Pre-Chamber Jet Ignition in an Optically-Accessible Constant-Volume Gasoline Engine

Dong Eun Lee, Purdue University

Abstract

In Chapter 2, an experiment has been developed to investigate the passive pre-chamber jet ignition process in gasoline engine configurations and low-load operating conditions. The apparatus adopted a modified 4-cylinder 2.0L gasoline engine to enable single-cylinder operation. To reduce the complexity, the piston position was fixed at a predefined position relative to the top dead center (TDC) to simulate thermodynamic conditions at ignition and injection timings. High-speed Infrared (IR) imaging was applied to visualize the jet penetration and ignition process inside the main cylinder and to investigate the cyclic spatial variability. Two passive pre-chambers with different total nozzle areas and numbers of nozzles were used. In addition, the pre-chamber volume and pressure at ignition timing were varied to examine their effect on jet ignition performance. Misfire behavior was observed in the main chamber of all test cases, and the results suggested that the main cause is a high Residual Mass Fraction (RMF) in the pre-chamber affecting the subsequent cycle. A larger total nozzle area, smaller volume, higher pressure, and fuel-lean operation tended to mitigate the misfire behavior. For a test case with a spark pressure of 6 bar, a reduced cyclic variability in terms of coefficient of variation peak cylinder pressure (COVPmax) from 10.03% to 7.38% and combustion phasing variation from 81 crank angle degree (CAD) to 12 CAD were observed with increasing pre-chamber volume-to-area (V/A) ratio from 59.37 m to 103.11 m, but slightly higher misfire frequency was observed, from 46.67% to 50.00%, suggesting an accurate combination of pre-chamber design parameters is needed to improve overall performance at low-load operation.In Chapter 3, it examines the influence of passive pre-chamber nozzle diameter and dilution level on jet formation and engine performance. Utilizing a modified constant-volume gasoline direct injection engine with an optically-accessible piston, we tested three passive pre-chambers with nozzle diameters of 1.2, 1.4, and 1.6 mm, while nitrogen dilution varied from 0 to 20%. With the help of high-speed imaging, we captured pre-chamber jet formations and subsequent flame propagation within the main chamber. Our novel findings reveal that asymmetric temporal and spatial jet formation patterns arising from pre-chambers significantly impact engine performance. The larger nozzle diameter pre-chambers exhibited the least variation in jet formation due to their improved scavenging and main mixture filling processes, but had the slowest jet velocity and lowest jet penetration depth. At no dilution condition, the 1.2 mm-PC demonstrated superior performance attributed to higher pressure build-up in the pre-chamber, resulting in accelerated jet velocity and increased jet penetration depth. However, at high dilution condition, the 1.6 mm-PC performed better, highlighting the importance of scavenging and symmetry jet formation. This study emphasizes the importance of carefully selecting the pre-chamber nozzle diameter, based on the engine's operating conditions, to achieve an optimal and balanced configuration that can improve both jet formation and jet characteristics, as well as scavenging.In Chapter 4, it investigates the influence of passive pre-chamber nozzle diameter on jet ignition and subsequent main chamber combustion under varying load conditions and dilution levels using a constant-volume optical gasoline direct injection engine. The results reveal that as the load decreases, both fuel availability and flow conditions deteriorate, leading to delayed and inferior jet characteristics that affect main chamber ignition and combustion processes. In high and medium load conditions without dilution, the smallest nozzle diameter pre-chamber (1.2mm-PC) shows improved jet ignition and main combustion due to earlier jet ejection, enhanced penetration, and intensified jet. This is facilitated by the smaller nozzle diameter enabling faster and higher pre-chamber pressurization. Conversely, under low load conditions, the largest nozzle diameter pre-chamber (1.6mm-PC) performs better, likely due to improved scavenging and reduced residual levels, resulting in less compromised pre-chamber combustion and subsequent jet characteristics. The nozzle diameter also has a significant impact on cycle-to-cycle variations, with smaller diameters enhancing jet ignition performance but increasing variability. The effect of external residuals (dilution) on jet ignition performance varies depending on the nozzle diameter, with the 1.6mm-PC exhibiting less degradation and demonstrating earlier jet ejection and CA50 timing compared to smaller nozzle diameter pre-chambers at higher dilution conditions. The improved scavenging and relatively lower residual levels in the larger nozzle diameter pre-chamber contribute to its increased resistance to dilution and potential extension of dilution tolerance.

Degree

Ph.D.

Advisors

Shaver, Purdue University.

Subject Area

Aerospace engineering

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