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Petrol or Diesel?

The days of the ‘clunk, clunk’ and belching smoke diesel-driven cars are well and truly over.

The modern diesel engine is virtually indistinguishable from behind the wheel of a normal family saloon.

The diesel’s enormous popularity in Europe is spreading to Australia, with passenger car sales increasing more than five fold in the last four years.

So why so popular?

Firstly a diesel engine now sounds almost as quiet, smooth and sophisticated as its petrol partner, and it’s arguably even more pleasant to drive.

In fact diesel engines produces much more torque, run at lower revs, give better pick up in higher gears and tow better.

They are also a lot more efficient.

They will burn much less fuel, so will be cheaper to run.

They are tough, clean and will probably take harsher treatment for longer.

So they should be the natural choice – except for a few drawbacks.

To start with it will cost you more when you buy one, probably between $2500 and $3000.

Then there’s the comparative cost of fuel. Diesel used to be cheaper than petrol, then it got more expensive (up to 30c a litre), now it’s about on a par.

So any comparison with petrol very much depends on the price of petrol v. diesel at the bowser.

You’ve also got a problem at the pumps. Usually less than one in six pumps dispense diesel fuel, so you may have to queue longer, especially on ‘Cheap’ days. But, of course, you won’t be visiting the pumps so frequently, as you can expect up to 1000kms. on a tank of fuel.

So is it worth it?

The answer depends on a number of factors, such as the type of vehicle you are considering; how far you travel each year, the type of driving you do, and the power and torque you need.

Diesel engines operate a little differently to petrol and liquefied petroleum gas engines. They do not use high voltage spark ignition (spark plugs). Rather, the engine compresses the air inside the cylinder to high pressures and temperatures. High compression ratios and throttleless operations generally result in greater efficiency than is achieved in most spark-ignited engines.

Being less flammable and explosive makes diesel more suited for use in armoured fighting vehicles. However, diesel viscosity increases quickly in cold temperatures, so starting a diesel engine in very cold weather can pose difficulty.

Diesel power has been popular in trucks for many years. That’s because diesel engines have a lot more ‘grunt’ to handle those heavy jobs. Because it produces more torque, a diesel engine is generally a better choice for heavy towing. If you are driving a 4WD up steep hills, a diesel engine is less likely to stall.

Diesel engines generally travel between 20% and 50% further on a litre of fuel than their petrol equivalent, so total fuel costs are considerably lower. And the higher initial cost of the vehicle should be outweighed by the fact that diesel vehicles hold their resale value better, and diesel engines last much longer.

Engine repairs to diesel motors are more expensive than to petrol engines, because the motors need to be more robust. You also need to change the oil more frequently, and regular oil filter replacements are critical. The fuel system will need attention at about 100,000 km. And you need to be careful to use clean fuel. Water or dirt in fuel causes premature wear in the injector pump and injectors.

Diesel cars warm up more quickly from a cold start than petrol, but once warm, a petrol engine is cleaner and gives out lower emissions. Diesel is great for lots of short trips, but it really isn’t the right choice for "soccer mums", or anyone doing lots of stop-start town or city driving. On the motorway, petrol and diesel engines perform similarly and achieve about the same fuel economy.

When it comes to environmental considerations, diesel cars produce less CO2, and that is recognized—in Australia at least—by applying a lower road tax. Because of fuel density, they produce higher toxic gas emissions per litre, but better fuel economy means lower greenhouse gas emission per kilometer travelled.

Recent improvement in diesel engines and diesel fuel composition have reduced sulphur content dramatically, reducing emissions. However, lowering sulphur content implies a need for additives to help lubricate the engine. Biodiesel and biodiesel/petrol blends are being developed to increase lubricity and improve overall performance, while controlling emission levels.

The advent of ‘common rail’ technology facilitated engine design changes to dramatically reduce emissions and noise in diesel engines. Noisy old smoke belchers are now part of history, and modern diesel cars are hard to distinguish from their petrol equivalents on the highway, but are noticeably more capable on steep climbs or when pulling weighty loads. When buying a truck or 4WD vehicle, or for those wanting to tow a good size caravan, diesel is certainly worth considering.

For the average car buyer, petrol may still have the edge. If you are buying a small runabout and travel less than 15,000 kms a year slower depreciation and a longer life engine are unlikely to outweigh the combination of added up-front costs, more costly engine servicing, and more frequent oil change requirements.

Despite these disadvantages, diesel cars are often the choice for company cars because their lower CO2 emisssion results in lower overall taxes.

The choice is not an easy one, and is further complicated by rapid technological advances that continue to produce more efficient engines and better quality fuels. Generally, diesel is likely to continue to be chosen for trucks and larger 4WD vehicles, and by those grey nomads wanting to pull large RV’s, while the average buyer of a family-size or compact car will opt for a petrol engine version. But circumstances are changing so rapidly. For example BMW have just launched their diesel powered Mini D (Click here for review) which claims a miserly 3.9L/100kmsfuel use. So if you travel arouind 10,000kms a year, at current prices you’ll spend little more then just $10 a week on fuel.