## 5 Hydrogen Only Combustion with Direct Injection
### Spark-Assisted Ignition  
A wealth of spark-assisted hydrogen DI combustion engine studies is available in literature, making this combustion concept the most investigated approach for hydrogen DI. The progress of hydrogen spark-assisted-ignition engines up to year 2013 was reviewed by [PERSON] [26] and thus only some highlights and recent development are included in this review. The most common engine configuration is very similar to that of glow-plug-assisted ignition shown in Figure 2, with the glow plugs replaced by one or more spark plugs. In many research studies, the injector and spark plug arrangement was confined by the geometry of production cylinder head used for modification. [PERSON] et al. [40] converted a single-cylinder automotive size SI engine into hydrogen DI engine and achieved an ITE of 40% at low and medium loads, only slightly inferior to the state-of-art light-duty diesel engine at that time. A significant conclusion was that injection timing directly affects mixture homogeneity, which more significantly influences engine performance and emissions than ignition timing. Retarding SOI timing, from 120\({}^{\circ}\) to less than 65\({}^{\circ}\) CA before top dead center (BTDC), was found to improve overall engine efficiency. This is attributed to the more stratified mixture, which results in a faster initial flame development and improved combustion phasing, despite the increased local wall heat flux caused by the fuel-rich zone near spark plug and short hydrogen quenching distance. Furthermore, compression work during the compression stroke can be reduced as well since hydrogen is injected into an already compressed charge. The influence of injection timing on NO\({}_{\mathrm{x}}\) emissions is engine load dependent--at medium and low loads, NO\({}_{\mathrm{x}}\) emissions increase with delayed injection timing, attributed to a less homogeneous mixture leading to diffusion combustion and the formation of lean and hot zones. To the contrary, at high loads, NO\({}_{\mathrm{x}}\) emissions increase significantly for early injection, attributed to the global equivalence ratio becoming favorable for NO\({}_{\mathrm{x}}\) production. The effect of EGR on the NO\({}_{\mathrm{x}}\) emissions has also been investigated and a trade-off of efficiency was reported similar to the results from studies using hydrogen PFI. The validity of injection timing effects on the efficiency and NO\({}_{\mathrm{x}}\) emissions at low and medium load in SI DI engines by [PERSON] et al. [40] has been confirmed by [PERSON] et al. [72], [PERSON] et al. [73] and [PERSON] et al. [74], despite differences in engine geometry and operating condition (e.g., engine displacement, compression ratio, injection pressure, injection timing). In addition, under the low and medium loads, increasing the injection pressure can reduce NO\({}_{\mathrm{x}}\) emissions by promoting air entrainment and mixing [73; 74] but at a cost of lower efficiency due to the increased engine wall heat losses associated with increased wall impingement arising from the longer jet penetration [73]. A more recent study by [PERSON] et al. [75] indicated that the injection angle has a strong impact on engine performance and needs to be optimized in order to reduce wall heat loss by ensuring best separation of the hydrogen jets from the chamber walls. The hydrogen injection strategies discussed above mostly aimed at generating a relatively well mixed charge. To allow a larger degree of fuel-air mixture stratification, close-coupled injection and  
Figure 2: Schematic of the glow plug assisted ignition of hydrogen direct injection. ignition strategies were proposed [76], namely the plume-head and plume-tail ignition. Both strategies utilize a late SOI--the plume-head ignition is triggered soon after the SOI, whereas the plume-tail ignition strategy triggers ignition of the jet just after the end of injection.