70 Engine dossier | Avadi Engines August/September 2024 | Uncrewed Systems Technology The MA-250 used a half-shaft to produce power, driven by the scissor-like motions of the piston’s two connecting rods valve approach, and without the half-shaft design, into what is known as the XMD-250 (Phoenix is also a working product name in the company). Drive evolution Dimitrios pointed out that Wilkinson and his team had designed unnecessary complexity into the MA-250 to achieve the piston, half-shaft and cylinder rotating in unison. Despite their assumptions that benefits would be gained by spinning the piston and cylinder together (particularly by preventing wear on the piston rings, which typically occurs near TDC as the dropping speeds reduce the protective effects of the oil film about the rings), he had many years of test data and experience indicating that there was no harm in the piston and cylinder rotating at different speeds, or one not rotating at all. “The original designer of the MA-250 didn’t know the extensive history of successful sleeve valve engine designs. In fact, most people don’t know the benefits of having a relative rotation between the piston and cylinder wall in a sleeve valve engine, although I’ve accounted for my test results of trialling them in several SAE papers,” Dimitrios says. “Landon originally asked me just to integrate my rotating liner technology into his engine, but I eventually asked for a clean sheet to make the whole power plant mechanically simpler.” How it works In the XMD-250, as the piston thrusts away from the cylinder head postcombustion, it still drives two con rods moving in a scissor-like motion, each one connected by conventional roller element bearings to a respective shaft, with each shaft affixed to the engine by three differently sized roller bearings. Each shaft drives a pinion gear with 20 teeth. As these gears counter-rotate, their teeth mesh with other teeth on the underside of the cylinder above, and on the top of an output shaft below (these latter two are ring gears with 40 teeth each; all four being cut as bevel helical gears). The cylinder runs in a large, single, deep-groove ball bearing for handling thrust and side loads, while the shaft runs in two smaller bearings. “Gears typically have 95-97% efficiency at high load when properly optimised, so a 3-5% loss is expected. However, any gear reduction to drive a propeller will have some efficiency loss,” says Wilkinson. The cylinder and shaft counter-rotate relative to each other, and as the shafts, con rods and piston maintain a fixed, non-rotational axis, the cylinder rotates about the piston. The cylinder’s rotation, compared with the non-rotating piston, helps keep the piston rings ‘hydroplaning’ over the oil film separating them from the cylinder walls, including near TDC, where (as Dardalis’ research indicates) friction between the cylinder and piston ring tends to be highest due to the much slower piston speed there – compared with mid-stroke and BDC speeds – dropping the surface tension of the oil film and allowing the ring metal through. Instead, the constant rotating motion helps maintain the oilpressure level necessary for forcing the piston ring and cylinder apart. Inside the rotating head atop the cylinder is a port (referred to here as a transfer port). As it rotates, it moves the transfer port opening over a stationary intake inlet and exhaust outlet located in a cylindrical ‘stator’ housing encircling the head. This exposes, and then closes, the inlet and outlet to the combustion chamber; thus the head is the rotary valve mechanism in the XMD-250. The MA250’s axially positioned valves created heat-transfer issues that their new position in the XMD-250 has eliminated. “Just as in sleeve valve engines, the rotation of the cylinder head – and In the XMD-250, gears such as this one are installed beneath the cylinder and atop the output shaft, driven by the gears on the two cranks
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