The advantage of using EGR is that a three-way catalyst can then be used to reduce the NO x to nearly zero because the overall exhaust mixture is stoichiometric. Natural gas engines create a new set of technical hurdles with the addition of EGR technology. While dilution with EGR can reduce NO x , there is a limit to the amount of dilution analogous to the lean-burn engine at the lean limit. The amount of EGR dilution is usually less than can be achieved with air so the NO x emissions produced by the engine are higher, but it enables a three-way catalyst to be used. Because the overall exhaust mixture is stoichiometric, the catalyst can significantly reduce the remaining NO x emissions so that the overall NO x emissions are nearly zero.
Development is still necessary to maximize the amount of dilution.
Another technical hurdle is thermal management. Because the amount of effective EGR dilution is limited, engine temperatures rise and heat transfer to critical components increases. Accurate metering of EGR under transient situations will be required, and the development of suitable EGR metering valves and coolers will be important. Diesel engine manufacturers, currently interested in using EGR to reduce NO x in diesel engines, might be able to provide some guidance by way of technology transfer. NO x emissions vary greatly with different engine configurations.