What car?

DUTR

Ultrasonic wrote: »

But those economy figures a crap compared to a decent diesel. I've got more than 60 mpg on long runs, and have averaged 55 mpg on all driving over the whole of the last year (including a 9 mile each way daily commute with lots of roundabouts to be slowed down for).

Driving style obviously has a big impact on fuel economy. It may be that if you buy a diesel to drive aggressively then you may as well buy a petrol (I don't know), but for people who buy a diesel for better fuel economy and drive accordingly I still would have thought they'd be better doing this than buying a petrol.

You missed out the essential part though, the laws of physics rule, two vehicles of similar power output but of different fuel types eg petrol and diesel with the same driving style will return similar MPG.
The figures I posted were round town, on a run from Birmingham to Preston at legal speeds my Golf returned 39mpg where as my friend on the same run with a diesel edition returned 42mpg, considering he was paying more for the diesel/litre me thinks the GTI required less ££s for the round trip.
I'm not suggesting deisel is rubbish and petrol is good, each variant will suit the owner's requirement. :beer:

Forgot to add, if typically deisel owners are returng often 60mpg , why are they complaining about fuel prices, as it would take a good amount of price change before any significant in miles/£ is noticed, eg £10/1.38 litre returns 95 miles for the tenner, if fuel suddenly jumped to £1.45/L then the driver would still get 91 miles for the tenner, next to no difference really

cepheus

DUTR wrote: »

You missed out the essential part though, the laws of physics rule, two vehicles of similar power output but of different fuel types eg petrol and diesel with the same driving style will return similar MPG.
The figures I posted were round town, on a run from Birmingham to Preston at legal speeds my Golf returned 39mpg where as my friend on the same run with a diesel edition returned 42mpg, considering he was paying more for the diesel/litre me thinks the GTI required less ££s for the round trip.
I'm not suggesting deisel is rubbish and petrol is good, each variant will suit the owner's requirement. :beer:

Sorry but you don't know much about the laws of physics!

There are many factors involved here, but the main one is that Diesels work at a higher compression ratio, which means more energy is extracted from the fuel (for conversion into mechanical work) during expansion of the gases.

Another important factor is that energy isn't being wasted throttling the air during intake with a Diesel.

The former is particularily important on Motorways, the latter during town driving.

mikey72

cepheus wrote: »

The fuel consumption difference isn't that great. Figures are usually exaggerated but both to a similar extent............

Diesel £6.31 a gallon, our C3 does 60 to 70mpg, so about 10p a mile to move. Petrol £6.04 a gallon, our Zafira, does 25 to 35mpg, so on average costs about 20p a mile to move. 10p a mile difference in the two. So that's £500 more ayear for 5000 miles, £1500 a year more for the 15000 we do in the C3, or a grand total of £2500 a year more for the op.

worried_jim

DUTR wrote: »

I will believe diesel is as good as petrol when I see diesel cars next to petrol in motor sports

Please refer to post 8.

DUTR

worried_jim wrote: »

Please refer to post 8.

I did see the picture in post #8, is that car featuring in the Monaco GP this afternoon?

getmore4less

DUTR wrote: »

You missed out the essential part though, the laws of physics rule, two vehicles of similar power output but of different fuel types eg petrol and diesel with the same driving style will return similar MPG.

Diesel contains more energy than Petrol

By your reasoning a gallon of wood would give the same MPG

cepheus

getmore4less wrote: »

Diesel contains more energy than Petrol

By your reasoning a gallon of wood would give the same MPG

Yeah for that reason as well.

PS the wood might be even more smoky! It can be done though



Wood gasifier on a Ford truck converted to a tractor

Ultrasonic

DUTR wrote: »

I did see the picture in post #8, is that car featuring in the Monaco GP this afternoon?

I'm pretty sure the R10 would break one or two of the FIA regulations .

DUTR

getmore4less wrote: »

Diesel contains more energy than Petrol

By your reasoning a gallon of wood would give the same MPG

Not sure what you are saying there, what I meant in the post and as others have pointed out, if you drive a deisel hard, it will be as poor economy as petrol, if you drive a petrol frugally , it will be near as economical as a deisel, what others are pointing out is there is little difference in running costs between the two for all but highest annual milages.
eg I fill up every two weeks , swapping a 2 litre GTI for a half the power A3 1.4 diesel would save me 4.4 gallons a month for the same journeys, however the extra £5ish a week fuel premium for the use of twice the BHP and ability to overtake a tractor or bus is justified for me, I couldn't do much with the £20 per month or even £240 per year

Ultrasonic

<This is pedantic but I feel professionally obliged given the posts above - feel free to completely ignore!>

getmore4less wrote: »

Diesel contains more energy than Petrol

The energy density of Diesel is higher than petrol if given in units of energy per unit volume, whereas energy density could equally well be given in units of energy per unit mass in which case that of petrol is higher. The former is the one of relevance to fuel economy measured in mpg though. See here:

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

p.s. I am a physicist. The detailed operation of Diesel engines is however a matter of engineering, where I defer to cepheus' excellent earlier post.