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Author Topic: What is the difference between diesel fuel and petrol (gasoline)?  (Read 128225 times)

Offline neilep

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In this country (UK) Diesel is more expensive than petrol (gasoline !...sheesh!! Ok US peeps!!)...but...you get many more miles to the gallon with diesel than petrol.

me is not a mechanic or a petro chem engineer..me is a wooly four legged trans-gender sheep.

Is it possible (in a john and jane fashion) to explain the differences between the two fuels and also the mechanics of the different engines !

How far would I get if I put petrol (gas...sheesh !!) in a diesel engine ?...and the other way round ?

I suppose I could wiki the answers but that would negate the premise of this forum and me wants to know the answers.

Spank Ewe

neil
xx

[MOD - Subject edited for clarity - CS]
« Last Edit: 27/05/2008 22:13:06 by chris »


 

Offline Karen W.

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Deisel is like kerosene and it is not as explosive as gas, so it doesn't have as much energy potential as petrol, whereas the gas has a higher energy potential.

If you put diesel into a regular gas powered engine, it would probably run crudy if at all!  I believe that I have that the right way!
 

Offline elegantlywasted

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Ha! A friend of mine did this, very funny.. and expensive.

First of all the nozzle sizes are different, diesel gas will not fit into a regular engine, but a regular gas (petrol) pump will fit into a diesel tank.


Now when that happens  you will be able to drive for a few clicks, then the engine will shudder then die. And it will cost you lots of money, lots.


Now in a bit of research I found this site http://www.straightdope.com/mailbag/mdieselvsgas.html It explains everything in more detail.


It basically explains the difference between the two types of engines. Gasoline is burned in a spark ignition system. Essentially the vapour of the gas and air mix, compress and then a spark is added and we have combustion... and exhaust. Now a diesel engine is burned in a compression ignition engine. Where air is drawn into the engine and the fuel and air compress (higher than regular fuel) and it self ignites.


Now from what I understand, and I am no mechanic (i've only dated a few) gasoline going into a diesel engine is a far worse (regular gas doesn't self ingnite) so you wont go anywhere and possibly will ruin your fuel pump and you will need to have all of the fuel lines in your car flushed out.


Diesel going into a gasoine engine will cause you have to flush the lines, but if you haven't put a large amount of diesel into the fuel tank the car should run, it may however shudder alot and run kindo of crappily (is that a word?) But if you have added alot or diesel, expect your car to stop, and a bill for mucho expensive car repairs.]
 

Offline eric l

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Sometimes a little amount (less than 5 %) of gasoline is added to diesel fuel to prevent parafin gelling in winter times.  This can only count as an emergency measure !
Diesel engines generally draw in air, rather than a mix of air and gasoline.  The compression heats up the air, and the fuel is injected into the combustion chamber as the piston nears the top dead center.
Under certain conditions, gasoline (petrol) engines can "diesel", which means they do not need a spark to ignite (the residual heat in the spark plug will be enough to ignite the compressed and hot mixture).  If this happens, even shutting off the engine will not stop it.  Unlikely to happen with modern engines, and almost impossible when you have fuel injection rather than mixing air and gasoline (petrol) in a carburetor. 
 

Offline Karen W.

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Good Job you two....I have a post around here somewhere where we were discussing the differences in the two fuels and types of engines.. both me boys are mechanics... and hubby drives a big rig and knows a great deal about these engine...
 

Offline daveshorts

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Both petrol and diesel are hydrocarbons, they are mae up of hydrogen and carbon. Hydrogen and carbon make up chains eg:

CH3-CH2 - CH2 ...

Although the chains are not allways straight, they can have brances and rings etc.

the big difference between the two is that petrol has 5-10 carbon atoms in it where as diesel has 17-20. this has various effects the first being that because diesel has longer chains they tangle and stick together better so it has a higher boiling point - it doesn't evaporate so quickly. This also means it is not nearly as flamable, because you have to evaporate the fuel to get it well mixed with air in order to burn. Hence you can drop a lighted match into diesel and it won't burn.

This difference in property makes how you ignite the two fuels in an engine different.
In a petrol engine you compress the air an fuel a bit, in order to get more of it in the cylinder then use the spark plug to cause ignition firing the piston down the cylinder.

With diesel a spark is not nearly enough to ignite it, what you do instead is compress the fuel and air very violently causing it to heat enormously and ignite.

There are two reasons diesels get a better mpg than petrol.
1. the fuel is denser so you get more of it in a litre.
2. The high compression cycle is in fact intrinsically more efficient.

 

Offline Karen W.

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 Cool Dave..Could you explain the compression cycle a tad bit??
 

Offline neilep

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Deisel is like kerosene and it is not as explosive as gas, so it doesn't have as much energy potential as petrol, whereas the gas has a higher energy potential.

If you put diesel into a regular gas powered engine, it would probably run crudy if at all!  I believe that I have that the right way!

Thank Ewe Karen mam...very helpful indeed

YAYYYYYYYYYYYY !!
 

Offline neilep

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Ha! A friend of mine did this, very funny.. and expensive.

First of all the nozzle sizes are different, diesel gas will not fit into a regular engine, but a regular gas (petrol) pump will fit into a diesel tank.


Now when that happens  you will be able to drive for a few clicks, then the engine will shudder then die. And it will cost you lots of money, lots.


Now in a bit of research I found this site http://www.straightdope.com/mailbag/mdieselvsgas.html It explains everything in more detail.


It basically explains the difference between the two types of engines. Gasoline is burned in a spark ignition system. Essentially the vapour of the gas and air mix, compress and then a spark is added and we have combustion... and exhaust. Now a diesel engine is burned in a compression ignition engine. Where air is drawn into the engine and the fuel and air compress (higher than regular fuel) and it self ignites.


Now from what I understand, and I am no mechanic (i've only dated a few) gasoline going into a diesel engine is a far worse (regular gas doesn't self ingnite) so you wont go anywhere and possibly will ruin your fuel pump and you will need to have all of the fuel lines in your car flushed out.


Diesel going into a gasoine engine will cause you have to flush the lines, but if you haven't put a large amount of diesel into the fuel tank the car should run, it may however shudder alot and run kindo of crappily (is that a word?) But if you have added alot or diesel, expect your car to stop, and a bill for mucho expensive car repairs.]


YAYYYYYYYYYYYY !!

Thank EWE Meg...and me appreciates the excellent link also...me knows stuff now ..YAYYYYYYYY !!
 

Offline neilep

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Sometimes a little amount (less than 5 %) of gasoline is added to diesel fuel to prevent parafin gelling in winter times.  This can only count as an emergency measure !
Diesel engines generally draw in air, rather than a mix of air and gasoline.  The compression heats up the air, and the fuel is injected into the combustion chamber as the piston nears the top dead center.
Under certain conditions, gasoline (petrol) engines can "diesel", which means they do not need a spark to ignite (the residual heat in the spark plug will be enough to ignite the compressed and hot mixture).  If this happens, even shutting off the engine will not stop it.  Unlikely to happen with modern engines, and almost impossible when you have fuel injection rather than mixing air and gasoline (petrol) in a carburetor. 

Excellent Eric...Many many thanks indeed !!....as always..a font of knowledge....YAYYYYYYYYYYY !!!
 

Offline neilep

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Both petrol and diesel are hydrocarbons, they are mae up of hydrogen and carbon. Hydrogen and carbon make up chains eg:

CH3-CH2 - CH2 ...

Although the chains are not allways straight, they can have brances and rings etc.

the big difference between the two is that petrol has 5-10 carbon atoms in it where as diesel has 17-20. this has various effects the first being that because diesel has longer chains they tangle and stick together better so it has a higher boiling point - it doesn't evaporate so quickly. This also means it is not nearly as flamable, because you have to evaporate the fuel to get it well mixed with air in order to burn. Hence you can drop a lighted match into diesel and it won't burn.

This difference in property makes how you ignite the two fuels in an engine different.
In a petrol engine you compress the air an fuel a bit, in order to get more of it in the cylinder then use the spark plug to cause ignition firing the piston down the cylinder.

With diesel a spark is not nearly enough to ignite it, what you do instead is compress the fuel and air very violently causing it to heat enormously and ignite.

There are two reasons diesels get a better mpg than petrol.
1. the fuel is denser so you get more of it in a litre.
2. The high compression cycle is in fact intrinsically more efficient.



This is wonderful Dave...THANK EWE so much....don't suppose you have any idea how hot Diesel gets to ignite under those conditions ?...are we talking about a lot lot lot hotter than petrol ?...does that mean Diesel engines run a lot hotter too ?
 

Offline daveshorts

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If you compress a gas it gets hotter, If you have ever pumped up a bicycle tyre enthusiastically you may have noticed the pump getting hotter..

Apparantly petrol will burn with a flame at -45 C but diesel will not do so until 65 C which is a big difference. This is why you couldn't ignite diesel with a spark unless you really pre-heated it.

Interestingly diesel will spontaineously ignite with no flame at 210 C but petrol not until 265 C, so a diesel probably wouldn't ignite if you filled it with petrol, and vice versa.
 

Offline lightarrow

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It basically explains the difference between the two types of engines. Gasoline is burned in a spark ignition system. Essentially the vapour of the gas and air mix, compress and then a spark is added and we have combustion... and exhaust. Now a diesel engine is burned in a compression ignition engine. Where air is drawn into the engine and the fuel and air compress (higher than regular fuel) and it self ignites.

Now from what I understand, and I am no mechanic (i've only dated a few) gasoline going into a diesel engine is a far worse (regular gas doesn't self ingnite) so you wont go anywhere and possibly will ruin your fuel pump and you will need to have all of the fuel lines in your car flushed out.
Diesel going into a gasoine engine will cause you have to flush the lines, but if you haven't put a large amount of diesel into the fuel tank the car should run, it may however shudder alot and run kindo of crappily (is that a word?) But if you have added alot or diesel, expect your car to stop, and a bill for mucho expensive car repairs.
Diesel fuel has a much lower Octane Number than gasoline. They infact have to behave in an opposite way: gasoline have to resist compression together with air as much as possible, without spontaneously ignite. Diesel have to ignite when compressed with air, as easily as possible. In the case of Diesel fuel this caracteristic is called Cetane Number.

If you put diesel fuel in a gasoline engine, it will ignite too early before the piston arrives at his maximum point, and this will cause an increase in the top temperature of the piston, fusing it; or can cause detonation, which can destroy the piston. This is worse than replacing pipes and fuel pump, believe me! However, modern engines have a lot of sensors to monitor the correct functioning, and this will not probably happen easily.

Anyway, your explanation of the two different kind of engines was very good, my compliments to you!

By the way, why are you "elegantlywasted"?
« Last Edit: 31/01/2007 18:31:50 by lightarrow »
 

Offline lightarrow

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This is wonderful Dave...THANK EWE so much....don't suppose you have any idea how hot Diesel gets to ignite under those conditions ?...are we talking about a lot lot lot hotter than petrol ?...does that mean Diesel engines run a lot hotter too ?
As Dave said, Diesel engines have a higher compression rate, because the higher it is, the greater the thermodynamical efficiency (gasoline cannot be compressed so much together with air, but Diesel engines compress air without fuel!).
So, Diesel engines run, intrinsically, hotter than gas engines; but they usually run at lower rpm, with a bit more fuel than what is required and have heavier pistons; these 3 things prevent them to get too hot.
« Last Edit: 31/01/2007 18:34:22 by lightarrow »
 

Offline neilep

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THANK YOU ALBERTO...and THANK YOU DAVE.


I really appreciate the explanations. Very helpful...and because of you..I now know.
 

Offline ukmicky

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The problem with desiel is less energy is released when it ignites ,so deisel cars have less power, so there drivers tend put their foot down a bit more and in the end they end up filling up more than petrol car owners.  And on top of that they polute more than petrol driven cars.
 

Offline neilep

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The problem with desiel is less energy is released when it ignites ,so deisel cars have less power, so there drivers tend put their foot down a bit more and in the end they end up filling up more than petrol car owners.  And on top of that they polute more than petrol driven cars.

Is that a fact ?...me did not know this !!

The premise behind this thread is because I am doing silly miles a week seeing my clients...and my car ?...a petrol guzzling  automatic 7 seater people carrier !!!.....my car takes 60 average to fill....surely there must be some compromise in pollution if I can do a lot less fuel stops and also save me money. I am thinking seriously about getting just a really small diesel...Toyota Yaris...or even  hyundai getz (with a 5 year warranty thrown in).

whajafink matey ?
 

Offline Karen W.

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Hey what does the hyundai  look like.. I have a huge gas guzzler also..8 passenger mini van.. I like my gas guzzler as far as comfort goes, but gas stinks.. LOL
 

Offline neilep

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Hey what does the hyundai  look like.. I have a huge gas guzzler also..8 passenger mini van.. I like my gas guzzler as far as comfort goes, but gas stinks.. LOL

The Hyundai is a just a really small car..I need it to be small car. I need it to be small for all the miles I do...and parking is hard with a people carrier. We don't have any of the space you have Karen mam.....But it does need to carry my exhibition cabinet and banner....

ho hum !!
 

Offline Karen W.

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Is that big enough to carry your things..? Mine only seats one more then yours
 

Offline lightarrow

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The problem with desiel is less energy is released when it ignites ,so deisel cars have less power, so there drivers tend put their foot down a bit more and in the end they end up filling up more than petrol car owners.  And on top of that they polute more than petrol driven cars.
Sorry Michael but I think what you say is not correct.
Diesel engines have a greater torque than petrol engines with the same cc. So, If they could run at the same rpm, their power would actually be greater. Infact they actually releases more energy than petrol engines when the same amount of air/fuel mix ignites, because of the greater intrinsic efficiency.
To compare how much fuel a Diesel consumes against a petrol fuel, we should have two engines with the same cc and the same weight of the cars.

Having said this, I personally hate Diesels, however, because of how much they pollute. Have you ever noticed that strange, bleach-like smell coming from modern diesel's exaust? If you haven't, try to do it. That is the smell of nitrogen dioxyde: NO2. Have you ever heard of the poison carbon monoxide CO? Ok. NO2 is 500 times more poisonous. And it's also carcinogenic. Nano particles of the smoke are carcinogenic too. Why these informations are not revealed? When will governments stop being paid from car companies and will tell us the truth?  [:(!]
« Last Edit: 02/02/2007 13:25:01 by lightarrow »
 

Offline Karen W.

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JEESE LOUISE THAT'S SCARY! I had no Idea it was that much higher in poisons.. and carcenogens.. Yikes. I knew they all held them but that is a lrge amount over the others!
 

Offline daveshorts

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Modern diesels are really nice, I drove a ford Focus estate about 6 years ago and it drove like a nice petrol engine (unless you are possibly a racing driver) and used very little fuel.

On a similar note, about 5 years ago I went on a science tour with a long wheelbase transit, and a petrol mondeo  estate, and despite the fact the van is huge and aerodynamiclly far worse they both cost the same in fuel.. do the maths.
 

Offline neilep

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I think Diesel is the way...and the engines are getting better and better...after all..if they were really that bad they would be banned wouldn't they ?...or am I being naive ?
 

Offline eric l

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I remember the days when the rotary piston engine (Wankel-engine)was heralded as the engine of the future.  What has happened to it ? 

At another moment, gasturbines were "te engine of the future" - it must have been in the late 1960's that a gas turbine powered car was racing at Le Mans.  Never heard of it again.  If i'm not mistaken, the latest winner at Le Manswas a Dieselpowered Audi.
« Last Edit: 03/02/2007 15:43:24 by eric l »
 

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