High-speed imaging analysis of misfires in a spray-guided direct injection engine
Introduction
The spray-guided spark-ignited direct-injection (SG-SIDI) engine offers the ability to improve fuel economy by unthrottled load control. The load is lowered by decreasing the fuel (rather than air) to create an overall lean charge, which is locally flammable due to charge stratification. For a SG stratified charge, fuel is injected late in the compression stroke, concentrating an ignitable mixture near the spark plug. Thus, optimum spark timing typically occurs directly after the spray event.
SG-SIDI engines are known to suffer from rare partial burns and misfires, (PB&MFs) [1]. It is observed that the injection event imposes high velocities and large velocity gradients near the spark plug at the time of spark. High velocities can create multiple spark re-strikes, which rapidly changes the location of the spark plasma and shortens the spark duration [1]. Local velocity fluctuations can cause undesirable spark motion and location [2]. The short time duration between the injection and spark event creates highly stratified mixtures including liquid fuel near the spark plug, and non-optimal timing between the injection and spark events are expected to cause an overly rich or overly lean mixture at the spark.
There does not appear to be one universal cause of PB&MFs; instead, they can be caused by multiple parameters [1], [2], [3], [4], [5]. Fuel imaging with planar laser-induced fluorescence (PLIF) has been used in a two-stroke SG-SIDI optical engine to image fuel distribution near the spark plug and have revealed that cyclic variation in fuel concentration near the spark plug are large enough to cause PB&MFs [3]. High-speed fuel LIF imaging was employed to measure the temporal evolution of the fuel cloud leading to misfires [4]. Fuel LIF imaging in a wall-guided direct injection engine showed that non-optimal spray and spark timing can cause over-mixing, producing mixture fractions beyond the flammability limits that lead to misfires [5]. Other LIF studies have shown adequate fuel distribution near the spark plug, but abnormally short spark durations in which a mixture was not ignited [4], [6]. Imaging of the spark and early flame kernel demonstrated that local flow fluctuations can cause unfavorable motion and location, and are a dominant source of random misfire events [2]. Imaging of the spray injection event demonstrated that the resulting high velocities near the spark gap lead to dramatic spark stretching and frequent re-strikes which shorten the spark duration, but did not show any significant correlation to misfire or partial burn cycles [1].
Whereas the previous studies addressed the flow and fuel concentration in separate experiments, this study employs simultaneous fuel and velocity measurements. High-speed particle image velocimetry (PIV) and PLIF of biacetyl tracer are used to measure the flow field and fuel concentrations, respectively, for cases of misfire, partial burn, and normal combustion. Normalized fuel concentration (equivalence ratio) and velocity magnitude data are extracted from a 4 mm × 4 mm region downstream but adjacent to the spark plug at the onset of spark to characterize the initial conditions near the spark plasma. In a separate study on the available spark energy as a function of mixture and flow conditions near the spark plug, relationships were characterized of available spark energy with equivalence ratio, flow velocity, strain rate, and vorticity [7]. Misfiring and partially burning cycles did not exhibit a correlation with either strain rate or vorticity. Therefore, the present study focuses on equivalence ratio and velocity only. The engine was operated to produce rare PB&MFs (five partial burns and nine misfires out of 1392 cycles) so that the other 99.4% of the cycles were near the normal steady-state operating condition. The results demonstrate that the fuel concentration, velocity, spark energy, and spark duration of these rare events were found to fall within the sample population of successful cycles. Thus, as a second step the flow and fuel images of the PB&MFs were compared with well-burned cycles, which have similar velocities and fuel concentrations at the time of spark is performed to further understand the cause of the ignition instabilities.
Section snippets
Experimental
Engine operating conditions, shown in Table 1, were chosen to mimic low-load idle operating conditions in a SG-SIDI engine at 800 rpm. The fuel used in the experiments was a mixture of 90% iso-octane, 10% biacetyl (by volume), which does not alter combustion characteristics in any significant manner [8], [9]. Experiments were conducted without any external dilution (i.e. neither nitrogen nor exhaust gas recirculation (EGR)). The combined high-speed flow and fuel imaging diagnostics have been
Inducement of ignition failures
The goal of this study was to identify in-cylinder events that lead to rare misfires and partial burns at a robust engine idle condition. To find a robust condition, the engine was operated with out external gas dilution of the air, and the end-of-injection (EOI) timing and spark timing were varied to maximize indicated mean effective pressure (IMEP) and minimize the coefficient of variance (COV) of IMEP. Figure 3 which shows the 375 cycle averaged IMEP and COV of IMEP, demonstrates that the
Conclusions
This study of misfires and partial burns in a spray-guided stratified-charge spark-ignition engine provides previously-unavailable, concurrent, cycle-resolved measurements of velocity using 2-D PIV and fuel PLIF during ignition and early flame development. The engine was operated at an idle condition with one degree of spark delay off optimum, which produced rare and random partial burns and misfires (0.3% and 0.5%, respectively); nine misfires and five partial burns occurred within 1392
Acknowledgements
This work was supported by the General Motors Corporation through the General Motors-University of Michigan Collaborative Research Laboratory on Engine Systems Research. The authors are grateful to Arun Solomon, Michael C. Drake and Todd D. Fansler at General Motors and Louise Lu at the University of Michigan.
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