ENGINE ALTERATIONS: Modifications in the engine chiefly relate to changes in the combustion process to reduce the formation of harmful pollutants. Though nox emissions are reduced while lowering the maximum temperature reached during combustion, it inhibits the complete oxidation of soot leading to higher particulate emissions.

• Injection timing retard, that is, delaying the start of fuel injection can significantly cut nox emissions by reducing the peak gas temperature, but this increases particulate emissions and reduces fuel economy.

• Achieving proper mixing of the air and fuel injected into the combustion cylinder by alterations to the cylinder reduces both nox and pm emissions, but this is difficult to achieve in small, high-speed diesel engines used in cars and light trucks. This is singularly important in view of the fact that diesel car manufacturers are increasingly targeting the small car segment.

• Exhaust Gas Recirculation (egr) consists of sending some exhaust gas back to the engine intake system. The recycled gases act as a dilutant, which reduce the peak gas temperatures. The limitation of this method is that it decreases combustion quality and increases particulate emissions with additional egr.

• Turbocharging is another technical option, which has been used to increase the power output of current direct injection diesels. But this elevates the temperature of the intake air, and this in turn cause increased nox emissions. This can be reversed by cooling the air used for turbocharging. However, this has a negative impact on fuel economy and possibly increases pm emission.

EXHAUST CONTROL TECHNOLOGIES: Lean nox catalysts facilitate the conversion of nox to nitrogen and require hydrocarbons in the exhaust gas to act as reducing agents. These catalysts are most effective when exhaust gas ratios of HC to nox are high. However, this ratio is very low in diesel exhaust generally much less than one, which casts a limitation on this system in the form of low fuel economy. Besides, the temperature of most of the diesel exhaust entering the catalyst results in making them inefficient over much of the driving cycle. High levels of sulphur in the fuel is another constraint of this technology.

• Selective catalytic reduction (scr) systems employ nitrogen-containing compounds (such as ammonia or urea) as reducing agents. scr systems are a proven, effective technology in stationary source applications, but application in the light-duty mobile sector faces serious challenges like need for an ammonia (or urea) supply infrastructure, sophisticated ammonia injection and control system, release of unreacted ammonia, etc.

• nox absorbers, which trap the gas and convert it into nitrogen, are also responsible for a loss in fuel economy. Although they display the potential to achieve high conversion efficiencies, they lack sufficient thermal durability. Besides, sulphur sensitivity poses a significant challenge. Prolonged exposure to even ultra-low sulphur fuels is likely to reduce their efficiency.

• Diesel oxidation catalysts (doc) reduce pm by targetting the soluble organic fraction of the particulates. The catalysts oxidise hydrocarbons to carbon dioxide and water, thereby reducing particulate mass emissions. However, docs also actively oxidise sulphur dioxide (so2) to sulphate particles, which actually increases PM emissions. The overall catalyst efficiency depends on the chemical composition of the particulates and the sulphur levels in the fuel. Some research also suggests that they may increase the number of ultrafine and nanoparticles emitted at lower exhaust gas temperatures.

• Particulate filters (or traps) physically capture the soluble organic and soot fractions of PM, but with time, the particles fill up and clog the filter, causing pressure in the exhaust system to increase (called backpressure), which reduces fuel economy and inhibits engine performance, thus creating the need for regeneration or cleansing process. Regeneration requires either passively or actively initiating combustion of the accumulated soot. Soot normally ignites around 500