Engine rating in kwh

Can someone--- maybe RickB--- explain to me how the kilowatt hour power rating for engines works?* Horsepower I understand (more or less). But with more engines--- particularly in commercial boat engine sizes--- being rated in kwh instead of hp, I'm curious how one uses a measure of electricity to describe the power output of a diesel engine.

Marin wrote:But with more engines--- particularly in commercial boat engine sizes--- being rated in kwh instead of hp, I'm curious how one uses a measure of electricity to describe the power output of a diesel engine.

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Watts*are a measure of power, not strictly an electrical measure. Remember it was Watt who figured out how much a horse could pull in a given time so he could collect royalties on the savings his steam pumps provided mine operators. So there is a direct connection with James.

It just expresses the power in metric or derived SI (Standard International)*units.

The difference is a horsepower in metric or SI units is just over 735 Watts while a regular old*mechanical horsepower is just under 746 Watts. One electrical horsepower is exactly 746 Watts.

It's the new world order but the propeller shaft can't tell the difference.


*Edit:* I just noticed that you used kwh.*Engine power output is rated in kiloWatts and fuel consumption is measured*by the*kiloWatt hour. An engine can produce X kW at any moment, it will burn Y grams of fuel to produce a kW for one hour.

That is*described as*brake specific fuel consumption or bsfc and is pretty much the only way to compare engine fuel efficiency.


-- Edited by RickB on Wednesday 13th of October 2010 02:10:38 PM

Years ago when I was selling Ag equipment for a living we held a seminar where a European gave a talk on tractors.
Over there engines are rated by KW. He kept talking about different KW size tractors. After the meeting farmer came up and asked me " are they really using electric tractors over there"
Steve W.

Rick--- Thanks for the explanation. I had not known that Watt had a connection with horsepower. So if I wanted to calculate the kw rating of an FL120 I would multiply 120 x 746 for 89,520 watts or 89.5 kw. Is that correct? (I understand what you said about a mechanical hp being a bit less than 746 watts but I used that number because it's there.)

Marin

The Cat website and detailed spec bulletins report ratings *in both bhp and kw. As Rick says the multiplier is 0.746. As long as I can remember the specs for Cat, Cummins, MTU etc*have been "dual" listed.

-- Edited by sunchaser on Wednesday 13th of October 2010 06:37:31 PM

0.746 or 746?

Marin wrote:

Rick--- Thanks for the explanation. I had not known that Watt had a connection with horsepower. So if I wanted to calculate the kw rating of an FL120 I would multiply 120 x 746 for 89,520 watts or 89.5 kw. Is that correct? (I understand what you said about a mechanical hp being a bit less than 746 watts but I used that number because it's there.)

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Yes, you got it. I also use 746 - everyone else I know does too - but there are degrees of geekdom.

Ah man, I am surprised you weren't familiar with* James Watt, being a techie in your own right. He needed a way to compare the work his pumping engines could do to the work that horses were doing so he could sell his engines. * It's a wonderful story that ties a lot of elements together and helps establish the relationship between a lot of the techiques and measures we commonly use. It really is a "rest of the story" thing.

When I was teaching steam propulsion I always started with the story of Watt and Newcomen and how they launched the* industrial revolution.* Being a history nut it was fun to make the connections for the cadets.

Edit: just saw the last post ...

One horsepower is equivalent to 746 Watts.

One horsepower is equivalent to .746 kW

One kW is equivalent to 1.34 horsepower

One Watt is equivalent to .00134 horsepower

*


-- Edited by RickB on Wednesday 13th of October 2010 07:02:19 PM

RickB wrote:
Ah man, I am surprised you weren't familiar with* James Watt...

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I am very familiar with Watt with regards to his steam engine work, but I did not know that he was the one who defined, or at least assigned a value to, horsepower.* I have always assumed that concept had been defined long before.

*

Then there is horsepower and brake horsepower

Then there is horsepower and brake horsepower


Go to any engine mfg site and you will find at least 4 HP ratings, from 24/7/360 for pumps and perhaps boats . to the really light duty ratings ,(tiny time) , which may easily be double the 24/7 rating

FF wrote: Go to any engine mfg site and you will find at least 4 HP ratings, from 24/7/360 for pumps and perhaps boats . to the really light duty ratings ,(tiny time) , which may easily be double the 24/7 rating

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**** "Then there is horsepower and brake horsepower"*

Except maybe for car manufacturers and home appliance dealers, engines are rated*by their shaft output*in brake horsepower. The power output is measured with a "brake."

In the olden days, again started by Watt as a means to compare the output of steam engines from his competitors, they used a voodoo measure called "nominal horsepower" that was based on the piston area and piston speed.

Indicated horsepower was calculated (still is) by measuring the cylinder pressure during a full "cycle" and calculating the mean effective pressure. Recording indicator readings are a regular procedure on large marine diesel engines (as it was on steam engines)*but thankfully the old spring and string method is long gone and a handheld or permanently installed device provides lovely printouts of cylinder performance.

The newer engine selection guides, CAT for example, provide engine output figures for metric horsepower (mhp), brake horsepower (bhp), and brake kW (bkW)

FF's apples to avacados comparison has nothing to do with the*measurement*of horsepower. It is a factory defined"service factor" that relates to the potential life of the engine and their willingness to provide warranty service.


-- Edited by RickB on Thursday 14th of October 2010 08:16:12 AM

If you like the history of how things came to be, there is an old book I have called Connections by James Burke. It was published in 1978 and goes way back to things invented in ancient Egypt and other places, then goes through time and explains how they affected some of the common machinery we see today. For instance, early mining efforts eventually lead to the invention of the spark plug and carburetor which leads to Henry Ford and beyond. Volta, Venturi, Newcomen, Watt and many others had a hand in this.
On this day in 1066, William the Conqueror defeats Harold at the Battle of Hastings; his advantage was the stirrup used by the cavalry which enabled them to hold a shield in one hand and make use of the lance with the other. One little invention helps to change history, facinating.
Anyway, in order to avoid being moderated, there is stuff on boats in the book too.
Mike
Baton Rouge

The derivation of horsepower has all sorts of claimants. Many centuries ago,* horsepower was used by Spanish miners to determine how many gallons per foot of wheel rotation could be bucket "lifted" by a horse. The deeper the mine, the more the buckets and the more horsepower required. Horses were selected and bred for this application. A "version" of torque entered into the equation by changing wheel size and adding horses. If the mine was too deep, pump stages were used by moving horses and wheels*underground. In many cases the horses never saw the light of day.

The Spanish miners of 1500 AD*got some of their horsepower pumping ingenuity from*pictures showing how the Romans did it 2000 or more years*ago.

Unless you are dealing in larger commercial duty engines there is a huge HYPE to lots of "HP" ratings.

SAE may not have mufflers alternator and even the water pump powered for the peak rating.

The Marine folks with smaller motors are usually realistic on the lower power ratings , the "pleasure " may be optomistic.

Back in the PM days Rick sent this,,,

http://www.ivecomotors.com/uploads/IM_EDITORIAL/20060403/1073751420/NEF 150_P3P04N002E.pdf

AS a good look at a common Mfg info .

AS it contains fuel burn vs rpm vs Hp charts it is far better than the crap "prop Graph" some mfg offer to confuse folks.

Still with only straight line graphs, not a useful graph like a fuel map it, will still be an approximation , if the boat is to be optomised at more than one cruising speed.

-- Edited by FF on Friday 15th of October 2010 05:16:20 AM