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S. Muralidhar

THE URGE to achieve the seemingly impossible in such areas as engine technology and performance, alternative fuels, driving comfort and vehicle stability is constantly driving innovation in the automotive world.

In the field of engine technology, barely a few years ago, diesels were synonymous with being dirty and they seemed to have reached a dead-end in terms of performance enhancement and meeting more stringent emission standards.

Diesel engines were inherently incapable of offering higher power that similar-sized petrol engines could muster, though they were always the more `torquey' of the two (which explains why load carriers are often run on diesel).

The difference lay in the way the two fuels were injected into the engine and their inherent combustion properties. No surprises then that diesels have just not been able to break into the world of motor sports in a big way and petrol continues to be the fuel of choice. But the day is not far when diesels rules the racetrack.

The last decade and more, there has probably been more development effort directed towards improving the diesel engine technology than to petrol burners. For companies that were attempting the breakthrough, the Holy Grail of diesel engine technology was the common rail direct (CRD) injection system, a technique so advanced and resulting in such a jump in performance for diesels that it made them almost on par with comparable petrol engines.

Yet, despite this new technique, CRD engines tend to be marginally noisier and more vibration prone than refined petrol engines, but the performance gap has been substantially bridged. What is more, there is a lot of effort going into related technologies for reducing the NVH (noise, vibration and harshness) parameters of CRD engines.

With diesel continuing to be cheaper than gasoline even in many of the developed European markets and with automobile companies keen on showcasing the latest technological innovations in their cars, this new injection system soon found its way first into luxury cars and then into mass-market vehicles.

Big Japanese, American and European brands such as Toyota, Honda, Ford, BMW, Mercedes Benz and Audi, some of which have never had diesels in their portfolios, have adopted this new technology for a number of their new cars.

The CRD injection system was first developed by companies such as Lucas Diesel Systems in the 1990s and has since been perfected by a number of component manufacturers into a reliable system that can be mass-produced and adopted even for most existing diesels after suitable changes are made to the engine design.

The ABC of diesel

To understand better how a CRD engine works, let us look at the basics first. Internal combustion engines need two important inputs for their performance — air and fuel. The more optimally the two ingredients are delivered and mixed in the combustion chamber, the more powerful and cleaner the performance of the engine will be.

As diesel's combustion properties are poorer than petrol's, optimising its injection and burn is more complex than the latter's. Unlike the simpler, less dramatic process of petrol injection, diesel needs to be sprayed into the combustion chamber for it to ignite and the whole process resembles an explosion leading to a lot of noise and vibration.

Conventional direct injection and indirect injection engines did just that. But the whole process had a lot of inevitable minuses.

Though the fuel efficiency of these engines was better or at least on a par with similar-size petrol engines (due to the manner of injection), the power output was low, the emission and the NVH levels were high and quite a bit of the fuel remained un-burnt. So, the mandate was clear when research began to fine-tune the process of injection of diesel, as that is what affects the noise, the fuel efficiency, the power and the emission.

Diesel had to be sprayed into the combustion chamber for it to ignite. That was not going to change. But what was being considered was the way in which diesel was being pressurised and injected and the amount of fuel that was being delivered.

The CRD difference

In conventional diesels, the engine had to build and generate pressure every time the fuel was injected into the cylinders. That did not give it much flexibility in terms of the intensity of pressure that could be built-up or the variation in pressure and fuel injected that may be required for responding to the vehicle's needs. So, in the older diesel engines, the amount of fuel injected rarely changed in accordance with the demands of the driver.

However, the CRD engine achieved the set goals by isolating the two functions — generating pressure and maintaining a real-time check on the amount of fuel injected. Under the CRD system, a common rail or pipe (also called a central accumulator) fitted alongside electronically controlled injectors acts as a shared reservoir of fuel.

Diesel is stored in the common rail at a constant pressure ranging from about 1,350 bar up to about 1,600 bar (approximately 24,000 pounds per square inch). The fuel available constantly at such high pressure eliminates the need for a build-up of pressure individually at each injector.

Connectors from the common rail deliver diesel already at a high pressure to each injector. At the end of the injector, a solenoid valve (a peizo valve in some of the newer ones) regulates the injection timing and the amount of fuel injected based on inputs from a microprocessor or an electronic control unit (ECU).

In the CRD system, as diesel is permanently available at very high pressure even when the engine rpm is low, the injection pressure is independent of the engine operating conditions.

This is what makes CRD engines more fuel efficient than the diesels with the older technology.

Once the fuel is injected into the cylinder at high pressure, the compression heats up the air to temperatures above the ignition point of diesel. As the fuel is being sprayed at high pressure, it ignites and a form of controlled, yet fierce, explosion takes place in the combustion chamber. This is what happens in a conventional diesel engine, too, and that is the reason for the signature rattling noise of many modern diesels even when the car is cruising.

However, there are two features that are different in a CRD engine. The pressurised spray of diesel is so fine in a common rail engine that it gets atomised when mixing with air in the combustion chamber.

So, there is complete combustion of fuel in a CRD engine, thereby boosting power output, even while eliminating wastage, keeping emissions down and improving the fuel economy.

To eliminate the noise and the vibration caused by the inevitable violent explosion accompanying the combustion process in diesel injection, most CRD engines employ what is known as a `pilot injection or pilot burn'. Seconds before the main fuel load is injected into the cylinders, a small amount of diesel is injected.

This pilot injection and ignition starts the combustion process even before the main load is injected, thus containing its explosive force. The rise in temperature and pressure is staggered and thereby less noisy.

Common rail diesel engines are a major breakthrough not just for meeting more stringent emission standards, but also for improving the level of refinement inside the vehicle.

Extremely efficient CRD engines can improve fuel efficiency by as much as 15 per cent compared to similar-sized turbo intercooler diesel engines.

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