If a gas engine produces 9.9 hp what motor size would be the equivalent

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B8rez 2g4

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Jun 21, 2021
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In order to do this:


Preferably a mass produced, widely available "inexpensive" motor option.

Voltage ? Amperage ?
 

MBF Dan

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May 14, 2021
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1 hp=746 watts. So 9.9 X 746 = 7.4kw electric motor
The issue is you actually need to know the torque and rpm requirements for the load. Often gas motors are oversized to provide the torque at lower rpms. An electric motor has close to full torque at zero rpm so often you can reduce the motor size when sizing for the torque curve of an electric motor vs a gas motor but without knowing the load torque curve it is just a guessing game
 

Korishan

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Often gas motors are oversized to provide the torque at lower rpms.
Unless it's a diesel engine. But even still, it's slightly oversized to compensate for the spool up to get torque

So if replacing a gas or diesel engine with an electric one, you need to know how much max torque is required. Then calculate the size of the electric motor.
 

MBF Dan

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Diesel engines usually have a higher torque but an even more narrow torque band than a gas motor. So their sizing is still very different than an electric motor. That is why an 18 wheeler has so many gears to match the ground speed to the torque curve of the motor.
Electric motors typically develop a pretty constant torque regardless of speed. As a result a gas and diesel motor are typically over sized because peak torque is not always at the usable rpm range. It is really hard generalize how oversized they are. This is a problem I have invested a fair amount of research in because I am in the process of planing my electric tractor and don't have a good torque curve for the pump I am replacing the gas motor on. I wish there was a simple answer but i haven't found it.
 

B8rez 2g4

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... torque and rpm requirements for the load....

I believe the older 9.9's produce 4000-5000 rpm. While the newer 9.9's produce 5000-6000 rpm.
I keep a separate electric trolling motor when low speed high torque is needed.
Only goal is getting there fast. Top speed
 

MBF Dan

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I believe the older 9.9's produce 4000-5000 rpm. While the newer 9.9's produce 5000-6000 rpm.
I keep a separate electric trolling motor when low speed high torque is needed.
Only goal is getting there fast. Top speed
A gas motor is going to peak at torque somewhere around 1600 -2500 rpm. It will change some based on the compression ratio the higher the compression the higher the peak torque rpm rating. But at 5000 rpm the gas motor will probably have less than half the peak torque.
Using a quick internet calculator if the gas motor can generate peak 9.9hp at 2500 rpm it will make 20ft lbs of torque.
But an electric motor rated for that same torque will make it from almost 0 rpm to full speed so it will be a far more powerful motor.
I am planing on replacing a 10hp diesel engine running a 12gpm hydraulic pump that was orginal powered by a 18 hp gas motor and is arguably under powered now could probably use a 12 or 14hp diesel motor under heavy load with a 4kw motor. I think as close as I can figure that should handle the torque load of my hydraulic pump. But a straight peak hp conversion would suggest I need a much bigger motor. Which is why you really need the load torque curve to size the motor not the side of a motor someone determined had an acceptable torque curve because you don't know where on the curve they where limited.
 

JosiahLuscher

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Jun 29, 2021
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As a very vague rule of thumb an electric motor can replace a gasoline motor thats three times the horse power. - Said another way: your electric motor probably needs 1/3 the horse power of the gasoline motor.

The big problem I see with making these comparasons is that horse power as measured on a gas motor is such a strange thing. (IMHO)

A nissan leaf has a motor that at max produces 80kw - that's 107hp. But have you driven a leaf? Any electric car? The instant torque off the line is addictive and fun. It very different than a gas car. If you wanted to have as much fun in a small gas car I think looking at 300hp engine would be very reasonable. The leaf would still win until the turbo was up to speed, but then the tables totally turn and the gas car is fun, and the leaf is boring.

I stand by my 1/3 rule of thumb , but it's about the power needed to do the work, NOT the power that you want to have to make your machine work. That's where torque and rpm become so critical to examine. Good luck!
 
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