Internal combustion engines fall under increased environmental and social pressure. However, they will still play an important role in future transport, especially in hybrid propulsion systems. As a consequence, efficiency of SI engines has to be further increased. Lean burn operation provides a promising way to reach this target.
An extremely downsized SI single cylinder research engine is used for the investigations. The engine features a stroke-to-bore ratio of 1.5, leading to higher piston speeds and hence increased tumble motion. The resulting increase in turbulent flame speed supports sufficient combustion performance of diluted mixtures. Although the mentioned provisions increase combustion stability for lean burn operation the reachable relative air/fuel ratio is limited. In order to extend the lean burn capabilities of the engine (λ ≥ 2.0) and further exploit the efficiency advantages of this combustion process the engine is upgraded with a hydrogen port fuel injection. With its high laminar flame speeds and low demanded ignition energy, hydrogen acts as a good combustion enhancer in order to achieve the aforementioned targets.
The conducted measurements show that small amounts of hydrogen are sufficient to reach relative air/fuel ratios of λ ≥ 2.0 for all investigated operating points. The increase in lean burn operation is accompanied by an increase of the indicated efficiency. The investigations reveal furthermore that efficiency does not peak with the highest reachable relative air/fuel ratio. This results in the highest achieved indicated efficiency of ηi = 45.6% at a relative air/fuel ratio of λ = 1.9.