Evaluating low NOx 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_x) 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_x exhaust emissions over the NRTC when compared to the diesel engine. Lowest NO_x emissions were observed for a spark ignition H2ICE with selective catalytic reduction and particulate filter (SCRF) aftertreatment. Tailpipe NO_x 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_x emissions observed for the H2ICEs over all power, torque, and speed settings. The frequency and magnitude of transient (<20 ms) increases in NO_x were also compared between diesel and H2ICE. A H2ICE using a hydrogen slip catalyst, but without SCRF aftertreatment, also emitted significantly lower tailpipe NO_x 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_x 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_x 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.