Coolant System

      The coolant system was chosen for its simplicity, cost effectiveness and robustness.  The system is modeled after a design commonly used in the developing world.   The system is passive and consequently uses a large amount of water, roughly 40 gallons, as the coolant.  It relies on basic thermodynamic principles. 

      The system is based on the thermal siphon principle.  As the water inside the water jacket of the engine heats up, its density drops.  The changing water density drives the coolant circulation. See Figure 4.  The low-density water rises up and out of the engine through the upper coolant hose.  Cooler water from the tank, acting as a non-compressible fluid is then pulled into the engine through the lower coolant hose, similar to a gravity-feed siphon.

      Two important factors keep this system working. First, a large thermal mass of water is needed so that the intake water is significantly cooler than the outgoing water.  The temperature gradient needs to remain high.  Second, both coolant hoses must extend upwards (between 10 and 25 degrees) from the engine to the coolant tank.  Kinks or leaks in the system will stop the siphoning action. 

      Our coolant system has an additional feature, a high temperature thermostat.  The NAPA 190º thermostat is located at the top of the water jacket to make sure the engine maintains the correct operating temperature.  The thermostat remains closed until the engine coolant rises to 190º.  This decreases the time it takes for the engine to arrive at operating temperature during start up and keeps the operating temperatures constant in extremely cold conditions.

      The thermal mass/sink for this system is a steel 50-gallon drum with welded 2” steel pipe-fittings at the bottom and 18” from the top.  Two CNCed flanges that match the engine pattern sit on the engine side.  These flanges are also welded to 2” diameter pipes with welded ¼” NPT female fitting that accepts thermocouples.  The flanges are attached to the engine with 3/8” bolts and sealed with 1/8” high temperature silicone gaskets.  Rubber radiator hoses that are 36” long 2” diameter slide over the welded 2” diameter pipes connecting the 50-gallon drum to the engine.