Question

Why is aluminium used to make an Engine

Answer 1

The engine block however is another story. The block forms a lot of the mass and weight of the engine so using aluminium to build engine blocks reduces the weight of the engine by many kilograms and assists the vehicle in obtaining a better power to weight ratio and assists in achieving a front to rear weight ratio of 50:50 or close to it.
  Due to the size of the block in relation to the entire engine, the cost of an aluminium engine block is significantly higher and is used on vehicles where the overall cost is not much of an issue.
  Pistons are also made of aluminium as the lighter pistons require less energy to be moved up and down during the four stroke cycle and this facilities higher revs. The issue with this is these pistons can't be used with the aluminium engine block as contact between two aluminium surfaces creates large amounts of friction so one must be cast iron, either the piston or the block. The common solution for this problem was to line the cylinder walls with cast iron.
  Aluminium is also used in intake manifolds to reduce their weight but another benefit is noise reduction. Heat resistant plastic is also used to make intake manifolds, its cheaper than aluminium and is lightweight. The problem with the plastic manifolds is that they generate noise as the air flows through them so luxury vehicles use aluminium or magnesium manifolds, both are lightweight without the noise.   Aluminium engines are becoming widely used in the automotive industry. The amount of parts used will vary with the manufacturer and the model of vehicle. Aluminium is also being used in other parts of the vehicle such as the chassis and the body of some high end vehicles. 

Answer 2

this is because Aluminium is lightweight, Strong and Resistant to corrosion