The air-cooled Gade engine is a bit of an enigma. Casual inspection reveals a basic air-cooled horizontal engine with an atmospheric inlet valve and a mechanically actuated exhaust valve. The confusion comes upon noticing the placement of the muffler just beyond the midpoint of the cylinder. For many people, this automatically suggests a two-stroke engine, yet this surely has to be a four-stroke owing to its use of intake and exhaust valves. Then it runs, puffing exhaust from both the muffler and an unchecked port in the cylinder head.
That basic feature is arguably the Gade's best-known mechanical attribute, and it was spelled out in the patent application filed in 1902 by Frank Hardenbrook and William Rice. Patent no. 760,333 was awarded in 1904, and engines manufactured by Gade Bros. Manufacturing Co., Iowa Falls, Iowa, hit the market the same year using the patented design.
The novelty behind the Gade engine lay in its use of a ported exhaust, a design copied and simultaneously developed by at least a few other companies. The impetus behind the design was the desire for a cooler-running engine. The Gade's governor is set up to hold the exhaust valve open on overrun. As with any atmospheric intake, this means the engine will not pull a fuel/air charge as there is no pressure differential to open the valve. But with the ported design, this also introduced an additional cooling mechanism.
With the exhaust valve held open, the engine draws fresh air through the exhaust port during over-run. This air in turn travels through the cylinder and vents through the port. Some quantity of air then pulls back into the cylinder through the port and vents through the exhaust valve. This combined current of fresh air draws heat away, keeping running temperatures at tolerable levels - and without the use of an auxiliary fan.
The port also supplies a secondary intake tract. The vacuum created during the intake stroke means the pressure inside the cylinder is less than atmospheric. When the port is exposed at the bottom of the intake stroke, a small charge of air enters the cylinder, mixing with the fuel/air mixture drawn through the intake valve.
With the ported design the lion's share of combustion gases are released at the bottom of the power stroke. Importantly, this means less work for the engine, since the piston doesn't have to push the hot exhaust gases through the poppet valve set in the cylinder head. This means less strain on corresponding valve train components and theoretically, at least, a more reliable engine.
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