Abstract
The paper deals with the optimisation of the charging efficiency in a small two-stroke high-speed engine according to well-established design guidelines. The aim is to present the method successfully used in the preliminary screening of the performance improvement attainable in a crank-case-compression engine by design modifications to the transfer ports and manifolds. The method applies a basic CFD model of the steady-state flow across the cylinder block validated against experimental tests at the discharge flow bench. This model is used to approximate the actual scavenging process through a transient simulation of the cold flow across the cylinder inside which the piston is fixed at the bottom centre. A fast assessment of the charging efficiency is permitted by a transported passive scalar implemented in the model to easily estimate all the parameters needed for monitoring the effectiveness of the scavenging process at each crankangle. This practical design approach has been applied to the geometry of a Schnürle-type loop-scavenged 125cc single-cylinder engine compliant with the 2018 FIA homologation form for the KF2 karting competition category. The maximum increase of indicated mean effective pressure expected according to the comparison between the CFD simulations of the original and the modified design of the transfer ports is approximately equal to 4%. This result demonstrates that the CFD analyses are sensitive to the limited modifications commonly needed to tune two-stroke racing-engines and confirms that the suggested design approach can be profitably employed by engineers and technicians involved in the design of small two-stroke highspeed engines.
Highlights
The effectiveness of the gas exchange process is crucial for two-stroke engines where the opening time of conventional piston-controlled transfer ports for small crankcase-compression engines hardly exceeds 110° [1]
The subject of this paper is the optimisation of the charging efficiency in an existing highspeed two-stroke engine for karting competitions and, in particular, the design approach used to improve the geometry of the transfer ports
Unsteady numerical simulations - Computational Fluid Dynamics (CFD) relying on Unsteady Reynolds Averaged Navier-Stokes (URANS) multi-cycle simulations on movinggrid domains with topology changes has become a reliable and widely used tool to support the optimisation of two-stroke engines
Summary
The effectiveness of the gas exchange process is crucial for two-stroke engines where the opening time (in crank angle degrees) of conventional piston-controlled transfer ports for small crankcase-compression engines hardly exceeds 110° [1]. Non-reactive multi-cycle computations [3] and moving-grid analyses of the blow-down to end-of-compression crank angle period [6] demonstrated the capability of quite accurate resolution in space and time of the gas exchange process Such cold-flow CFD approaches applied to small two-stroke racing engines achieved good reliability in the early 2000s [10, 11]. Neither the numerical analysis expertise and experience in using the commercial CFD tools to setup and manage very complex numerical simulations nor the experimental facilities recalled above are accessible to many researchers and manufacturers of small two-stroke racing engines This evidence justifies the practical design approach proposed here as a rapid and cost-effective method to improve loop-scavenged crankcase-compression engines. The successful application of the design approach is briefly reported in the third section that precedes the final summary
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