When does Diesel become more efficient - a quick query

dulpit

Hey all, I've heard differing theories/explanations regarding when a Diesel car becomes more efficient. Some have told me it's if you are driving a certain milage, others have said it has to be if you do long drives.

What I'm wondering is if you do a lot of milage, but it's over short distances (7 - 10 miles, which can be repeated 6 - 8 times a day on occasion - which is the better engine type to have?

I'd go diesel if I was you but only if I was spending less than €10,000. I would get a DPF equipped diesel unless I was going on a longer run regularly. Most diesels would return 40+mpg on the runs you describe no bother at all. Most petrols won't unless they are super mini type yokes.

I do less than 200 miles a week 3 out of every 4 weeks ish and a diesel version of my car would save me €30/week on fuel. Admittedly my car only does 20mpg ish.

-Chris-

Diesel is always more efficient, but it takes a certain amount of mileage to compensate for the higher purchase price and potential bigger bills.

Based on the short trips you're doing, I'd go with petrol.

EPM

On shorter trips the extra efficiency of a diesel is minimal. If you are starting from cold, it could take 10 minutes before it reaches the mid level and your journey could be almost over by then. My diesel is actually less economical for the first few miles compared to the petrol I had before it.

MugMugs

EPM wrote: »

On shorter trips the extra efficiency of a diesel is minimal. If you are starting from cold, it could take 10 minutes before it reaches the mid level and your journey could be almost over by then. My diesel is actually less economical for the first few miles compared to the petrol I had before it.

Is your vRS Diesel ?

EPM

MugMugs wrote: »

Is your vRS Diesel ?

Yeah:o

But only because I do 1500km a week:pac:

EI_Flyboy

Driving a 99 C250 2.5 turbodiesel Merc, I could get approx 44mpg or around 600miles out of one tank doing mixed driving. It would take about 2 mins to warm up, never going above 2000rpm, driving slow enough to rarely have to use the brakes. The question is, have you the patience to drive like a granny!? I had to do this because even tipping the brakes put the engine into emergency mode.

ShadowHearth

Petrol in your case.

You are looking at turbo diesel, which means that it will have a turbo which needs to be heated up. Constant on and off with short trips will destroy it.

Petrol - you pay more for mpg, but less on repair bills
Diesel - you pay less for mpg, but you more on repairs.

EI_Flyboy

EI_Flyboy wrote: »

.......... I had to do this because even tipping the brakes put the engine into emergency mode.

What's this emergency mode you speak of?

EI_Flyboy

RoverJames wrote: »

What's this emergency mode you speak of?

Emergency or safe mode, it's when the EMS or engine management system senses there's something amiss, it limits performance to a minimum to avoid any or further damage. It's usually triggered by doing burnouts, dumping the clutch, any hooning around that would void a warranty. In my case, the engine would not go above idle once it kicked in, which was good for about 40mph on a flat road in top gear. This was after having the engine replaced, the guy who replaced it didn't have the necesary diagnostic equipment to reset the EMS.

Oh and the reason Diesel gets higher economy is because Diesel fuel has a higher calorific value, i.e. it contains more energy, then petrol. Modern engines are a lot more reliable then 10 or 15 years ago so repairs are less of a worry. Peugeot Diesels have good reputation and are used by many other manufacturers, their auto gearboxes too. An old 406 or 306 is a great runner if you can get them in good condition. I do most of my own servicing, buy most supplies online and use the truck fuelstation on the old airport road for extra savings, Tesco Diesel dosen't give as good mileage.

Max Power1

EI_Flyboy wrote: »

Modern engines are a lot more reliable then 10 or 15 years ago so repairs are less of a worry.

I had to laugh at this part

10-15 years ago was IMO the peak of motoring developments.

Now we have gone too far (DMF/DPF etc)

kermitpwee

EI_Flyboy wrote: »

Modern engines are a lot more reliable then 10 or 15 years ago so repairs are less of a worry.

No offense mate but you have that arseways. Diesels made 10-15 years ago are way more reliable then any modern diesel and will cost a lot less in any repairs.

MugMugs

Max Power1 wrote: »

EI_Flyboy wrote: »

Modern engines are a lot more reliable then 10 or 15 years ago so repairs are less of a worry.

I had to laugh at this part

10-15 years ago was IMO the peak of motoring developments.

Now we have gone too far (DMF/DPF etc)

Isn't DPF a revolution?

El_Flyboy, you're speaking of "limp mode" ?
Isn't safe mode what happens windows when you watch a lot of p0rn ?

YouTookMyName

MugMugs wrote: »

El_Flyboy, you're speaking of "limp mode" ?
Isn't safe mode what happens windows when you watch a lot of p0rn ?

Either Or. Something goes into "Limp Mode"!

EI_Flyboy

Max Power1 wrote: »

I had to laugh at this part

10-15 years ago was IMO the peak of motoring developments.

Now we have gone too far (DMF/DPF etc)

Engines and cars are now being designed as throw away units, diagnostics and repairs are being simplified to cut on training costs. But I share your laugh, I too find it humerous when someone displays their ignorance

I advocate doing research on any model you think of buying, reliability, maintenance and repair costs, safety issues, customer satisfaction, resale value, etc. I think that's only sensible.

conzy

EI_Flyboy wrote: »

Oh and the reason Diesel gets higher economy is because Diesel fuel has a higher calorific value, i.e. it contains more energy, then petrol.

Petrol has a much higher calorific value than diesel, that's why similar size diesel engines need turbo charging to produce an equivalent power output. The main reason diesels are more efficient is because you spend a lot more time at lower revs and most of them are turbocharged which also adds efficiency if driven economically.

EI_Flyboy

conzymaher wrote: »

Petrol has a much higher calorific value than diesel, that's why similar size diesel engines need turbo charging to produce an equivalent power output. The main reason diesels are more efficient is because you spend a lot more time at lower revs and most of them are turbocharged which also adds efficiency if driven economically.

Diesel fuel 46 MJ kg−1
Petrol 44.8–46.9 MJ kg−1

Hmmm... I'm more right than you!

conzy

Diesel 44,800 kJ/kg

Petrol 48,000 kJ/kg

I got my infos here must be wrong..

But the small difference in calorific content is not the primary factor behind the increased efficiency.. its the stuff I said

eagerv

If I remember correctly Diesel contains more energy per litre than petrol.

Interesting article http://en.wikipedia.org/wiki/Fuel_efficiency

mrroboito

EI_Flyboy wrote: »

I too find it humerous when someone displays their ignorance.

Good work commenting on someone else's ignorance dick but learn to spell.

MugMugs

eagerv wrote: »

If I remember correctly Diesel contains more energy per litre than petrol.

Interesting article http://en.wikipedia.org/wiki/Fuel_efficiency

And soon us Oily Diesel vermon will take over the world with our smoke blowing power plants.

EPM

mrroboito wrote: »

Good work commenting on someone else's ignorance dick but learn to spell.

Banned for personal abuse.

pajo1981

eagerv wrote: »

If I remember correctly Diesel contains more energy per litre than petrol.

Interesting article http://en.wikipedia.org/wiki/Fuel_efficiency

Yes, diesel is denser than petrol and contains more energy per volume.

EI_Flyboy

conzymaher wrote: »

Diesel 44,800 kJ/kg

Petrol 48,000 kJ/kg

I got my infos here must be wrong..

But the small difference in calorific content is not the primary factor behind the increased efficiency.. its the stuff I said

You may well be right, calorific value differs even from one supplier to another. Quality is a very important factor in fuel when it comes to Diesel injectors and may be one reason it never caught on in America as the quality of their forecourt Diesel can be dodgy.

The main place I got the idea that Diesel fuel has a higher value was from the first transatlantic flight of a model airplane which used a wee engine modified to operate on the Diesel principle because it's much more economical, they said the fuel has a higher calorific value weight for weight. It actually used kerosene or "stove fuel."

Diesel engined light airplanes running on Jet-A1 do enjoy significant endurance gains over their Av-gas brothers but getting injectors to work reliably when used with lower quality jet fuel was the main problem in their design.

SilverBell

conzymaher wrote: »

Petrol has a much higher calorific value than diesel, that's why similar size diesel engines need turbo charging to produce an equivalent power output. The main reason diesels are more efficient is because you spend a lot more time at lower revs and most of them are turbocharged which also adds efficiency if driven economically.

The reason a diesel engine is more efficient than a petrol engine is due to the higher compression ratio. Nothing to do with revs.

Petrol engines would be as efficient if they had the same compression ratios, but they cant because of pre-ignition.

Jimmy Bottlehead

Just to add to this, my (frankly, pretty dense) sister is going to sell her '06 Corsa, and is strongly considering buying a newer Diesel Corsa.

Her mileage is roughly 8,000 miles per year.

*sigh*

EI_Flyboy

SilverBell wrote: »

The reason a diesel engine is more efficient than a petrol engine is due to the higher compression ratio. Nothing to do with revs.

Petrol engines would be as efficient if they had the same compression ratios, but they cant because of pre-ignition.

It could also be due to some of the petrol in the not fully burnt fuel/air mix being expelled as exhaust for cooling purposes. Petrol engines don't utilise a stochiometric mixture, they run rich most of the time, as it would burn too hot and damage the engine, the valves in particular.

Diesels on the other hand run lean to prevent soot in the exhaust so even though it has a marginaly better calorific value, all the Diesel oil is burnt up in the combustion process.

SilverBell

EI_Flyboy wrote: »

It could also be due to some of the petrol in the not fully burnt fuel/air mix being expelled as exhaust for cooling purposes. Petrol engines don't utilise a stochiometric mixture, they run rich most of the time, as it would burn too hot and damage the engine, the valves in particular.

Diesels on the other hand run lean to prevent soot in the exhaust so even though it has a marginaly better calorific value, all the Diesel oil is burnt up in the combustion process.

Yes, in practice, the petrol engine wastes a bit. Under load, the diesel engine also cannot combust fully, and wastes fuel too.

In the basic air standard analysis, the higher efficiency of a diesel engine compared to a petrol engine is down to its higher compression ratio. I cant find my 3rd year diesel engine notes but found these ones from Plymouth Uni which explain the same thing.
http://www.tech.plym.ac.uk/sme/THER205-web/IC-ENGS1.PDF