egt & air fuel ratio tuning

[bnaellis]

Hi all,

From my understanding on a bridgeported rotary engine part of the intake charge goes straight down the exhaust port. When putting a bridgeport to4r on the rollers to get tuned. I have egt guage and afr gauge but how reliable will these readings be as they do not just measure the compressed charge but also what come from intake and straight out the exhaust ports. so maybe afr of 11.5 is something much richer or maybe even leaner in reality. Bill shuvrinton if your around could you reply please as I know you understand this kind of stuff pretty well. Thanks in advance.


rgds

[bill shurvinton]

No shuvrinton, just lil ol me :-)

I\'ll try and think up something tonight and post

Bill

[bnaellis]

Cheer bud,

rgds

[Glenn Butcher]

Hi Brian,

Don\'t forget when the revs rise etc. the fuel doesn\'t go straight from the intake port to the exhaust port - it is sucked/forced around.

Hence when idling you get the "bap bap bap" sound from the engine, but when the engine is under load it smooths out.

So you don\'t have to compensate your AFR/EGT\'s etc.

[bill shurvinton]

Right, few notes about EGT and AFR. I am not overly keen on EGT measurement in rotaries other than a sanity check for fuel balance and to measure TIT (turbine inlet temp), but I\'ll get onto that.

Anyway all you know about EGT is it peaks at a stoich mix. Richer than stoich and the excess fuel cools things down. Leaner than this and the incomplete combustion of all the air cools things down. Aviators use this to tune for cruise, as they find peak then lean off by a certain number of degrees.


The history behind EGT goes back to the days when it was the only way of measuring what came out of an engine. bear in mind that 20 years ago the best you had was a fuel flow meter and an air flow meter (hence air/fuel ratio), and that was too expensive for many so EGT was used. However it takes 10-20 seconds to stabilise, so can only be accurately used on an engine dyno, where you can hold a particular load point without head soak in the engine.

So what does this mean for the bridged turbo. You have realised that an ECU that supports timed sequential injection is a good idea, but if you haven\'t the following happens.

1. When the inlet opens, fresh charge scoots straight in and then straight out again. This cools the turbo down (good and in fact used in big turbo deisels for this very reason). If you are up to temperature then the fuel will combust in the headers.

2. The bridge porting also lowers the dynamic compression ratio, which is handy on higher boost apps. (not related to EGT but useful to keep in the back of your mind.

Now, given that you will running very rich when under boost, if you look at the overlap period as a total percentage of the inlet period, the distortion of the measured AFR will be minor, especially given the inherent 100ms response time of a nernst cell. With a small bridge you have some 150degrees of overlap in a 400 odd degree inlet, BUT the flow during this early part is much lower than after the exhaust closes.

Upshot: you really don\'t know the EXACT AFR that you are running in this sort of case, but neither do you know what the engine actually wants. luckily it will tell you when it is happy. At worst the errors will be in the order or around 0.3AFR.

Note on TITs. These are important to look at to make sure that you are not about to melt your turbine blades.  Inconel blades are available in these cases. There are types of turbines that use the heat of blowdown to be more efficient, but that\'s another story.

Hope this helps

[bill shurvinton]

Glenn,

The brapp is inlet reversion, so you still get blowthrough of inlet charge once running.

Bill

[Glenn Butcher]

Hi Bill,

I just checked with someone here,  you are right - I\'m sorry about that.

But aparently it is very minimal once the revs get up there, so you still don\'t have to compensate on your AFR/EGT\'s.

.ps Brian, you should be aiming for around 900 degrees c (if the sensor is mounted around 6-8inches from the exhaust port).

[bnaellis]

Thanks Glen, Thanks Bill. I always learn something new when reading one of your posts. Much appreciated.

rgds

[Carl]

Originally posted by bill shurvinton
2. The bridge porting also lowers the dynamic compression ratio, which is handy on higher boost apps. (not related to EGT but useful to keep in the back of your mind.

 

Hi Bill and Brian,
Small point is that the volumetric efficincy goes up on a bridge port at revs,even one that is boosted so the chamber intake charge is increased by quite a bit over a non ported engine.
Kind regards
Carl:)

[bill shurvinton]

True,

But VE and dynamic compression are 2 different things. Both aiming at the same goal (power) and related at least in piston engines as you raise compression as you increase cam duration (which generally raises VE, but thats another topic). Rotaries its hard to drop CR as you up boost, except by the porting.

Which leads back to the fact that all the text books say that bridge ported turbos are a no no in all but out and out race engines. However the engines haven\'t read these books :-)

Bill

[Carl]

I know what you mean bill,
I was not saying that the dynamic compression was raised but that at revs the intake charge with a bridge port can be greater than with convential porting under certain conditions.
it is almost like miller cycling,but I know from experience that the intake charge does increase contrary to what the books say.
This is because the port is early in the cycle and there is such a short duration in the overlap on a bridge port that at revs the charge no longer goes down the exhaust port.This is because you have a high pressure zone of exhaust gas there to prevent the intake charge going down the exhaust.Then what happens is the  bridged inlet port is allowing this hot exhaust gas to try to go into the intake charge and dilute it.If the porting is done correctly this should reach an optimum point of balance between pressure and timing of intake and exhaust so that the mixing of gases either way is nonexistant.That is the ideal perfect world scenario.
Also due to the fact that the exhaust is going to atmosphere,under power a turbo engines exhaust wants to go the line of least resistance or lower pressure so it favours the exhaust over the boosted by now inlet(At Revs),which has a lovely turbine that is not only driven by exhaust gas but pulls it too as the pulses vary in pressure.
If on the other hand you went extreme like a peripheral port you would have to be doing around 7-8000 revs before this even started to take place.
In the miller cycle rotary that Mazda played with the, port was purely to lower the dynamic compression under certain conditions and it was around halfway through the compression phase,it allowed the some of the compression to return to the intake port near the intake manifold which made the engine behave like a smaller capacity motor.When this port was was shut it went back to being a high capacity motor.
This is a highly complex subject with many answers that can be correct too.The theory is not allways right as I have found out on the race engines I have produced.
These sort of topics are great.
Keep up the good work Bill.
Kind regards
Carl
:wave

[bnaellis]

Hi Carl,
How you doing mate? hope all is well.
I\'ll be honest quite a bit of this stuff is over my head:confused:
makes me research and learn more. I guess I should understand all this better having been a car mechanic for first 7 years of leaving school. Would a person with a bridge then be better off running it at say 18 psi driving hard rather than 13-14 psi driving normal as far as engine health and efficiency is concerned?

rgds

[Carl]

Hi Brian,
I am fine thanks,you?

Basically when you tune the engine run the A/Fs as you would if the engine was not ported as under any reasonable boost it will run fine.

Don\'t think that you need to run high boost to run safely,that is not necessary.
Just map the car for whatever boost you want to run.

In my post above I was talking of the charge going down the exhaust,what I meant to say as Bill has rightly pointed out to me is the charge from the exhaust going into the intake charge and diluting it.On overun the the inlet  charge sometimes is pulled down the exhaust and gives funny readings on the A/F\'s,don\'t worry about that.

Kind regards
Carl

[maximtaylor]

Good info chaps

Top banana!

Regards

Max

[bnaellis]

I\'m sure I won\'t be keeping it for myself even though I want to. There are too much people that want it. But if ever there was a beast for street fight challenge this is the one. Either way I will have a beast ready for rotorstock. I might take two down actually.
Anyway appreciate all this info guy\'s i\'m sure this will benefit many on this forum not just me.

rgds