The Engine Block is the backbone of every internal combustion engine. It is the largest and arguably the most intricate metal part in an engine. All the other engine parts are either placed inside the block or are bolted to its surface.
So, what are engine blocks made of? The engine blocks are made up of either of the two materials – Cast Iron or Aluminium.
While the motorcycles of earlier decades were dominated by Cast Iron engine blocks, Aluminium is now the preferred material owing to its low weight. Aluminium engine blocks also help in reducing the emissions.
Now, simply saying Cast Iron or Aluminium as the materials used to manufacture an engine block is quite misleading and inaccurate.
This is because within Cast Iron, there are numerous grades and classes. Similarly, for Aluminium, there are hundreds of different alloys.
To be more accurate, let us deep dive into what kind of iron and what kind of aluminium are used in manufacturing the engine blocks.
Cast Iron Engine Blocks
Iron engine blocks are typically cast by Gray Cast Iron.
Gray Cast Iron is a type of cast iron that has a graphitic micro-structure and is named after its grey color which is due to the presence of graphite.
Gray Cast Iron is one of the cheapest possible iron to have and are used extensively in different machinery across industries. Within motorcycles, the gears are also made up of Gray Cast Iron.
The constituents of Gray Cast Iron are as follows.
| Constituents | Gray Cast Iron |
|---|---|
| Carbon | 2.5 – 4 % |
| Silicon | 1 -3 % |
| Manganese | 0.2 – 1% |
| Sulfur | 0.02 – 0.25 % |
| Phosphorus | 0.02 – 1 % |
Now, within this Gray Cast Iron there are several different classes. The classes used typically for engine blocks are Class 20 and Class 25.
The tensile strength of these iron classes ranges between 20,000 to 25,000 psi.
Advantages: Engine Blocks made from Gray Cast Iron have relatively low cost of manufacturing. In addition, Gray Cast iron provides good machinability and good galling and wear resistance. These engine blocks also have higher tensile strength when compared to Aluminium engine blocks.
Disadvantages: The main disadvantage of a Cast Iron engine block is its heavy weight. The engine weight plays a crucial role in the performance of motorcycles. Large weight means more fuel requirement and also increased emissions, which are lower in Aluminium engine blocks.
Aluminium Engine Blocks
In Aluminium engine blocks, the most commonly used aluminium alloys are 319, A356 and A357.
These aluminium alloys do not have suitable strength required for an engine block. Therefore, these alloys are subjected to receive a T5 or a T6 heat treatment to achieve sufficient strength required for manufacturing.
Without the heat treatment, these alloys will be too weak and unsuitable for the engine blocks.
So, it is imperative for the aluminium alloys to undergo the heat treatment.
The typical tensile strength of 319, A356 and A357 aluminium alloys are between 10,000 to 14,000 psi.
The constituents of these aluminium alloys are as follows.
| Constituents | 319 | A356 | A357 |
|---|---|---|---|
| Aluminium | 85.8 – 91.5 % | 91.1 – 93.3 % | 90.8 – 93 % |
| Silicon | 5.5 – 6.5 % | 6.5 – 7.5 % | 6.5 – 7.5 % |
| Magnesium | 0.1% | 0.25 – 0.45 % | 0.4 – 0.7 % |
| Copper | 3 – 4 % | 0 – 0.2% | 0 – 0.2% |
| Titanium | 0.25% | 0 – 0.2% | 0.04 – 0.25% |
| Iron | 1% | 0.2% | 0.2% |
| Zinc | 1% | 0.1% | 0.1% |
| Nickel | 0.35% | – | – |
| Manganese | 0.5% | – | – |
Advantages: Aluminium engine blocks are of lower wight when compared to cast iron engine blocks. The low weight translates to high fuel economy and lower emissions in the motorcycles with aluminium engine blocks.
Disadvantages: Aluminium alloys are costlier to manufacture and does not have good machinability as that of Gray Cast iron. Their tensile strength even after undergoing heat treatment is still lower than cast iron engine blocks.
Comparison of Engine Block Materials
Here we compare the two engine block materials against different parameters – weight, cost of manufacturing, fuel efficiency, wear resistance, noise and vibration, ease of repair, and ease of rebuilding.
Weight
A low weight engine block provides many advantages to a vehicle. Less weight means better fuel economy, better acceleration, and low emissions as well.
Aluminium as a material is always lighter than the cast iron. The density of these materials is as below.
Density of Gray Cast Iron: 6900 to 7350 kg/m3
Density of Aluminium: 2550 to 2800 kg/m3
However, density is not an accurate measure to compare since Aluminium engine blocks use more aluminium material.
This is because reinforcements are used in aluminium engine blocks to increase their tensile strength. Cast Iron engine blocks, on the other hand, do not require any reinforcements.
Still, Aluminium engine blocks always weigh less when compared to Cast Iron engine blocks. In general, aluminium engine blocks are 40% lighter despite the additional reinforcements used to strengthen them.
Cost of Manufacturing
Aluminium alloys are costlier than Gray Cast Iron. Add to it, the complex casting process that aluminium requires for the engine block manufacturing. The cost can easily be 2 to 3 times of a cast iron block.
Earlier, the sand casting of an aluminium engine block was a laborious task since aluminium alloys react in the air to form aluminium oxides.
However, as the years have progressed, the casting technology has improved vastly to carry out aluminium casting easily now.
Still, the casting process of an aluminium engine block remains to be an expensive process when compared to the easy and low-cost manufacturing of cast iron engine blocks.
Fuel Efficiency
Aluminium engine blocks are low weight and have better thermal conduction properties. These characteristics allow the engine to start quickly and to provide better acceleration. The low weight aids in increasing the fuel economy as well.
For these reasons, an aluminium engine is far more fuel efficient than an iron engine block.
Wear Resistance
Cast Iron have a far better wear resistance capabilities than any of the aluminium alloys.
Due to its low wear resistance, Aluminium engine block liners are often made from cast iron and not aluminium. The block liners are subjected to high wear and tear due to the continuous movement of piston along the cylinder lining. The block needs a strong resistance to withstand the wear.
Noise and Vibration
This is one of the advantages of Cast Iron engine blocks. They absorb noise and vibrations better than the aluminium blocks.
Aluminium engine blocks can improve their noise and vibration absorption through complex and intricate casting. However, in general, cast iron engine blocks fare better in absorbing noise and vibrations.
Ease of Repair
When an Aluminium engine block cracks, it is easier to repair using TIG welder.
However, if an iron engine block cracks, it can still be repaired, but it requires great skill since a furnace welder is needed.
Typically, aluminium engine blocks are easier to repair when compared to iron engine blocks.
Ease of Rebuilding
Iron engine blocks are typically easier and cheaper to rebuild. The boring of an iron engine block can be machined easily. A simple equipment shop can handle this.
However, rebuilding an aluminium engine is far more complex. The boring is not at all simple and cannot be handled in a small equipment shop. Also, rebuilding an aluminium engine block requires great skills. The success rate is not too high as well.
Comparison Table
Here is a comparison table highlighting the differences between a Cast Iron engine block and an Aluminium engine block.
| Parameters | Gray Cast Iron | Aluminium Alloy |
|---|---|---|
| Weight | High | Low |
| Manufacturing Cost | Low | High |
| Fuel Efficiency | Low | High |
| Wear Resistance | High | Low |
| Noise & Vibrations | Low | High |
| Ease of Repair | Difficult | Easy |
| Rebuilding | Easy | Difficult |
Manufacturing Process
The manufacturing of engine blocks is mainly done using sand casting.
Although die casting can be used, sand casting is much more cost effective since die casts wear out quickly. The high temperature of the molten metal will wear down the dies and hence are an expensive process when compared to sand casting.
Earlier, the sand casting of aluminium was a laborious task since aluminium alloys react in the air to form aluminium oxides. However, as the years passed, the casting technology has improved vastly to carry out aluminium casting easily now.
Since Aluminium engine blocks are not as strong as Cast Iron blocks, some manufacturers use cast iron in the liner or in some other portion of the engine block. In addition, to improve the tensile strength of the Aluminium engine blocks, reinforcement frames are added in the design of the blocks.
Once the casting process is finished, the casted engine block is further taken for machining and surface finishing.
Computerized milling and boring machines are used to arrive at the correct dimensions for the block and to provide smooth finish.
Surface Modifications To Improve Performance
To enhance the performance of engine blocks, certain modifications are made depending on the engine and the manufacturer. Here are some of the common surface modifications:
- Chrome plating: A thin layer of chromium is electroplated on to the engine block surface to improve wear resistance. Chrome plating is most commonly used in Aluminium engine blocks.
- Ni-SiC (Ni-Silicon Carbide) composite plating: The Ni-Silicon Carbide plating hardens the surface and improves its capability to withstand heat.
- Thermal Spraying: Thermal spraying provides a coating layer to improve the surface of the engine blocks and protects the layer from wear, corrosion, abrasion and heat. It extends the life of the engine blocks as well.
