Ammonia Plus Hydrogen as Fuel in a S.I. Engine: Experimental Results 2012-32-0019
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.
Citation: Frigo, S., Gentili, R., and Doveri, N., "Ammonia Plus Hydrogen as Fuel in a S.I. Engine: Experimental Results," SAE Technical Paper 2012-32-0019, 2012, https://doi.org/10.4271/2012-32-0019. Download Citation
