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undersquare engine design

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BBerson

Light Plane Philosopher
Joined
Dec 16, 2007
Messages
17,548
Location
Port Townsend WA
Reading about the problems associated with making and using a propeller speed reduction unit on other threads, I got to thinking this:

How about forgetting the PSRU and designing the engine for good low rpm torque with long stroke?

First, some definitions according to wikipedia,
Square engine is: stroke=bore
Oversquare is : stroke smaller than bore
Undersquare is : stroke longer than bore

The book "Aircraft Power Plants" by James L. McKinley says: "It has been found that a square engine provides the proper balance between the dimensions of bore and stroke."

But the small engines I have owned are oversquare. (continental A-65 and Limbach)

Why not more square or undersquare designs for low speed flight such as ultralights or other high drag designs?



Here is a chapter from wikipedia about auto engines:
Undersquare, or long-stroke engine
An engine is described as undersquare or long-stroke if its cylinders have a smaller bore (width, diameter) than its stroke (length of piston travel) - giving a ratio value of less than 1:1.
For example an engine which has 90 millimetres (3.54 in) bore, and 120 millimetres (4.72 in) stroke has a bore/stroke value of:
90 mm / 120 mm = 0.75:1
This can be a negative trait,[citation needed] since a longer stroke usually results in greater friction on the cylinder walls, more stress on the crankshaft,[citation needed] and a smaller bore requires smaller valves which restrict gaseous exchange. An undersquare engine usually has a lower redline than an oversquare one, but it generates more low-end torque.
Engines can be modified with a "stroker" crankshaft, which increases an engines stroke from stock, increasing torque.
Undersquare engines typically are, proportionally, shorter in length, heavier,[citation needed] and taller than equivalent oversquare ones, which is one of the reasons why this type of engine is not generally used.[citation needed]
[edit]Undersquare engine examples
Many British automobile companies used undersquare designs through the 1950s, largely because of a motor tax system that taxed cars by their cylinder bore.[citation needed] Therefore, many of the most famous cars of that era use this design. This includes the Austin A-Series engine, and many Nissan derivatives.
The Chrysler Slant-6 engine, in its most common 225 cubic inch (3.7 litre) version, is a massively undersquare engine, with a 86 millimetres (3.39 in) bore and a 105 millimetres (4.13 in) stroke, producing most of its power right on the peak of its torque curve. The achilles heel of this engine, otherwise known for its exceptional durability, is being over-revved by inexperienced drivers. Red line for a factory engine is under 4,500 revolutions per minute (rpm); red line with aftermarket connecting rods is about 5,500 rpm. On the other hand, a well-maintained Slant-6 can be made to idle as low as 75 rpm (though this is not a recommended speed - neither the alternator nor the oil pump will function adequately).[original research?] In some circles, the Slant-6 is nicknamed "The Stump-Puller" for its diesel engine-like low-speed torque. Appropriate gearing and driving skill is required for performance use.
Willys also used mostly undersquare engines, in fact the L134 and F134 engines, with their fairly small 79.4 millimetres (3.13 in) bore and 111.1 millimetres (4.37 in) stroke, are probably the most undersquare engines ever built (for Jeeps).
The Dodge Power Wagon, among other vehicles, used a straight-six Chrysler Flathead engine of 230 cubic inches (3.8 litre) with a bore of 83 millimetres (3.27 in) and a stroke of 117 millimetres (4.61 in) - yielding a substantially under-square stroke ratio of 0.70.
Undersquare engines tend to be less common than oversquare, but this form of engine is still used in some applications. For example, a modern 8.4 litre Valmet 645 inline-6 tractor diesel engine is a longstroke/undersquare engine, but has an output of over 224 kilowatts (305 PS; 300 bhp) with turbocharger and intercooler.
Numerous Volkswagen Group petrol engines are undersquare.
The popular Mazda Miata also uses an undersquare engine.
 
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