Ammonia Plus Hydrogen as Fuel in a S.I. Engine: Experimental Results

Abstract

Storing hydrogen is one of the major problems concerning its utilization on board vehicles. Today hydrogen can be compressed and stored at 200 or 350 bar (it is foreseen that in a near future storage pressure will reach 700 bar, according to new expected regulations and using tanks in composite materials) or cryogenically liquefied. An alternative solution is storing hydrogen in the form of ammonia that is liquid at roughly 9 bar at environmental temperature and therefore involves relatively small masses and volumes and requires light and low-cost tanks. Moreover, ammonia contains almost 18% hydrogen by mass and, by volume, liquid ammonia contains 1.7 times as much hydrogen as liquid hydrogen. It is well known that ammonia can be burned directly in I.C. engines, however a combustion promoter is necessary to support combustion especially in the case of high-speed S.I. engines. Among the potential promoters, hydrogen is worthy of note, since it is carbon free and counteracts ammonia combustion characteristics. As a matter of fact, hydrogen has high combustion velocity and wide flammability range, whereas ammonia combustion is characterized by low flame speed, low flame temperature, narrow flammability range, high ignition energy and high self-ignition temperature. The experimental activity shown in this paper is correlated with a project that is focused on a range-extended electric vehicle involving an ammonia-plus-hydrogen I.C. engine and where hydrogen is obtained from ammonia by means of on-board catalytic reforming. Accordingly, the test engine is a 505 cm₃ Lombardini twin-cylinder S.I. engine that is well suited to power the onboard electric generator and the activity is aimed at determining proper air-ammonia-hydrogen mixture compositions at actual operating speeds and loads of the engine connected to the electric generator. Hydrogen and ammonia are separately injected in the gaseous phase. The only mechanical modification of the engine involves the intake manifold, where electro-injectors for hydrogen and for ammonia (conventional ones for CNG application with appropriate modification to inner parts) are added to the original ones for gasoline. The experimental results confirm that it is necessary to add hydrogen to air-ammonia mixture to improve ignition and to increase combustion velocity, with ratios that depend mainly on load and less on engine speed. Brake power is less than with gasoline, due to mixture poor volumetric heating value and to ammonia low flame speed that penalizes engine brake thermal efficiency. The amount of hydrogen needed by the engine is compatible with the flow rate provided by the reformer, except at cold start. The maximum NOx emission is 11.5 g/kWh at half load and 4500 rpm, without catalytic reduction

Performance and Emissions of an Ammonia-Fueled SI Engine with Hydrogen Enrichment

“…Ammonia combustion enhancement by in-situ NH3 dissociation was also investigated by Frigo et al [22][23][24] not in a CFR but in a commercial two-cylinder SI engine (CR = 10.7:1) equipped with a dedicated catalytic reformer. The stability of the whole system was proven for stoichiometric mixtures over a range of engine speeds and loads.…”

Laminar burning velocity and Markstein length of ammonia/hydrogen/air premixed flames at elevated pressures

“…Liu et al [20] performed a numerical simulation on ammonia flame assuming the compression ratio of 15 conditions and investigated the laminar burning velocity and NO mole fraction in burned gas of ammonia flame. Recently, Frigo et al [21] investigated the applications of ammonia/hydrogen in an SI engine and showed that the possibility of improvement of engine brake thermal efficiency by the increase in compression ratio although the efficiency decreases with the increase in ammonia concentration in the fuel. Westlye et al [22] investigated the emission characteristics of an ammonia/ hydrogen engine in detail using an FT-IR analyzer and showed the difference of emission gas characteristics between an ammonia/hydrogen engine and a gasoline engine.…”

Laminar burning velocity and Markstein length of ammonia/air premixed flames at various pressures

“…The application of ammonia in an actual combustor is also being studied. Frigo et al [6,7] applied hydrogen-doped ammonia as a fuel for the spark ignition engines. The power and thermal efficiencies of a hydrogen-doped ammonia fielded engine were compared with those of a gasoline engine.…”

“…In order to improve the lower combustion intensities of ammonia, hydrogen-added ammonia flames have been studied [3][4][5][6][7]. Lee et al [3,4] experimentally investigated the laminar burning velocity of hydrogen-added ammonia/air premixed flames.…”