How can an engine run on both gasoline and diesel?

When I was a volunteer fire fighter, we had a few old army trucks that had been converted to tanker trucks. There was a plate on the dash of these trucks that said that they could run on either gasoline or diesel and that mixing fuels was permissible. Jet fuel was a no-no. Given that gas and diesel engines operate in completely different ways (as Una explains here), how did these engines work?

The 11th picture on this page shows an example of the plate. Sorry, I couldn’t find a way to link directly to the image.

they’re basically just diesel engines but with low enough compression ratio to where they can safely burn gasoline with compression ignition.

https://en.wikipedia.org/wiki/Multifuel

Gasoline was only supposed to be an “emergency” fuel because it would not lubricate the injection pump. and the ones you describe should have been able to run on jet fuel, since jet fuel is essentially kerosene which is almost identical to #1 diesel fuel oil.

https://en.wikipedia.org/wiki/M35_series_2½-ton_6x6_cargo_truck

https://en.wikipedia.org/wiki/Multifuel#Military_multifuel_engines

It looks like it’s a compression-ignition engine, i.e. what most people would call a diesel engine. No spark plugs, it relies on heat developed during the compression stroke to ignite fuel injected into the cylinder near top-dead-center. That being the case, the fuel had to be stuff that wasn’t terribly resistant to autoignition - which is basically the defining feature of diesel fuel. If you wanted to use gasoline, it had to be shitty low-octane stuff; going by the Wikipedia page, you’d either need to blend in some diesel fuel or motor oil to assure ignitability. The dashboard placard you linked to says MIL-G-3056D, which is (or was) a particular specification for gasoline. The description says it covers “gasolines suitable for…use in all gasoline engines other than aircraft…”. Piston-engine aircraft use high-octane fuel (AKA “avgas”), which implies that MIL-G-3056D gasoline was low-octane. This is bolstered by the other warning on your placard that says “CAUTION - DO NOT USE AVIATION GRADE GASOLINE.” Note that avgas is not the same as (and not interchangeable with) jet fuel.

It appears the M35 was a similar vehicle, and to hear these posters, you could burn just about anything in them; if it didn’t run good, you just needed to add a bit of diesel to whatever crap you had in the tank.

Alternately, you could go the route used in Abrams tanks: They can run on absolutely anything at all that’s flammable, because they don’t use piston engines, but turbines. Apparently they pulled a publicity stunt once where they ran a tank on perfume.

I hadn’t heard that about the Abrams tank, but I heard the same thing about Chrysler’s turbine car. I also heard that they took it to Mexico and ran it on tequila.

This was one of the most interesting episodes of Jay Leno’s Garage IMO, where he talks about his 1963 Chrysler Turbine. 1963 Chrysler Turbine: Ultimate Edition - Jay Leno's Garage - YouTube

I have a little camping stove like that. The liquid fuel is vaporized, mixed with air, and burned. What may or may not be relevant is that when burning diesel, as opposed to kerosene, jet fuel, or automotive gasoline, one is supposed to screw in a different jet.

That is partially true for liquid fuels. But even some liquid fuels that have high vanadium (common with some crudes and residues), turbines don’t work at least not for a long time.

As to gaseous fuels, Gas turbines have limits on the Wobbe index. Wobbe index - Wikipedia
So a gas turbine designed for natural gas will not work with coke oven gas or pure hydrogen. You need a different design.

Gas turbines also make a lot of NOx. Turbine manufacturers will guarantee NOx emissions for a narrow range of fuels although the turbine can run with other fuels.

I guess, tanks don’t go through emissions testing like civilian vehicles.

Oh BTW I worked once with a elderly researcher, who in his prime day had run a gas turbine on coal (pulverized coal of course) for short intervals of time. This was part of a cool DOE study.

Aviation grade gasoline, as noted above, is not the same thing as jet fuel. Jet fuel is basically kerosene/#1 grade fuel oil or just a lighter weight of diesel fuel.

Aviation grade gasoline is high, 100/130 octane gasoline. Octane rating shows the fuels resistance to pre-ignition or detonation by compression, so that is what makes it unsuitable for a diesel type engine that operates on compression ignition.

FWIW, another important feature of AVgass is low freezing point. Typically around -100C, must be at most -60C ???

It has to be volitile to work correctly in the cylinders (which is releated to a low freezing point), and, of course, in a carburated engine it has to not freeze in the carburator (where it gets cold due to evaporation)