Lean Burn Natural Gas Operation vs. Stoichiometric Operation with EGR and a Three Way Catalyst

Exhaust Emissions from lean burn natural gas engines may not always be as low as the potential permits, especially engines with open loop lambda control. These engines can produce much higher emissions than a comparable diesel engine without exhaust gas after treatment. Even if the engine has closed loop lambda control, emissions are often unacceptably high for future emission regulations. A three way catalyst is, today, the best way to reduce hazardous emissions. The drawback is that the engine has to operate with a stoichiometric mixture and this leads to; higher heat losses, higher pumping work at low to medium loads, higher thermal stress on the engine and higher knock tendency (requiring lower compression ratio, and thus lower brake efficiency). One way to reduce these drawbacks is to dilute the stoichiometric mixture with EGR. This paper compares lean burn operation with operation at stoichiometric conditions diluted with EGR, and using a three way catalyst. The results show that nitric oxides (NO_X) and hydrocarbon (HC) emissions are several orders of magnitude lower than at lean operation. Higher loads can be achieved, and brake efficiency is higher than lean operation optimized for low NO_X production. A fast burning (high turbulence) combustion chamber is used to allow high amounts of dilution.