Development of a Control-Oriented Ignition Delay Model for GCI Combustion

Abstract

Increasingly stringent pollutant emission limits and CO2 reduction policies are forcing the automotive industry toward cleaner and decarbonized mobility. The goal is to achieve carbon neutrality within 2050 and limit global warming to 2 °C (possibly 1.5 °C) with respect to pre-industrial levels as stated in both the European Green Deal and the Paris Agreement and further reiterated at the COP26. With the aim of simultaneously reducing both pollutants and CO2 emissions, a large amount of research is currently carried out on low-temperature highly efficient combustions (LTC). Among these advanced combustions, one of the most promising is Gasoline Compression Ignition (GCI), based on the spontaneous ignition of a gasoline-like fuel. Nevertheless, despite GCI proving to be effective in reducing both pollutants and CO2 emissions, GCI combustion controllability represents the main challenge that hinders the diffusion of this methodology for transportation. Several works in the literature demonstrated that to properly control GCI combustion, a multiple injections strategy is needed. The rise of pressure and temperature generated by the spontaneous ignition of small amounts of early-injected fuel reduces the ignition delay of the following main injection, responsible for the torque production of the engine. Since the combustion of the pre-injections is chemically driven, the ignition delay might be strongly affected by a slight variation in the engine control parameters and, consequently, lead to misfire or knocking. The goal of this work was to develop a control-oriented ignition delay model suitable to improve the GCI combustion stability through the proper management of the pilot injections. After a thorough analysis of the quantities affecting the ignition delay, this quantity was modeled as a function of both a thermodynamic and a chemical–physical index. The comparison between the measured and modeled ignition delay shows an accuracy compatible with the requirements for control purposes (the average root mean squared error between the measured and estimated start of combustion is close to 1.3 deg), over a wide range of operating conditions. As a result, the presented approach proved to be appropriate for the development of a model-based feed-forward contribution for a closed-loop combustion control strategy.

Experimental Characterization of Hydrocarbons and Nitrogen Oxides Production in a Heavy-Duty Diesel–Natural Gas Reactivity-Controlled Compression Ignition Engine

“…A further advancement in SOI (from −35 to −50 deg aTDC) modifies the RoHR shape, characterized by a smoother energy release (Gaussian profile). Such behavior is mainly related to the HRF ignition delay increment because of the unfavorable thermodynamic conditions in the combustion chamber when the HRF is injected [28,29]. The very low NOx emissions shown by Figure 11 at advanced SOIs confirms a potential link between the combustion shape (i.e., the existence of a distinct first stage of RoHR) and NOx emissions.…”

“…With early SOIs, the shape of the apparent rate of heat release (RoHR) curve is "Gaussian" and it generates very low pollutants, especially NOx, and high indicated efficiency. However, dual-fuel combustion at very advanced SOIs leads to the ignition process (dictated by the HRF injection) becoming driven more by in-cylinder chemical reactions that are especially driven by the local air-fuel ratio [28][29][30]. By using advanced SOIs, the HRF ignition delay increases due to the local air-fuel ratio "over leaning", and it generates engine instability as evident from high values of the coefficient of variation (COV) of indicated mean effective pressure (IMEP).…”

“…As reported in Figure 6, the spread of the cyclic IMEP and CA50 are seen by analyzing a subsystem of 300 consecutive cycles (same considerations can be extended to the entire dataset) in the two identified engine conditions at low load: "Advanced SOI" (SOI −60 deg aTDC) and "Retarded SOI" (−10 deg aTDC). For dual-fuel combustion at very advanced SOI, the ignition process (as discussed previously) is driven more by in-cylinder chemistry, which in turn, is affected by the local air-fuel ratio [26][27][28][29][30]36]. To better clarify this aspect, the cyclic IMEP of two groups of 300 consecutive cycles with different SOIs ("Retarded SOI" and "Advanced SOI") are shown in Figure 7.…”

“…Such behavior is summarized in Figure 8, where the two random groups of engine cycles (the group are highlighted through the yellow box) highlighted in Figure 7 are reported. As explained in the previous section of the paper, due to the different thermodynamic conditions when the HRF is injected, the For dual-fuel combustion at very advanced SOI, the ignition process (as discussed previously) is driven more by in-cylinder chemistry, which in turn, is affected by the local air-fuel ratio [26][27][28][29][30]36]. To better clarify this aspect, the cyclic IMEP of two groups of 300 consecutive cycles with different SOIs ("Retarded SOI" and "Advanced SOI") are shown in Figure 7.…”