Which are better: diesel or petrol engines?

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 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.

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.

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!