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In a cross-scavenged two-stroke engine, a piston with a deflector is often utilized to flow the fresh charge toward the cylinder head and away from the exhaust port. But flow-visualization studies have shown that the fresh charge flows toward the cylinder head even without the deflector. To find why the fresh charge flows toward the cylinder head, we used Fluent, a general purpose computational fluid dynamics (CFD) software, to simulate two- and three-dimensional flows in a cross-scavenged two-stroke engine. While the fresh charge entered from the scavenging port into the cylinder and flowed across the cylinder, we investigated the instantaneous fresh charge flow, velocity, and pressure distribution in the cylinder.

During the scavenging of a two-stroke engine, it can be assumed that a very small quantity of fresh mixture flows from the scavenging ports and also from the crankcase through the gap between the cylinder and the piston to the exhaust port, in order to assess the effects of these leakages on fuel economy and hydrocarbon emissions, the authors calculated the quantity of mixture lost using the time-areas of the flow paths and the pressure-time history in the crankcase, and found that this quantity was in the order of 1-3% of the inducted fresh mixture.

The scavenging picture means here the velocity distribution of the scavenging flow in the cylinder measured by a pitot tube. The picture is often used to investigate the suitability of the design of the scavenging system in a two-stroke cycle engine. In measurements by pitot tube only the upward component of the velocity has been measured until now. By this improvement, however, it is well possible to measure the velocity in three dimensions. In other words, it is now possible to measure also the scavenging flow toward the exhaust port after the flow turns downward at the cylinder head.

Two-stroke engine cylinders have ports to exchange gas. While the engine runs, the piston and its piston rings slide over these ports in the cylinder walls, and the rings may project into the ports. This paper explores this, first, by reporting a simple model of material mechanics that predicts rings might project into ports, and second, our experimental verification. We installed strain gauges on the bottom of the top and second rings, over the intake and exhaust ports, and ran signal wires out of the engine. We then examined the variations of strain while running the engine. Our analysis confirmed how the dimensions and the tension of the rings, and the dimensions of the ports, affect ring projection into ports as static displacements.