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

65 Briggs & Stratton Vanguard EFI/ETC vee twins | Dossier “The Small Block’s exhaust relies on aluminium pushrods, because we need better control of the valve lash that can come with temperature on that side,” Bruener says. “That is particularly important for achieving proper function of the compression release, which is required for rope-starts.” “The iron cam lobes’ surfaces are treated using induction hardening. Some companies prefer flame hardening, but for high production volumes induction hardening followed by water quenching produces better results for us and our customers. It particularly hardens the raised part of the lobes, which is the part that’s in danger of being stressed.” Each pushrod lifts one end of a rocker arm, which pushes down against a valve stem and spring to open its respective poppet valve. Standard rockers are used, with no rollers; set screws with locknuts are located on the rocker arm above each pushrod, and are used to set valve lash. There are a few minor differences between the valvetrains of the 23 and 40 bhp engines – in particular, the latter’s cam lobes are shaped to slightly prolong opening times. Intake valve seats on both engines feature a three-angle cut, to enable a greater throughput of intake gases in line with the higher power output. Naturally, the valve diameters are greater on the 40 bhp engine – 36.8 mm for the intake, 32.5 mm for exhaust – compared with 32.5 and 27.5 mm respectively on the 23 bhp engine. The Big Block also uses exhaust valves made from a steel alloy coated with a Stellite material on their sealing surfaces, largely to reduce valve-to-seat wear. In addition its pushrods are aluminium, but aside from that the rockers, tappets and springs are identical. Combustion chamber As mentioned, the cylinders in the vee twins have had their displacement increased incrementally over the years, the intention being to supply more power in a given package. “Our customers often ask us for more horsepower, so we went up from 479 cc and 16 bhp with the original Small Block, to 570 cc and 18 bhp with the next version, then up again to 627 cc and 23 bhp, continuing with the Big Block up to 40 bhp,” Bruener explains. “The reason there was something of a bigger jump in power output for the latest versions was that the ports were angled,” he says. “Our cylinders are die- cast, so it’s been easier to manufacture the cylinder heads with a 90 º port – that is, the angle between the centreline of the intake port and that of the valve – but with this latest 627 cc version of the Small Block engine and the 993 cc Big Block we chose to introduce angled ports. “It’s the first time we’ve supplied an engine with that kind of porting. It’s more complex to manufacture but it gives much better power efficiency than 90 º ports since it’s now easier to get airflow into the combustion chamber, so it’s definitely been more than worth the cost increase,” he says. Duch notes also that on the 40 bhp engine, “We do an additional machine cut on the short-side radius of the intake port to help with the comparatively greater intake airflow its cylinders need, along with a few other geometric adjustments for greater airflow in the heads.” In addition to using Ansys Fluent for simulating the different fluid dynamics, Briggs & Stratton’s engineers have applied GT-Power for accurate 2D simulations of engine models and performance. The cylinders are die-cast along with the crankcase as a single block from standard-grade aluminium. The cylinder heads are also die-cast from the same material but as separate parts – thick and square-like in shape, with cooling fins around their circumferences, angled ports for charge and exhaust, and internal passages for the valves and pushrods. They are then fastened atop the cylinders with four screws per head on the Small Block, and six on the Big Block. A cast valve cover sits atop each cylinder head to seal the oil, rocker arms and other valvetrain parts, with a head gasket sitting between the cylinder head and cylinder (designed with a large surface area to handle the higher operating pressures). The gasket is a sandwich steel piece made from a flat plate between two shim layers, with a thin graphite coating. Unmanned Systems Technology | April/May 2021 The newest engine cylinder heads feature angled ports, to improve power efficiency over the 90 º ports formerly used