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I've now owned three diesel cars and a number of petrol cars. In my personal opinion, the advantages of petrol power and diesel power can be summarised as follows:
The disadvantages of the diesel engine can of course be deduced from the advantages cited for the petrol engine, and vice versa. In comparing the relative merits of petrol and diesel, there are at least four issues to consider:
Taking the issue of performance first, many of us will have memories of old-technology diesels and we have all seen lorries and buses - and lots of cars - holding up lines of traffic. However, the latest diesels with turbochargers and intercoolers and other technological aids cannot be put into this class. The Audi A4 3.0 TDi puts out about 230 BHP, which isn't too bad! The very latest diesels put out even more than this.
So, diesels no longer lose out to petrol in the numbers game of power output. Having said this, at the upper extremes of performance, petrol cars will always have the potential to develop a greater power output, simply because of their ability to rev higher. Coupled with lower gearing, the highest performance motors will always be petrol cars (in my opinion). However, diesels produce much greater levels of torque in their optimum rev range (say 2000 RPM to 4000 RPM). This makes for much more relaxed driving, and greater levels of acceleration within this rev band. So, if you want outright performance, petrol is for you. Conversely, if you favour relaxed driving, then diesel would be the choice.
(Note April 2009: My remarks above about petrol cars being the choice for performance may now be out of date as my current Audi seems faster than my previous V6 petrol Vectras, and my 150 BHP Astra didn't feel that far off from the Vectras either - see other videos for some comparisons of performance between these cars).
One aspect of diesels that is indisputable is their potential for greater levels of economy over petrol-engined models of similar power output. This stems in part from the greater thermal efficiency of diesel engines (as a consequence of their higher compression ratio). The economy advantage depends of course on your driving style, but let me give you my experience. If I drove my SEAT Toledo gently, I could expect to see about 52 MPG. My previous car was a petrol-engined Rover 420, developing 136 BHP. Driven gently, I would be delighted to see 35 MPG. My Vectra 3.2 used to manage about 30 MPG with gentle driving. The Audi 3.0 TDi averages in the high 30's in mixed driving, and will give 45 MPG on a gentle cruise.
My Astra managed about 47 MPG, which I found slightly disappointing - though the car I had after that, a 2.8 V6 Vectra, could only manage about 27 MPG overall. Of course, the amount of money you save in fuel costs will depend on how many miles you drive per year. The more you drive, the greater the saving. While on the subject of saving money, it is worth noting that insurance costs (in the UK) for diesels are generally lower than for a similarly priced petrol model. However, the cost of a brand new diesel is often greater than the equivalent model, and of course diesel fuel (in the UK) is more expensive than petrol. Therefore, to find out if you will be better off driving a diesel, a fairly complex calculation may be required!
Having stated that the economy advantage of a diesel is undisputed, it seems (to me) equally clear that petrol engines are generally quieter than most diesels. My Astra was by far the noisiest car I have driven. (My petrol V6 Vectras were easily the quietest). When reading road tests of diesel cars, most testers seem interested in the amount of noise generated when the car is first started up in the morning. Most testers note that Volkswagen diesels are noisy, but that they do quieten down when the engine is warm. To be fair, if I was cruising in the Astra on the motorway, for example, then I did not hear much engine noise. The Audi 3.0 TDi is very quiet when warmed up, but can be a bit noisy when first starting on a cold morning.
A few years ago, Mercedes introduced a diesel engine into their range of S-class limousines, and most people agree that the refinement in this application is up to limousine standard. Nevertheless, I think it is fair to say that your average diesel will be slightly less refined than your average petrol. For those who like noise, particularly the sound of a howling sports car, diesels are not for you. My Astra did not sound at all inspiring as you approached the red line, though the Audi invariably sounds good, especially at around 3,000 RPM.
The arguments about the relative merits of petrol cars and diesel cars with respect to exhaust emissions is a complex subject and is currently ongoing. In the minds of the public, diesels seem inherently more dirty because you can actually SEE the emissions emerging from the exhaust pipe, in the form of grey or black smoke. Except on a very cold morning when the car has first been started, you don't see very much from the exhaust pipe of a petrol-engined car. But don't be deceived - the emissions are there alright. Its just that the particulate matter emitted by the exhaust is too small to see.
Without going into all the arguments currently being debated, it seems that two clear truths have emerged. First, petrol-engined cars emit more carbon dioxide than diesels. Secondly, diesels emit higher NOx emissions than petrol engines. Carbon dioxide in the atmosphere is of concern because of the greenhouse effect and the consequent climate changes that could result from excess levels. NOx gases have a more immediate effect on humans, leading to breathing difficulties and other bronchial effects. Consideration of the ill-effects of the various gases emitted is further complicated by the fact that uptake of particulate matter in the lung depends critically on the size of the particulate matter inhaled. I think it is fair to say that both diesels and petrol cars produce particulate and gaseous emissions, and that further research is required to reduce emission levels for both types of engine.
To summarise then, if you like relaxed, economical motoring, but are not too concerned about ultimate refinement, then a diesel may be the car for you. On the other hand, if you prefer outright performance with refinement, but are not too fussed about fuel economy, then petrol may be the choice for you.
Of course, my discussion here only touches on many of the issues to be considered, and is only my personal opinion, but I will simply say that I will be sticking with my Audi A4 3.0 TDi for a while, because I love cars that are fast AND economical!