Various mixture control strategies for a light duty gas engine with scavenged pre-chamber

Abstract

This work presents research and development of an advanced ignition system with the scavenged pre-chamber for extremely lean mixture combustion in a natural gas fueled engine for a light duty truck. The goal of the work is efficiency improvement and reduction of greenhouse gas emissions of an engine with a simple and robust after-treatment system, capable to comply with current and future emission limits. The results of experiments show that the scavenged pre-chamber significantly expands the flammability limit compared to the conventional gas fueled SI engine. Low temperature combustion of extremely diluted charge enables to achieve high engine efficiency and low NOx emissions in raw exhaust gases and reduces the necessity of using a special NOx after-treatment. At full load conditions, the pre-chamber engine is able to operate with the stoichiometric mixture to achieve the highest possible power density. The stoichiometric operation is also compatible with a simple three-way catalyst. The proposed control strategy for the pre-chamber engine therefore assumes the use of a combination of stoichiometric operation at high load and lean combustion concept in part and low load region. The paper describes an experimental mapping and optimization in steady state engine operation in selected points of the full performance map with stoichiometric and lean combustion. Tests with activated and de-activated scavenging of the pre-chamber were performed as well. Optimum combustion phasing was considered. Wide range of air excess ratio was investigated to obtain the NOx and combustion efficiency trends for subsequent considerations. Based on the measured results, a detailed engine model was thoroughly calibrated. Transient cycle modelling of proposed fueling control strategy in the World Harmonized Transient Cycle (WHTC) was carried out. Multiple levels of air dilution at low load region were tested and the optimal setting was chosen regarding the combustion efficiency. The results were compared with the conventional SI engine operated with the stoichiometric mixture full time. The results show the operating region where the lean strategy is beneficial in terms of NOx and CO2 emissions. At the current stage, the potential for 5% WHTC CO2 emissions reduction is shown, maintaining NOx emissions level below the current emission limit.