# Why do petrol(gasoline) and diesel cars exist? **URL:** \ **Category:** Factual Questions\ **Created:** [July 31, 2012, 11:34am UTC](https://boards.straightdope.com/t/why-do-petrol-gasoline-and-diesel-cars-exist/629941 "2012-07-31T11:34:58Z")\ **Posts on this page:** 10\ **Page:** 2
**Author:** ![AaronX](https://avatars.discourse-cdn.com/v4/letter/a/7bcc69/32.png) [@AaronX](https://boards.straightdope.com/u/AaronX)\ **Post date:** [August 1, 2012, 6:39am UTC](https://boards.straightdope.com/t/why-do-petrol-gasoline-and-diesel-cars-exist/629941/21 "2012-08-01T06:39:48Z")
Kerosene? Isn’t that different from diesel? ---
**Author:** ![Princhester](https://avatars.discourse-cdn.com/v4/letter/p/3e96dc/32.png) [@Princhester](https://boards.straightdope.com/u/Princhester)\ **Post date:** [August 1, 2012, 8:15am UTC](https://boards.straightdope.com/t/why-do-petrol-gasoline-and-diesel-cars-exist/629941/22 "2012-08-01T08:15:33Z")
> [@si\_blakely](#): > > The Cox .049 was fun, though. I wouldn’t use that word. I always found them a right, royal PITA. So hard to start. ---
**Author:** ![Dog80](https://avatars.discourse-cdn.com/v4/letter/d/6bbea6/32.png) [@Dog80](https://boards.straightdope.com/u/Dog80)\ **Post date:** [August 1, 2012, 9:29am UTC](https://boards.straightdope.com/t/why-do-petrol-gasoline-and-diesel-cars-exist/629941/23 "2012-08-01T09:29:28Z")
> [@AaronX](#): > > Kerosene? Isn’t that different from diesel? Kerosene is a cleaner type of Diesel, but other than that their properties are similar. Truckers add kerosene to their truck prior to an emissions test when they know that their truck won’t normally pass. ---
**Author:** ![kferr](https://avatars.discourse-cdn.com/v4/letter/k/71e660/32.png) [@kferr](https://boards.straightdope.com/u/kferr)\ **Post date:** [August 1, 2012, 10:31am UTC](https://boards.straightdope.com/t/why-do-petrol-gasoline-and-diesel-cars-exist/629941/24 "2012-08-01T10:31:33Z")
> [@AaronX](#): > > I’ve heard diesel fuel has a flash point higher than room temp, so it won’t ignite normally, is this true? That fact was used in an episode of _Sherlock_. Some baddie had captured Watson, poured fuel all over him and was going to torch him (after the required baddie confession monologue). The fuel wouldn’t light and Watson said something like “Ha-ha, it’s diesel you moron!” ---
**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 1, 2012, 12:45pm UTC](https://boards.straightdope.com/t/why-do-petrol-gasoline-and-diesel-cars-exist/629941/25 "2012-08-01T12:45:59Z")
> [@AaronX](#): > > I’ve heard diesel fuel has a flash point higher than room temp, so it won’t ignite normally, is this true? Yeah, you can hold an open flame as close as you like to the surface of a pool of room-temperature diesel fuel, and it won’t ignite. If you’re trying to start a campfire, gasoline is an extremely dangerous accelerant to use; it’ll build up a cloud of vapor that can blow your campfire to pieces; [here’s good example.](http://www.youtube.com/watch?v=ttdg2vX9AtA) Diesel fuel, OTOH, requires the continuous application of flame until the diesel is hot enough to vaporize, and usually some sort of wicking arrangement (e.g. a log that has soaked up some diesel) before you can get a fire to light off. If you atomize room-temperature diesel finely, yes, you may be able to light it off with a match, as the atomized, distributed fuel approximates a gaseous fuel-air mixture in the same way that a [cloud of flour dust can do so in a grain elevator](http://en.wikipedia.org/wiki/Grain_elevator_explosion#Elevator_explosions), or a handful of sawdust can do so when thrown at a campfire. This brings up the Hollywood trope of exploding big-rigs, e.g. in Terminator 2 when the T-1000’s truck explodes in a ball of fire after crashing into a bridge abutment. A truck like that can indeed catch fire and burn to the ground, but it won’t be a sudden, instantaneous explosion, triggered by a tiny spark, that blows pieces of it a quarter-mile away; it’ll be a fire that starts small (e.g. some diesel fuel from a severed line drips onto a hot exhaust manifold) and grows over the space of minutes. ---
**Author:** ![Kevbo](https://avatars.discourse-cdn.com/v4/letter/k/e47774/32.png) [@Kevbo](https://boards.straightdope.com/u/Kevbo)\ **Post date:** [August 1, 2012, 3:42pm UTC](https://boards.straightdope.com/t/why-do-petrol-gasoline-and-diesel-cars-exist/629941/26 "2012-08-01T15:42:19Z")
> [@TriPolar](#): > > …but the difficulty in igniting kerosene is what makes the combustion controllable since it’s occuring throughout the fuel much faster than the reaction in a gasoline engine. This is not correct. When ignition occurs, all the fuel in the combustion chamber pretty much detonates, and this is what gives Diesels their characteristic clatter/knock sound. But most of the fuel producing the power is NOT yet in the combustion chamber when ignition occurs…it has yet to leave the injector. Once ignition has happened, the remaining fuel burns as it is injected, and THAT is what controls the burn rate and prevents the detonation of the full fuel charge from damaging the engine. This is also why true Diesels can never get away from the need for complex, expensive high pressure injection systems. Many modern Diesels are now splitting the injection into two events…a small “priming” squirt to light the fire, then the main charge. This reduces noise, emissions, and wear, and may slightly improve efficiency. ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [August 1, 2012, 3:49pm UTC](https://boards.straightdope.com/t/why-do-petrol-gasoline-and-diesel-cars-exist/629941/27 "2012-08-01T15:49:33Z")
> [@Kevbo](#): > > This is not correct. When ignition occurs, all the fuel in the combustion chamber pretty much detonates, and this is what gives Diesels their characteristic clatter/knock sound. But most of the fuel producing the power is NOT yet in the combustion chamber when ignition occurs…it has yet to leave the injector. Once ignition has happened, the remaining fuel burns as it is injected, and THAT is what controls the burn rate and prevents the detonation of the full fuel charge from damaging the engine. This is also why true Diesels can never get away from the need for complex, expensive high pressure injection systems. > > Many modern Diesels are now splitting the injection into two events…a small “priming” squirt to light the fire, then the main charge. This reduces noise, emissions, and wear, and may slightly improve efficiency. Yes, that sounds right. I was trying to hedge because the combuston process is more complicated than the idealized concept of all of the fuel detonating at once. ---
**Author:** ![60wag](https://avatars.discourse-cdn.com/v4/letter/6/7ba0ec/32.png) [@60wag](https://boards.straightdope.com/u/60wag)\ **Post date:** [August 1, 2012, 7:15pm UTC](https://boards.straightdope.com/t/why-do-petrol-gasoline-and-diesel-cars-exist/629941/28 "2012-08-01T19:15:51Z")
So back to the original question of why have both gas and diesel engines? It’s my understanding that we have fuel for both. As in, when you refine crude oil into different weight components, you’ll have a bunch of mid weight molecules that become gasoline. And a bunch of heavier weight molecules that become diesel, jet fuel and oil, and a bunch of small lightweight molecules that’ll vaporize at very low temperatures that become industrial solvents. If the only engines in use burned diesel, we would have a bunch of refined gasoline like products left over. Now we could convert them into different weight molecules to make them into diesel, or plastic or whatever but that is very energy intesive and expensive. The more efficient (cheap) way to create and consume fuel is to be able to use less altered petroleum products. So, some people burn gas, some diesel and some people ride bikes. Do I have this right? ---
**Author:** ![si\_blakely](https://avatars.discourse-cdn.com/v4/letter/s/d9b06d/32.png) [@si\_blakely](https://boards.straightdope.com/u/si_blakely)\ **Post date:** [August 1, 2012, 9:17pm UTC](https://boards.straightdope.com/t/why-do-petrol-gasoline-and-diesel-cars-exist/629941/29 "2012-08-01T21:17:01Z")
> [@60wag](#): > > So back to the original question of why have both gas and diesel engines? > … > If the only engines in use burned diesel, we would have a bunch of refined gasoline like products left over. Now we could convert them into different weight molecules to make them into diesel, or plastic or whatever but that is very energy intesive and expensive. The more efficient (cheap) way to create and consume fuel is to be able to use less altered petroleum products. So, some people burn gas, some diesel and some people ride bikes. No. Petrol and diesel engines have different characteristics that make them suitable for different tasks. In some cases, they do overlap (like for car engines). But for many uses, a diesel or petrol engine is the right engine for the job. Si ---
**Author:** ![jz78817](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jz78817/32/1028_2.png) [@jz78817](https://boards.straightdope.com/u/jz78817)\ **Post date:** [August 2, 2012, 12:12am UTC](https://boards.straightdope.com/t/why-do-petrol-gasoline-and-diesel-cars-exist/629941/30 "2012-08-02T00:12:50Z")
> [@bldysabba](#): > > In that case, what is it about the nature of the two fuels that make them better suited for one or the other cycle? gasoline/petrol is rated (via octane number) by its resistance to autoignition. The higher the number, the less likely it is to autoignite. Diesel is rated (via cetane number) by its ability to autoignite. The higher the number, the more likely it is to autoignite, thus reducing ignition delay. [Previous page](https://boards.straightdope.com/t/why-do-petrol-gasoline-and-diesel-cars-exist/629941.md?page=1)