The multi-field coupling effect of fuel injection duration on gas exchange stability for a free piston engine generator

Abstract

• The detailed process of fuel injection duration affected the gas exchange stability through multi-field coupling effect was revealed. • Increasing the fuel injection duration is beneficial to improve scavenging efficiency, and the maximum scavenging efficiency is 97.9%. • Shortening the fuel injection duration is beneficial to improve the trapping efficiency, and the maximum trapping efficiency is 41.3%. • The afterburning effect and the backflow phenomenon were found. • The EGR mass fraction is inversely proportional to the fuel injection duration. Free piston engine generator (FPEG) is a new energy conversion plant to substitute for conventional combustion engine. However, the fluctuation of gas exchange is a key problem to induce the FPEG to run unsteadily and restrict its engineering application. This paper presents a study to explore the multi-field coupling effect of fuel injection duration on the gas exchange stability of a diesel compression-ignition, direct-injection, looped scavenging FPEG. A full-cycle gas exchange model of the FPEG is carried out by coupling the zero-dimensional dynamics, multi-dimensional combustion model and scavenging model, and an iterative calculation method is proposed for numerical simulation of the model. Meanwhile, the effect of injection duration schedule lasting from 0.50 ms to 0.80 ms on the scavenging and gas exchange is analyzed. The results show that the shorter injection duration brings about higher fresh trapping efficiency due to its faster reciprocating frequency which reduces the available time for fresh gas to flow out of the cylinder during gas exchange process, and the trapping efficiency is 41.3 % and the operating frequency achieves 36.6 Hz in the injection duration of 0.5 ms condition. However, the influence of injection duration on exhaust gas cleaning is opposite, and the scavenging efficiency varies in positive correlation with the injection duration, because the long injection leads to low motion velocity and causes the exhaust port to be opened and closed slowly, which provides long exhaust time of the residual burned gas in the cylinder. The maximum scavenging efficiency occurs in the injection duration of 0.8 ms condition and features with 97.9%. Suggesting, relatively high scavenging efficiency and trapping efficiency can be obtained at the same time under appropriate injection duration condition.