What factors determine the maximum power of a 4-stroke engine per cylinder?

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Discussion Overview

The discussion centers around the factors that determine the maximum power output of a 4-stroke engine per cylinder. Participants explore various equations and methodologies for estimating engine power, considering aspects such as fuel characteristics, engine displacement, and efficiency.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Exploratory

Main Points Raised

  • One participant inquires about an equation for calculating maximum power, suggesting factors like octane rating, RPM, engine displacement (CC), bore and stroke dimensions, and an assumed efficiency of 100%.
  • Another participant mentions using brake specific fuel consumption, displacement, and volumetric efficiency as means to estimate engine power.
  • A detailed calculation is provided for the Boss 302 Mustang, including assumptions about volumetric efficiency and air-fuel ratios, leading to a comparison of calculated power with published values.
  • Questions arise regarding the constant "3456" used in calculations, with clarifications provided that it serves as a conversion factor for unit consistency.
  • Participants discuss the variability of brake specific fuel consumption as a measured parameter, noting that it can vary in published values.

Areas of Agreement / Disagreement

Participants generally agree on the factors that influence engine power but do not reach a consensus on the specific equations or constants used in calculations. Questions remain about the constants and their application.

Contextual Notes

Some calculations rely on assumptions about volumetric efficiency and air-fuel ratios, which may not apply universally. The discussion does not resolve the variability in brake specific fuel consumption across different engines.

oldboyonrgv
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Is there any equation for calculating the maximum power possible for a given 4 stroke engine? per cylinder.
I guess it would involve:
Octain rating of fuel used
RPM
CC
some factor of bore X Stroke
And an efficiency of 100%!
 
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Yes, you can figure from brake specific fuel consumption, displacement, and volumetric efficiency. I have a book I will try to find and get back to you...
 
OK, here is a simple approach for rough estimating. Actual engine power is based on many more variables that this method does not account for.

Lets try this for the new Boss 302 Mustang rated at 444 hp at 7400 rpm.

Compute the air flow rate at peak horsepower:

CFM = (VE)(CID)(RPM) / 3456 = (0.85)(302)(7400)/3456 = 550 cfm

A volumetric efficiency of 0.85 was assumed.

The mass flow rate of air:

mass_air = density X CFM = 0.076 lb / hr X 550 cfm = 2506 lb/hr

Now air and gasoline burn at a ratio of about 15:1, so the rate fuel is burned:

mass_fuel = mass_air / AF ratio = 2506 lb/hr / 15 = 186 lb/hr.

We convert the mass_fuel to power with "book value" of brake specific fuel consumption:

brake_hp = mass_fuel / bsfc = 186 lb/hr / 0.37 lb /hr-hp

brake_hp = 452

which is reasonable compared to published value of 444 hp.
 
Cool but I have some questions:
what is the 3456 number ? is it just a constant that should be used?
The book value is that a constant...
 
oldboyonrgv said:
Cool but I have some questions:
what is the 3456 number ? is it just a constant that should be used?
The book value is that a constant...
The 3456 number is a constant "conversion factor" to make all the units work.

And the book value can also be considered constant although bsfc for gasoline is a measured parameter and you will see some variation in the published values.
 
oldboyonrgv said:
Cool but I have some questions:
what is the 3456 number ? is it just a constant that should be used?
The book value is that a constant...

12 cubed (converting feet to inches), multiplied by 2 = 3456
 
OK thanks guys got it.
 

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