Probably the most researched model and also the most well documented, direct injection with spark plug ignition.

In many ways, it is very similar to the model described in the previous paragraph, ignition with glow plugs. What has largely been done is that the glow plug/one has been replaced with a spark plug. There may be more spark plugs than one. Studies carried out on a single cylinder engine converted for direct injection and igniting the hydrogen from a spark from a spark plug show a thermal efficiency of 40% at low to medium engine loads, which is only slightly less when compared to an equivalent Diesel engine.

What has also been noted with this model is that injection timing affects mixture homogeneity, which significantly affects engine power and emissions more than ignition timing. Delaying the injection timing from 120 degrees before TDC to 65 degrees before TDC further increases engine power. Unfortunately, this results in increased emissions of NOx, they have then tried injecting more air to lower the content, it has worked but then the effect has been reduced as a result. What has also been shown to have a major impact on engine performance is the angle at which the injector injects the fuel into the combustion chamber.[1]


[1] Ibid. s. 10 f.