MSefi.com read only forum (Jan.15,2018)

First of all, keep in mind that I'm running Alpha-N, because my MAP isn't much informative due to my throttlebodies.

I´ve noticed that my TPS% varies in my cruise area.
When I first started to tune my VE tables, I drove 120km/h with 20% tps.
After several days of tuning, I noticed that my TPS percentage dropped a bit, to 15% with the same speed.

The last few days the mixture in the cruise area (80-100-120km/h) was leaned a bit more to reduce fuel consumption (target afr was guessed at 16.4).
The tps% at the mentioned speeds went up again, to about 20%. There wasn't a lean surge though, car drove fine and the AFR was about that 16.4.
After setting the values richer a bit, it's now back to 15% while cruising.

Now, my theory is:
Driving at a constant speed means (always) a constant RPM. Driving at those speeds means I need a specific amount of horsepower (not taking slight weight differences, wind etc in account). So the RPM and horsepower are constant.
So when I had 20% tps in cruise, my engine put out the same HP as it did when I had 15% after the tuning.
That means the engine gets less air, and still has the same power, thus running more efficient.

So that would mean that at the same speed, with the same RPM, the engine runs most efficient at the lowest possible TPS percentage.

Although leaning the mixture should decrease fuel consumption, the higher tps% compensates because the cruise area is thereby shifted up in my VE table.

Since I don't know the optimal AFR values, I checked what the current AFR was during cruise...and that's about 15.9... so I'll keep 15.9 as target AFR.

What I'd like to discuss is if my theory is correct.
The engine runs the most efficient with the lowest possible TPS percentage, at a constant speed, thus using the least fuel.

Can someone think of any arguments why this isn't true? I couldn't..... as I said, I can lean the mixture without having lean surge. I don't think that the optimal target AFR is the leanest without lean surge.... and that's how it's said in the manual... lean the cruise AFR until you feel the surge, and then richen it a minor bit.

If my theory is correct, I could try to develop some sort of algorithm to find the optimal AFR for the engine in cruise, based on TPS.

Interesting observations. I'll just throw out a couple thoughts.

For engine efficiency, I'm not sure air flow is going to be a reliable indicator. What you really want to look at is brake specific fuel consumption. At a given power output (speed), how much fuel is being consumed? Air is an unlimited resource that you don't pay for so air flow is not directly important. What is the injector pulsewidth under these conditions? That's what I would be looking at.

The other complicating factor here is ignition timing. At leaner fuel mixtures the combustion rate is slower so the ignition timing has to be advanced to keep torque output at it's optimum level. If you aren't advancing the timing for the leaner mixtures it's possible that it's not operating as efficiently as it is with the richer mixtures.

C|

good discussion... how far would this advance be, and how do we know when we hit it? Say we shoot for 100km/hr reference speed. Do we shoot for the lowest kPa we can get while holding that exact speed? What are the safe limits... will it knock audibly while just cruising? I currently run a max of 44 degrees @ 2200rpms and up, very low map, with 15.9:1 fuel ratio.

I would have to agree with cygnus, and tune your engine to use the lowest possible fuel pulse duration, to save money, but this will prolly have a negative affect on emissions (to much nox?) but who cares, unless you have air care regulations like me

vinister wrote:good discussion... how far would this advance be, and how do we know when we hit it? Say we shoot for 100km/hr reference speed. Do we shoot for the lowest kPa we can get while holding that exact speed? What are the safe limits... will it knock audibly while just cruising? I currently run a max of 44 degrees @ 2200rpms and up, very low map, with 15.9:1 fuel ratio.

You know when you hit it because adding more timing doesn't decrease the MAP (or TPS) any further. You hit the point of maximum efficiency so going leaner decreases available power to where you can't maintain speed and need more fuel.

C|