Issue 37 Unmanned Systems Technology April/May 2021 Einride next-gen Pod l Battery technology l Dive Technologies AUV-Kit l UGVs insight l Vanguard EFI/ETC vee twins l Icarus Swarms l Transponders l Sonobot 5 l IDEX 2021 report

68 plain iron ring with a scraper base for oil scraping, and the bottom ring provides oil control, being made from two rail pieces and one expander piece. In addition to these functions, the rings, pistons and crankshaft have been designed for balance and light weight with a view towards minimising vibration as well as sensitivity to temperatures, which are vital for UAV applications. Pomeroy adds that simplicity of use has been a key driver in choosing these and most of the other components in the engines. As another example, the crankcase has an oil drain plug with a square feature so that it can be removed using a standard adjustable wrench. Crankcase The cylinders lead into the crankcase at a 45 º angle, forming a single engine block as mentioned, without a vertical or horizontal split into two halves. The block’s interior is instead accessible via a cover (also die-cast from aluminium) that bolts over the PTO face of the crankcase. That open PTO side is where the die sits during casting, and where it is removed from once casting is finished. The crankshaft is made as a single piece, as makes sense with split con rods, with the PTO and flywheel ends each connecting to a counterweight. Each counterweight is parallel to the other and connected by a single journal. The shaft is initially sand-cast from high-strength ductile iron alloy before going through a rough-machining process. A finishing machine-cutting stage is then carried in-house, then the oil passages are drilled from the flywheel end to the crank pins. Also, the crank pins are compression-rolled to improve their fatigue strength. “We used to harden the crank pin surfaces as well, but recent studies showed that it wasn’t necessary,” Brunelli notes. “It’s enough for us to just do our machine finishing, which includes superfinishing to ensure the surfaces of the crank pins and bearings are extra- fine and smooth. That really minimises the rate of wear, as well as any undesired effects on the direction of oil flow that might result from any microgrooves created during surface finishing.” Also, since both con rods sit on the same journal (at about 90 º to each other), a pressure roller is applied to its radius to induce a compressive load. That reduces the bending stress under WOT, enhancing the crankshaft’s durability. Future plans At the time of writing, the 23 bhp engine’s trials with Windracers were ongoing, and Briggs & Stratton was looking for more ways to tune its engines for unmanned systems. “One advantage of having to apply our engines to hundreds of utility applications is that each new one brings an opportunity for our engineers to learn how to improve them,” Pomeroy says. “UAVs, UGVs and USVs are a totally new breed of vehicle for us – we never thought we’d see our engines going into them, but we’re excited to see who else in that sector might benefit from them and what we can learn from each other.” * All brake horsepower figures quoted in this article are as stated by Briggs & Stratton for gross output at 3600 rpm, as measured by the company in accordance with SAE J1940 April/May 2021 | Unmanned Systems Technology Dossier | Briggs & Stratton Vanguard EFI/ETC vee twins Both engines use two-piece con rods; the smaller engine’s rods are squeeze-cast aluminium, while the larger ones are forged 2024-T6 aluminium alloy The crankshaft is sand-cast as a single piece from a high-strength ductile iron alloy, then put through several machining stages