The Bigger Bang: Diesel or Petrol? --Research

[Mark]

Here Diesel fuel is about 5p more than petrol. With Diesel cars typically £1,000 more to buy.

[D-Type]

I think it comes down to how many miles you do. A diesel will give you better fuel consumption. But, and it's a big but, despite the manufacturers' best efforts a diesel is simply not as refined as a perol engined car. And frequently, although the torque (or power) at low revs is better with a diesel, they normally fall down on power (or torque) at higher revs and they don't rev so freely. The last was a big disadvantage when I had an automatic Golf diesel. The auto simply didn't match the engine.

[Mark]

The auto was the problem. Not the Diesel, for which you really need a manual transmission. As you do with all engines of course but for a Diesel it's essential.

[ioan]

That's strange that you had issues with a diesel with automatic transmission. The higher torque provided by the diesel makes it very very flexible in terms of transmission settings.
The multitronic A4 diesel I had until last year was a dream to drive and the 2 liter diesel in the Espace is also doing great with the automatic transmission even though the car weighs close to 2 tons.

Anyway I agree that it come down to how many miles and in what kind of circumstances you use it.
I travel mostly by plane and usually take the car for short distances and mostly within the city, as such the torquey diesel has an advantage with the stop and go nature of the traffic, however an electric/petrol hybrid car would be even better and more environment friendly especially given that most of the electricity produced here is from renewable sources. The only problem is the price.

[D-Type]

Yes. a diesel produces higher torque (and power) at lower revs than a petrol engine. So as long as you are in the right rev band all is well. A diesel engines "sweet spot" is at lower revs than a petrol engine's.

The trouble is that sensors for the automatic transmission appear to work on revs (it may be on the negative presure in the inlet manifold, but the effect is the same). So, as the revs drop it changes down a gear. And the revs for the changes seemed to be optimised for a petrol engine. The car always felt it was in rthe wrong gear. It didn't matter around town or on a motorway but on twisty country roads it was a disaster.

Road tests I have read seem to say that the only manufacturer to get to grips with the issue and match an auto transmission to a diesel was Citroen a few years ago and they have since discontinued the model. I think, from a faint memory, that the "good" auto diesel was the BX.

[ioan]

Well, if it was the BX than it was more than a few years ago, more like over 10 years ago.
I drove two automatic diesels for quite a few thousands of kms and they were both excellent, maybe they changed the way they are actuating the gear changing.
The A4 it was a double clutch one so maybe that explains why it was doing exceptional, but the Espace is just a normal automatic gearbox and is really doing fine.

[Malbec]

[url=http://www.bbc.co.uk/news/business-18017591]I'm still 3-4 years off changing my car, and I wonder at the time will it be better for me to get another Diesel, or a small turbo petrol engine?

I had this conundrum a year ago. Bought a Skoda Octavia estate and had a choice of a TSi turbo/supercharged petrol or a Diesel. Plumped for the 1.4 TSi in the end as we drive mostly in town and the petrol would be so much nicer to use and not that bad for economy. As for cost, Skoda were offering 20% off for petrol but not Diesel and we'd have taken 4 or 5 years to recoup the difference in cost (the equivalent Diesel was about £1k more expensive without the discount) so simply wasn't worth it. Ford has just released a turbo triple that should be very similar to the TSi, and FIAT has their multi-air engine though not sure whether thats turbo or not. I'll be quite happy with any of them.

I've test driven lots of Diesels in small cars and I simply don't find them pleasant to drive, but then again I'm accustomed to very revvy engines so I'm probably quite biased.

[Mark]

Agreed. That many peoples view of Diesels are outdated. The only difference is the sound at idle and they produce more low down torque. And that's it really.

[Malbec]

Agreed. That many peoples view of Diesels are outdated. The only difference is the sound at idle and they produce more low down torque. And that's it really.

Never driven an old Diesel so I wouldn't really know. However I find the narrow power band and discernable though small turbo lag inferior to that of most petrol engines, and if you drive one hard you can notice the effect of the extra weight in the nose, both through handling and ride quality.