The purpose of turbo charging is not primary to increase engine power, but to increase Air Fuel Ratio, AFR, at intermediate to high load conditions to make it possible to use Exhaust Gas Recirculation, EGR, without significant increase in soot formation.
The purpose of an intercooler is to cool down the compressed air downstream of the turbocharger. The cooled air will reduce the combustion temperature as well as NOx formation, and at the same time also increase Air Fuel Ratio, AFR, due to higher density of the compressed inlet air. Cooled inlet air will also bring down the thermal stress on the combustion components.
Exhaust Gas Recirculation, EGR, system
The purpose of Exhaust Gas Recirculation, EGR, is to introduce exhaust gas, containing CO2 and water vapor, into the combustion chamber. As a result EGR absorbs heat from the combustion process, lowering the temperature and reducing NOx formation. The EGR system must be able to deliver the required EGR rates to reach the target of Euro 3 emissions in combination with turbo charging.
The EGR-coolers purpose is to decrease the emissions of nitrogen oxides, carbon monoxide, hydrocarbons and particulates. The cooling has to be chosen carefully for optimum reduction of nitrogen oxide emissions.
Diesel Oxidation Catalyst, DOC
An oxidation catalyst is used to reduce emissions of CO and HC. It also affects the heavy hydrocarbons adsorbed on the particulates. Depending on the amount of hydrocarbons adsorbed the total particulate matters can be reduced by 10-30 %. The performance of an oxidation catalyst is dependent on the exhaust gas temperature.