Modern technology has overcome these drawbacks and the direct injection diesel engine for passenger car is becoming a priority for automakers all over the world. This article briefly highlights the design changes responsible for this success.

First, the diesel engine (heavy by design) is forced to shed weight when greater mobility is the criteria! An effective way of achieving this goal is to use a downsized engine and make up for the lost power by forced induction of pressurised air.

Such power boosting is not a new idea, as the practice of pressurising the induction air was adopted in the early aircraft piston engines (petrol) to make up for lost power while flying high in low-density air. In terrestrial applications, this was implemented only for racing cars.

But, today, diesel engines for passenger cars are invariably downsized engines, which are "turbo-charged" using the energy in the exhaust to drive a turbine and compressor for forced induction of air into the cylinder. In some special applications, independently run compressors off the power shaft (called superchargers) are also used.

Turbo-chargers are more energy efficient as they use the exhaust waste heat, and the fuel economy is increased if the turbo is positioned compactly in the right way, thus enhancing the performance of a smaller engine to do the work of a bigger one. The main drawback of the turbocharged diesel engine is the poor throttle response in achieving the required acceleration, causing inconvenience to the driver to manoeuvre in the congested traffic flow.

This is because of the phenomenon called the "turbo lag", or the time lag for the turbocharger to provide the needed pressure boost to the intake air when the driver steps on the accelerator pedal in search of more power.

At this time, the turbine is spinning at lower speed and takes a little time to speed and supply the needed air for better acceleration. The variable geometry turbocharger helps to reduce the turbo lag responsible for the nonlinear acceleration.

The turbine is provided with guide vanes with adjustable entry angles that respond to an electronic control system. This innovation provides a more matching level of pressure boost, even to a slow spinning turbine, without producing too much of a spurt at higher speeds. The variable-geometry turbocharger is a smart design and is becoming quite popular in European diesel passenger cars, but not yet in petrol driven cars, because the high exhaust temperature characteristics of petrol car makes the design of movable turbine blades a difficult proposition.

But the lower exhaust temperature of a diesel engine, which is a boon for turbo control, is antagonistic to the exhaust emission control process. The low temperature will create a problem for the effective use of catalytic converter with oxidation catalyst. This needs a minimum temperature (called "light off") to become active for the destruction (oxidation) of CO and HC.

Since the turbocharger sits between the engine and the catalytic converter, it absorbs the heat that the catalyst needs to light off on cold start and aggravates further the emission control. To mitigate this problem, the turbine housing is integrated with manifold casting, thus reducing the heat absorbing mass of the exhaust system and also providing better packaging. Alternatively, the use of stainless steel sheet metal housing that has less thermal inertia and higher loadings of the noble metals on the catalyst surface are other options available. Water-cooled bearing housings protect the lubricant from overheating.

Oil-less designs are also upcoming innovations that will permit installation of the turbocharger in any orientation. This saves space for better packing flexibility in the crowded engine bay of a car. Other refinements for compact packing are use of screw compressors and Roots-type blowers instead of space-occupying centrifugal compressors.

Regardless of variable vane technology, there is bound to be some lag in turbo chargers, resulting in nonlinearity between throttle control and resulting acceleration. In order to provide instant boost on demand without delay, a new innovation called " E-boosting" is under development on the laboratory breadboard.

Conceptually, it is an electric motor providing the device that boosts the air, the electric power being derived from the exhaust energy or other means.

With the introduction of 42-volt infrastructure to automotive electronics, e-boosting will be a practical proposition as the 12-volt system has limitations to power an e-booster. A computer-controlled e-boost system could be engineered to get power on demand.

Challenges in developing a clean diesel engine for cars have also been met. The key development is the common rail high-pressure (1,800 bar) fuel injection system with its compact solenoid design and electronic controls for multi-jet injection of fine sprays with pilot injections. Recently, Siemens VDO Automotive developed the first common rail using piezo-electric actuators, which are faster in action. These developments, in conjunction with an optimised combustion chamber and four-valve per cylinder with helical intake ports creating strong swirl, would give clean and smooth combustion resulting in a quietness comparable to gasoline engines.

The use of ultra low sulphur (15 ppm) and optimised combustion with electronically controlled EGR (exhaust gas circulation) aided by electronic injection control for a proper trade off reduces both NOX and particulates. The improved course of combustion eliminates the specific hydrocarbons responsible for the odour.

In addition, use of oxidation catalysts, particulate filters and selective catalytic reduction have qualified diesel engines as a clean and efficient prime mover.

(The author is a former Professor, IIT Madras.)

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