Much of the early work on H2ICEs was targeted at
trying to avoid these anomalies. The most frequently cited
anomaly is called Bbackfire,[ which happens when the
hydrogen–air mixture already ignites during the intake
stroke. This results in a combustion event in the intake. In
the best case, this is manifested by a loud Bbang,[ and in
the worst case, it results in damage of the intake. Care
must thus be taken to prevent unwanted ignition sources
such as hot spark plug electrodes, exhaust valves, etc.,
which implies some hardware modifications to the engine.
Also, avoiding the presence of ignitable mixtures when
there might be an ignition source, is a means to prevent
backfire. Thus, variable valve timing and carefully
controlled injection timing have been used to avoid
backfire occurrence [17], [18] by allowing a cooling phase
with fresh air while limiting the presence of hydrogen
during the early intake phase. Directly injecting hydrogen
into the combustion chamber after the intake valves have
closed, avoids backfire completely and is one of the
advantages of DI engines.