# The Naked Scientists Forum

### Author Topic: Horsepower question  (Read 12538 times)

#### Somes J

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##### Horsepower question
« on: 06/08/2011 22:03:17 »
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 newbielink:http://www.gizmag.com/volvo-premieres-worlds-most-powerful-truck/10724/ [nonactive] the world's most powerful commercial (hauling?) truck has a 700 hp engine, now some sports cars have hp that is around half that (e.g. newbielink:http://en.wikipedia.org/wiki/Ferrari_California [nonactive] has 460 hp), and even a relatively modest performance car like the newbielink:http://"http://en.wikipedia.org/wiki/Nissan_Leaf" [nonactive] 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 newbielink:http://www.howstuffworks.com/question381.htm [nonactive] 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?

Thanks.

#### Geezer

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##### Horsepower question
« Reply #1 on: 06/08/2011 23:11:33 »
Yes, you are correct about the numbers. The difference between truck engines and automobile engines is that truck engines can produce their maximum horsepower for very long periods without wearing out. On the other hand, automobiles spend most of their time producing only a fraction of their maximum output.

At the other extreme from trucks you'll find dragsters that produce phenomenal amounts of power, but their engines have to be rebuilt after every run.

#### SeanB

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##### Horsepower question
« Reply #2 on: 07/08/2011 17:25:58 »
Also remember that a truck tends to take quite a while to reach top legal speed, most are quite slow on take off, and have a lot of gear ratios to keep the engine in the range it is comfortable in. Thus a car might have 4, 5 or even 6 gears, whilst a truck can have 16 or more gear ratios to use. As to the power, a truck tractor is amazingly overpowered when not connected to a load, and is capable of some serious acceleration from a standstill, often much more than any pocket racer. Look at truck racing to see just how much power a street legal unit can generate.

#### CliffordK

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##### Horsepower question
« Reply #3 on: 07/08/2011 18:13:23 »
The RPM at peak HP is different for the different engines.

Truck Diesels tend to have peak HP at 2000 to 2500 RPM.

Automobile peak HP may be at 5000 to 6000 RPM.

The torque on the Diesel engine at peak HP must be much higher on the big Diesel than the auto engine.

I'm not sure how that all works out.  Once it goes through the transmission, it should all equate to the same.  However, it is likely that the truck is driven much more near the peak HP, while the car rarely hits those high RPMs.

It sounds like the Nissan is over-powered.
My FIAT puts out a whopping 20 HP on a good day.

#### Geezer

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##### Horsepower question
« Reply #4 on: 07/08/2011 18:46:41 »

The torque on the Diesel engine at peak HP must be much higher on the big Diesel than the auto engine.

The whole torque business is a bit confusing (I think the car manufacturers have managed to get everyone really confused  )

The "torque range" of an engine is important because it determines what kind of gearing arrangement you need and how often you'll have to change gears, but assuming you have appropriate gearing, at the end of the day, the only thing that really matters is power.

Truck engines are made slow and large so that they last a long time. You could actually replace a truck engine with a much lighter high speed engine, but it would wear out far too quickly.

#### CliffordK

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##### Horsepower question
« Reply #5 on: 10/08/2011 08:04:16 »
Horsepower ratings in modern society are "interesting"...  to say the least.

Buy a 1 HP grinder from 50 years ago, and it is a BEAST.  A new one, and you can stop it with a feather.

I think the old motors are essentially 1 HP, 100% duty cycle.  The new ones are perhaps a 1 HP instantaneous.  Or, maybe a calculated value based on maximum amp draw.

Here is a quote I saw recently about a very small dozer from the 1920's (gasoline engine, also rated at 1600 RPM).
the 9-16 hp rating for the tractor proved rather conservative when it was tested at Nebraska in April 1922, yielding over 13 hp at the drawbar and nearly 20 hp on the belt.

I've got a John Deere Lawn Tractor.  I recently got a notice about a class action lawsuit for over-reporting the hp.  And, I'll say that it is EASY to bog it down.  In fact, depending on the fuel, it will barely spin up its own mower deck.

I wonder if some of the same thing happens with cars vs trucks.  A Corvette may be able to spin its wheels for a few seconds with 500HP.  Your semi will be able to push out 500 HP pulling a load uphill for miles.

#### Geezer

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##### Horsepower question
« Reply #6 on: 10/08/2011 20:52:50 »

I wonder if some of the same thing happens with cars vs trucks.  A Corvette may be able to spin its wheels for a few seconds with 500HP.  Your semi will be able to push out 500 HP pulling a load uphill for miles.

I don't think they would get away with that, although it would actually be quite difficult to find a situation where you could produce 500 hp in a car for any length of time. The transmission would probably overheat if you could. With a pickup truck it's not difficult to maintain maximum power for quite a while if you are pulling a trailer up a long incline. That's why they include additional transmission cooling when you buy a "tow package" option.

#### The Naked Scientists Forum

##### Horsepower question
« Reply #6 on: 10/08/2011 20:52:50 »