Unmanned Systems Technology 017 | AAC HAMR UAV | Autopilots | Airborne surveillance | Primoco 500 two-stroke | Faro ScanBot UGV | Transponders | Intergeo, CUAV Expo and CUAV Show reports

66 using a 12 V battery supply and four ignition boxes, each containing two coils (one for each cylinder’s pair of plugs). Those boxes are controlled by the ECU. The Primoco One 150 has 28 as well as 12 V electrical systems, the former serving the starter and the payload while the latter supports the ignition, ECU and autopilot. Thus the voltage output from the generator is reduced to charge the 12 V lithium-polymer battery serving those applications. In the event of generator failure the battery will last for at least four hours. As noted above, essentially the 500 is a pair of boxer twins working in tandem. Each opposing pair has its own sealed section of crankcase and its own individual carburettor/reed valve feeding into that. However, the exhaust primaries are paired on each bank. As standard, the two cylinders on each side exhaust via a short primary into a common canister, those horizontal canisters flanking twin intakes (one for each pair of cylinders) on the underside and in turn each having a single short vertical exit (at the mounting end). That system does not have noise suppression but, remarks Smat, silencing will be a future development, according to customer requirement. This twin carburettor-equipped engine has its fuel supply line “lightly pressurised against any build-up of bubbles in the fuel supply that can be caused by altitude and temperature variations,” Smat explains. In terms of mixture control (via the fixed-in-flight carburettor jetting) the engine will normally be optimised for operating at up to 10,000 ft of altitude, as expected, with spark advance adjusted as necessary by the ECU to cope with other altitudes. The ECU itself is used purely to govern ignition timing. Its inputs are crankshaft position and ambient temperature and pressure as measured in the engine bay. Magnetic rather than Hall effect speed sensing is used, which Smat considers to be more durable. The magnetic sensors have often been applied, with good redundancy results, he adds. The ECU controls the ignition timing according to its mapping, with a limiter at 6500 rpm and a cut-off at 7000 rpm. The limiter takes the form of cutting out every other spark; the cut-off is of all the sparks. Smat notes that the ECU also records the data it acquires, which is a useful diagnostic tool. Ignition timing will be varied within the range of 20 and 2 º BTDC. With the ECU controlling the ignition, Smat says, “We don’t anticipate any danger of detonation.” He notes that for two-stroke race engines detonation can be a real danger, but he explains that this engine is not on the limit in the same way – “it is a workhorse rather than a racehorse” – and moreover, as altitude increases, air pressure falls, and as such the effective compression ratio also falls, and with that the danger of detonation is reduced. A detonation sensor is not considered necessary by Smat. A thermal sensor is incorporated into each spark plug washer to monitor cylinder head temperature, while there is also a thermal sensor on each exhaust outlet. The data from those sensors is sent to the autopilot rather than the ECU, and thence to the ground control station. That means the vehicle operator sees the temperature of each cylinder head and the temperature of each exhaust gas outflow in real time. If they see a temperature rise that concerns them, they can ask the autopilot December/January 2018 | Unmanned Systems Technology Dossier | Primoco 500 four-cylinder, 500 cc two-stroke Integrating the Primoco 500 into the One 150 UAV The ECU purely governs ignition timing. Its inputs are crankshaft position and pressure as measured within the engine bay