HP can also be expressed in terms of Watts, there are 746 watts to one HP.
The Horse Power of an engine is usually expressed as Brake Horse Power which is a measure of power delivered at the crank and available to do work for the operator. Occasionally the term Indicated Horse Power may come up, this is the theoretical amount of power produced by the engine based on cylinder size and amount of fuel burned (or steam pressure in a steam engine). An engine consumes a certain amount of the power it produces to carry it through to the cycle to the next point of power production. If the Indicated Horse Power of an engine is 10 HP, and it takes 3 HP to drive the engine itself then the Brake Horse Power is 7 HP.
Now that we've gotten over the boring bit of Horse Power, we can look at how HP is measured to establish the true HP of an engine. There are three popular devices to accomplish this measure:
- The Fan Dynamometer
- Electrical Dynamometer
- The Pony Brake Test
The Fan Dynamometer
The Fan Dynamometer is simply a paddle fan which is driven by the engine. The faster that the fan turns, the more power has to be put into it to keep it turning. The HP required to drive the fan is in direct relationship to the speed, so as long as the speed is known, the HP can be calculated. The speed is shown using a revolution counter or tachometer driven from the same shaft as the fan. This measurement method produces are good approximation of HP but is not exact nor is it 100% repeatable. Simple mechanical differences such as air pressure and the amount of water present in the air can affect the measurement.
The Electrical Dynamometer
A dynamo (note: not a generator) can be used to measure HP by measuring how much electrical work can be done by an engine driving the dynamo. A volt meter and an Amp meter are in the circuit along with the load. The measured volts multiplied by the measured amps gives the calculated watts, which in turn gives HP. Of course the dynamo itself takes some power to drive, but typically will be around 90% efficient. The wastage of the dynamo is known in advance and is included in the final calculation. This method is reliable and repeatable and would typically be used for measuring an engine to generate its vital statistics for marketing, or for proving that a modification had the expected positive effect.
The Pony Brake Test
The final example is the Pony Brake. To use this method a certain sized pulley is fixed to the engine (the same one for every test) and a clamp arrangement is bolted to the pulley. An arm comes of the clamp which has a weight hanging from it. After the engine is running and warm, the clamp is tightened down until the engine is dragged down to the speed where HP is to be measured. The height that the weights have been lifted is measured and that is used to calculate the HP.
Andy Gline's Huber Traction Engine on the Dynamometer at Portland engine show
This is an example of the first kind of dyno, the fan type. This is Andy Gline's Huber #8213 belted up to the dyno at the Portland Indiana (Tri-State) Engine Show.