## 9 Conclusions  
This study reviews the current development of hydrogen internal combustion engines, with a focus on the hydrogen direct injection strategy. The port fuel injection engine configuration suffers many limitations, including pre-ignition, knocking, backfiring, low volumetric efficiency and compression loss problems. This limits the engine achievable load and efficiency. A potential solution to circumventing or alleviating these limitations is hydrogen direct injection. Despite being a promising strategy, the low engine compression ratio of typical SI engines limits the thermodynamic efficiency. In a compression-ignition engine, the spark ignition can be replaced by pilot fuel ignition, which induces multiple ignition kernels to promote fast combustion of gaseous fuel. This combustion mode, known as dual-fuel hydrogen-diesel direct injection, can potentially alleviate the power and compression ratio limitations reported for hydrogen applications in spark-ignition engines. When using green hydrogen, such combustion mode will greatly reduce the reliance on fossil fuels and thus reduce carbon-based emissions. However, hardware development for commercialization and research towards enhanced understanding of the mechanisms governing the engine efficiency and pollutant formation in such dual-fuel combustion mode is needed:  
Figure 10: Jet tip penetration prediction for hydrogen and methane at (**a**) different injection pressures and 1 mm nozzle diameter and (**b**) different nozzle diameters and 150 bar injection pressure. The ambient density, pressure and temperature used in this model are 20.8 kg/m\({}^{3}\), 60 bar and 1000 K, respectively.  
* Metal engine testing is required to prove the effectiveness of this combustion concept in terms of emissions and performance, and to investigate the effect of different operating parameters, for instance injector configuration and operation strategy.
* Fundamental optical and laser-based investigation as well as numerical simulations are needed to understand the governing mechanisms to facilitate engine performance optimization.
* For research purposes, this combustion mode can be studied by using single-fuel injectors--prototype injectors for hydrogen while using commercial diesel injectors to deliver pilot fuel. An integrated dual-fuel injector for hydrogen and pilot fuel may be needed as a long term solution.
* Further technological advancement towards a complete, compact, cost-efficient and robust on-board fuel delivery system is required for commercialization.  
Conceptualization, H.L.Y.; A.S. and Q.N.C.; resources, A.C.Y.Y.; S.K.; R.A.T. and G.H.Y.; writing--original draft preparation, H.L.Y.; A.S. and Q.N.C.; writing--review and editing, A.C.Y.Y.; S.K.; R.A.T.; G.H.Y. and P.R.M.; visualization, H.L.Y. and A.S.; supervision, G.H.Y. and Q.N.C.; project administration, A.S.; A.C.Y.Y.; S.K.; G.H.Y. and Q.N.C.; funding acquisition, S.K.; G.H.Y. and Q.N.C.  
This research was funded by the Australian Renewable Energy Agency (ARENA) 2018/RND011 and Australian Research Council (ARC) Industrial Transformation Training Centre under grant number IC170100032.  
The first author would like to acknowledge the financial support of UNSW Sydney through the University International Postgraduate Award (UIPA) scheme.  
The authors declare no conflict of interest.