This article presents experimental results obtained with a disruptive engine platform, designed to maximize the engine efficiency through a synergetic implementation of downsizing, high compression-ratio, and importantly exhaust-heat energy recovery in conjunction with advanced lean/dilute low-temperature type combustion. The engine architecture is a supercharged high-power output, 1.1-liter engine with two-firing cylinders and a high compression ratio of 13.5: 1. The integrated exhaust heat recovery system is an additional, larger displacement, non-fueled cylinder into which the exhaust gas from the two firing cylinders is alternately transferred to be further expanded.
The main goal of this work is to implement in this engine, advanced lean/dilute low-temperature combustion for low-NOx and high efficiency operation, and to address the transition between the different operating modes. Those include well-mixed charge compression-ignition at low-load, and a mixed-mode combustion at higher loads, before transitioning to boosted homogenous and stochiometric spark-ignited combustion. Here, the mixed-mode combustion strategy is composed of a deflagration of a stratified mixture created by a late direct injection, then triggering a controlled autoignition of the surrounding gas, improving the robustness of lean/dilute combustion. The paper describes the key features of the engine and details regarding the combustion and multi-mode valve strategies.
The experiments were performed under steady-state operation at 2000 rpm, from 1 to 11 bar IMEPn and naturally aspirated conditions. The engine demonstrated great efficiency gains compared to a conventional naturally-aspirated and downsized-boosted spark-ignited engines. The piston-compounding exhaust-heat recovery system contributes to up to 10% of the total efficiency improvement, while lean/dilute advanced combustion increases the fuel economy by up to 38% compared to a naturally aspirated engine, and up to 22%, compared to a downsized-boosted engine. NOx emissions target was met using high-levels of internal and external dilution in mixed-mode combustion operation, as well as by optimizing the injection and ignition strategy. Finally, the analysis shows that a seamless transition between the different valving strategies is achievable in support of robust transient operation.
