Why the exhaust pipe of the engine  will emit black smoke,?

 Why the engine will be a shot or an external fire, and the engine body overheat? 

You will get the main reasons after you read the following points:

1. Poor fuel burn
Low injection pressure, fuel injector dripping, poor injection atomization, and too late fuel supply can cause poor fuel combustion and cause knocking. The knocking sound caused by such faults is low and hoarse, accompanied by overheating of the engine, black smoke or explosion of the exhaust pipe, and fire. The injector should be removed for inspection and adjustment, adjust the oil supply time, and replace the new parts if necessary to make them comply with the regulations.

2. Fuel supply time is too early
When the piston of the engine has not reached the specified injection position, the injector starts to inject oil, so fuel is burned ahead of time. High-pressure oil in the cylinder impacts the piston and sends a sound“da, da,da” crisp knocking cylinder very obvious.
The elimination method is: adjusting the fuel supply advance angle, increasing the gasket between the fuel injection pump and the gear box to delay the oil supply time to meet the prescribed injection time. For each 0.1 mm increase in gasket, the oil advance angle can be delayed.

3. Oil valve wear out
After the oil outlet valve is worn,.
on one side, the clearance between the pressure reducing ring belt and the inner hole of the valve seat becomes larger, the sealing performance is deteriorated, and the pressure reducing effect is deteriorated; 
on the other side, the residual pressure in the high pressure oil pipe is too high, and the oil supply is increased, causing The fuel detonated and knocked on the cylinder. The knocking of the cylinder is especially noticeable when the throttle is reduced.