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Tech talks: Diesel power versus petrol power

| Car Maintenance, Performance, Sri Lanka | January 5, 2013

Tech talks: Diesel power versus petrol power

Have you ever wondered why diesel engines are used on large applications? The benefit of diesel is much more apparent than just the cost; which is primarily the reason why diesel cars are so popular in our country.

Over the last few years, there has been a splurge of new diesel cars in our market, including the top marques producing high refinement levels on the oil burning engines. (Audi, BMW, Mercedes etc)

Even traditional purists like Subaru, who have mastered the boxer engine, have developed the worlds first flat-4 turbo diesel, install in their cars in some markets.

The Europeans in their countries offer many tax incentives/ benefits for diesel car users and companies. The technology has evolved to such a stage that on modern engines, it is extremely difficult to say what engine it is due to reduction in NVH (noise, vibration and harshness)

The fuel consumption figures whilst driving on the highway also correspond to dizzying figures which seem near impossible to attain in a petrol engined car.

However, when it comes down to repairs and servicing, it is a totally different story. Almost all the high output passenger car diesel engines are turbocharged, intercolooled and run on a few thousand bar of fuel pressure. (In comparison to around 4 bars on a conventional petrol engine and a few hundred on a direct injection petrol engine)

This directly corresponds to high repair and maintenance costs, and inevitably means that these specialized components in most cases cannot be repaired; only replaced.

Let us understand the basics of diesel versus petrol.

The diesel engines operate at much higher compression ratios due to the oily nature of the fuel, and diesel engines are CI (compression ignition) which means that the engine has to physically attain this stage and undergo excessive stress and strain. This is achieved by having engines which are much heavier and are able to spin at lower revolutions and exerts tremendous load on the oil. Additionally, the combustion of the heavier fuel adds a lot of soot and other by products to the oil and exhaust gases. This is why on diesel engines you would see lots of black smoke and darkening of the engine oil almost immediately.

However, most new engines are fitted with dual stage catalytic converters and complicated EGR (exhaust gas recirculation) systems to purify the exhaust gases.

Diesel engines also produce a lot of torque, which is basically pulling power, at very low revolutions. This means the engines struggle less to pull load, and is the reason why you can drive your van at 40 km/h on 4th gear without any noticeable strain.

It is very important to note that

  • Torque is proportional to acceleration
  • Power is proportional to top speed

This is why diesels “feel” quicker through the gears at low speeds. In all honesty, it is not possible to get the maximum of a petrol car on our roads. The low rev band also allows them to reach the maximum operating speed very quickly.

For example: the Audi Q7 V12 TDI has a 1000NM of torque with a 6 litre twin turbo diesel. The petrol version 6 litre of the same engine has 560 NM of torque. (It is almost double albeit with twin turbos). Your average 1.5 liter car has around 150 NM by comparison.

This is all very well, but using diesels on our roads in stop/start conditions gives them extreme loads which means they need servicing and care earlier than recommended. The servicing should also be done in close intervals to prevent damage to the internals (especially turbocharged engines)

It is important that you decide which is best for you. The pros and cons of each engine are different and individualistic and the levels of care needed can vary for each application.

By Sadath Nizar

Motor Magazine
(Source: Motor Magazine)