Diesel fuel in a FI gas motor = more power?

[Masher Manufacturing]

So says a reasonably reliable source some years ago. Seems a customers 90 ish Jeep with a straight 6 came in with a difficult to start when cold issue.

Truck had great power when hot, more than normal but not a 9 second car. After doing the normal checks it was determined the truck had been missfueled with diesel. ( fuel smell, credit card recipt)

Gasoline has ~ 125,000 BTU / gal
Diesel ~ 138,000 BTU / gal
Kerosene ~ 135,000 BTU / gal

The theory was the feed back FI / knock sensor kept things under control.

Not having a FI car in the fleet, is the power increase at all possible? I'd think the octane rating would drop too far to realize any power gains.

There are spark engines that run on kerosene. These were primarily pre 50's farm tractors and some air cooled engines for countries where gas is expensive. These engines had reduced compression and a small tank for gas to start the enigne when cold. In either case the motors had reduced power output. ( Kero has 135,000 BTU / Gal )

[icebluecuda]

From what I understand, diesel does not mix with air as a vapor like gasoline does. It is actually in very small droplets that burn radially inward. You can probably run it in limited amounts and actually gain power up to the point that you get incomplete burns or knocking.

These droplets are also why Diesel is RPM limited. The burn time of diesel is directly proportional to the droplet size. Over a certain RPM, you don't burn all your fuel before the BDC. I don't remember the exact numbers for droplet size and injector pressure, but you can increase the pressure to the point where the droplets allow for 9k RPM. Beyond that, you reach diminishing returns again.

One of the other reasons that diesel is more efficient than otto cycles is that diesel always run lean of peak. In theory every fuel molecule has enough O2 molecules. Otto engines typically run rich of peak, meaning there are extra fuel molecules that don't get used. Diesel is also higher compression, which gives a few percent more overall thermal efficiency.

It would be interesting to play with, but I don't think you are going to see any gains offsetting the higher cost of diesel.

[PackardV8]

Two words - urban legend. Not saying an EFI Jeep 6-cyl engine won't run with some diesel in the tank, albiet roughly and at reduced power. It will not make more power than gasoline.

FWIW, this is the second time this week I have seen this come up on performance sites. On another forum of guys who should know better it was insisted adding diesel to gasoline is today's perfomance tip. It just won't die!

jack vines

[Warpspeed]

Not so sure about that, I have heard people ran gasoline engines on all sorts of things during World War Two.

Performance may not have been wonderful, but I bet it beat walking.

[bobqzzi]

I has a Honda generator that I used to bring to the track. During one memorable thrash we were running it for quite a while and I refued it while it was going. Ran it for another couple of hours then shut if off for the night.

Came back the next morning and tried to fire it up, but it wouldn't start. After much mucking around I discovered I had filled it with diesel form the spare can I kept for my truck. Didn't seem to actually run any different. Once it put gas back in it , it ran fine.

That said, since the octane of diesel is about 25, I thinking running it in a gas engine is not wise.

[jmarkaudio]

I has a Honda generator that I used to bring to the track. During one memorable thrash we were running it for quite a while and I refued it while it was going. Ran it for another couple of hours then shut if off for the night.

Came back the next morning and tried to fire it up, but it wouldn't start. After much mucking around I discovered I had filled it with diesel form the spare can I kept for my truck. Didn't seem to actually run any different. Once it put gas back in it , it ran fine.

That said, since the octane of diesel is about 25, I thinking running it in a gas engine is not wise.

I think you mean Cetane, different from Octane.

[bobqzzi]

I has a Honda generator that I used to bring to the track. During one memorable thrash we were running it for quite a while and I refued it while it was going. Ran it for another couple of hours then shut if off for the night.

Came back the next morning and tried to fire it up, but it wouldn't start. After much mucking around I discovered I had filled it with diesel form the spare can I kept for my truck. Didn't seem to actually run any different. Once it put gas back in it , it ran fine.

That said, since the octane of diesel is about 25, I thinking running it in a gas engine is not wise.

I think you mean Cetane, different from Octane.

No, I mean octane

[Masher Manufacturing]

Two words - urban legend.

jack vines

Jack, this info came to me directly from the shop owner and was not part of another post on a board. The shop owner is a racer reasonably technically astute so I filed his info under something to investigated.

Diesel in a carbed nonfeedback engine seems like a looser, the mix in a FI engine might have a chance since the fuel is under pressure. I like icebluecuda's droplet size info. Being it was a gas / diesel mix at a unknown ratio I'd start at 50% and go up / dn from there.

[PackardV8]

Three thoughts for your consideration:

1. Both the cited examples, the Jeep and the Honda generator are low-specific-output. low speed engines.
2. Diesel was added to a tank containing some gasoline, thus the percentage of diesel-to-gasoline is unknown.
3. In neither case would they start from cold.

thnx, jack vines

[Lazy JW]

You need to get the compression ratio under 5:1 to run low-octane fuel such as kerosene or distillate in a spark-ignition engine. Kerosene conversions were popular during The Big War for engines in the Model A Ford category. Some means of pre-heating the intake is mandatory, also a gasoline source for starting. Specific power output suffers considerably. Kerosene octane rating is around 35 or so.

My 1941 John Deere tractor burns it nicely with its 4.45:1 compression. I have obtained several tanks of fuel from folks who inadvertently put gasoline in diesel vehicles.
Joe

[Harte3]

Diesel in gas for performance? Throw in a box of moth balls too.

Someone that does this routinely may have cat converter problems not too far down the road.

[Lazy JW]

I has a Honda generator that I used to bring to the track. During one memorable thrash we were running it for quite a while and I refued it while it was going. Ran it for another couple of hours then shut if off for the night.

Came back the next morning and tried to fire it up, but it wouldn't start. After much mucking around I discovered I had filled it with diesel form the spare can I kept for my truck. Didn't seem to actually run any different. Once it put gas back in it , it ran fine.

That said, since the octane of diesel is about 25, I thinking running it in a gas engine is not wise.

I think you mean Cetane, different from Octane.

Of course, diesel is normally thought of in reference to its Cetane number, but if one wishes to burn it in a spark-ignition engine that compresses an air/fuel mixture then the octane rating (very low) needs to be considered as well.

The only valid reason to do this is when gasoline is too expensive/scarce compared with kerosene or diesel.
Joe

[GT50]

Once l went to the gaxs station to fill the deposit of my pick up, the attended said how much I said fill her up but before I want to put Molly Liquid to improve the cetane number and clean the injectors and the fuel system. He told me that most people put 2liters of gasoline to 30 liters of fuel. I said that is their car they can do whatever they want but for sure they don't know what they are doing technically.

[4vpc]

It is very temperature dependent. I've run petrol engines on a % of paint thinners before now, whether they started or not from cold depended on the % of thinners to petrol, yet once they were hot they would run fine on a high % of thinners.
Getting it running and keeping it running are two totally different things, that's why we have glow plugs.

[David Redszus]

Gasoline has ~ 125,000 BTU / gal
Diesel ~ 138,000 BTU / gal
Kerosene ~ 135,000 BTU / gal

The above comparisons are invalid.
The heating values are based on BTU/gal (volume), while engines run on fuel mass.
The reason that diesel seems more potent is that a gallon of diesel weighs more than
a gallon of gasoline.

The limiting factor of power production is the amount of fuel that can be burned.
The amount of fuel that can be burned is limited by the available air.
A proper comparison would be the specific power as determined by (BTU/lb)/lb air.

Now the useable energy appears as nearly identical.

Diesel fuel
BTU/lb = 18,300
Stoich value = 14.5
SpGravity = .850
Specific power = 1264 BTU/lb air

Gasoline
BTU/lb = 18,600
Stoich value = 14.7
SpGravity = .735
Specific power = 1267 BTU/lb air

Fuel components by type, of both are somewhat similar; paraffins, isoparaffins, napthenes, and aromatics.
But the carbon numbers for diesel run from C10 up to C20. Gasoline carbon numbers are typically C5 to C9.
Diesel is both more dense and has a higher range of boiling points which poses an ignition problem
for low compression engines.

Simply raise your gasoline engine compression ratio up to 24:1, and use a direct injection fueling system.
No problem.

Oh wait. It's been done before; it's called a diesel engine.