Gas vs Electric Engine: HP requirements

[Charles84]

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!

 

[RNeumann]

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.

 

[Charles84]

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.

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]

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.

 

[Charles84]

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.

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]

It kinda baffles me that when it comes to tractor specs, I rarely ever see torque mentioned.

 

[4570Man]

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.

 

[Charles84]

Yes, 3600 rpm Gas is what I will be running.

 

[MossRoad]

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.

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]

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!

Why 2 pumps for 4 wheel motors?
And why a variable speed for the FEL and backhoe?

How are you planning to turn it?

 

[Charles84]

I will be using the 35 HP Briggs Vanguard (not efi). It's only capable of electric start. I doubt i will ever use this machine in -5 degree weather as we only get a few days of 7-10 degree weather here in Central Va each year. Since i'm going to have to do a significant amount or wiring and rewiring for this job, I might look at including a 120v electric start like i've got on my Crafstman snowblower. The 120v electric start via electrical cord is the only way i can get that thing started any time.

I've been studying the PT425. They look like neat machines and the specs seem very similar to what i'm looking to do.

Right now it looks like 2 Hydrogear PR or PY Piston pumps in tandem mounted to the crank and 1 PR or PY mounted to a Flywheel stub shaft. I'm planning for a re Power and a 4wd Full track conversion of my JD 420. But i'm still in the early design stages and in no rush because i want to get this done right the first time. I'll start a new thread to document this once i get ready to start building.

Are you familiar with Lunchboxsessions.com? It's got some great free content regarding hydraulics and electrical, and then for $29 a month you can use all their resources which allows you to Study, and put together hydraulic components into a schematic type puzzle to test whether your system is functional.

Thanks for your replies everybody.

 

[Threepoint]

It kinda baffles me that when it comes to tractor specs, I rarely ever see torque mentioned.

Not to derail the OP's thread, but maybe it's because of so many variables in translating torque to a tractor's various capabilities. :confused3: I suspect there's also an understandable desire of the manufacturers' not to get too far down in the weeds if it would mean disclosing engineering design algorithms and assumptions.

Racenut, since you're shopping for a tractor, the following thread might interest you if you haven't already seen it. I started it, and learned a lot from the replies. https://www.tractorbynet.com/forums/buying-pricing-comparisons/416609-engine-pto-hp-rated-rpm.html

 

[Charles84]

Why 2 pumps for 4 wheel motors?
And why a variable speed for the FEL and backhoe?

How are you planning to turn it?

I would prefer to do variable speed for the FEL and backhoe for smoother operation of the equipment.

2 pumps for 4 wheel/sprocket motors. 1 pump for the left side, 1 pump for the right side. .I think the correct term would be connected via series, or parallel? 4 Wheel/sprocket motors because i want to have as much torque and power to turn and use it as a mini bulldozer as well. The terrain has lots of hills here also. I realize I'm going to have to have a larger reservoir/oil cooler/fan, which is also one of the reasons I want the backhoe /FEL to be on variable displacement so that It's not constantly cycling oil heating it uselessly. Luckily this JD 420 is large for a garden tractor so it's the perfect candidate, and i'm gutting it nearly completely to the frame to start from scratch (Traded for the JD 420 with a friend who had a significant amount already disassembled which he was going to restore until he found out it had a severely blown engine..which blew up 2 connecting rods, sheared part of both pistons, bent a valve and damaged the block!)

I will be turning with control valves. It will be a zero turn, not articulated like the PT.

 

[MossRoad]

How are you going to control the demand for more speed for the FEL when needed? That would require either an extra lever/pedal, or a very expensive/complex torque sensing setup to sense demand. Sounds like a great project. I'll watch for updates. Good luck. :thumbsup:

 

[Charles84]

How are you going to control the demand for more speed for the FEL when needed? That would require either an extra lever/pedal, or a very expensive/complex torque sensing setup to sense demand.

I'm not sure I understand what you mean by more speed. Could you elaborate?

 

[MossRoad]

I'm not sure I understand what you mean by more speed. Could you elaborate?

Variable volume pump on the FEL. How are you going to control the speed of the loader arms up/down and dump/curl? With the joystick changing the angle of the swashplate?

Think about FEL operations. There are 2.
1. up/down
2. dump/curl

How are you going to provide those 2 operations with only 1 variable volume pump? You can only change directions of one operation with a swashplate. You'd need two variable volume pumps for the FEL. One for up/down, and the second for dump/curl.

Mine is a fixed volume pump. The farther you push the lever forward, the faster the FEL arms lower. The farther you pull the lever back, the faster the FEL arms raise. Anything not used by the valve goes back to the tank, as it's open center. If the pump were variable volume, you'd need 1 circuit to control the up/down, 1 circuit to control the dump/curl, and one circuit to control the volume of the pump.

Perhaps I'm missing something?

 

[oldnslo]

I believe the hydro-gear pumps are closed loop or HST pumps. These will work for the travel drive but will not work well for any auxiliary functions like FEL.

 

[Charles84]

Thanks for the advice guys. I'll look into the FEL pump requirements further.

 

[MossRoad]

Thanks for the advice guys. I'll look into the FEL pump requirements further.

Just for knowledge, my PowerTrac has a 25hp Kohler. There's a variable volume pump on one side that feeds 4 wheel motors. Left front and right rear are in series. Right front and left rear are in series. Then those two series circuits are in parallel. On the other side of the gas engine there is a two section pump. One section supplies 8gpm @ 2500 psi and that is used to power any attachments like mowers, brush cutters, trenchers, tillers, brooms, etc... and it is activated with an electric solenoid and has motor spool function, so it lets motors coast when shut down. The other section supplies (as I recall) 4gpm @ 2500 psi, and it supplies power to the steering valve, out a power beyond port, to a 3 section valve bank that supplies the FEL up/down, dump curl, and aux PTO. The aux PTO is for things like power-angle snow plow, grapple jaw, 4-in-1 bucket... things like that, all cylinders, and no motors. Steering has priority over FEL and aux PTO functions.

So my setup uses the engine's standard shaft, plus a stub shaft on the other side. Newer models have the two-section pump piggybacked off of the variable volume tram pump, so everything is on one side of the engine. I suppose it saves on the cost of the stub shaft setup, at the expense of having to find a tram pump with the piggy back shaft for more sections.

 

[WranglerX]

Might want to get away for "tractor" specs and look at it in more of engineering concepts....

Hydraulics and Pneumatics

Dale