Analysis of scavenging and mixture formation in a 2-stroke direct-injection hydrogen engine with poppet valves

Ultra-lean spark-ignited hydrogen combustion is a proven technique for achieving near-zero pollutant emissions, along with high brake thermal efficiency. However, conventional small-bore 4-stroke engines are affected by a relevant reduction of their power density. The 2-stroke cycle offers a clear advantage, but it also needs an effort to extract its full potential. Among the various 2-stroke designs, the selected one features intake and exhaust poppet valves and external supercharging and/or turbocharging (no crankcase pump, standard lubrication). This solution has the major advantage of requiring minimal modifications compared to a conventional 4-stroke engine, except for the cylinder head and valves, which must be redesigned from scratch. The goal of this study is to explore the above-mentioned concept for an automotive application, delivering a maximum brake power of about 120 kW along with near-zero emissions. The study is particularly focused on the optimization of the scavenging and air–fuel mixing processes. Engine development was based on specific theory and state-of-the-art numerical simulation framework, combining both 0D/1D and 3D-CFD models. Combustion analysis was carried out through a quasi-dimensional model, embedded in the 0D/1D engine model and calibrated with experimental data. The numerical results show that the optimized design guarantees the formation of a strong counter-tumble vortex (tumble ratio = 3.1) into the combustion chamber, with a limited amount of short circuit of fresh charge (about 15%). Late direct hydrogen injection, when both exhaust and intake valves are closed, and high in-cylinder turbulence are the keys to provide an optimal fuel–air mixing at spark timing, avoiding any loss of fuel from the combustion chamber. Introducing the scavenging characteristics calculated by the 3D-CFD analysis into the 0D/1D engine model, the latter predicted that the proposed 3-cylinder 1.2 ℓ 2-stroke direct-injection spark-ignited hydrogen engine can easily fill the performance gap between hydrogen and Diesel, typically found on 4-strokes. Moreover, the 2-stroke engine maintains the same excellent standards of efficiency and emissions of an reference 4-stroke 4-cylinder 2.0 ℓ direct-injection spark-ignited hydrogen engine. The current study provides a foundation for the development of hydrogen valved 2-stroke engines and highlights key design features and challenges. Clearly, further research is needed to include accurate combustion modelling and evaluate real engine performance and emissions.