Lizard928

Greg while this is true, the heat that is put into the fuel via the pump is very small, every little amount of heat put into the fuel does have an effect, and while you are trying to tune from scratch (not LH), it makes it FAR more challenging when the fuel temp is rising and leaning out the mixture slowly.

You know, I have spent ALOT of time trying to decipher why they would have done this. And after a few discussions with Louie, and looking at the hot fuel issues we have seen, I believe I have finally figured it out.

Porsche put the fuel cooler on the back of the engine on the return line in an attempt to keep the fuel in the tank from getting too hot. As it is better to keep the fuel in the tank a cooler tempature as suppose so that when you start the cooler fuel cools the rails down right away enabling a shorter time running really lean.

I couldn't agree more.

Z

The options are kind of limited for larger injectors that are the correct physical size for the stock fuel rails and manifold openings, and that are also of the high impedance type. There are some 52lb ones currently sitting in a box waiting to go in to see how they'll work. The main concern in regards to the maximum size capability with the stock ECU is how short of a duty cycle can consistently be achieved to get a stock quality idle at stock fuel pressures.

I thought I read somewhere that at least part of the reason for the cooler being installed in the return line is for emissions purposes. Heat is picked up by the fuel in the engine compartment, so the fuel returning to the tank is warmer than when it went to the engine compartment. Warmer fuel vaporizes much easier. On the way to the engine the fuel is under pressure and cooler, so much less likely to vaporize. On the way back to and in the tank it would be warmer without the cooler, and it wouldn't be under that higher pressure, so would be much more likely to vaporize. Besides there being more vaporized fuel then, it would be under some pressure above atmospheric. Those vapors would be much more likely to escape into the atmosphere then. Think along the lines of the escaping vapors that you hear when you open a closed gas can that's warmed from sitting in the sun compared to when that gas can is cooler and you open it. It's part of some sort of total vehicle emissions thing, where the emissions from the entire car are considered, rather than just what comes out of the tailpipe. It's like the car is run then shut off in a sealed room, and the emissions in the room are monitored.

mark kibort

Louie,

Sounds like you have some other issues going on. Ive never had any issues with hot start or any kind of running abnormalities at any temp, and ive been racing at 110F (140 degree on track temp) and down to near freezing. The only car that had an issue was my 84, and it turned out to be a leaking plennum seal.

If now you agree that the density of gas is near 5% for a 50degree F change, you will also agree for a given injector duration, the less dense fuel, will now have an increased flow. probably the two factors might equal out (or cancel each other out). So, you are really left with the rising tempurature of the fuel due to the engine bay heating. Now, is this really a factor in mixture variance? I already gave the spec for Gasoline going from 300 to 400K degrees. near 11%, for over 180F degree temp change! (about .5% change in density for every 5 degrees C change in temp)

as i mentioned, the total density that you can expect with the fuel tempurature rise is 5% (and thats at the extreme end of the spectum) . what is the variance after the car is warmed up, hot day vs cold day? probably a lot less.
No where near the 15% that has been mentioned. NO WAY!!!

Now, going back to basics, I know you are an expert in this area, so why isnt the closed loop system working? the o2 sensor should easily take care of ANY range of mixture issues based on what the output of the stock o2 sensor system. I seem to remember you might have an aftermarket, or highly modified chip system, where maybe you are not using the stock stuff any more. For idle, you cant beat the stock system. You can tell mine is fighting the big injectors during idle, but anything off idle is fine with the o2 sensor system reacting to Air/Fuel ratio changes. WOT is fine too, based on near stock fuel pressures and pre programed fuel maps, with my larger injectors and larger displacement.

edit: I had to change some values. I'm watching the rose bowl and cant multiplex very well!

mk

GregBBRD

I still think you guys are kidding about the fuel temperature caused by increasing the fuel pressure 10 or 15 lbs. is going to make a bit of measurable difference. Actually, I don't think you guys are kidding, I hope you are kidding.....

Go look at a Porsche Cup Car and see where they put the fuel filter. That will tell you how concerned Porsche is about fuel temperature.

Louie928

Hi Mark,
I don't doubt that you don't have any noticeable starting or running issues at whatever temperatures you have experienced. Normal running, you'll not see any appreciable difference with the fuel temps encountered. The difference does somewhat cancel out because as the air gets hotter it is less dense so requires less fuel, or less dense fuel. Also, as you mentioned, the closed loop system will take care of small changes.

What I mentioned, and I shouldn't have bothered, is that on my particular engine, I do have to be concerned with the fuel temp in the fuel rails. It's especially problematic when the fuel temp gets above 50C. On a stock 928, the fuel rails are thin metal and have small thermal mass and that allows the fuel in the rails to very closely follow the fuel temp coming in from the tank. On my engine the rails are large aluminum and it takes about 3 minutes for the rail temp to drop down to the incoming fuel temp. During that 2 to 3 minute period after a hot engine start, I have severe lean out condition. It's completely different than on a stock 928 engine.
Agree that a 100F fuel temp change will cause about 2% fuel density change. I don't agree that for a given injector pulse width the less dense fuel will have increased flow. The flow rate will depend on viscosity and I've not heard that gasoline changes viscosity with temperature to any degree. We really have to be concerned with fuel mass flow since it's fuel mass that has to be within certian limits to provide the desired air(mass) to fuel(mass) ratio. You are right that my problem was due to fuel temp rising due to engine bay heating. It does seem to be a factor when the fuel temp gets to be greater than around 50C. For sure at 60C.

I think what happens is that with hot (50C - 60C) fuel in the injector, I use constant 60 psi fuel pressure, The fuel is liquid. As soon as the pintle begins to open, there is a pressure drop across the injector orifice. There is 60 psi on the upstream side and less than atmospheric pressure on the downstream side of the orifice/discharge nozzle or whatever you want to call it. Somewhere within that region of the nozzle, some of the fuel factions flash to vapor. It's not all liquid anymore and the vapor is hundreds, or thousands, of times less dense than even the hot liquid fuel. The injector may be open for only 4ms and that's not enough time for enough mass of the combination of liquid fuel, and fuel vapor, to give the proper AFR. The only remedy is to keep the injector open longer. In my case it can take up to 20% longer. After the engine (fuel pump) has run for 2 or 3 minutes, the fuel (rail) temp comes down and there is none of this problem and the mixture richening isn't needed anymore. I can't depend on the closed loop system for this correction because it takes too long for the sensor to heat and for the system to become functional. The % correction required is beyond it's limit too.

Here is a graph of the temperature correction I need for this ECU & engine. There is no direct measurement of air mass on this engine so it all has to be done by compensation tables. Note that the mixture is richened with a cold engine, but that compensation goes away at normal engine operating temps. If the engine temp gets too hot, I add more fuel to help cool combustion temps. Intake air temp is compensated richer with cold air because cold air is denser than warm air so requires more fuel. I chose to have my "standard" temperatures be at 20C so air temp compensations are zero there. When the intake air gets hotter than 40 to 50C I add more fuel for help in cooling the combustion temp for detonation control. The fuel temp compensation follows fairly closely to the normal fuel density formula until it gets to 50C, then I have to have the injectors open longer so it is compensated richer. The AFR doesn't actually go richer. It stays about the same. This after hundreds of hours of testing and measuring over a two year period and it's still not exactly right.

https://rennlist.com/forums/attachme...1&d=1230855302

CanAmJohn

Fuel coolers are put in the return line because of a few reasons.

1) it's safer to have it flowing low, close to atmospheric pressure than have it run at 3BAR.
2) having a flow restriction from the turbulators in the cooler in the feed line to the injectors would increase the pressure drop between the pump and injectors, making the pump work harder to provide the FPR regulated pressure.
3) having a flow restriction in the feed line to the injectors would produce a variable pressure drop through the cooler depending on flow rate, giving more lag in FPR control, making it harder to maintain known pressures for the ECU programming to reference, this is especially so the higher the flow and pressure you run.
4) you are passing the fuel at it's hottest condition through the cooler, after it has been through the FPR (that acts as a heat generator due to the way fuel has to squeeze past the FPR valve aparture), so you gain more of an efficient drop in temperature as the fuel is at a higher differential to the ambient temperature after it's been subjected to all it's work.

GregBBRD

Wow. Thanks for the insight.

John Speake

You will certainly have very acceptable injector opening times with ST and 60# injectors, due to the technique we have used. There is a little margin in there to increase the injector size above that somewhat. You could certainly drop the fuel pressure down to the rated pressure for most injectors, around 45psi and that would give mpre margin for injectors above that size.

The S4 uses 55psi or so even though the injectors are rated 45psi. I think that is because they are pretty well max'd out at max power, even at 55psi.

mark kibort

What about mine at 72psi? 335 to 320rwhp and this is over a 6 year period, 105 race days and no fuel related issues. a/f ratios in the mid 12s

mk

123quattro

You probably aren't noticing the issue because your car runs pretty rich.

It is very noticeable on my turbo Audi. On really hot days after several hours of driving my lean cruise A/F ratio can be off nearly a whole point due to this phenomenon. Since I'm already lean it makes it worse, but it's the same idea.

John Speake

I don't see a problem with that set up, running at 72psi. As Greg says there are several ways to skin the cat.

I was responding to Z's comments about running very large >42# injectors

bronto

I got my ride in Shawn's shark yesterday... wow! Flooring it in 1st is utterly useless except for creating gobs of smoke - you just don't go anywhere until he lets off the gas. Hard acceleration in 2nd produces significant g forces. And it sounds great, although nearly stock sounding. A little deeper rumble than normal, but not to raise any suspicions. Sounds only like different mufflers in use.

Z

I think that I know what technique you're talking about with the SharkTuner and the large injectors John. Is that technique used for all of the fuel injector settings that can be selected in the SharkTuner software, or is it only used for some of the ones above a certain size? If so, above what injector size selection point does it start to be utilized? Is it used if custom sizes are used, or just with the sizes in the drop down list which can be chosen from?

Dropping fuel pressure some would help, but would then reduce fuel flow at the top end, which is why the larger injectors would be put into the car in the forst place. Part of the idea is to be able to leave the stock fuel pressure regulator in there too, so that an adjustable one wouldn't have to be purchased and installed.

mark kibort

Lean cruise is handled via the close loop fuel mixture circuit. it would be almost impossible to see a full point of ratio change for a hot day vs cool day temp difference. What you are seeing is due to something else, but not the mass flow difference in the fuel flow.

Again, to have a 11% change in fuel density, you would have to have a near 180F degree temp difference. are you seeing this type of temp change in the fuel rails? 180F? Hmmm normal is 140F..... you would need to have the rails go to near 300 degrees for this kind of fuel density change!

mk

123quattro

Mark, my car has a stand alone controller in it. It's running open loop at cruise with a target of about 15.5:1 A/F at light load. I have a wideband in the car, but currently it's not feeding my controller.