Engine problem?

What is your question?

I have a big problem in my bmw 325 2002 model..every week im adding 1kg of oil to the engine..there is no leakage from the engine..no oil on the ground.but when input my car on N nd pull on the throtle black smoke comes out from the exhaust wen rpm reaches 5..otherwise no white smoke nor there is white on the cap of te engine..what is my problem?? Does the engine needs to be changed?? Orr there is smthg else?? If someone can help me ill be grateful..