The Effect of Fuel Spray Momentum on Performance and Emissions of Direct-Injected Two-Stroke Engines

In recent years, it has been demonstrated that E-TEC direct injected two-stroke engines are capable of meeting the toughest emissions standards for marine outboard engines. Proper in-cylinder mixture distribution and preparation are essential for achieving low emissions, high performance, and good run-quality.

The mixture distribution is driven largely by the momentum exchange between the fuel spray and the scavenging flow. It has been found that different engines can exhibit significantly different behaviors with similar fuel sprays. This difference is attributed to the difference in scavenging flow patterns and its effect on the momentum balance between the fuel spray and the air flow.

In order to investigate this phenomenon, a test fixture was designed and built to evaluate fuel sprays into air-counter-flows with velocities of up to 40m/s by recording spray images and measuring spray penetration. Two different sprays were tested in the fixture and in a variety of engines. White light images of the fuel sprays were recorded and analyzed. The test results were used to evaluate an existing KIVA spray model. The model provides a good match for quiescent conditions and it captures the trends of the effect of drop size and counter-flow on penetration. However, the spray model quantitatively under-predicts those effects into higher velocity counter-flows. Spray and engine test data show that sprays with smaller droplets and less spray momentum are more sensitive to the scavenging pattern than sprays with larger droplets and more momentum. If the spray momentum is not large enough, Hydrocarbon emissions can increase significantly due to increased mixture short-circuiting despite the improved fuel vaporization. Similar trends were predicted by CFD modeling with KIVA.