Why does diesel produce better fuel consumption than petrol?

[baypond]

This has really stumped me. Is it to do with compression ratios with diesel being much higher? It certainly isn't to do with octane where diesel is about 15 RON vs 95 RON for petrol.

[Dog]

It's got a higher calorific value.

[andybear]

Does that mean that hob-nobs and mars bars would be good !!

( sorry could't help it - things seem to have been getting so serious lately)

[Théière]

Diesel has only a slightly higher calorific value 10% or so but the higher compression squeeze's the mixture and causes a bigger explosion.

Diesel engines are heavier built to handle the extra compression.

Petrol engines rev quicker and easier which is generally why they are faster accelerating because of the lower compression.

Modern diesels are quickly narowing the gap and some are even better than the petrol models due to the improvements in fuel injection systems, that has also increased the fuel efficiency further.

[Chancer]

They are thermodynamically more efficient or to put it another way, more of the bang goes towards propelling the vehicle and less into creating waste heat.

Nonetheless after a century of development both petrol and diesel engines are far more efficient at heating than they are at traction.

[Théière]

[quote user="J.Rs gone native"]

They are thermodynamically more efficient

[/quote]

Remind me never to play scrabble with you [:P]

[AnOther]

[quote user="J.Rs gone native"]Nonetheless after a century of development both petrol and diesel engines are far more efficient at heating than they are at traction.[/quote]Maybe so but put into perspective, even fully laden with 4 adults plus baggage, a medium sized modern turbo diesel car will will easily top 50mpg which, put another way is 6.25 miles to the pint, or in terms I'm sure everyone can instantly relate to, a tad less than a 70cl bottle of wine [:D]

Not too bad really when you look at it that way is it ? 

[Gluestick]

[quote user="teapot"][quote user="J.Rs gone native"]

They are thermodynamically more efficient

[/quote]

Remind me never to play scrabble with you [:P]

[/quote]

If you would like yet another strange word, teapot:

A Diesel Engine provides greater Adiabatic Efficiency.

Def. The ratio of actual work output of a heat engine to the ideal output.

 

[AnOther]

Provide you operate with the optimum stoichiometric ratio of course [:P]

[Frederick]

 But what effect will it have on the coefficients of linea expansion and the specific heat ratios ?

[Chancer]

That was the word I was searching for in the depths of my memory but couldnt find G.S.

Another answer to the original question is that diesel is (still) cheaper than petrol in France.

 

[Gluestick]

[quote user="ErnieY"]Provide you operate with the optimum stoichiometric ratio of course [:P][/quote]

As the Bard might have said, Little Ern; "Aye. There's the rub!"!

Diesels, of course run ideally beneath the ideal theoretical Stochiometric

Even Lean Burn Petrol engines have only improved from the sort of ideal theoretical 13:1 in my day, to now, 14-14.5:1.

Some good stuff in these two refs.

http://franzh.home.texas.net/lean.html

http://hhopower.wordpress.com/diesel-engine-overview/

 

[Chancer]

[quote user="ErnieY"]Provide you operate with the optimum stoichiometric ratio of course [:P][/quote]

But sadly modern petrol and turbo diesel engines are not allowed to thanks to the concerted efforts and lobbying of the likes of Johnson Matthey under the guise of a green future, so we are stuck with engines that operate at the ideal conditions (lamda 1.0) for the catastrophic convertor to operate rather than maximum economy at part throttle.

But for their intervention most of us would now be driving lean burn petrol engines and not lining the coffers of the precious metal manufacturers having to buy a new cat as a consequence of minor engine fuelling malfunctions.

[Anton Redman]

The original Lotus 4 valve twin cam just got under the regs in California  by virtue of being efficient so they change the regs to make it fail.

[Théière]

Well I'm not playing any of you at scrabble now [:P]

[Gluestick]

JR Wrote:

[quote]But sadly modern petrol and turbo diesel engines are not allowed to thanks to the concerted efforts and lobbying of the likes of Johnson Matthey under the guise of a green future, so we are stuck with engines that operate at the ideal conditions (lamda 1.0) for the catastrophic convertor to operate rather than maximum economy at part throttle.

But for their intervention most of us would now be driving lean burn petrol engines and not lining the coffers of the precious metal manufacturers having to buy a new cat as a consequence of minor engine fuelling malfunctions.

[/quote]

I well remember the promises about ceramic pistons, bores, valves and head surfaces, in the early days of Lean Burn, JR.

Still waiting.............................

But not holding my breath you understand!

[Www]

[AnOther]

Will you make the tea whilst we play then teapot [:P]

The biggest con trick of all in the emissions game of cat and mouse was with the last of the MGB's which were exported to the US.

Already strangled by a single Stromberg carburettor reducing BHP to 65, to fully comply with emission standards they incorporated an air pump which simply injected air into the exhaust ports on the overrun ensuring of course that what came out of the tail pipe was within spec, farcical [:'(]

[Gluestick]

The thing I remember most about the B, Little Ern, was how in order to make it conform to the then US standards, they changed loads of stuff like the chrome bumpers etc.

And then it was pointed out that if left alone it would have passed: since its kerb weight was beneath the mandatory limit!

But, by adding all the nasty rubber impact bumpers and etc, BL took the B above the limit!

Ah me...............

Back to bothering to actually read the regs right through from cover to cover.

 

[AnOther]

Good old BL doing what they were best at then Gluey, shooting themselves in both feet !

Apart fom making some truly cr*p cars of course [blink]

[baypond]

I will never remember all this !

Teapot, game of snap ?

[just john]

And it took the little freelancer Costello to make the B a good car[8-|]

[Théière]

The "B" is only a two seater morris minor [:D]

[Iancharlton]

I remember when I bought my first car, my Gran saying "I've always wanted to know how these things work". So I explained, in very simple terms, how an engine worked.

She gave me a blank look, so I said "It's all done by science, Nan". She replied "That's all right then". Bless her little cottons.

[Gluestick]

[quote user="teapot"]The "B" is only a two seater morris minor [:D][/quote]

Is that a Scrabble answer, teapot?

[:D]

How d'yer work that out?

[8-)]

[Chancer]

Two seater Austin/Morris Oxbridge more like.