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Know Your Engines - II - Diesel Engines...

28 November 2010, 12:00 AM

This is the part two of our series - Know Your Engines. We covered the Petrol Engines last week and this week we write about diesel engines. If you haven’t read that article, please take a look at it first, as much technology is common between petrol and diesel engines.

India is a very price sensitive market where customers are ready to shell out an extra penny only if it provides more reliability or durability. Diesel engines require an extra investment but may reduce cost of ownership for the owner, thus explaining their huge popularity.

Customers’ opinion on diesel engines generally comes under two broad categories. Some render them to be noisy, messy and suffocating and have no liking to them. Others opt for them for their fuel efficiency, high mileage and more value for money, making the diesel engine a large market.

Invented by Rudolph Diesel, the world knows the diesel engine as a powerful powertrain with outstanding pulling power, fuel efficiency and longevity.

Diesel engines are used in automobiles, generators, boats and have varied applications with agricultural applications. Turbo-charged diesel engines are also used in two-wheelers like motorcycles.

The Working of the Diesel engine

Diesel engines and petrol engines share several similarities. They have number of components in common such as the crankshaft, pistons, valves, camshaft, and coolant and and oil pumps. However, a key difference between the two is that the diesel engine lacks an ignition system. Instead of relying on a spark for ignition (as in petrol engines), a diesel engine uses heat produced by compressing air in the combustion chamber to ignite the fuel. All other systems used in the diesel powered vehicles are essentially the same as those used in gasoline vehicles, and we refer to the article published last week on petrol engines.

Ignition system in diesel engine

In a diesel engine, fuel injection is used. Fuel is supplied to a Fuel injection pump and from there to injectors positioned on each cylinder. Timing and pressure is set to inject a fine spray of diesel at the end of the compression stroke. The heat of compressed air entered into the cylinder then ignites the fuel and thus begins the power stroke.

Fundamentally, using compressed, hot air as well as diesel spray ensures that the air and diesel particles can mix better so that ignition can take place.

Glow plugs are used in diesel engines only to warm the combustion chamber when engine is cold. As the above explained, the air in the cylinder needs to be hot to ignite the diesel spray. Hence, cold starting is impossible without these plugs because even the high compression ratios cannot heat cold air enough to cause combustion. Thus when you’re just starting your engine, these glow plugs will warm up the air in the combustion chambers.

Schematic comparison of Diesel and Petrol technology

  Gasoline Diesel
Intake Air/Fuel Air
Combustion Spark ignition Compression ignition
Air/Fuel Mixing point In intake manifold near intake valve In cylinder near Top dead center by Injection
Compression ratio 8 -10 to 1 13-25 to 1
Pressure 464 Psi 1200 Psi
Exhaust 704- 982 degree centigrade, CO =1% 371-482 degree centigrade CO=0.5%
Efficiency 22-28 % 32-38%

Let’s take a look at the differences and clarify on why they exist.

First of all, as explained before, in diesel engines, there is a separate injection for diesel fuel into a diesel engine’s cylinder, as opposed to the common intake valve in petrol engines, where fuel and air are injected in one go into the intake valve.

This also means that in petrol engines, explosion takes place near the fuel intake valve itself, whereas for diesel engines this takes place properly in the chamber. For the curious ones, it is this property which makes the two stroke engine still allowable for large power diesel engines (such as in trains or ships) as the combustion is much more complete even in a two stroke engine (less leakage), thus combining the power of the two stroke engine with a high efficiency.

The compression in diesel engines needs to be much higher to ensure a proper combustion, compared to the more easily ignitable petrol fuel. As you see, the compression is almost double.

As a result the pressure (1200 Psi vs 464 Psi) is also more than double!

The temperature of the diesel’s exhaust fumes is less than petrol engines, due to the technology used, and also, the amount of noxious gases is also less in petrol gases.

Finally, all this culminates in a higher fuel efficiency for diesel engines.

Types of Diesel Engines

Diesel engines come in both two stroke or four stroke engines, differing from petrol engines only in their ignition systems. Large diesel vehicles usually use 2-stroke configuration for simplicity though passenger vehicles tend to have 4-stroke configuration. Over the years, variations have been developed for diesel engines. Let’s take a look at some engine types.

CRDi Engine

Common Rail Direct Engines are favourites amongst Mahindra automobiles in India. Globally, they are widely used by Fiat automobiles which are known to be extremely robust cars. In India, apart from Mahindra, Hyundai Accent, Ford Motors and BMW automobiles widely use CRDi engines. They are variation of direct injection engines but are less noisy, less messy and cleaner.

More specifically, in CRDi engines, there is a ‘common rail’ containing the diesel to be injected into the cylinders. This common rail serves as an accumulator for fuel and pressure to spray the diesel is built up in this common rail, instead of in the individual injectors thus allowing better control of the pressure and uniform pressure while injecting. Importantly, as the pressure in the diesel (needed for spraying) is built up in a different component, the pressure can be maintained consistently, so that the engine is more efficient and smoother.

TDI Engine – Turbo-charged Direct Injection

The TDI engine is a technology developed by the Volkswagen group. In these engines, a turbocharger is added to the multi-jet fuel injection engine which helps in increasing the performance of the engine. This turbo charger further compresses the air which is provided to the cylinder, while keeping it cooled as well, thus increasing the amount of air which is available to ignite the diesel spark.

The result of the larger amount of air is that more fuel can be injected for combustion, while this combustion is also more complete. It thus reduces emissions drastically making diesel engines cleaner, while the power of the car is also increased.

Initially seen in VW Group cars, turbodiesels are now also used by BMW, among others.

Multi-jet Fuel injection Engine

Multi jet diesel engines are what Fiat Group companies call their common rail diesel engines. These are the engines that in India power Maruti cars, such as the Maruti Swift Dzire, Suzuki Swift Diesel, Tata Cars, such as the Indica, and of course Fiat Group cars, including the Palio.

These engines were developed by Fiat Group and feature Common Rail technology, but also Turbo technology, compressing the air further before combustion.

Naturally Aspirated

They are direct injection engines without a turbocharger. Then engine draws the air through air filter which passes through a monitoring meter into the combustion chamber. They produce less power and also have low pulling power. They are generally used to make automobiles cheaper as turbocharged engines are more expensive than naturally aspirated ones.

Benefits of Diesel Engines

  • Fuel Efficiency

    In a weight to weight comparison, diesel per volume unit contains more energy than petrol, i.e. it has a higher calorific value. Thus, if a complete ignition of diesel takes place, the engine will deliver more power than a petrol engine. Equivalently, a smaller amount of diesel is needed to deliver a certain amount of power. As a result, the diesel car’s mileage is better than the petrol car’s.

    At the same time, with diesel fuel cheaper than petrol fuel (since diesel is unrefined and thus requires less work from the oil company), the running cost of a diesel engine becomes lower than a petrol engine’s.

    Hence, if you have been thinking of a hybrid car but limited by its price, then think of diesel engines. Although diesel engines cannot deliver the eco-friendliness of hybrid models they can definitely match the fuel efficiency.
  • Safety

    Diesel engines are safer than gasoline engines. They are less volatile than the petrol engine which means they are less likely to explode in case of a car crash
  • Longevity

    Diesel engines last longer as they are less prone to wear and tear. This makes diesel automobiles more cost effective. Moreover, vehicles used for long runs opt for diesel engines. Diesel engines require less maintenance and are robust in nature.

    However, maintenance when undertaken for a diesel engine, will be more expensive than for a petrol engine, as it is a more complex technology. Similarly, the initial investment for a diesel vehicle is also more, for the same reason.

    Thus, the buyer should ideally offset the investment and maintenance expectation against the savings to be expected from the better mileage.
  • Torque or Pulling Power

    Diesel engines have greater pulling power which clubbed with their tendency to last longer converts commercial and passenger vehicles to more value for money. However, acceleration is less for diesel engines.
  • Environment

    It may be surprising to you, but diesel engines are considered as cleaner than petrol engines. This is due to a more complete combustion. Hence, though the emission in diesel engines is more visible (dusty), petrol engines emit invisible noxious gases.

    An interesting recent development is bio-diesel, which is a animal or vegetable fat based fuel which has properties equivalent to diesel.

Do read this more thorough analysis and comparison of petrol and diesel engines for more information. as well as this comparison of petrol, diesel and LPG.


With the steep increase in price of petrol every fortnight, there is an increased tendency to go for diesel engine cars as the trade off between the investment of the diesel engine and its lower running cost is tipping. This has resulted in almost all manufacturers coming up with both petrol and diesel version their car models. The present generation diesel engines have started to overcome earlier defects such as cold starting problem (waiting for the glow plug to heat the air before starting the engine) as well as improved noise reduction technology to overcome excessive noise from diesel engines.

The improvements have resulted in smoother operation of the diesel engines, though the difference to the petrol engines version remains around rs 75,000. Hence customers may remain in dilemma whether to invest high for diesel engine version or to go for petrol version.

Our earlier articles on selection between petrol and diesel engines and alternative fuels will certainly help in this regard.

We hope the above has shed some light on the working of the diesel engine and its advantages. We remind you that much technology between diesel and petrol is similar, and thus you should read both articles in conjuction.

We welcome your questions and comments.and will do our best to answer them.

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