Diesel vs Petrol Fuel

For enhanced Fuel Efficiency and lesser burden on Mother Earth, you don't have to look far ahead for hybrids and other alternative fuel sources. Modern diesel engines - common rail direct injection engines with micro processor controlled engine management system offer lower emissions and exceptional fuel economy. The NVH (Noise Vibration and Harshness level) associated with older diesel engines has drastically come down. About 10 years ago VW introduced the Pumpe Duse or pump nozzle injection system in their TDI (turbo charged Direct Injection) engines powering Jetta and Passat cars. Even the luxury SUV Touareg sports diesel engines. VW is about to change this PD-TDI to Common Rail DI technology to meet the stringent Euro 4 and North American standards. The upshot is the modern diesel engine is frugal and comparatively greener compared with its petrol guzzling siblings.

The price advantage of diesel over Petrol has been steadily declining over the years and may probably not amount to much in the future. As of now the price difference in Chennai is barely Rs.14/- Still it makes sense to go for a diesel car for many reasons as outlined below:

Diesel VS Gasoline / Petrol

Advantage of Diesel Technology:

Diesel is derived from petroleum like gasoline is. Its density is 850 gms/liter whilst the density of gasoline is about 15 % less at 720 gms/ liter. It also releases more energy than petrol. Compare this - Diesel puts out about 40.9 MJ (mega joules) per liter while petrol puts out about 34.8 MJ per liter. In other words the high energy density of diesel fuel is seen in 38685 BTU as against petrol's figure of 32895 BTU. As a result, diesel engines are far more fuel efficient than petrol engines. Not only that, they have far more torque available at a lower rpm than their gasoline engine counterparts. Diesel Engines also offer lower carbon dioxide emissions per kilometer than petrol engined cars.

Take the case of our first Indian SUV - Tata Safari as an example:

Average fuel consumption for the Variable Turbine Technology 2.2 L DICOR diesel Safari: 13.5 km per liter
Average fuel consumption for the 2092 CC Petrol Safari: 7 km per liter
It is the main reason why larger engines are invariably powered by diesel.

Diesel engines have a compression ratio of 20-22:1 compared with 7-9:1 of petrol engines. So the engines have to be bigger and built on sturdier engine blocks. The injection pumps which are critical for the performance of the engine are precision engineered. This makes these engines last longer and require fewer engine overhauls. In the land of Automobiles - USA, trucks using diesel engines have reported quarter million miles in service - which shows the longevity of heavy duty diesel engines. As they deliver more torque (turning power) at lower engine rpm, the engine tends to operate within its intended power delivery phase. Petrol engines tend to rev high and don't exhibit low end grunt which results in mixed results. A 'better' driver can keep the engine at the right rpm getting optimum mileage as well as not stressing the engine and the transmission.

As the diesel engines have their air intake always open unlike the petrol driven engines, they can idle for longer duration without gulping down the fuel. The low end grunt or the higher torque available at the lower rpm makes the diesel engine haul reasonable load easily at low engine speeds.

Turbo charged inter cooled diesel engines often can surpass the peak power/torque of many naturally aspirated petrol engines of comparable size.

In water logged roads of many parts of India, diesel cars have little chance of dying out due to electrical problems. Another factor which works in favor of diesel cars in monsoon India is the lower exhaust temperatures of diesel engines compared with petrol cars. This would result in fewer problems in the exhaust pipes getting exposed to water.

Disadvantage of Diesel:

Initial price difference is high - by at least Rs.50,000. Engines are heavier than petrol engines resulting in more wear in tires.

Oil changes are frequent - at every 5000 km instead of 10,000 km in the case of petrol engines. Modern common rail technology makes the engine management systems complex rendering the engines serviceable only with their authorized dealers.

But in these days of fuel injected petrol car engines - complex engine management control systems exist in both engine types obviating the need for specialized service facilities.

So the bottom line is - it still depends on the usage. If you commute long distances, drive around often in water logged roads or plan to retain your car for a longer duration - diesel is the best option for you.