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Author Topic:   Horsepower vs Electrical Power
jimh posted 04-22-2006 01:49 AM ET (US)   Profile for jimh   Send Email to jimh  
One horsepower is about the equivalent of 750 watts of electrical power.

I got a good demonstration of this relationship this afternoon. The Cummins Diesel folks fired up our new emergency generator at work. It is a half-megawatt generator, i.e. 500-kW--but it sure sounds cooler to call it a half megawatt, doesn't it?

If 1-HP = 750 watts and 500-kW = 500,000 watts, then in order to generate 500-kW, the diesel has to have about

HP = 500,000/750
HP = 666

To give the engine some margin it is rated at 750-HP!

The engine is a straight six cylinder with 15-liter displacement. To get the frequency of the generated power to be 60-Hz, I believe the engine runs at 1,800-RPM.

The diesel contractor was on site today to start the engine up, and the electrical contractor was there to check the generator and change over equipment. The big Cummins came to life and ran smoothly. What a beast. We had no electrical load, so it was just running at very low throttle, and still it had a real roar to it.

If all goes well we will test the system under load on Monday, and run our technical and emergency lighting plant off the generator. This should put about 300-kW of load on it. I anticipate a little more of a rumble from the big beast.

jimh posted 04-22-2006 02:08 AM ET (US)     Profile for jimh  Send Email to jimh     
The fuel system of this big diesel is also interesting. We have a large in-ground fuel tank. This feeds a smaller "day tank" in the generator enclosure. A large electrically operated lift pump pulls fuel from the ground tank to the day tank. I think the motor is rated at several horsepower!

The diesel draws fuel off the day tank to its high pressure pump which feeds the common-rail injector system. Excess fuel is returned to the day tank. The day tank overflows into a return to the ground tank.

If you figure the Brake Specific Fuel Consumption of this diesel is around 0.4-lbs/HP/Hr, and assume it runs at 666-HP under load, this implies

666 x 0.4 = 266.4-lbs of fuel per hour

Diesel weighs about 7-lbs/gallon, so the engine should burn about

266.4/7 = 38 gallons per hour

In the summer of 2003 there was an Edison blackout in our area for over 48-hours. To run the generator for that time would require

38 x 48 = 1,824 gallons of diesel fuel.

Chuck Tribolet posted 04-22-2006 08:44 AM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
Watts is a measurement of power, not just electrical power.
Outboards are rated in KW in some parts of the world.

And it would be clearer if:

HP = 750 watts

instead read

1 HP = 750 watts

(and, folks, don't gets confused because 750 shows up in two
independent ways in Jim's example.


jimh posted 04-22-2006 09:52 AM ET (US)     Profile for jimh  Send Email to jimh     
Chuck, I don't see where "750" is used except as a factor relating horsepower and electrical power. Can you explain what you meant?

I changed my presentation slightly and incorporated your suggested of "1-HP =" in the text. Thanks.

The speed of the engine determines the frequency of the generated power. The goal is to obtain 60-Hz power:

1 HZ = 1-cycle per second

1-minute = 60 seconds

Thus 1,800-RPM = 30-Hz

The generator is probably constructed with two poles in its rotating coils, so that each rotation produces two cycles of output. (Actually, it is a three-phase generator so it probably has six poles.) The alternative would be to either gear the engine up 1:2 to turn the generator, or to run the engine at 3.600-RPM. In the former case there would be a loss of torque, and in the latter case the engine speed would be too high. A big diesel engine likes to run around 1,800-RPM much more than at 3,600-RPM.

The precise relationship between horsepower and electrical power is

1-HP = 0.7457 kW

This is also interesting in terms of the amount of power needed to run the charging system in an outboard. Bombardier rates some E-TEC engines as having an 1,800 watt alternator. This implies the engine is providing

1,800/745.7 = 2.4-HP

to turn the alternator. In a 12-volt system, 1,800 watts is 150-amperes of current.

jimh posted 04-22-2006 12:09 PM ET (US)     Profile for jimh  Send Email to jimh     
Chuck's observation that horsepower and watts are both units of measurement of power is a good one. And indeed the kilowatt can be used to measure mechanical power, as in the rating of an outboard. I was giving some consideration to whether or not the CONTINUOUSWAVE website ought to begin to employ the kilowatt as an alternative unit of power. For example, when mention is made of an outboard engine's power, such as 225-HP, I considered whether or not the website should include the equivalent rating in kilowatts, in this case 300-kW, as an alternative dimension, much like is done for linear measurements like feet and meters.

My finding on this issue was that even in Europe where the metric system has been in use for more than a generation, people still tend to refer to mechanical energy output from engines in terms of horsepower, and, guided by this, I decided that it would not be particularly beneficial for readers to have the kilowatt rating of engines given whenever the horsepower rating was mentioned.

Tom W Clark posted 04-22-2006 12:14 PM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
"Outboards are rated in KW in some parts of the world"

Actually, outboard motors are rated in kW in ALL parts of the world. All new outboards sold in North America today have their ratings expressed on both units of measure.

boatdryver posted 04-22-2006 12:16 PM ET (US)     Profile for boatdryver  Send Email to boatdryver     
The fuel consumption estimate of 900 gal/24 hours fits with a recent experience my son-in-law had in Saudi Arabia. He and his crew were sent from the US to a city on the Red Sea to install 150 televisions and several home theaters in a second or third palace owned by a member of the Royal Family where the outside air temperature was 120 degrees at noon.

They were told to bring coats. Why, you ask? Because the interior where 500 Foreign workers had been working for months was kept at 54 degrees thanks to two V-12 Diesel Caterpillar generators, each burning 1000 gal per day.


Chuck Tribolet posted 04-22-2006 12:17 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
Jimh: To give the engine some margin it is rated at 750-HP!


davej14 posted 04-22-2006 03:34 PM ET (US)     Profile for davej14  Send Email to davej14     
Interesting, I didn't realize that these large generators depended upon the diesel RPM's for the output frequency. That is the most direct way to get the power out but if the motor speed varies you risk frying all the electronics connected to it. I would have thought that the output would be rectified to a DC bus which would power an inverter. You could still run with 95+% efficiency and have no dependence upon the motor RPM's. As an added benefit you could add power factor correction and get the maximum usable power out of the big guy.

Jimh, you may want to have someone check the power factor when you do your factory test. By adding some relatively inexpensive power factor correction capacitors you will significantly reduce the load on the motor and use less diesel.

Bulldog posted 04-23-2006 01:02 AM ET (US)     Profile for Bulldog  Send Email to Bulldog     
I'm responsible for a large hospital's generation plant with eigth generators, 5 of which are Caterpillar 1250KW diesel units in common bus setup, that have to run at 1800 RPM to make 60 hertz and each one burns about 100 gallons an hour. Our tool room portable 2500 watt Kohler generator also has to run at 1800 RPM to make 60 Hertz. How much diesel do I have on site for generation....about 42,000 gallons, which I'll burn at about 9,050 gallons a day, so after about 96 hours, the tankers better start coming!......Jack

Speedo66 posted 04-23-2006 08:51 AM ET (US)     Profile for Speedo66  Send Email to Speedo66     
A 15L 750HP Diesel! I'd love to know the torque rating of that motor. Diesels are known to have incredible amounts of torque, a truer rating of ability to move objects.
Chuck Tribolet posted 04-23-2006 09:07 AM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
That would be one MONDO inverter, esp. it made the nice sine
waves that electronics like. Far easier to regulate the
motor RPM, with a simple back up tach, and, perhaps most
importantly, the tech's well tuned ear.


jimh posted 04-23-2006 09:24 AM ET (US)     Profile for jimh  Send Email to jimh     
Jack--I figured you'd be familiar with this kind of power generation!

Here are the specifications:

Here is a data sheet on the QSX 15 G9 nonroad diesel, which is turbo-charged and charge air cooled:

The data sheet shows fuel consumption at 36.7 gallons per hour when running at 755 BHP--that is quite close to my seat-of-the-pants estimate of 38 gallons per hour.

We use this generator as a standby emergency power source.

Bulldog posted 04-23-2006 06:30 PM ET (US)     Profile for Bulldog  Send Email to Bulldog     
Jim, looks good, to give you an idea the Cat generators that we put in this year, were 26,000 lbs each! Just be sure that the unit is maintained , hospitals are required to transfer load monthly and we run them up to operating tempature every week. Labor just to check them weekly is about 6 hours....Jack
Jerry Townsend posted 04-23-2006 07:53 PM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
Interesting information - and I'm glad that the discussion has turned more positive.

Now, your 36.7 gal/hr at an average of 7.38 lb/gal means that the diesel engine is burning 270.93 lb/hr of fuel - and at an average of 19,250 Btu/lb means that the input energy is 5,215,400 Btu/hr. And - Horsepower, Watts - and joules, Btu/lb, ft-lb/time - are all units of power and used as such since day one. Now, the 500,000 Kw is 1,706,700 Btu/hr. So, the efficiency of that DG set is about 32.7% (1706700/5215400)- which is the ball-park for internal combustion engine/generator units.

Jack - thanks for your information. Those Cat engines are GOOD!! I would guess that all eight DG sets are not running simultaneously - but that there is also redundancy in your emergency power supply system. If multiple DG sets are running simultaneously - how are the DG sets synchronized?

Are these DG sets use air start engines? ----- Jerry/Idaho

Moe posted 04-23-2006 10:56 PM ET (US)     Profile for Moe  Send Email to Moe     
Torque = (HP x 5252)/rpm

755HP @ 1800 rpm = 2,203 ft-lbs


Bulldog posted 04-24-2006 11:15 AM ET (US)     Profile for Bulldog  Send Email to Bulldog     
Jerry, I have two generator plants in one building, one has three 1250KW Cats and the other has two 1250KW Cats. Both systems Parallel the generators together into basically one hugh system. There is a 24volt computer controlled Paralleling system that actually runs off the best battery bank of the three generators, which it picks itself, on power fail all generators start, first generator to 1800 RPM is the setpoint and the other generators will run until the system synchronizes them, then close their breaker to parallel with the others. It is pretty neat to be in the room during an outage as the system involves 21 breakers closing and banging. Code requires the whole system to be on line within 10 seconds, we are at 8 seconds, installing a second starter, and using a second bank of Cat high output batteries at each generator, took a second off our times. We always had electric starters, although I thought about adding air starters instead of the second electric. The set with three generators has one redundent generator, which will start then shut off after about ten minutes, both systems also load shed if generators start failing, dropping the non critical first, then we turn off the power to the patient support and the last thing off is the exit lights.
There is a lot of expense to maintaining the generators at a hospital, because the two oil tanks could each take out a set of generators we have the oil cleaned once a year, by filtering it through a set of filters and water removing equipment. Batteries are replaced every year, cost just to replace the batteries $8,000. Oil is changed yearly also, each generator has about a 70 gallon sump, so we get about 350 gallons at one shot! I donate the used engine oil, and batteries to local business people. So what does this have to do with Whalers, think of the twin engine Revenge I have,I know that the batteries and that one fuel tank are the weak spots!.......Jack
davej14 posted 04-24-2006 08:45 PM ET (US)     Profile for davej14  Send Email to davej14     

That is an impressive system. $8K/year for batteries makes replacing my Optima battery every 5 years sound like a good deal. No wonder my medical insurance bills are so high ;-)

How many batteries are you replacing every year? Would it be worth considering replacing them with ultra-caps. It would be more costly in the short run, but they have much greater life and no maintainence. Some of the larger wind turbine manufacturers are replacing SLA's with ultra caps for their blade pitch control backup.

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