Traditionally, the line in the sand between diesel and petrol power was that cars used petrol while commercial vehicles ran on diesel. That all changed in some countries many years ago when the price of fuel spiked, leading many European countries to take the diesel engine to their hearts for passenger cars, purely because a diesel engine squeezed a little (sometimes a lot) more from each litre.
In other parts of the world – namely the USA and Middle East– where fuel remained cheap until very recently, the petrol-for-cars-diesel-for-trucks mantra continues to be the rule rather than the exception.
But let’s go back to the start and talk about these different fuels and the engines they work in. Both petrol and diesel fuels are made by refining crude oil. Petrol is a more refined product than diesel which still has an oily feel if you’ve ever spilled it on your fingers. Diesel is theoretically cheaper to make, although you wouldn’t know it from the pump price lately.
Beyond that, they both work the same way in that they combine with oxygen inside the engine, explode and that explosive power turns the crankshaft and, eventually, the car’s wheels.
The main difference is that a petrol engine uses a spark plug to ignite the fuel inside the engine and start the combustion process. In a diesel, the fuel and air mixture explodes when it’s compressed; there are no spark plugs.
To achieve a sufficient squeeze to create this ignition, a diesel needs a very high compression ratio and that places physical strains on the whole engine. That’s why, until very recently, most diesel engines were made from cast iron rather than lighter aluminium, and everything else about them had to be physically tougher. So, they had heavier internals and a much higher overall mass than a petrol. It was this internal mass that meant the diesel was usually a slow revver, vibrated and was noisy compared with a petrol engine which had much lighter internals and could spin faster, but still safely.
If you tried to run a petrol engine with a diesel’s compression ratio, you’d find the fuel-air would explode before the spark plug told it to and the whole engine would be a hand-grenade thanks to the internal forces. Forcing the fuel into the high-compression diesel also meant that engine designers had to use high-pressure fuel-injection to cram the fuel into the engine, while the petrol engine was perfectly happy drawing in its fuel-air mixture through a simple, cheap carburettor. These technical considerations all made the diesel more expensive to produce, and car-makers don’t like that.
And that’s pretty much how things were until the last couple of decades when the turbo-diesel arrived in passenger cars. Turbocharging a diesel is not new. Engine builders have bene doing it for decades in heavy equipment, trucks, boats and all sorts of cranes, busses and earth-movers. Forcing more air and fuel into the engine via the turbocharger is a way to offset the lower energy (and therefore lower power output) of a diesel engine compared with one burning the more volatile petrol.
And eventually, designers began to get the hang of making a turbo-diesel engine that was light and small enough – and cheap enough – to power even a tiny hatchback. Some luxury brands had tried diesel cars before (marketed in Australia mainly to farmers who could fill their car from the same tax-free diesel drum that the tractor used) but in the 2000s, the concept really started to fly and diesel was suddenly sexy.
Even more recently, though, petrol engine technology has made some serious advances to the point where the petrol engine with a turbocharger can be almost as fuel efficient as a diesel, but also quieter, much faster and more powerful and much more fun to drive.
Along the way, we saw some manufacturers offering both a petrol and turbo-diesel engine option on the same model. The Mazda CX-5 is a great example. And it’s still happening with the South Korean makers very keen to offer you a choice of petrol or diesel engine in your next Hyundai Santa Fe or Kia Sportage. But that trend is winding back with many makers moving to a hybrid layout that suits a petrol engine better.
So why isn’t every car running on petrol again? Well, it’s certainly heading back that way, but the whole trend is being upstaged by EV tech which will eventually take over anyway. Fundamentally, in a mass-market, passenger-car sense, both diesel and petrol engines are on borrowed time.
That said, they’ll still be around for a while yet, and both have pros and cons you need to be aware of when choosing which way to go.
For the petrol engine, the pros start with the fact that it’s a more fun powerplant. It revs better, makes a better noise and is the default choice for anything vaguely sporty. The petrol is also cheaper to make, so the cars it’s fitted to can be cheaper to buy. The petrol is also cheaper to maintain over the long term and will tolerate purely urban running where the car never gets out of third gear.
On the minus side of things, a petrol engine doesn’t generate the torque that a good turbo-diesel can, so it’s not so great for moving heavy towed loads. That’s why most dual-cab utes are diesel powered; because they are required to tow up to 3.5 tonnes.
For the diesel engine, the pros start with that ability to lift heavy things. They are generally tougher and will cope with the demands of off-roading, while their lower fuel consumption means greater range in the outback (which is why a lot of off-roaders are diesel-powered) and are happy to sit and idle for ages. That relaxed, effortless surge of torque is why they also work well with today’s transmission of choice, the automatic. Diesel engine life is also a little better than a petrol, but, again, that gap is closing.
On the downside, modern diesels can be a bit more expensive in a service cost sense, as maintaining their complex, high-pressure fuel-injection systems needs some pretty specific tending. Fuel injectors can require replacement and the high-pressure fuel pumps and filters don’t last forever either.
However, it’s probably modern emission controls that the diesel hates most. To clean up the more toxic exhaust emissions (compared with a petrol engine) engineers have developed a few methods for the diesel to clean up its act. One of these is the EGR system where the engine consumes a portion of its own exhaust to burn it twice and reduce harmful emissions.
Unfortunately, combine that with the crankcase ventilation system that also sees the oil fumes from the crankcase consumed by the engine, and you have a great recipe for the formation of thick, black gunk that can actually block the engine’s airways. When that happens, it has to be cleaned out manually, and that costs plenty.
The other elephant in the room is the Diesel Particulate Filter (DPF) which traps some of the soot in the exhaust, stores it and then burns it off later at high temperature. These DPF systems are supposed to regenerate (burn off) automatically, but they have been known to clog up and block, needing either manual cleaning or even costly replacement.
Toyota Australia, right now, is in the throes of a class action brought by thousands of HiLux owners complaining about the DPF system fitted and its unsuitability for the job. You’ll be hearing plenty more about that, but it proves that the DPF issue is a real one.
It can be made worse when owners use their cars only for urban work where the engine speed and load never allows the exhaust system to get hot enough to regenerate properly. But even when that’s not the case, DPFs can cause all sorts of problems including poor running, poor fuel economy and the vehicle going into limp-home mode.
So here’s the bottom line in the diesel engine vs petrol engine comparison: If you need to torque of a turbo-diesel, you can live with the less entertaining feel and sound of the diesel and you travel at highway speeds for at least 45 minutes every couple of weeks, then maybe a diesel is for you. If not, the modern petrol is probably a better idea and since the fuel economy gap is narrower than ever before, you might be surprised that going diesel suddenly doesn’t seem such a trendy idea.
The flip side to all this, of course, is that sometimes, the model you want in the layout you want is available only with a petrol or diesel engine. That is, if you want a two-wheel-drive SUV versus a four-wheel-drive one, you might be stuck with a petrol engine, and vice-versa. Maybe you want a manual transmission. If so, in many cases, that will force you into the petrol-engined version of a particular car. Sometimes the choice of a diesel vs petrol engine you thought you had doesn’t exist at all.
What about when you’re finished with the car? The resale value of petrol vs diesel cars is a closer race than ever as many folks understand that a diesel is not what they want after all. In light commercial vehicle terms, diesel still commands a premium, but in passenger cars, the difference is a lot smaller than you might think and is often more about things like whether the car is two or four-wheel-drive and what transmission and options are fitted.
By David Morley