Copyright © 2009 KSAE 1229−9138/2009/049−01 International Journal of Automotive Technology, Vol 10, No 6, pp 645−652 (2009) DOI 10.1007/s12239−009−0076−3 HCCI COMBUSTION CHARACTERISTICS DURING OPERATION ON DME AND METHANE FUELS Y TSUTSUMI , A IIJIMA , K YOSHIDA , H SHOJI and J T LEE 1)* 1) 1) 1) 1) 2) Department of Mechanical Engineering, College of Science and Technology, Nihon University, 1-8-14 Kanda-Surugadai, Chiyoda-gu, Tokyo 101-8308, Japan School of Mechanical Engineering, Sungkyunkwan University, Gyeonggi 440-746, Korea 2) (Received 28 July 2008; Revised 19 December 2008) ABSTRACT−The Homogeneous Charge Compression Ignition (HCCI) engine has attracted much interest because it can simultaneously achieve high efficiency and low emissions However, the ignition timing is difficult to control because this engine has no physical ignition mechanism In addition, combustion proceeds very rapidly because the premixed mixture ignites simultaneously at multiple locations in the cylinder, making it difficult to increase the operating load In this study, an HCCI engine was operated using blended test fuels comprised of dimethyl ether (DME) and methane, each of which have different ignition characteristics The effects of mixing ratios and absolute quantities of the two types of fuel on the ignition timing and rapidity of combustion were investigated Cool flame reaction behavior, which significantly influences the ignition, was also analyzed in detail on the basis of in-cylinder spectroscopic measurements The experimental results revealed that within the range of the experimental conditions used in this study, the quantity of DME supplied substantially influenced the ignition timing, whereas there was little observed effect from the quantity of methane supplied Spectroscopic measurements of the behavior of a substance corresponding to HCHO also indicated that the quantity of DME supplied significantly influenced the cool flame behavior However, the rapidity of combustion could not be controlled even by varying the mixing ratios of DME and methane It was made clear that changes in the ignition timing substantially influence the rapidity of combustion KEY WORDS : Internal combustion engine, Combustion, HCCI, DME, Methane, Spectroscopic measurement INTRODUCTION et al., 2006; Sato et al., 2006) This study examined the method of using a blend of two types of fuel The test fuels used were dimethyl ether (DME), which tends to autoignite easily because of its active low-temperature oxidation reactions, and methane, which does not autoignite readily, as it has no low-temperature oxidation reaction mechanism The heat release rate was analyzed to investigate the influence of each type of test fuel on combustion behavior when the fuel mixing ratios were varied Spectroscopic techniques (Shoji et al., 1994, 1996) were used to measure the light emission intensity and absorbance of HCHO, which is rapidly produced in cool flame reactions The Homogeneous Charge Compression Ignition (HCCI) engine (Thring, 1989) can simultaneously reduce nitrogen oxide (NOx) and particulate matter (PM) emissions (Aoyama et al., 1996) because, among other factors, the air and fuel are premixed homogeneously and operation is possible in the lean mixture region Another reason for the increased interest the HCCI combustion process is that it achieves thermal efficiency on par with that of diesel engines However, it is difficult to control the ignition timing of HCCI combustion because the fuel is ignited by the temperature rise resulting from compression Furthermore, the fact that combustion occurs simultaneously throughout the combustion chamber causes the pressure to rise too quickly Various methods of controlling HCCI combustion have been proposed, including varying the compression ratio (Hyvonen, 2005), varying the intake air temperature (Yoshida et al., 2005), applying exhaust gas recirculation (EGR) (Urushihara et al., 2003; Urata et al., 2004; Persson et al., 2004; Iijima et al., 2007), and using two types of fuel having significantly different ignition characteristics (Ozaki TEST FUELS 2.1 Characteristics of DME and Methane The properties of DME and methane are shown in Table (Glassman, 1996) DME has drawn interest as an alternative fuel for compression ignition engines because its high cetane number allows for compression ignition It also has a negative temperature coefficient region in which the ignition delay is not shortened even though the mixture reaches a higher temperature due to compression For that reason, it displays a multi-stage heat release pattern attribut- *Corresponding author e-mail: shoji@mech.cst.nihon-u.ac.jp 645 646 Y TSUTSUMI et al Table Properties of test fuels Fuel DME Methane Molecular Formula CH3OCH3 CH4 Cetane Number >55 Auto Ignition Temperature [K] 623 905 ed to low-temperature and high-temperature oxidation reactions Methane has vastly different ignition characteristics from DME It does not autoignite easily because it has a cetane number of zero and displays only a single-stage heat release pattern ascribable to high-temperature oxidation reactions 2.2 DME and Methane Reaction Mechanisms Figure shows the oxidation reaction process of a blended DME and methane fuel (Pilling , 1997; Konno, 2003) DME reactions (denoted as A in the figure) are divided into two processes One reaction process (1) begins from the first O2 addition and, depending on the temperature region, follows a path to a second O2 addition; the other reaction process (2) proceeds without any addition of O2 At low temperatures below 800 K, reaction (1) takes place and is accelerated by a chain-branching reaction (cool flame region) As the temperature rises further, the process switches to reaction (2), which is a chain propagation reaction, such that acceleration of the reaction ceases (negative temperature coefficient (NTC) region (Leppard, 1998; Shoji , 1992)) in spite of the temperature rise A subsequent increase in temperature induces reaction (3), resulting in excessive production of OH radicals and causing an acceleration of the reaction, leading to autoignition In relation to the temperature rise, two-stage ignition (Koyama , 2001) occurs owing to the progression from a cool flame through the NTC region to autoignition as the reactions proceed from (1) to (3) In the case of a blended DME and methane fuel, the OH radicals produced by the reaction of DME are consumed by et al et al et al Figure Oxidation reaction process of blended DME and methane fuels the initial H-atom abstraction reaction (5) of methane This is said to influence the progress of the oxidation reaction of DME In the cool flame region of DME, rapid production of HCHO occurs, and therefore attention was focused on HCHO in this study in order to investigate cool flame behavior EXPERIMENTAL PROCEDURE 3.1 Experimental Equipment Specifications for the test engine are given in Table 2, and the configuration of the test equipment is shown schematically in Figure A 4-cycle air-cooled single-cylinder diesel engine was used as the test engine The engine inducted a premixed mixture that was ignited by compression to accomplish HCCI combustion Mass flow controllers (denoted as (C) in the figure) were used to control the respective supply of DME and methane The cylinder pressure was measured with a crystal pressure transducer (P) In order to investigate the engine operating condition, K-type sheath thermocouples were used to measure the combustion chamber wall temperature and the intake air temperature The equipment shown in Figure was attached between the cylinder head and the cylinder as well as to the piston crown for measurement of light emission and absorption Flame light was extracted through a quartz window and introduced into a spectroscope via an optical fiber cable Light was separated at a wavelength of 395.2 nm, corresponding to the light emission wavelength of HCHO The inside of the combustion chamber was also irradiated with light from a xenon lamp and the transmitted light was introduced through an optical fiber cable into the spectroscope Light was separated at a wavelength of 293.1 nm correTable Specifications of test engine Number of cylinders Bore×Stroke 76×66 mm Displacement 299 cm3 Compression ratio 12:1 Intake valve close 54 deg ABDC Exhaust valve open 56 deg BBDC Figure Configuration of test equipment HCCI COMBUSTION CHARACTERISTICS DURING OPERATION ON DME AND METHANE FUELS Figure Schematic of spectroscopy system sponding to the absorption wavelength of HCHO (Gaydon, 1974) The wavelength resolution of the spectroscope used in the light emission and absorption measurements was 4.0 nm in terms of the half-bandwidth value The separated light in each case was input into a photomultiplier for conversionto an electric signal The output voltage of the photomultiplier was regarded as the emission intensity of the flame light For the transmitted light from the xenon lamp, absorbance AHCHO was calculated using Equation (1) below, where E0 denotes the baseline output voltage of the photomultiplier at bottom dead center and E denotes the output voltage at each crank angle E0 – EAHCHO= -(1) E0 In the experiments, the test engine was operated at 1400 rpm, and the intake air temperature and the combustion chamber wall temperature were controlled to 313 K and 353 K, respectively The quantity of fuel supplied was kept within the range where misfiring and knocking did not occur 3.2 Method of Calculating Heat Release Rate In a combustion process with a fast burning velocity, the rate of change in the specific heat ratio influences the calculated heat release rate Accordingly, it is important to take into account that rate of change when calculating the heat release rate (HRR) for HCCI combustion, which proceeds extremely rapidly Therefore, in this study, the change in the in-cylinder gas composition and the temperaturerelated change in the specific heat ratio were factored into the HRR calculation (Shudo et al., 2000; Muto et al., 2006) The specific heat ratio κ (n , T) was calculated based on the in-cylinder gas composition ni and average gas temperature T at crank angle θ Taking into account the rate of change in the specific heat ratio dκ /dθ, the HRR was calculated with Equation (2) below PV - ⋅ d -κdV- ⎞ − -1 ⎛ -dQ - = -V dP + κ P -(2) dθ ⎠ ( κ – 1) dθ dθ κ – ⎝ dθ 647 In calculating the HRR, the composition and number of moles of the gaseous body filling the cylinder were determined from the intake air mass and quantity of DME and methane consumed The change in the number of moles of the fuel was calculated, under the assumption of complete combustion, by finding the cumulative heat release from the measured cylinder pressure data Using the change in the number of moles of the fuel, the respective change in the number of moles of O2, CO2, and H2O was found In the case of a blended DME and methane fuel, the autoignition temperature of methane is higher than that of DME, as indicated by the fuel properties in Table Therefore, the change in the in-cylinder gas composition was calculated on the assumption that methane burned after the DME had burned The average temperature of the in-cylinder gas was calculated using the equation of state for an ideal gas The specific heat of each component was calculated at that temperature (Prothero, 1969; Fujimoto et al., 2006) and then the average specific heat ratio of the working gas was found 3.3 Experimental Conditions Figure shows the range of the injected heat value of the fuel per cycle Experiments were conducted under the conditions defined in the four cases below in order to investigate in detail how changes in the injected heat value of DME and methane influenced combustion Case 1: Only DME was supplied and the injected heat value of DME QDME was varied This condition was used to investigate the basic combustion characteristics when DME was supplied as a single component fuel Case 2: Both DME and methane were supplied The injected heat value of methane QCH4 was varied while keeping that of DME QDME constant This condition was designed for investigating the combustion characteristics of methane as a single component fuel However, the test engine could not be operated under this experimental condition i Figure Operating map 648 Y TSUTSUMI et al because the high autoignition temperature of methane gave rise to misfiring Therefore, a constant amount of DME was injected Case 3: Both DME and methane were supplied The methane share of the injected heat value γCH4 (=QCH4/Qin) was varied while keeping the total injected heat value Qin (=QDME+QCH4) constant Because the injected heat value of the fuel has a large influence on ignition characteristics, the influence of the mixing ratio of DME and methane was investigated while keeping the quantity of fuel injected constant Case 4: Under the conditions of Case 3, the intake air temperature was adjusted so that the ignition timing for each level of the methane share of the injected heat value γCH4 was 10 degrees or degrees before top dead center (BTDC) The influence of the ignition timing was excluded in this case because of its large influence on combustion characteristics RESULTS AND DISCUSSION 4.1 Investigation of Separate Control of Ignition Timing and Operating Load Figure shows the HRR results for Case With only DME as the test fuel, heat release of the high-temperature oxidation reactions increased as the injected heat value was increased Simultaneously, the ignition timing was advanced considerably to an earlier crank angle (X in the figure) These results indicate that the load and ignition timing cannot be varied independently with a single-component fuel of DME Additionally, increasing the injected heat value of DME results in extremely rapid combustion The indicated mean effective pressure (IMEP) relative to the injected heat value is compared in Figure for Cases and For Case 1, the IMEP increased due to the increase in heat release until the injected heat value reached point A However, it was observed that IMEP stopped increasing after point A because the ignition timing advanced too far Even though the injected heat value was increased, it did not increase the load owing to the advance of the ignition timing The HRR results for Case are shown in Figure The ignition timing (Y in the figure) did not change appreciably even though the injected heat value of methane was increased It was also seen that the heat release of the hightemperature oxidation reactions increased These results indicate that varying the injected heat value of methane alone can change the load, without changing the ignition timing As is also clear from the IMEP graph in Figure 6, the IMEP continued to increase because the ignition timing did not change even though the injected heat value was increased Furthermore, the knock limit was higher compared with Case (i in Fig 6) because combustion did not become extremely rapid owing to the fact that the ignition timing did not change An investigation was made of the ignition timing θ ign and the interval τ from the occurrence of a cool flame until ignition, under a condition where the quantities of fuel supplied were varied The definitions of θ ign and τ are shown in Figure The fuel supply conditions were those of Case with only DME as the fuel, Case in which the injected Figure Influence of QDME on HRR in Case Figure Influence of QCH4 on HRR in Case Figure Injected heat value (Qin) vs IMEP in Case and Case Figure Definitions of cool flame used for analysis HCCI COMBUSTION CHARACTERISTICS DURING OPERATION ON DME AND METHANE FUELS 649 HRRcool as a function of the injected heat value of DME It can be seen that the plots of HRRcool are arranged along the same line in relation to the increase in the injected heat value of DME under all of the conditions examined Accordingly, the following reason can be inferred for the dependence of τ and θ ign on the injected heat value of DME, as shown in Figure 10 This is attributed to the fact that the level of cool flame activity is strongly dependent on the injected heat value of DME and is little influenced by that of methane Figure Influence of QDME on ignition timing (θ ign) and ignition delay after occurrence of a cool flame (τ ) heat value of methane was varied (while keeping that of DME constant at values of QDME=240, 260, 278, and 297 J/ cycle), and Case in which the mixing ratios of DME and methane were varied while keeping the total injected heat value Qin constant at 357, 387 and 417 J/cycle, respectively Figure shows θ gn and τ in relation to the injected heat value of DME as the parameter The results in this figure show that the plots of θ ign and τ continued along the same line even though the injected heat value of methane differed, indicating that θ ign and τ were dependent on the injected heat value of DME This result suggests that, under the condition used in this study (γCH4