Issue 45 | Uncrewed Systems Technology Aug/Sept 2022 Tidewie USV Tupan | Performance monitoring | Bayonet 350 | UAVs insight | Xponential 2022 | ULPower UL350i and UL350iHPS | Elroy Air Chaparral | Gimbals | Clogworks Dark Matter

88 as homogeneous a fuel-air mixture as possible, to minimise the risk of detonation. “We’re fortunate that our partners in Ostend have a lot of experience in rally car engines – it’s their expertise that informs key design targets for the cylinder interior,” Denorme explains. “For instance, the shape formed between the piston crown and cylinder head must always push the charge mixture specifically towards the spark plugs, so we try to keep the spark plug right in the middle and we cut and forge the two sides into a ball-like shape for the detonation. “Getting it perfect wasn’t easy, as we needed to compromise on how spherical the shape is in some sections of the chamber, to be able to fit two spark plugs and two poppet valves inside. But we’re very satisfied with the very tight compression that the piston imparts on the gases against the plugs now.” The exhaust gases also exit through a poppet valve. The exhaust system sitting downstream is kept particularly simple, using COTS parts most of the time. Either a four-into-one exhaust pipe system or a muffler can be installed, depending on customer preference. The company notes that different exhaust systems are available. Since the fuel maps are modified to match the exhaust system, it is important that customers decide which exhaust system is best suited for their aircraft. “A four-into-one will get maybe 2 bhp more out of the engine than a muffler,” Defoer comments. “Granted, that isn’t much, but many customers will want it in order to squeeze every last bit of energy out of the combustion chambers, be it for payload carrying, data processing, AI functions or manoeuvres. “Of course though, there are lots of different kinds of UAV designs, so some customers will prefer separate exhaust pipes on the left and right for integration purposes, or mufflers for noise and vibration benefits. “Naturally we can run tests in-house if a customer wants a special exhaust or has an idea for an exhaust manifold or muffler that hasn’t been paired with our engines before. We’ll install Lambda sensors on the exhaust system to test if the power and air-fuel levels are running the same, then adjust the fuel-air maps if necessary.” Valvetrain As mentioned, overhead poppet valves are used for the intake and exhaust, with one valve for each – hence two valves total – per cylinder being sufficient for the relatively low rpm (compared with most other gasoline engines we have featured) that the UL350 runs at. “If we were building higher speed gasoline engines that ran upwards of 6000 rpm, we’d switch to four valves per cylinder: two for the charge intake and two for the exhaust,” Defoer says. “For now though, two valves per cylinder is enough, and they sit at about 15 º offset from the stroke. The intake valve is about 40 mm in diameter and the exhaust is around 32 mm.” The valvetrain starts with a single camshaft, which is gear-driven by the crankshaft at half the latter’s speed. Denorme comments that the camshaft mounts in a chamber below the crankshaft, the crankshaft runs in the crankcase’s aluminium chamber and is held in place by five plain bearings that are directly greased via channels running from the main oil gallery. It features eight cam lobes, as well as a driven gear fastened onto the back, with a standard sequence of valvetrain parts leading down to the cylinder heads. First, a cam pushes on a cam follower, which sits atop a pushrod that acts on one end of a rocker by way of a tappet adjustment screw. The opposite rocker arm pushes down on a valve stem to open the valve, each valve being held in a spring that in turn is held in place by retainers top and August/September 2022 | Uncrewed Systems Technology The cylinder heads are designed with a squish zone in the combustion chamber that narrows to a gap of less than 1 mm with the piston