Understanding the differences in horsepower

There is a world of difference among horsepower ratings, which can cause confusion when replacing engines and motors.

Engineers like to say that if the results do not match the math, then go back and check the math, because the sum cannot be less or more than the total of the parts. This brings us to the subject of horsepower and all the different types found on the farm. 

There is engine, PTO, drawbar, electric, hydraulic, and wheel horsepower. It is important to understand how horsepower is determined for each power source. For example, you may have a 5-hp. gas engine you want to replace with an electric motor. What would be the gas engine horsepower equivalent in watts?

Mechanical horsepower 

When it comes to internal combustion engines (of any fuel), the horsepower it generates is based on the amount of work it does over a period of time. The faster the engine can perform the same amount of work (load), the more horsepower it has. 

Horsepower in an internal combustion engine is calculated from its torque production working against a load that is called a brake (either water-based or electrical) via an eddy current. Though we refer to horsepower, the true metric of an engine’s power is its torque. 

Mechanical horsepower (determined on a dynamometer) is determined by multiplying torque by engine rpm and then dividing that sum by 5,252. Though this equation stays the same, there are different test protocols that control the inlet air, coolant, and water temperatures; barometric pressure; humidity; rate of acceleration; and how quickly the load is applied to the engine. 

In North America, engines are rated using the Society of Automotive Engineers (SAE) standard. Yet, there are multiple SAE standards. The difference among standards is found in the barometric air pressure, humidity, and ambient temperature values. These are called correction factors. So when considering an internal combustion engine, it’s important to determine which standard and correction factors were used to rate it. 

As air temperature is changed, horsepower is altered by about 1% for every 10°F. In simplest terms, engines consuming cooler air can produce more power. 

For example, Engine Company A may use an ambient air temperature of 50°F. in rating its engine, which results in 400 hp. But Engine Company B could employ a standard of 90°F. resulting in an advertised engine output of 390 hp. 

If you didn’t know the ambient air standard used when rating these engines, you’d assume that Company A’s engine is more powerful than Company B’s. But Company A’s engine would produce 384 hp. if tested under the same standard as Company B’s engine.

With the international presence of agriculture, you may see engines rated with correction factors and test protocols. Instead of SAE, the standards are from JIS (Japan) or DIN (Germany), or they are metric. 

There are also engines that rate mechanical horsepower in kilowatts (1,000 watts). To convert kilowatts (kW) to SAE horsepower (hp.), you multiply an engine’s horsepower by 1.34. For example, 500 kW × 1.34 = 670 SAE hp.

To convert from SAE hp. to kW, you multiply by 0.746. For example, 670 SAE hp. equals 499.82 kW.

Gross and net horsepower

Gross and net horsepower are measured at the crankshaft and determine the way the engine is configured. Up until 1971, auto and truck engines were rated using the gross scale. That is the total amount of horsepower generated by an engine without losses caused by an air filter, exhaust system, accessories, cooler inlet air, or a higher barometric pressure with lower humidity. Net horsepower ratings factor in all installed components in an engine as well as ambient air temperature, humidity, and barometric pressure.

If you bought a pickup with an engine rated in 1970 using the gross standard, you would have found the exact same engine appeared to have less power when rated under the net rating in 1971. 

PTO, wheel horsepower 

Unlike mechanical horsepower, which is tested at the engine’s crankshaft, PTO and wheel horsepower are measured at their respective location. Either of these ratings will be lower than mechanical horsepower since a portion of the horsepower at the crankshaft is consumed by the components it passes through resulting in a parasitic loss. The general rule is that there is about a 20% loss of horsepower from the crankshaft to the PTO or drive wheel. For example, 500 crankshaft hp. × 0.80 = 400 PTO hp. 

When working backward from the PTO or wheel horsepower to the crankshaft, there is a 25% gain. For example, 400 PTO hp. × 1.25 equals 500 crankshaft hp. 

Drawbar horsepower is a measurement of a tractor’s ability to pull an implement determined by pulling a weighted load that is fitted with a strain gauge to determine pull. Maximum ground speed is also measured. In many ways, drawbar horsepower is a true measure of the power the tractor can use to pull the implement. It is not only a function of engine power and drivetrain loss, but also the machine’s ability to transfer the load to the tire.

Hydraulic motor horsepower

Hydraulic horsepower is calculated on hydraulic pressure in pounds per square inch (psi) multiplied by gallons per minute (gpm) in American measurement units with a multiplicative constant of 0.0005834. The formula to determine hydraulic motor horsepower is psi × gpm × 0.0005834. Thus, a hydraulic motor that turns out 400 psi at a flow of 100 gpm would turn out 231∕3 hp.

Also, a hydraulic motor is sensitive to its displacement and the pressure-flow relationship that it receives. If you ever need to replace a hydraulic motor, check all the specifications against the original and the fluid circuit output of the host machine.

Electric motor horsepower 

To determine the horsepower of an electric motor, you need to know the voltage input, current draw in amps, and the rated efficiency of the motor. Also remember that 746 watts equals 1 hp. The input voltage and current draw in amps is from Ohm’s Law, which states that volts × amps = watts.

To calculate the horsepower of an electric motor, use this formula: volts × amps × rated efficiency ÷ 746 watts.

Let’s take, as an example, an electric motor rated at 480 volts, 50 amps that has a 92% rated efficiency. Thus, the horsepower of this motor is 480 volts × 50 amps × a 92% rated efficiency ÷ 746, which = 29.59 hp.

Now, some electric motors such as those used on shop air compressors will have their power stated as peak horsepower. This represents the power the motor has just as it starts, which sometimes can be five to seven times the rated continuous horsepower. This bump in horsepower is caused by the motor’s start windings being engaged. 

It’s also important to know that an electric motor has full torque output at 0 rpm. This compares with an internal combustion engine that requires a certain operating speed to achieve peak torque.

Be mindful of the efficiency rating of replacement electric motors. One that has a higher rating will be more powerful, run cooler, and cost less to operate than a less-efficient motor.

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