Workshop: Petrol vs Diesel


Words and image by Martin Pretorius

In the traditional marketplace, diesel-engined off-roaders used to exist only for certain, specialised buyers. Farmers, miners and lumberjacks found them useful for their fuel efficiency, toughness and longevity, and overland adventurers knew that diesel was often the only fuel to be found out in the bush. For everybody else, there was always a petrol-powered alternative, with more power, less noise, and no smoke.

Then the turbocharger made its appearance; and, after microchips also got to work their magic on the humble oil burner, the performance difference between petrol- and diesel-powered off-roaders was just about erased. Whatever modern diesels lose, in comparison to their petrol counterparts, in throttle response and noise levels, they compensate for it with a lovely wave of mid-range torque, and lower fuel consumption.

So diesel engines have finally caught up to petrols?

Not quite, and they probably never will, for a reason which lies in the fuel itself. Petrol is extremely volatile, and only a little spark is enough to ignite it extremely quickly. This quick burn allows for high engine speeds, and Turbochargers cram more air through an engine’s intake valves, which allows more fuel to be injected, which (in turn) liberates extra torque. But that torque is not always available when the accelerator pedal demands it: if the engine speed is too low, the exhaust gas flow simply isn’t enough to get the turbo up to speed, often leading to an annoying dead spot just above idle.

Even when the engine speed is high enough, there’s still a momentary delay while the turbo speeds up to do its job (we recognise this as turbo lag) which sometimes makes it difficult to accurately modulate the engine’s power output. Too sharp a boost response isn’t a great idea either, for it could lead to just enough wheelspin at the wrong moment to dig you into a sandy river bed.

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