# Tell us an interesting random fact you stumbled across (Part 2) **URL:** \ **Category:** Miscellaneous and Personal Stuff I Must Share\ **Created:** [July 25, 2024, 3:06pm UTC](https://boards.straightdope.com/t/tell-us-an-interesting-random-fact-you-stumbled-across-part-2/1004785 "2024-07-25T15:06:02Z")\ **Posts on this page:** 20\ **Page:** 13
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [August 20, 2024, 9:11pm UTC](https://boards.straightdope.com/t/tell-us-an-interesting-random-fact-you-stumbled-across-part-2/1004785/241 "2024-08-20T21:11:17Z")
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. ---
**Author:** ![Slithy\_Tove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/slithy_tove/32/18248_2.png) [@Slithy\_Tove](https://boards.straightdope.com/u/Slithy_Tove)\ **Post date:** [August 20, 2024, 9:15pm UTC](https://boards.straightdope.com/t/tell-us-an-interesting-random-fact-you-stumbled-across-part-2/1004785/242 "2024-08-20T21:15:29Z")
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](https://www.shell.com/business-customers/aviation/aviation-fuel/military-jet-fuel-grades.html) ---
**Author:** ![WildaBeast](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/wildabeast/32/6_2.png) [@WildaBeast](https://boards.straightdope.com/u/WildaBeast)\ **Post date:** [August 20, 2024, 10:06pm UTC](https://boards.straightdope.com/t/tell-us-an-interesting-random-fact-you-stumbled-across-part-2/1004785/243 "2024-08-20T22:06:39Z")
> [@Chronos](#): > > 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. 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](https://en.wikipedia.org/wiki/Chrysler_Turbine_Car) 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. ---
**Author:** ![eschereal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eschereal/32/18939_2.png) [@eschereal](https://boards.straightdope.com/u/eschereal)\ **Post date:** [August 20, 2024, 11:38pm UTC](https://boards.straightdope.com/t/tell-us-an-interesting-random-fact-you-stumbled-across-part-2/1004785/244 "2024-08-20T23:38:50Z")
> [@Lancia](#): > > Since diesel and jet engines both use compression to ignite the fuel mixture rather than a spark plug 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. ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [August 21, 2024, 12:51pm UTC](https://boards.straightdope.com/t/tell-us-an-interesting-random-fact-you-stumbled-across-part-2/1004785/245 "2024-08-21T12:51:19Z")
> [@Lancia](#): > > Sorta… a gasoline with a higher octane rating means said fuel has a higher resistance to _self_-ignition under pressure. Since diesel and jet engines both use compression to ignite the fuel mixture rather than a spark plug, as a gasoline engine uses they both, by design, ignite well under pressure – the opposite of what gasoline is supposed to do. 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](https://www.researchgate.net/figure/Exemplary-pressure-trace-of-a-rapid-compression-machine-experiment-with-a-two-stage_fig6_361022475)**, 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: - Reduce the compression ratio, so that the pressures and temperatures at top dead center (TDC) aren’t so high. - Throttle the engine down, so that temperatures/pressures at bottom dead center (BDC) are reduced, causing a similar reduction in temperatures and pressures at TDC. - Reduce the combustion chamber diameter, so that it doesn’t take as long for the flame front to travel all the way from the spark plug to the edges. - Run at higher RPM, increasing the turbulence that stretches/distorts the flame front and results in a faster burn. - Redesign the intake port so that it fosters increased in-cylinder turbulence at TDC, likewise resulting a faster burn. - Use higher octane fuel, buying you more time to complete the burn. > [@Lumpy](#): > > Aviation gasoline, the stuff that WW2 fighters and bombers ran on, _is_ ultra-high octane because those are piston engines. Jets and turboprops use the modern jet/diesel/kero. In fact aviation gas is among the last holdouts of leaded fuel because it’s genuinely needed in that application, in far fewer vehicles than automobiles and the exhaust is more dispersed. 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: > **[EPA Determines that Lead Emissions from Aircraft Engines Cause or Contribute...](https://www.epa.gov/newsreleases/epa-determines-lead-emissions-aircraft-engines-cause-or-contribute-air-pollution)** > > EPA News Release: EPA Determines that Lead Emissions from Aircraft Engines Cause or Contribute to Air Pollution  > [@Lancia](#): > > Recently I stumbled upon the random (well, random to me, not random to people who actually need to know this) fact that jet fuel, diesel, and kerosene are basically the same fuel – at least, they have similar octane ratings (or would, if an octane rating for these fuels were a thing). There _are_ differences: jet fuel has additives that help it lubricate better and not gel in sub-zero temps, kerosine also does not gel as easily in the cold, and diesel has some lubricants that kerosene doesn’t have but different than the ones found in jet fuel and etc. But my dad was certainly wrong about the perceived octane rating of jet fuel, which makes sense considering that jet engines use compression of the fuel vapor mix for ignition, as do diesel engines. 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. > [@eschereal](#): > > 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. 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. ---
**Author:** ![eschereal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eschereal/32/18939_2.png) [@eschereal](https://boards.straightdope.com/u/eschereal)\ **Post date:** [August 21, 2024, 2:27pm UTC](https://boards.straightdope.com/t/tell-us-an-interesting-random-fact-you-stumbled-across-part-2/1004785/246 "2024-08-21T14:27:39Z")
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. ---
**Author:** ![Pardel-Lux](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pardel-lux/32/20182_2.png) [@Pardel-Lux](https://boards.straightdope.com/u/Pardel-Lux)\ **Post date:** [August 21, 2024, 3:21pm UTC](https://boards.straightdope.com/t/tell-us-an-interesting-random-fact-you-stumbled-across-part-2/1004785/247 "2024-08-21T15:21:17Z")
> [@Machine\_Elf](#): > > 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. 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. ---
**Author:** ![enipla](https://avatars.discourse-cdn.com/v4/letter/e/54ee81/32.png) [@enipla](https://boards.straightdope.com/u/enipla)\ **Post date:** [August 21, 2024, 5:20pm UTC](https://boards.straightdope.com/t/tell-us-an-interesting-random-fact-you-stumbled-across-part-2/1004785/248 "2024-08-21T17:20:15Z")
This might be interesting for the current discussion - [Glow plug](https://en.wikipedia.org/wiki/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. ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [August 21, 2024, 5:22pm UTC](https://boards.straightdope.com/t/tell-us-an-interesting-random-fact-you-stumbled-across-part-2/1004785/249 "2024-08-21T17:22:24Z")
> [@eschereal](#): > > 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. 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: [![](https://img.youtube.com/vi/pukra9Ch3h8/hqdefault.jpg "Diesel combustion, double injection") ](https://www.youtube.com/watch?v=pukra9Ch3h8) 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. > [@eschereal](#): > > 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. 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. > **[Heat engine | Everyday examples](https://en.wikipedia.org/wiki/Heat_engine#Everyday_examples)** > > Everyday examples of heat engines include the thermal power station, internal combustion engine, firearms, refrigerators and heat pumps. Power stations are examples of heat engines run in a forward direction in which heat flows from a hot reservoir and flows into a cool reservoir to produce work as the desired product. Refrigerators, air conditioners and heat pumps are examples of heat engines that are run in reverse, i.e. they use work to take heat energy at a low temperature and raise its tempe... 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](https://en.wikipedia.org/wiki/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](https://en.wikipedia.org/wiki/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. ---
**Author:** ![Al128](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/al128/32/16541_2.png) [@Al128](https://boards.straightdope.com/u/Al128)\ **Post date:** [August 21, 2024, 5:35pm UTC](https://boards.straightdope.com/t/tell-us-an-interesting-random-fact-you-stumbled-across-part-2/1004785/250 "2024-08-21T17:35:16Z")
thx for those quality posts/debate!!! ---
**Author:** ![Loach](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/loach/32/350_2.png) [@Loach](https://boards.straightdope.com/u/Loach)\ **Post date:** [August 21, 2024, 7:45pm UTC](https://boards.straightdope.com/t/tell-us-an-interesting-random-fact-you-stumbled-across-part-2/1004785/251 "2024-08-21T19:45:28Z")
> [@Slithy\_Tove](#): > > 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](https://www.shell.com/business-customers/aviation/aviation-fuel/military-jet-fuel-grades.html) 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. ---
**Author:** ![Lumpy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lumpy/32/446_2.png) [@Lumpy](https://boards.straightdope.com/u/Lumpy)\ **Post date:** [August 21, 2024, 8:50pm UTC](https://boards.straightdope.com/t/tell-us-an-interesting-random-fact-you-stumbled-across-part-2/1004785/252 "2024-08-21T20:50:58Z")
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. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [August 21, 2024, 9:35pm UTC](https://boards.straightdope.com/t/tell-us-an-interesting-random-fact-you-stumbled-across-part-2/1004785/253 "2024-08-21T21:35:25Z")
> [@Machine\_Elf](#): > > 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. Would I be right in guessing that the cetane rating originated by comparison of various fuels with whale oil? > [@eschereal](#): > > 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. All of these things are heat engines. ---
**Author:** ![markn\_1](https://avatars.discourse-cdn.com/v4/letter/m/f9ae1b/32.png) [@markn\_1](https://boards.straightdope.com/u/markn_1)\ **Post date:** [August 21, 2024, 10:52pm UTC](https://boards.straightdope.com/t/tell-us-an-interesting-random-fact-you-stumbled-across-part-2/1004785/254 "2024-08-21T22:52:44Z")
> [@Chronos](#): > > Would I be right in guessing that the cetane rating originated by comparison of various fuels with whale oil? Dubious.cetacean needed ---
**Author:** ![Dogginit](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dogginit/32/16920_2.png) [@Dogginit](https://boards.straightdope.com/u/Dogginit)\ **Post date:** [August 21, 2024, 11:18pm UTC](https://boards.straightdope.com/t/tell-us-an-interesting-random-fact-you-stumbled-across-part-2/1004785/255 "2024-08-21T23:18:03Z")
> [@Machine\_Elf](#): > > 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. 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. ---
**Author:** ![Al128](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/al128/32/16541_2.png) [@Al128](https://boards.straightdope.com/u/Al128)\ **Post date:** [August 21, 2024, 11:33pm UTC](https://boards.straightdope.com/t/tell-us-an-interesting-random-fact-you-stumbled-across-part-2/1004785/256 "2024-08-21T23:33:54Z")
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. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [August 21, 2024, 11:45pm UTC](https://boards.straightdope.com/t/tell-us-an-interesting-random-fact-you-stumbled-across-part-2/1004785/257 "2024-08-21T23:45:20Z")
> [@markn\_1](#): > > Dubious.cetacean needed Hm, looking it up, “cetane” is linear C16H34 (whatever the source), but the [origin of the name](https://www.ahdictionary.com/word/search.html?q=cetane) is in fact whale oil. ---
**Author:** ![Slithy\_Tove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/slithy_tove/32/18248_2.png) [@Slithy\_Tove](https://boards.straightdope.com/u/Slithy_Tove)\ **Post date:** [August 21, 2024, 11:52pm UTC](https://boards.straightdope.com/t/tell-us-an-interesting-random-fact-you-stumbled-across-part-2/1004785/258 "2024-08-21T23:52:50Z")
If I open the back of my great grandfather’s pocket watch it might smell blubbery. ---
**Author:** ![Spectre\_of\_Pithecanthropus](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/spectre_of_pithecanthropus/32/12343_2.png) [@Spectre\_of\_Pithecanthropus](https://boards.straightdope.com/u/Spectre_of_Pithecanthropus)\ **Post date:** [August 22, 2024, 3:07am UTC](https://boards.straightdope.com/t/tell-us-an-interesting-random-fact-you-stumbled-across-part-2/1004785/259 "2024-08-22T03:07:00Z")
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!” ---
**Author:** ![Exapno\_Mapcase](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/exapno_mapcase/32/1051_2.png) [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)\ **Post date:** [August 22, 2024, 3:20am UTC](https://boards.straightdope.com/t/tell-us-an-interesting-random-fact-you-stumbled-across-part-2/1004785/260 "2024-08-22T03:20:34Z")
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. [Previous page](https://boards.straightdope.com/t/tell-us-an-interesting-random-fact-you-stumbled-across-part-2/1004785.md?page=12) [Next page](https://boards.straightdope.com/t/tell-us-an-interesting-random-fact-you-stumbled-across-part-2/1004785.md?page=14)