Back Common rail injection systems — Advanced technology for diesel vehicles
B. S. Murthy
The V-6 engine, equipped with Bosch injectors, is used by cars such as the new Audi-A6 and is rated one of the most successful engines in terms of performance, emissions and combustion noise. — Photo: Courtesy Bosch
Mahindra has already launched Scorpio CRDe with help from MICO Bosch and plans to offer this model to major cosmopolitan cities very soon. Delphi-TVS has earmarked a substantial investment to create a new facility for the CRDe to meet the growing demand in the car segment. While signing the agreement with Tata Motors for supply of CR fuel injection systems, the Tata group Chairman, Mr Ratan Tata, extolled the `Think Big' attitude of Indian auto industries, which are poised to make India self-reliant and an active global player.
What makes the CRDe so attractive? In the common rail fuel system, all the injectors are supplied by a common fuel supply line or a manifold (reservoir), called the common rail, where the fuel is charged by a high-pressure pump to 1800 atmospheres, depending on engine operating conditions and boost pressure.
This keeps the fuel delivery system delinked from load and speed variations of the engine, since the pressure charging is independent of camshaft. The injectors, which are connected to the pressure rail by short tubes, are solenoid-operated by timed electric pulses of varying width being modulated through a free mapping of the injection timing, fuel quantity and injection pressure. This is only possible in CR system by electronic control.
But in the conventional pump-line-nozzle system, a reciprocating fuel pump (called the jerk-pump) is engine-camshaft driven and literally enslaved to the crank-shaft. There is always a lag in response in this mechanical control and difficult mechanical adjustment for changing injection timing.
More important, the pressure build up and regulation of the quantity is by a complicated barrel and plunger system, with a centrally drilled hole and helical slots. All diesel technicians and engineers know this mechanism, the design and manufacture of which is a feat in precision engineering and fine technique.
Unfortunately, this time-honoured system has inherent disadvantages associated with mechanical control in governing. First, the pressure build up and release is done through every power stoke.
Fuel reaches the injector through long tubes with pressure waves travelling back and forth till the valve opening pressure is reached. This results in another injection delay. Fuel delivery and atomisation is not precise, causing combustion delay. No wonder, there is ample chance for incomplete combustion, soot and gaseous emission and combustion noise. Combustion noise can be controlled with a pilot injection of small quantity of fuel prior to main injection, so that the pressure rise during combustion is not steep, resulting in low noise.
But this is difficult to achieve, though not impossible, in a mechanical system.
Common rail system can allow multiple injections. Noise can be controlled by pilot injection, and the particulate/soot and gaseous emissions can be minimised by post injection after the main injection. The efficacy of the catalytic converter is increased as unburned hydrocarbon is destroyed by the hot exhaust. As active lean catalysts (called deNOx catalysts) are developed, nitrogen oxides (NOx) emission will also be reduced by appropriate post injection. Now it is possible to have five injections — two pilot, one main and two post injection. With such a CRDe, the quietness of a petrol engine (spark ignition) can be achieved.
With the advent of variable geometry turbochargers (VGTs), the necessity to achieve high injection pressures at low speeds and loads are required to take advantage of potential improvement in mid-range torque and, hence, the drivability of the vehicle.
This has to be optimised in the conventional diesel injection as the whole diameter rated for the rated power-operating point is too large, leading to unfavourable mixture formation.
This classic conflict in a DI combustion system tuning is easily overcome in CR systems.
Another recent trend to further refine CRDe is to replace solenoid actuation by the so-called piezo-actuators, using the reverse piezo-electric effect.
Now piezo-electric actuators have been developed by automotive industries, concurrently with solenoid type of injections — and five injections instead of three are reported to give a very quiet and low emission performance.
The new Audi-A6 with a brand of new V-6 engine, equipped with Bosch injectors of both types, is rated as one of the successful engines in terms of performance, exhaust emissions and combustion noise. Sidemen's VDO is another example of development of this high-tech injection system.
It is interesting to know that the concept of common rail technology is nearly as old as the diesel engine itself. It was the dream of Dr Rudolf Diesel to squirt pulverised coal into the cylinder by compressed air stored in a pressure reservoir — which is literally the common rail for compressed air! Soon he realised that it is more convenient to pressurise liquid fuel in the reservoir. This is the birth of the common rail principle for liquid fuel.
A fuel system based on common rail principle was originally introduced commercially by Vickers in 1913 for Atlas Imperial Company! But this did not prove practicable for large-scale commercial exploitation as there were many drawbacks in design and manufacture of these early systems.
This excellent CR concept waited for a long time till electronic controls, advancements in material and precision engineering techniques improved.
This technology was revived all of a sudden, when stringent emission control became regulatory.
Today, this forgotten CR technology has become synonymous with low emission and high performance diesel engines, thanks to advancements in electronic control.
More significantly, present-day spark-ignition engines started borrowing this diesel-originated concept of air-assisted injection in the development of new engines like Orbital engine and gasoline direct injection using air-assisted injection stored in a reservoir!
As a bonus, air assistance for injection also resulted in better torque and cleaner combustion!
(The author is a former Professor, IIT Madras. Feedback can be sent to bsmurthy@saeindia.org)
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