Turbo: It forces air into the engine so that the combustion can take place. It is very common in cars with Diesel engines.
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The turbocharger works like a small and powerful fan since its blades move very fast. It takes the natural air, compresses it and forces it inside of the cylinders. When it gets damaged, it may produce a very loud noise, similar to a screech, because the axle brushes against the bodywork. White or black smoke may also come out from the exhaust pipe.
My 2009 2.5 litre turbodiesel pathfinder has recently started surging when I am going up hills... Initially it started happening randomly only around the 2500 rpm range however it seems to do it now when the engine is warm and between 2000 and 2500 rpm. I noticed today when I was going up a hill and it was doing this that it was also blowing a lot of black smoke... it only does it under load and any other time drives fine...