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48 Dossier | HFE H70 marine powertrain Type of engine Consideration was given to the use of a compression ignition base engine “but we couldn’t find one small and light enough,” reports West. “We did find an air-cooled diesel that gave similar power, but even that was too big and it was quite a bit heavier.” On top of that was the need for water cooling in view of the engine bay temperature requirement. “We looked at converting an air-cooled engine to water-cooled but the complication involved in that would have made it a much bigger project than we had time for,” remarks West. Should not a two-stroke be smaller in displacement for a given power level, hence inherently lighter and more fuel- efficient? West replies, “It would have been lighter but the bsfc [brake-specific fuel consumption] would not have been as good in our operating range. “There are off-the-shelf two-stroke outboard engines, but they are rare in the size that we needed. Also, they tend to radiate more heat from the crankcase and tend to be partially air-cooled. In addition, the customer did not want to mix oil into the petrol, as is required for most small capacity two-strokes. The two stroke pre-mix also contributes to the pollution in the exhaust gas mixture that would have been deposited into the marine sanctuary.” How about an alternative engine type, such as a rotary? “This was a project with less than three months to complete – that narrowed the choices to existing marine engines,” West replies. “Also, we wanted the user to be able to do their own maintenance, which is one reason we chose a conventional-type engine. A mechanic needs to be able to step in and understand the engine. Rotaries are unconventional, and require some special training for maintenance. “The engine we chose was specifically designed to be cooled by seawater, which gets rid of the need for a radiator system typically required by rotary engines. That was a key driver in our selection of it. We looked at a range of seawater-cooled four-stroke possibilities; of those that could do the job the engine we selected was the smallest and lightest available.” Thus the H70 is based on an undisclosed off-the-shelf outboard engine “to which we have made a lot of changes,” notes West. “This was for a specific customer, for a specific application, and it is one of the most interesting projects we have undertaken.” Using a proven base engine from a major outboard engine manufacturer “means we are leveraging millions of dollars of investment in design and development” West says. “That is a much better bang for the buck than developing that aspect of the engine ourselves from scratch.” The engine in question is a gasoline- fuelled, naturally aspirated 70 cc four- stroke. It is an all-aluminium single- cylinder with pushrod activation of two overhead valves and, converted from a magneto ignition to capacitor discharge ignition (CDI) and from carburettor to throttle body injection, is run by HFE’s own engine management system. HFE retains the stock cylinder and cranktrain while removing the lower half of the stock crankcase, replacing it with an alternative unit including the sump and the water pump housing, designed to help make the overall package more compact. HFE’s lower crankcase unit includes a gear set that drives a rearward-facing power October/November 2016 | Unmanned Systems Technology The H70 is based on an outboard motor with a vertical crankshaft; HFE replaces the bottom end