Thank you for the heads-up. Big day at the doctor shop today. (Good
news, too) But there's really no excuse for not re-reading this stuff
before posting it.

-Bob

On Jun 9, 9:47*am, Veeduber wrote:
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 crank rotation,
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

"Veeduber" wrote

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.

-R.S.Hoover

I'm going to go out on a limb here Bob and say that the RATE of valve
opening and closing is a function of RPM and cam lobe contour only. Mass of
the valve train would primarily affect the max RPM attainable without
destructive valve float.
Tom

On Jun 9, 8:35*pm, "Tom Wait"
wrote:

I'm going to go out on a limb here Bob and say that the RATE of valve
opening and closing is a function of RPM and cam lobe contour only. Mass of
the valve train would primarily affect the max RPM attainable without
destructive valve float.
-------------------------------------------------------------------------------------------------------

Dear Tom,

Not a problem. There's plenty of room on the limb for the two of us.
(See my error as caught by Charlie.)

With regard to the rate at which the valve opens, what I'm trying to
say is that a valve train having greater mass will open more slowly
than a valve train having less mass. The assumption here is that all
else -- including the factor you've mentioned -- are equal.

The object here was ways to increase Volumetric Efficiency, for which
the rate at which the cylinder fills is a critical factor. According
to Taylor (or possibly Litton) the shape of the combustion chamber,
especially with regard to the shrouding of the intake valve, AND the
mass of the valve train components, are the only ways of improving VE
without going to super-charging.

I can see where you're coming from with regard to RPM but as you must
know, VE falls as rpm increases. The idea behind lighter valve train
components is to allow more time for charging the cylinder at a given
RPM. For the purpose of this exercise, RPM is fixed.

So put away the saw -- there's plenty of room for the two of us on
this branch :-)

In a more serious vein, I apologize for stating my explanation so
poorly.

-R.S.Hoover

Going back to some hot rodding roots,I believe V.E. was increased
in the late '60's-70's thru the use of a specially designed double
cone affair placed in the collector pipe of a tuned exhaust
system,which created a stronger vacuum effect , creating stronger
scavenging of exhaust, and to some extent , helping draw more fuel/air
mix into the cylinder.

I believe that Ballenger Headers had the "pickle" located in the
collector of 4 tube equal length headers and there was also the
"coanda" effect which is specifically what I think you were speaking of.
It was used experimentally in a truly weird exhaust setup in the early
days by of one of the major racing teams back in the mid 60s, but my
memory is a bit foggy for specifics that far back, such as who or on
what car, sorry.


cmyr wrote:
Going back to some hot rodding roots,I believe V.E. was increased
in the late '60's-70's thru the use of a specially designed double
cone affair placed in the collector pipe of a tuned exhaust
system,which created a stronger vacuum effect , creating stronger
scavenging of exhaust, and to some extent , helping draw more fuel/air
mix into the cylinder.

"cmyr" wrote in message
...
Going back to some hot rodding roots,I believe V.E. was increased
in the late '60's-70's thru the use of a specially designed double
cone affair placed in the collector pipe of a tuned exhaust
system,which created a stronger vacuum effect , creating stronger
scavenging of exhaust, and to some extent , helping draw more fuel/air
mix into the cylinder.

There have been a number of things that improved VE: Generally, higher
compression ratios help especially at higher RPM, roller tappets seem to
withstand much faster ramp angles and can stay open further during the open
part of the valve cycles, and anti-reversion cones in the exhaust are said
to work very well in the mid-range of RPM for any given four cycle engine.
In addition, intake and exhaust port shapes play a major role; as do other
aspects of head ad piston crown design.

In a nut-shell, there has been a lot of progress over the past three
quarters of a century, and the only place that I can think of in which
aircraft engines have led the way has been in the area that we used to call
"blue printing" in which the ports are more carefully caste, machined, and
finished to closely match the design drawings for the engine. Today, every
late model engine that I have seen is done that way at the factory; but
forty years ago, automotive engines were really crude.

Peter

On Thu, 11 Jun 2009 07:40:10 -0700 (PDT), cmyr
wrote:

Going back to some hot rodding roots,I believe V.E. was increased
in the late '60's-70's thru the use of a specially designed double
cone affair placed in the collector pipe of a tuned exhaust
system,which created a stronger vacuum effect , creating stronger
scavenging of exhaust, and to some extent , helping draw more fuel/air
mix into the cylinder.

The anti reversion cone was a dirty fix for a crappy header design.
Better than a manifold, but not as good as a proper "tuned" header.