Got three minutes? Here’s a clear and simple explanation of the radial engine that anyone can understand, including the important points you want to know.
To car-oriented enthusiasts, the radial engine might be a stange-looking beast. But while there are some significant differences in its construction, at its heart it’s the same Otto four-stroke engine we’ve all come to know. The differences are the interesting and often mysterious part, and since radials are most commonly used in aircraft, we went to an aviation authority: the Boldmethod online aviation school and its Youtube channel, where we found the best and briefest explanation we’ve found to date. Of course, we had to share.
While the clip is a quick three minutes, you’ll learn some key facts car guys often don’t know. (The same questions seem to always come up.) For instance, why the classic radial engine always has an odd number of cylinders per row, how the cylinders are numbered, and the universal firing sequence that is common to all engines of its type. While the animated demo shows a five-cylinder layout for clarity, the operating principles are identical for seven and nine-cylinder rows, too.
Also covered are the important historical advantages and drawbacks of the radial, and there’s a brief but informative segment on the queen of American radials, the mighty Pratt & Whitney Wasp Major, a 71.5-liter, 28-cylinder (four rows of seven) masterpiece that developed up to 4,300 hp in turbocompound form. There’s a lot more to know about these fascinating engines, their distinctive valvetrains and connecting rod arrangements for example, but here’s a great start. Enjoy the video.
In the late 1920s Packard introduced a large radial aircraft engine that used Diesel Fuel! Note the motor used only one valve for both the intake and exhaust. Read on for further explanation.
In the book “Diesel and Other Internal Combustion Engines” by Howard E. Degler, published by the American Technical Society in 1937, here are excerpts of the discussion of the Packard Diesel radial . . .
The Packard-Diesel aircraft engine is of the radial air-cooled type having nine cylinders with a bore of 4-13/16 inches and a stroke of 6 inches. It gives a displacement of approximately 980 cubic inches per minute. The engine is rated at 225 horsepower at 1950 revolutions per minute and weighs 510 pounds.
An inspection reveals a radical departure from the usual engine in that each cylinder is provided with only one valve, which serves for both inlet and exhaust. The single rocker-arm box, which is slanted in the direction of the spiral of slipstream, contributes considerably to the clean external appearance of the Packard- Diesel engine, and what is more important, to its low parasitic drag.
In the Packard engine, an advance that has been effected over previous Diesel practice consists in the ability to extend the range of engine speeds possible with the Diesel cycle. Heretofore Diesel engines in stationary and marine service have been of the low-speed type, 100 to 500 rpm. Even so-called high-speed Diesels of modern type have been limited to a maximum speed of about 1500 rpm. With this engine the speed has been increased to more than 2000 rpm., which has been attained by an engine design that produces a turbulence never before approached in this type of engine. The engine design and the highly efficient and quick-acting fuel pumps that were developed to go with it are the means that produce the accelerated co-mingling of the fuel and air which brought about this greatly increased engine speed. The fuel pumps give a positive and metered supply of fuel.
The most interesting aspect of the design is undoubtedly a consideration of the features that reduced the weight of the engine to practically the same level as that of gasoline engines of equivalent power. Heretofore (1935), even Diesel engines of the so-called light-weight modern type weighed about 20 pounds per horsepower, whereas this engine weighs but one-tenth as much, i.e., 2.3 pounds per horsepower.
A minimum of weight is essential for any successful aircraft engine; new methods of construction were employed in the Packard-Diesel to reach this desired objective. Important weight economies were secured; first, by the elimination of carburetors and magnetos; and second, by an intensive simplification of design, as shown in Figs. 65 and 66. Evidence of the latter are found in the one-piece crank case construction of extremely light weight and single-valve arrangement which automatically halves the number of parts required for valve operations as used on conventional gasoline engines.
The fact that only air is drawn into the cylinder on the intake stroke of the Diesel engine permits the novel use of a single valve for both inlet and exhaust purposes in the Packard aircraft engine. The use of one valve per cylinder contributes in a marked degree to the lightness of the cylinder itself, since the cylinder head is weakened by only one port instead of two, as in the conventional engine. The single-valve arrangement was favored not only on account of the general gain in simplicity and saving weight, but also in the interests of reliability, as this valve operated at a much lower temperature than the conventional exhaust valve due to the cooling effect of the incoming air passing over the same valve. The available time for opening and closing the valve can also be utilized, thus saving wear and tear on the whole mechanism. Bolted to the top of each cylinder is a light aluminum cylinder head carrying cooling fins and supporting the valve-operating mechanism as well as forming the combined inlet and exhaust port.
For more info on the Packard DR-980 motor, the Smithsonian has one in the Air & Space museum, and they also have photos on their webpage.
Thanks for the detailed info on the Packard aero diesel. We hope to do a feature on it one of these days. Packard was a great engineering company.
I had a non-running [locked up solid] example in my Packard collection, sadly it was destroyed in a lightning-induced building fire in 1995. I had been doing research on the engine, hence I have the info I shared. Also have the factory original brochure somewhere.
Great loss to America with the closure of Packard. The WW II Merlin Packard engines were a hallmark of modern production technology.