Generator KW Rating Based Upon Engine Horsepower

[Ravenvalor]

Hello,

Does anyone know whether or not generator KW ratings are still based on the horsepower of the engine. It seems like the generator manufacturers are no longer displaying the engine horsepower as prominently as they used to.

Thanks

 

[steve66]

I'd say it depends as much on the alternator as it does on the engine HP.

The engine has to be able to turn the alternator of course, but if we can assume that's true (and it should be), than the KW and KVA ratings will depend on the current rating of the alternator.

As far as not seeing engine HP as often, since there are a lot of turbocharged generators now, I'm not sure comparing HP is always apples to apples.

 

[drcampbell]

An alternator requires the same mechanical power input, whether the engine is turbocharged or not.

But with credible ratings for the electric power output of the genset, it isn't important to know the engine's mechanical power output.
(assuming that we're talking about a genset, not piecing together random generators and engines )

 

[Hv&Lv]

Generator kw should be kw electrical.
engine kw should be kw mechanical.
while a kw is a kw is a kw, the formulas to figure the power isn’t.
engine losses are more so the generator engine may need to be 150% of the generator itself.

 

[Ravenvalor]

Generator kw should be kw electrical.
engine kw should be kw mechanical.
while a kw is a kw is a kw, the formulas to figure the power isn’t.
engine losses are more so the generator engine may need to be 150% of the generator itself.

I have a 10hp diesel engine running my 7.5kw alternator which I believe is the standard calculation for hp to kw design (1 hp - .75 kw).
However I believe some people recommend a formula closer to 1 hp - .5 kw for gasoline and diesel engines.

 

[Besoeker3]

Generator kw should be kw electrical.
engine kw should be kw mechanical.
while a kw is a kw is a kw, the formulas to figure the power isn’t.
engine losses are more so the generator engine may need to be 150% of the generator itself.

Wouldn't the engine be rated at its output so losses would already be accounted for?

 

[retirede]

Wouldn't the engine be rated at its output so losses would already be accounted for?

But the alternator rating is also output so its losses would not be accounted for.
One possibility: Given that everything is rated in nice round figures, they are conservative enough to account for the losses.

 

[Hv&Lv]

Older engines only had to meet the 85% requirement of declared hp. Somewhere about 5-6 years ago the standard changed and the newer engines have to meet 95% of declared hp.
A lot of engines, even at the 95% rating, is engine only rating. Add something to it such as a fan, or generator, or alternator and it goes down quickly.

 

[powerpete69]

From an engineering standpoint, KW should represent power at the shaft in a perfect world.
However, from the comments above, you will get slightly less at the shaft.
So apparently a 50 HP generator will struggle feeding a 50 HP motor. It's a sad day for engineering.

 

[kwired]

From an engineering standpoint, KW should represent power at the shaft in a perfect world.
However, from the comments above, you will get slightly less at the shaft.
So apparently a 50 HP generator will struggle feeding a 50 HP motor. It's a sad day for engineering.

Biggest struggle in many cases would be starting. You won't need a soft start controller though, the generator kind of does that function by not allowing the same starting current the utility probably would be able to deliver.

Now if that 50 HP motor is loaded continuously to 50 HP the driving engine may be overloaded while trying to deliver that load. Depending on design it may be able to deliver, but with additional heating or it may slow down and not ever quite deliver the demand.

 

[drcampbell]

... Add something to it such as a fan, or generator, or alternator and it goes down quickly.

Um, no. The definition of SAE Net Horsepower recognizes the losses incurred by "all the accessories necessary to perform its intended functions ".

SAE net horsepower definition | The Free Automotive Dictionary

SAE net horsepower according to the free Automotive Dictionary.

www.automotivedictionary.org

... It's a sad day for engineering.

It's not "sad", just the inherent detail-oriented nature of engineering work. There are a beezillion variables to consider, many of which are glossed over by sales & marketing people.
For example: Higher elevations result in less power being developed because each intake stroke sucks in less air. Likewise higher ambient temperatures. Except in some engines that are turbocharged and normalized. And it results in more power being available because less energy is needed to expel exhaust gasses into less-dense air and a single-speed belt-driven radiator fan consumes less energy in thinner air. Et al., et al., et al., et al., et al. ...

 

[powerpete69]

Thinner air. Good point. Ok, I'm only a little sad.

 

[kwired]

Um, no. The definition of SAE Net Horsepower recognizes the losses incurred by "all the accessories necessary to perform its intended functions ".

SAE net horsepower definition | The Free Automotive Dictionary

SAE net horsepower according to the free Automotive Dictionary.

www.automotivedictionary.org

It's not "sad", just the inherent detail-oriented nature of engineering work. There are a beezillion variables to consider, many of which are glossed over by sales & marketing people.
For example: Higher elevations result in less power being developed because each intake stroke sucks in less air. Likewise higher ambient temperatures. Except in some engines that are turbocharged and normalized. And it results in more power being available because less energy is needed to expel exhaust gasses into less-dense air and a single-speed belt-driven radiator fan consumes less energy in thinner air. Et al., et al., et al., et al., et al. ...

If that accessory item is directly related to operation of the motor and not an additional driven load for the final application.

Here on irrigation systems is common to have a diesel engine, alternator for the 12V engine electrical system - main output shaft drives the well. Often there is an additional 10-12 kVA belt driven alternator added to power the center pivot machine, or in some cases a hydraulic pump for some center pivots that are hydraulically driven systems. That alternator or pump is an additional load to the engine system.

 

[Besoeker3]

But the alternator rating is also output so its losses would not be accounted for.
One possibility: Given that everything is rated in nice round figures, they are conservative enough to account for the losses.

My point was about the engine losses. If it is called a 100HP engine, for example, you would reasonably expect it to be able to output 100HP.

 

[Hv&Lv]

Um, no.

http://www.smallenginewarehouse.com/Changes%20in%20Small%20Engine%20Horsepower%20Ratings.pdf

 

[Ravenvalor]

Older engines only had to meet the 85% requirement of declared hp. Somewhere about 5-6 years ago the standard changed and the newer engines have to meet 95% of declared hp.
A lot of engines, even at the 95% rating, is engine only rating. Add something to it such as a fan, or generator, or alternator and it goes down quickly.

Will you please be kind enough to explain how engine only rating is determined? Thanks...

 

[Hv&Lv]

Engine only. Gross hp. No air cleaner, exhaust or other accessories installed.
net hp. Air cleaner installed, exhaust installed, cooling fans, alternators, etc.

 

[Ravenvalor]

Engine only. Gross hp. No air cleaner, exhaust or other accessories installed.
net hp. Air cleaner installed, exhaust installed, cooling fans, alternators, etc.

Is HP calculated based on the size of the engine block?

 

[retirede]

Is HP calculated based on the size of the engine block?

No. Without getting too deep into design, it’s a function of bore, stroke and cylinder BMEP (Brake Mean Effective Pressure).

 

[LarryFine]

How to Calculate Horsepower: 12 Steps (with Pictures) - wikiHow

Horsepower is a unit of measurement that calculates power. It was originally coined by a Scottish engineer to compare the power of steam engines with the power of horses. This article will show you how to calculate the horsepower of your...

www.wikihow.com