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64 was, “We could not get a uniform mixture across the two cylinders when we tried using a single carburettor. Owing to the rotation of the crankshaft, you will inevitably get unequal filling of the cylinders. The situation is much better with the use of an intake for each cylinder.” Of the overall design process Limbach notes, “Anything that was unnecessary was left off in the quest to minimise weight.” The L 275 EF is designed to run on automotive gasoline, with lubrication by means of premix using a 50:1 ratio of gasoline to high-quality fully synthetic two-stroke oil. The engine is loop scavenged, which means the transfer ports are located, dimensioned and shaped to direct the incoming charge towards the combustion chamber. Having reached that area, the charge is deflected downwards again, creating a loop that prevents it escaping through the exhaust port. This avoids the need for a deflector on the piston crown, while the swirling turbulence in the chamber assists combustion. Limbach recalls that Prof Blair advised that the provision of two transfer ports was quite adequate for the envisaged rpm level. Engine components The propeller end of the L 275 EF is considered the front of its crankshaft, and the engine is mounted using bosses at the rear of the crankcase such that the unit is cantilevered. The aluminium alloy crankcase is split vertically and longitudinally into mirror-image halves, which are brought together by means of eight bolts and a liquid gasket. In turn, the two aluminium alloy cylinders flanking it are each attached by four bolts with a paper gasket at the junction. No other structural engine component is bolted to the front or rear of the crankcase, and the cylinders have integral heads. Each cylinder has a flat top (one could say ‘end’, given the 180 º bank angle) to which is attached a finned heat sink. Originally there was no such addition, but the introduction of this extruded aluminium part made the cylinder easier to cool; it had previously been difficult to maintain low cylinder temperatures, Limbach admits. Machined in-house, the finned cylinders are cast in a 14% silicon aluminium alloy similar to alloy 4032, as is commonly used for pistons. That ensures the cylinder is durable at high temperatures and that the coefficient of expansion is similar to that of the piston, which is cast from an 18% silicon aluminium alloy. Low-pressure die casting is used for the cylinders, providing a good finish for the hard-to-access transfer ports (which simply need to be deburred, Limbach notes). Likewise, the combustion chamber is as cast. The crankshaft runs in two main bearings, those ball bearings being of the same diameter, although the front February/March 2018 | Unmanned Systems Technology The L 275 EF’s two-piece crankcase The L 275 EF’s cylinder, which has an integral head. A finned heat sink is attached to the top