Franklin engineers called their system "direct air cooling." To them, a water-cooled engine used "indirect air cooling."
Any directly air-cooled engine hides within it several inherent advantages and compromises. The most obvious advantages are no freezing or boiling points. The normal operating temperature of a Franklin engine is 250-300 degrees Fahrenheit, while water-cooled engines normally run at between 180 and 245 degrees.
The hotter an engine can run without overheating or detonating, the more efficiently it can burn fuel. Thus the Franklin was a good deal more thermo-dynamically efficient than the typical water-cooled engine of its day.
Another advantage to direct air cooling comes in weight reduction, since there's no radiator, no water, etc. As for complexity, that's probably a toss-up: The absence of hoses and waterjacketing is offset by ducting, baffles, fins, oil coolers, and so forth in an air-cooled engine. Liquid coolants can corrode passages in the block and radiator, but air passages can cake up with oil and dirt.
However, air cooling carries several disadvantages, too. The cylinders in an air-cooled inline engine need to be spaced relatively far apart, with enough room between them for large volumes of air to flow.
This means a long crankshaft, and long cranks tend to whip. That's why Franklin never produced a straight eight. (The company did experiment with them and even built one unsuccessful eight-cylinder racer in 1905).
It's also why the company's inline sixes used seven wide main bearings, and why most modern air-cooled auto and motorcycle engines have opposed cylinders with short crankshafts.
Air cooling also limits cylinder-bore diameters, because too large a bore traps heat at the middle of the piston. The piston center heats up and eventually melts. This bore restriction was one reason the Corvair engine couldn't grow and why Porsche switched to water cooling.
Limiting bore diameter also restricts valve size. Small bore means small valves, which cut volumetric efficiency (breathing) and thus restrict power output. Franklin engineers were always fighting too-small valves, and it was Shoemaker who suggested making the six-cylinder Airman engine's exhaust valves smaller so the intakes could be bigger.
Another problem: Air-cooled power-plants usually need more exotic and expensive metals than water-cooled engines. Franklin metallurgists tried various aluminum alloys over the years, along with copper cooling fins.
As mentioned, all Franklin engines used overhead valves, and in the early days, valvetrain noise tended to be a problem. This was caused by the different heat-expansion rates of the metals used. Franklin solved this by using a simple compensating-stud device that maintained valve clearances throughout the engine's heat range.
Another early handicap involved the cooling fan absorbing quite a lot of power. It's been estimated that early Franklin fans drew as many as 20 bhp.
But again, by 1928, Franklin engineers had come up with scirocco fans attached to the front of the crankshaft, plus baffles within the cooling stream, that drew only four to six horsepower -- no more than the fan and water pump of a conventional water-cooled engine.
Air cooling does make an engine louder than water cooling. That's partly because the water acts as an insulating material and partly because a big fan blasting large volumes of air past fins creates noise.
Because most aircraft used air-cooled engines -- and airplane engines were known for performance and reliability -- Franklin capitalized on that link. Factory literature stated in 1932, "The Franklin Twelve is fundamentally an airplane type engine, with all its inherent reliability and high power characteristics."
Charles Lindbergh, Amelia Earhart, and Frank Hawks were internationally known aviators who drove and promoted Franklin automobiles. Even the series name Airman capitalized on Lindbergh's historic transatlantic flight of 1927.
Go on to the next page to learn about the production of the Franklin V-12.
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