The use of exhaust gas recirculation (EGR) in spark ignition engines has been shown to have a number of beneficial effects under specific operating conditions. These include reducing pumping work under part load conditions, reducing NOx emissions and heat losses by lowering peak combustion temperatures, and by reducing the tendency for engine knock (caused by end-gas autoignition) under certain operating regimes. In this study, the effects of EGR addition on knocking combustion are investigated through a combined experimental and modeling approach. The problem is investigated by considering the effects of individual EGR constituents, such as CO2, N2, and H2O, on knock, both individually and combined, and with and without traces species, such as unburned hydrocarbons and NOx. The effects of engine compression ratio and fuel composition on the effectiveness of knock suppression with EGR addition were also investigated. A parametric, experimental matrix of diluents, compression ratio, and fuels was tested to measure knock-limited combustion phasing of each combination. The resulting knock limits were evaluated in the context of thermodynamic effects on the closed cycle, chemical interactions between the EGR constituents and the fuel-oxidizer mixture, and the effect of altered pressure-temperature trajectories on fuel-autoignition behavior. This paper provides an overview of the experimental results, and uses chemical-kinetic modeling to investigate the behavior of a particular fuel - diluent combination which had a strong sensitivity to compression ratio variation. The numerical results shed light on the complex interactions between fuel chemistry, the engine's thermodynamic cycle, and the effect of residence times on the autoignition chemistry which leads to knock. An important and fuel-dependent role of thermal stratification in the end-gas is also suggested by the chemical-kinetics modeling of the experimentally observed knock limits.
This study evaluates the applicability of the Octane Index (OI) framework under conventional spark ignition (SI) and “beyond Research Octane Number (RON)” conditions using nine fuels operated under stoichiometric, knock-limited conditions in a direct injection spark ignition (DISI) engine, supported by Monte Carlo-type simulations which interrogate the effects of measurement uncertainty. Of the nine tested fuels, three fuels are “Tier III” fuel blends, meaning that they are blends of molecules which have passed two levels of screening, and have been evaluated to be ready for tests in research engines. These molecules have been blended into a four-component gasoline surrogate at varying volume fractions in order to achieve a RON rating of 98. The molecules under consideration are isobutanol, 2-butanol, and diisobutylene (which is a mixture of two isomers of octene). The remaining six fuels were research-grade gasolines of varying formulations. The DISI research engine was used to measure knock limits at heated and unheated intake temperature conditions, as well as throttled and boosted intake pressures, all at an engine speed of 1400 rpm. The tested knock-limited operating conditions conceptually exist both between the Motor Octane Number (MON) and RON conditions, as well as “beyond RON” conditions (conditions which are conceptually at lower temperatures, higher pressures, or longer residence times than the RON condition). In addition to directly assessing the performance of the Tier III blends relative to other gasolines, the OI framework was evaluated with considerations of experimental uncertainty in the knock-limited combustion phasing (KL-CA50) measurements, as well as RON and MON test uncertainties. The OI was found to hold to the first order, explaining more than 80% of the knock-limited behavior, although the remaining variation in fuel performance from OI behavior was found to be beyond the likely experimental uncertainties. This indicates that the effects of specific fuel components on knock which are not captured by RON and MON ratings, and complicating the assessment of a given fuel by RON and MON ratings alone.
The need to avoid SI engine knock often comes with an efficiency penalty, motivating efforts to understand its causes. In this study, the relationship between knock and end-gas autoignition is examined based on experiments in a DISI engine with CR = 12, using three different gasoline fuels. The three gasolines are an alkylate blend (RON=98, MON=97), a blend with high aromatic content (RON=98, MON=88), and a blend of 30% ethanol by volume (RON=98, MON=87). Fuel/air-equivalence ratio (φ) sweeps show that the response of the knock limits to changes in φ varies between the fuels. Furthermore, it is observed that the statistics of knock and heat-release vary in a complex manner between the fuels. Since knock originates from end-gas autoignition, a robust heat-release-based metric of "trace autoignition" is developed to determine knock occurrence. The metric is based on the observation that autoigniting cycles exhibit a faster heat-release rate decay at the end of the combustion event, even if no knock oscillations are detected. In general, the acoustic knock trends match the trends of the "trace autoignition" metric. However, for rich operation, the Alkylate fuel needs to be operated with an average CA50 that is somewhat retarded relative to trace autoignition. Furthermore, the data reveal that the dependence of autoignition on φ varies in manner that relates to the RON-MON octane sensitivity of the fuel. For example, the Alkylate fuel with low octane sensitivity displays no benefit of fuel enrichment, which is in strong contrast to the two high octane-sensitive fuels. Further, the Alkylate fuel shows a strong reduction of its anti-knock quality for lean operation, which correlates with the development of low-temperature heat release.
Fundamental engine research is primarily conducted under steady-state conditions, in order to better describe boundary conditions which influence the studied phenomena. However, light-duty automobiles are operated, and tested, under heavily transient conditions. This mismatch between studied conditions and in-use conditions is deemed acceptable due to the fundamental knowledge gained from steady-state experiments. Nonetheless, it is useful to characterize the conditions encountered during transient operation and determine if the governing phenomena are unduly influenced by the differences between steady-state and transient operation, and further, whether transient behavior can be reasonably extrapolated from steady-state behavior. The transient operation mode used in this study consists of 20 fired cycles followed by 80 motored cycles, operating on a continuous basis. The intention of the cycle is to provide a significant transient condition, namely the change from motored to knock-limited fired operation, while also maintaining a repeatable cycle that allows for the collection of statistics during quasi- steady-state operation. This study investigates the effect of transient operation on Knock-Limited Combustion Phasing (KL-CA50) compared to steady-state operation. Three compositionally dissimilar matched Research Octane Number (RON) = 98 fuels are used in this study, allowing for the assessment of fuel-specific effects on differences between steady-state and transient operation. This study first characterizes the 20/80 firing cycle described above, before comparing the transient KL-CA50 measurements to the steady-state KL-CA50 measurements. Analysis of both the steady-state and transient results are used to gain insights into the effects of transient operation on end-gas autoignition, relative to steady-state operation and as a function of fuel composition. The results of this study indicate the significant effect that transient operation has on KL-CA50 behavior of a fuel. This is both universal, in that all fuels show responses to the differences in compression temperatures of the charge, as well as fuel specific, in that the fuel response varies based on the fuel's sensitivity to temperature, [O2], and trace species. All fuels showed a significant load extension under transient operation, based on tolerance of higher intake pressures. However, transient operation moved operating conditions to "beyond RON" (Octane Index K < 0) conditions, which favored higher-sensitivity fuels. Based on the analysis of system time constants (e.g. cylinder head temperature dynamic response, exhaust gas temperature dynamic response), it is expected that transient operation, and the benefits for knock-limited operation, are highly influential on drive-cycle performance.
The overall objectives are to provide the science-base needed by industry to understand how emerging alternative fuels impact highly efficient DISI light-duty engines being developed by industry and how engine design and operation can be optimized for most efficient use of future fuels.
The overall objectives are to provide the science-base needed by industry stakeholders to understand how engine design and operation can be co-optimized with future fuels for highest overall system efficiency.
Implementation of spray-guided stratified-charge direct-injection spark-ignited (DISI) engines is inhibited by the occurrence of misfire and partial burns. Engine-performance tests demonstrate that increasing engine speed induces combustion instability, but this deterioration can be prevented by generating swirling flow during the intake stroke. In-cylinder pressure-based heat-release analysis reveals that the appearance of poor-burn cycles is not solely dependent on the variability of early flame-kernel growth. Cycles can experience burning-rate regression during later combustion stages and may or may not recover before the end of the cycle. Thermodynamic analysis and optical diagnostics are used here to clarify why swirl improves the combustion repeatability from cycle to cycle.The fluid dynamics of swirl/spray interaction was previously demonstrated using high-speed PIV measurements of in-cylinder motored flow. It was found that the sprays of the multi-hole injector redistribute the intake-generated swirl flow momentum, thereby creating a better-centered higher angular-momentum vortex with reduced variability. The engine operation with high swirl was found to have significant improvement in cycle-to-cycle variations of both flow pattern and flow momentum.This paper is an extension of the previous work. Here, PIV measurements and flame imaging are applied to fired operation for studying how the swirl flow affects variability of ignition and subsequent combustion phases. PIV results for fired operation are consistent with the measurements made of motored flow. They demonstrate that the spark-plasma motion is highly correlated with the direction of the gas flow in the vicinity of the spark-plug gap. Without swirl, the plasma is randomly stretched towards either side of the spark plug, causing variability in the ignition of the two spray plumes that are straddling the spark plug. In contrast, swirl flow always convects the spark plasma towards one spray plume, causing a more repeatable ignition. The swirl decreases local RMS velocity, consistent with an observed reduction of early-burn variability. Broadband flame imaging demonstrates that with swirl, the flame consistently propagates in multiple directions to consume fuel-air mixtures within the piston bowl. In contrast, operation without swirl displays higher variability of flame-spread patterns, occasionally causing the appearance of partial-burn cycles.