Publications

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Well-mixed lean or dilute SI engine operation can provide efficiency improvements relative to that of traditional well-mixed stoichiometric SI operation. However, the realized gains depend on the ability to ensure stable, complete and fast combustion. In this work, the influence of fuel type is examined for gasoline, E30 and E85. Several enabling techniques are compared. For enhanced ignition stability, a multi-pulse (MP) transient plasma ignition system is compared to a conventional high-energy inductive spark ignition system. Combined effects of fuel type and intake-gas preheating are examined. Also, the effects of dilution type (air or N2-simulated EGR) on lean efficiency gains and stability limits are clarified. The largest efficiency improvement is found for lean gasoline operation using intake preheating, showing the equivalent of a 20% fuel-economy gain relative to traditional non-dilute stoichiometric operation. The reason for gasoline’s larger efficiency improvement is its lower octane number which facilitates the use of end-gas autoignition to produce mixed-mode combustion. For these conditions, such mixed-mode combustion is required for rapid completion of the inherently slow lean combustion event prior to piston expansion. The fuel-economy gains are somewhat smaller for both E30 and E85 because of higher resistance to end-gas autoignition under lean conditions. To avoid knocking cycles when mixed-mode combustion is used, the deflagration-based combustion must be very repeatable to ensure consistent compression of the end-gas reactants. Multi-pulse transient plasma ignition is used beneficially to stabilize the combustion, especially for dilute operation which suffers from low flame speeds. However, even with an enhanced ignition system, the best fuel-economy gains of dilute stoichiometric operation with mixed-mode combustion are on the order of 11-12%, which is substantially less than for lean operation.

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Climate change and the need to secure energy supplies are two reasons for a growing interest in engine efficiency and alternative fuels. This project contributes to the science-base needed by industry to develop highly efficient DISI engines that also beneficially exploit the different properties of alternative fuels. Our emphasis is on lean operation, which can provide higher efficiencies than traditional non-dilute stoichiometric operation. Since lean operation can lead to issues with ignition stability, slow flame propagation and low combustion efficiency, we focus on techniques that can overcome these challenges. Specifically, fuel stratification is used to ensure ignition and completeness of combustion but has soot- and NOx- emissions challenges. For ultralean well-mixed operation, turbulent deflagration can be combined with controlled end-gas auto-ignition to render mixed-mode combustion that facilitates high combustion efficiency. However, the response of both combustion and exhaust emissions to these techniques depends on the fuel properties. Therefore, to achieve optimal fuel-economy gains, the engine combustion-control strategies must be adapted to the fuel being utilized.

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Modern spark-ignition (SI) engine technologies have considerably changed in-cylinder conditions under which fuel autoignition and engine knock take place. In this paper, fundamental HCCI engine experiments are proposed as a means for characterizing the impact of these technologies on the knock propensity of different fuels. In particular, the impacts of turbocharging, direct injection (DI), and downspeeding on operation with ethanol and gasoline are investigated to demonstrate this approach. Results reported earlier for ethanol and gasoline on HCCI combustion are revisited with the new perspective of how their autoignition characteristics fit into the anti-knock requirement in modern SI engines. For example, the weak sensitivity to pressure boost demonstrated by ethanol in HCCI autoignition can be used to explain the strong knock resistance of ethanol fuels for turbocharged SI engines. Further, ethanol's high sensitivity to charge temperature makes charge cooling, which can be produced by fuel vaporization via direct injection or by piston expansion via spark-timing retard, very effective for inhibiting knock. On the other hand, gasoline autoignition shows a higher sensitivity to pressure, so only very low pressure boost can be applied before knock occurs. Gasoline also demonstrates low temperature sensitivity, so it is unable to make as effective use of the charge cooling produced by fuel vaporization or spark retard. These arguments comprehensively explain literature results on ethanol's substantially better anti-knock performance over gasoline in modern turbocharged DISI engines. Fundamental HCCI experiments such as these can thus be used as a diagnostic and predictive tool for knock-limited SI engine performance for various fuels. Examples are presented where HCCI experiments are used to identify biofuel compounds with good potential for modern SI-engine applications.

This study investigates combustion variability of a stratified-charge direct-injection spark ignited (DISI) engine, operated with near-TDC injection of E70 fuel and a spark timing that occurs during the early part of the fuel injection. Using EGR, low engine-out NOx can be achieved, but at the expense of increased combustion variability at higher engine speeds. Initial motored tests at different speeds reveal that the in-cylinder gas flow becomes sufficiently strong at 2000 rpm to cause significant cycle-to-cycle variations of the spray penetration. Hence, the fired tests focus on operation at 2000 rpm with N2 dilution ([O2] = 19% and 21%) to simulate EGR. In-cylinder flow, spray, and early-flame measurements are correlated to reveal their effect on the combustion variability. Results reveal two types of flow/spray-interactions that predict the likelihood of a partial burn. (1) Proper flow direction before injection with a more collapsed spray leads to high kinetic energy of the flow during injection, thus generating a rapid early burn, which ensures complete combustion, regardless of the EGR level. (2) Improper flow direction and less collapsed spray generate low flow energy during the early phase of combustion. For this second type of flow/spray-interaction, application of EGR results in a partial-burn frequency of 30%, whereas without EGR, early combustion is shown to be insensitive to flow variations. Flame-probability maps illustrate that the partial-burn cycles for operation with EGR have a weak flame development in that the flame does not develop uniformly and reliably from the spark plug. Without EGR, the flame development is more repeatable regardless of the type of flow/spray-interaction, at the expense of higher NOx emissions.

Due to concerns about future petroleum supply and accelerating climate change, increased engine efficiency and alternative fuels are of interest. This project contributes to the science-base needed by industry to develop highly efficient DISI engines that also beneficially exploit the different properties of alternative fuels. Lean operation is studied since it can provide higher efficiencies than traditional non-dilute stoichiometric operation. Since lean operation can lead to issues with ignition stability, slow flame propagation and low combustion efficiency, focus is on techniques that can overcome these challenges. Specifically, fuel stratification can be used to ensure ignition and completeness of combustion, but may lead to soot and NO_x emissions challenges. Advanced ignition system and intake air preheating both promote ignition stability. Controlled end-gas autoignition can be used maintain high combustion efficiency for ultra-lean well-mixed conditions. However, the response of both combustion and exhaust emission to these techniques depends on the fuel properties. Therefore, to achieve optimal fuel-economy gains, the combustion-control strategies of the engine must adopt to the fuel being utilized.

Well-mixed lean SI engine operation can provide improvements of the fuel economy relative to that of traditional well-mixed stoichiometric SI operation. This work examines the use of two methods for improving the stability of lean operation, namely multi-pulse transient plasma ignition and intake air preheating. These two methods are compared to standard SI operation using a conventional high-energy inductive ignition system without intake air preheating. E85 is the fuel chosen for this study. The multi-pulse transient plasma ignition system utilizes custom electronics to generate 10 kHz bursts of 10 ultra-short (12ns), high-amplitude pulses (200 A). These pulses were applied to a custom spark plug with a semi-open ignition cavity. High-speed imaging reveals that ignition in this cavity generates a turbulent jet-like early flame spread that speeds up the transition from ignition to the main combustion event. Performance testing shows that lean operation with heated intake air enables a 17% improvement of fuel economy at ϕ = 0.59 for both ignition systems, relative to that of stoichiometric operation. Moreover, multi-pulse transient plasma ignition offers more stable ultra-lean operation, with IMEPn variability less than 5% down to ϕ = 0.49. The ability to operate stably at such lean conditions is attributed to a more stable flame initiation offered by both the increased charge temperature and the multi-pulse transient plasma ignition that allows a later spark timing due to the very fast transition to fully turbulent deflagration.

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In a companion study [1], experimental observations in a stratified-charge DISI engine operated with late injection of E70 led to the formation of two hypotheses: (1) For highly stratified spray-guided combustion, the heat-release rate of the main combustion phase is primarily controlled by mixing rates and turbulence level associated with fuel-jet penetration. (2) During the main combustion phase, the role of the in-cylinder flow field generated by the intake and compression strokes is primarily its stochastic disturbance of the mixing and flow associated with the fuel jets, thereby causing cycle-to-cycle variations of the spray-guided stratified combustion. Here, these hypotheses are tested. An optical engine was operated skip fired at 1000 and 2000 rpm, and exhibited the same combustion properties observed in the steady-state all-metal engine tests. High-speed particle image velocimetry (PIV) and spray imaging are used to quantify the intake-generated in-cylinder flow momentum, the spray induced momentum, and the resulting liquid spray variability. The PIV measurements reveal that the spatially-averaged gas-flow speed (momentum) without injection at 2000 rpm is twice that of 1000 rpm. In contrast, just after injection the gas flow spatial average speed at 2000 rpm is only 24% higher due to the dominance of spray momentum. This is comparable to the 16% increase of the measured ensemble-averaged heat-release rate (in kW). The cyclic variability of the in-cylinder flow speed prior to injection is measured to be considerably higher at 2000 rpm compared to 1000 rpm. Though the injected liquid spray reduced the flow-speed cyclic-variability after injection, the higher variability did persist. The spray imaging reveals that the increased flow-speed variability at 2000 rpm causes increased variability of the spray jet trajectory, jet coalescence, and spray rotation from cycle to cycle. This work supports both the hypotheses that motivated this study. Copyright © 2014 SAE International.

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