It used to be so easy, didn’t it? If you were interested in performance, you bought a petrol car while thrifty drivers opted for diesel.
But then it all got a bit confusing as some manufacturers launched sporty diesels that could streak to 60mph in the blink of an eye, while others promised us that their economical petrol engines could rival, or even beat, their slower, noisier opponents for economy.
Yet it isn’t just about economy because the fact that diesel exhaust fumes kill thousands of people a year is beyond doubt, but it took the Volkswagen scandal to shatter the diesel illusion and tilt the balance back in favour of petrol, possibly irrevocably.
But what are the facts? Is one conclusively better than the other? The answers are, as you might expect, not straightforward.
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Economy and general running costs
Diesel cars are generally more economical than their petrol-engined equivalents, as diesel is about 10% more thermodynamically efficient than petrol, which enables it to do more work for the same quantity of fuel, or the same work for less fuel, which is much the same thing.
However, I suspect that the differential between the two fuels will start to narrow as the research money that went into developing more fuel-efficient diesel engines – a situation that was driven entirely by the government levying a lower rate of fuel tax on diesel – is now being funneled into petrol engine development and we’re starting to reap the benefits.
Servicing costs should be slightly lower for the diesel engine owner too. This simplicity makes them more reliable too, enabling them to chalk up another win in this category.
Diesel wins its first victory.
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A diesel-engined car costs significantly more to buy than one with a petrol engine and most manufacturers suggest that 10,000 miles a year is a good indicator of which you should choose if you’re buying new; if you drive fewer miles then the lower initial purchase price makes petrol the cheaper option, even given the increased fuel consumption.
If you drive more than 10,000 miles a year, then you are probably better off buying a diesel-engined car as the running costs will more than offset the higher initial purchase price.
Of course, not everything can be simplified into financial terms, but if running costs are important to you, you might not need to read any further…
Overall, the result is a draw.
Diesel burns more slowly than petrol in the engine, which limits the number of times you can inject it into the cylinder per second; this is why diesel cars have a lower rev limit than a similarly powerful petrol engine and being able to rev higher enables a petrol engine to develop more power than a diesel of the same capacity.
However, diesel engines typically produce more torque than a similar petrol engine, making mid-range acceleration better.
Lots of torque also makes towing easier, which is why so many caravanners plump for a diesel car.
So, another draw.
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Diesel engines eke out a few more miles per gallon but produce more pollutants – especially diesel particulates and nitrogen dioxide – that are expelled in the exhaust gas. So while they score a point for being more economical, they lose it for producing more dangerous exhaust fumes, even if there are fewer of them.
While neither engine is exactly environmentally friendly, I think that the petrol engines slightly cleaner exhaust gives it a win in this category.
The smog that you see in even the most developed cities in the world, which the World Health Organisation estimates kills 29,000 people a year in the UK, comprises 70% diesel particulates. This death toll is more than ten times the number killed on the entire UK road network and is more than alcohol and obesity combined.
That’s not all. The UK Government’s Committee on the Medical Effects of Air Pollution has suggested that smog might also play a part in a further 200,000 deaths annually.
An easy win for petrol.
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So, there are answers, but none of them are easy. Diesel is, rightly, demonized for the appalling effect that it has on our health and yet petrol is no innocent either.
The reality is that if you tow a heavy caravan or cover tens of thousands of miles a year, then a car with a diesel engine might make sense. Otherwise you are probably better sticking to a petrol-fuelled car. If protecting the environment and those who live in it is your priority, petrol is the easy winner.
Or you could just buy an electric car, obviously…