Choose Petrol or Diesel?

In my personal opinion, the advantages of petrol power and diesel power can be summarised as follows:

Petrol

Diesel

1. Much higher rev range, allowing for greater power output;

2. Generally more refined, in terms of engine noise;

3. Much more pleasant to listen to when worked hard.

4. Lower NOx (nitrous oxides) emissions.

1. Much greater torque levels low down in the rev range;

2. Greater economy;

3. Lower carbon dioxide emissions.

4. As fast as equivalent petrol models, in "real world" driving

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 four issues to consider:

1. Performance;

2. Economy;

3. Refinement;

4. Emissions.

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. My father and I have cars developing 150 BHP and 230 BHP, and various other manufacturers produce cars with similar or higher power outputs.

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(Note November 2007: My remarks above about petrol cars being the choice for performance may now be out of date as my father's Audi seems faster than my previous Vectra, and my 150 BHP Astra doesn't feel that far off from the Vectra - see other videos).

One aspect of diesels that is indisputable is their potential for greater levels of economy over petrol-engined models. 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.

That is, the economy advantage of Toledo over Rover was about 50%. My Astra manages about 47 MPG and my father's Audi manages about 35 MPG - not bad for a car of that power output and performance. 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 is by far the noisiest car I have driven. (My previous Vectra 3.2 was 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 am cruising in my Astra, I do not hear much engine noise.

I note that Mercedes have introduced a diesel engine into their range of S-class limousines, and that 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 does not sound at all inspiring as you approach the red line!

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 will 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 petrols 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 is the choice for you. Of course, my discussion here only touches on many of the issues to be considered, but I will simply say that I will be sticking with my diesel Astra for a while, because I love cars that are fast AND economical!