i know that load controlled by dyno and load cell and will affects the speed, performance, etc but how do we know what is load?
You can get reading of load applied by load cell on a digital indicator.
The load is basically the amount of work delivered by the engine. Depending on the application, we impose various requirements for the engine, for example, a vehicle running at a certain speed requires that we overcome the aerodynamic and rolling losses of the vehicle. This requirement is the load imposed on the engine. In the laboratory, we impose various loads on the engine by adjusting the settings of the dynamometer in order to measure the performance of the engine. Typically, this is done with an arm attached to the dynamometer and some sort of scale or load cell holding the arm from rotating. Multiplying the arm length by the load on the scale gives a measure of the load called the torque, usually given in Newton-meters.
To determine the work done in this experiment, we find product of the force and the distance through which it is exerted. The distance is 2 pi times the arm radius, and the work done is then 2 pi times the arm radius times the load cell or scale reading. This gives the work done per revolution of the engine. Usually when we talk about the load, we mean the torque produced by the engine, or another form way of expressing the work, which is the brake mean effective pressure, bmep. This is the work produced per engine operating cycle per unit displacement, and has units of pressure. It is useful because it reduces engine size dependence, and tells us how much work the engine is producing for each liter of displacement. Most engines, regardless of size, produce a maximum bmep of between 1000 and 3000 kPa. The bmep is the term I personally prefer to use in referring to the load, but torque and power are used by many.
Another aspect is how rapidly we produce this work, that is the power. To obtain the power, we take the work (Newton-meters per revolution) times the number of revolutions per second, to get Newton-meters per second, which is Watts. For engine speed in revolutions per minute, the power is then 2*pi*Torque*RPM/60 (watts). Divide by 1000 to get Kilowatts. With most dynamometers, the arm length is incorporated into a term called the dynamometer constant such that the torque times the speed in RPM divided by the constant gives the power. From that you can determine the torque.
I hope this explains what we refer to when talking about load. How is it controlled in the diesel engine? It is controlled by adjusting the amount of fuel injected into the cylinder each operating cycle. Diesel engines always operate with an excess of air due to the type of combustion, and so to get more power, more fuel is injected. Limits to power can be poor combustion with too much fuel, mechanical load restrictions due to high pressure and other factors. In modern diesel engines this is done with electronic control of fuel injectors, older engines used ingenious mechanical designs of high pressure pumps.
load is an external force acting on the engine which depends on the type of dynamometer used. You can perform either a constant speed test or a constant load test.
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