Petrol or Diesel? Your opinion matters.

[Dieselman]

Diesels don't work too well from cold. How many drivers actually wait for
for the glow plugs before going?.

???????????????
What has that to do with economy?

According to Bosch technical manual 1987722104 revision 1999.
A 1.1 litre petrol engine of 37kw output and a 1.5 litre diesl of 37kw output.
The graph clearly shows the fuel consumption of the diesel engine to be nearly liinear over the first 10 km of a journey from a cold start, whereas the graph for the petrol engine is very steep initially and gradually becoming more linear.

Sample reference points:
1km the diesel has used 0.06 litres, the petrol 0.27 litres.
2km, diesel = 0.1 litre, petrol = 0.38 litre
3km, diesel = 0.16 litre, petrol = 0.47 litre
4km, diesel = 0.2 litre, petrol = 0.53 litre
6km, diesel = 0.28 litre, petrol = 0.62 litre
8km, diesel = 0.35 litre, petrol = 0.72 litre

If you look at the first 1km you will see the consumption to be 4 times as great in the petrol engine, at 3km this has dropped to 3 x the consumption, at 6km it is 2.2 x the consumption, and at 8km the petrol consumption is twice that of the diesel.

From this and from actual driving I deduce that particularly for short journeys especially from a cold start the diesel is significanlty more efficient then a petrol engine of the same output.

There are two reasons for this, firstly a petrol engine requires a richer mixture for cold starting due to condensation of the fuel, whereas a diesel does not, and secondly a diesel engine produces more torque so the stop start nature of town driving has less impact on fuel economy. Diesels always score against petrols in town driving due to higher efficiency. This is partly due to the petrol having throttle pumping losses which are reduced once cruising at higher speed.

Maybe Bosch got it wrong.

As a quick check to the relative economy think how quickly does your car warm up after a cold start? 1 mile or so, 2 miles, 5 miles, 10 miles?

 

[glojo]

Diesels don't work too well from cold. How many drivers actually wait for
for the glow plugs before going? My E starts at 20 to 24 mpg over the
first mile or so and then starts improving. I've never seen it reach 80
to 96 mpg (it hits 35 on gentle long runs) so whatever the mixture
strength might be on a cold start the actual impact on real world
consumption isn't 4x.

Are you really trying to tell me that you get payback over 6K miles in
an average diesel?

If I was willing to pay a bit more for a higher performance E then my
choice would be a diesel 280 or 320 but that's based on flexibility
(outright performance or economy based on how it is used) rather
than unsentimental economics.

However in terms of raw economics I reckon that there a lot of diesels
out and about that are bought based on false assumptions about cost
of ownership.

I note you have a 211, all I will say is at the time I bought my 211 which is a 320CDI it was over £1000 CHEAPER than the 320 petrol engined equivalent E-class. I wonder how much these 2004 vehicles are selling for now?

I read what your saying, but unfortunately I don't understand it.

Diesel can be a pain if it spills onto the fuel pump, but even worse is if it gets on the concrete floor by the pump. Getting diesel onto your shoes is a mega pain in the butt. I have told my wife to look before she approaches a pump. If the floor is oily then walk away.

Always, always carry some spare disposable gloves. ALWAYS.

John

 

[Bobby Dazzler]

Dieselman knows his onions - well, cars.

 

[Dieselman]

Most people I know (friends/family) who buy diesels don't really work out the costs properly. ('It saves me money on long journeys'. Yes it does. But if you only do 6000 miles a year of three holiday trips of 1200 miles and the rest is pottering about within a 3 mile radius of your driveway then you're not thinking it through properly).

I dont want to labour this too much but take the 3 mile journey senario.
By following the graph and taking the 6km mark which is 3.72 miles we can see that the consumption of the petrol is 2.5 times that of the diesel.

Take 6000 miles at say 40mpg for the diesel, that gives 16mpg for the petrol.
This equates to £630 for the diesel cost and £1500 for the petrol cost.

As you can see this equates to £870 difference over the year.

I have chosen reasonable figures but given the relationship of fuel consumption if you reduce the mpg figures the situation becomes worse for the difference in fuel cost.

Also dont forget that lots of short journeys creates a situation known as bore wash and oil dilution in a petrol engine. This will prematurely wear the engine.

This is why it was always said better to buy a high mileage motorway car than a low mileage shopping car.

 

[Dryce]

???????????????

As a quick check to the relative economy think how quickly does your car warm up after a cold start? 1 mile or so, 2 miles, 5 miles, 10 miles?

I reckon on 1 mile to a mile and a half is my car's norm. Figures I've seen elsewhere usually say the car consumes double when cold which fits with my experience.

Following link gives some figures for cold start.

http://www.green-car-guide.com/news/official-mpg-v-real-life-mpg.htm

They don't match the Bosch numbers. Different test conditions / driving style? My experience doesn't match the Bosch numbers either.

The greater torque argument is countered by the fact that you have to push the car to benefit from it. I doubt most drivers use more than a given % of the engine's capability most of the time. So just because the engine can deliver X lb/ft doesn't mean the driver uses it in stop start driving.

I'm not arguing diesels are bad, just that ownership costs are not actually being measured properly by many owners.

 

[Bobby Dazzler]

The greater torque argument is countered by the fact that you have to push the car to benefit from it. I doubt most drivers use more than a given % of the engine's capability most of the time. So just because the engine can deliver X lb/ft doesn't mean the driver uses it in stop start driving.

But isn't that the very reason that diesels have a fair turn of pace in real world driving??

(1) Because diesels have relatively more torque than a petrol; and

(2) It's all low down in the rev range ready and waiting to be used at normal engine speeds.

Meaning the diesel offers greater flexibility due to the fact the torque is there immediately without waiting for it to build as revs rise.

 

[Dryce]

I
Take 6000 miles at say 40mpg for the diesel, that gives 16mpg for the petrol.
This equates to £630 for the diesel cost and £1500 for the petrol cost.

As you can see this equates to £870 difference over the year.

I have chosen reasonable figures but given the relationship of fuel consumption if you reduce the mpg figures the situation becomes worse for the difference in fuel cost.

Frankly I don't think you've chosen reasonable figures.

What I would expect (and you may accuse my figures of being no less reasonable) is that if I saw 40mpg from the diesel I would see 30mpg from the petrol car.

That shifts the fuel cost to £630 vs £800. A difference of only £170.

 

[Dryce]

But isn't that the very reason that diesels have a fair turn of pace in real world driving??

(1) Because diesels have relatively more torque than a petrol; and

(2) It's all low down in the rev range ready and waiting to be used at normal engine speeds.

Meaning the diesel offers greater flexibility due to the fact the torque is there immediately without waiting for it to build as revs rise.

I think this is true (especially in the last few years).

But it's not an argument for economy -- that comes down to the amount of energy in the fuel, it's cost, and the ability of the engine to convert it efficiently. (And how you drive it.)

 

[Dryce]

I note you have a 211, all I will say is at the time I bought my 211 which is a 320CDI it was over £1000 CHEAPER than the 320 petrol engined equivalent E-class. I wonder how much these 2004 vehicles are selling for now?

Well in my experience the used market the diesels do rather well compared with their petrol brethren.

 

[Robbo]

Also dont forget that lots of short journeys creates a situation known as bore wash and oil dilution in a petrol engine. This will prematurely wear the engine.

Just a thought, I believe diesels take longer to warm up than petrols, if one is doing lots of 4-5 mile trips and the diesel doesnt warm up properly, is there a chance of condensation build up in the engine?

Take 6000 miles at say 40mpg for the diesel, that gives 16mpg for the petrol.
This equates to £630 for the diesel cost and £1500 for the petrol cost.

I dont think your figures are reasonable. I've never seen 16mpg in mine even on my 5 mile trip to work through some traffic (from cold), I usually get 22mpg on that trip. I suspect an equivalent diesel engine (say a 270CDI) would do about 30-35mpg. In the petrol A class I have got 38mpg on that trip!

Diesels are probably the way to go though, even though i do wonder about the long term running costs of modern CDI engines with their propensity to need replacement injectors and turbo replacements are not exactly unheard of either.

 

[Deleted member 2227]

Basically this arguement seems to boil down to, if you do loads of miles buy a diesel and a pair of gloves, if you dont then buy a petrol and then you wont sound like a tractor from cold!

What I want is a hybrid C Class....Please Mr MB!

 

[glojo]

Well in my experience the used market the diesels do rather well compared with their petrol brethren.

You've missed my main point. The 320CDI was cheaper to buy when new if we look directly at the E320 petrol. there is no paying extra for the benefits of the diesel. Being more economical when compared to the petrol equivalent is a bonus, having better residuals than the petrol speaks even louder when thinking about what is the better buy. I fully accept in other brands, models etc this might not be the case, but you own an E-class and are using that as an example. I was anti diesel until we tried this new generation CDI technology. We have now owned numerous different Mercedes diesel powered vehicles and would unlikely go back to petrol.

Each to their own, but don't knock it till you try it.

John

 

[KLP 92]

Here's my 2p worth.
I had a W202 C230 (non kompressor) estate first. I then changed to a C250 TD estate so i can comment accurately as i've owned 2 cars rated with similar horsepower,

Firstly the petrol model struggled to give me 280 miles per tank whereas the diesel model gives 400+ miles.

Secondly the power of the diesel makes it feel alot quicker than the petrol, power's always there low down, don't have to wait to build up revs as on the petrol.

The petrol is quieter on idle, smoother when cold, but once the diesel is warm its not really noticable and once driving its fine.

Also on a full load the diesel doesn't struggle in terms of performace as the petrol did.

Lastly the diesel value is significantly more, yes in terms of used value it cost more, but then again when it sells its worth more.

I have no regrets buying a diesel, i would choose a MB diesel over a petrol equivalent any day of the week. The only time i would buy a petrol is if i wanted a V8/V12 or an AMG version.

 

[hawk20]

And now there is the coingestion charge argument to add in favour of diesels. (see separate thread for details) My S320cdi has emissions below band G. BUT many petrol engines of only 2 litres will be in band G. Note that band G cars will cost £25 per day to drive anywhere in the Congestion charge area, from 2009. Being band F will be £8 per day. A powerful argument for diesel in the capital.

And don't forget when pricing roads at so much per mile the Government plans to charge more for higher emissions cars, just like Mr Livingstone.

 

[Robbo]

thats why my plans to get a Nissan 350Z have been shelved in favour of a big diesel from either MB or BMW.

 

[DieselE]

You've missed my main point. The 320CDI was cheaper to buy when new if we look directly at the E320 petrol. there is no paying extra for the benefits of the diesel. Being more economical when compared to the petrol equivalent is a bonus, having better residuals than the petrol speaks even louder when thinking about what is the better buy. I fully accept in other brands, models etc this might not be the case, but you own an E-class and are using that as an example. I was anti diesel until we tried this new generation CDI technology. We have now owned numerous different Mercedes diesel powered vehicles and would unlikely go back to petrol.

Each to their own, but don't knock it till you try it.

John

The economics of buying a new E320CDI in 2004, when compared to its petrol equivalent were unassailable. For £1000 less you got the same 0-60 performance as the E320, mid-range overtaking ability only matched by the E500 and much better economy. Also depreciation was much lower. If you bought it on the standard 16" wheels, it was even in a lower road tax band. Unfortunately, MB seem to have realised how much of a bargain it was because that price differential is no longer there.

For somebody making the same purchase decision and still looking at the 2004 cars, the price advantage will now be with the petrol version but all the other diesel advantages are still there.

 

[Dryce]

Basically this arguement seems to boil down to, if you do loads of miles buy a diesel and a pair of gloves, if you dont then buy a petrol and then you wont sound like a tractor from cold!

And the number of miles that constitutes 'loads' is currently lower when buying new.

 

[big x]

Frankly I don't think you've chosen reasonable figures.

What I would expect (and you may accuse my figures of being no less reasonable) is that if I saw 40mpg from the diesel I would see 30mpg from the petrol car.

That shifts the fuel cost to £630 vs £800. A difference of only £170.

I think Dieselman may have been drinking at the office party ...16 mpg indeed.
Using the tank to tank method I got 24 mpg over 1 year running a BMW 740i, an old 4 litre V8 in a heavy 1800 Kg body. I do lots of short 2 mile trips into my local town and the occasional 160 mile trip up to London.
My C180 comes in at around 35 mpg and an old diesel golf at 42 mpg.



adam

 

[big x]

Meaning the diesel offers greater flexibility due to the fact the torque is there immediately without waiting for it to build as revs rise.

Not always.The new BMW 335i Coupe produces peak torque at only 1300 rpm and holds that right through the rev range.

295 lb ft at 1300-5000 rpm
302 bhp @5800

adam

 

[Mactech]

Diesels can be thirsty!

I had the privilege of engineering this diesel in August. It only did 9mpg but then it also did over 350mph. Diesel technology is advancing at a much greater rate than petrol. I would not try to keep up with that speed on the road, but I would with the technology and I am getting over 40mpg from my diesel Mercedes...