The exhausted gas recirculation improved brake thermal efficiency and combustion characteristics under different intake throttling conditions of a diesel/natural gas dual fuel engine at low loads

Abstract

Previous studies have shown that the introduction of EGR into diesel pilot-ignited natural gas engine can increase brake thermal efficiency (BTE) and emissions. And intake throttling can also increase BTE and reduce THC and CO emissions as shown in previous studies. However, diesel/natural gas dual fuel engines are usually modified from pure diesel engines without throttling valve. To increase BTE and reduce emissions, a detailed study on EGR coupled with intake throttling was conducted on a 6-cylinder turbocharged intercooler diesel/natural gas dual fuel heavy-duty engine at low load operations. The experiments kept the total fuel quantity, gas injection strategy and diesel injection strategy unchanged. The time-sequenced characteristic and the heat release rate (HRR)-imbalanced characteristic were discussed. The results show that the BTE increased and brake specific THC, NOx and CO decreased as EGR rate increased without intake throttling, CA10 and CA50 shows a slightly delayed trend which are similar to previous studies. EGR with intake throttling (throttling opening degree (TOD) = 30.11%) will continue to increase BTE and reduce emissions as EGR rate increased. While, the BTE increases first and then decreases, NOx still shows a significant decrease trend, THC first falling slightly and then rising slightly, CO first falls slightly and then significantly up as EGR rate increased with further intake throttling (TOD = 25.1%). When TOD is 25.1% and EGR rate is 13.39%, the engine can achieve highest BTE of 31.88%. THC and CO are lower, but NOx is slightly higher at this condition compared with EGR without intake throttling condition. In addition, the BTE can be further improved to 34.75% by advanced diesel injection timing with EGR and intake throttling. In summary, EGR with proper intake throttling can improve the BTE and reduce emissions at a relatively low level on diesel/natural gas dual fuel engine at low load. These provide research guidance for the practical application of diesel/natural gas dual fuel engine.