mawingho
Topic Author
Posts: 41
Joined: Sun Jul 22, 2012 4:23 pm

Throttle & Propeller RPM And Detonation

Tue Aug 07, 2012 3:49 pm

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.
 
nomadd22
Posts: 1566
Joined: Fri Feb 22, 2008 7:42 pm

Throttle & Propeller RPM And Detonation

Tue Aug 07, 2012 4:48 pm

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.
Anon
 
KELPkid
Posts: 5247
Joined: Wed Nov 02, 2005 5:33 am

Throttle & Propeller RPM And Detonation

Wed Aug 08, 2012 8:34 pm

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 :-)
 
SAAFNAV
Posts: 337
Joined: Wed Mar 17, 2010 5:41 pm

RE: Throttle & Propeller RPM And Detonation

Thu Aug 09, 2012 3:42 pm

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
 
DC8FriendShip
Posts: 235
Joined: Fri Apr 29, 2005 7:35 pm

RE: Throttle & Propeller RPM And Detonation

Mon Aug 20, 2012 5:03 pm

(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
 
KELPkid
Posts: 5247
Joined: Wed Nov 02, 2005 5:33 am

RE: Throttle & Propeller RPM And Detonation

Mon Aug 20, 2012 6:43 pm

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 :-)
 
Mir
Posts: 19092
Joined: Mon Jan 05, 2004 3:55 am

RE: Throttle & Propeller RPM And Detonation

Tue Aug 21, 2012 3:08 am

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 avatar
Starlionblue
Posts: 17058
Joined: Fri Feb 27, 2004 9:54 pm

RE: Throttle & Propeller RPM And Detonation

Tue Aug 21, 2012 10:09 am

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." - John Ringo

Who is online

Users browsing this forum: SAAFNAV and 10 guests