Emissions analysis for a hydrogen-fueled low-pressure-ratio split-cycle engine

Recuperated low-pressure-ratio split-cycle engines represent a promising engine configuration for applications like transportation and stand-alone power generation by offering a potential efficiency as high as 60%. However, it can be challenging to achieve the stringent NOx emission standard, such as Euro 6 limit of 0.4 gNOx/kWh, due to the exhaust cylinder high intake temperature. This paper presents experimental investigation of hydrogen-air combustion NOx emissions for such engines for the first time. Experiments are carried out using a simplified constant-volume combustion chamber with glow-plug ignition. Two fuel injection techniques are performed: direct injection and injection via a novel convergent-divergent injector. For the direct injection scenario, NOx levels are unsatisfactory with respect to the Euro 6 standards over a range of operating temperatures from 200 °C to 550 °C. Recorded NOx levels can reach twice the permissible limit which necessitates the implementation of after-treatment techniques. Interestingly, NOx emissions are found to be less by around 50% in the case of the novel injector. The enhanced fuel-air mixing mechanism using the convergent-divergent injector increases the ignition delay and shifts the combustion flame towards the premixed conditions and results in lower NOx levels.