Petrol or Diesel?

1st Nov 2011 by Admin

The number of cars now being powered by Diesel engines has increased to the stage that most vehicle manufacturers now offer at least one in their lineup. But what is the latest news on diesel and petrol motors and what should I be driving?

Research undertaken by Roy Morgan and published in the telegraph this month indicated that almost 1 in 5 private vehicles sold this year in Australia will be powered by a diesel. This is in contrast to less than 1 in 100 for hybrid vehicles.

Traditionally the argument for diesel has been focused on two things - better fuel economy and longer engine life. Both of these advantages mean that, over the life the engine, you will tend to save money with a diesel. However, the NRMA suggests, that this maybe a simplistic view and once you take into account the initial higher cost of the engine then the difference between Petrol and Diesel over four years is minimal.

The better fuel economy argument for diesel is still legitimate. Diesel engines definitely use less fuel than a petrol engine performing the same work. Petrol engines are typically 30 percent efficient while diesel engines can convert over 45 percent of the fuel energy into mechanical energy.


But as General Motor’s director of global advanced engineering stated, "There is certainly a convergence in efficiency levels between petrol and diesel engines, and while diesels will always maintain a slight advantage, the gap will continue to get smaller”. Petrol motors over the past decade, have introduced once-exotic efficiency enhancing hardware such as, direct fuel injection and turbochargers and while these technologies aren’t new, the incremental improvements in electronics and materials have pulled them into the mainstream. And there’s more on the way.

But don’t expect the diesel engine to lie down. "We’ll continue to see incremental improvements in diesel efficiency," says Marc Trahan, Audi’s North American director of quality and technology. Trahan says these smaller gains will come from hardware such as variable valve timing and independent cylinder combustion control, as well as improved after-treatment systems. 


So let’s look at the historical problems that have held diesel engines back:

  1. Performance - One of the past criticisms of diesel motors has been their perceived lack of performance but Audi and Peugeot have used diesel-powered cars in Le Mans since 2006. Audi, using their diesel TDI power, have won this premium 24 hour race in 2006, 2007, 2008, 2010 and 2011. And, like all marques, many of the innovations from this racing are now used in your everyday Audi TDI road car engines.
    It should also be remembered that diesel engines offer more torque or pulling power and with the introduction of turbocharging, that power is much smoother and across a broader range.
  2. Diesel produces smoke and smells funny - The image of the black smoke being blown out by diesel cars no longer applies. Now it’s about Clean Diesel. Thanks to the use of DPF’s (diesel particulate filters) on almost all diesel powered cars, manufacturers estimate that between 80 and 100 per cent of particle emissions are cut by the filters.
  3. Diesel engines are much noisier and tend to vibrate. The characteristic noise of a diesel engine often called diesel clatter, is caused largely by the sudden ignition of the diesel fuel when injected into the cylinders. Engine designers are now reducing the noise through a combination of technologies such as multi-stage injectors and higher injection pressures.
  4. Limited choice - While in the past there were fewer diesel models, today the choice is in the hundreds. Audi for instance now have the option of Diesel Engines in almost any model now available in Australia, from the A3 right through to the Q7. Soon even the little A1 will have this option.
  5. Diesel engines tend to be more expensive. This still applies with an average of a $2000 premium for the diesel option.
  6. Diesel fuel is less readily available than petrol. This is not such a problem anymore but depending on where you are can still be an issue.
  7. Messy: Diesel fuel is oily and smelly and is difficult to get it off of your hands and clothes. Unlike petrol (which evaporates), diesel fuel remains thick and consistent (like syrup).
  8. In summary, cost savings are only part of the decision process, you also need to consider what the vehicle is being used for and so balance the pulling power, initial costs, environmental factors and noise plus the traditional aspects of design, comfort and features.

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