Why diesel vehicles are better for towing

Are diesel vehicles better for towing? The short answer is yes. There is a reason Mack trucks and container ships don’t use petrol engines.

Why? They are more fuel efficient and have more bottom-end pulling power.

Notwithstanding current fuel prices, diesel efficiency will ease pain at the bowser. Bottom-end pulling power means you can tow a horse float up a Gundagai hill without a down shift.

Diesel has a 15 per cent higher energy density than petrol. One litre of diesel has more energy than one litre of petrol. This means you can travel the same distance with less fuel, which, all things being equal, improves your fuel economy.

All things aren’t equal and diesels do better in most other fuel economy considerations than petrol, doubling down on their natural energy advantage.

The main differences are compression ratios and pumping losses.

Diesels don’t have throttles. The accelerator controls how much fuel goes in, instead of how much air goes in.

Throttles are what petrol engines use to control the amount of air flowing through them, and thus, the power. When a throttle butterfly is fully open the air flows smoothly into the cylinder with minimal pumping losses, operating at peak efficiency as a result.

When they are closed or partly closed, the pumping losses increase. This is why a car slows down off throttle with engine braking. The engine is sucking against the closed throttle valve.

Diesel engines don’t have throttles. Like a petrol engine that is full throttle all the time. Full throttle means minimal pumping losses and peak efficiency. The amount of fuel you pump into the engine controls the power.

Anytime you aren’t full throttle, a diesel engine will have an efficiency edge, which unless you are on a racetrack, is pretty much always. In stop/start city driving, this difference is even more noticeable.

No spark plugs means you use really high compression ratios to auto-ignite (or detonate) the fuel.

The higher compression ratio gives a higher thermal efficiency which means more cylinder pressure pushing the piston down after firing for a given amount of fuel.

In other words, extracting more punch from the fuel when the cylinder fires.

To give you an idea, compression ratio of petrol engines range from 8 to 12:1 depending on turbocharging and performance level, while diesels engines range from 15 to 20:1.

Auto ignition is bad in a petrol engine. Okay, not entirely bad as most engines run some ‘det’ (detonation) to give the best fuel efficiency, but too much will break piston crowns and blow head gaskets, so petrol engines are carefully monitored to make sure they don’t run lean.

In a petrol engine, you want the spark plug to control the explosion.

Diesel burns and petrol explodes. If you’ve ever lit a bonfire with too little diesel in your mix, you (and your eyebrows) will know what I mean.

Diesel takes longer to combust. At low rpm this means you are getting more grunt during the window your crank is at 90 degrees. Diesels tend to have a longer stroke, making further use of the slow burn at low rpm.

Unlike the petrol engine, diesels run lean which heats up the surrounding air, causing more expansion as things progress during the stroke. This further adds to cylinder pressure and is the reason that petrol engines can’t run as lean, as this heating sets off a detonation chain reaction.

Running lean means all of the fuel is getting used up in the burn, inherently resulting in better fuel economy.

It also means more nitrous oxides. AdBlue is used to offset those nasties, but presumably produces some of its own. Diesels can also run rich but they get smoky quickly, and nobody likes particulates on their clean business shirts.

On the flipside, the slower burn means poor combustion starts to rear its head as the revs increase. So diesel engines aren’t as revvy as their petrol counterparts.

Power and torque are often misunderstood. Power is torque multiplied by rpm. Torque is power divided by rpm.

Torque is literally a rotating force. It’s the arm wrestle between the piston and the crank that shoves your car forward. It is how strong your engine is at a given rpm.

As we’ve seen, diesels are very strong at low revs, right in the purple patch for towing.

Power takes into account how many times your engine will be firing as it speeds up. Your diesel engine might get weaker with more revs, but the cylinders will be firing more often.

Imagine a rower. He is really strong when paddling slowly and gets progressively weaker when he is paddling faster. This is the torque curve. Time needs to be factored in. Each stroke might be getting weaker, but he is doing more of them. This is the power curve.

The tyres don’t care about the torque curve, they only see the power curve. On an engine dyno, the force you are seeing at the roller is the power curve. The computer then calculates the torque by dividing the rpm.

Same thing when you are towing uphill. Your caravan only cares about the power curve. The fact is diesels have more power at low revs and petrols have more power at high revs.

So, that’s the long answer. Diesels produce a lot of grunt at low engine speeds and do it more efficiently than petrol. They are more of a steam engine than a whipper snipper motor. More of a Clydesdale than a Hummingbird.

They produce a lot of force at low revs and are built strong to handle it. They are the perfect accompaniment to your towing tastes.

Diesel engines – the bottom line

• Ideal choice for when you need torque for towing and lugging heavy loads
• Less efficient over short distances, but perfect for country touring with a van behind