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Horsepower question

  1. Aug 6, 2011 #1
    Probably a stupid question, but here goes:

    Horsepower always measures the amount of work an engine can perform in one minute and can be converted into watts by horsepower X 746 = engine power in watts, right?

    I ask this because I was wondering about something. According to http://www.gizmag.com/volvo-premieres-worlds-most-powerful-truck/10724/" has 100 hp. I'm pretty sure this truck is designed to haul a lot more than 2-7 times the weight of those cars (the legal limit for truck hauling in the US is I think 36 tons - 24 times the weight of the Leaf), so it seemed kind of odd that big truck engines would have such relatively low hp if it translated directly to engine output in watts and I was wondering if there was more going on here.

    According to http://www.howstuffworks.com/question381.htm" big diesel engines usually have long strokes with high torque but few piston cycles per minute while smaller engines tend to have more piston cycles per minute, but I don't think that would matter here (the horsepower is work per minute and even a long stroke engine should have many cycles per minute, so you're just seeing the same energy divided over many vs. fewer cycles).

    Just wanted to check that I can plug in 700 X 746 = 522 kW for the big truck's engine and this would be an accurate answer. Any problems here?

    Last edited by a moderator: Apr 26, 2017
  2. jcsd
  3. Aug 6, 2011 #2
    Looks good, unless you want the truck to do 0 to 60 mph in 3 seconds.
  4. Aug 6, 2011 #3


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    The key comparison between the output of the Ferrari and the Volvo truck is that the truck develops 3150 N-m of torque compared to the Ferrari's 485 N-m. Because the Ferrari has a higher revving motor (7750 rpm), its power output is due not so much to the amount of torque developed as to the higher rpm at which it is delivered. The truck's engine, on the other hand, is designed to deliver its huge torque at much lower rpms. The truck's engine also is designed to last for a longer period of time in service than the Ferrari's.
  5. Aug 6, 2011 #4
    Couple of interesting additional tidbits that you may or may not know:

    Larger displacement-lower rpm engines are more efficient than smaller displacement engines of the same HP because: Pumping losses. All engines are air pumps first of all. Compression ignition (CI) engines pick up a little here because they run WOT (for air) all the time. (Wide Open Throttle.)

    Biggest: ring friction: goes up with piston velocity. Largest mechanical loss in IC reciprocating engines.

    An aircraft engine (recip.) will be more efficient at any given output (say 75% power) at a higher altitude than at sea level. Reason: At, say 8,000 feet, it will require full throttle which will reduce the pumping losses. (Bigger opening for the air to come in.) You have to limit the RPM's (variable pitch propeller) for this to work well.


    There is a great book that has page after page of this stuff in it. "Internal Combustion Engines, Analysis and Practice." Jennings and Obert The copy I have was published in 1944, but there were several more editions, much later dates. Check Amazon if interested.
    Last edited: Aug 6, 2011
  6. Oct 1, 2011 #5
    Not so fast, Its true that 745.7 watts= 1 electrical hp, but when taking mechanical work and converting into electrical output you have a waste.

    Most engines need to run at a 3600 rpm's (60hz) for a single phase generator, this is also the wasted energy to keep the rpm's at a constant. A good rule is for each 1kw, it takes 2 mechanical hp.

    This is also why you see a 5kw home generator using 9-11 hp engines depend on surge output.
  7. Oct 2, 2011 #6
    I'd check over that reasoning again, if I were you.
  8. Oct 2, 2011 #7


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    The OP's question doesn't appear to me to involve any conversion to electrical power. He appears to just want to know the SI equivalent output of a truck's engine.
  9. Oct 2, 2011 #8


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    I think you're confusing conversion efficiency with conversion units.
  10. Oct 2, 2011 #9

    So your saying that 5 mechanical horse power will produce 3.7 kw?
    745.7*5=3728.5 watts..

    Please explain yourself, and why you think you can create this electrical energy without a loss.

    You will have a mechanical loss, due to friction, heat loss and constant load on the motor.

    I would like to see you make 7457 watts with a 10 mechanical hp.....

    Like I have said, every 1kw of energy will need about 2hp, yet your saying is 1 mechanical hp will make 745.7 watts of electricy.
  11. Oct 2, 2011 #10
    "Just wanted to check that I can plug in 700 X 746 = 522 kW for the big truck's engine and this would be an accurate answer. Any problems here?"

    700hp will produce about 350kw, not 522kw!
  12. Oct 2, 2011 #11


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    746 watts equals 1 hp, the same way that 12 inches equal 1 foot. They are just different measurements of the same thing.

    Converting energy from one form to another is a different issue. Sure you can't convert 1 watt of mechanical energy into 1 watt of eletrical energy either, with any real-world machine (you lose some energy as heat, etc), but that doesn't mean that "1 watt is not equal to 1 watt".
  13. Oct 2, 2011 #12


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    When comparing the power to weight ratio of a car and truck, it might seem as if the truck engine is woefully undersized. One key difference however is that the truck engine can produce the rated power on a relatively continuous basis whereas a car engine can't generally be run at full power for very long. You couldn't put a car engine in a truck and expect it to last very long, even if they were both rated at the same power.
    Last edited by a moderator: Apr 26, 2017
  14. Oct 2, 2011 #13


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    No, 5 hp is 3.7 kW. No one said anything about converting to electrical energy.
  15. Oct 3, 2011 #14
    I think everyone else has done a fine job of explaining already. This is a numerical conversion from one unit system to another. Not any physical process.
  16. Oct 3, 2011 #15

    I have this rant with you, calling me a scammer.....

    Who the hell do you think you are? I find that most Engineers milk people for more money then ever before.

    The Chevy Volt is just one of a true scams, and thosands more out there. Just shows how you guys will say anything just to keep milking us for money. a wastful project....

    When someone don't speak up about a fales project like the Chevy Volt scam, being part of a party of engineers, you are also part of that scam, and lies also.

    Remember when GM claimed 230mpg, in the new Chevy Volt. You guys knew that was not true, and yet you did not make any complaint did you. Why? Becasue you would have been ban for the party of lies (mechanical engineers)

    I would like to see your projects, and make fun of them too..

    I'm sick of these people that think they know so much, and yet energy is wasted because they love to lie just to make more money.

    The real truth is engineering is not what it should be, its a part of true lies that controls every in-efficient stuff we have today.

    You sir, have no room to talk...
    Lies can send you to hell, lets see if you can put out that fire!
  17. Oct 3, 2011 #16
    Oh christ, here we go. I said yours was one of the only ones that didn't seem like a scam.

    Last edited: Oct 3, 2011
  18. Oct 3, 2011 #17
    We'll maybe I jumped the gun.....I get alot of heat on my projects and looking for more efficient engines...I'm sorry if I called you out.....maybe I need a break.

    Working on all the calculation for a gaslline version of the Rotary Piston Engine can get to anyone, I'm studying a great book on "The Internal Combustion Engine in Theory and Practice, Volume1" to get the data you ask for......

    Again, I'm sorry......My apology xxChrisxx
  19. Oct 3, 2011 #18


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    Calm down Tommy. You're the only person in this thread who's mentioned anything about electricity, so your only quarrel is with yourself.
  20. Oct 3, 2011 #19

    Ranger Mike

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    welcome..it is good to see some one who has passion in their beliefs ..also good to see some one who can step back and re-evaluate ..i gotta say...yours is one of the more refreshing starts i have seen in this forum.....
    hang in there

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