Ambient temps and air ratios vs. combustion temps

[Guest bastaad525]

Well... it's learning time again here on the Hybrid Z forums

 

Another theoretical, crazy tech question from yours truly... asked for no particular reason other than wanting to know, and knowing someone here probably already does, so here goes.

 

I'm wanting to find out how ambient temp affects the threshold at when detonation/ping would occur.

 

Basically... I'm thinking like this. As ambient air and thus the intake charge temp drops, the air gets more dense (more oxygen per volume). We all know that. But, how does it affect the efficiency of the I/C, for one? It would seem to me that colder air across the I/C would make it more efficient. Or not... I dunno... either way, you end up with a colder charge in the cylinders.

 

Okay... now... lets assume, as is probably the case with a stock 280ZXT EFI running WOT at 15psi of boost, that the ECU is no longer easily compensating for changes in air temp and density (probably the case on stock injectors as they are maxed out, probably running full duty cycle). So, essentially, the amount of fuel is staying the same, though the air density (oxygen) is increasing... so, leaner mixture. And leaner mixture = hotter combustion, correct?

 

let's throw some numbers in, for the heck of it. Lets say you tuned your car (okay still dealing with stock EFI, so lets say we're using an FMU or AIC, something like that) on the wideband on a hot day... 90* F, to give a nice flat 12:1 mixture. Now... lets say you're out zooming around on a really cold morning... 40* F (really cold by CA standards), and that the EFI is still providing the exact same amount (volume) of fuel, what would the new mixture be?

 

Now here's my real question... what would happen to the combustion temps? Would the leaner running cancel out the change (lower) in the intake temperature and still end up giving you hotter combustion temps than when you tuned to 12:1 on the 90* day? Or would the temps still be lower? Same?

 

What I'm getting at is... when does the colder air cease to be a benefit, when dealing with an augmented stock system like this, and start possibly posing a threat such as detonation from running too lean?

 

I'm running an FMU... and I tuned it on a hot day. Lately... it's been mid 40's out... I don't have a wideband yet to monitor what's going on. I can say... the car is MUCH punchier in the cold, but at the same time, I worry that the leaner air is putting me in the danger zone.

[Tony D]

On a non-intercooled turbo application, the discharge temperature of the turbo is easily figured, through sijmple physics.

The compression ratio will determine the temperature along with a variable like the compressor's efficienfcy. Simply put, 100 degree ambient air pushed to Xpsi will be approx 270 degrees.

Add that temperature to the compression ratio of the engine, and you will quickly see the compression in the engine being closer to 7.5 or 8:1 will also raise the air temperature some more.

 

And BOOM! the gas lights off if you have cheap stuff...

 

In reality due to camshaft overlap and how dynamic compression works on a rotating engine, the compression ratio (actual) in the engine while running is somewhat lower until peak torque where it reaches nearer the actual design ratio.

 

So basically, you are limited in a non-intercooled application to the compression ratio, it's resultant heat, and then the engine's effective compression ratio raising the temperature in steps to a critical point where iognition wold occur.

 

This is why some Diesels use rad high-boost turbos with no intercooling due to the heat produced through the turbocharging.

 

 

Throw in an intercooler, and all bets can efectively eliminate the turbocharger's heat, and all you have is intercooler out temperature being the effective ambient temperature for purposes of determining ignition temperatures in the combustion chamber.

 

Basically, the Turboicharger acts like a "variable compression ratio" component. With the cumulative heat, a low-boost turbo's output can be equated (as well as having detonation characteristics comparable to) a high-copmpression engine (14:1 and up!)

 

But it's late, I got a killer neckknot, and can't expound any longer on any more of the question. maybe tomorrow! not enough energy left right now to get into density and AFR variations.

 

But this heat is how the ping and ignition happens. Simple Physics.~

[Guest bastaad525]

LOL leave me with a cliffhanger

 

 

I wouldn't have figured the I/C would be dropping the charge air all the way back down to ambient. Then again... on a cold morning... that thing is ice cold to the touch even after driving *shrug*.

 

 

So, at about what temperature would 91 octane (best I get ) be prone to preigniting?

 

If ambient air comes in at 40* F, how hot is it leaving the T3 at 14psi? How much of that would the I/C realistically pull back out? And finally, how much affect on that final temperature would the fuel mixture have?

[cygnusx1]

Being in the Northeast and my engine being a virtual shadow of yours, Bast., I have to chime in. When I was at Limerock last week, I was running the FMU at about 55psi, the boost at 14psi, and timing at 27btdc (according to the pulley marks). The air temp on the track was about 43 degrees. She was running great pedal to the floor. I didn't hear ping. I am not sure how much effect the air temp sensor has on the mixture with the stock ECU but my gut feeling says that is has to continue to adjust mixture at least down to zero deg F. So as long as you are not at the limit of the injectors, the EFI should be managing the colder air properly. With the added IC this becomes a gray area because that same temp sensor is measuring air before the turbo and before the IC. You just have to assume that Intake air temp varies about linearly with air entering the TB. In other words...

ex. IF...

Ambient=40, @TB=60 @Xboost (deltaT 20)

THEN...

Ambient=90, @TB=110 @Xboost (delta T 20)

 

If you assume it's nearly linear, then the ECU and Air Temp Sensor know how to handle it already. That is, as long as we set up a good cruise mixture with the AFM spring to compensate for the IC in the first place.

 

Assuming it's linear is probably a pretty fair assumption in this case.

 

I could be way off base but I think AS LONG AS YOU DONT 100% the injectors, the cold air will not put you over the edge. It will only lower your redline (leanline) due to injector capacity being used to compensate for denser air.

 

Maybe.

[Guest bastaad525]

yeah but think about it... the injectors must be maxed out at WOT at 14psi or even lower boost, as low as 9-10psi they are already maxing out at higher revs (5k+) as the mixture is already at 13:1 and leaning at that point. Hence the need for the FMU to force more fuel thru them. I have no doubt the ECU and air temp sensor do a good job of correcting for ambient temps at anything other than WOT. But if what I saw on the dyno was correct, the AFM or injectors or both are maxed at 12psi at about 4000rpm... the mixture just climbed leaner and leaner from that point, and only adjusting the FMU was really affecting the mixture at all. So, since the FMU on it's own cannot compensate for ambient temperature changes... I figure my WOT/top end mixture MUST be leaner on a cold morning... but how to know just how much leaner it's running, or how much hotter?

 

I mean... do I need to be more worried about pinging on a cold morning than a 100* day?

 

Really it makes sense that the FMU is very much NOT a 'set it and forget it' device, but rather would be better off monitoring it constantly and tuning it for changes in elevation and weather and such, just like a set of SU's.

 

Santa please bring me a WB02!!!!

 

Cyg if you lived close I'd say go halves on one with me....

[cygnusx1]

Yeah we should start a hybridz WBO2 loaner program like the spindle pin tool.....anyone know where the spindle pin tool is??