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Jay

Engine management

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Does anyone know of a good reference that describes the relationship between manifold pressure, mixture, CHT, RPM and load for a typical modern 4 stroke small aircraft engine?

 

 

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Thanks, worthwhile reference - interesting that CHT reduction is more sensitive to leaning rather than riching mixture around peak EGT

 

 

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Don't apply that technique to Jabiru engines. There was a popular view that mixture lean of peak is the way to go. I don't agree with it. Lean mixtures detonate more easily than rich ones and the surplus oxygen looks for things hot,, to oxidise. ( including any trace of oil on the cylinder surface). Stick with the individual engines recommendations. Nev

 

 

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Nev

 

If you read Mike Busch's articles (Avweb) you may change your mind and come over to the new world. :-)

 

 

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The Avweb article and the Savvy Webinar both by Mike Busch push the same point ; peak EGT and max CHT occur close to one another in relation to mixture , so how to reduce CHT from a mixture perspective (only)? ; use CHT and fuel flow ( lean of peak to get economy, rich of peak to get airspeed )

 

 

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Oooohhh you are playing with fire jake:oh yeah:004_oh_yeah.gif.82b3078adb230b2d9519fd79c5873d7f.gif.David

Yeah, I know but the truth is out there when you question entrenched theory - we can learn a lot by asking why.:big_grin:

 

 

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Nev - wasn't questioning your input, I just thought it was a good opportunity to generalise on this topic.022_wink.gif.2137519eeebfc3acb3315da062b6b1c1.gif

 

 

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I'm not personally worried jake. There is a dangerous view sometimes that if you are not "with it" on the latest fad you are "passe". What happens inside engines is not subject to passing 'phases'. There is a great deal of utter rubbish written about ignition advance, mixtures etc. (mainly from America). All you have to do is run an engine on a dynamometer and watch the results. Too much theorising and not enough "hands on " experience leads to wrong assumptions that seem plausible but don't deliver. The "lean mixture" experience gave Jabiru a bad name, that had to be unravelled., painfully... Nev

 

 

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Remember that most of what these guys are talking about refers to fuel injected engines with manual mixture control. Carburettors often don't give the even mixtures and granular control needed to safely run lean of peak, as Jabiru found to their detriment.

 

 

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Going back to the original question, "small, modern,engine,etc" . Most would see a FADEC system, as the answer. Fully Automated Digital Electronic Control, injected, probably with anti knock sensors too. A really high specific output engine would need such a system to look after itself. It would also need to have a constant speed prop, the same way as a car would work better ( engine management wise), with an automatic trans.

 

What do we end up with? Cost and complexity. Possible loss of reliability in the field

 

The other end of the equation is to have a simple engine, that is not too critical with it's operating parameters. Both Jabiru and Rotax are there using a Bing Carb that by tweeking the taper of the long needle and relating it's position to the airflow, (CV) you get an approximate mixture to mass airflow set-up that gives some altitude compensation and a richer mixture at high power... The biggest problem with this is the fact that ONE carb is feeding more than one cylinder with atomised (partly) fuel and the mixture distribution is never EVEN, so you will lose some fuel efficiency, as the most adversely affected cylinder(s) will have to be looked after. (Kept rich enough). Air cooled motors are more critical for localised overheating.

 

IF you are going to manually lean the motor, you need a fair bit of information to do it well. If the engine is a robust design you just pull the mixture out till it splutters and then shove it in till it smooths out and then give it a bit more for MUM. Pretty hit and miss. You can rely on the EGT and of course you will get a "lean of peak" lowering of the EGT reading because you are putting the fire out by starving the engine of fuel.

 

Otherwise you need an accurate fuel flow meter and performance CHARTS. Fuel flow relates to power and that is not easy to determine unless you have a TORQUEMETER. RPM and Torque give you a power output figure. Nev.

 

 

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Just to put my 10 bobs worth (using old money ... I must be old world too) ... running the Jab engine LoP is only going to end in tears. The majority (not all) of engine problems in Jabs is because of lean running... I also think there is a lack of understanding of how to manage the engine in flight even with a CV Bing fitted.

 

I say, "a fat Jab is a cool Jab".

 

Cheers

 

Vev

 

 

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