pablo
29th Jun 2004, 19:06
Hi!
After reading and searching about the topic i dont understand yet the MAP vs RPM settings.
Why do we use a maximum of 1" per 100rpm?
How does the MAP/RPM setting affect the engine performance?
Why it does harm the engine set a too low MAP?
Thanks in advance!!!
Oktas8
2nd Jul 2004, 10:26
I'm not sure what other sites or books you have read on this subject. See the bottom of this post for a reference I have found very helpful.
However, I will explain some basics as briefly as I can. Sorry if this is too basic - your question does not tell me your background at all.
1" per 100rpm is just one rule of thumb that happens to work reasonably well for ungeared, normally aspirated, horizontally opposed engines - most of the general aviation fleet in other words! It gives a low enough manifold pressure to avoid detonation, and a low enough rpm to offer efficient operation. Usually the rule gives us power settings close to those recommended by the manufacturer for cruising flight. There's no magic to it; it's just the way the numbers come out when the engine is designed.
1" per 100rpm is certainly not the maximum manifold pressure allowed. Look at your engine at take off - probably about 30" MAP and 2700rpm, and it won't hurt the engine. Read your flight manual or Pilot's Operating Handbook, section 5, for various combinations of MAP & rpm recommended by the manufacturer.
In general, high MAP and low RPM (eg 30"MAP, 1800 rpm) may cause detonation. Detonation is the name given to explosive burning of the fuel-air mixture inside each cylinder head, and may happen when the air pressure is high and the piston is moving relatively slowly - low rpm in other words. You can usually hear it happening in a car if you try to accelerate rapidly in fourth or fifth gear; this gives a ticking (or pinking) sound. You can't hear it in the noisy aeroplane, but it can cause very rapid damage to the piston, and is highly undesirable. It can also be caused by leaning the mixture at high power, even if the MAP and rpm are set correctly.
Low MAP and high RPM (18"MAP, 2700 rpm) is undesirable because it is inefficient (high rpm means lots of power going to turn the engine quickly, and less power left over to give thrust) and noisy (more rpm -> more noise). Also, propellers are usually most efficient in the cruise at a lower rpm setting, say 2000 to 2400 rpm for many light twins.
Prolonged operation at very low MAP, such as throttle closed and propeller windmilling in a descent, is bad for an engine. The engine is designed so the crankshaft absorbs power from the pistons in rough proportion to rpm - high rpm at low MAP (throttle closed or nearly so) upsets this relationship, and in an extreme case can "detune" the "counterweights" - that is, cause the weights on the crankshaft to move from their set positions. Also, spark plugs can become dirty from operating at a low temperature, and the engine is less efficient when running at low temperature. However, this kind of operation with a modern engine will not cause catastrophic failure of the engine, although the engine may well need new spark plugs and a tuning afterwards!
The best site that explains the effect of running an engine at high manifold pressure, low rpm, and various mixture settings is this site (http://www.avweb.com/news/columns/182084-1.html), by John Deakin, and some other sites linked from that page. You may or may not agree with his conclusions on how best to run your engine though!
Hope this helps,
Oktas8 :)
HI OKTAS!
First of all i want to thank you, i have understood all the explanation. It's not basic, it's just perfect. I'm a little bit fed up of most people doing things 'cos it's a tradition and dont know why they do that. I prefer to understand a little bit more why it's harmful, wrong or whatever.
I'll have a look to the site.
By the way... which are the recommended leaning procs? I mean for most a/c.
Thanks!
Pablo
Sorry!
Disregard last part of last msg. I've started reading the article and realized it was about mix, prop and map.
Thanks anyway!!!
Best regards!