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Re: JMOR - Explain Horses and Horsepower

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Author  [Modern View]

11-12-2012 08:15:51

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something to add.

The newer lawn tractor will have compression ratio around 8.5:1 compared to the 8N of 6.7:1

Using proportioning and the 8N as a base case
and the ratio of engine displacements, rated rpm & compression ratios

HP(lawn tractor) = 20 HP (8N) x (49.8/119.7)x
(3800 rpm/2000 rpm) x (8.5/6.7) = 20 HP(lawn tractor)

where 49.9 = lawn tractor engine displacement 119.7 = 8N engine displacement 3800 rpm = rated lawn tractor engine speed 2000 rpm = rated 8N engine speed 8.5 = compression ratio of lawn tractor 6.7 = compression ratio of 8N tractor

Therefore, the lawn tractor measures up to its specs at 20 HP based on the 8N as a base .

If the lawn tractor and 8N are fully loaded that is putting out every thing its has, both machines will do the same amount of work in one hour, assuming the power train efficiency in each machine are equal.

However, in a tractor pull the 8N will pull the lawn tractor all around the barn yard!

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11-12-2012 17:37:08

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 Re: JMOR - Explain Horses and Horsepower in reply to Bulldozer, 11-12-2012 08:15:51  
Why does compression ratio come into it? HP=C x torque x rpm. That"s independent of compression ratio.

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11-13-2012 06:52:28

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 Re: JMOR - Explain Horses and Horsepower in reply to Jerry/MT, 11-12-2012 17:37:08  
The torque an engine develops is a fuction of many variables but compression ratio of the engine is a major contributor.

Consider two engines with equal displacement, one has a compression ratio of 6.7:1 and another engine has a compression ratio of 11:1. Which engine will develop the most torque?

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11-13-2012 09:08:30

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 Re: JMOR - Explain Horses and Horsepower in reply to Bulldozer, 11-13-2012 06:52:28  
If two engines are rated at 20 hp and at the same rpm, it will produce the same torue at that rpm independent of the displacement or the compression ratio. Horsepower is horsepower.

An engine with a higher compression ratio will produce a given horsepower with less displacement than one with lower compression ratio. A large displacement low compression ratio engine can produce the same horsepower as small displacemnt high compression ratio engine. How ever if they are rated at the same horsepower and they have the same OUTPUT SHAFT RPM, THEY WILL PRODUCE THE SAME TORQUE.

Torque = HP/C X RPM!

These are facts. You can believe what you want.

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11-13-2012 07:42:05

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 Re: JMOR - Explain Horses and Horsepower in reply to Bulldozer, 11-13-2012 06:52:28  
Torque ~ engine displacement, compression ratio

For example what engine develops the most torque
having equal displacement, the aircooled gasoline engine with CR = 6.7:1 or the diesel engine with CR = 22:1 ?

The engine output shaft torque is proportional to the engine compression ratio but not directly portional but enters the equation as a major contributor.

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