TURBOCHARGING

Presently, almost all the modern light, medium and heavy-duty diesel engines are turbocharged.  A turbocharger consists of a turbine driven by the engine exhaust gas which is directly coupled to a compressor.  The fresh air from the atmosphere is drawn into the compressor of turbocharger where its pressure and hence density are raised before it enters the engine cylinder The higher mass flow of air in the turbocharged engines compared to the naturally aspirated engines of the same swept volume, results in an increase in engine power, lower fuel consumption, better transient operation response and lower specific exhaust emissions (in g/kW-h). Advantages of turbocharging are;

• With turbocharging, excess air of more than 50% can be easily used in DI engines while still developing enough engine power.
• Due to higher air density and higher temperatures at the end of compression stroke shorter ignition delay period is obtained.
• As a result of shorter ignition delay period , the fraction of fuel burned during premixed combustion phase decreases resulting in lower NO_x emissions.
• A shorter delay period gives lower HC emissions.
• With turbocharging, the injection timing can be retarded to further lower NO_x emissions without compromising fuel efficiency and power.
• High excess air results in lower soot emissions.
• Inter-cooling of the boosted air charge further increases charge density and lower intake temperatures helps in reducing NO_x emissions.

The effect of turbocharging (TC), aftercooling (TCA) and injection retard typically observed on the NO_x – particulate trade off compared to naturally aspirated diesel engines is shown in Fig. 6. 8.