8 ms·

Diesel engines have a very high compression ratio, like 20:1 or more, but diesel fuel has a very high octane rating in comparison with gasoline, so it still com

Diesel engines have a very high compression ratio, like 20:1 or more, but diesel fuel has a very high octane rating in comparison with gasoline, so it still combusts, not detonates, at these high ratios. What makes a diesel different than a gas engine is that the heat generated from the compression of the air/fuel mix is what actually causes the mix to start burning in the cylinder. So, for this reason you don't see any ignition system on a diesel usually, it doesn't need it (though maybe glow plugs or a block heater in cold areas, but not an actual ignition system). But the mixture still combusts in a diesel. Detonation is incredibly destructive to any engine, even a big beefy diesel. It can destroy any engine very quickly.

TheSpiceIsLife 6y ago
Thanks all for clearing that up.
esaym 6y ago
>but diesel fuel has a very high octane rating in comparison with gasoline, "octane" is the ability of a fuel to "resist combustion" from compression. Diesel fuel is not rated in "octane" but the opposite: the ability for it to combust from compression. For some reason in the "old days", it was commonly thought by some that by adding some diesel to gasoline that it would help "lubricate" the motor (as diesel does have an oily feel). However mixing diesel with gasoline only lowers its octane rating.
- erdos4d 6y ago
  Yeah, I am not sure if octane is the right word here, you are probably right in what you say. I just know that diesel fuel is much more resistant to detonation at high compression ratios, given how a diesel works. In a gas motor, octane gives a measure of this, so I used the word there, but as you say it is probably not the right word.
kazinator 6y ago
Diesel has a low octane number. Though octane isn't used for diesel fuel, it can be determined and is around 15-25. Diesel engines avoid pre-ignition knocking by compressing only air. There is no compression of air/fuel mix. The fuel is injected into the hot, compressed air at the top of the compression stroke (at something like 3000+ PSI, IIRC). The fuel does not have to be particularly volatile because of the high compression and the injector's atomization. In theory, diesel engines can run on "anything". Diesel engines have been run on crude oil taken directly from the ground (of good purity), and on vegetable oil.
- TheSpiceIsLife 6y ago
  Modern diesel injectors: 30,000 to 40,000 PSI https://www.buyautoparts.com/blog/4-common-diesel-fuel-injection-pump-problems/ https://www.buyautoparts.com/blog/4-common-diesel-fuel-injec...
- icefo 6y ago
  You can do that with old diesel engines but new ones are too sensitive and destroy themselves if you try to run them on anything but diesel. It mostly because the high pressure pump use the Diesel for lubrication so if you use something else the pump destroy itself sending tiny metal shards down the pipe and into the injectors. That's why accidentally running a diesel car on gas turn expensive fast. The contrary is much cheaper as just have to flush the fuel lines and you're good to go.

TheSpiceIsLife 6y ago

Thanks all for clearing that up.

esaym 6y ago

>but diesel fuel has a very high octane rating in comparison with gasoline, "octane" is the ability of a fuel to "resist combustion" from compression. Diesel fuel is not rated in "octane" but the opposite: the ability for it to combust from compression. For some reason in the "old days", it was commonly thought by some that by adding some diesel to gasoline that it would help "lubricate" the motor (as diesel does have an oily feel). However mixing diesel with gasoline only lowers its octane rating.

erdos4d 6y ago
Yeah, I am not sure if octane is the right word here, you are probably right in what you say. I just know that diesel fuel is much more resistant to detonation at high compression ratios, given how a diesel works. In a gas motor, octane gives a measure of this, so I used the word there, but as you say it is probably not the right word.

kazinator 6y ago

Diesel has a low octane number. Though octane isn't used for diesel fuel, it can be determined and is around 15-25. Diesel engines avoid pre-ignition knocking by compressing only air. There is no compression of air/fuel mix. The fuel is injected into the hot, compressed air at the top of the compression stroke (at something like 3000+ PSI, IIRC). The fuel does not have to be particularly volatile because of the high compression and the injector's atomization. In theory, diesel engines can run on "anything". Diesel engines have been run on crude oil taken directly from the ground (of good purity), and on vegetable oil.

TheSpiceIsLife 6y ago
Modern diesel injectors: 30,000 to 40,000 PSI https://www.buyautoparts.com/blog/4-common-diesel-fuel-injection-pump-problems/ https://www.buyautoparts.com/blog/4-common-diesel-fuel-injec...
icefo 6y ago
You can do that with old diesel engines but new ones are too sensitive and destroy themselves if you try to run them on anything but diesel. It mostly because the high pressure pump use the Diesel for lubrication so if you use something else the pump destroy itself sending tiny metal shards down the pipe and into the injectors. That's why accidentally running a diesel car on gas turn expensive fast. The contrary is much cheaper as just have to flush the fuel lines and you're good to go.