HST Power Consumption

[CJONE]

I looked into this a bit more thoroughly. Simple enough really. If the tractor stops without stalling out while under heavy load with the pedal down (e.g. hill climbing) then the appropriate forward/reverse relief valve has been activated. Pushing the pedal further merely increases the pump displacement hence flow, diverting more flow through the relief valve and hence delivering less power to the HST motor and the tractor slows. Back off and you are generating maximum torque at maximum efficiency. Up the RPMs and you should be able to deliver a bit more torque out, although inefficiently and with some abuse to the HST if the relief has been activated (but a bit more aggressive ground speed and momentum may help get up that hill). Eventually you have to shift to a lower range which will allow the HST to operate more efficiently at a lower pressure.

So to answer my own question, yes the HST relief valves will kick in if one pushes it hard enough. Could also happen due to shock loading. But it is still possible to stall an HST tractor under heavy load, so the relief pressure must be set near the limit of what the engine can deliver.

Yep it is simple, 3 things are going to happen. You will either run out of horsepower, run up to the relief setting or move the wheels. The relief setting is usually specd to the system max pressure and the pump is sized to engine horsepower. Most hydrostatic systems are sized larger than the engine HP because the cool thing with a hydrostat is you can reduce the swashplate angle thus reducing GPM flow and reducing HP requirements keeping the engine from stalling. That is how the new fancy stall guard feature from Kubota and Deere work. An experienced operator or one that is not half asleep would just back off the pedal a little!! I work with these things almost every day, building and troubleshooting them. CJ

 

[Redneck in training]

There are those who argue that maximum torque rpm equals best fuel economy is only the case when an engine is on a dyno and not in a real world situation, and that lower rpm below torque peak will usually result in a fuel savings. To be honest, I have never seen what a torque curve looks like for a small diesel engine like my Yanmar and I am now curious enough to wonder what it's like.

The key is "specific fuel consumption". The unit is g/kW or oz/HP. Running engine at maximum torque rpm gives you the best fuel consumption. HST excel at it because you can keep the engine RPM constant regardless of rpm.

 

[TBarD]

Yep it is simple, 3 things are going to happen. You will either run out of horsepower, run up to the relief setting or move the wheels. The relief setting is usually specd to the system max pressure and the pump is sized to engine horsepower. Most hydrostatic systems are sized larger than the engine HP because the cool thing with a hydrostat is you can reduce the swashplate angle thus reducing GPM flow and reducing HP requirements keeping the engine from stalling. That is how the new fancy stall guard feature from Kubota and Deere work. An experienced operator or one that is not half asleep would just back off the pedal a little!! I work with these things almost every day, building and troubleshooting them. CJ

Simple enough but also subtle! So the key thing is to be in the right range lo/med/hi. In the right range the HST operating oil pressure is lower and the limit is likely to be max HP; the engine could stall or lug and stall guard or a good operator can maximize power output by backing off a bit. Hitting the relief pressure is less likely and the engine would be near max HP in any case. But if the gearing is too high there is not enough system pressure to turn the HST motor and increasing flow just dumps waste HP off through the relief valve.

This explains why HST is different than normal open center hydraulics even though they are based upon the same principles. With HST in the proper gear range one can work the tractor up to the max engine HP or easily spin the tires (but in the wrong gear it will just blow the relief). With normal hydraulics like on a backhoe or bucket, it is easy to hit the relief pressure and have a cylinder stop without even straining the engine.

 

[Redneck in training]

Simple enough but also subtle! So the key thing is to be in the right range lo/med/hi. In the right range the HST operating oil pressure is lower and the limit is likely to be max HP; the engine could stall or lug and stall guard or a good operator can maximize power output by backing off a bit. Hitting the relief pressure is less likely and the engine would be near max HP in any case. But if the gearing is too high there is not enough system pressure to turn the HST motor and increasing flow just dumps waste HP off through the relief valve.

This explains why HST is different than normal open center hydraulics even though they are based upon the same principles. With HST in the proper gear range one can work the tractor up to the max engine HP or easily spin the tires (but in the wrong gear it will just blow the relief). With normal hydraulics like on a backhoe or bucket, it is easy to hit the relief pressure and have a cylinder stop without even straining the engine.

Big backhoes, large tractors etc have constant pressure or pressure on demand hydraulic system with variable delivery pump that is controlled by pressure or flow controller with pressure limiting. HST is similar to system with pressure on demand. Otherwise your statements are correct. Some , like my HST, will stall the engine and might go to relieve only running at top rpm.

The advantage of constant pressure system is that you can operate all cylinders concurently. Disadvantage is compexity.
Open center systems are simple but allow only limited concurent operation of cylinders.

 

[bigdeano]

Keep in mind that the losses in a hydro go up with wear as it's used, while losses in a gear transmission don't, at least very little. Hydros are great if you do a lot of reversing. For field work gears are best in my opinion.

 

[SPYDERLK]

The relief pressure is set that does not change, your pedal application or flow is what changes. Say 1/2 pedal is 10 gpm at 4000psi that is 27 hp, full pedal 20 gpm at 4k is 54 hp. If you are in high range and you hit 4k at half pedal the relief opens. If you back off to 1/4 pedal you drop to 1/4 flow and if the resistance is low enough to keep below 4000psi you move. If not you have to drop down a range, effectivly lowering resistance and pressure. Now if you only have a 30 hp tractor you will only be able to use 1/2 pedal travel at the relief pressure, if you go over that in ANY range at the relief the engine will stall. CJ

Yep it is simple, 3 things are going to happen. You will either run out of horsepower, run up to the relief setting or move the wheels. The relief setting is usually specd to the system max pressure and the pump is sized to engine horsepower. Most hydrostatic systems are sized larger than the engine HP because the cool thing with a hydrostat is you can reduce the swashplate angle thus reducing GPM flow and reducing HP requirements keeping the engine from stalling. That is how the new fancy stall guard feature from Kubota and Deere work. An experienced operator or one that is not half asleep would just back off the pedal a little!! I work with these things almost every day, building and troubleshooting them. CJ

I like your posts because they draw the key factors together cohesively. Others, tho presenting factual data, skip over a key issue now and then. The thread then moves on and its hard to draw the factors back. ...
...Sure it is possible to design an HST for the loads that a gear can benignly support in an overload engine stalling condition. However; due to the nature of an infinitely variable transmission, it must be either made hugely robust, or include built in protection against itself. This protection thwarts extreme hi torque performance.

...Regardless of tires/traction there will always be a situation where they cannot be spun on any tractor. Universally, to my knowledge, this will happen first with a fluid drive. This is because you dont have a mechanical connection between engine and wheels - the connection passes thru fluid and fluid pressure is limited by a relief valve. So the torque to the wheels is limited by the combination of the relief setting and the displacement of the hyd motor being driven and then the final drive [geared] ratio to the wheels. This is the ratio you select when you change ranges.

With an HST, regardless of the relief setting the engine cannot be stalled without abusive operation of the pedal because you are varying the displacement of the hyd pump. Without the relief, infinite forces would be available in any range just by adjusting the pedal, [and thereby pump displacement], so that the HP driving the pump would be able to deliver whatever pressure required to give the torque needed at the wheels. Of course the pressures would have to be proportionately higher in the higher ranges to get the torque needed at the wheels. Well, darn it anyway, we cant just keep kicking the forces up thruout the powertrain like that. We have to limit each part according to its ability. We limit the pump/motor with a protective relief valve, and we make the gears in the ranges amply strong to take the [thereby limited] hyd motor torque. We adjust the range gear ratios to give usable speed and wheel torque combinations. This part of the hst tractor is exactly analogous to a gear tractor if you recognize that each gear ratio is a range.
>>Not surprisingly, on modern gear tractors that usually have at least 8gears, the ratios are much more closely spaced thruout than they are with hst which has just 2 or 3 gears/ranges to cover the full range of conventional speeds. So you dont find a lo range gear ratio on an HST as low as 1st on an 8 speed. The hst will go faster in 1 of 3 than the gear in 1 of 8. That means the HST is effectively in a higher gear. And that means that the HST cannot deliver as much torque to the wheels if the relief valve is set safely. To realize this you have to take a close look at the interaction of the engine, pump, relief valve, and motor. Since the pump is variable from 0-100% the engine will be capable of driving it to deliver HUGE pressure at low pedal. Losses by leakage and other friction will limit the theoretical infinite pressure capability. However not to a low enuf level that destruction would not be inevitable if the wheels didnt turn. The relief has to be set at a level low enuf so that it will relieve if the tractor is stopped dead in a pull. If it didnt something requiring repair would give. The wheels would spin? In lo range maybe, but not in hi. If the relief doesnt give and the wheels dont spin the engine stops dead. This kind of impulse must be prevented. For maximum and "safe" power delivery it is done by setting the relief at a pressure that will stall the full engine power gracefully [say, about 1second] at full pedal. Somewhat more than full engine torque is being delivered to the pump in this situation, and an approx 2 or 3x multiple of engine HP is flushing fluid to the sump. The hyd pressure seen here is as high as it can go in the protected transmission. This pressure defines the max torque available from the motor that is driving the final ratio. Changing the flow with the pedal has only a tiny effect on this torque as the engine bogs. Because of the relief, backing off the pedal to let the engine work easy does only that - the pressure and therefore torque limit at the motor remains the same. That motor is working against a higher speed gear than 1st on the gear tractor. From all this, it follows directly that the HST cannot deliver as much torque to the wheels as the gear. For example - if the pressure protected HST goes 2mph at full pedal and rpm in lo range, and 1st on the gear goes 1mph topped out, the gear will deliver about twice the wheel torque. [And yes, a gear can be overloaded to failure] It is essentially an HST set for 1MPH and having no relief. But this only occurs in 1st on the gear. This setting is possible in any range on the HST. Whereas the gear would bog and stall while its clutch slipped a few revolutions, the HST operator could just lift the pedal and apply destructive hydraulic pressure without even bogging the engine. This is and must be prevented with the relief valve.
...larry

 

[CJONE]

Big backhoes, large tractors etc have constant pressure or pressure on demand hydraulic system with variable delivery pump that is controlled by pressure or flow controller with pressure limiting. HST is similar to system with pressure on demand. Otherwise your statements are correct. Some , like my HST, will stall the engine and might go to relieve only running at top rpm.

The advantage of constant pressure system is that you can operate all cylinders concurently. Disadvantage is compexity.
Open center systems are simple but allow only limited concurent operation of cylinders.

HST's do not generate pressure in neutral other than charge pressure on the forward and reverse lines. Pressure is a result of resistance just like on a open hydraulic system. The systems on most of the large equipment like you are referring to are set pressure with the pumps generating a max psi like 4500psi then compensating back to 0 flow when pressure droppes the pump provides the correct amount of oil [flow] to maintain set pressure with the used oil returning back to tank. With the HST the oil does not leave the loop unless it is bypassing the pistons or flowing over the charge relief. That is why clean oil is so important in these systems. The bigger systems that I build have 2 charge pump reliefs 1 for off stroke and 1 for on stroke for cooling and filtration for the pressure loop. I am not sure for the small all in one units like in our cuts. I have not looked into it that far yet. CJ

 

[dennis52]

I have never seen the name "Greenfield" as applied to power lawn equipment. Hmm, maybe somebody needs to start a new thread about the power consumption of a belt/pully friction clutch transmission vs. a hydro....

A link to an Australian forum that will do the trick.
http://forums.whirlpool.net.au/archive/1326833

 

[CJONE]

One other thing, alot of people are using the PTO HP rating for HP loss on a hydrostat saying like 1.5 HP that is simply not true. That measurement is taken without the hydrostat being used. It is just parasitic loss from the hydrostatic charge pump. The real figures are like what has been stated 20-30% so take that in to consideration. CJ

 

[xtn]

Not surprisingly, on modern gear tractors that usually have at least 8gears, the ratios are much more closely spaced thruout than they are with hst which has just 2 or 3 gears/ranges to cover the full range of conventional speeds. So you dont find a lo range gear ratio on an HST as low as 1st on an 8 speed. The hst will go faster in 1 of 3 than the gear in 1 of 8. That means the HST is effectively in a higher gear. And that means that the HST cannot deliver as much torque to the wheels if the relief valve is set safely.

The hst pump/motor combination, separate from any geared range mechanism, is infinitely variable within it's own range. So if you push the pedal down 1/3, for example, you are getting an "effective" gear ratio that is 1/3 of the highest available. (The available range can be played with by specifying different displacements for the pump/motor.) So effectively by moving your foot you are choosing an input/output ratio.

The "safe" limit in a hydraulic system is mostly determined by how much you want to spend on the robustness of parts. If you want to go all out and use super strong hoses, fittings, coolers, valves, pumps, etc., then you can use a relief that is much higher.

Combine those two facts and the result is that an hst system can be designed to put down as much torque as you want and effectively geared as low as you want. You want enough torque on the ground to slowly spin the treads of the biggest track hoes... it can be done. Maybe your BX can't do deliver as much torque as its geared twin. But please stop suggesting that there is some quality inherent in hst systems that automatically prevent them from ever transmitting as much torque as a geared system can. After all, a gear box must be engineered to handle some design goal amount of torque too, and as it can be, so too can an hst system.

xtn