76 reduced emissions, including approaches in which fuel was sprayed into the transfer ports. “Semantically, DI means the fuel is injected into the cylinder from TDC. We looked into different approaches for this during our evaluation process, but the weight penalty of all of them was too high for a small engine like this,” Kehe says. “DI takes high-pressure injection and so quite a bit of equipment to work, including a heat-protected injector, a high-pressure fuel pump and a powerful means for driving that pump, whether mechanical or electrical. “Maybe one doesn’t notice all that weight as much on a big compressionignition engine, but we determined it just couldn’t be the way forward on our two-cylinder spark-ignited engine. It would’ve also meant having injectors sticking out of the tops of our cylinder heads, which isn’t good for making a very compact UAV engine.” The sideways-positioned injector and pump in the SDI approach are similar to those used in gasoline systems, although with a different injection pressure, but within the range for a conventional EFI. As Suter claim, positioning the injector sideways relative to the stroke prevents the need for extremely high-pressure injection. In DI, fuel injects when the piston is closing on TDC, and pressure in the chamber is high. But the placement and angle of Suter’s injector enables injection before pressure builds in the chamber (the piston then compressing the mixture), and protects the injector after combustion as the piston opens the exhaust port first, reducing heat impact and lowering cylinder pressure on the injector. Additionally, a key issue with throttle body-injected heavy fuel is the spray can contact the wall before vaporising (or combusting), resulting in poor combustion and build-up of heavy fuel in the crankcase. Hence, SDI provides a more direct route for the spray to enter the combustion system allowing better mixture vaporization and combustion. “During combustion, the piston is fired back down, exposing the injector to the cylinder interior again so it can spray. Then, the piston is delivered back up, covering the injector as the atomised kerosene is pushed into the combustion pocket,” Kehe says. “Naturally, the most critical part of engineering all of this was just getting the timings right. We needed to maximise the vapouring time, spray at exactly the right moment, make sure the fuel entered the combustion chamber at the right moment, and so on.” As one might expect, the cylinders on the 288-SDI had to be almost entirely redesigned from the TOA-288’s. Copious testing with cylinder-pressure transducers and other sensors fed into optimisations of the internal geometry, the compression ratio, fuel-injection pressure and other parameters. “As much as we worked to optimise the system, when you switch a given engine from gasoline to heavy fuel, there is inevitably a power loss due to the latter’s higher molecular weight and lower octane rating, and the power efficiency tends to further drop because you need to design for a lower compression ratio to prevent knocking,” Kehe explains. “That’s what motivated the creation of the 330. Alex posited that we could try making a larger engine, and that’s when the work to make a 330 cc engine really started, so the next step will likely be porting the heavy fuel innovations to the 330 design to make a HF TOA 330-SDI.” Oiling optimisation As the HF TOA 288-SDI’s heavy fuel is sprayed directly into its cylinders, oil must be sprayed separately into the engine, rather than using the 50:1 premixing approach, as used in the gasoline TOA-288; otherwise, no oil would ever enter the crankcase to lubricate the bearings and piston rings. Hence, in the 288-SDI, a single injector sprays oil onto the crankshaft’s main bearings, with the movement of internal air and the centrifugal force of the crankshaft carrying oil up to the piston February/March 2025 | Uncrewed Systems Technology Semi-direct injection (SDI) makes use of sidewayspositioned injectors that achieve significant weightsaving and fuel-mixing benefits over direct injection (DI)
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