Evaluation of massive exhaust gas recirculation and Miller cycle strategies for mixing-controlled low temperature combustion in a heavy duty diesel engine

Abstract

The future of compression ignition engines depends on their ability for keeping their competitiveness in terms of fuel consumption compared to spark-ignition engines. In this competitive framework, the Low Temperature Combustion (LTC) concept is a promising alternative to decrease NOx and soot emissions. Thus, this research focuses on implementing the LTC concept, but keeping the conventional mixing-controlled combustion process to overcome the well-known drawbacks of the highly-premixed combustion concepts, including load limitations and lack of combustion control. Two strategies for implementing the mixing-controlled LTC concept were evaluated. The first strategy relies on decreasing the intake oxygen concentration introducing high rates of cooled EGR. The second strategy consists of decreasing the compression temperature by advancing the intake valves closing angle to reduce the effective compression ratio, compensating the air mass losses by increasing boost pressure (Miller cycle). These strategies were tested in a single-cylinder heavy-duty research engine. Additionally, 3D-CFD modeling was used to give insight into local in-cylinder conditions during the injection-combustion process. Results confirm the suitability of both strategies for reducing NOx and soot emissions, while their main drawback is the increment in fuel consumption. However, they present intrinsic differences in terms of local equivalence ratios and temperatures along combustion.