Characteristics of Cycle-to-Cycle Combustion Variability at Partial-Burn Limited and Misfire Limited Spark Timing Under Highly Diluted Conditions

Diluted combustion with exhaust gas recirculation (EGR) has been widely employed to improve the fuel economy of spark ignition engines. The combustion kinetics, however, are affected and the flame propagation speed is decreased. In order to compensate for this adverse effect, the spark timing needs to be recalibrated to achieve maximum brake torque (MBT). At high levels of EGR dilution, the spark timing is constrained by two ignition limits: 1) the partial-burn limit where the spark timing is retarded from MBT and 2) the misfire limit where the spark timing is too advanced. This work uses a probabilistic framework to capture the differences between both ignition limits. In particular, it introduces the concept of a nominal indicated mean effective pressure (IMEP) distribution based on the stochastic properties of the cycle-to-cycle variability (CCV) at nominal stable conditions. By defining a nominal band where fully burned cycles occur with high probability, we introduce a cycle classification method that can be used to 1) determine the level of randomness of misfire and partial-burn events, and 2) measure CCV. The new CCV metric based on the density of the nominal band is compared with the traditional coefficient of variation of IMEP (CoVIMEP). It is shown that the nominal band concept, together with the CoVIMEP, can help to discern between partial-burn limited and misfire limited conditions. Furthermore, the Kullback-Leibler divergence is used to demonstrate that the IMEP distribution is significantly different between nominal and partial-burn/misfire limited conditions. Experiments are carried at various EGR levels and spark timings while recoding in-cylinder pressure at steady state. Although the emphasis of this work is to characterize the differences of both ignition limits from a probabilistic point of view, similarities between partial-burn cycles at either limiting conditions are also discussed.

Experimental investigation on ignition limits of plasma-assisted ignition in the propane–air mixture

In recent past, the plasma-assisted ignition has been explored for applications on a variety of engines. The plasma ignition has been shown to possess special advantages such as reducing the ignition delay time, improving the reliability, and reducing the NOx emissions. By using a plasma jet ignition experimental system, the plasma jet ignition of argon-discharge arc has been investigated. Owing to the characteristics of high temperature, the mixture can be easily ignited by the plasma jet. Through the propane–air mixture ignition experiments, the ignition limits of the plasma jet and spark ignition are investigated. The results show that the plasma jet ignition could extend the ignition limits of propane–air mixture obviously. The ignition limit extends with the increase in the air flow rates. The average ignition limit (the gap between rich and lean limit) of spark ignition and plasma jet ignition are 2.34 and 2.57, respectively. The average ignition limit of the propane–air mixture extends by 9.8%. The plasma jet ignition limit extends with increasing arc current, and the degree of extending plasma jet ignition limit increases with increasing air flow rates. The average ignition limits of 5.7 A and 20.3 A are 2.57 and 2.79, respectively. The average ignition limit of the propane–air mixture extends by 8.5%. The plasma jet ignition limit extends with increasing argon flow rates. The average ignition limits of 200 L/h and 250 L/h are 2.79 and 3.08, respectively. The average ignition limit of the propane–air mixture extends by 10.4%.

Auto-Ignition of In-Line Injected Hydrogen/Nitrogen Fuel Mixtures at Reheat Combustor Operating Conditions

De-carbonization of the power generation sector becomes increasingly important in order to achieve the European climate targets. Coal or biomass gasification together with a pre-combustion carbon capture process might be a solution resulting in hydrogen-rich gas turbine (GT) fuels. However, the high reactivity of these fuels poses challenges to the operability of lean premixed gas turbine combustion systems because of a higher auto-ignition and flashback risk. Investigation of these phenomena at GT relevant operating conditions is needed to gain knowledge and to derive design guidelines for a safe and reliable operation. The present investigation focusses on the influence of the fuel injector configuration on auto-ignition and kernel development at reheat combustor relevant operating conditions. Auto-ignition of H2-rich fuels was investigated in the optically accessible mixing section of a generic reheat combustor. Two different geometrical in-line configurations were investigated. In the premixed configuration, the fuel mixture (H2 / N2) and the carrier medium nitrogen (N2) were homogeneously premixed before injection, whereas in the co-flow configuration the fuel (H2 / N2) jet was embedded in a carrier medium (N2 or air) co-flow. High-speed imaging was used to detect auto-ignition and to record the temporal and spatial development of auto-ignition kernels in the mixing section. A high temperature sensitivity of the auto-ignition limits were observed for all configurations investigated. The lowest auto-ignition limits are measured for the premixed in-line injection. Significantly higher auto-ignition limits were determined in the co-flow in-line configuration. The analysis of auto-ignition kernels clearly showed the inhibiting influence of fuel dilution for all configurations.