And the US military generally tries to supply all of their vehicles, jets, diesels, and flex-fuel*, with the same fuel, because having one kind of fuel for anything simplifies logistics.
*The Abrams tank uses a turbine engine that can run on just about anything flammable. In a pinch, they could use raided gasoline, or whatever the enemy uses for fuel, or even perfume. But when the military is supplying fuel through their normal supply lines, it’s jet fuel.
I used to have a bunch of roommates who wore purple sweatshirts and pumped JP 4 into helicopters. I see things have changed since then
I believe that’s true for pretty much any turbine engine. Of course the FAA certainly wouldn’t allow it for a civilian airliner, but I believe they would theoretically run on anything flammable as well, at least for a little while.
When Chrysler was testing their experimental Turbine Cars in the 1960s, as a publicity stunt they ran them on tequila in Mexico and perfume in France, at least according to what Jay Leno said on his YouTube channel.
That seems slightly misleading, though. A turbine engine feeds compressed air into the combustion chamber where the combustion is already taking place, perhaps ignited by a sparkplug initially but burning continuously, like a flame. This is quite a bit different from the way a diesel engine actually uses the heat of compression to ignite the spray of fuel injected into the cylinder.
It’s mostly related to temperature, and it’s a question of time. Gasoline has an autoignition temperature, and if you take a combustible mixture above that temp, it’s going to light off, but only after a slight delay, on the order of milliseconds. The trick is that a higher octane fuel confers a slightly longer delay before lighting off. This has been demonstrated in laboratory rapid-compression machines, which quickly compress a mixture to a specified pressure/temperature, and then measure the time it takes before the mixture auto-ignites. Here’s an example plot, showing the ignition delay for a gasoline-air mixture in a rapid-compression machine. Hydrocarbon combustion is surprisingly complex: you can see in that plot that there are two distinct stages to the combustion event, and there are many, many chemical reactions taking place before and during each stage.
Once you trigger combustion with the spark plug, the unburned part of the mixture gets compressed and adiabatically heated by the burned part of the mixture. At some point, the unburned mixture becomes hotter than the autoignition temperature, and now the clock starts ticking: you’ve got to get that mixture burned with the inbound flame front before it autoignites and causes knock. If you can’t do that, then you’ve got options:
Aircraft piston engines need high octane because they typically use big cylinders and operate at high loads and modest RPM (see list above). Older engines were made with soft valve seat inserts that assume lead will be present to act as a lubricant. If you get rid of the lead, you need to replace the valve seat inserts with ones made of Stellite, a very hard wear-resistant material. That’s a costly changeover.
Once the lead is gone, how do you boost octane to match what avgas currently provides? Well, people are working hard on that, and just in time:
Whereas the octane rating of gasoline is correlated with its ignition delay, diesel fuel comes with a cetane rating, which is a measure of how quickly it ignites when heated past its auto-ignition temperature. A higher cetane number means you have a diesel fuel with a shorter ignition delay: after injection and vaporization, a high cetane diesel fuel will start burning sooner than a low cetane diesel fuel. A low cetane number results in rough running: you’ll get a lot of the fuel injected and vaporized before the burn really gets started, and then a lot of it will burn very quickly. Diesel-engine passenger cars are popular in Europe not just for their fuel economy, but also because the diesel blend sold in Europe has a higher cetane number than the diesel blend sold in North America. That means a diesel-powered Mercedes sedan in Europe sounds/feels a lot more like a gasoline-powered sedan than it would in North America.
Gasoline and diesel fuel are both hydrocarbons, but beyond that, they’re pretty different. They’re both blends, but on average, a gasoline molecule has a little less than eight carbon atoms, and a diesel fuel molecule has around 13 carbon atoms. The latter is much more difficult to vaporize. You can start a gasoline puddle on fire with a spark, but if you want to ignite a diesel puddle, you’ll need to blast it with a propane torch until it’s warm enough to supply vapor in sufficient quantity to sustain a flame.
This. At maximum power, a gas turbine engine can have a compression ratio that’s hot enough to ignite the fuel spray, but unlike a diesel-powered piston engine, the compression ratio of a gas turbine engine falls off when it’s running at part load or at idle. Under those conditions, the compressed air entering the combustion chamber may or may not be hot enough to ignite the fuel, but that’s fine, because the fuel spray is already burning, as you noted. You do need an igniter (basically a spark plug) to get it lit when you first start the engine, but AIUI, the igniters don’t run after that unless there are operating conditions where a flameout is more likely or would be more problematic than usual.
I was given the impression that the compression level in a diesel engine is high enough that fuel combustion occurs upon injection. Diesel injectors produce an incredibly fine mist of fuel, to maximize the air/fuel interface area, so that most of the injected fuel is exposed to the compressed oxygen upon injection and burns immediately. In essence, an injector is squirting a broad flame into the cylinder and there is very little combustion that takes place after injection.
This is in stark contrast to a gasoline engine, in which the spark plug initiates explosive combustion of a fuel/air mixture in the cylinder. A diesel engine is called a heat engine because of the fact that it is adding heat to the compressed air rather than creating an explosion.
Now that is really interesting! And explains a lot about the difficulties diesel engines have experienced in the USA compared to Europe, something I often wondered about.
This might be interesting for the current discussion - Glow plug
My tractor has them. They help a lot.
The old Cox engine, used for small model airplanes as well. They should come with a box of Band-Aids.
The air in a diesel combustion chamber at TDC is certainly hot enough to ignite a mixture of fuel vapor and air, but of course nothing is instantaneous. It takes a non-zero amount of time for each droplet to vaporize and mix with the air, and then there’s a non-zero ignition delay similar to that of gasoline: it takes a bit of time for the chemistry to spool up. That ignition delay is shorter with a higher cetane number, but it’s never zero. What you end up with is a fuel injection event that lasts maybe a couple of milliseconds (at 1200 RPM, that’s about 14 degrees of crankshaft rotation), and the injection actually starts well before the piston reaches TDC, maybe 15-20 degrees early. All of that results in combustion mostly taking place just after the piston reaches TDC.
Here’s a slow-motion video (26 seconds) of a diesel injection/combustion event in a running engine, recorded from below through a piston with a transparent crown:
This is a two-stage injection event, and you can see that the that the first injection is completed before there’s any visible sign of combustion.
A couple of things:
A heat engine is any mechanism that takes heat, converts part of it to mechanical work, and then throws away the unused part of the heat. This means that internal combustion engines (including gasoline, diesel, and gas turbine) and external combustion engines (steam locomotives, power plants) are all heat engines.
In the end, it doesn’t matter how the heat is added. Could be burning fuel, nuclear fission/fusion, or even an electrically-powered heating element that delivers heat to a heat engine. If you’re burning fuel, it doesn’t matter how you burn it. Deflagration (in which the reaction front travels through the combustible mixture at subsonic speed) is the usual approach in an IC engine. But you could just as easily employ detonation (in which the reaction front travels through the combustible mixture at supersonic speed). Strictly speaking, a grenade is a heat engine: the heat of the explosive, released through a detonation event, generates hot gas that expands and does mechanical work, propelling metal fragments and a high-speed destructive shock wave away from the grenade.
And if you’re into the details of combustion science, the word “explosion” kind of blurs things. The spark plug in a gasoline engine may ignite the mixture, but the resulting combustion event is still a subsonic burn, qualitatively different from what happens in a grenade. The pressure-versus time curves for a gasoline combustion event and a diesel engine have much more in common with each other than they do with a detonation event.
thx for those quality posts/debate!!!
When I was in aviation we used JP8. When I was in armor we mostly used JP4 in the tanks. We would also take larger than normal “fuel samples” to run our heaters.
Now I have this bizarre vision of combining 1950s atom-mania with the classic car era and imagining a proposal to use ionizing radiation to instantly detonate fuel mixtures in V8 engines.
Would I be right in guessing that the cetane rating originated by comparison of various fuels with whale oil?
All of these things are heat engines.
Dubious.cetacean needed
There are cetane booster additives available in the US for correcting this problem. Available most any place that sells other fuel additives, STP, and the like.
somewhat related:
in the 1970-80ies, when dinos still roamed central europe and winter-diesel was just becoming a thing, it was common knowledge/practice to put 1 liter of regular gas in the tank when fuelling up a diesel car/truck … kept the diesel from gelling/turning into wax at cold temps …
in the mid 90ies, when winterdiesel was well established, I still had problems on an extremely cold morning in my 197x 50hp Diesel Golf I. Started right up, but later on when climbing a bridge it would stall out… had to call a toe truck (stranded on the bridge) … and when the truck came, I tried to start the car once again, and it would start normally … an in-situ-headscratcher …
later it dawned on me, that the fuel-lines (running below the body) would freeze/turn to wax in the extreme cold and doing 70-90 km/h … once standing for an hour or so, the wax would convert back into diesel-oil again … so I got back into 30L diesel and 1L gas pit-stops.
Hm, looking it up, “cetane” is linear C16H34 (whatever the source), but the origin of the name is in fact whale oil.
If I open the back of my great grandfather’s pocket watch it might smell blubbery.
I glanced at the BFHS yearbook for 1935, and I would say I’m astonished that they printed the students’ home addresses, but I have seen other yearbooks of the era that do likewise.
“Just fantastic”, more than one must have grumbled. “Now the whole school knows I live on Craptastic Alley!”
Newspaper letters to the editors of that era often included the full address of the writers, presumably to show that they weren’t made up.
Today that would be a license for harassment.