Cylinder to Cylinder Variation Related to Gas Injection Timing on a Dual-Fuel Engine

The natural gas/diesel dual-fuel engine is an interesting technique to reduce greenhouse gas emission. A limitation of this concept is the emission of un-combusted methane. In this study we analyzed the influence of PFI gas-injection timing on cylinder to cylinder gas-distribution, and the resulting methane emissions. This was done on a 6 cylinder HD engine test bench and in a GT-power simulation of the same engine. The main variable in all tests was the timing of the intake port gas injection, placed either before, after, or during the intake stroke. It showed that injecting outside of the intake window resulted in significant variation of the amount of trapped gaseous fuel over the 6 cylinders, having a strong impact on methane emissions. Injecting outside of the intake stroke results in gas awaiting in the intake port. Both testing and simulation made clear that as a result of this, cylinder 1 leans out and cylinder 6 enriches. The simulation showed how this is caused by the airflow into the manifold, which enters the manifold close to cylinder 1. This flow picks up gas residing before the first cylinders, and distributes it over the cylinders further downstream. The richer cylinders have a higher absolute methane emission. By calculating the ratio between trapped and emitted methane, we found operating points where the enrichment caused by unequal gas distribution led to lower relative methane emission. This can be attributed to the better combustibility of methane under richer conditions. Although the injection timing was placed outside the intake window on purpose in our experiments, the same distribution problem occurs in situations where the gas injection is longer than the intake stroke. This is a common situation with currently available dual-fuel systems. The solution can be found in a gas injection system with significantly higher capacity.