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Measuring Horsepower

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On a test stand engine horsepower is derived by measuring the twisting force (torque) that the engine exerts on a load.  In fact horsepower is defined as simply the product of torque times RPM.  Therefore if our piston engines were equipped with real torque transducers we could utilize RPM and compute real horsepower. While such torque transducers are commonly used to calibrate turbo-prop engines they actually cost more than a typical piston engine. Sadly there is no practical way to directly measure horsepower in a piston aircraft engine.  But this hasn’t stopped at least one engine instrument manufacturer from making bogus claims to the contrary!

REMEMBER:  There is no practical way to truly measure horsepower.

If you planned to use your horsepower display to gauge take off performance or measure engine health you can forget it. In fact, dynamometer tests of so-called horsepower displays revealed NO CHANGE in indicated “horsepower” even when individual cylinders were completely disabled!

The Insight Approach to Power

Despite some limitations we elected to put a power display in the G3 graphic engine monitor.  To prevent any misconceptions we called it “Theoretical Horsepower”, and under certain conditions it actually does a pretty good job of reporting horsepower. One dependable metric of power is fuel consumption. At peak EGT and leaner where ample or excess oxygen available to consume the available fuel horsepower is a linear function of fuel flow. At richer mixtures where some fuel remains unconsumed and doesn’t contribute its full power potential the simple linear relationship falls apart. Prior to leaning to peak the display is challenged to deliver an accurate horsepower display but once peak has been reached the rich-of-peak accuracy will improve.

One limitation of the fuel method is that it is not sensitive to engine faults that can significantly affect the power produced. For example, a cylinder without ignition won’t combust the fuel to produce power, but will nevertheless continue to consume it. So don’t expect to diagnose engine performance issues with the fuel flow approach to horsepower calculation.

What good is a fuel flow based horsepower calculator? The intended purpose of the theoretical horsepower display is to provide an indication of power for leaning purposes. For example, lean-of-peak operation is not recommended at power settings greater than 65%.  It is interesting to see the reduction in horsepower that accompanies lean of peak operation.

The Insight Approach to Performance Measuring