Most drivers’ relationship with diesel fuel begins and ends with the garage fuel pump. It’s what they put in their diesel cars. It’s dirty, smelly, oily stuff (it’s sometimes called diesel oil) which is why garages have rubber glove dispensers so you can avoid getting your hands dirty as you fill your car.
It’s dangerous, too, because it can lie on the forecourt without evaporating as petrol quickly does, making the surface slippery. Motorcyclists are always looking out for the telltale rainbow appearance of oil on the road when it’s sloshed out of overfilled fuel tanks on trucks.
So it’s nasty stuff but it’s also quite brilliant. Three reasons: it has a higher energy density than petrol (basically, it’s more powerful for a given quantity) so can power your car for longer, it can be used in an engine without a spark plug (unlike a petrol engine which needs several) and it can be obtained from a variety of sources.
What is diesel fuel?
Most diesel is distilled from crude oil by heating the oil to separate out its various component fuels, among them diesel. As a member of the petroleum fuel oil family it is also known as petrodiesel. It’s the most common type of diesel.
However, there’s another type called biodiesel produced not from crude or fossil oil but from vegetable oil, in particular rapeseed oil.
However, very few manufacturers recommend or permit its use in their cars, at least in undiluted form. Some do permit it at levels of 5% of standard diesel but you should check with your car’s manufacturer before using it.
How does diesel do its job in a car engine?
A diesel engine is like a petrol engine in that both have pistons that go up and down inside cylinders, and breathe and exhaust via holes in the top that are alternately opened and closed by valves.
The explosion that provides the power to turn the pistons takes place between the top of the piston and the top of the cylinder.
In a petrol engine it’s triggered by a spark igniting a mixture of petrol and air. In a diesel, it’s caused by the air being compressed by the rising piston. This heats up the air so that when the diesel is forced into the compressed space via the fuel injector, the mixture instantly combusts.
However, when the incoming air is really cold, for example in winter, it can be difficult for it to heat up sufficiently to make the diesel combust. So car makers give it a hand with a small heater or glowplug that heats up the air when the engine first starts.
What are the upsides of diesel fuel?
The major one is how much father you can go on a gallon of diesel than a gallon of petrol. Despite the increasing efficiency of petrol engines, the gap is in the order of 40% farther in favour of diesel.
As we’ve seen, this is because the energy released by diesel fuel is greater than that released by petrol. In fact, diesel is so efficient, the engine can run ‘lean’, that is, the fuel mixture has a higher ratio of air to diesel.
It’s another reason a diesel car can travel farther on a gallon of fuel and why, as a result, it produces less CO2 per kilometer than a petrol.
For a long time, CO2 was the bogeyman of the environment, a key contributor to global warming. Governments rewarded cars that produced less CO2 by taxing them less.
It’s the logic behind today’s UK road tax rules (2016), where a car attracts a tax charge based on a sliding scale of CO2 emissions. As a result, diesel cars are cheaper to tax than petrols. (The rules will change significantly from April 2017.)
As of September 2016, diesel fuel was around the same price as petrol (having previously been much more expensive), making it even more attractive given its greater efficiency.
A diesel engine doesn’t require spark plugs, so there’s an immediate saving in parts.
What are the downsides of diesel fuel?
We’ve heard diesel is messy and dangerous when spilled. It’s also very polluting, and difficult and expensive to treat after it’s been combusted and is passing through a car’s exhaust system.
In recent years, concern has switched from CO2 emissions to equally dangerous pollutants such as nitrogen oxides (NOx) and particulate matter (PM) that are present in large quantities in diesel exhaust at ground level and which are a risk to human health.
Diesel cars tend to be more expensive both new and used than petrol ones. Dearer used because they cost more as a new car and also because their fuel economy benefits are widely known and attract a premium on the used car market.
They’re more expensive as new cars because the technology required to clean their exhausts of dangerous pollutants, such as particulate filters (DPF) and selective catalytic reduction systems that use ammonia to reduce NOx emissions, is expensive to develop. The technology is set to get even more expensive as tougher anti-pollution limits approach in 2020.
Finally, when this technology wears out, it is expensive to replace. For example, AaDPF can cause the engine to run badly if it is allowed to become clogged with diesel soot.
The answer is to run the car for an extended period to get rid of the soot or as some people used to do, remove the DPF altogether.
However, the MOT test now requires that where a DPF is standard equipment, it must be fitted and functioning correctly for the car to be tested.