A short while ago I was giving a talk to a student group about some of the challenges of hydrogen internal combustion engines (HICE) technology and mentioned that managing excessive NOx emissions were a problem. Surprisingly, no questions were forthcoming during the presentation, but immediately afterward, several students confronted me for an explanation. My statement contradicted what they understood about the zero emissions credentials of hydrogen combustion. I’ve worked in technical education long enough to know that when one student asks, half a dozen others likely have the same question. So, here’s a simplified explanation.
H2 Stoichiometry
Most automotive technicians understand the concept of stoichiometry: simply put, it is combustion in which the ratio of fuel delivered to an engine cylinder is precision-balanced with the mass of air required to completely burn it. No more, no less.
When hydrogen is introduced to an engine cylinder at a stoichiometric ratio, the gas dispersal and mixing efficiency with air is haphazard, resulting in insufficient time to completely oxidize the fuel. This results in incomplete combustion and wasted energy consisting of costly H2 fuel. Excess air can address this problem. Most research indicates that hydrogen in-cylinder combustion efficiency is optimized when the excess air factor is managed at 2X the fuel mass, but this creates another problem. High excess air ratios result in oxidation of the nitrogen in the air mixture and NOx dump higher than that of a pre-2010 diesel engine.
Solutions
Are there solutions? Modern diesel engine technology has shown that high NOx emissions can be mitigated onboard a vehicle using a combination of electronic management of combustion combined with selective catalytic reduction (SCR). This can also achieved when combusting H2 in an engine cylinder. The big questions that will determine the feasibility of HICE in the future of transportation are:
•Cost and sources of hydrogen fuel
•Upfront costs of HICE and maintenance