The New Harvest of Mid-Sized Diesel Tractors

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It isn't necessary to be a full-fledged diesel mechanic to make a good tractor choice, either. There are, however, a few features often quoted by manufacturers (and listed in the chart that follows) which probably could use a little explanation:

BORE, STROKE, AND TOTAL ENGINE DISPLACEMENT: The diameter of one engine cylinder is referred to as the bore, and the distance the piston moves within the cylinder in its up-and-down travel is called the stroke. Since the bore is actually nothing more than the diameter of a circle the formula pie r 2 (or 3.14 times Me square of half the bore) can be used to calculate the cross-sectional area of an engine cylinder ... and this area multiplied by the length of the stroke equals the displacement volume of one cylinder. To get the total engine displacement, multiply this figure by the number of cylinders in the engine. This is the specification that's quoted in most manufacturer's information tables.

COMPRESSION RATIO: The compression ratio is also a function of bore and stroke. If a cylinder holds 44 cubic inches of air when the piston is at the bottom limit of its stroke, and only 2 cubic inches at the top of the stroke, then the compression ratio is 44 to 2, or 22:1.

Generally speaking, compression ratios are higher in small diesel tractor engines than in larger industrial powerplants. The little diesels—which are less massive—lose heat faster ... so, in order to maintain the temperature needed to ignite the fuel, small tractor engines generally feature relatively high compression ratios.

HORSEPOWER: This is one specification that's often misleading ... because of the way it's expressed. You see, the horsepower of a diesel engine varies with the total displacement, the pressure in the cylinders, and the revolutions per minute. Hence, any useful indication of horsepower should include a statement of the RPM as a point of reference.

Unfortunately, there isn't much uniformity among agricultural implement manufacturers when it comes to expressing horsepower. Most U.S. firms indicate the PTO horsepower which—because of frictional loss of power in the engine and drive train—is lower than "maximum bare horsepower", or "manufacturer's estimated maximum horsepower". Therefore, in the accompanying table I've quoted the "maximum PTO horsepower at rated engine speed"—based on independent University of Nebraska tests—wherever possible.

For a more accurate "yardstick" to measure your prospective tractor's muscle, I suggest you take another look at "displacement" . . . and simply compare machines on that basis if the horsepower ratings seem a bit confusing. Better yet, your dealer (or the local library) may have a copy of the January 1979 Implement and Tractor Red Book. You'll find an excellent summary of the University of Nebraska tests—with accurate information on whatever tractor you choose—included there.

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