An, Yanzhao; Jaasim, Mohammed; Vallinayagam, R; AlRamadan, Abdullah; Sim, Jaeheon; Chang, Junseok; Im, Hong G.; Johansson, Bengt(SAE Technical Paper Series, SAE International, 2018-09-10)[Conference Paper]
Partially premixed combustion (PPC) is an operating mode that lies between the conventional compression ignition (CI) mode and homogeneous charge compression ignition (HCCI) mode. The combustion in this mixed mode is complex as it is neither diffusion-controlled (CI mode) nor governed solely by chemical kinetics (HCCI mode). In this study, CFD simulations were performed to evaluate flame index, which distinguishes between zones having a premixed flame and non-premixed flame. Experiments performed in the optical engine supplied data to validate the model. In order to realize PPC, the start of injection (SOI) was fixed at -40 CAD (aTDC) so that a required ignition delay is created to premix air/fuel mixture. The reference operating point was selected to be with 3 bar IMEP and 1200 rpm. Naphtha with a RON of 77 and its corresponding PRF surrogate were tested. The simulations captured the general trends observed in the experiments well. The flame index was noted to be an indicator to evaluate and quantify the in-cylinder combustion development under PPC engine operating condition. The evolution of premixed flames shows the same two-stage ignition behavior as the rate of heat release. Premixed flames are surrounded by the non-premixed fuel/air mixtures and distribute in the piston top-land region as isolated clouds. The proportion of premixed flames increases from low temperature heat release (LTHR) region first and decreases in negative temperature coefficient (NTC) region then increases to high temperature heat release (HTHR) region at PPC mode.
Zhou, Qiyan; Jaasim, Mohammed; Mohan, Balaji; Lu, Xing-Cai; Im, Hong G.(SAE Technical Paper Series, SAE International, 2018-09-10)[Conference Paper]
The purpose of present numerical study was to extend the operating range of alcohol (methanol and ethanol) fueled Homogeneous Charge Compression Ignition (HCCI) engine under low load conditions. Ignition of pure methanol and ethanol under HCCI mode of operation requires high intake temperatures and misfires at low loads are common in HCCI engines. Three methods have been adapted to optimize the use of methanol and ethanol for HCCI operation without increasing the intake temperature. First, blending methanol and ethanol with ignition improver, namely di-methyl ether (DME) and di-ethyl ether (DEE), was used to increase the cetane number and ignitability of premixed charge. Second, based on the blended fuels, the spark assistance was used to reduce required intake temperature for auto-ignition. Third, DME and DEE were directly injected to methanol and ethanol operated HCCI engine, in the form of Reactivity Controlled Compression Ignition (RCCI) combustion. Negligible improvement in reducing intake temperature was observed in spark-assisted HCCI combustion due to the slow flame propagation speed under the lean premixed condition with blended fuels. In all three methods, it was found that RCCI combustion was more effective at reducing the required intake temperature compared to HCCI and spark assisted combustion, in spite of the fact that they are operated at same lambda (3.3) operating conditions.
Luo, Yueqi; Jaasim, Mohammed; Huang, Zhen; Im, Hong G.(SAE Technical Paper Series, SAE International, 2018-09-10)[Conference Paper]
Ignition quality tester (IQT) is a standard experimental device to determine ignition delay time of liquid fuels in a controlled environment in the absence of gas exchange. The process involves fuel injection, spray breakup, evaporation and mixing, which is followed by auto-ignition. In this study, three-dimensional computational fluid dynamics (CFD) is used for prediction of auto-ignition characteristics of diethyl ether (DEE) and ethanol. In particular, the sensitivity of the ignition behavior to different injection rate profiles is investigated. Fluctuant rate profile derived from needle lift data from experiments performs better than square rate profile in ignition delay predictions. DEE, when used with fluctuant injection rate profile resulted in faster ignition, while for ethanol the situation was reversed. The contrasting results are attributed to the difference in local mixing. The fluctuant injection profile yields larger spray velocity variations promoting fuel evaporation and local turbulent mixing. The suitable ignition conditions were reached earlier for DEE with fluctuant injection profile, whereas ethanol exhibits pseudo-homogeneous mixing due to its lower cetane number. Ignition was faster for square rate profile due to ignition in end tube for ethanol. The fluctuant injection leads to a better homogeneity for ethanol due to longer time available for mixing. The nature of heat release rate, auto-ignition and combustion were altered by the fluctuant injection rate profile when compared to square rate injection profile.
Jaasim, Mohammed; Hernandez Perez, Francisco; Sow, Aliou; Im, Hong G.(SAE Technical Paper Series, SAE International, 2018-04-03)[Conference Paper]
Super-knock that occurs in spark ignition (SI) engines is investigated using two-dimensional (2D) numerical simulations. The temperature, pressure, velocity, and mixture distributions are obtained and mapped from a top dead center slice of full cycle three-dimensional (3D) engine simulations. Ignition is triggered at one end of the cylinder and a hot spot of known temperature was used to initiate a pre-ignition front to study super-knock. The computational fluid dynamics code CONVERGE was used for the simulations. A minimum grid size of 25 μm was employed to capture the shock wave and detonation inside the domain. The Reynolds averaged Navier-Stokes (RANS) method was employed to represent the turbulent flow and gas phase combustion chemistry was represented using a reduced chemical kinetic mechanism for primary reference fuels. A multi-zone model, based on a well-stirred reactor assumption, was used to solve the reaction terms. Hot spots introduced inside the domain at various initial temperatures initiated a pre-ignition front, which resulted in super-knock due to detonation of the end-gas. The detonation speed was around 2000 m/s. The detonation was induced for temperatures greater than 1000 K during the start of pre-ignition flame propagation. For temperatures between 800 K to 1000 K detonation was exhibited when almost all the fresh gases are consumed by the propagating pre-ignition front. Multiple auto-ignition sites in the end-gas region were observed at higher temperatures. High peak pressures were generated during the detonation onset. The low temperature case, 700 K, exhibited a deflagration mode of flame propagation without detonation development. The results were analyzed and reported by comparison with Bradley diagram which predicted a deflagration propagation mode for the lowest temperature case and developing detonation mode for all other cases considered in this study.
An, Yanzhao; Jaasim, Mohammed; Vallinayagam, R.; Vedharaj, S.; Hernandez Perez, Francisco; Sim, Jaeheon; Chang, Junseok; Im, Hong G.; Johansson, Bengt(SAE Technical Paper Series, SAE International, 2018-04-03)[Conference Paper]
The experimental in-cylinder combustion process was compared with the numerical simualtion for naphtha fuel under conventional compression ignition (CI) and partially premixed combustion (PPC) conditions. The start of injection timing (SOI) with the single injection strategy was changed from late of 10 CAD aTDC to early of 40 CAD aTDC. The three-dimensional full cycle engine combustion simulation was performed coupling with gas phase chemical kinetics by the CFD code CONVERGE™. The flame index was used for evaluating the combustion evolution of premixed flame and diffusion flame. The results show that the flame index could be used as an indicator for in-cylinder homogeneity evaluation. Hydroperoxyl shows a similar distribution with the premixed combustion. Formaldehyde could be used as an indicator for low temperature combustion.
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