Fuel pre-heating

[jtanguay]

Has anyone done any research on this?  Would it increase mileage?

My guess is that it would give better combustion since the molecules would be moving quicker than usual and thus much easier to be broken apart.  

One way to heat would be through engine coolant (during winter this would take a while... maybe 10 minutes or more)  or from the exhaust heat. (run the fuel lines into a copper flex tube wrapped around the exhaust pipe and sealed with some sort of epoxy in place.  

Is there any documentation backing up fuel pre-heating?  I looked around but I cant find much.  I have found some info on gasoline pre-heating schemes with not much luck.

Thanks guys!! and p.s.   I'm almost done converting the german PDF!!!

[srivett]

I've seen a cab-over transport that had its 3" exhaust loop around under the fuel tank and it was stock.  

I would think that having warm fuel would lessen the amount of fuel per injector shot due to it expanding.  I'm sure you've seen a car shooting fuel out of it's filler when it gets really hot out.  My 92s excess fuel goes back to the fueltank to warm up the tank/filter...this is stock.  

Steve

[Dr. Diesel]

most of the later A2 vehicles have a factory fuel heating loop built into the fuel filter, probably with similar effects. I guess the VW engineers found good reason for it!

[VWRacer]

All you guys up in our Great White North Neighbor can probably benefit from somewhat heated fuel. Diesel fuel ranges from a carbon number of C9 to C23, centered on C16 (gasoline ranges from C3-C10, centered on C8 ). All hydrocarbons solidify at a rate proporsional to their carbon number, with the higher diesel compounds solidifying (freezing) as high as 15C (60F). By -5C (~20F) half of diesel compounds are near or below their freezing point, and the fuel can really benefit from external heating.

Conversely, diesel compounds have quite high boiling points, ranging from 175C (~350F) for the lightest compounds to more than 370C (700+F), so even if it's heated to the standard operating temp of a TD (95C or 200F) it is just slightly thinner, but in no danger of boiling. There isn't much change in volume as diesel heats up, so that isn't much of an issue, either. (The coefficient of thermal expansion for diesel is 0.0046 per degree F, so heating a gallon of it from 32F to 212F (0C to 100C) increases its volume to just 1.083 gallons.) Moreover, heated diesel has lower drag in fuel lines and injectors, makes for a better spray pattern, and ignites easier in the cylinder.

In a normally running diesel engine, all the conponents heat up to operating temp eventually, so once the engine is thoroughly warmed up, there is probably little need for additional fuel heating, but it won't hurt any.

[jtanguay]

wow thanks for the scientific data vwracer!

I think that I might test out the heated fuel theory and see how it goes!  I also wanted to try heating the engine coolant from the exhaust (engine would warm up a heck of a lot sooner than by engine temps alone)

Heh I can leave my car warm up for 5 minutes, drive home, and still when I reach home its only luke warm air coming out!!!  So unless I feel like driving on the highway for heat, I have to look elsewhere!

For this I think a solenoid would work best.  I could shut off the flow to the exhaust system when the temp gets hot enough (better for summer because my engine would probably overheat and die)

anyways, thanks for all the help guys!

[fspGTD]

Varying the temperature of fuel changes it's viscosity and density.  We have found here that hot fuel has "less performance" than cold fuel with all other things being equal, but that because mostly it's density is greater when cold and so the injection pump injects more fuel when cold, all other things being equal.  The newer ECU-controlled fuel pumps have fuel temperature sensors that in theory compensate injected quantity for changes in fuel density.

Short of having an ECU-controlled fueling rate with a temperature sensor, I believe that for us mechanical pump people, having some sort of regulated fuel temperatures would be best for the most consistent performane.  If you regulate it to coolant temperature, you will always find it's rich until the coolant is fully warmed up, but once it's warmed up it should be highly consistent.  Did you know that the A4 TDIs with automatic transmissions have thermostatically controlled fuel coolers mounted underneath the car?  I don't know what temperature it regulates the fuel temps to, but it would be interesting to know.

In my race class, fuel coolers aren't allowed, but relocating the fuel filter from the firewall to inside the fenders seemed to have helped a lot in keeping the fuel temps cooler and more consistent.  I saw a lot more consistency in the engine's performance since doing this.

Whether the spray pattern or ability of the fuel to vaporize and combust quickly might be better hot vs cold, although I haven't verified it myself through testing, could very well be true.

Has anyone rigged up a thermometer or thermocouple to the fuel circulating into and/or out of their fuel pump, and monitored the temps and if so, what kind of number did you see?

[VWRacer]

Yep, Jake is no doubt right. If the heat rise results in a 5% increase in the volume of a starting weight of fuel, the engine will be down on power 5%, all other things being equal. That's axiomatic. But that wasn't the point of my exercise. I was just showing that heating the fuel can have some real benefits in very cold weather.  

[fspGTD]

That is some good info you gave Stan, thanks.  That is also very interesting for me to see that you quantify that diesel fuel expands by roughly 8% when heated from 32 deg F to 212 deg F!

I found another tid bit of info that backs up pretty much what has already been said, but is interesting to see it as written from a very credible source.  Taken from Edwards Ralbovsky's "An Introduction to Compact and Automotive Diesels" section on Diesel Fuel:

"Viscosity

The viscosity of diesel fuel directly affects the spray pattern of the fuel into the combustion chamber and the fuel-system components.  Fuel with a high viscosity produces large droplets that are difficult to burn.  Fuel with a low viscosity sprays in a fine, easily burned mist, Figure 4.4.  [figure shows an injector spraying a wide-angle cone, says "correct viscosity equals proper dispersion" vs a narrow cone where is says "high viscosity equals poor dispersion"]  Also, if the viscosity is too low, it does not adequately lublicate and cool the injection pump and nozzles."

[jtanguay]

Hmmmm seems quite complicated to me.

So if I would need to achieve a specific temp range to benefit the pump, as well as burning process. hmmm.

Peltier heater/coolers seem to come into play then!!  What would be a neat idea is to use the seebeck effect from exhaust heat to power the peltier to regulate fuel temps (cool in summer, heat in winter) providing maximum efficiency

Good idea, but the setup would cost quite a bit.  Would need a peltier controller, but I do have some 350W peltier coolers  

[VWRacer]

Jake, you'd have to really heat diesel for it to loose enough lubricity to negatively affect the pump and injectors. Diesel's ideal viscosity is about 4 centistokes, which it has at 55C (130F), and falls so little off that in either direction to normal temp extremes that you'd literally have to get it up into boiling temp (over ~400F) to lower its viscosity enough to damage anything.  :shock: