It isn't a matter of homework, it is a matter of balance...or wisdom, in my opinion. All inline, four-cylinder engines have inherent vibrations or disharmonies, if you will. I read a great article in Road & Track magazine touching on the subject of engine balance about 15 years ago. This isn't just a Kubota issue. Cat, Cummins, Deere, Yanmar, Ford, BMW, Honda or anybody else who designs an inline four-cylinder has to contend with these vibrations. (I once had a BMW K100 motorcycle that had a four cylinder engine. At certain rpms, there would be a weird throbbing & vibrating through the whole bike...which explained why the handlebars were mounted with rubber isolators.) There is a reason why it would need balance shafts. Balance shafts typically spin counter to the engine's crank rotation at twice engine speed. These balance shafts are necessary to negate the vibrations set up by a four-cylinder engine. Across the whole diesel engine industry, it is rare to encounter a inline-four cylinder diesel that exceeds 4.5-liters of displacement.
In contrast, an inline-six is a perfectly balanced engine, as is an inline-eight, a V-12, and a V-16. Why aren't more cars using these engine configurations? Packaging under the hood, visibility, and overall length are high priorities. In applications where packaging is not a high priority (unlike cars, skid steer loaders, or other compact equipment) an in-line six is often the design of choice. This is the case in off-highway equipment, over-the-road trucks, and tractors. Inline sixes work and work very well with low-end torque and strong bottom ends.
All other tractors in the
M135X's 115-hp class are using six cylinder engines. For me, I would tend to be very skeptical of a four-cylinder engine in any tractor in this horsepower class. Just like I am skeptical of Sisu's brand new SEVEN-cylinder tractor engine. That's just the way I roll...