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How to Calculate Horsepower on a Small Gasoline Engine

By Pauline Gill
Motorcycle engines have many similarities with mower engines.

Small gasoline engines are the key power source for many mobile, portable or remotely located machines and implements such as mowers, pumps, generators and small transportation venues. Their key advantages are their ability to operate completely independently of external power sources, very high power to weight ratio, adjustable power output, or throttling, and relatively simple implementation. Principle disadvantages are relatively low overall efficiency and their inherent vibration and noise, especially when compared to inductive electric motors. Gasoline engine horsepower can be calculated based on several measurement methods.

Work Performed Calculation Method

Rotary mowers may have from 3 to 7 horsepower.

Define the force measurement application. A construction crane lifts a 400-pound weight at the steady rate of 6 feet-per-second. The crane is powered by a small gasoline engine running at 3,600 rpm. If the crane mechanism has a documented 86-percent efficiency, you can calculate the actual horsepower of this small gasoline engine.

Calculate the actual work performed by the engine. The work performed by the crane is 400 pounds X 6-feet/sec = 2,400 foot-lb/sec. The work performed by the engine is 2,400 foot-lb/sec/0.86 crane efficiency factor = 2,790.7 foot-lb/sec

Riding mowers may have up to 24 horsepower.

Calculate gasoline engine horsepower from work performed. By definition, one horsepower equals 550 foot-lb/sec. Therefore, engine horsepower = 2,790.7 foot-lb of work/sec/550-foot-lb/sec/hp = 5.074 hp. This engine would be rated at 5 hp at 3,600 rpm.

Electrical Power Produced Method

Emergency generators have engines very similar to this mower.

Define the electrical generator measurement scenario. A gasoline-powered portable generator has a maximum rated continuous output of 5,000 watts (5 kW-hr). If the generator efficiency is 84 percent, you can calculate the engine’s horsepower.

Determine the amount of power being supplied the generator to provide the 5,000-watt output. Dividing 5 kW-hr watts by the 0.84 (84-percent) efficiency factor to yield 5.952.4 kW-hr.

Calculate the gasoline engine’s horsepower output. Since 1 hp equals 0.746 kW-hr, then dividing the 5.952.4-kW-hr engine output by 0.746 equals 7.97 hp, close enough to call it an 8-hp engine with minor operational adjustment.

Water Pumping Method

These small gasoline engines can power many types of devices.

Define the pump application. A small gasoline engine is directly attached to a water pump for draining basements. If the pump moves 15,000 gallons-per-hour 24 feet higher than it originally was, and the pumping system efficiency including hoses is 32 percent, you can calculate the gasoline engine’s horsepower.

Determine the amount of work performed moving the water. If the weight of water is 8.33 lb/gallon, then 124,950 lb of water is lifted 24 feet vertically up and out of the basement. The work performed is 99,960 lb X 24 feet = 2,998,800 foot-lb/0.32 efficiency factor = 9,371,250 foot-lb of work in one hour.

Calculate engine horsepower by dividing the work by 1,980,000 foot-lb/hr/hp which works out to 9,371,250/1,980,000 = 4.73 hp.


Things You Will Need

  • Calculator

About the Author


Pauline Gill is a retired teacher with more than 25 years of experience teaching English to high school students. She holds a bachelor's degree in language arts and a Master of Education degree. Gill is also an award-winning fiction author.