Units of Torque

• Torque is measured in terms of force per distance. This can take the form of pounds per feet or pounds per inches with similar variations under the metric system. A small engine with a rating of 5 ft/lbs of torque is capable of exerting 5 lbs. of rotational force at a distance of 1 foot. Car engines by comparison are much more powerful and are capable of exerting hundreds of pounds of force.
  
  

Considerations

• While is torque is a useful measurement, it is not always the best measurement. Torque is not used to measure the power rating of small engines used in certain machines. Since riding mowers consist of several components powered by a single engine the torque rating of the engine can not accurately measure its cutting power since some of that power is used for other purposes. Therefore, riding mowers and other machines that use transmissions---such as cars---rate engine strength by horsepower.
  
  

Mathematics of Engine Torque

• At the most basic level, torque is calculated by multiplying the amount of force applied and distance from the center where it is applied. So, if 10 lbs. of force is exerted against a bolt using a 2-foot wrench then the torque exerted would equal 20 ft/lbs of torque. Increasing either the amount of force or the distance from the center will increase torque.
  
  

Torque Measuring Tools

• Torque in small engines is measured in one of a few ways. It is directly measurable using a torque meter rated for small engines or it can be calculated using the RPM (revolutions per minute) and horsepower ratings of the motor. The equation for deriving the torque of a small engine is 5252 multiplied by the horsepower rating and divided by the number of RPMs. Since RPM is a variable measure based on how fast the engine is turning, this measure of torque only measures the torque output of the engine at one speed.
  
  

Applications for Small Engines

• Torque is a useful measurement for determining the power of small engines commonly found in machines ranging from generators to lawn mowers. Generally, the higher the torque rating the stronger the engine is and the more work it is capable of. Therefore, it is most useful in situations where the engine applies all of its force directly to one task. Objects ranging from lawnmowers to generators are rated using torque.