Fuelling the great petrol or diesel debate

The diesel or petrol dilemma gives real fuel for thought with today’s hi-tech, modern engines. Motoring editor Andy Russell explains why.

Caption: The 1.0-litre turbo petrol EcoBoost engine powers more than 40% of Fiestas sold in the UK.

Whenever people question me about whether they should buy a diesel the first question I ask is how many miles they do a year.

Over the years various figures have been bandied around about how many miles a year you need to do to make the extra outlay of deciding on diesel pay dividends.

But now there is another reason why mileage has to be taken into consideration – one that can have serious consequences on the longevity and reliability of the diesel engine itself.

As the EU puts more and more stringent emissions regulations on modern cars, car-makers are having to crank up the technology to meet them.

The result is that modern diesel cars – now meeting Euro V and VI emissions standards – are fitted with diesel particulate filters or DPFs for short.

These particulate filters do exactly what it says on the tin – they trap particles of soot to prevent them being released in the atmosphere. This soot is then burned off when the vehicle is driven at a constant speed for a decent distance or time when the exhaust gets really hot.

If the filter needs to ‘regenerate’ itself, to prevent it getting partially blocked by soot, but the exhaust is not getting hot enough to burn off the collected soot the engine’s electronic control unit (ECU) initiated the regeneration process.

The ECU will inject extra fuel into the system to trigger regeneration by increasing the exhaust temperature. If the journey is a bit stop and start or you take your foot off the accelerator while regeneration is taking place, it may not complete the process. The AA says it should be possible to start a complete regeneration and clear the DPF warning light by driving for 10 minutes or so at more than 40mph.

If regeneration is unsuccessful the extra fuel injected will not burn and will drain into the sump. This means the oil level will rise and quality will deteriorate. and the danger is that it will rise above the maximum level on the dipstick which can cause serious damage.

It sounds scary and certainly caused me some concern when the mileage on my wife’s diesel car dipped considerably when our sons bought their own and mum’s taxi was no longer required.

And it certainly gave some fuel for thought when it came time to change our car. With our mileage down to no more than 7,000 a year petrol was the best bet.

That was a couple of years ago but now car-makers are making their petrol offerings even more appealing with small-capacity turbo petrol engines that give diesel-like economy and low CO2 emissions so are not only kind to the environment but also kind on the wallet with no annual road tax to pay.

Because petrol cars warm up faster than diesel ones they are better suited to short trips as well as generally being priced lower and petrol costs less than diesel.

Most major car-makers now offer such engines but let’s take Ford’s 1.0-litre three-cylinder EcoBoost engine because it goes into a range of cars including the UK’s best-selling Fiesta, where it accounts for more than 40% of sales, and the Focus and has won international engine of the year two years running – no mean achievement.

In 100PS guise – there’s also a 125PS version – it’s as powerful as a non-turbo 1.6-litre petrol engine and has official combined economy of 65mpg with 99g/km of CO2. By comparison, the new 1.5-litre Ford turbo diesel engine has an output of 75PS, official combined economy of 76mpg with 98g/km of CO2.

And the EcoBoost petrol Fiesta is £500 less than the equivalent diesel version to buy. Even if the MPG is not quite as good, £500 buys a lot of fuel especially when it costs seven or eight pence a litre less at the pumps.

Twitter: @andyrussellauto

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