EGR and Backpressure Effects on Knock Behavior in Stoichiometric Natural Gas Engines

Abstract

Increasingly restrictive limits on NOx levels are driving the change from lean-burn to stoichiometric combustion strategies on heavy-duty on-highway natural gas engines in order to take advantage of inexpensive and effective three-way catalyst technology. The change to stoichiometric combustion has led to increased tendency for engine knock due to higher in-cylinder temperatures. Exhaust Gas Recirculation (EGR) has been proposed as a method to suppress knock via charge dilution while maintaining a stoichiometric air-fuel ratio. Two of the more common EGR driving architectures and the challenges associated with each architecture are described. A series of engine tests were devised and performed on a 7-liter heavy-duty natural gas engine to explore the relationships between EGR knock suppression and engine backpressure. A unique concept for an external EGR pumping cart which allowed for the exploration of higher EGR rates independent of backpressure is also described. Results showed that for the conditions tested, increasing EGR rates beyond a certain point did not result in decreased knock tendency. 1D Simulation showed that the effectiveness of the EGR is limited by trapped hot residual gasses which resulted in higher in-cylinder temperatures and nullified the cooling effects of the EGR. These results suggest that attention must be paid to reducing backpressure via efficient EGR system architecture design in order to achieve the highest possible efficiency.