Most drivers’ relationship with diesel fuel begins and ends with the pump at the filling station. It’s what they put in their diesel powered cars, or more often, what they've seen truck drivers put in their rigs. It’s dirty, smelly, oily stuff (it’s sometimes called diesel oil) which is why gas stations 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 pavement without evaporating as gasoline quickly does, making the surface slippery. Motorcyclists are always looking out for the telltale rainbow glimmer of oil on the road when it’s sloshed out of the overfilled fuel tanks on trucks.
So it’s nasty stuff but it does have its benefits:
1) It has a higher energy density than gasoline (basically, it’s more powerful for a given quantity) so can power your car for longer.
2) It can be produced from many different sources, including fast food fryers.
3) It can be used in an engine without a spark plug (eliminating quite a few parts and pieces).
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 many different vegetable oils, most commonly rapeseed oil.
However, very few manufacturers recommend or permit its use in their cars, at least in pure form. Some do permit it at levels of up to 15% mixed with 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 gasoline 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 gasoline (or methane, propane, ethanol) engine it’s triggered by a spark igniting a mixture of fuel and air. In a diesel, it’s caused simply 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 called a 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 gasoline. Despite the increasing efficiency of gas engines, the gap is in the order of 40% farther in favor of diesel.
As we’ve seen, this is because the energy released by diesel fuel is greater than that released by gasoline. Plus, because of how it burn, the engine can run ‘lean’, that is with a higher ratio of air to diesel fuel to produce the same power. As a result, it produces less CO2 per kilometer than a an engine burning gasoline.
Recently, CO2 was the bogeyman of environmentalists because it is a key contributor to global warming. This went to far that in Europe governments rewarded cars that produced less CO2 by taxing them less, which made diesels less expensive to drive year after year..
It’s the logic behind the updated 2016 UK road tax rules, where the annual tax charge for registration is based on a sliding scale of CO2 emissions. As a result, diesel cars pay much less tax than gasoline powered versions.
Typically, diesel fuel is around the same price as gas per gallon, making it even more attractive given its greater efficiency.
A diesel engine does without spark plugs, and the entire ignition system, so there are less parts to fiddle with or break.
What are the downsides of diesel fuel?
We’ve heard diesel is messy and dangerous when spilled. It’s also very polluting, but in a different way than burning gas, because it produces particulates and soot. These are 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 more immediately dangerous pollutants such as nitrogen oxides (NOx) and particulate matter (PM) that are present in much larger 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 gasoline burning ones. They’re more expensive as new cars now because the technology now required to clean their exhausts of dangerous pollutants, such as particulate filters (DPF), and urea or diesel exhaust fluid (DEF) injection to reduce NOx emissions, is expensive to develop. The technology is set to get even more expensive as tougher anti-pollution limits come into play.
Finally, when this technology wears out, it is expensive to replace. For example, a clogged DPF can cause the engine to run badly if it is allowed to become clogged with diesel soot.
The proper solution is to run the car for an extended period to burn out the soot, but what some people used to do is just remove the DPF altogether.
However, if your state has a smog test for diesel vehicles, they will spot a missing DPF and it must be fitted and functioning correctly for the vehicle to be tested.