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shotdownski
10-13-2005, 06:55 AM
I'm on a listserve for the 303rd BG (H) Association and one of their former pilots has been gladly answering all sorts of technical questions about flying B-17s in combat. He recently sent this explanation of manifold pressure that finally made sense to me. Thought some of you guys might find interesting. If so I think I've saved some of his other discussions about flying formation, turbo superchargers, etc.


Gentlemen and Ladies,

Our B17s had a 9 cylinder rotary air cooled 4 cycle engine. At 2400 RPMs, where we operated right after take off and some other times when we needed that much power. There were 20 power strokes and 20 intake stroke every second. That means the intake valve was open about 1/20th of a second to receive the fuel charge for the following power stroke.

We could run the manifold pressure at 42 inches at that rpm to blow the fuel into the combustion chamber. Following the intake stroke the intake valve closed and the piston came up on the compression stroke. By the time the piston reaches near top dead center the spark plugs fires from electricity generated by the magnetos and ignites the fuel air mixture compressed therein to about 1/7th of its original volume.

The fire moves across the combustion chamber rather slowly raising the pressure and temperature as it burns to well over 1000 pounds per square inch. If the piston was 6 inches in diameter it would have over 28 sq. inches of
surface on the piston head, which means the piston would be pushing down through the connecting rod to the crankshaft with a force of some 28000 pounds (14 tons)

Now, if we leave the manifold pressure at 46 inches and with the prop governors (pitch control) slow the engine down to say 1200 RPM the intake valve will be open 1/10th of a second, twice as long as at 2400 RPMs and the combustion chamber will have twice as much fuel in it. Now when the spark plug fires we will
have so much fuel in there the pressures and temperature will be so high it will bend
the rod, blow the cylinder head off and/or break the piston.

So one thing the pilots have to learn. WHEN YOU ARE GOING TO REDUCE POWER ALWAYS REDUCE THE MANIFOLD PRESSURE FIRST. Then the RPMs.
When you are going to increase power increase the RPMs first then you can increase the Manifold pressure.

When the pressure in the combustion chamber is a bit to high or the octane rating of gasoline to low the pressure and temperature therein can cause the unburned fuel there in to explode instead of burning and you get a metallic
knock like someone is in your engine with a hammer. We often hear this knock in an automobile engine. It can cause an engine failure very quickly.

Thought you might find this interesting. I did over 60 years ago.

-HH-Dubbo
10-13-2005, 11:53 AM
Cool read. Thanks http://forums.ubi.com/images/smilies/25.gif