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Increasingly stringent policy on carbon dioxide have proved to be a severe constraint on the design of light two-stroke (2S) engines. The enhancement of efficiency is claimed to be the main target to make these devices compliant with future regulations. Low-Pressure Direct Injection (LPDI) was found to be effective in the reduction of fuel short circuit, thus improving efficiency and mitigating pollution. Innovative combustion technologies are required to exploit further the fuel potential. Jet Ignition (JI), i.e., ignition provided by means of hot turbulent jets, was found to improve the fuel ignition process, leading to a faster and more uniform combustion. Several benefits are shown in over forty years of Literature, from the higher performance to wider flammability limits. However, few studies have been performed on small 2S engines, for which only full-load data are available. This paper aims at experimentally evaluating benefits and challenges of adopting the JI technology on light 2S LPDI engines at both full- and low- load operation. Different configurations of passive prechambers have been investigated in terms of indicated cycle, brake efficiency and cycle-to-cycle variation. A sensitivity analysis on the spark timing was performed to fine-tune the combustion process. Results show the adequacy of the JI for use in devices operating close to full torque, like garden tools; however, issues related to the excessive amount of residual gas or to the rapid pressure decrease during the expansion phase were highlighted. Different solutions for solving these challenges are proposed.

An accurate estimation of rotating stall is one of the key technologies for high-pressure centrifugal compressors. Several techniques have been proposed to detect the stall onset; inter alia, few dynamic pressure probes have been shown to not only properly detect the phenomenon, but also reconstruct the stall characteristics after an ensemble averaging approach. The massive use of this technique in the field is, however, not a common practice yet. In the present study, the use of dynamic pressure probes has been combined with that of an environmental microphone to evaluate the prospects of this latter for a possible stall onset detection. To this end, experimental tests have been carried out in the experimental test rig of the Department of Industrial Engineering (DIEF) of Università degli Studi di Firenze. Results show that the microphone was able to distinguish the onset of rotating stall accurately and promptly, even though – differently from dynamic pressure sensors - it does not provide sufficient information to determine the characteristics of the stall pattern. On this basis, the use of acoustic measurements could find room for automatic detection of rotating stall onset.

Modern engines require continuous and detailed monitoring in order to minimize variations in performance and emissions during their operation. To this aim, the engine thermodynamic cycle must be brought into focus, with special emphasis on the combustion process. The in-cylinder pressure is the parameter that is most directly associated with the engine thermodynamic cycle. Direct measurements by means of dynamic pressure sensors are commonly used for research and development purpose. Currently, however, the cost and intrusiveness of such sensors, together with the harsh operating conditions that limits their lifetime, make direct measurements of the in-cylinder pressure not yet suitable for mass production applications. As a consequence, there’s great interest in developing cost-effective and reliable alternative solutions to extract pressure trace and combustion indicators such as the indicated mean effective pressure (IMEP). The paper presents a technique for reconstructing the pressure cycle of each cylinder of a multi-cylinder engine by combining the instantaneous crankshaft speed information and a 0D thermodynamic model. Crankshaft position, angular velocity and angular acceleration, coupled with the engine inertia, are used to estimate the effective torque oscillation. The average indicated torque and the combustion torque contribution of each cylinder are then evaluated by coupling the information coming from engine speed and the 0D model. Eventually, through the kinematic relationship between the effective torque and the indicated pressure, the in-cylinder pressure trace is reconstructed. The methodology proposed by the authors allows to evaluate the indicated pressure of each cylinder: this is especially useful for detecting cylinder-to-cylinder variations and misfiring events. Data from a 1D calibrated numerical model of a four-cylinder engine is used for preliminary validation at different engine speeds and loads.