When testing engines, some method to ‘load’ it is required. If the engine crank shaft is not connected to anything, then the engine will just be able to run up to maximum speed very quickly, but will not be producing much torque. If you want to begin testing how the engine performs if you actually want to do some work with it, then you need a way of making the engine produce torque and power.
In a car, you do this by putting the vehicle in gear and driving away. With professional engine test apparatus, a machine called a dynamometer (or ‘dyno’) can be used which is designed to place a controllable load on the engine. There are many different types of dynamometer, ranging in complexity and controllability, but many of the modern ones are some variation of electric motor/generator.
For my engine test rig, I have a car alternator (basically a generator) available as my dynamometer. This can be connected to the engine via pulleys and a belt, and will rotate when the engine rotates. To control the amount of torque required to rotate the alternator, we place an electrical load on it. This can be any type of electrical load from a heater to an LED or another motor. The power dissipated in the electrical load is proportional to the power required to drive the alternator, hence we can vary the load placed on the engine by varying the size of the electrical load. The load I will be using is an array of 12 V 50 Watt halogen light bulbs. Banks of these can be switched on and off to control the size of the electrical load and hence the load on the engine.