74 Dossier | Suter Industries without meticulous CFD to guide high-velocity flow to sensitive points around the cylinder head (such as the narrows between the valves and spark plugs), the two-stroke is mechanically simpler, and hence it can be redesigned for water cooling relatively easily. “We’d produced many water-cooled engines before, so we had lots of experience without needing direct inspiration from any one past project. We basically just designed an initial version of the jackets, optimised it in CFD software, and then built and validated prototypes, all within a few weeks. Most of the work was just making the prototypes,” Jäger says. Suter also opted for an electricallydriven pump to drive the liquid coolant, rather than driving it mechanically (off a crankshaft gear, for instance); thus the ECU tracks the engine’s CHT sensor and directs the pump to increase or decrease flow velocity accordingly. The end result of development here is an engine that is 1.1 kg heavier than the TOW 288 (stemming from the 500 g pump, and a slightly heavier exhaust, silencer and other parts), but producing an extra 2.4 kW (3.2 bhp) at 6500 rpm with the same 1 kW of electricity output from the alternator, and specific fuel consumption (SFC) improved to 306 g/kWh. Sizing up As Giussani explains, going from a 288 cc version of the engine to a 330 cc one was also generally straightforward. CAD versions of the TOW 288 were redesigned with an increased stroke in the cylinder and a lengthened con rod to match (as well as some very minimal adjustments to the length and geometries of the transfer ports). Prototypes of the 330 cc engine were then manufactured and tested, with the performance numbers largely matching those produced in computer simulations. “There really were no major issues in producing the TOA 330. Far more difficult was making a version of the TOW 288 that could run on heavy fuel – our HF TOA 288-SDI,” Giussani says. “The engine behaves very differently on heavy fuel due to factors like the HF TOA 288-SDI having to handle larger quantities of air and fuel despite running at similar speeds to the TOA 288. There were several similar sorts of issues, so optimising each parameter of the engine would have taken quite intense work and far more time than the project customer was happy with. “In some aspects of heavy fuel engineering, we just made educated guesses to fix appropriate points and geometries, and move forwards into prototype testing. We may have left some points unoptimised, but we also consolidated the design in a very timely fashion, enabling us to start optimising some really important enabling technologies for heavy fuel operations.” Semi-direct injection Arguably, the most important revolved around how best to get heavy fuel into the cylinder with sufficient atomisation for a clean and uniform combustion. “We all had great experience working with heavy fuel and direct injection [DI], so we evaluated different approaches to the problem of heavy fuel delivery in the 288’s form factor, eventually ending up with something called semi-direct injection [SDI],” Kehe says. Suter’s SDI is achieved largely through positioning a fuel injector in the cylinder-head wall, pointed at a proprietary angle to the piston stroke, which then sprays more or less directly into the cylinder. A variety of SDI systems have been used, with several notable examples among two-stroke snowmobile engines (some of which came from BRP Rotax, Ski-Doo and Polaris) to achieve February/March 2025 | Uncrewed Systems Technology Suter had produced many water-cooled engines before, so the company had much experience helping it to develop the TOW 288 version of the engine Far more difficult was making a version of the TOW 288 that could run on heavy fuel – our HF TOA 288-SDI. The engine behaves very differently
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