Reducing vehicle weight and frictional losses in the powertrain can help improve overall vehicle efficiency, which is one reason why many manufacturers have adopted aluminium alloy engine blocks as an alternative to traditional cast iron blocks.
The primary advantage of aluminium is the reduction in weight. Yet, because typical casting aluminium alloys do not have similar characteristics of cast iron for wear, many automakers have spent significant time engineering solutions to address the tribological deficiencies introduced by aluminium cylinder blocks.
The Early Solution: Cast Iron Sleeves
The classic solution has been to sleeve the cylinder bores of an aluminium block with cast iron. Generally, this solution can compensate for the poor tribological characteristics of aluminium. The issues that arise from using cast iron sleeves include packaging the liner into the block, the weight, and poor interface of the liner to the block itself.
The most significant disadvantage of a cast iron liner is the thermodynamic properties of the liner and its interface with the aluminium. Not only is the thermal conductivity an issue but also the thermal capacitance, which can hamper the ability to maintain efficient piston and bore temperatures for optimal combustion.
This may be further degraded if the interface of the liner to the block material is not ideal, the insulation points between the liner and block become hotspots on the bore that may cause pre-ignition events. Even if the insertion of the liner to the block material is ideal, thermal conductivity and thermal capacitance can result in less-than-optimal engine performance.
Plasma Transferred Wire Arc Thermal Spray
A more recent solution to address these tribological issues is the utilisation of Plasma Transferred Wire Arc (PTWA) thermal spray coating to coat aluminium cylinder bores with a fine controlled deposition of molten metal particles.
A critical step in the successful production of thermal spray coating is the substrate surface preparation, in other words, preparing the uncoated bore wall. There are a number of commonly available surface preparation techniques available, with notable differences in terms of bond strength, ease of application and cost. The most common of which for aluminium blocks are water jet and mechanical roughening. Both of which have consistently demonstrated high bond strength.
It should be kept in mind that consistent bond strength is required for all engine bore applications. Depending on the manufacturing preparation processes, bond strength has historically been from 35 to 70MPa, although generally anything above 25MPa is sufficient. By adopting mechanical roughening in a standard boring machine or machining centre, a process which engine manufacturers are familiar with, manufacturers can generate a robust surface preparation with the lowest processing and investment costs.
To be continued... Enhancing Engine Performance With Thermal Spray Coating (Part II)
APMEN Sept 2016, Features
