# Diesel engine, gasoline fuel
**URL:** https://boards.straightdope.com/t/diesel-engine-gasoline-fuel/48713
**Category:** Factual Questions
**Created:** [January 6, 2001, 6:53am UTC](https://boards.straightdope.com/t/diesel-engine-gasoline-fuel/48713 "2001-01-06T06:53:12Z")
**Posts on this page:** 20
**Page:** 1
### Author:  [@Ms2001](https://boards.straightdope.com/u/Ms2001)
#### Post date: [January 6, 2001, 6:53am UTC](https://boards.straightdope.com/t/diesel-engine-gasoline-fuel/48713/1 "2001-01-06T06:53:12Z")
OK, so a gasoline engine works by compressing a fuel/air mixture, then igniting it with a spark. If the compressed mixture gets too hot, it will ignite by itself, which is bad. We use higher-octane fuel to prevent this from happening.
A diesel engine works by just compressing air, then injecting fuel. The fuel ignites by itself due to the temperature of the compressed air, which is good.
So would it be possible to design an engine that compresses air, then injects gasoline, which then ignites by itself? What would the advantages and disadvantages be?
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### Author:  [@Adolph\_Peewee](https://boards.straightdope.com/u/Adolph_Peewee)
#### Post date: [January 6, 2001, 11:06am UTC](https://boards.straightdope.com/t/diesel-engine-gasoline-fuel/48713/2 "2001-01-06T11:06:26Z")
I would have thought that gasoline is too unstable under pressure, unless using an extremely high octane fuel. I’ll probably get shot down in flames now, but hey…that’s why I’m a nurse…mechanics isn’t really my forte!!
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### Author:  [@zen101](https://boards.straightdope.com/u/zen101)
#### Post date: [January 6, 2001, 11:39am UTC](https://boards.straightdope.com/t/diesel-engine-gasoline-fuel/48713/3 "2001-01-06T11:39:37Z")
> [@](#):
>
> The main differences between the gasoline engine and the Diesel engine are:
>
> ```
> A gasoline engine intakes a mixture of gas and air, compresses it and ignites the mixture with a spark. A Diesel engine takes in just air, compresses it and then injects fuel into the compressed air. The heat of the compressed air lights the fuel spontaneously. A Diesel engine uses a much higher compression ratio than a gasoline engine. A gasoline engine compresses at a ratio of 8:1 to 12:1, while a Diesel engine compresses at a ratio of 14:1 to as high as 25:1. The higher compression ratio leads to better efficiency. Diesel engines use direct fuel injection, in which the Diesel fuel is injected directly into the cylinder. Gasoline engines generally use either carburetion, where the air and fuel is mixed long before the air enters the cylinder, or port fuel injection in which the fuel is injected just prior to the intake valve (outside the cylinder).
>
> ```
>
> One big difference between a diesel engine and a gas engine is in the injection process. Most car engines use port injection or a carburetor rather than direct injection. In a car engine, therefore, all of the fuel is loaded into the cylinder during the intake stroke and then compressed. The
> compression of the fuel/air mixture limits the compression ratio of the engine - if compressed too much, the fuel/air mixture spontaneously ignites and causes knocking. A diesel compresses only air, so the compression ratio can be much higher. The higher the compression ratio, the more power is generated.
>
> Some Diesel engines contain a glow plug of some sort that is not shown in this figure. When a Diesel engine is cold, the compression process may not raise the air to a high enough temperature to ignite the fuel. The glow plug is an electrically heated wire (think of the hot wires you see in a toaster) that helps ignite the fuel when the engine is cold so that the engine can start. According to Cley Brotherton, a journeyman heavy equipment technician, “All functions in a modern engine are controlled by the ECM communicating with an elaborate set of sensors measuring everything from R.P.M. to engine coolant and oil temperatures and even engine position (i.e. T.D.C). Glow plugs are rarely used today on larger engines. The ECM senses ambient air temperature and retards the timing of the engine in cold weather so the injector sprays the fuel at a later time. The air in the cylinder is compressed more, creating more heat, which aids in starting.” Smaller engines and engines that do not have such advanced computer control use glow plugs to solve the cold-starting problem.
This is all from [http://www.howstuffworks.com/diesel1.htm](http://www.howstuffworks.com/diesel1.htm)
And from what I gather while regular gas may ignite (probably would actually) the diesel fuel has a much higher potential energy than regular gas and therefore you may get flame but it would either underpower the motor or not move the pistons at all (probably depends on the size of the piston vs the size of the crankshaft).
Glad you asked that, I always wondered about this myself and this was a good reason to look it up.
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### Author:  [@Coldfire](https://boards.straightdope.com/u/Coldfire)
#### Post date: [January 6, 2001, 2:43pm UTC](https://boards.straightdope.com/t/diesel-engine-gasoline-fuel/48713/4 "2001-01-06T14:43:41Z")
[I think it exists already.](http://www.mitsubishi-motors.co.jp/inter/products/CARISMA/carismatext/csagdi7.html)
I’m not much of a technician, but I think the (Dutch built) Mitsubishi Carisma qualifies with its GDI (Gasoline Direct Injection) engine.
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### Author:  [@galen](https://boards.straightdope.com/u/galen)
#### Post date: [January 6, 2001, 4:25pm UTC](https://boards.straightdope.com/t/diesel-engine-gasoline-fuel/48713/5 "2001-01-06T16:25:55Z")
What I’ve always wondered about is why, if Diesels are so much more efficient than gasoline engines, do they have such dirty exhaust. Diesels blow clouds of soot, i.e., unburned carbon. I would think that this would lower the efficiency. What’s the deal here?
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### Author:  [@Una\_Persson](https://boards.straightdope.com/u/Una_Persson)
#### Post date: [January 6, 2001, 4:37pm UTC](https://boards.straightdope.com/t/diesel-engine-gasoline-fuel/48713/6 "2001-01-06T16:37:20Z")
> [@](#):
>
> \*Originally posted by galen \*
> \*\*What I’ve always wondered about is why, if Diesels are so much more efficient than gasoline engines, do they have such dirty exhaust. Diesels blow clouds of soot, i.e., unburned carbon. I would think that this would lower the efficiency. What’s the deal here? \*\*
The problem is confusing cycle effciency with the type or category of the pollution emissions. Gasoline engines produce unburned hydrocarbons as well, but they are not the long-chain, soot-producing hydrocarbons that you see in diesel engines. It is the nature of the diesel fuel itself that trends towards producing these soot particles. Gasoline engine hydrocarbons are typically of smaller quantity, true, and said emissions are often invisible (unless there is very poor tuning or engine control). But overall the diesel cycle can still be more efficient.
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### Author:  [@Una\_Persson](https://boards.straightdope.com/u/Una_Persson)
#### Post date: [January 6, 2001, 4:44pm UTC](https://boards.straightdope.com/t/diesel-engine-gasoline-fuel/48713/7 "2001-01-06T16:44:35Z")
> [@](#):
>
> _Originally posted by zen101_
> **And from what I gather while regular gas may ignite (probably would actually) the diesel fuel has a much higher potential energy than regular gas…**
Not true, unfortunately. They are very close, with gasoline being slightly higher:
Fuel Heating values for Automobiles (Una)
```auto
Higher Heating value Lower Heating value
Fuel MJ/kg MJ/kg
Gasoline 47.3 44.0
Light Diesel 46.1 43.2
Heavy Diesel 45.5 42.8
Methanol 22.7 20.0
Ethanol 29.7 26.9
Natural Gas 50.0 45.0
Propane 50.4 46.4
```
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### Author:  [@Una\_Persson](https://boards.straightdope.com/u/Una_Persson)
#### Post date: [January 6, 2001, 4:47pm UTC](https://boards.straightdope.com/t/diesel-engine-gasoline-fuel/48713/8 "2001-01-06T16:47:01Z")
> [@](#):
>
> \*Originally posted by Mr2001 \*
> \*\*So would it be possible to design an engine that compresses air, then injects gasoline, which then ignites by itself? What would the advantages and disadvantages be? \*\*
The real disadvantage would be that the burning characteristics of the gasoline would lead to a lower combustion efficiency in such a system, unless somewhat radical changes were made in combustion chamber (and even cycle) design. I read once of a six-stroke engine that used such a principle in my grad class on IC engines, but can find no reference currently.
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### Author:  [@Spiny\_Norman](https://boards.straightdope.com/u/Spiny_Norman)
#### Post date: [January 6, 2001, 5:04pm UTC](https://boards.straightdope.com/t/diesel-engine-gasoline-fuel/48713/9 "2001-01-06T17:04:49Z")
Most tanks without gas turbines are powered by diesels - and at least some of those diesels will run on quite a variety of combustibles, gasoline included.
The Russian T-72 described here: [http://www.armscontrol.ru/atmtc/Arms\_systems/Land/Tanks/T-72/T-72\_Fam\_from\_MP.htm](http://www.armscontrol.ru/atmtc/Arms_systems/Land/Tanks/T-72/T-72_Fam_from_MP.htm) had the only link I could find that specified gasoline (and rocket fuel!!), but plenty of tank engines are described as “multifuel”.
Of course, emission controls or fuel efficiency aren’t that much of a concern here.
S. Norman
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### Author:  [@Ms2001](https://boards.straightdope.com/u/Ms2001)
#### Post date: [January 6, 2001, 9:07pm UTC](https://boards.straightdope.com/t/diesel-engine-gasoline-fuel/48713/10 "2001-01-06T21:07:52Z")
**Coldfire** :
> [@](#):
>
> I’m not much of a technician, but I think the (Dutch built) Mitsubishi Carisma qualifies with its GDI (Gasoline Direct Injection) engine.
Thanks… that was exactly what I was imagining. I guess it’s just trickier to get direct injection to work with gasoline than with diesel, so it hasn’t been done until recently.
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### Author:  [@Adolph\_Peewee](https://boards.straightdope.com/u/Adolph_Peewee)
#### Post date: [January 9, 2001, 2:40pm UTC](https://boards.straightdope.com/t/diesel-engine-gasoline-fuel/48713/11 "2001-01-09T14:40:15Z")
> [@](#):
>
> \*Originally posted by galen \*
> \*\*What I’ve always wondered about is why, if Diesels are so much more efficient than gasoline engines, do they have such dirty exhaust. Diesels blow clouds of soot, i.e., unburned carbon. I would think that this would lower the efficiency. What’s the deal here? \*\*
was having a chat with my old man tonite, who’s a truckie. He drives a late model Scania 144. He’s got a mate who worked out, that on the basis of his fuel consumption versus the load he’s towing (approx 65 tonnes gross) that it would be the equivalent of a car getting around about 250 miles per gallon in so much as fuel consumption is concerned. So having said that…does this not make diesel engines more fuel efficient??
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### Author:  [@Milton\_De\_La\_Warre](https://boards.straightdope.com/u/Milton_De_La_Warre)
#### Post date: [January 9, 2001, 6:04pm UTC](https://boards.straightdope.com/t/diesel-engine-gasoline-fuel/48713/12 "2001-01-09T18:04:00Z")
I owned a Toyota Diesel Pickup for a while (like about 300,000 miles) and once lent it to my son, who mistakenly put in about five gallone of gasoline. It didn’t seem to hurt it or cause any more (or less) smoke. (It didn’t smole all that bad in the first place.) BTW: it had zero guts; 0-60 in about half a day.
I have heard about vehicles that are intentionally multi-fuel (as opposed to my old truck), too. I think a lot of military vehicles are supposed to be that way these days. Now, as to what I don’t definitely know as a solid fact: I read on a site (which I think was either Federation of American Scientists or a link thereto) that in one of these ex-Soviet republics, someone’s military confiscated the output of someone else’s liquor plant to fuel their military vehicles. I also believe that Aviation Week once described the Su-25 “Frogfoot” attack aircraft as being multi-fuel capable so that it could operate from forward areas. Anthracite or one of the other brilliant chemists/physicists might know if it is possible to run a jet engine on diesel fuel, gasoline, or even liquor. And if it woud run, would it run safely enough to fly (produce about the same power ratings as before)? Wouldn’t it smoke like a pile of tires on fire? To quote the film “Dirty HArry”: “I gots to know!”
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### Author:  [@WickedAngel](https://boards.straightdope.com/u/WickedAngel)
#### Post date: [January 9, 2001, 6:44pm UTC](https://boards.straightdope.com/t/diesel-engine-gasoline-fuel/48713/13 "2001-01-09T18:44:02Z")
slightly off topic…
My 1st car was an 1980 300 Benz, and my current car is a 1984 300 Benz. \>Dad has a thing for German cars\< Anyway, both are diesels with approximately 21-gallon tanks. Going at an average rate of say 70mph, I can get about 30 miles to the gallon. I’d say that’s somewhat efficent. Both are in great shape too, although the 84 has a better exhileration rate then the 80, but that’s back to talking about engine designs and the such…
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### Author:  [@AWB](https://boards.straightdope.com/u/AWB)
#### Post date: [January 9, 2001, 8:27pm UTC](https://boards.straightdope.com/t/diesel-engine-gasoline-fuel/48713/14 "2001-01-09T20:27:10Z")
> [@](#):
>
> \*Originally posted by WickedAngel \*
> \*\*slightly off topic…
>
> My 1st car was an 1980 300 Benz, and my current car is a 1984 300 Benz… Both are in great shape too, although the 84 has a better _exhileration_ rate then the 80, but that’s back to talking about engine designs and the such… \*\*
I’m glad your car excites you so much. Does it do this while you’re _accelerating_?
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### Author:  [@Una\_Persson](https://boards.straightdope.com/u/Una_Persson)
#### Post date: [January 9, 2001, 8:47pm UTC](https://boards.straightdope.com/t/diesel-engine-gasoline-fuel/48713/15 "2001-01-09T20:47:41Z")
> [@](#):
>
> \*Originally posted by Adolph Peewee \*
> \*\*was having a chat with my old man tonite, who’s a truckie. He drives a late model Scania 144. He’s got a mate who worked out, that on the basis of his fuel consumption versus the load he’s towing (approx 65 tonnes gross) that it would be the equivalent of a car getting around about 250 miles per gallon in so much as fuel consumption is concerned. So having said that…does this not make diesel engines more fuel efficient?? \*\*
No.
Well, I would seriously like to see what his mate actually worked out, because there is simply not that much of a difference between diesel and gasoline Otto-cycle efficiency. Not nearly that order of magnitude - we are talking about around or less than a 10% difference, IIRC from my rules of thumb from my Grad IC engines class.
Was he just working on a premise that it takes a fixed amount of horsepower to tow a 65 tonne load, and comparing that to what a car can tow, and trying to derive an efficiency from that? Because I cannot even begin to tell you why that analogy is wrong…
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### Author:  [@Adolph\_Peewee](https://boards.straightdope.com/u/Adolph_Peewee)
#### Post date: [January 10, 2001, 1:20pm UTC](https://boards.straightdope.com/t/diesel-engine-gasoline-fuel/48713/16 "2001-01-10T13:20:14Z")
But on the premise that a car can tow a one tonne gross load, it would mean that 65 cars would be towing the load.
I think what I’m trying to say here, is that in-so-much as trucks are concerned, generally you would end up with less pollution from the one DIESEL vehicle towing a load as opposed to 65 GASOLINE cars towing the load.
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### Author:  [@WickedAngel](https://boards.straightdope.com/u/WickedAngel)
#### Post date: [January 10, 2001, 6:16pm UTC](https://boards.straightdope.com/t/diesel-engine-gasoline-fuel/48713/17 "2001-01-10T18:16:29Z")
[QUOTE]
\*Originally posted by AWB \*
\*\*
> [@](#):
>
> I’m glad your car excites you so much. Does it do this while you’re _accelerating_? \*\*
the 80 doesn’t
the 84 does
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### Author:  [@Una\_Persson](https://boards.straightdope.com/u/Una_Persson)
#### Post date: [January 10, 2001, 8:04pm UTC](https://boards.straightdope.com/t/diesel-engine-gasoline-fuel/48713/18 "2001-01-10T20:04:01Z")
> [@](#):
>
> \*Originally posted by Adolph Peewee \*
> \*\*But on the premise that a car can tow a one tonne gross load, it would mean that 65 cars would be towing the load.
>
> I think what I’m trying to say here, is that in-so-much as trucks are concerned, generally you would end up with less pollution from the one DIESEL vehicle towing a load as opposed to 65 GASOLINE cars towing the load. \*\*
You see, this is the wrong premise entirely. You are mixing the analogy of how many cars it would take to pull the load with whether or not diesel engines are more efficienct than gasoline engines. As per your previous post:
> [@](#):
>
> \*Originally posted by Adolph Peewee \*
> **He’s got a mate who worked out, that on the basis of his fuel consumption versus the load he’s towing (approx 65 tonnes gross) that it would be the equivalent of a car getting around about 250 miles per gallon in so much as fuel consumption is concerned. So having said that…does this not make diesel engines more fuel efficient??**
Herein, you are mixing here the real-world towing ability of a car versus a truck, with whether or not diesel engines are more fuel efficient. That is the error. If you said “Is it more efficient to use 65 cars to tow 1 tonne each, versus one truck to tow 65 tonnes”, then the answer is an obvious “yes”. But it has nothing to do with the relative merits and efficiency of the diesel engine versus the gasoline engine. It is also, by counter-example, much more efficient to have one gasoline truck tow 65 tonnes, than having 65 diesel cars each tow 1 tonne. See?
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### Author:  [@Adolph\_Peewee](https://boards.straightdope.com/u/Adolph_Peewee)
#### Post date: [January 11, 2001, 2:56pm UTC](https://boards.straightdope.com/t/diesel-engine-gasoline-fuel/48713/19 "2001-01-11T14:56:35Z")
someone obviously has too much time on their hands!
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### Author:  [@Adolph\_Peewee](https://boards.straightdope.com/u/Adolph_Peewee)
#### Post date: [January 11, 2001, 3:03pm UTC](https://boards.straightdope.com/t/diesel-engine-gasoline-fuel/48713/20 "2001-01-11T15:03:12Z")
> [@](#):
>
> \*Originally posted by Anthracite \*
> Herein, you are mixing here the real-world towing ability of a car versus a truck, with whether or not diesel engines are more fuel efficient. That is the error. If you said “Is it more efficient to use 65 cars to tow 1 tonne each, versus one truck to tow 65 tonnes”, then the answer is an obvious “yes”. But it has nothing to do with the relative merits and efficiency of the diesel engine versus the gasoline engine. It is also, by counter-example, much more efficient to have one gasoline truck tow 65 tonnes, than having 65 diesel cars each tow 1 tonne. See? \*\*
which is exactly the analogy I was making. i have no idea what analogy your brain was imagining.
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