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

58 Dossier | Limbach L 275 EF here is so called as the ‘L’ represents Limbach, 275 the displacement in cc, ‘E’ the use of a single plug per cylinder (‘D’ is used to indicate double, which is an option for the company’s four-stroke engines) and ‘F’ the use of fuel injection (an earlier carburettor version would simply be L 275 E). The Limbach company, nowadays Limbach Flugmotoren, is still based in Königswinter, where its current facility occupies 12,000 sq m and the staff headcount is 17, and growing. A few years ago the Limbach family sold the operation to Shuide Chen, who these days is managing director, while Peter Limbach (Jnr) continues as a consultant. Other key engineering staff include Armin Sacha as head of engine testing, and Peter Stolinski as head of four-stroke development. Design The L 275 EF is based on the original design of 1972. Fuel injection was introduced in 2015, at which stage more than 2000 examples of the original carburettor engine had been sold, as witness to the integrity of its performance, including its dependability. It had started life with battery ignition, and then a switch was made to a magneto that ran on the nose of the crankshaft (between the crankcase and the propeller hub). With that came the option of an electric starter. The current port fuel injected version does away with the magneto and has a full engine management system run in conjunction with a battery or rear-mounted generator, according to customer preference. The electric starter remains an option. Aside from the ignition and fuelling developments there have been no fundamental changes to the L 275 EF since 1972. Peter Limbach relates that the twin- cylinder configuration was chosen on the basis of the (apparently motor-glider inspired) performance target. “A single- cylinder engine would not have been large enough to provide the power requirement,” he says. “The design was focused on available components, and we used the largest pistons and cylinders we could get that were suitable for our needs. Any motorcycle engine cylinder would have been far too heavy at that time; making our two-stroke extremely light was one of our design goals.” Indeed, he notes that a single-cylinder engine of comparable displacement would have had excessive bore and stroke dimensions, considering factors such as flame propagation and piston speed. Limbach adds that a larger bore brings other challenges. “Very large bore two-stroke engines have been known to experience thermal problems,” he says. “The larger the bore, the more difficult it is to control thermal distortion and obtain good scavenging.” In fact, at the time of the design work there had been a four-cylinder two-stroke on the market that produced around 25 bhp. Limbach’s aim was to offer that level of performance from a far lighter and more compact twin. Having opted for a twin, the logical route was a boxer configuration, which is beneficial in respect of air cooling and vibration. With the bank angle being 180 º , the crankshaft has separate crankpins for each cylinder arranged at 180 º such that the pistons ‘box’ towards and away from each other. Since this is a two-stroke they fire concurrently while the primary force generated by one piston February/March 2018 | Unmanned Systems Technology The L 275 EF seen from the front The design was focused on what components were available, and we used the largest pistons and cylinders that were suitable