A parametric study to improve first firing cycle emissions of a gasoline direct injection engine during cold start

Abstract

A parametric study was carried out for the first firing cycle of a 4-cylinder, 2.0-liter, turbocharged gasoline direct injection (GDI) engine. The primary goal was to see how changes in the fuel injection parameters would affect the GDI engine combustion and emissions for the first four combustion events that constitute the first firing cycle. Experimental studies were carried out with a custom-designed powertrain control system to measure the HC emissions and pressure development for the first firing cycle. The quantitative experimental results were accompanied by simulations of the detailed temporal and spatial fuel concentration profiles using Converge CFD engine simulation software. An alternative calculation method was used to calculate the average combustion equivalence ratio for each of the four cylinders. This method showed that the majority of the cold start HC emissions during the first firing cycle was unburned gasoline and its possible decomposition products, which did not contribute significantly to the combustion and heat release. For the same amount of fuel injected into a cylinder, increased fuel rail pressure resulted in better evaporation and combustion, while slightly increasing the HC emissions during the cold start process. A multiple injection strategy was studied that split the fuel delivery between the intake stroke and the compression stroke with either one or two injections in each of those strokes (two or four injections total). The quadruple injection strategy led to better first cycle combustion, with higher engine IMEP and lower HC emissions. This resulted from a richer fuel mixture in the region near the spark plug due to better fuel evaporation and a better spatial fuel distribution. While increasing fuel rail pressure with either injection strategy failed to significantly lower the HC emissions given the same amount of injected fuel mass, higher rail pressure with the quadruple injection strategy resulted in higher IMEP for the same amount of injected fuel; this may provide the possibility to reduce the total fuel injection mass which may have benefits for both fuel consumption and emissions.

Development of a fractal engine simulation model in a multidimensional simulation for the cold start process of a gasoline direct injection engine

“…HC emissions are a primary concern for engine cold start, especially for the first several cycles, as mentioned in the introduction section. The simulation results of the fuel tracking using the FES model for the very first firing cycle are presented as the Sankey graphs in another paper 51 of ours, along with experimental measurements. That study showed that there are thick fuel films (26% of total injected fuel) on the cylinder and piston walls at a very late crank angle of the very first firing cycle.…”

A detailed multidimensional simulation for the cold start process of a gasoline direct injection engine using a fractal engine simulation model

“…A comparison of the engine speeds using the 2-inj and 4-inj strategies is provided as Figure 3 in our previous paper. 14 In particular, for the intake strokes of cylinders 2 & 1, seen in that figure, the engine speed with the 4-inj strategy significantly surpassed that of the 2-inj strategy, enhancing the engine intake turbulence intensity, making a direct comparison of the effects of the normalized turbulence more complicated. Nevertheless, the overall effect is presented in Figure 4 of that same paper 14 : the combustion process for all four cylinders was greatly enhanced in the improved strategy, with the least increase of peak pressure for cylinder 1 (the last one to fire), where the engine speed increased the most.…”

“…The high-resolution transient engine speed was captured and calculated based on the pulse signals of a rotational incremental encoder, which was installed on the flywheel. Figure 3 depicts a schematic diagram of the experimental setup, and more information can be found in previous papers 12,14 from our team.…”