Influence of Trapped Residual Gasses in Air-Diluted Spark Ignited Combustion

Journal article, 2022

Homogeneous air-diluted spark ignition is one advanced combustion concept that is recognized for its potential of efficiency improvement which can be utilized in the next generation, affordable, light duty propulsion systems. However, cyclic dispersion and high load end-gas autoignition of diluted combustion remains a challenge, preventing necessary nitric oxide emission suppression which in turn obstructs market penetration. It is well known that trapped residual gasses in the cylinder influence the cyclic dispersion of combustion by contributing to the total amount of charge dilution which influence the total mean flame speed and thereby the combustion sensitivity to cyclic perturbations. However, the amount of trapped residual gasses in the cylinder is difficult to measure and therefore its influence is complicated to assess. In lean combustion research, the presence of residual gasses is often acknowledged, but few studies have investigated the influence of a combined dilution of residual gasses and air. This paper aims at assessing the influence of trapped residual gasses on lean combustion through combining engine experiments with 1D-computer simulations using a three-pressure analysis. A three-pressure analysis utilizes experimentally acquired data such as crank angle resolved port- and cylinder pressures to minimize the scope of discretization and predictability of the model, to improve accuracy. The experimental results were replicated in the simulation and quantities, such as residual gas fraction and total trapped in-cylinder mass, were estimated.
From the performed engine experiments and corresponding simulations, it has been concluded that residual gasses have a substantial influence on combustion. The total dilution, the blend of residuals and air, is highly correlated to NOx emissions at all investigated operating conditions. Additionally, at low loads, the total dilution correlates with the dilution stability limit, rather than air-dilution solely. At high loads, residuals contribute little to the total dilution, but has been linked to increased propensity of knock.