Horsepower in a 2.5 liter engine?

Answer 1

Piston engine horsepower of any kind cannot be calculated by engine displacement alone.

Horsepower is a broad term which simply denotes 745.7 Watts of power output by any given mechanical or electrical device.

First off, there are three (3) types of horsepower involved in a piston engine, they are:

1.) Indicated Horsepower (IHP).

2.) Brake Horsepower (BHP).

3.) Friction Horsepower (FHP).

Indicated Horsepower = Theoretical horsepower of a piston engine at a given RPM.

Brake Horsepower = Available useful horsepower output of a piston engine's crankshaft while at a given RPM and Torque.

Friction Horsepower = Horsepower needed to keep a piston engine running at a given RPM.

Indicated Horsepower = Brake Horsepower + Friction Horsepower.

For all intents and purposes the Brake Horsepower (BHP) of a piston engine is the useful horsepower which should be used to measure a piston engine's power output. BHP is derived by the combined measurements of Ft-Lbs Torque and RPM at the crankshaft.

To answer this question correctly using just engine displacement alone is not possible. More engine data is required

Therefore, I will post an example of an existing automotive gasoline piston engine and demonstrate how to calculate its Brake Horsepower.

Most if not all automobile piston engines are based on a 4-stroke piston duty cycle. This means that one duty cycle of each cylinder's piston is based on: Intake, Compression, Power & Exhaust piston strokes.

The significance of mentioning this is that the combustion powerstrokes per minute (N) are exactly half of the engine's RPM in regards to a 4-stroke piston engine.

Take for instance a Chrysler Mopar 2.5 Liter In-Line 4-Cylinder, 4-Stroke, Gasoline Automotive Piston Engine with the following specifications:

Engine: Chrysler Mopar / In-Line 4-Cylinder / 4-Stroke / Gasoline

Displacement: 2.5 Liters (2,500 cc)

Piston Bore Diameter: 3.44 inches

Piston Stroke Length: 4.09 inches

Maximum Torque Output: 135 Foot-Lbs @ 2,800 RPM

Compression Ratio: 8.9:1

The most accurate method to calculate BHP based on Maximum Torque output on automotive piston engines is the following:

BMEP - 4-stroke = [(150.8 x Torque) / (CID)]

BHP = [(BMEP x L x A x N x K) / (33,000)]

BMEP = Brake Mean Effective Internal Cylinder Pressure in PSI per powerstroke.

Torque = Maximum torque = 135 Ft-Lbs @ 2,800 RPM

CID = Total Cubic Inch Displacement = [(2,500 cc / 16.387)] = 152.56 cu. in.

L = Piston stroke in feet = [(4.09 / 12)] = 0.341 feet.

A = Piston head area in sq. in. = [(3.14159 x (3.44 / 2)^2] = 9.29 sq. in.

N = Number of powerstrokes per minute = 4-stroke = [(2,800 / 2)] = 1,400 p/m.

K = Number of Cylinders = 4.

BMEP - 4-stroke = [(150.8 x 135) / (152.56)] = 133.44 PSI.

BHP = [(133.44 x 0.341 x 9.29 x 1,400 x 4) / (33,000)] = 71.7 BHP.

So this particular automotive gasoline piston engine will develop a Brake Horsepower of 71.7 BHP while at its Maximum Torque RPM of 2,800.

[65 / 234]^3 x [Vehicle Weight in Lbs] = 71.7

0.0214334705 x [W] = 71.7

W = [(71.7) / (0.0214334705)] = 3,345.24 Lbs Vehicle Weight

If this engine maintained a steady 2,800 RPM with a Brake Horsepower output of 71.7 BHP it will keep a 3,345.24 Lbs vehicle moving at a steady cruise speed of 65 MPH.

Many different 2.5 Liter gasoline-powered, 4-stroke, 4-cylinder automotive piston engines all operating at the same RPM can have completely different BHP & Torque outputs.

Useful horsepower (BHP) of a piston engine is primarily a function RPM and Torque and cannot be calculated by engine displacement alone.

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