Mirko Baratta IC Engines Advanced Laboratory (ICEAL), Politécnico di Torino, C.SO Duca degii Abruzzi, 24, Torino 10129, Itaiy e-mail: miri(o.baratta@poiito.it Stefano d'Ambrosio iC Engines Advanced Laboratory (iCEAL), Politécnico di Torino, C.SO Duca degii Abruzzi, 24, Torino 10129, itaiy e-maii: stefano.dambrosio@poiito.it Daniela Misul IC Engines Advanced Laboratory (ICEAL), Politécnico di Torino, C.SO Duca degii Abruzzi, 24, Torino 10129, Itaiy e-maii: daniela.misul@poiito.it Ezio Spessa IC Engines Advanced Laboratory (iCEAL), Politécnico di Torino, C.SO Duca degii Abruzzi, 24, Torino 10129, Italy e-mail: ezio.spessa@polito.it Effects of H2 Addition to Compressed Natural Gas Blends on Cycle-to-Cycle and Cylinder-to-Cylinder Combustion Variation in a Spark-Ignition Engine An experimental investigation and a burning-rate analysis have been performed on a pro- duction 1.4 liter compressed natural gas (CNG) engine fueled with methane-hydrogen blends. The engine features a pent-roof combustion chamber, four valves per cylinder, and a centrally located sparkplug. The experimental tests have been carried out in order to quantify the cycle-to-cycle and the cylinder-to-cylinder combustion variation. There- fore, the engine has been equipped with four dedicated piezoelectric pressure transducers placed on each cylinder and located by the spark plug. At each test point, in-cylinder pressure, fuel consumption, induced air mass flow rate, pressure, and temperature at dif- ferent locations on the engine intake and exhaust systems as well as "engine-out" pollu- tant emissions have been measured. The signals related to engine operation have been acquired by means of a National Instruments PXI-DAQ system and software developed in house. The acquired data have then been processed through a combustion diagnostic tool resulting from the integration of an original multizone thermodynamic model with a computer-aided design (CAD) procedure for the evaluation of the burned-gas front geom- etiy. The diagnostic tool allows the burning velocities to be computed. The tests have been performed over a wide range of engine speeds, loads, and relative air-fuel ratios (up to the lean operation limit (WL)). For stoichiometric operation, the addition of hydrogen to CNG has produced a brake-specific fuel combustion (bsfc) reduction ranging between 2% and 7% and a brake-specific total unburned hydrocarbons (bsTHCs) decrease up to 40%. These benefits have appeared to be even higher for lean mixtures. Hydrogen has shown to significantly enhance the combustion process, thus leading to a sensibly lower cycle-to-cycle variability. Hydrogen addition has generally resulted in extended operation up to relative air-to-fuel ratio (RAFR) = J.8. Still, the LOL consis- tently varies depending on the considered cylinder. [DOI: 10.1115/1.4026163] Introduction The growing employment of alternative fuels and the necessity for further reduction of engine-out emissions so as to meet the even more stringent pollutant regulations establishes the need to thoroughly investigate the use of hydrogen-blended fuels. Hydro- gen represents a promising alternative to gasoline, even though, at present, the lack of distribution infrastructure limits its use as a mere additive [1]. More specifically, hydrogen-enriched methane (HCNG) allows combining the advantages of both methane and hydrogen in terms of lower pollutant emissions and increased combustion velocity. Many research activities were carried out on the use of hydrogen-methane blends, mainly focusing on the effect of hydro- gen addition at various ignition timings as well as at different air- to-fuel ratios. Nagalingam et al. [2] investigated HCNG blends featuring from 20% to 50% of hydrogen added to methane. The HCNG engine displayed a reduction on the peak power output Contributed by the Combustion and Fuels Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received March 28, 2013; final manuscript received November 29, 2013; published online January 2, 2014. Assoc. Editor: Paolo Chiesa. and spark advance (SA) had to be reduced to achieve the maximum brake torque (MBT) timing. Hydrogen addition showed to be almost ineffective on the main combustion parameters (peak firing pressure (PFP), in-cylinder temperature, mass fraction burned (MFB),...) while attaining considerable bsHC reduction with little variations in bsCO and bsCOa [3]. Similar results were obtained in Ref. [4], where a comparison was carried out for blends ranging up to 30% of hydrogen blended with natural gas. NOx emissions proved to increase with the increasing H2 percent- age for a given ignition timing, whereas lower values were attained for lean operations. The hydrogen wider range of flamma- bility allows the use of new control strategies, such as those inves- tigated in Refs. [5] and [6]. In Ref. [5], the adoption of hydrogen- enriched mixtures led to the evaluation of the potentialities of the lean bum combustion as a factual alternative to throttling the flow of intake aii- in order to control the engine power output. In Ref. [6], an additional survey on the effects of the load, speed, and spark timing on the engine lean operation limit (LOL) was carried out. As expected, the higher pressure and temperatures together with the decreased residual gas fraction produced by the increased loads sped up the combustion process, thus further extending the engine LOL. Increase in engine speed appeared to exert positive Journal of Engineering for Gas Turbines and Power Copyright © 2014 by ASME MAY2014, Vol. 136 / 051502-1