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.
Highlights
High power density, light weight, small market price, minimal maintenance requirements, simplicity of construction and high reliability are typical features of two-stroke (2S) engines, which make them well-suited for several applications, from motorcycles to professional garden equipment
The present paper aims at evaluating the benefits and challenges of adopting the Jet Ignition (JI) combustion technology on small 2S gasoline Low-Pressure Direct Injection (LPDI) engines for commercial use, in order to understand its applicability on low-weight appliances, e.g., motorcycles, chainsaws, blowers, brush cutters, and drones
3.1 Sensitivity analysis on spark timing The preliminary experimental campaign for the fine-tuning of the JI engine was performed by studying the sole PC-1B prechamber at the three operating conditions (L1, L2, and L3)
Summary
Light weight, small market price, minimal maintenance requirements, simplicity of construction and high reliability are typical features of two-stroke (2S) engines, which make them well-suited for several applications, from motorcycles to professional garden equipment. The high fuel consumption and unburned hydrocarbons emissions conflict with the increasingly stringent international environmental policy. The reason of these issues relies on the layout of transfer and exhaust ports, which leads to the loss of a significant amount of unexploited fuel at high engine loads, and to a noticeable cycle-to-cycle variation at low loads due to poor cylinder scavenging [1]. A consistent reduction of fuel consumption and pollutant emissions was observed by adopting the LPDI from mid-to-high engine loads, even if the weak flow field was found not to be able to sufficiently homogenise the fuel droplets at low loads, increasing further cycle-to-cycle variation
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