Hi. I've read many knowledgeable users on this forum state calculations for HP requirements to drive Hydraulic Pumps, yet to my inexperienced mind they seem to contradict the calculations from most websites, Surplus Center being one example. Surplus Center Calculator states that Gas Engines require 2 times the horsepower as Electric Motors to drive certain pumps, yet all the posts I've read by experienced folks here seem to be advising GPM, PSI, HP calculations that coincide with Electric motors, not Gas. Could someone help me understand? Thanks!
Gas vs Electric Engine: HP requirements
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RNeumann
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I’m no expert in this field but.......
Typically the petroleum based HP is at the flywheel- and often without a fan belt or accessories. So when drivetrain, fan belt and accessories are factored the HP that say a PTO pump can drive is decreased. An electric motor has nothing but the output/flywheel to rob power.
The second issue is mechanical sympathy. Without over complicating things, electric motors apply 100% power when they get energized. Conversely, a petroleum engine isn’t typically designed to run at 100%, 100% of the time.
With those two factors combined I could see why a petroleum engine would need twice the HP. That would make for a reliable setup.
Typically the petroleum based HP is at the flywheel- and often without a fan belt or accessories. So when drivetrain, fan belt and accessories are factored the HP that say a PTO pump can drive is decreased. An electric motor has nothing but the output/flywheel to rob power.
The second issue is mechanical sympathy. Without over complicating things, electric motors apply 100% power when they get energized. Conversely, a petroleum engine isn’t typically designed to run at 100%, 100% of the time.
With those two factors combined I could see why a petroleum engine would need twice the HP. That would make for a reliable setup.
Thanks for the info. That makes a lot of sense about the belts and accessories decreasing output. In the Hydro setup I'm designing, there would be 2 Piston Pumps in Tandem mounted directly to the crankshaft with no other accessories, so I think I'll take that into consideration and probably aim for somewhere between the Surplus Center Calculations and what most people recommend in this forum.
California
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An electric motor has its maximum rated torque starting from zero rpm. This is important if it has to start that pump turning against resistance. In contrast a gas or even diesel motor needs to get up some rpm before a load won't stall it, and doesn't reach rated hp until approaching maximum rpm.
Then there's the difference in noise in a stationary application if you are going to work next to the power source.
Then there's the difference in noise in a stationary application if you are going to work next to the power source.
Good to know. I'm assuming the startup torque requirement will be minimal because i'm using variable displacement piston pumps. Would it be safe to assume that calculators such as the one at Surplus Center are very generalized and don't take into account many nuanced or real world applications? Consider this Recommendation by J_J ''To pump 10 GPM at 3000 psi, will require about a 21 HP engine." Using Surplus Center's calculator, it gives an estimate of 19.4HP for electric and 38.8HP for Gas engine to pump 10 GPM at 3000psi. After reading a lot of posts, it's clear to me that J_J and others know what they are talking about so I'm just trying to figure out what I'm missing as I see a lot of recommendations similar to the numbers J_J stated. I'd like to maximize the efficiency of the Hydraulic system I'm building and get it right the first time. (2 Variable speed piston pumps powering 4 wheel motors, and 1 variable speed piston pump for FEL, Backhoe, etc.) Thanks for the replies so far!
Racenut
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It kinda baffles me that when it comes to tractor specs, I rarely ever see torque mentioned.
4570Man
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I think RPM has a lot to do with it. If you’re comparing a 3600 rpm electric against a 3600 rpm gas there shouldn’t be a lot of difference. Also electric motors will run way above their rated hp for a brief period but the gas engine will stall.
MossRoad
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On that note, how large of a HP motor and pumps are you planning on, and, how are you planning to start that gas engine? Pull start or electric start? The reason I ask is that I have a PowerTrac PT425 machine. It has a 25HP Kohler engine, and 3 pumps, coupled directly to the engine. The electric start works well in warm weather, or if the hydraulics are already warmed up in winter. However, when the temps drop below freezing, it gets proportionally harder to start the colder it gets. While I have started it down to -5F, others with different engines, like the Subaru Robin engine, have great difficulty at sub freezing temps.
I couldn't image pull starting something like that.
On the other hand, I have a pull start log splitter that is easy to start at all temps. But it's only a small, 6hp gas engine and one pump.
So just an FYI on cold temps and your planned method of starting the machine.
A clutched system would be ideal, but ads to cost and complexity.
MossRoad
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Why 2 pumps for 4 wheel motors?
And why a variable speed for the FEL and backhoe?
How are you planning to turn it?