Is aluminum engine better than cast iron engine?

In a car, material plays important role and the overall value that we spend on material is about 47% of the total car cost. Among the materials used, metals have a larger portion, as they are used in making body, chassis, engine, and other parts. Previously cast iron was largely used in car manufacturing but today aluminum has become leading material for making foothold in wheels and transmission casings to cylinder heads and suspension joints. In Automobile industry, Aluminum is now considered as ‘material of the choice’ for all types of cars including mass-market vehicles like Ford and Nissan to outstanding luxury models like Benz and Land Rover, because of its increased performance, fuel economy, energy efficiency and reduced emissions.
So, lets us see what makes these two metals different from each other by comparing their properties:

Properties comparison

  • Weight
    Density of cast iron (7.2gm/cc) being more, will add more mass to the engine and to the entire car. Aluminum (2.7gm/cc) being less dense will have a same shape but with less weight. With aluminum engine block, fuel consumption per distance traveled will be less and makes driving easy.
  • Tensile strength
    As the automobile engine is subjected to strain which in turn causes stress to the material, so material with higher tensile strength can withstand the higher stress. As Iron has high Young’s modulus and ultimate tensile strength, it makes cast iron engine blocks to deform less. To achieve same strength in aluminum, one must add more material. So, engine blocks made of cast iron are robust in construction and can survive even in case of heavy accident.

  • Thermal conductivity
    Engine of a car produces a large amount of heat inside cylinder. To some extent the heat is helpful in burning air/fuel mixture easily, but if heat produced is very high it must be dissipated. Material with good thermal conductivity can easily dissipate heat from the engine cylinder. Aluminum(205W/m-k) has very good thermal conductivity which makes it very easy to dissipate heat from engine block and this also helps in reducing strain on the cooling system of the car.
  • Melting point
    Temperatures produced inside cylinder are very high and to withstand these, a material with high melting point temperature is required. Iron has a high melting point (1090oC) which can with stand high temperatures. On the other hand aluminum (529oC) having good thermal conductivity does not raise its temperature as the produced heat gets dissipated at a higher rate.
  • Thermal expansion
    Metals expand when their temperature is raised and for engine blocks, thermal expansion must be very less. As Iron has less thermal expansion value, it expands less when heated to high temperatures. Hence using iron gives more life to the engine as well as the car.

Cost comparison between cast iron and aluminum engine blocks

Aluminum (82300 ppm) is the third most abundant element in the earth’s crust and iron (56300) is the forth. Even though aluminum is more abundant than iron in earth’s crust, it’s engine blocks have higher price. Reason for this is, extraction of aluminum is difficult and its processing involves more cost. For this reason aluminum engine blocks are mostly used in the high end cars.

Comparison of fuel consumption and CO2 production

Lets take values of weight and fuel consumption in case of cast iron as standard. Now we have a aluminum engine block, this reduces the overall weight of the car as density of aluminum is less. Lets assume by using aluminum engine block overall weight of car is reduced by 100 Kgs, then a car can save

  • 0.35 liters of fuel for every 100 kms distance
  • Production of 9 grams of CO2  for every 1 Km distance as it saves 0.0035 liters of fuel
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