The effect of Exhaust Gas Recirculation (EGR) on the emission from a lean-burn gas engine

Per Pedersen, March 1998, R9806

Exhaust gas recirculation (EGR) is a well known method of controlling emission of nitric oxides from engines. It was used on petrol engines before introduction of the three-way catalyst. Recently, it has been re-introduced in combination with the three way catalysts.

On automotive petrol engines smaller rates of EGR are utilised.

When larger rates of EGR are applied, it is known from various sources in the literature that the THC content in exhaust gases increases, and, eventually, severe misfires occur at about 30% EGR. In this project, only smaller EGR-rates were investigated.

The project should demonstrate an indirect effect of EGR on the content of total unburned hydrocarbons (THC) in the exhaust gases from a lean-burn natural gas engine.

When running a specific engine on lower air excess (without EGR), it is known that the THC content in the exhaust gases is often lower compared to the THC content when running the engine on higher air excess. On the other hand, nitric oxides will increase rapidly at decreased air excess. The formation of nitric oxides can be suppressed by the use of EGR.

A small rate of EGR has little effect on the emission of THC according to the literature. The indirect effect of replacing air excess with EGR should thus be a reduction of the emission of THC.

The tests showed that a small amount of about 1% (EGR) reduces the nitric oxides significantly, with no detectable undesired effect on fuel consumption or THC emission.

This is the case for gas engines running moderately lean (max. 50% air excess), and EGR can thus be recommended for these engines. It lowers the content of nitric oxides, or the engine can be operated on a richer mixture leading to reduced THC emission.

On engines running on ultra lean mixtures, EGR has little effect, mostly disadvantages.

he experiments reported here were carried out on a supercharged natural gas lean-burn engine installed in the laboratory at DGC. The engine has been built into a mini co-generation unit comprising control unit, asynchronous generator and heat exchanger. Emission characteristics have been reported in an earlier project report.