Smart Buying:Petrol vs Diesel

Buying a new car is an exciting process... But it can also be confusing and at time downright confronting. In this first of a new series on smart new car buying we examine the pros and cons of petrol versus diesel...

Every time petrol prices surge past $1.50 per litre, the rattle of diesel engines in suburban driveways becomes louder. Europe with its historically-high gasoline prices has for decades embraced the diesel-fuelled passenger car but many Australians remain sceptical.

Diesel is a lower-volatility fuel than petrol, requiring higher engine compression and special heaters within the combustion chamber for cold starting. Once operating, the heat within a diesel engine is sufficient to ignite metered injections of vaporised fuel.

Diesel engines used in light vehicles are typically smaller than 3.0-litres and circa-2011 are almost universally turbocharged to deliver higher levels of performance. Illustrating the advancements made in diesel technology, the winners of every Le Mans 24 Hour endurance race since 2006 have been diesel-powered. Modern diesels produce lower levels of carbon monoxide and are significantly more environmentally-friendly than the smoke belchers of old.

So what are the questions to consider before buying a diesel-engined ‘family’ vehicle:

Will it save money?
In the longer-term, yes. Diesels typically use 30 per cent less fuel than a comparable petrol engine. Their heavier construction and lower operating speeds promote longevity but they must be assiduously serviced using correct lubricants and with filter-change intervals strictly observed. Though diesel is more expensive that unleaded petrol in most Australian cities (and remote areas) it typically costs the same or less than the Premium Unleaded required in a growing proportion of modern petrol engine designs.

What about the noise and smell?

The ‘rattle’ generated by large diesel engines has been virtually eliminated in passenger-vehicle applications. When cold and near idle they will generate more noise than a petrol motor but at suburban or highway speeds, with some exceptions, the engine is virtually inaudible. Modern, car-based diesels have been developed to minimise smoke and odour and use maintenance-free filters to eliminate soot emissions. Any smoke or smell should disappear almost completely once the engine is at operating temperature.

Diesels are slow aren’t they?

Not any more, especially in passenger models. Europe has led the way in development of high-efficiency engines that deliver substantially greater low-speed torque than petrol engines and, when turbocharged, comparable power output. A Jaguar XF twin-turbo diesel accelerates from 0-100km/h in six seconds and more mainstream models like the Volkswagen Golf and Hyundai i30 deliver better overall performance than petrol versions.

Do diesel-engined cars cost more to buy?
As new vehicles, the diesel may cost 10-15 per cent more than a similarly-equipped petrol car. That said in some cases the premium is considerably less. Despite a belief to the contrary generally in Australia’s used car market, diesel vehicles currently depreciate at higher rates than petrol models. As the cost of petrol rises that situation is likely to change in favour of the ‘oil-burner’.