Why are tractors so under-powered?

[RoyJackson]

One should consider longevity too. Tractor engines are not stressed at all (assuming the operator isn't an idiot). You'll find decades old diesels still running and have never been overhauled.

 

[sixdogs]

One should consider longevity too. Tractor engines are not stressed at all (assuming the operator isn't an idiot). You'll find decades old diesels still running and have never been overhauled.

It shocks me every time I see it but there are tractors and such around here that I often see with 15,000 or so hours on them and never an overhaul. On top of that they sell for good money at auction.

 

[Oldpath05]

Field work might be the only place where more hp is needed/wanted, but power to weight ratio will come into play here somewhere. I see stuff like this on Power Block TV, they take a 4 cylinder out of a Ford Ranger and put a big V8 back in, why so they can spin tires in high gear and go like a bat out of heaven.

For my tractor work, mostly hauling wood, some FEL chores, going fast will just break the tractor. The only time I can speed travel up is when I have snow pack woods road, even then I have all the hp I need and then some, so if someones tractor is under powered, try orange.....

 

[Henro]

Did we all get sucked into a response?

The truth is that tractors are NOT under powered.

Most as delivered will spin the wheels before stalling. With extra weight, perhaps a lot of it, they may have enough traction to lug the engine when working, but I am not sure about that. ( I have stalled my engine on occasion when taking a big bite with my loader, but that is a rare occurrence).

Myself, I don't think tractors are generally under powered at all. Sure one must size his tractor for the work he wishes to accomplish, but other than that...tractors are NOT under powered. Period...

 

[Oldpath05]

"Did we all get sucked into a response?"<<<<<<YES, that's why I'm glad to be the last one to respond, but I was hoping for more funny replies.....

 

[hayden]

Just in case it's not clear, there are really two different questions being answered here. I actually don't know which the OP meant to ask.

Question 1 is why tractor engines are rated for so much lowered HP per liter of displacement compared to pickup diesels. The answer has to do with duty cycle for the engine in its application.

Question 2 is why don't our tractors have more horsepower than they do. The answer comes down to traction, and the max size implement the tractor can handle and its horsepower requirements.

In my experience snow blowers and field mowers are the only things that really put a load on the tractor engine. For any kind of pushing or pulling the wheels will spin before the engine bogs down. A snow blower is the only thing I've used that can seriously bog down a tractor.

 

[hayden]

Comparison of some diesels. Kubota diesels are from the Grand L 60s lowest and highest HP versions. Caterpillar engines are from their web site. Matter of physics for the diesel cycle, more power per unit displacement as engine size grows - within limits. Comparing a small tractor engine vs a pickup sized engine is not a valid comparison. Marine vs industrial - marine applications are based on virtually unlimited cooling ability using the water in which the boat/ship is running. Something larger ag tractors today make use of is a power boost. A New Holland T7.210 for example is rated at 165 engine horsepower but has a 210 horsepower power boost for a short time. Its an equivalent of the marine engine unlimited cooling - through electronics the ag engine can make use of the higher power capability for a short period but the cooling system can't keep up for long periods.

1.8 Kubota 20.5
2.4 Kubota 25.8
4.4 Cat 39.8
7.1 Cat 43.1
9.3 Cat 49.0
7.1 Cat M 70.4 (Marine application, same basic engine as the 7.1 industrial above)

I think you are mixing different duty cycle rating in this comparison. The Cat C7 marine number of 70hp/liter is for their highest output power level, and correspondingly lightest duty cycle for recreational boats. The same engine in what they call a "B" rating (allows for up to 80% of the time at full power) is between 26-33hp/liter. At the low end of 26, that's comparable to the Kubota tractor engine ratings. By comparison, one of my marine engines is 17hp/liter, and the other is 36hp/liter.

 

[CalG]

Efficiency (fuel burn) must be paramount in design criteria for any industrial power plant.
I've not studied the Compression ignition engines, but in the early days of carburetted Otto cycle engines it was known that the highest efficiency was obtained when the throttle plate was fully open on an inlet bore that provided the desired output. Inlet tract proportions were optimized.

For a stationary engine running a fixed load such as a water lift pump etc. The matching of required performance to peak efficiency was made easy. Flexibility of power matching to a variety of service needs meant compromise.

And just as when tuning for PEAK horsepower output, The final output was a compromise of taking from one operation regime and giving to another. High speed HP vs low end grunt being the typical trade off.

Comments in this regard to the Diesel would be of interest.
I am quite sure that every combustion chamber design has conditions of inlet velocity and mass that contribute to maximum efficiency. Anti chamber injection designs must complicate or even extend limits to the acceptable variations.
Even bore to stroke relationships and hence combustion chamber geometry are instrumental in achieving goals.

There are many factors to consider, but I will not pay extra for a horsepower figure.
In fact, the Hurlimann 435 is offered in a 445 configuration. That is, the Same Mitsubish K4F engine, only in the latter case fitted with a turbo compressor. I admit that in high range of the gear box, It is possible to overload the engine when climbing or pulling loads. Of curse this is at speeds over 20 mph! ;-)

Bonus comment in reply to those wondering why 4wd tractors are only 3wd? The hurlimann has locking diff on both front and rear axles! True Four Wheel Drive! It's a climbing fool! ;-)

 

[MHarryE]

I think you are mixing different duty cycle rating in this comparison. The Cat C7 marine number of 70hp/liter is for their highest output power level, and correspondingly lightest duty cycle for recreational boats. The same engine in what they call a "B" rating (allows for up to 80% of the time at full power) is between 26-33hp/liter. At the low end of 26, that's comparable to the Kubota tractor engine ratings. By comparison, one of my marine engines is 17hp/liter, and the other is 36hp/liter.

I'm going by what Engine Division told me about why I had to use a higher cost engine to get the power rating I needed in my machine. Other than the small Kubotas and the marine application, the others are all engines for which I have worked on the application and verified their ability to operate at 100% power up to 40C (119F). With engine improvements for emissions, we have increased power density. My first application of a Perkins (before Cat bought them) was the 4 at 107 HP with only 7% torque backup. Now that engine is at 175 HP with 25%+ torque backup. As for the marine version, I can only repeat what Engine Division told me when I wanted it for a mill. They said I couldn't get the high marine rating due to the cooling capabilities. Having recently talked with the captain of a river boat equipped with twin C32s, however, his application has not been smooth. First, the C32s have the power of their older boats but not the torque backup. Cooling has also been a problem because river water gets higher than projected. Still he said that he can run 100% continuously if needed now that the cooling has been resolved. I didn't include large engines like that because the discussion had been small diesels.

Personally on my toys, I have gone from a L3710 to a L5740 to a L6060 Kubota. The 6060 is the first one to appear to have decent torque backup. Still nothing like my M135GX or JD7720 but I believe the 7720 has a 70% backup. I agree that most CUTs are lacking power, but power is cost. There is a rather steep hill between my home and the major portion of our farm. The CUTs will pull down from 17 mph to 12 mph climbing that hill. The 7720 and M135GX will drop a fraction of a mph. I blame it on the lack of torque backup, not low power.

 

[CalG]

Harry

How to delineate torque back up from "power"? To me "power" is encompassing.

Also, Is your example of road speed "pull down" from full output rpm under governor?
It would be curious to compare specific output (hp/displacement) of the machines you offer in example.