Unmanned Systems Technology 018 | CES show report | ASV Global C-Cat 3 USV | Test centres | UUVs insight | Limbach L 275 EF | Lidar systems | Heliceo DroneBox | Composites

62 Dossier | Limbach L 275 EF counteracts that generated by the other at all times, resulting in primary and secondary balance. However, since there is an offset between the cylinder bores caused by each having its own crankpin, there is a rocking couple along the crankshaft. The outer crankshaft throws are each formed as a counterbalancing web such that no other damping is required. A four-cylinder boxer does have the advantage of imparting a lower level of vibration to its mounts. Limbach says that a two-cylinder boxer with the same power level as the equivalent four would impart ten times the magnitude of excitation. At the same time, it is superior to a single of comparable power level. It should also be noted that given a relatively low-compression two-stroke running at around 6000 rpm, such as this, the mass forces are more dominant than the spiky firing forces. Limbach recalls that the overarching goal of the design process was to obtain the required performance, including durability, for the minimum possible weight. “Our engine is still one of the lightest twins at its power level,” he remarks. As an example of the quest for lightness, Limbach notes, “In Germany at the time [1972], every single and twin two-stroke had a built-up crankshaft – that was the state of the art then. We purposely decided against a built-up crankshaft. Our reasoning was that putting in the space for the press-fits you need to transmit the torque would have made the engine longer, so we opted instead for a one-piece crankshaft and therefore to use split con rods.” Then there was the question of induction, which could have meant the use of a single central carburettor or an individual carburettor for each cylinder. The design team discovered that a pair of 25 mm carburettors was actually lighter than the single 36 mm alternative, with the added advantage that one could be mounted just above the respective cylinder, whereas the single carburettor would logically be centrally mounted and have heavier intake manifolding. It is clear that a two-stroke must have some means to stop the charge returning back through its intake system under crankcase compression. If a single central carburettor on a boxer engine is being used it is logical to supply the charge directly into the centre of the common crankcase, with then a reed or rotary valve closing the intake under compression. When using twin carburettors, an alternative is for each cylinder to have an independent intake port feeding in through the cylinder, such that the port can be controlled by the piston skirt, in the manner of the exhaust and transfer ports. This is the case with the L 275 EF, which forgoes the use of reed or rotary valves. This publication has as a sister magazine, Race Engine Technology , and Professor Gordon Blair was its Technical Consultant from its inception in 2003 until his death in 2010. He was well-known and highly respected by the author of this article. Indeed, Prof Blair’s tome, Design and Simulation of Two-Stroke Engines , is renowned as the premier source of information for anyone undertaking such work, and draws on his huge personal experience in the field. Limbach recalls that having assessed the requirements of the Limbach twin, Prof Blair recommended that the complication of a rotary or even a reed valve was unnecessary. That advice reflected its operating speed range, in particular the fact that the drive would be going to a propeller operating mostly at a steady speed. The absence of a reed or rotary valve saved weight and complication. Any complication represents a reliability risk; avoiding it saves the customer cost. Limbach says a further advantage February/March 2018 | Unmanned Systems Technology The L 275 EF crankshaft assembly with con rods, propeller hub and trigger assembly We purposely decided against using a built-up crankshaft, and opted for a one- piece crank and therefore used split con rods