Mixing diesel with petrol

[Crashy]

I saw something about a new engine design that combines petrol and diesel to give better performance.

Has anyone tried adding some diesel to their petrol engine?

[oil pan 4]

Some engined can make use of low octane fuel, ones that can widely vary the cam timing.
I wouldn't try it as most gasoline engines depend on knock resistant gasoline.
Probably better off with lean burn and the search function because we know both of those things work.

[19bonestock88]

Some “back of the hand” calculations to debunk this myth that adding diesel to gasoline(in a gas car) to gain economy...

Gasoline is 120,405 BTU/US gal and diesel fuel is 128,700 BTU/US gal, or ~11% more than gas...
Since the two fuels are formulated for the opposite forms of combustion, we’ll assume diesel has an “octane rating” of 0

If you mixed 5% diesel (for ease of math) into 93 octane, non ethanol fuel, you’d end up with a fuel that could be supposed to then have an “octane rating” of 88, which will run in most gas engines. Averaging the BTUs of the fuels, your new mixed fuel would contain 120,820 BTU/US gal, or 0.34% more than standard pump gas...

You’d have to be measuring your distance to the meter, and your fuel to the gram(or mL), in a controlled laboratory environment, to be able to maybe see any gain in efficiency from fuel alone, and this is ignoring any potential negative effects on catalyst efficiency, or the slightly enhanced lubricity of the fuel...

In addition, you’re now mixing 93 octane non ethanol fuel which I’m betting is at least a dollar more than regular unleaded, with on road diesel, which is usually on par price wise with premium unleaded, to make a fuel that no high compression or high boost engine will tolerate...

Can we call this myth, “BUSTED”, yet?

[Crashy]

Originally Posted by 19bonestock88

In addition, you’re now mixing 93 octane non ethanol fuel which I’m betting is at least a dollar more than regular unleaded, with on road diesel, which is usually on par price wise with premium unleaded

In NZ, until recently, unleaded was double the price of diesel. Petrol gets heavily taxed at the pump, while diesel users pay a tax per km. In theory, if you could go 51% petrol, 49% diesel then you would save 25% overall. But diesel gets 35% better mpg, so you would need to add another 17% to your km/$ figure. Australia is similar I think.

Can we call this myth plausible yet?

[Crashy]

[samwichse]

Originally Posted by Crashy

In NZ, until recently, unleaded was double the price of diesel. Petrol gets heavily taxed at the pump, while diesel users pay a tax per km. In theory, if you could go 51% petrol, 49% diesel then you would save 25% overall. But diesel gets 35% better mpg, so you would need to add another 17% to your km/$ figure. Australia is similar I think.

Can we call this myth plausible yet?

Except you're off by an order of magnitude on the amount of diesel you can add. So no.

[19bonestock88]

Originally Posted by Crashy

In NZ, until recently, unleaded was double the price of diesel. Petrol gets heavily taxed at the pump, while diesel users pay a tax per km. In theory, if you could go 51% petrol, 49% diesel then you would save 25% overall. But diesel gets 35% better mpg, so you would need to add another 17% to your km/$ figure. Australia is similar I think.

Can we call this myth plausible yet?

It wasn’t fair of me to consider thatfuel prices abroad aren’t necessarily the same as they are here in WV... I will still assume that higher octane gasoline will cost more than regular...

Best possible case scenario, you live at a really high elevation, and are not bothered by the cost of obtaining race fuel... with 100 octane unleaded race fuel, and at an elevation of a mile or more, you *may* get away with 15% diesel(85 octane at that point) and have a lower compression engine live... but the fuel would still only be about one percent more energy dense than gasoline and any gains from that would be really hard to quantify, and here in the US, race fuel costs more than $7 a gallon, or double the cost of diesel...

Diesel engines get ~30-35% better mileage than an equivalent gas engine because the Diesel combustion cycle is more thermally efficient than the Otto combustion cycle, NOT just because the fuel is more energy dense

Originally Posted by Crashy

Okay, fair enough, but he painted over Alaska, lol

[Crashy]

I watched an episode of fifth gear
https://youtu.be/GL9-i9tcESU and the guy put diesel in a petrol car and petrol in a diesel car to see what happens. In both cases they ran for quite a long time. So even 100% works (with possible damage) so I'm not buying that there is a maximum mixture. Where did you get this data anyway?

The question still has not been answered: has anyone tried it?
https://youtu.be/pCr6bjQMrgU

[arcosine]

Don't do it. It stinks!!!

[19bonestock88]

I googled the energy densities of the two fuels. I did have to guess that diesel has 0 octane rating, based on the fact that an Otto cycle engine and a diesel cycle engine use the opposite forms of combustion, I couldn’t find a rating...

I guessed at the ratio to still keep the mixture at at least 87 AKI octane, to prevent definite engine damage, averaging diesel in at 0 octane... if it were, say, rated at 60, things would change some, in that you could run more diesel in gasoline and still have a mixture average of 87, but it wouldn’t boost the energy density of the resultant fuel enough to bother with, IMO... if mixing some diesel in makes the fuel mix cheaper than straight gasoline, then it could be a feasible option for reducing your fuel bill but I don’t believe it will boost efficiency enough to see from tank to tank