Jet ignition in small two-stroke engines: an experimental survey on benefits and challenges

Abstract

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.

Experimental investigation on the potential of passive prechambers for use in 2S engines

“…This increase is aimed at enhancing volumetric efficiency while maintaining a 2 % CO level, consequently leading to a reduction in indicated efficiency and an increment in fuel consumption. As explained in [20] to achieve this enhanced efficiency, a delay in the ignition advance (IGA) is required. Such a delay, in turn, shifts the point of peak pressure away from the ideal timing, thus resulting in a reduction of the useful work provided by the piston and an increase in the temperature of exhaust gases.…”

Numerical Investigation on the Performance of a 4-Stroke Engine with Different Passive Pre-Chamber Geometries Using a Detailed Chemistry Solver

“…In the more traditional case of slow natural gas engines for power generation, where the size of valves is not so relevant, the pre-chamber shape is usually stumpy [14] and can fit easily above the engine's head. Large and flattened shapes are also used in two-stroke engines, thanks to the absence of head valves [15,16]. Having a low height-to-diameter ratio, they seem to facilitate the scavenging process [17], especially when using multiple orifices.…”