EFI University

Johnny_9 · Joined: 17 May 2006 Posts: 212

Anyone got a graph or data?

Does EGT drop all the way to knock limit on a NA?

Turboivo · Joined: 09 Mar 2006 Posts: 672 Location: Bulgaria

There was a EFIUni video in the past made by Ben and a former EFIUni instructor where a Toyota Yaris, Motec managed was ign advanced on a dyno till knock was detected. EGT was dropping all of the time.

Matt Cramer · Joined: 25 Sep 2007 Posts: 275

Turboivo · Joined: 09 Mar 2006 Posts: 672 Location: Bulgaria

I remember an old thread where same topic was discussed and someone's experience was increased EGT drop trend when knock occur.

MrDomino · Joined: 01 Oct 2009 Posts: 188 Location: Indianapolis, IN

That makes sense since you're going to be releasing more heat into the combustion chamber sooner when you're knocking so there's more time for the heat to transfer to the engine coolant, head, and piston crown prior to exiting the engine.
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Flr Power · Joined: 05 Jun 2007 Posts: 100

I think this is also due to the air boundary layer effect...

MrDomino · Joined: 01 Oct 2009 Posts: 188 Location: Indianapolis, IN

Explain.
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Flr Power · Joined: 05 Jun 2007 Posts: 100

Because of the way detonation breaks down the boundary layer of protective gas surrounding components in the cylinder thus transfering heat quicker to the piston crown and combustion chamber driving EGT down.

MrDomino · Joined: 01 Oct 2009 Posts: 188 Location: Indianapolis, IN

Gasoline engines are premixed combustion. The flame goes from the center all the way to the walls of the combustion chamber. In Diesels you only burn in the bowl of the piston. I'm not sure what protective layer you're talking about.
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StantonWarrior · Joined: 10 Dec 2010 Posts: 188 Location: UK

Theory aside, the reality is, a more advanced ignition means peak heat release is closer to TDC, and as such experiences a larger exchange due to larger expansion as piston travels to TDC.
Also consider larger crank-angle and thereofre time delay 'til exhaust valve opens.

Johnny_9 · Joined: 17 May 2006 Posts: 212

Lets say on a brake dyno MBT EGT is x at 50% load at 3000rpm

Would be a good strategy to get to MBT, to tune all 50% load site at the same EGT x ?

Roberto Arano · Joined: 22 Nov 2005 Posts: 361 Location: colorado

Turboivo · Joined: 09 Mar 2006 Posts: 672 Location: Bulgaria

Dyno · Joined: 14 Aug 2011 Posts: 7

Detonation will normally reduce EGTs further. EGT isn't a way to check for knock, in the same way as AFR.

Roberto Arano · Joined: 22 Nov 2005 Posts: 361 Location: colorado

without that insulating layer things start to melt, it's thin but very important.

i'm an engineer but i'd hate to sound like one, we sometimes come across very poorly...classic well known attribute that one learns to moderate over the years
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Vic Dyno Hire · Joined: 30 Sep 2009 Posts: 53 Location: Ballarat, Victoria

Interesting topic.
Would it be possable to increase this boundary layer thickness by changing the surface finish. and would this have an effect on say, reducing piston crown temps

Roberto Arano · Joined: 22 Nov 2005 Posts: 361 Location: colorado

complete conjecture on my part, but: I would guess that the less turbulence one has the thicker the possible boundary layer. I would think sacrificing turbulence (and the resulting slower burn speed &increase in negative work) would not be worth it. But I am certainly no expert in this matter.
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StantonWarrior · Joined: 10 Dec 2010 Posts: 188 Location: UK

MrDomino · Joined: 01 Oct 2009 Posts: 188 Location: Indianapolis, IN

I'm not sure how I feel about calling it a boundary layer since it really has nothing to do with traditional boundary layer theory- think flow parallel to a flat plate. What they're referring to as a boundary layer is (at least to me) the quench area where the flame sort of dies out because the wall temps are low enough that combustion cannot be sustained. I don't think knock really affects this too much since you're talking about a small amount of fuel compared to the amount that is burned although it's enough that the government cares about it. The high temperatures as a result of knock occur because the mixture ignites in several places near the walls and burns very rapidly. Because the mixture towards the wall is ignited earlier in the cycle there's more time for it to transfer heat to the walls. As more heat is transferred to the walls the bulk gas temperature goes down and thus your EGTs are lower.
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Johnny_9 · Joined: 17 May 2006 Posts: 212

I came across this on the web, my question is how would you do it?

From the WEB - Tuning Ignition Advance at part throttle

Tuning ignition advance at part throttle is more difficult than full throttle because it is difficult to accurately determine the correct settings. In general the default calibrations are suitable for part throttle, otherwise an EGT gauge can be used to determine the best ignition advance