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Throttle & Propeller RPM And Detonation  
User currently offlinemawingho From Hong Kong, joined Jul 2012, 41 posts, RR: 0
Posted (2 years 2 months 2 weeks 7 hours ago) and read 4853 times:

I have a question "Why reduce the throttle before the propeller rpm when reducing power?"

The answer is "Reducing the throttle first when reducing power ensures you do not over-charge or over-boost the engine, which can cause detonation"

However, I do not understand why and how it can cause detonation.

Maybe I even not quite understand what is detonation and how it can occur.

I hope someone can explain it. Thanks in advance.

7 replies: All unread, jump to last
 
User currently offlinenomadd22 From United States of America, joined Feb 2008, 1881 posts, RR: 0
Reply 1, posted (2 years 2 months 2 weeks 6 hours ago) and read 4825 times:

The same throttle at a lower rpm could allow more mixture into the cylinders causing greater pressure. Enough pressure and the mixture will detonate itself before the spark has a chance to. That's why diesels are injected and not carburated.


Andy Goetsch
User currently offlineKELPkid From United States of America, joined Nov 2005, 6407 posts, RR: 3
Reply 2, posted (2 years 2 months 1 week 6 days 3 hours ago) and read 4623 times:

When dealing with a constant speed prop, it is best to avoid being "oversquare", meaning manifold pressure is higher than acceptable for the current RPM setting. That is why you usually move the throttle first before reducing engine RPMs.

Piston engines are much happier when turning at their natural RPM, but the constant speed prop robs them of the ability to do so (to a degree...you will get to a point where, no matter how the govenor in the prop hub is set, the engine just can't maintain the RPM setting). You will find that, without a computer controlling things, you can make the engine unhappy quickly, and detonation is one of the symptoms that you have   It is similar to lugging the engine in an old car with a carburetor and a manual transmission, or downshifting that same car without giving the engine throttle. There are certain parts of the engine's "envelope" (and I use that word loosely, I just invented it   I've never heard it in the context of the engine before) where the combination of fuel to air, engine rpm, and spark advance just weren't designed to go   You don't find that much in modern cars, because the engine control computer is more flexible than any carburetor setting, timing adjustment, etc. to fix those problems for us. Even fuel injected aviation engines are 100% mechanical (unless someone has certified an aviation EFI system while I wasn't paying attention!).

Turbo aircraft engines are even more sensitive than naturally aspirated ones...



Celebrating the birth of KELPkidJR on August 5, 2009 :-)
User currently offlinesaafnav From South Africa, joined Mar 2010, 281 posts, RR: 1
Reply 3, posted (2 years 2 months 1 week 5 days 8 hours ago) and read 4498 times:

Quoting mawingho (Thread starter):

Maybe I even not quite understand what is detonation and how it can occur.

Detonation is when the fuel in the cylinder combusts when it isn't yet time. This could be due to localized hot spots in the cylinder or the pressure being to high.
As the detonation occurs at the wrong place, it will create a downwards force on the piston when it is still moving up, and some other bad things for engines as well.

http://en.wikipedia.org/wiki/Engine_knocking

Erich



On-board Direction Consultant
User currently offlineDC8FriendShip From United States of America, joined Apr 2005, 243 posts, RR: 3
Reply 4, posted (2 years 2 months 1 day 6 hours ago) and read 4053 times:

(unless someone has certified an aviation EFI system while I wasn't paying attention!).

Yes, there are FADEC powered piston engines now...



Come fly the Friendly Skies of United
User currently offlineKELPkid From United States of America, joined Nov 2005, 6407 posts, RR: 3
Reply 5, posted (2 years 2 months 1 day 5 hours ago) and read 4006 times:

Quoting DC8FriendShip (Reply 4):
Yes, there are FADEC powered piston engines now...

In what sense? I know about a system a few years back that replaced one of the mags with an electronic, computer controlled iginition system, but it obviously only worked on the ignition system...is there something that also controls mixture and throttle automatically now?



Celebrating the birth of KELPkidJR on August 5, 2009 :-)
User currently offlineMir From United States of America, joined Jan 2004, 21677 posts, RR: 55
Reply 6, posted (2 years 2 months 20 hours ago) and read 3902 times:

Quoting KELPkid (Reply 5):
is there something that also controls mixture and throttle automatically now?

The diesel (Jet-A) version of the DA-42 had two power levers. They set a percentage of power, and the FADEC took care of the throttle, mixture and prop settings in order to get that percentage of power.

-Mir



7 billion, one nation, imagination...it's a beautiful day
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17068 posts, RR: 66
Reply 7, posted (2 years 2 months 13 hours ago) and read 3859 times:

mawingho, There's a book that I would very highly urge you to buy and read. It's the "Pilot's Handbook of Aeronautical Knowledge", issued by the FAA. It answers all the questions you have had (so far) in a well laid out, easy to understand, excellently illustrated manner. You can order it here http://www.bookdepository.co.uk/Pilo...n-Administration-FAA/9781560277507 with free delivery to HK.

Quoting KELPkid (Reply 2):
There are certain parts of the engine's "envelope" (and I use that word loosely, I just invented it   I've never heard it in the context of the engine before)

The expression for car and motorcycle engines is more or less "power band" but that isn't quite the same.

BTW driving a somewhat older (1998) motorcycle I've had to go back in time with "engine management". Carburetors, warming up the engine, manual choke, blipping the throttle while downshifting, the whole thing. I love it. Motorcycle engines lagged way behind car engines in sophistication for the longest time.



"There are no stupid questions, but there are a lot of inquisitive idiots."
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