Petrol Vs Diesel

[Dr Jekyll]

How much of the extra mileage you tend to get from diesel fuel is down to a litre of petrol weighing around 700 grams, while a litre of diesel is about 900 grams?

How does the amount of energy compare?

[Krikkit]

The energy density is around 37MJ/l for petrodiesel and 34 for petrol, so 10% difference there, but the combustion method for diesel is much more efficient than petrol.

Edited by Krikkit on Thursday 12th September 09:33

[torqueofthedevil]

My mate did an engineering degree and said something about how if petrol could be ignited properly through compression (I know it does but in more of a way that can be harnessed - or something) then that would be a better engine

[torqueofthedevil]

My mate did an engineering degree and said something about how if petrol could be ignited properly through compression (I know it does but in more of a way that can be harnessed - or something) then that would be a better engine

[Simpo Two]

torqueofthedevil said:

My mate did an engineering degree and said something about how if petrol could be ignited properly through compression (I know it does but in more of a way that can be harnessed - or something) then that would be a better engine

I was thinking the same - and don't even have an engineering degree!

But of course the less fuel we use the more the tax will be increased so HM Govt gets the same loot, so no point really...

[Derek Smith]

torqueofthedevil said:

My mate did an engineering degree and said something about how if petrol could be ignited properly through compression (I know it does but in more of a way that can be harnessed - or something) then that would be a better engine

I once worked on a friend's car which had a compression ignition petrol engine. When i took the head off i found that pre-ignition burnt a hole through one piston. That is literally a hole.

I own a piston crown from an Ford DFV engine from 1971. All over the surface there are little marks from pre-ignition. It seems the main problem for petrol engines is stopping it igniting through compression.

[Mojocvh]

Simpo Two said:

torqueofthedevil said:

My mate did an engineering degree and said something about how if petrol could be ignited properly through compression (I know it does but in more of a way that can be harnessed - or something) then that would be a better engine

I was thinking the same - and don't even have an engineering degree!

But of course the less fuel we use the more the tax will be increased so HM Govt gets the same loot, so no point really...

Indeed. Some people [not the above btw] just can't understand that rather basic principle which rules fuel pricing in the uk.

[Flibble]

Dr Jekyll said:

How much of the extra mileage you tend to get from diesel fuel is down to a litre of petrol weighing around 700 grams, while a litre of diesel is about 900 grams?

How does the amount of energy compare?

The heavier weight of diesel would make it have worse mileage surely, as you're carrying more weight. Aside from that weight is unrelated as it's distance travelled per volume of fuel, not weight of fuel.

So I'm going to go with:
How much of the mileage? None.
Amount of energy: 37 vs 34 MJ/l.

The main differences are not having a throttle plate and forced induction though.

[Dr Jekyll]

Flibble said:

The heavier weight of diesel would make it have worse mileage surely, as you're carrying more weight. Aside from that weight is unrelated as it's distance travelled per volume of fuel, not weight of fuel.

No, because you are getting more diesel in a litre than you get petrol. Another way of asking original question would be 'If we bought fuel (and measured consumption) by weight instead of volume, how much advantage (Miles per KG) would diesel have?

[Flibble]

Dr Jekyll said:

No, because you are getting more diesel in a litre than you get petrol.

No you aren't; you're getting a litre of fuel each way. The fact that the diesel weighs more is irrelevant.

You get slightly more energy in a litre of diesel than petrol because the energy density is higher, but this is far from the only reason diesels have higher mpg and is in any event not directly related to the differing masses of the fuels.

Dr Jekyll said:

Another way of asking original question would be 'If we bought fuel (and measured consumption) by weight instead of volume, how much advantage (Miles per KG) would diesel have?

An odd way to ask the question since we don't buy fuel (or indeed any liquids commonly sold) by weight.

Since you asked though:

1 kg of petrol is 1.43 l by volume. So to convert petrol mpl to mpkg multiply by 1.43.
1 kg of diesel is 1.11 l by volume. So to convert diesel mpl to mpkg multiply by 1.11.

Relatively this is a difference of 1.43/1.11 = 1.286.
So comparatively petrol would gain a 28.6% advantage over diesel if we measured fuel consumption by weight instead of volume.

Edited by Flibble on Tuesday 17th September 20:11

[Gandahar]

It's not down to how much it weighs in real life but how much volume the engine burns per second to go distance X. ie how efficient the engine is. Diesel engines are just more efficient.

The weight of a tank of fuel is only about 1/20th of the weight of a car for instance and so this is negligible compared to the engine efficiency.

[Odie]

http://en.wikipedia.org/wiki/Specific_fuel_consump...

That might be worth having a look at. Diesel engines are about 10% more efficient than petrol engines(otto).

[Mr2Mike]

torqueofthedevil said:

My mate did an engineering degree and said something about how if petrol could be ignited properly through compression (I know it does but in more of a way that can be harnessed - or something) then that would be a better engine

Compression ignition petrol engines have been in development for a while. It's far from an easy task though, it makes the control strategies in a modern common rail diesel look trivial.

[marksx]

Simpo Two said:

I was thinking the same - and don't even have an engineering degree!

But of course the less fuel we use the more the tax will be increased so HM Govt gets the same loot, so no point really...

There is if you want the oil reserves to last longer.

[Watchman]

[Mr2Mike]

Watchman said:

That shows just how crap E85 "Gasohol" is compared to the proper stuff.

[Kozy]

Another reason for diesels efficiency gain over petrol is that they are unthrottled.

Throttling a petrol engine raises pumping losses which increases BSFC. Diesels simply reduce the fuel injected while the MAP remains high, reducing pumping losses and increasing BSFC.

I thought fuel was 47.3MJ anyway.

[Tiggstar]

Hi - I'm unable to create a new topic due to only just registering but I had a question.

I'm currently looking to buy a new car (2011+) preferably diesel mainly because of the MPG difference with petrol; which is all good and well because my current job means I do a 70+ mile round trip a day which takes on average 45 mins each way, 35 of that is motorway.

However, I am likely to be moving to a job where it is a 25 mile round trip, 20-25 mins in total, 10 mins of it will be on the motorway. Would I still get the benefits of the MPG and will I likely encounter a problem because with the newer diesels DPF's??

Any help/advice would be greatly appreciated

[Kozy]

Tiggstar said:

Hi - I'm unable to create a new topic due to only just registering but I had a question.

I'm currently looking to buy a new car (2011+) preferably diesel mainly because of the MPG difference with petrol; which is all good and well because my current job means I do a 70+ mile round trip a day which takes on average 45 mins each way, 35 of that is motorway.

However, I am likely to be moving to a job where it is a 25 mile round trip, 20-25 mins in total, 10 mins of it will be on the motorway. Would I still get the benefits of the MPG and will I likely encounter a problem because with the newer diesels DPF's??

Any help/advice would be greatly appreciated

Most people seem to agree that 12k/year is about the balancing point in terms of overall ownership costs between two equivalent models of either fuel.

In your case, save your sanity and buy a nice petrol.

[MiseryStreak]

Tiggstar said:

Hi - I'm unable to create a new topic due to only just registering but I had a question.

I'm currently looking to buy a new car (2011+) preferably diesel mainly because of the MPG difference with petrol; which is all good and well because my current job means I do a 70+ mile round trip a day which takes on average 45 mins each way, 35 of that is motorway.

However, I am likely to be moving to a job where it is a 25 mile round trip, 20-25 mins in total, 10 mins of it will be on the motorway. Would I still get the benefits of the MPG and will I likely encounter a problem because with the newer diesels DPF's??

Any help/advice would be greatly appreciated

DPF problems are totally model/engine dependent. There are no issues with the DPF on my Mazda 6 because it has the 2.2 but there are lots of issues with the 2.0 (Ford Mondeo) engine. Many of the issues arise due to not being replaced frequently enough and lots of short journeys where it doesn't get chance to clear and burn off particulates.

If it's your only car I would go for a petrol but only if it's a good one, no point suffering the worse fuel economy if it's a lacklustre eco stbox. There are some very good diesel engines now that are actually a better option for motorway driving, mpg aside. If you want to do any sort of track driving then a petrol is nearly always better.

Do you have a particular model of car in mind?