Differences in Horsepower

This week I’d like to go back to some basics. The other day I was reading an article about cranes and there was a section on horsepower. It dawned on me that even though it is a term we use often, it is not a term that we all really understand. Do we know the difference between base, brake, and chassis horsepower? What factors are used to calculate horsepower? This tip will attempt to explain all those and more. So as usual, grab a drink, sit back, relax and let’s learn together.

Horsepower Defined

The term horsepower was established and defined by an engineer named James Watt in 1782 as a way to rate the power of a steam engine. Watt realized that a horse could perform 33,000 foot-pounds of work every minute. As a result, Watt theorized that one horsepower is based on the ability of an animal to raise 330 pounds of coal, 100 feet in a minute. The mathematical expression is shown as:

1 Horsepower = (33,000 ft/lb) / Minute

How do we get from a horse dragging a weight to horsepower rating a mechanical device, specifically an engine? Read on!

Base Horsepower

Engine horsepower is the amount of power transmitted to the pistons as a result of combustion. It is not determined on a dynamometer but mathematically calculated. Engine base horsepower is as follows:

Base horsepower = (P x L x A x N x C) / 33,000

where:

P = Mean indicated pressure in PSI

L = Length of Stroke in Feet

A = Piston cross section area of the piston in feet

N = Number of power strokes / cylinder / minute

C = Number of cylinders in the engine

 

Let’s examine a four stroke, 8-cylinder gasoline engine, with a 6 inch bore and a 6 inch stroke.

P = Average cylinder pressure (185 PSI)

L = Stroke in Feet (6 inch / 12 inch = .5 Feet)

A = Bore Diameter (6 inches), Bore radius (3 inches)

     Circumference = 2 x pi x r = 2 x 3.14 x 3 inches = 18.84 inches

N = 3000 rpm / revolution = 1500

C = Number of Cylinders = 8

Base Horsepower = (185 x .5 x 18.84 x 1500 x 8) / 33,000 = 634 HP

Base horsepower may also be expressed as indicated horsepower.

Brake Horsepower

The engine described above is stout, but base horsepower is not necessarily a number that mobile equipment customers need to worry about. Our customers are more concerned with something called brake horsepower. This is the horsepower the engine produces at the flywheel, and is typically 15% lower than base horsepower.

Brake horsepower takes into account frictional efficiency losses an engine encounters during operation. It is calculated using a dynamometer, which is a braking device that measures horsepower developed during operation. Let’s look at the calculation for brake horsepower:

Brake Horsepower = (Force x distance x time) / (33,000 x 1 ft x 1min)

Remember that engines rotate, so the distance is circular. This simplifies the equation as follows:

Brake Horsepower = [Torque (1 lb - ft) x Time (rpm)] / 5252

Torque is the product of force on the crankshaft and its perpendicular distance of the piston from the shaft center. Peak torque occurs when cylinder pressures are at their highest and are proportional to the length of the piston stroke. Torque values are typically published by the engine manufacturer, and for the engine above are 1,500 lb-ft at 2,000 rpm. Brake horsepower calculates as follows:

Brake Horsepower = (1500 lb ft x 2000 rpm) / 5252 = 571 HP

Remember, the dynamometer used to calculate brake horsepower considers the engine only and is measured at the flywheel.

Chassis Horsepower

People often confuse brake horsepower with chassis horsepower or horsepower at the wheels. Horsepower at the wheel is technically called chassis horsepower. It is brake horsepower (flywheel) minus the efficiency and frictional losses experienced as the power travels from the flywheel, through the transmission, driveshaft and differential, to the wheels. It is measured on a dynamometer that sits directly beneath the drive tires, differing from the dynamometer that is used to calculate BHP, which attaches to the engine flywheel.

Summary

As you can see, horsepower can be a complicated subject and is dependent upon where it is measured. One thing we did not discuss is that brake and chassis horsepower are dependent on how well the engine is running. Fuel supply, compression, and timing must be optimal to obtain the maximum value. Measure gives an honest assessment of the actual engine power when compared to design parameters and may indicate when a rebuild is needed.

I hope you enjoyed reading this week’s Tip and learning a little about horsepower and the various parameters. Also check out our additional content on our new Mobil Lubricants North America LinkedIn page. Until next time!

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