Petrol -vs- diesel in Australia in 2015 | Auto Expert John Cadogan

You're looking at an increasingly diesel Australia - diesel cars, diesel SUVs and diesel utes and light commercials are all selling in record numbers.

There’s been a meteoric rise in diesel vehicle sales. Here in Australia when you add diesel cars to diesel SUVs the sales growth is staggering: up more than 70 per cent - in seven years. Diesel cars, SUVs and utes have sold to private buyers in record numbers - up a staggering 130 per cent there. What, exactly, is behind the rockstar rise of diesel?

More people than ever are doing the diesel vs petrol comparison for themselves, and discovering diesel motors increasingly suit them better - as in better real world performance and much better fuel efficiency. In fact diesels are among the most fuel efficient cars money can buy.

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.

Nothing’s free though, and diesel engines cost more up front. Generally two or three grand more for ordinary cars. And the diesel fuel pump is always filthy, smelly and slippery underfoot.

But the fundamental difference between petrol and diesel engines is still where the fuel actually mixes with the air. In a standard multi-point injected petrol engine, the fuel injector does its spraying in the inlet port, just upstream from the valve. In a diesel, the fuel gets injected directly into the combustion chamber, just in time. There’s four or five precise little fuel injections per combustion event - we’re talking precision down at the millisecond level in the time domain. There’s no spark plug - the fuel just burns spontaneously.

(Some of the latest direct injection petrol engines inject like that. But they still need spark plugs; they’re not auto-ignition engines like diesels. So, in a sense, petrol engines are playing a game of combustion catch-up.)

Diesels cost about $2500 more. Some people get obsessed with calculating a break-even point - the hypothetical distance you might need to drive to save enough fuel to offset the extra cash you pay up front, for the diesel engine. It’s a fundamentally flawed economic analysis. For starters, the diesel goes better. It’s therefore intrinsically worth more. More importantly, when you sell the car down the track, the extra cost of the diesel is reflected in the value of the car. You always get a proportion of the price premium back.

You’re probably wondering if the extra cost is justified. It certainly is. Two reasons: First: economies of scale. Carmakers make more petrol engines, and that reduces the per-unit cost of each petrol engine. And, second, the diesel is more complex. It’s turbocharged. It’s intercooled. It’s got a 2000-atmosphere fuel rail. There are peizo-electric injectors, and it’s got to be built to withstand higher internal pressures because of the greater compression. Often, because of the greater torque, diesel vehicles need a beefier driveline as well. So that extra cost is actually fair enough.

It costs more to service a diesel, too … but not really that much more. In fact, in terms of total cost - fuel, depreciation, servicing - cost of ownership is very similar. Too close, actually, for the cost to be a factor in your decision to buy the petrol or the diesel.