Effects of Diluents on Knock Rating of Gaseous Fuels

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587 Effects of diluents on knock rating of gaseous fuels S O Bade Shrestha ∗ and R Rodrigues Department of Mechanical and Aeronautical Engineering, Western Michigan University, Kalamazoo, Michigan, USA The manuscript was received on 16 November2007 and was accepted after revision for publication on 29 May2008. DOI: 10. 1243/09576509JPE554 Abstract: Concerns on energy security, emissions, and the recent hike in the price of fossil fuels have prompted the rapidly growing interest in the use of
    JPE554 © IMechE 2008 Proc. IMechE Vol. 222 Part A: J. Power and Energy587 Effects of diluents on knock rating of gaseous fuels S O Bade Shrestha ∗ and R Rodrigues Department of Mechanical and Aeronautical Engineering, Western Michigan University, Kalamazoo, Michigan, USA The manuscript was received on 16 November2007 and was accepted after revision for publication on 29 May2008. DOI: 10. 1243/09576509JPE554 Abstract: Concerns on energy security, emissions, and the recent hike in the price of fossilfuels have prompted the rapidly growing interest in the use of various alternative and renew-able fuels, including low heating value fuels such as land-filled gases, biogases, coal bed methanegases, and others. Generally, the low heating value (btu) fuels contain substantial amounts of diluentssuch as carbon dioxide, nitrogen, water vapour, and other trace gases in the fuel composition. Thepresent contribution describes the results of the investigation of knock in a single-cylinder variablecompression ratio (CR) spark-ignition engine fuelled with gaseous fuels, such as natural gas,methane, and hydrogen, in the presence of different amounts of diluents, specifically carbon dioxideand nitrogen, in the fuel mixture in order to represent closely the general composition of landfilled and biogases in practice. The knock characteristics of the fuels were quantitatively evaluated interms of the methane number using various methods. Generally, the addition of either diluentcarbon dioxide or nitrogen in the fuel mixtures augmented the knock resistance characteristicsextending the engine operational limits. With every 10 per cent increase of carbon dioxide in the fuelmixture, the CR was increased by one point, whereas for a 25 per cent of nitrogen content in the fuelmixture, the CR was augmented by a half point in the operating conditions considered. Keywords: landfill gases, biogases, knock rating, low-btu gases, alternative fuels, methanenumber 1 INTRODUCTION Knock in spark-ignition engines occurs when thefuel  –  air mixture in the end gas region ignites aheadof the flame front that srcinates from the spark of the spark plug. As a result, multiple flame fronts col-lide creating a shock wave [ 1 ] that reverberates in thecombustion chamber, which thereby creates a char- acteristic metallic ‘pinging’ sound. The high pressure waves resonance in the cylinder is heard as a distinct metallic ‘pinging’ sound . The resonance frequencies of these pressure oscillations are usually between 4 and10 kHz, depending on the engine application [ 2 ].Knock is a phenomenon that is affected by variousfactors such as spark timing (ST), compression ratio ∗   Corresponding author: Department ofMechanical and Aeronau-tical Engineering, Western Michigan University, Parkview Cam- pus, 4801 Campus Drive, Kalamazoo, MI 49008, USA.email:bade.shrestha@wmich.edu  (CR), equivalence ratio, fuel type, mixture temperature,end gas temperature, combustion chamber pressureand volume, heat transfer, and others. Knock is exten-sively researched as it is a major barrier to achievinghigher thermal efficiency and increased power outputin spark-ignition (SI) engines. Knock due to auto-ignition causes abrupt pressure changes in the cylin-der and generates extreme temperature and pressurespikes due to rapid combustion of the gaseous mix-ture. Auto-ignition in an SI engine can cause potentialdamage to pistons, piston rings, connecting rods, headgaskets, bearings, spark plugs, and cylinder heads [ 3 ]. 1.1 Knock rating In spark-ignition engines, knock is a harassing traitcharacterized by a pinging sound. Knock results inloss of engine efficiency, performance, increased emis-sions, and potential damage to engine components.The resistance of the fuel to the incidence of knock is acritical factor in the consideration of fuel selection,    588 SO Bade Shrestha and R RodriguesProc. IMechE Vol. 222 Part A: J. Power and Energy JPE554 © IMechE 2008 and consequently an experimental procedure forrating fuels was established through the use of octanenumber (ON), methane number (MN), and the lesswidely used butane number (BN) [ 4 ]. Hence, knock rating refers to a numerical classification of motor fuelantiknock characteristics that is established empiri-cally with a specialized set of operating conditions.ON is typically used to characterize the knock resis-tance quality of gasoline, whereas MN or BN is usedfor the characterization of gaseous fuels. 1.1.1 Methane number  Gaseous fuels, such as natural gas, landfill gas, or bio-gas, when utilized to run in an internal combustion(IC) engine instead of gasoline or diesel can createentirely different variations in knock parameters. MNis defined as a measure of the resistance of a gaseousfuel to auto- ignition (‘knock’) when ignited in an SI engine. The reference fuels utilized to establish theMN are methane and hydrogen, where 100 per cent methane equals ‘100 MN’ and 100 per cent hydrogenequals ‘0 MN’ [ 5 ]. 1.2 Knock intensity To accurately determine the MN of a specific fuelblend, an established measure is required to quan-tify the onset and intensity of knock to identify aknock condition. This parameter is acknowledged asthe knock intensity, and the empirical value employedas the threshold to define a knock condition variesfrom authors to applications and it is subjective. 2 EXPERIMENTAL SET-UP An ASTM-CFR (cooperative fuel research) enginewas utilized for knock investigation. This engine isapproved by the ASTM and is specifically designed andextensively used throughout the world for researchand testing of liquid fuels for the IC engines. The mostimportant features of this type of engine are a single-cylinder, a variable CR, a variable ST, and a constantspeed [ 6 ]. The ST can be varied over a wide rangefrom 40 ◦ before top dead centre (BTDC) to 40 ◦ aftertop dead centre (ATDC). The engine geometric detailsare given in Table 1. The engine runs at constant speedof 600 r/min. The fuel under research can be tested atvarious CRs (4: 1  –  16:1) and various STs.The engine utilized was srcinally designed toresearch and conduct experimental tests on liquidfuels. For the current research, the engine intake andfuel delivery system had to be modified to allow for theability to operate on gaseous fuels. Table 1 CFRengine details[ 7 ] Make WaukeshaCompression ratio 4:1  –  16:1Cylinder bore 82.55mm (3.25 in)Stroke 114.3mm (4.5 in)Connecting rod length 254mm (10 in)Displacement volume 0.611l(37.33 in 3 ) 2.1 Metering panel The air metering panel was designed separate fromthe fuel and diluent metering panel as the air flowcontrol requirements were distinct. Air flow controlconsisted of an electronic mass flowmeter to mea-sure the intake flow and a 0.075 m 3 (20 gal) surgetank (Fig. 1) to reduce the high-intensity pressurepulsations generated during engine operation.The fuel and diluent metering panel was developedto safely and accurately meter the desired mixtureratios at the engine intake. The panel consisted of four sets of electronic mass flowmeters to regulatethe desired concentrations of four gases, especiallymethane, hydrogen, carbon dioxide, and nitrogen. TheOmega FMA series and TSI 4000 mass flowmeters uti-lized were specifically calibrated for the individualgases. Additionally, the panel consisted of flashback arrestors, gas filters, regulating needle valves, pressuregages, check valves, and a mixing manifold (Fig. 1). 2.2 Data acquisition The electronic mass flowmeters were coupled to theengine data acquisition system and monitored viasoftware. This set-up allowed the accurate delivery of fuel  –  air mixture to the engine intake. The data acqui-sition system was also utilized to analyse and monitorvarious other engine related parameters such as crank angle, in-cylinder pressure, intake temperature, andexhaust temperature.A BEI model HS35 incremental optical rotaryencoder mounted on the crank shaft of the engine was utilized to record the crank angle data. The encoders’ disc resolution of 4096 enabled data logging of in-cylinder pressure transmitted by the Kistler model7061B flush-mounted pressure transducer for every0.088 ◦ of crank angle. All devices were connected to theNational Instruments SCB-68 terminal block whichwas coupled to the computer using a PCI-MIO-16e-4data card and Labview software as the data acquisitioninterface. 3 EXPERIMENTAL PROCEDURE    Biogas and landfill gas composition varies with timeand geographical location. Therefore, in order to    Effects of diluents on knock rating of gaseous fuels 590Proc. IMechE Vol. 222 Part A: J. Power and Energy JPE554 © IMechE 2008   Fig.1 Schematic diagram of experimental set-up closely represent the landfill and biogas compositions,experiments were conducted for various compositionsof fuel mixtures and different percentages of carbondioxide and nitrogen (Table 2). The experiments wereperformed to determine the effect of carbon dioxideand nitrogen on knock characteristics of the fuel mix-ture, as it accounts for a significant portion of thediluents in these fuels.To investigate the onset of knock in the presenceof diluents, the knock limited spark timing (KLST)and knock limited compression ratio (KLCR) had tobe determined with different volumetric composi-tions of diluents. This involved the variation in STfor KLST while keeping all other parameters such asequivalence ratio, CR, and intake temperature con-stant. Unless otherwise specified, the KLST valueswere determined for a constant CR of 12:1, equiva-lence ratio of 1.0, and an intake temperature of 303 K.The ST was varied from 40 ◦ BTDC to 40 ◦ ATDC, which isbased on the fuel  –  diluent composition and operatingconditions. All the tests were conducted with full openthrottle. Table 2 General composition of landfill gas [ 8 ] No. Gas Volume (%)1 Methane 45  –  602 Carbon dioxide 40  –  603 Nitrogen 2  –  54 Oxygen and other NMOCs <  1 Similarly, the CR was varied for KLCR while keep-ing all other parameters such as equivalence ratio,ST, and intake temperature constant. Unless otherwisespecified, the KLCR values were determined for a con-stant ST of 13 ◦ BTDC, equivalence ratio of 1.0, andan intake temperature of 303 K with full open throt-tle. The CR was varied from 4.5 to 16.0 for the variousfuel  –  diluent compositions and operating conditionsuntil the predetermined knock intensitywas achieved.The initial sets of experiments conducted were todetermine the baseline knock characteristics, specif-ically KLST and KLCR of methane and hydrogen of MN 0, 20, 40, 60, 80, and 100. The baseline was criti-cal to the determination of the optimum ST and CR tocover the range of ST and CR parameters to conductuseful analysis. With the baseline data generated, theknock parameters were further investigated with thepresence of 5, 10, 15, 20, 25, 30, 40, and 50 per centCO 2 or N 2 . 3.1 Knock detection The accurate determination of the MN of a fuel relieson a reliable and repeatable knock detection method.Several methods exist in industry and research thatcan be employed, such as human ear, engine vibra-tions, and in-cylinder pressure, which are some of themore common methods. Additionally, less sparinglyused knock detection methods are ion current sensingand wall thermal losses [ 9 ].
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