Diesel Vs Petrol: What Engine Should You Buy?

Diesel cars (and trucks, utes and 4WDs) are well and truly mainstream. So should you buy a diesel car? John Cadogan from AutoExpert offers his tips.

If you’re on the ‘mature’ side of 30, you probably grew up in a world where 99 per cent of family cars ran on petrol. That’s just not the case any more – diesel proliferates increasingly across the car new vehicle market, standing shoulder-to-shoulder with petrol counterparts. Diesel is no longer solely in the domain of trucks, utes and 4WDs – plenty of cars are diesel too.

The Mazda3 and Hyundai i30, two of Australia’s most popular cars, are available with diesel engines. So is the Mazda6, the Hyundai i40, the Kia Carnival, the Holden Cruze, the Ford Mondeo and the Volkswagen Golf, among others.

More and more ordinary Australians are saying ‘yes’ to diesel cars. The sales growth in diesel cars is staggering: up well beyond 100 per cent in the past seven years.

Why Would You Want A Diesel Engine?

Here are the top six reasons you might consider a modern diesel car:

Diesel engines grew up

A host of technological improvements has really pumped up the performance of diesel engines. Among them: High-pressure, common rail injection with fast-acting piezoelectric injectors, electronic injection control, turbocharging and intercooling. Today’s diesel engines are refined and very responsive.

Cruising range and economy

Diesels typically deliver about 30-40 per cent better fuel efficiency, compared with petrol. This is mainly because the fuel tolerates more compression inside the engine, and therefore gets to expand through a greater range when it burns. This means you get more kilometres – 30-40 per cent more – from each tank. For city drivers it means fewer stops to refill at service stations (and we all know how uplifting that generally is) and four long-haul drivers it means more than 1000 kilometres from a full tank in many of today’s diesel cars.

Low RPM power

Diesels deliver their power differently, and the most profound difference is at low revs. A modern diesel is typically delivering three or four times the power of its petrol counterpart at 1500-2500rpm, and as a result they feel very strong at low revs. Petrol is still better for boy (or girl) racers intent on setting a lap record – but you need to rev a petrol engine very high, typically, to extract its peak performance. In ordinary driving situations, however, diesel beats petrol, and it feels effortless.

Smaller Carbon Footprint (CO2)

If you are concerned about greenhouse, then it might cheer you up to know that the flipside of that 30-40 per cent economy benefit described above is translated directly into a reduction in CO2 emissions. Getting more distance per litre out of diesel also means emitting less CO2 because the emission of CO2 is fairly rigidly related to the mass of fuel an engine burns, so burning less fuel means emitting less greenhouse gas per kilometre.

However (and there’s always a ‘however’) there are typically more pollutants in diesel exhaust – and one of the ways to deal with the more harmful particles is using exhaust filtration systems, which are increasingly standard in new diesel vehicles.

Towing performance of diesel

If you tow a boat, or a caravan or anything else heavy (car trailer, horse float, etc.) the diesel engine is a winner, every time. Petrol engines typically don’t deliver their peak torque until they get to 3500-4000rpm, and diesels generally do it at about half that. This means that when you get to a hill, with a heavy load behind, the diesel is already at the revs where it can deliver its peak output, whereas the petrol might need to shunt back one or even two gears to get there, which feels a lot less dignified.

Cost of diesel

Diesel drivelines often cost $2000-$3000 more up front. (There’s a lot of additional technology in there, compared with an average petrol engine.) As a result, many people do a kind of long-winded ‘break even’ calculation, wherein they calculate the distance you need to drive in order for the money saved in fuel consumed offsets the greater acquisition cost.

This is interesting in theory, but generally irrelevant to the way people buy new cars (using finance). When you add up the payments, take into account the fuel consumption and factor in the truth that some of diesel’s up-front acquisition cost premium is reflected in the trade-in value when you sell the car, the result is generally too close to call. Certainly it’s too close for the all-up difference in cost to be a major factor tipping the decision one way or the other.

The bottom line is that whichever you choose – petrol or diesel – the decision should be made more on the performance characteristics and cruising range/economy differences, which are profound, rather than on the balance sheet, which is a virtual photo finish.

If you’re in the market for a new car, and you’ve never driven a diesel, perhaps it’s time you did.

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