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57 opens under the pressure of the exhaust flow. So long as the engine is running, the exhaust and coolant flows discharge into the sea; once the engine stops, the lack of exhaust flow allows the spring-loaded flap valve to close, shutting them off. At the mid-point of the tubular coolant/ exhaust passage, where it is above sea level, is another check valve. When the exit flap is shut, a vacuum is drawn upstream, due to the rapid cooling of the engine, and this causes this upper check valve to open, letting in air. That in turn balances the pressures, avoiding the back-flow of water that would otherwise occur. “The upper check valve keeps the engine from sucking the water back in when it cools,” notes West. “Otherwise, when you shut off the exit flap valve and the engine cools, it will draw the water back. The upper check valve lets air in when there is a loss of pressure beneath it. When the engine is running and water is passing beneath it, that water can’t come out and air can’t come in. When the engine stops, the coolant pump stops operating, and cooling air is drawn in through the upper check valve.” Galvanic challenge West points out that it was important to manage the galvanic reaction that occurs between certain metals in contact with each other and salt water. Galvanic corrosion is an electrochemical process in which one metal corrodes preferentially to another when both are in electrical contact and in the presence of an electrolyte. “All the materials in the H70 coolant system are designed to have a minimal galvanic reaction with one another,” he says. “The spacing of insulators between the metals can cause a charge that will remove material from the anodes, and deposit it on the cathodes. Therefore, the engine is assembled with specific grounding and electrical connectivity between all components.  “Also, the goal with materials selection was to find similar compatible materials to reduce the reaction. The materials in the coolant system are aluminium, plastic, stainless steel and silicon; the fasteners are chrome-plated stainless steel. The aluminium is the most susceptible to the galvanic reaction and is therefore carefully assembled with hard, clear anodise and epoxy paint to prevent degradation due to galvanic reaction with the salt water.  “There is also a sacrificial anode inside the coolant system to draw the reaction away from the vulnerable components in the engine. That will dissolve before the aluminium starts corroding; you just keep replacing it.” The project The H70 project began at HFE in October 2014, and the power unit had been designed and built by the end of the year. Then, in January 2015, the client supplied a hull so that it could be tested in an in-house water tank with the impeller loading it. Engine management system calibration was done at this stage. By March 2015 it was being tested in an actual buoy on a lake in Tucson, and the following month it was put through its paces at sea by the Los Angeles County Fire Department Lifeguards, which uses the Emily platform and does a lot of beta testing for Hydronalix. The brief having been for 2 bhp, the upshot was a 2.5 bhp (1.86 kW) 5700 rpm engine that at wide-open throttle consumes only 380 g/kWh and meets EPA emission regulations. A 2.5 gallon fuel tank allows the engine to run for 24 hours continuously. Station-keeping endurance is subject to electrical power draw and ocean currents. The first customer was the Los Angeles lifeguards, which already had the electric version for search-and-rescue missions. They use it with sonar to scan the ocean floor in the event of water rescue for downed aircraft or sunken ships. Various other (undisclosed) customers are now using the buoy. Interestingly, the Emily platform has been deployed to help rescue refugees in the Mediterranean. Since the AMB went into service in April 2015 there has been a development programme to improve the H70’s reliability through endurance testing, entailing some detail design changes. The most significant of these was that the water jet impeller drive ratio has been increased from 2.0:1 to 2.5:1 so that the engine runs slower for a given impeller speed. “We found that the engine was revving more than we liked,” remarks West. We asked him: was this project a spin- off from another you’ve done? “No,” he replied, “this is a very good example of what we at HFE do – a customer has a need and we will make an engine suitable for their specific application.” That, as can be seen from this case, is not necessarily as straightforward as it might first appear to be. Unmanned Systems Technology | October/November 2016 HFE is a service provider modifying third-party engines to suit specific applications, many of which are for unmanned vehicles. HFE makes mechanical modifications, adds electronics including engine management equipment and devises new control strategies, according to the requirements of the application. “We don’t manufacture engines ourselves; we produce modified engines and components for them,” notes Tom West, lead designer of the H70. “The engine management system, the sensors and the engine harness are all produced in-house.” HFE is based in Tucson, Arizona, and employs half-a-dozen people. The design of the H70 was supported by design engineer Dawn McClain, mechanical engineer Kyle Gratien and electrical engineer Ryan Denney. HFE profile