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

66 Also, the cylinder liners are spun-cast from iron, and inserted into the cylinders during die-casting to bond them in place. The process of creating the liners is largely similar to centrifugal mould casting, with a porous pipe being spun inside the cylinder and liquid iron poured into it, forming a long cylindrical shape as it cools. “The casting mould process allows spines to form on the outside of the spun-cast piece, 1-2 mm long,” Bruener explains. “That makes the piece structurally easier to cut into individual cylinder sleeves, which we then put into our own die-casting machines. “The liners are held in position by bore cores, and the molten aluminium is injected under high pressure into the die. It isn’t easy to inject it with precision but it’s critical for engines like these, and the spines will later help hugely with thermal transfer during combustion and keep us from having to switch to liquid-cooling.” Bruener notes that cylinder liner materials such as nickel silicon carbide or sintered metals are highly effective for reducing engine weight, but iron is far more cost-effective and helps keep the price of their engines suitable for utility applications. Also, the finished iron sleeve is only 5 mm thick after machining, so it doesn’t add considerable weight compared with other liner options. The pistons are an aluminium alloy and have been gravity-cast – a necessary deviation from most of the rest of the engines’ parts as die-casting would not provide the material homogeneity the company wanted. Also, the aluminium con rods on the 23 bhp version are squeeze- cast, a process similar to die-casting but with greater densification of its material through application of higher pressure. “It’s a two-piece con rod, bolted either side of the big end,” Pomeroy says. “At the small end is a slide pin that engages it to the piston, and circlips hold the pin in place. If we were to go to 6000-7000 rpm we might have to use a different kind of rod, but for this application it’s worked very well.” Duch adds, “On the bigger engine, we forge the rods from 2024-T6 aluminium alloy, for reduced porosity and improved strength, and add a tapered small end to help with the increased stress on the rod and piston coming from the higher power output.” Above this, the Big Block’s combustion chamber is slightly more ‘open’ than that of the Small Block. It has more of a dome shape compared with the pocket-like chamber of the smaller engine’s cylinder heads and is designed to induce higher squish during compression. “The pistons in both versions have a dish about 2 mm deep to accommodate the higher fluid volumes,” Duch says. Oil management Notably, neither the small nor big ends of the con rods use bearings to engage with the piston or crankshaft. Similarly, the iron camshafts use no bearings, and simply sit in finely machined aluminium pockets. While Briggs & Stratton’s engineers generally aim to minimise friction by using different materials for surfaces in contact with each other, a consistent and comprehensive lubrication system has also been designed, and is critical to the smooth running of the engine. The company recommends an oil change April/May 2021 | Unmanned Systems Technology Dossier | Briggs & Stratton Vanguard EFI/ETC vee twins The pistons are gravity-cast from stainless steel Once the valve lash is set it’ll run a really long time before it needs to be adjusted again – typically 250 hours but it could be much longer