Enhancements in Hydrogen Low Pressure Injection on Small Two-Stroke Engines

The use of small 2-stroke crankcase scavenged engines running on hydrogen is very attractive for low power rates, when low cost and compact dimensions are the fundamental design constraints. However, achieving optimal performance with hydrogen fuel presents challenges, including uneven air-fuel mixtures, fuel losses, and crankcase backfiring. This research focuses on a small 50cc 2-stroke loop-scavenged engine equipped with a patented Low-Pressure Direct Injection (LPDI) system, modified for hydrogen use. Experimental results demonstrate performance comparable to the gasoline counterpart, but further optimizations are needed. Consequently, CFD-3D simulations are employed to analyses the injection process and guide engine development. The numerical analysis focuses on a fixed operating condition: 6000 rpm, Wide Open Throttle (WOT), with a slightly lean mixture and injection pressure fixed at 5 bar. A numerical model of the entire engine is set up with the primary objective of improving injection efficiency by modifying the position and orientation of the injector, along with the piston dome shape. Seven configurations under the same operating conditions and injected mass are investigated to assess the impact of these modifications and find the best compromise. The methodology considers the following parameters: fuel trapped within the cylinder, fuel lost through the exhaust, fuel mass in the crankcase, and mixture uniformity before spark ignition. The best-performing configuration, featuring a standard piston dome but with a repositioned injector, achieves a notable reduction in fuel short-circuiting (up to 20%), while ensuring a relatively uniform air-fuel mixture at spark timing.