What is the difference between gas and diesel?

[burythehatchet]

and what is different in the combustion chambers of engines that use each type of fuel?

[stopbush]

1. Check Wikipedia or How Things Work

[Captain Hilts]

2. Flash point of diesel is MUCH lower. Safet to handle, etc. nt

[LostinVA]

11. Yeah, it's the exact same thing as heating oil (just with a few additives)

You can put it in your oil tank for your furnace. I've put a few gallons in before myself.

[Captain Hilts]

22. I mean the flash point is much HIGHER!!!!

[Winterblues]

3. About $2.00 per gallon

:shrug:

[LostinVA]

13. Less than a dollar now, thnak God.

It's only 30 cents away from where it was for three years until last Fall. Hopefully, it'll drop to closer to $1.85-2.00.

[phantom power]

4. In a gasoline engine...

the fuel/air mixture has to be ignited with a spark-plug. In a diesel engine, the fuel/air mixture is ignited purely by temperature increase from adiabatic compression.

[LostinVA]

9. glowplugs

[TheWraith]

5. Diesel is a thicker, more simplistic type of fuel

It's easier to refine than gas, can be produced from a variety of sources, and provides better torque and fuel efficiency. It's also less prone to explosion than regular gas. However, it's rather prone to gelatinizing in low temperatures, making it difficult to use in northern climates, and traditionally it's contained a lot of sulfur, though that's been changing.

[BobRossi]

6. I'll try.

Gasoline engines have low compression ratio combustion chambers, on the order of 8:1 whereas diesel engines are high compression ratio. > 10:1. Gas engines require a spark to ignite the air/fuel mix in the chamber, diesel engines relay on the mixture self-igniting due to the high compression. (All things heat up when under great pressure). Diesel fuel has a higher energy density than gasoline so in theory it is more efficient.

See http://auto.howstuffworks.com/diesel3.htm for more.

[Puzzler]

17. I'm no mechanic...

... but I'm pretty sure that diesel compression ratios are much higher than 10:1 . 15, 18:1 is more the norm.

(it's not uncommon for a modern gasoline engine to be 10: 1 or more)

[slackmaster]

7. Diesel = bigger molecules, requires much higher compression

And no spark to make it go off once the correct temperature is reached.

[Selatius]

8. Diesel engines don't need spark plugs to ignite fuel that is injected into the chambers.

Petrol engines do need spark plugs. The high compression ratio in diesel engines is what causes the fuel to ignite. A side-effect of the high compression is better fuel economy, typically 30% or better compared to a petrol engine of the same size. It's one reason why now 50% of all cars sold in Europe are diesel-based. It's sensible to meet rigorous fuel efficiency standards in Europe.

I'll let the others fill in the other blanks for you.

[LostinVA]

10. The difference is I get 45-55 mpg with my VW

[burythehatchet]

16. If my next car isn't the Civic Hybrid it will be the Jetta TDI

[LostinVA]

18. Do you drive more highway or in-town?

If less start-and-stio, the TDI is the way to go!

The only downside: it takes longer to be able to put the heat on in the Winter -- about 10-15 minutes when it's really cold (15-20 degrees).

[burythehatchet]

19. I hardly drive at all anymore because I ride my Vespa everywhere

I need the car for the coldest days and for my son. I think the electric hybrid is best for my environment, though having driven a Golf GT for several years and BMW's for many years I really love the German driving experience.

[LostinVA]

20. I love the DSG "no clutch: manual transmission that feels like automatic!

[tammywammy]

21. I average between 42-46 mpg

And get heated seats, that helps when it's cold outside. :)


The new Jetta TDI is really clean, you can hold a white towel over the exhaust and rev the engine and it'll still be white.

[LostinVA]

12. There is also no carb, and basically very few moving parts

[Ganja Ninja]

14. Diesel is a much denser fuel than gas.

It produces more energy per gallon than gas. Gasoline in more volatile and burns more easily. A typical gas engine has a compression ratio of around 10 to 1 while a diesel's compression ratio is around 22 to 1. Gasoline engines do not compress the fuel/air mixture to the point of combustion. They use a spark plug to ignite the mixture. A diesel will compress the fuel/air mixture to the point where it will burn without a spark. Although some will use a glow plug for easier starting they don't necessarily need one once they're warmed up. The heat and density of the oxygen in the compressed air is all a diesel needs to set off combustion.

[elifino]

15. BTU content gasoline/diesel

# Gasoline: US gallon = 115,000 Btu = 121 MJ = 32 MJ/liter (LHV). HHV = 125,000 Btu/gallon = 132 MJ/gallon = 35 MJ/liter

* Metric tonne gasoline = 8.53 barrels = 1356 liter = 43.5 GJ/t (LHV); 47.3 GJ/t (HHV)
* gasoline density (average) = 0.73 g/ml ( = metric tonnes/m3)

# Petro-diesel = 130,500 Btu/gallon (36.4 MJ/liter or 42.8 GJ/t)

* petro-diesel density (average) = 0.84 g/ml ( = metric tonnes/m3)

# Note that the energy content (heating value) of petroleum products per unit mass is fairly constant, but their density differs significantly – hence the energy content of a liter, gallon, etc. varies between gasoline, diesel, kerosene.

[ThomWV]

23. A couple of errors here and another difference that hasn't been mentioned

First the error. It is not true that with a diesel the air/fuel mixture is compressed causing the heat which is required for combustion. If that were the case the engines wouldn't last very long at all before the tops of the pistons were blown clean out of them. What actually happens is that air is compresssed with no fuel present until - and past - the point where it will burn and then at the correct time (engine timing, which means at the correct point of crankshaft rotation) the fuel is injected and then burns more or less immediately.

Here is the other big difference that was not mentioned. With a gasoline engine, even a fuel injected one, the incomming air flow is controlled, generally with some sort of butterfly valve. With a diesel engine there is no attempt made at control over the input airflow (although compressors of various sorts are often used to increase the flow) and power production is controlled as a function of how long the injector is held open up to its service duty limit.