Cooled v/s Un-cooled EGR

Cooling of EGR before mixing with air has the following possible effects;

• Lower intake charge temperature would further reduce NO_x formation.
• Higher intake charge density, higher volumetric efficiency and higher oxygen content in charge would give higher flame temperatures resulting in higher oxidation of soot.
• A higher volumetric efficiency provides lower BSFC.
• Lower charge temperatures would result in longer ignition delay period, more premixed combustion and higher unburned HC emissions.
• For the same reduction in NO_x, loss in fuel economy is lower with cooled EGR compared to un-cooled EGR.

Due to a better NO_x - BSFC trade-off, the cooled EGR is more commonly employed in practice.

EGR Systems for Turbocharged Engines

Two methods are used for application of EGR in the turbocharged engines. The layouts of the two systems given below are shown on Fig. 6.10.

Low- Pressure Route: Exhaust gas from downstream of turbine is tapped and inducted to the intake side of the compressor. In this method, the required pressure difference across the EGR valve is easily obtained and EGR is possible over a wide range of engine operation.   The main disadvantage is that   the exhaust gas along with air also passes through the compressor and intercooler resulting in durability problems of compressor due to presence of soot particles.

High-Pressure Route: Exhaust gas from the upstream side of the turbine is tapped and led to the exit side of the compressor. The EGR gas does not pass through the intercooler or compressor. EGR is additional to the intake charge of compressor and amount of oxygen in the charge remains constant. With this method, higher reductions in NO_x can be achieved with lesser increase in smoke and particulate emissions.
In the high-pressure route of EGR sufficient pressure difference between pressure upstream of turbine and downstream of compressor is not available under all the engine speed-load conditions. At high loads the pressure at the exit of compressor may be higher than the pressure upstream of turbine.  To introduce EGR at high engine loads:

(i)       Turbine back pressure is increased, by use of a variable geometry turbocharger,  and

(ii)      A venturi is fitted between the compressor and intake manifold. The EGR is introduced at the throat of           venturi.