Mixing-controlled compression ignition of ethanol using exhaust rebreathe at a low-load operating condition—Single cylinder experiments in a heavy-duty diesel engine

Abstract

As pollutant emissions regulations in the heavy-duty sector become further stringent, the energy requirements in this sector continue to increase. Due to the high-power density and operating period requirements within this sector, full electrification is challenging. A viable solution to meet the stringent requirements for criteria pollutant and greenhouse gas (GHG) emissions is the use of low carbon renewable fuels. Low carbon renewable fuels are desirable due to their lower carbon content per unit energy compared to conventional fossil diesel fuel. However, implementing some low carbon renewable fuels in the heavy-duty sector poses challenges due to their low reactivity and the prevalent mixing-controlled compression ignition (MCCI) combustion strategy. Because of this, ignition assistance is needed to ignite low reactive renewable fuels in MCCI. This work investigates the use of a variable valve actuation (VVA) strategy, exhaust rebreathe, as an ignition assistance method to ignite pure fuel grade ethanol (E98) in MCCI. Exhaust rebreathe utilizes the hot combustion products from the previous cycle by reinducing the exhaust back into the cylinder during the intake stroke from a secondary exhaust valve event. The reinduction of exhaust into the cylinder increases the in-cylinder bulk gas temperature aiding in the ignitability of the low reactive fuel. The exhaust rebreathe strategy as an ignition assistance method is investigated in this work by conducting single cylinder engine experiments on a heavy-duty diesel engine fueled with E98. Exhaust pressure is varied for the exhaust rebreathe strategy to achieve combustion characteristics with E98 similar to diesel at a low-load, high engine speed operating condition. Additionally, an elevated intake air temperature strategy is implemented to compare to the exhaust rebreathe strategy as another form of ignition assistance with the objective to induce heat in-cylinder prior to combustion without the presence of diluent. Furthermore, zero-dimensional (0D) constant pressure chemical kinetic simulations are conducted to evaluate the most reactive conditions for ethanol to achieve ignition delay times similar to n-heptane. The experimental results demonstrate that at a high speed, low load condition—the elevated intake air temperature strategy with ethanol requires 120°C intake temperature to yield an ignition delay similar to diesel. When accounting for the energy required to heat the intake air, the brake thermal efficiency is 40% lower than the diesel baseline. On the contrary, the exhaust rebreathe strategy with an exhaust pressure of 1.5 bar-a, is able to achieve the same ignition delay with an MCCI combustion process but maintain a brake thermal efficiency within 5% of the baseline diesel engine.