Diesel versus Petrol

It’s always a challenge trying to fit specifications with popular perceptions when it comes to trailer towing engine choices. An Australian AutoExpert, John Cadogan, has a lecture on YouTube that contains a few hints: Diesel Australia – the Diesel vs Petrol story  — do read the text that goes with the video as it is a bit more complete than many such video explanations.

Diesel vs petrol engines: Comparable petrol engines make more peak power – but diesels deliver huge torque at low revs. That means more low-rpm power from the diesel – maybe three or four times as much down at 2000rpm. That makes diesel feel unfussed and effortless in traffic. Diesel motors are about 30-40 per cent more fuel efficient. That means more cruising range out of a diesel, and less spent every week on transport.

This doesn’t help the confusion. There is a clue given in the video that does provide some help. The first item to consider is a proper definition of terms. It is the power that determines how fast you can get up the hill and the engine torque that determines the gear you need to use. Torque is a force whereas power is the rate of energy flow. So torque needs to have both distance and time figured in to be able to compare it to power. That is one problem with the quote. Another is that comparisons such as “maybe three or four times as much” are useless without a proper referent. The key item is that a diesel is a low speed engines while petrol (gas) engines produce best at higher rotational speeds (RPMs). The power range in a diesel covers only about half the RPM of a gas engine. That means that the transition from cruising on the flat to climbing a hill is going to cause the diesel to speed up much less than a gas engine and that means that the gas engine is more likely to need a shift of gears. 

It all comes back to the transmission. Big rigs need a lot of gears despite being diesels because they need all the power they can get and the gears help optimize the narrow torque curve. Choosing the best engine for long haul means it’s likely a bit small for grades and start stop traffic and the transmission compensates. The typical RV isn’t as heavy and can use an engine that has a better power to vehicle weight ratio to handle mixed traffic situations with fewer gears.

That’s just one example. John covers a lot of ground in 12 minutes so you need to listen carefully to catch things a bit different than you think. It’s not as simple a debate and there is a lot of nuance to catch.

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