A 27-year reign as fuel king
When John Deere’s latest effort in adapting diesel power to tractors arrived at the Nebraska Tractor Test for evaluation late in the summer of 1956, it must have caused a stir among test engineers. Rated at 50½ belt horsepower, the 720 would be the largest row-crop tractor evaluated at Nebraska up to that time.
What happened after the evaluations were completed and performance numbers tallied confirmed what the engineers suspected about Deere fuel-miserly diesels. The 720 had set a new economy record of 17.97 horsepower hours per gallon (hhpg), an industry standard for determining fuel efficiency.
What the engineers didn’t know at that time – and what few people would have expected – is that the 720 record would rule supreme among all tractors for an amazing 27 years.
Not until 1983 would the 720’s accomplishment be bested, appropriately, by another Deere. That would be the Model 1650, which set an economy record of 18.64 hhpg in 1981.
Les Larson, head of the Nebraska Tractor Test when the 720 was evaluated, told me in an interview years ago that, in a way, he wasn’t surprised the 720 set the record. After all, he recalled, the Deere Model 70 had established the previous economy record of 17.74 hhpg set two years earlier at the track.
The fuel economy record holder prior to the 70 was another Deere. This time it was the Model 80 (the 70’s big brother) that hit 17.58 hhpg in 1955. To give the Deere diesels’ accomplishments some perspective, consider that at this time the only tractor to come close to meeting the records was the Case 401, which tallied 15.95 hhpg in 1955. That was a full 2 hhpg less than the 720.
So what was the secret to Deere engineers’ success at producing such miserly diesels? The two-cylinder platform. With its huge cylinder volume (376 cubic inches of displacement split between two vs. four cylinders) combined with a 16:1 compression ratio (the ratio of maximum piston volume to minimum compressed volume), it was efficient by its nature. Yet, this feature by itself was not enough to generate such an efficient engine.
Advances in cylinder head and piston configuration were the key to throttling fuel use.
Deere engine designers had altered the geometry of the cylinder head and piston to increase turbulence in the combustion chamber. This boosted swirling action in the chamber (what Deere engineers termed cyclonic), which increased whirling action and more thoroughly mixed intake air with diesel fuel.
The end result was a more complete combustion of that air-fuel mixture, which generated more horsepower from less fuel. This advance was first introduced on the Model 80 in 1955. Deere engineers had devised an intake valve venturi that incited the swirling action within the combustion chamber, which Deere called a cyclonic fuel intake.