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Which are better: diesel or petrol engines?

Updated on March 18, 2011

Introduction

More than three decades ago, there was a big push to make everyone switch to diesel engines in their cars. Diesel fuel was taxed less heavily in the UK to encourage people to buy diesel cars so that they could take advantage of the cheaper diesel fuel; and the greater efficiency in terms of miles per gallon was heavily pushed. However, the diesel engine never supplanted the petrol engine, and now diesel fuel is more expensive in the UK than petrol is. So what went wrong?

The problems with diesel engines

The problem with diesel engines is that they are not the great panacea that they were claimed to be. They have a number of drawbacks, which, added together, make them generally less attractive overall, compared with petrol engines, for use in cars.

Firstly, they used to be very noisy, so ride comfort in a car was diminished by the constant sound of a noisy engine. That has been largely overcome now as an issue, as manufacturers have found ways to make them as quiet as petrol engines.

Secondly, they are not as environmentally friendly as they were initially made out to be. Their advantage comes from greater mileage to the gallon, but that is only true once they are warmed up, and cruising. For short journeys when they are still cold, they are far less efficient than a petrol engine. Additionally, their waste products are far more damaging to the environment than from a petrol car. They produce oxides of nitrogen, which are greenhouse gases 298 times more potent than CO2, and they emit particulate matter which is carcinogenic. Thus, you should only buy a diesel-engined car if you are going to be doing lots of high-mileage journeys across the countryside, where the exhaust gases have room to disperse. Driving them in traffic jams through towns is highly polluting, much worse than driving a petrol car.

Thirdly, they are slow to accelerate compared with petrol cars. They produce very high torque, but not high power, and this is why they are used in large vehicles - vans, truck, lorries - instead of petrol engines, because of their huge load-pulling capability. No one puts a diesel engine in a performance sports car, or a high performance saloon car, though, because of the poor power levels produced.

Fourthly, diesel engines tend to be heavier than their equivalent petrol engines.

Fifthly, diesel engines tend to be more expensive than their equivalent petrol engines.

Sixthly, diesel fuel is prone to waxing in very cold temperatures, so they are potentially less reliable in cold weather. In temperate countries this is not an issue, but in countries with severe winters, then it is.

Is there anything good about diesel engines?

Yes there is.  As previously mentioned, once you get them up to cruising temperature and speed, they will use less fuel than an equivalent petrol engine, but now that diesel fuel is more expensive than petrol, this is not going to save you much money, unless you are a very high-mileage driver.  And as you would have paid more for the car in the first place in order to have a diesel engine, then that also is offsetting any savings you are making.

The other good thing about them is that they tend to last "forever" compared with a petrol engine.   They just keep going on and on, mile after mile.   Again, this is only an advantage if you are a very high mileage driver, as you will be able to do several hundred thousand miles before the engine is worn out.  However, if you are a low-mileage driver, having a ten year old car with a still pretty pristine engine is no advantage, as the rest of the car is likely to be worn out, and you will trade it in for a new one.

Conclusions

If you are a high mileage driver, who spends your days pounding up and down the motorways, then a diesel car could be a good investment for you.    However, for everyone else, you are best to stick with petrol engines.

Technology advances all the time, and diesel engines are getting better and better, but then so are petrol engines!    I don't see diesel engines ever replacing petrol engines completely.  However, turbojet engines might - but that's the subject of another hub which I'm going to write!

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    • CHRIS57 profile image

      CHRIS57 5 years ago from Northern Germany

      "The World Health Organisation, in June of 2012, has classified diesel fumes as a carcinogen.."

      This is why Diesel powered cars are now a days equipped with particle filters to hold back the carcinogen exhaust. I am not sure what rules and laws apply for trucks, but everyone may have seen the dark soot coming out of truck exhausts while climbing steep hills.

      Soot, dust and fumes are a problem. Here in Germany cars and trucks without exhaust filtering are no more allowed in downtown areas of bigger cities. But then - someone smoking a cigarette is inhaling more fumes and tar than waiting hours at a bus stop on your favourite highway. Things are relative.

    • profile image
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      *Voice_Of_Reason* 5 years ago

      The World Health Organisation, in June of 2012, has classified diesel fumes as a carcinogen, so it will be interesting to see how this affects the sales of diesel cars, and indeed government regulations in many countries. Here is an extract from an article by James Gallagher, Health and Science Reporter, BBC News, from 12 June 2012. For the full article, go to : http://www.bbc.co.uk/news/health-18415532.

      ---------------------------------------------------------------------

      "Exhaust fumes from diesel engines do cause cancer, a panel of experts working for the World Health Organization says.

      It concluded that the exhausts were definitely a cause of lung cancer and may also cause tumours in the bladder.

      It based the findings on research in high-risk workers such as miners, railway workers and truck drivers.

      However, the panel said everyone should try to reduce their exposure to diesel exhaust fumes.

      The International Agency for Research on Cancer, a part of the World Health Organization, had previously labelled diesel exhausts as probably carcinogenic to humans.

      IARC has now labelled exhausts as a definite cause of cancer, although it does not compare how risky different carcinogens are. Diesel exhausts are now in the same group as carcinogens ranging from wood chippings to plutonium and sunlight to alcohol.

      It is thought people working in at-risk industries have about a 40% increased risk of developing lung cancer.

      Dr Christopher Portier, who led the assessment, said: "The scientific evidence was compelling and the Working Group's conclusion was unanimous, diesel engine exhaust causes lung cancer in humans.

      "Given the additional health impacts from diesel particulates, exposure to this mixture of chemicals should be reduced worldwide." "

      ---------------------------------------------------------------------

      This would support my assertion above that diesel vehicles are better outside of urban areas, where the fumes can disperse more easily, and petrol cars are better for people's health in towns and cities. Of course, petrol fumes are not good for you either, but diesel fumes are now officially worse!

    • profile image
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      *Voice_Of_Reason* 6 years ago

      Chris57, I've just realised that I didn't thank you for your very useful and informative comment, so I'll do it now: thank you - a very useful contribution to the debate.

      By the way, I put in a teaser at the end of the hub about turbojet engines to power cars, and said that I was going to write another hub about that. I did, ages ago, but didn't put a link here. Go to: https://hubpages.com/technology/What-is-the-most-e... to read that one.

    • profile image

      *Voice_of_Reason* 6 years ago

      Mark,

      Thank you for your comment. I actually meant in production cars, sold to the public as "sports cars". There may be very good reasons to use diesel engines in endurance racing - superior miles to the gallon springs to mind as the most obvious. Your point is useful for clarification.

      I did actually say in the hub that diesel engines are superior for long journeys.

    • profile image

      Mark 6 years ago

      The first comment about not seeing Diesel engines in Sports cars is just plain wrong - Audi and Peugeot only use Diesel engines in Sports car endurance racing and it's only diesel powered cars which win Le Monds now.

    • CHRIS57 profile image

      CHRIS57 7 years ago from Northern Germany

      There are two Nikolaus and one Rudolf that should lead you on the right track. You better go for Diesel, because whenever there is any technical progress, both Diesel and Otto engines will benefit similarly. But it is physics, better thermodynamics that always keep the Diesel engines ahead of Otto engines.

      Lets pick the first Nikolaus, sorry: Nicolas Carnot. He gave name to the ideal thermodynamic cycle which presses most mechanical energy out of thermal energy. The closer a combustion cylce gets to this cylce, the less fuel this cycle consumes.

      Rudolf Diesel created a combustion cycle that performed closer to Carnot than the "Nikolaus" Otto cycle in petrol engines.

      The closer to the Carnot cycle, the larger the cycle volume has to be to give the same power output. This is why Diesel engines tend to be a little heavier than Otto engines. For the dead weight of a car this can be neglected though. And higher shaft torque easily compensates and makes Diesel cars more agile at low to medium speed.

      The Diesel requires a high overdose of air/oxygen for self iqniting combustion. Modern engines are all equipped with turbo chargers or compressors to feed high volumes of air into the combustion chambers. After optimising the use of superchargers, Diesel engines are on the same power/weight ratio as Otto engines are nowadays.

      Of course it is up to every individual to make his choice. I made mine some 30 years ago and am driving only Diesel powered cars. I never ran into problems with waxing of fuel or such legends. In recent years efficient exhaust filters and catalyzers hold back dust, particles and nitrogen oxides.

      The carbon footprints of Diesel engines are much smaller than from Otto engines. Modern engines cannot be compared to old engines in trucks of the 70ties.

      Mercedes never sold their cars with Diesel engines more expensive that Otto powered versions, other manufacturers do though and fuel taxing leads to an optimisation calculation that gives Diesel powered cars and advantage only with more than 10 ..15.000 km/year. That is why small cars have much higher percentage of Otto engines than larger cars have, small cars do less km/year.

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