When will drivers with big trailers and motorhomes switch from diesel to petrol?


If you have a lot of weight to shift like a big trailer, or an RV, then ask around … the answer has always been diesel power. But rather than just accept that on face value, let’s take a look at exactly why diesel beats petrol for heavy-duty work, and maybe then we can start to predict the future of recreational vehicle propulsion.

If you did ask why diesel beats petrol, chances are the term ‘torque’ will come up. And chances are, the sage intoner of said advice cannot actually define torque. So let’s give it a shot. Torque is simply a turning force, nothing more, nothing less. When you undo a wheel nut you apply torque to the nut, and in fact you apply torque regardless of whether the nut turns or not. Here’s a fun fact – an adult human can generate 300Nm of torque easily enough using a lever between about one or two metres long – if you weigh 100kg and stand on the end of a 300mm long spanner, well there’s your 300Nm. How about that? You can out-torque a medium-sized car engine!  

So if you can produce the same torque as a car engine, how come you don’t like the idea of pulling a car and trailer by yourself? The answer is power. How quickly you apply torque is power – so going back to our long lever example, sure you can move that one metre long lever, but how many rotations could you do per minute? One, maybe? Now think about how quickly an engine spins. The idle speed is probably 700 revolutions per minute (RPM), and it can go to several thousand RPM. You certainly couldn’t, and there’s the difference. Despite you and the car engine being able to generate the same amount of torque, the car engine is more powerful as it can produce that torque at far higher revs per minute.

Now we understand torque and power, it’s time to get into the petrol and diesel debate. Naturally, diesels produce torque and petrols produce power, right? And torque’s what you need for big loads, so job done, diesel it is.

Except it’s not that simple. These days, both petrol and diesel engines produce huge amounts of torque, and power. The days when diesels would struggle to keep up with petrols are gone, and petrols make pretty decent towing machines. Helping both is the magic of gearing – what’s important is not so much the torque or power output of the engine, but the torque and power at the wheels.

With six-speed gearboxes standard and up to eight speeds common, then smart, efficient gearboxes can maximise the engine’s potential in ways that the older four-speeders could never achieve. Wider torque bands thanks to the magic of computer-controlled engines help too; it’s not so much the peak torque as the torque that’s spread around the peak.

Nevertheless, some differences between petrol and diesel remain. All else being equal, diesels use less fuel for any given job. The reason is twofold: first, there’s more energy in a litre of diesel than a litre of petrol. The figures are about 36 megajoules per litre of diesel, and a 34 per litre of petrol.

Second, compression. Both diesels and petrols compress air before creating ignition, but the diesels compress further because they compress only air, and the act of adding fuel creates the ignition which forces the cylinder down. Petrols compress a mixture of air and fuel, and add a spark to ignite. That places a limitation on how much compression can be done, because if you compress the fuel/air mixture too much it’ll ignite before the spark, something known as “pre-ignition” which is all sorts of bad news. The diesel’s ability to handle greater compression is one reason why it is such a good match for turbocharging, which involves compressing air before it is introduced into the engine for combustion.

So we have the diesel compressing much more than a petrol, and the greater energy in its fuel – combined, that means a stronger ‘explosion’ so there’s more force as the cylinder goes down. Diesel is also slower-burning than petrol, and combustion in a diesel is controlled by the amount of fuel injected as opposed to the petrol’s spark.

The end result is that the diesel engine simply pushes its cylinder down with more force than the petrol – think of the diesel as a strong, slower push, as opposed to a faster but less powerful push with a petrol. This is also why diesels have a longer stroke than petrols – that’s the length of the connecting rod between the piston and the crankcase, and that longer stroke means more leverage.

These differences are clearly shown when you look at two Toyota LC200s, one petrol one diesel, both V8s. The bore of the cylinder (diameter of the piston) is 94mm for the petrol, 86mm for the diesel. But the stroke is only 83mm for the petrol, and 96mm for the diesel. The petrol compresses its fuel/air mixture at a ratio of 10.2:1, the diesel compresses its air at 16.8:1.

The result for the petrol is 227kW @ 5500rpm and 439Nm @ 3400rpm. The diesel offers 200kW @ 3600rpm, and 650Nm @ 1600rpm. And the diesel weighs another 45kg, 2585kg vs 2630kg, due to the stronger engine construction to deal with the higher compression and torque, and add-ons like turbos.

Translating that into reality, as you’d expect, the diesel wins fuel consumption but it’s interesting to look at exactly how. The freeway cruise cycle is 8.5 and 10.7L/100km for the diesel and petrol, a difference of 26% and 2.2L/100km. But we get into the urban stop/start cycle and as load increases on the diesel, the difference widens. The diesel’s urban figure is 11.2, but the petrol shoots up to 18L/100km – that’s 61% and 6.8L/100km. And to round it out, the combined cycle is 9.4 diesel, 13.4 petrol which is 41% and 3.9L/100km apart.

Now at this point all I’ve done is explain some of the theory behind what is a commonly accepted fact – diesels tow and shift large loads better than petrols. In short, the reasons are: more torque developed at a lower RPM, which means the engine doesn’t need to rev as hard and the drive is generally more relaxed as a result. And diesels offer considerably better fuel economy which translates into increased range between fills, particularly as load increases. But only for the moment.

The future is clear, and it’s not diesel. The basic problem is diesel’s cancer-causing Class 1 carcinogen pollution, which is leading country after country to ban them from city centres. Diesels are also disappearing up their own (dirty) exhaust pipes with complexity such as AdBlue, particulate filters and exhaust gas recirculation, thanks to every new round of legislation tightening the emissions controls further and further.

In contrast, petrols have a relatively easy ride, and are much simpler as a result. Their capabilities are edging closer to diesels as technology progresses, not least helped by sophisticated multi-speed gearboxes. But that gap will never close until the real quantum leap in capability arrives, and it’s almost here. In fact, you’ve probably used it if you’ve had an Uber or taxi ride of late.

Petrol-electric hybrids. It’s a match made in RV heaven. Electric engines offer huge torque at nil RPM, perfectly compensating for the petrol’s relative lack of low-rev grunt. And every time the hybrid slows down, it harvests energy to recharge its batteries for the next pull, so fuel efficiency is improved too – not so much in cruise, but certainly in terrain where it’s on and off the throttle, which is exactly where diesels hold the greatest advantage over petrols.

Our recent tow test of the all-electric Tesla Model X SUV proved the point. Strong electric engines are fantastic for shifting heavy loads – lots of lovely, seamless pulling power. But the test also proved that electric range is, for the moment, nowhere near what recreational drivers in Australia will accept. The solution is simple: the vehicles will switch to petrols with small electric motors, then bigger electric motors, then electric with petrol backup, and eventually no petrol at all. Sound far-fetched? Think about what the horse riders said to the early drivers of the steam-driven motorcars back in the day… we’ve squeezed the internal combustion engine about as far as we can, and as steam gave way to petrol and diesel, it’s time for a new vehicular power plant: our friend the petrol-electric hybrid.