13/09/2006 00:00 | By By MSN cars

Petrol or diesel?

 (© image © PA Motoring)

The debate at the pumps has been raging for years. But how do you choose between petrol and diesel power?

Bring up this subject and you will quickly realise that there are no grey areas in drivers' minds on the matter of petrol versus diesel. As both diesel and petrol technology improves, the difference increasingly comes down to personal driving preferences.

 (© image © empics)

Petrol v Diesel, it can cause arguments

One group will insist that petrol is far better, largely because petrol cars are faster, quieter and cheaper. Diesel fans will, of course, highlight the vastly better fuel consumption plus the improved torque from a diesel engine, which gives good pulling power and less gear changing, making driving in everyday, rather than race-track, conditions a pleasant experience. There are also the muddy waters of exhaust emissions, with strong arguments on both sides of the fence.

So what is the objective truth? Diesel engines inherently use less fuel than petrol cars, and so will undoubtedly save you money in the long run. This is particularly true in town driving, if you make a lot of short runs, or if you do towing, where the differences are even more marked. On long trips at motorway speeds the differential closes. Diesel fuel in the UK is usually priced close to unleaded petrol, so figure out the difference in hard figures comes down to a straight MPG comparison. However, if you travel a lot through France, you'll find that diesel is approximately two-thirds the price of petrol, enhancing the savings even further.

 (© image © PA Motoring)

Ideal for diesel: Off-roaders - economy and low-down power pays huge dividends

There is also a commonly held belief that diesel engines will run for much higher distances than petrol engines before giving any trouble. Whether this is actually true of today's sophisticated engines is arguable, but a consequence of it is that there is a keen market for diesels that have covered more than 75,000 miles than for similar petrol-engined cars. It means that if you are a high mileage driver, a diesel car will be easier to sell when the time comes, than a petrol version.

There are disadvantages to owning a diesel car too. One is the noise, which despite many advances in recent years, is always louder when starting from cold and is certain to be noticed by all your neighbours on a winter's morning. A related problem is vibration - rarely a real problem today, but always more noticeable on cold starting than with an equivalent petrol engine.

Manual or auto

Then there is the performance. A few years ago it was fair to say that a diesel offered poor performance compared with a petrol engine of a similar size. That's still true of what are referred to as normally-aspirated diesels - those without turbo-chargers. But virtually all new diesels cars today are turbo-charged which gives them particularly healthy performance in the mid-range. The result is that a modern turbo-diesel has the sort of get-up-and-go in fourth and fifth gear that knocks a petrol engine for six and is particularly useful for motorway driving.

A "Common rail injection" Diesel engine

A "Common rail injection" Diesel engine

But diesel engines do vary quite markedly between manufacturers, so it is vital to look at the MSN Cars Research Centre to be sure you are buying one of the better engines. The buzz words of the new millennium are `direct inject' and common rail'. Direct injection diesels give improved mpg over the older indirect injection models but are inherently noisier. Some manufacturers have dealt with this noise problem better than others. Common rail diesels are the latest and best application of direct injection technology, offering better performance and fuel economy; not everyone has developed these yet.

 (© image © PA Motoring)

Petrol engines still give more you more bang for your buck

Petrol-engined cars still usually give more horsepower for any given size, which usually means a higher top speed and a better 0-60mph acceleration figure - although even this is not always the case with some of the most modern new diesel engines. Those `headline figures' can be misleading, though - to get the best performance you have to drive a petrol engine hard, and sometimes the power and flexibility of a diesel engine at low engine speeds can make it more pleasurable to drive. It also means that it can be virtually as quick in practical conditions on the road as its petrol equivalent. However, petrol engines undoubtedly offer better refinement than a diesel, and crucially, they are cheaper. And in the end, the argument often comes down to cost. It's great to drive a family car with a diesel engine that returns 45mpg, knowing that the petrol equivalent would struggle to reach 35mpg. But against that has to be weighed the cost of buying the car. Diesels are almost inevitably more expensive, in some cases substantially so.

You should also take into account servicing costs. These used to be higher for diesels, though with today's engines they tend to be similar to petrol costs. It's worth noting that proper servicing is essential if you want to keep your engine in good condition - without it, the economy and performance will decline and you may suffer an expensive turbocharger failure. As with petrol engines, it's also important to make sure that your car's camshaft drive belt is changed if necessary - a failed belt can cause very expensive damage to your engine.

Alternative fuel cars

To be fair, there is a reason for a slightly higher purchase price of diesels - a turbo-charger costs something, after all. Car manufacturers know that buyers are prepared to pay a bit more in order to save money in the long run, and they are very happy to take advantage of that fact.

The exception to this is off-roaders, where the diesel may be a similar price to the petrol version. Although torquey diesels are highly suited to off-road driving and their economy advantage is keenly felt in these inherently thirsty vehicles, the problem is that the diesel 4x4 will inevitably be slow. It is a source of continued surprise how buyers of large expensive off-roaders are prepared to put up with performance they would hate if it applied to a family saloon. But that's the diesel-petrol argument in a nutshell - there is as much emotion in the choice as there is logical argument.


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