Shawn M. Grannell e-mail: sgrannel@umich.edu Dennis N. Assanis Stanislav V. Bohac Donald E. Gillespie Department of Mechanical Engineering, University of Michigan, Ann Arbor, MI 48109-2133 The Fuel Mix Limits and Efficiency of a Stoichiometric, Ammonia, and Gasoline Dual Fueled Spark Ignition Engine An overall stoichiometric mixture of air, gaseous ammonia, and gasoline was metered into a single cylinder, variable compression ratio, supercharged cooperative fuel research (CFR) engine at varying ratios of gasoline to ammonia. The engine was operated such that the combustion was knock-free with minimal roughness for all loads ranging from idle up to a maximum load in the supercharge regime. For a given load, speed, and compression ratio, there was a range of ratios of gasoline to ammonia for which knock- free, smooth firing was obtained. This range was investigated at its rough limit and also at its maximum brake torque (MBT) knock limit. If too much ammonia was used, then the engine fired with an excessive roughness. If too much gasoline was used, then knock-free combustion could not be obtained while the maximum brake torque spark timing was maintained. Stoichiometric operation on gasoline alone is also presented, for compari- son. It was found that a significant fraction of the gasoline used in spark ignition engines could be replaced with ammonia. Operation on about 100% gasoline was required at idle. However, a fuel mix comprising 70% ammonia/30% gasoline on an energy basis could be used at normally aspirated, wide open throttle. Even greater ammonia to gaso- line ratios were permitted for supercharged operation. The use of ammonia with gasoline allowed knock-free operation with MBT spark timing at higher compression ratios and higher loads than could be obtained with the use of gasoline alone. DOI: 10.1115/1.2898837 Introduction An initial version of this work was presented at the ASME 2006 International Mechanical Engineering Congress and Exposition as IMECE2006-13048. One option for phasing out the use of petroleum-derived hydrocarbon fuels is to produce hydrogen with nuclear power, using the sulfur-iodine thermochemical process, which requires high temperature nuclear heat sources. This would enable the domestic production of a motor fuel, which does not involve carbon dioxide emissions anywhere in its life cycle. Wind or solar power could be used to make hydrogen, although their contributions are expected to be small, and biofuels are also avail- able in limited capacity 1. Although hydrogen gas does not have favorable energy density by volume, it can be combined with atmospheric nitrogen to make ammonia, which is stored as a liq- uid at a modest pressure near 10 bars. For the same energy con- tent, liquid ammonia has 2.6 times the volume and 2.3 times the mass of gasoline. The higher efficiency, at which an ammonia fueled engine can be run, will partially offset ammonia’s reduced energy density relative to that of gasoline. Ammonia is an irritant that can cause injury if the liquid is splashed onto the skin or eyes, or by prolonged breathing of the vapor. Care must be taken to avoid spills by the appropriate design of the fill connectors on the vehicle and on the dispenser. How- ever, ammonia is much less flammable than hydrogen. Also, am- monia, unlike hydrogen, does not require extreme pressures to contain it at a reasonable energy density. The authors have years of experience with handling liquid anhydrous ammonia, and hold the opinion that, with the right equipment, the handling, delivery, and use of liquid anhydrous ammonia can be made much less problematic than are the handling, delivery, and use of high pres- sure hydrogen gas. The principal limitation on the use of ammonia as an engine fuel is the slow flame speed of ammonia/air mixtures. Ammonia can also be difficult to ignite, and it has a high autoignition tem- perature and narrow flammability limits. The challenge of ammo- nia is to make it burn completely, so that efficient engine opera- tion with minimal power loss can be achieved. These challenges have caused ammonia to be overlooked as a solution to the hy- drogen storage problem. However, the logistical and operational penalties of using hydrogen as stored by other means may delay its use even as crude oil and natural gas become more expensive than hydrogen produced by nuclear power 2. This dual fuel in- vestigation is intended to provide a means by which ammonia can be used as the principal fuel, while achieving engine efficiency and power, which are superior to those obtained with the use of gasoline alone. Related Work The use of ammonia in diesel engines has been investigated by others using various configurations and with different fuel addi- tives to improve the engine operation 3–5. Efforts to improve the combustion of ammonia in diesel engines have produced recom- mendations for the use of spark ignition, elevated compression ratios, and gaseous phase ammonia induction at a constant equiva- lence ratio 4. Mozafari and co-workers gave similar recommen- dations 6,7. Liquid phase ammonia induction gives better volumetric effi- ciency because the ammonia cools the intake mixture, and the liquid ammonia does not displace as much air. However, the the- oretical efficiency gain for liquid induction is small 8, and the Contributed by the Internal Combustion Division of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received February 7, 2007; final manuscript received February 9, 2008; published online April 28, 2008. Review conducted by Thomas W. Ryan III. Journal of Engineering for Gas Turbines and Power JULY 2008, Vol. 130 / 042802-1 Copyright © 2008 by ASME Downloaded 28 Apr 2008 to 141.213.244.223. Redistribution subject to ASME license or copyright; see http://www.asme.org/terms/Terms_Use.cfm