Let's say we have a 6 cylinder diesel engine having 385 KW power. Now I am running this engine at high idle of 1900 rpm. What will be power produced by this engine? Is it equal to max power or less than that and when I will apply load than it will produce max power?
That's a difficult thing to answer without knowing the power characteristics of the engine. Best measured with a dynamometer. Peak horse power and torque are 2 different things and dont necesarily occur at the same rpm. Increases in power output is seldom a straight line, ie doubling the revs doesn't necessarily mean doubling the power. Power at half revs isn't necessarily half of the peak output either. The only way to accurately measure the power output of your engine at a given rpm is to put in on a dyno.
What will be power produced by this engine? It is putting out net zero power if you are in neutral. Gross power output is just enough to overcome friction and any load from the alternator, a/c or other accessories.
What will be power produced by this engine? See Peter's answer
Is it equal to max power or less than that and when I will apply load than it will produce max power? Max power is based on a chart that come with diesels and shows the curve of torque vs rpm. The curve looks like a mountain, where the torque increases as the RPMs increase, then there is a peak and the engine looses efficiency and the curve slopes down as the RPMs increase. The maximum torque is attained under wide-open throttle when you have enough load that your RPMs are at the peak in the curve on the chart. This is commonly around 80% of the max-rated RPMs on a diesel.
To give an example, let's say this engine is on truck and the peak torque for your engine is 2400 RPMs. You are driving up a hill that is getting steeper and you have the pedal to the floor (wide open throttle). When you hit 2400 RPMs your engine is putting out it's maximum amount of torque.