Evaluating low NO hydrogen engines designed for off-road and construction applications

Hydrogen internal combustion engines offer a near-term decarbonisation pathway for hard to electrify sectors such as non-road mobile machinery (NRMM). However, few hydrogen-specific engines have ever been developed with the twin-goals of maximising low carbon energy efficiency and delivering air quality co-benefits. We present analyses of dynamometer-derived nitrogen oxides (NO) tailpipe emissions from four variants of a ∼55 kW four-cylinder port fuelled injection spark ignition hydrogen internal combustion engine (H2ICE) suitable for a range of uses within the NRMM industry. Engine out (pre-aftertreatment) emissions are also reported for one of the H2ICE variants. The emissions were compared over the Non-Road Transient Cycle (NRTC) with an equivalent contemporary Stage V emissions compliant 55 kW diesel engine. All four H2ICE variants were configured to operate under lean burn conditions generating substantially lower NO exhaust emissions over the NRTC when compared to the diesel engine. Lowest NO emissions were observed for a spark ignition H2ICE with selective catalytic reduction and particulate filter (SCRF) aftertreatment. Tailpipe NO emissions over the full NRTC for this configuration were 1.90 mg kWh^-1, a greater than 99% reduction compared to diesel (3340 mg kWh^-1) with lower average NO emissions observed for the H2ICEs over all power, torque, and speed settings. The frequency and magnitude of transient (<20 ms) increases in NO were also compared between diesel and H2ICE. A H2ICE using a hydrogen slip catalyst, but without SCRF aftertreatment, also emitted significantly lower tailpipe NO than the diesel equivalent (63.7 mg kWh^-1), a factor of greater than 50 times improvement over the NRTC. This creates a systems level dilemma: whether the additional small absolute reductions in NO achieved using SCRF would have a net benefit that outweighed the broader financial and environmental costs of the SCR and exhaust fluid manufacture, distribution and possible small in-service ammonia slip from exhaust. Irrespective of aftertreatment system, the adoption of low NO emitting H2ICE in NRMM, and particularly construction equipment, would appear to offer much greater near-term air quality benefits for cities when compared to switching to other low carbon alternatives such as biodiesel or hydrotreated vegetable oil.