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Knock and Pre-Ignition Limits on Utilization of Ethanol in Octane-on-Demand Concept

King Abdullah Univ. of Science & Tech.-Eshan Singh, Robert Dibble
Saudi Aramco-Kai Morganti
Published 2019-09-09 by SAE International in United States
Octane-on-Demand (OoD) is a promising technology for reducing greenhouse emissions from automobiles. The concept utilizes a low-octane fuel for low and mid load operating conditions, and a high-octane additive is added at high load operating conditions. Researchers have focused on the minimum ethanol content required for operating at high load conditions when the low-octane fuel becomes knock limited. However, it is also widely known that ethanol has a high tendency to pre-ignite, which has been linked with its high laminar flame speed and surface ignition tendency. Moreover, ethanol has a lower stoichiometric air-fuel ratio, requiring a larger injected fuel mass per cycle. A larger fuel mass increases the potential for oil dilution by the liquid fuel, creating precursors for pre-ignition. Hence, the limits on ethanol addition owing to pre-ignition also need consideration before the technology can be implemented. In this regard, experiments were performed using light naphtha (RON 68) and ethanol in direct and port-fuel injection configuration, respectively. The engine load was parametrically swept by simultaneously increasing the intake air and fuel quantity until the…
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Mechanism Triggering Pre-Ignition in a Turbo-Charged Engine

King Abdullah University of Science & Technology-Eshan Singh, Robert Dibble
Published 2019-04-02 by SAE International in United States
Pre-ignition in modern engines is largely attributed to oil-fuel mixture droplets igniting before the spark timing. Researchers have also found pre-ignition events to be triggered by high hydrocarbon emissions from the previous cycle as well as late spark timing in the previous cycle. Additionally, an ideally scavenged engine was not found to be limited by pre-ignition. These observations point to a significant role of residuals in triggering pre-ignition events. Current work studies pre-ignition in a probabilistic approach. The effect of residuals and in-cylinder thermodynamic state is studied by varying the exhaust back pressure and intake air temperature respectively. Experiments were performed with a fixed mass flow rate of air + fuel and intake air temperature while the exhaust back pressure was varied. Intake air pressure varied in response to fixed intake temperature. Pre-ignition and super-knock count increased with increasing exhaust back pressure. In the next set of experiments, mass flow rate of air + fuel and intake air pressure were fixed, while the exhaust back pressure was varied. Intake air temperature was varied to fix…
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Effect of Different Fluids on Injection Strategies to Suppress Pre-Ignition

King Abdullah University of Science & Technology-Eshan Singh, Ponnya Hlaing, Hao Shi, Robert Dibble
Published 2019-04-02 by SAE International in United States
Pre-ignition is an abnormal engine combustion phenomenon where the inducted fuel-air charge ignites before the spark ignition. This premature combustion phenomenon often leads to heavy knocking events. The mixture preparation plays a critical role in pre-ignition tendency for a given load. Literature shows efforts made towards improving pre-ignition-limited-IMEP by splitting the injection pulse into multiple pulses. In this study, two direct injectors are used in a single cylinder research engine. A centrally mounted direct injector was used to inject Coryton Gasoline (RON 95) fuel early in the intake stroke. A second fluid was injected late in the compression stroke to suppress pre-ignition. The fluids used in the second direct injector was varied to see the effects of the molecule and its physical and chemical property on pre-ignition suppression tendency. Methanol, ethanol, water, and gasoline were tested as second fluid. Engine tests were conducted at 2000 rpm and at an intake pressure of 2.1 bar (abs). Although alcohols show high pre-ignition tendency as fuels, they were most effective at pre-ignition suppression when injected later in the…
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A Path towards High Efficiency Using Argon in an HCCI Engine

King Abdullah University of Science & Technology-Abdulrahman Magdy Mohammed, Jean-Baptiste Masurier, Ali Elkhazraji, Robert Dibble, Bengt Johansson
Published 2019-04-02 by SAE International in United States
Argon replacing Nitrogen has been examined as a novel engine cycle reaching higher efficiency. Experiments were carried out under Homogeneous Charge Compression Ignition (HCCI) conditions using a single cylinder variable compression ratio Cooperative Fuel Research (CFR) engine. Isooctane has been used as the fuel for this study. All the parameters were kept fixed but the compression ratio to make the combustion phasing constant. Typical engine outputs and emissions were compared to conventional cycles with both air and synthetic air. It has been found that the compression ratio of the engine must be significantly reduced while using Argon due to its higher specific heat ratio. The resulting in-cylinder pressure was lower but combustion remains aggressive. However, greater in-cylinder temperatures were reached. To an end, Argon allows gains in fuel efficiency, in unburned hydrocarbon and carbon monoxide, as well as in indicated efficiency.
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Effectiveness of Fuel Enrichment on Knock Suppression in a Gasoline Spark-Ignited Engine

King Abdullah University of Science & Tech.-Eshan Singh, Robert Dibble
Published 2018-09-10 by SAE International in United States
Knock, and more recently, super-knock, have been limiting factors on improving engine efficiency. As a result, engines often operate rich at high loads to avoid damage resulting from knock and protect the after-treatment system from excessive thermal stress. In this work, port-fuel injection and direct injection of excess fuel is explored as a mechanism to suppress knock and super-knock. Under naturally aspirated conditions, increasing the fuel enrichment initially increases knock intensity. However, further increasing fuel enrichment subsequently decreases knock intensity. The competing mechanism from calorific value and latent heat of vaporization can be used to explain the phenomenon. However, when directly injecting the excess fuel after the spark plug has been fired, knock intensity monotonically decreases with increasing fuel quantity. This decrease is shown to be due to fuel quenching the flame that is propagating from spark location. Under boosted conditions, the amount of fuel injected is of critical importance in avoiding super-knock. A lower fuel quantity leads to knock suppression. But beyond a critical value, higher quantities of fuel result in more interaction with…
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Effect of Mixture Formation and Injection Strategies on Stochastic Pre-Ignition

King Abdullah University of Science & Tech.-Eshan Singh, Mohammed Jaasim Mubarak Ali, Adrian Ichim, Robert Dibble
Saudi Aramco-Kai Morganti
Published 2018-09-10 by SAE International in United States
Stochastic pre-ignition remains one of the major barriers limiting further engine downsizing and down-speeding; two widely used strategies for improving the efficiency of spark-ignited engines. One of the most cited mechanisms thought to be responsible for pre-ignition is the ignition of a rogue droplet composed of lubricant oil and fuel. This originates during mixture formation from interactions between the fuel spray and oil on the cylinder liner. In the present study, this hypothesis is further examined using a single cylinder supercharged engine which employs a range of air-fuel mixture formation strategies. These strategies include port-fuel injection (PFI) along with side and central direct injection (DI) of an E5 gasoline (RON 97.5) using single and multiple injection events. Computational fluid dynamic (CFD) calculations are then used to explain the observed trends. Overall, this study reinforces that interactions between the fuel spray and oil on the cylinder liner can be an important contributor towards stochastic pre-ignition. The occurrence of pre-ignition, as shown by CFD calculations, is successful after completion of two stages. The first stage involves the…
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Effect of Timing and Location of Hotspot on Super Knock during Pre-ignition

King Abdullah University of Science and Technology-Mohammed Jaasim Mubarak Ali, Francisco Hernandez Perez, S. Vedharaj, R. Vallinayagam, Robert Dibble, Hong Im
Published 2017-03-28 by SAE International in United States
Pre-ignition in SI engine is a critical issue that needs addressing as it may lead to super knock event. It is widely accepted that pre-ignition event emanates from hot spot(s) that can be anywhere inside the combustion chamber. The location and timing of hotspot is expected to influence the knock intensity from a pre-ignition event. In this study, we study the effect of location and timing of hot spot inside the combustion chamber using numerical simulations. The simulation is performed using a three-dimensional computational fluid dynamics (CFD) code, CONVERGE™. We simulate 3-D engine geometry coupled with chemistry, turbulence and moving structures (valves, piston). G-equation model for flame tracking coupled with multi-zone model is utilized to capture auto-ignition (knock) and solve gas phase kinetics. A parametric study on the effect of hot spot timing and location inside the combustion chamber is performed. The hot spot timing considered are -180 CAD, -90 CAD and -30 CAD and the locations of the hot spots are in the center and two edges of the piston surfaces. Simulation results for…
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α-Pinene - A High Energy Density Biofuel for SI Engine Applications

Vedharaj Sivasankaralingam, Robert Dibble, S. Mani Sarathy
KAUST-Vallinayagam Raman
Published 2016-10-17 by SAE International in United States
This study proposes a novel biofuel for spark ignition (SI) engine, α-pinene (C10H16), which is non-oxygenated and thus has a gravimetric energy density comparable to that of hydrocarbon fuels. The ignition characteristics of α-pinene were evaluated in an ignition quality tester (IQT) under standard temperature and pressure conditions. The measured ignition delay time (IDT) of α-pinene is 10.5 ms, which is lower than that of iso-octane, 17.9 ms. The estimated research octane number (RON) for pinene from IQT is 85. A temperature sweep in IQT showed that that α-pinene is less reactive at low temperatures, but more reactive at high temperatures when compared to isooctane. These results suggest that α-pinene has high octane sensitivity (OS) and is suitable for operation in turbocharged SI engines.With these considerations, α-pinene was operated in a single cylinder SI engine. The engine combustion characteristics of α-pinene are compared with FACE A gasoline (RON = 85), Euro V gasoline (RON = 97) and ethanol (RON = 109). The experimental investigation reveals that the spark timing and start of combustion for α-pinene…
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Experimental and Numerical Investigation of Ethanol/Diethyl Ether Mixtures in a CI Engine

KAUST-Vedharaj Sivasankaralingam, Vallinayagam Raman, Mohammed Jaasim Mubarak Ali, Adamu Alfazazi, Hong Im, S. Mani Sarathy, Robert Dibble
Univ of Connecticut-Tianfeng Lu
Published 2016-10-17 by SAE International in United States
The auto-ignition characteristics of diethyl ether (DEE)/ethanol mixtures are investigated in compression ignition (CI) engines both numerically and experimentally. While DEE has a higher derived cetane number (DCN) of 139, ethanol exhibits poor ignition characteristics with a DCN of 8. DEE was used as an ignition promoter for the operation of ethanol in a CI engine. Mixtures of DEE and ethanol (DE), i.e., DE75 (75% DEE + 25% ethanol), DE50 (50% DEE + 50% ethanol) and DE25 (25% DEE + 75% ethanol), were tested in a CI engine. While DE75 and DE50 auto-ignited at an inlet air pressure of 1.5 bar, DE25 failed to auto-ignite even at boosted pressure of 2 bar. The peak in-cylinder pressure for diesel and DE75 were comparable, while DE50 showed reduced peak in-cylinder pressure with delayed start of combustion (SOC). Numerical simulations were conducted to study the engine combustion characteristics of DE mixture. A comprehensive detailed chemical kinetic model was created to represent the combustion of DE mixtures. The detailed mechanism was then reduced using standard direct relation graph (DRG-X)…
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Knock Prediction Using a Simple Model for Ignition Delay

KAUST-Robert Dibble
Kaust-Mani Sarathy
Published 2016-04-05 by SAE International in United States
An earlier paper has shown the ability to predict the phasing of knock onset in a gasoline PFI engine using a simple ignition delay equation for an appropriate surrogate fuel made up of toluene and PRF (TPRF). The applicability of this approach is confirmed in this paper in a different engine using five different fuels of differing RON, sensitivity, and composition - including ethanol blends. An Arrhenius type equation with a pressure correction for ignition delay can be found from interpolation of previously published data for any gasoline if its RON and sensitivity are known. Then, if the pressure and temperature in the unburned gas can be estimated or measured, the Livengood-Wu integral can be estimated as a function of crank angle to predict the occurrence of knock. Experiments in a single cylinder DISI engine over a wide operating range confirm that this simple approach can predict knock very accurately. The data presented should enable engineers to study knock or other auto-ignition phenomena e.g. in premixed compression ignition (PCI) engines without explicit chemical kinetic calculations.
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