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Why you should consider a diesel as your next car

By Kenny Yeo - 24 Jul 2016

Diesel vs petrol engines

How are diesel and petrol engines different?

The diesel and petrol engines that are used in most cars are highly similar. In essence, they are internal combustion engines that work using a two or four-stroke cycle. In an internal combustion engine, the power cycle is made up of four phases: intake, compression, power and exhaust.

In the intake phase, air is drawn into cylinder through the opening intake valve. In the compression phase, the intake valve closes and air is compressed with fuel. At this point, the mixture of fuel and air is ignited to cause an explosion. It is this explosion that causes the piston to downwards and drive the crankshaft to produce motion. This is the power phase. The final phase is exhaust where the spent air-fuel mixture is expelled out of the cylinder through the opening exhaust valve so that a new cycle may begin.

Petrol and diesel engines operate using the same principle, but the difference lies in the spark plug. Diesel engines don't have any.

The main difference between diesel and petrol engines is that petrol engines use spark plugs to ignite the air-fuel mixture, while diesel engines rely solely on heavily compressed air. As mentioned earlier, Rudolf Diesel found that the temperature of air could be made to increase high enough if it was heavily compressed. The temperature would rise so high to the point where it could cause the ignition of diesel fuel. 

Therefore in diesel engines, air in cylinder would be very heavily compressed, typically to around 14 to 23 times its original volume. In petrol engines, the compression ratio is generally much lower, because they rely more on the spark plug to begin the power phase. The compression ratio is petrol engines is typically around only around 7 to 10, with high performance vehicles having higher compression ratios of up to 13.

A graph showing the relationship of thermal efficiency and compression ratios. Efficiency continues to increase significantly from compression ratios of 10 to 20. (Image source: Duke Engines)

High compression ratios are desirable because it results in higher thermal efficiency. In other words, more energy can be extracted out of the air-fuel mixture. This also explains why diesel engines are considerably more efficient than petrol engines. In fact, diesel engines have the highest thermal efficiency of any internal combustion engine.

 

The Pros and Cons

Besides being very efficient, what other advantages do diesel engines have over their petrol counterparts? And what disadvantages do diesel engines have? Let’s take a look at some of the important ones briefly here.
 

Pros

  • Not only are diesel engines more efficient, diesel fuel is also cheaper to purchase. At the time of writing, diesel fuel is around 40% cheaper per liter than petrol. This means diesel vehicles will be cheaper to run, which also explains why buses and most taxis have diesel engines.

     
  • Because diesel engines are so efficient, cars can get incredible mileage out of them. It is not uncommon for passengers cars with modest 50 liter fuel tanks to be able to travel over 1000km on a single tank. This means more time spent traveling and less time spent refueling.
     
  • To withstand the high compression of gases within the cylinders, diesel engines are built to be extremely hardy and will generally last longer than their petrol counterparts. They can also go longer between maintenance.
     
  • Diesel engines can be made to run on alternative and renewable fuels like biodiesel with little or no modifications. Biodiesel generally refers to used vegetable oil that has been used for cooking, and is then recycled and treated so that it can be used to power diesel cars.

 

Cons

  • Diesel engines need to be built stronger to withstand the high compression of gases, as a result, they usually cost more to manufacture. Consequently, diesel cars can sometimes cost more than their petrol equivalent. This depends heavily on the manufacturer.
     
  • Diesel engines produce a distinctive knocking sound that is referred to as diesel clatter. This sound is the result of the sudden ignition of fuel which causes a pressure wave. It makes diesel engines sound less refined and noisier.
     
  • Diesel engines are heavier and less eager to rev than petrol engines, which makes them undesirable in sports cars. This also makes diesel powered cars less peppy and engaging to drive.
     
  • In Singapore, diesel cars are subjected to a special tax on top of the typical road tax which can add to the running costs of the car. Since this tax can be substantial, we will address it in greater detail below.

 

Special Tax

Diesel cars used to be a rarity in Singapore, and that’s not surprising considering the bad reputation it has had. Diesel cars are often considered to be polluting and slow, and it didn’t help that a huge special tax was levied on diesel cars. This special tax is placed on diesel cars because there’s no duty on diesel fuel. Petrol, on the other hand, are subjected to a petrol duty which, according to the LTA, encourages fuel conservation and discourages excessive use of petrol cars that may contribute to congestion and pollution.

But times have changed dramatically in the past decade. Diesel technology has improved rapidly, and governments are recognizing the environmental benefits that diesel cars have and have put legislations in place to promote their sales. As a result, sales of diesel cars have been on the rise. In many European countries, such as Austria, Belgium and Germany, sales of diesel cars are on a par with or have even surpassed that of petrol cars.

In Singapore, petrol passenger cars greatly outnumber diesel ones. But diesels are becoming more popular amongst more savvy car owners lately.

In Singapore, sales of diesel cars are on the rise, albeit slowly. Knowledgeable car buyers are now open to trying out diesel and that has been helped in part by the government, who has revised the special tax on diesel cars. For modern diesel cars which meet the Euro V standards, the special tax has been drastically reduced. To get a sense of how much cheaper it is to drive a diesel car now, one only has to look at the tax rates.

For a pre-Euro IV compliant car, the special tax is a whopping 6 times the road tax of the petrol equivalent. In other words, if we take a 1600cc diesel car as an example which has a 6-month base road tax of S$372, the special tax levied on a pre-Euro IV compliant diesel car would be a whopping S$2,232. The total 6-month tax would therefore be S$2,604.

For a Euro IV compliant car, the special tax is calculated to be $0.625 per cc of engine capacity and subjected to a minimum of S$625. This means that on top of the 6-month base road tax of S$372, we have to add a special tax of S$1000. Hence, the total road tax would be S$1,372 for 6 months. It’s substantially less, but it’s still a significant markup over a petrol equivalent car.

However, if you got a new diesel which is Euro V or JPN2009 compliant, the special tax is calculated at a lower rate of $0.20 per cc and subjected to a minimum of $200. This means that for a 1600cc diesel, the special tax levied would be just S$320, bringing the total 6 month road tax to a much more reasonable and affordable S$692.

Here's a table to recap the various tax payable using a car with a 1600cc engine as a comparion.


Car Type Total tax
Pre-Euro IV compliant S$2,604
Euro IV compliant S$1,372
Euro V compliant S$692
Petrol S$372

This meant that in the past and with older diesel cars, you’d have to cover huge distances to be able to justify the additional costs of the special tax. But thanks to the more favorable tax rates, it is now more affordable and financially practical to drive a diesel.

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