Back in the mid seventies, one of my first jobs out of college was developing master pump settings for various engine applications on the dyno. What I recall is that a relatively high power turbocharged tractor would have a torque rise of 15% at the time. NA engines had less torque rise, as they would quickly run out of air and be smoke limited as to the shape of the torque curve.Torque rise is the difference between torque at rated engine speed and torque peak. Fifteen percent allows the all speed governors to maintain constant engine speed when the plow or implement encounters difficult going in the field.
combines had no need for much torque rise between rated and peak torque , just based on the nature of operation, ie running on what is called the overrun curve between high idle and rated.
Some fuel systems didn’t have much sontrol of delivery at different engine speed, so you were essentially hardware limited as to the shape of the curve. The drivetrain and cooling system guys were also pretty much in control of what the final shape of the power curve looked like. Putting out a tractor that wouldn’t cool, or blew up geartrain wasn’t a financially viable option from a warranty or customer satisfaction perspective.
Torque rise today is usually a lot more than 15%. Nebraska changed their testing some in the early 1990s and started to publish engine torque numbers and torque rise between rated speed and peak torque. I'll pick on some tractors I've run that have been tested at Nebraska and have figures available for peak torque and torque rise. They are definitely not high power by modern standards but the New Holland would have been a pretty potent tractor back in those days and the others' size would have put them right in the middle of the ag market back then, so not completely inappropriate.
- Deere 5075E (Final Tier 4, common-rail turbocharged and intercooled): Rated speed 2100 RPM. Peak torque 209 ft-lb at 1249 RPM, torque rise 31.6%
- Deere 5083E: (Tier 3, mechanical injection, turbocharged): Rated speed 2400 RPM. Peak torque 204 ft-lb at 1499 RPM, torque rise 41.3%
- Deere 5410: (Tier 1, mechanical injection, naturally aspirated): Rated speed 2400 RPM. Peak torque 204 ft-lb at 1094 RPM, torque rise 42.5%
- New Holland T6030 (Tier 3, mechanical injection, turbocharged and intercooled): Rated speed 2200 RPM. Peak torque 422 ft-lb at 905 RPM, torque rise 63.6%.
Going back to the original question as to why engines are rated according to HP rather than torque is because horsepower is what accomplishes work. You can have a high-torque engine that turns slowly or a low-torque engine spinning fast and both will do the same amount of work. For example, CNH's Workmaster 70/Farmall 70A are competitors to the Deere 5075E above and they take the opposite strategy with engines:
- Deere 5075E (FT4): 179 cid turbocharged (17.7 PSI), intercooled, common-rail three-cylinder. Rated speed 2100 RPM, redlines at 2200 RPM. Produced 64.2 PTO HP at 540 PTO RPM (2083 engine RPM), 209 ft-lb torque at 1249 RPM, torque rise of 31.6%. Fuel efficiency 14.2 HP-hr/gallon.
- CIH Farmall 70A (2019): 136 cid turbocharged (18.2 PSI), intercooled, common-rail three-cylinder. Rated speed 2600 RPM, redlines at 2800 RPM. Produced 63.5 PTO HP at 540 PTO RPM (2575 engine RPM), 167 ft-lb peak torque at 1798 RPM, torque rise of 29.2%. Fuel efficiency 13.8 HP-hr/gallon.
The much smaller engine with a much higher operating speed has noticeably less torque but makes almost exactly the same PTO power, has a similar torque rise, uses about the same turbocharger boost, and even has very similar fuel efficiency. I would thus expect very little difference in performance between the two engines despite the torque difference.
