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Fuel Cooler - Worth it on turbo gasoline engine?

wgknestrick (Mechanical)
11 Jun 07 20:07
I know that some drag teams run fuel coolers to cool their fuel to sub ambient temps prior to runs and get a HP gain out of this.  I am building an all-out effort turbo 4-cyl.

My questions are these:
  What is the average temperature increase on fuel in an EFI engine running around 45-75psi(fuel pressure) (using a Walbro 255l in tank)?

  Have any of you guys run one a fuel cooler in a turbocharged (non-diesel)application and did you experience any performance gains?  

  Is a normal air cooled fuel cooler worth it or are sizable gains only seen by super-cooling the fuel?
patprimmer (Publican)
11 Jun 07 21:10
The gains by cooling the fuel will be more than offset by the weight of the cooler.

If you do a site search with the google feature you will find this has already been covered in detail with all the math done to support the arguments.

Real rough ball park summary from memory.

a:f ratio 15:1 by mass.

Forget about specific heats as I can't remember them.

15 deg drop in fuel temp = 1 deg drop in charge temp, but evaporation rate is also hurt.

Net effect, two tenths of a gnats whisker power increase for a pound or two weight increase

Regards

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wgknestrick (Mechanical)
11 Jun 07 21:59
If only I had a nickel for every time someone posted "please use the search" function on this forum, and then not provided any real information to the original question.

I have done multiple searches but "fuel" and "cooler" tend to return many unwanted results.

Does anybody have a link to this "detailed" thread, because I can't seem to find it.  This is the only thread that I could find, but it isn't very scientific, and no real data to prove either side correct.

http://www.eng-tips.com/viewthread.cfm?qid=72191&page=44

Just one note on turbocharged cars.  They all run richer than required because they use excess fuel as an in-cylinder cooling agent.  My theory is that any cooling done to the fuel prior to injection is a measurable benefit to performance since the fuel itself is used as a coolant.

It would be helpful to have information on the average injected fuel temp so I could see if it is worth the effort to make a air-cooled fuel cooler.
patprimmer (Publican)
11 Jun 07 22:16
That is not the thread I was thinking of.

I guess you want me to search and find it for you.

I will see if I can find that amount of time tonight.

Regards

eng-tips, by professional engineers for professional engineers
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FoMoCoMoFo (Automotive)
11 Jun 07 22:42
to the extent that the fuel is being used to cool, it's the evaporation of the fuel rather than the starting temp. of the liquid that does the most.  by far
IceStationZebra (Mechanical)
11 Jun 07 23:03
And my guess on the drag racing scene is consistency. If the fuel is a consistent temp it helps them dial in the combination a little better, which results in more HP. And along these lines I assume that the temp/viscosity relationship is more important as that can affect actual pressure relief settings, pump leakage, flow through an orifice, etc. A couple of degrees shouldn't make a big difference in the cooling provided by state change.

You also have to realize that some set-ups run a lot of fuel through a pressure regulator and back to tank - which turns the pump energy into pressure and then into heat. (This is common on diesel injection pumps, the fuel is used as a coolant.)

"What is the average temperature increase on fuel in an EFI engine running around 45-75psi(fuel pressure) (using a Walbro 255l in tank)?" I would figure that ~80% of the energy the pump uses will end up heating the fuel, but then you have to consider the mass of fuel you are starting with (takes XX minutes to heat XX lbs of fuel XX degrees), the cooling affect of the metal fuel lines, heat absorption in the engine compartment, etc. Very difficult to predict. I suggest measuring the temp rise with a thermocouple during your debugging runs to see what the actual temp rise is and if it is worth "fixing."

ISZ
ISZ
patprimmer (Publican)
12 Jun 07 0:41
The most significant effect in drag racing is that when nitro methane and methanol blends are used, the volume:volume percentage of nitro changes with temperature as nitro methane has a different co-efficient of expansion to methanol. A change in nitro ratio can have a significant effect on power.

Even with a straight fuel, as the mechanical fuel injection systems metre fuel by volume as the fuel expands the mixture goes richer if the air temperature remains constant.

Regards

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dcasto (Chemical)
12 Jun 07 15:08
The limiting factor on street cars is getting enough oxidizer (air) into the cylinder, not getting enough fuel.  As patprimmer was pointing out, the heat capcity of fuel is.54 btu/lb - F and the latent heat of vaporization is 125 BTU/lb.  The heat capcity of air is .25 BTU/lb-f.  1 lb fuel vaporized gets 125 BTU cooling, with 15 lb of air this will cool the whole mixture about 30 F.  At 30 F thats about 5.5% increase in the amount of air than with out the fuel cooling effect

If the fuel was cooled down by 50 F, that would give 27 more BTU/lb or 21% more cooling or 6 F or 1.0% more air than before.  

If you could just cool the air down by 6 degrees you would get the same effect (or close anyway) Drop the after cooler temp from 120 to 110 F and get the same effect.

With dragsters, the oxidizer is in the fuel, so cooling the fuel is also cooling the "air" too.
wgknestrick (Mechanical)
13 Jun 07 6:54
Thanks for the detailed post.
Fabrico (Automotive)
13 Jun 07 16:55

One of the more recent and lengthy threads started out under "Fuel Warming".

http://www.eng-tips.com/viewthread.cfm?qid=155457

hemi (Automotive)
14 Jun 07 13:27
patprimmer,

"Even with a straight fuel, as the mechanical fuel injection systems metre fuel by volume as the fuel expands the mixture goes richer if the air temperature remains constant. "
Right principle, but you've stated the effect backwards, I think...
patprimmer (Publican)
14 Jun 07 18:22
I just re read that post. I did state it backwards. morals of story, don't post late at night. Don't post if composition was interrupted.

Regards

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dcasto (Chemical)
19 Jun 07 10:39
TH AC condensor would warm the fuel.  I could see running the fuel through a set of finned tubes at the front of thr car in order to cool the fuel after it was pumped up (the heat of pumping).  This would assure that you would not vapor lock (cause partial vaporization) beofe the injectors.  If the fuel was heat exchanged with the cold freon, that means the AC would be running all the time and then it would be a loseing proposition.
wgknestrick (Mechanical)
19 Jun 07 17:01
I have seen multiple applications that have cooled after the injectors (which makes no sense to me at all).  Why would you not cool before injecting?  The fuel will just be heated up again once it is pumped.

This is like intercooling your exhaust to me.
patprimmer (Publican)
19 Jun 07 22:56
Cooling after the injector is to cool the fuel being returned to the tank, thereby reducing heat build up in the tank.

Hot non pressurised fuel is much more prone to evaporative losses than hot fuel pressurised being injected into the intake tract. This hot fuel in the tank makes the fuel pump more prone to cavitation.

Regards

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caddaddy (Mechanical)
27 Jun 07 22:50
Reading these posts have been very interesting.  It leads me to the question: Can someone tell me the optimum fuel (gasoline) temperature for an IC engine?  There seems to be two schools of thought - Cool fuel vs. Hot fuel (vapor).

Thanks for the input.  I'm looking forward to reading more on the topic.

wgknestrick (Mechanical)
28 Jun 07 13:26
I still think that injecting cool fuel is always better than hot fuel with respect to a FI engine.  Getting more HP is much easier if you are not detonation limitted and cooler fuel has to contribute something to this theory.  Considering that cooling fuel before injecting is almost a "free" item, I would think it would have to return a measurable gain.  I just think that cooling the inside of the cylinder has more effect than better atomization before compression.

My fuel lines are hot to the touch with my car just idling and they are only physically attached to the engine via the rails, so most of the heat must be coming from the thermodynamics of pumping.
GregLocock (Automotive)
28 Jun 07 23:53
But, at some point the fuel must vaporise before it burns. If you can start that vaporisation process using 'free' energy then there might be an efficiency advantage. I agree that for max power this is the wrong way to go.

Cheers

Greg Locock

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