Unmanned Systems Technology 021 | Robot Aviation FX450 l Imaging Sensors focus l UAVs Insight l Liquid-Piston X-Mini l Riptide l Eurosatory 2018 show report l Zipline l Electric Motors focus l ASTS show report

61 Liquid-Piston X-Mini | Dossier combustion volume near TDC results in higher peak pressure and efficiency than a piston engine operating with SI. That is related to the slower variation of displacement in proximity to TDC. “The dwell in volume at TDC allows the engine to more closely achieve true constant-volume combustion compared to a piston implementation of the Otto cycle. Over-expansion further increases efficiency.” Conventional piston engines using Otto and Diesel cycles still contain energy at the end of expansion, as inherently the charge is not fully expanded. That energy is typically wasted through the exhaust process, but it can be partially recovered by turbocharging or over-expanding, as in the Miller and Atkinson cycles. Over-expansion means that the effective expansion ratio is higher than the compression ratio, allowing more energy of the charge to be converted into useful work. That also has the effect of lowering average temperature in the combustion chamber, reducing cooling needs. Also, the exhaust pressure can be near to atmospheric, which means it has a lower temperature than a conventional engine’s exhaust. “We are sucking air into the chamber but at the same time, in order to achieve over-expansion, we push some of it back out, so we are effectively trapping a smaller volume,” explains Shkolnik. “We sacrifice some volumetric efficiency, and with that we lower the effective compression ratio, but that means the effective expansion ratio becomes greater than the compression ratio. “We continue expanding until there is no energy left – as per the Atkinson or Miller cycle – and we can incorporate that concept in a simple mechanical way.” As is the case with a piston engine using valve timing to achieve Miller or Atkinson cycles, the X-Mini’s working chamber volume at intake port closing is much smaller than at exhaust port opening. “By locating the intake and exhaust ports asymmetrically [relative to the rotor centreline] we have a longer effective expansion ratio and a lower effective compression ratio,” Shkolnik explains. “So the trapped volume is 19 cc [57 cc total] and the expansion volume is 23 cc [69 cc total].” Thus the port timing of the X-Mini uses an effective expansion versus compression ratio of 1:1.2. With different port locations, larger over-expansion ratios that would be closer to the ideal cycle are possible. “The X-Mini port timings were selected to maximise power density rather than fuel efficiency,” Shkolnik reports. X-Mini sealing “The Wankel apex, side and corner seals have manufacturing tolerances and need an allowance for thermal expansion,” Shkolnik says. “That creates gaps at the corners, which allows a lot of blow-by. Also, leakage occurs when an apex seal travels over the spark plugs. “Our seals are in the housing, whereas theirs are on the rotor. On the rotor means they move at high speed, so they have a lot of dynamic forces on them, they tend to bounce around – chatter – and they are very difficult to lubricate. Usually, oil is injected into the charge to lubricate them. You need a lot of oil, as much of it gets burnt before it can lubricate the seals.” The centrifugal force to which the Wankel’s rotating apex seals are subject is recognised as a major design constraint. Liquid-Piston’s stationary apex seals have no such constraint, and can be lubricated with small quantities of oil metered directly at the seal location through the stationary housing. “Our apex seals are stationary – we don’t have the same dynamic forces on them,” Shkolnik emphasises. “Furthermore, they interface directly with the side seals, so there is no gap between them. We have an almost perfect sealing grid and comparable blow-by to a piston engine. We have three to five times less blow-by than a typical Wankel engine, and that means higher efficiency [5].” The X-Mini’s rotor housing is aluminium alloy, while the rotor is steel, as is the eccentric shaft and the rotor’s planetary gear. The rotor’s peripheral working surface is polished, then DLC coated, and it runs against laser-treated chilled cast iron apex seals, with Liquid-Piston currently investigating the use of bronze alternatives and various coatings. The side housing working surfaces against which the rotor-mounted cast iron side seals operate is chromed. On the larger X engine, minute amounts of oil are metered directly Unmanned Systems Technology | August/September 2018 Design work at the Liquid-Piston facility in Bloomfield, Connecticut