Petrol Engine - Design - Power Measurement

Power Measurement

The most common way of engine rating is what is known as the brake power, measured at the flywheel, and given in kilowatts (metric) or horsepower (USA). This is the actual mechanical power output of the engine in a usable and complete form. The term "brake" comes from the use of a brake in a dynamometer test to load the engine. For accuracy, it is important to understand what is meant by usable and complete. For example, for a car engine, apart from friction and thermodynamic losses inside to the engine, power is absorbed by the water pump, alternator, and radiator fan, thus reducing the power available at the flywheel to move the car along. Power is also absorbed by the power steering pump and air conditioner (if fitted), but these are not installed for a power output test or calculation. Power output varies slightly according to the energy value of the fuel, the ambient air temperature and humidity, and the altitude. Therefore, there are agreed standards in the USA and Europe on the fuel to be used when testing, and engines are rated at 25 ⁰C (Europe), and 64 ⁰F (USA) at sea level, 50% humidity. For a marine engine, as supplied it will not have a radiator fan (usually) and often not an alternator either. In such cases the power rating quoted will not allow for losses in the radiator fan and alternator. The SAE in USA, and the ISO in Europe publish standards on exact procedures, and how to apply corrections for deviating conditions like high altitude.

Car testers are most familiar with the chassis dynamometer or "rolling road" installed in many workshops. This measures drive wheel brake horsepower which is generally 15-20% less than the brake horsepower measured at the crankshaft or flywheel on an engine dynamometer. Youtube video showing workshop measurement of a car's power. The measured power curve in kW is shown at 3:39.

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