Providing better temperature control for the piston at the crown and top ring land helps to reduce the · engine’s sensitivity to k

Providing better temperature control for the piston at the crown and top ring land helps to reduce the engine’s sensitivity to knocking and pre-ignition, allowing for increased compression ratios and boost levels.9 Similarly, all aircooled VW and many aircooled Porsche models used cast iron for its cylinders. One of the major problems with the use of cast iron for cylinders in an aircooled engine is that they can easily become thermally overloaded. Porsche’s 911 engine, at a maximum displacement of 2.4L, produced about 22 horsepower per cylinder, when fitted with cast iron cylinders. Even Porsche realized that cast iron was not the optimal cylinder material. They eventually turned to aluminum cylinders for their aircooled engines, just like engineers switched from cast iron and steel to aluminum for pistons many decades earlier for its significantly improved thermal conductivity.10 In the evolution of the watercooled internal combustion engine, the next logical step was to go to an aluminum engine block. Manufacturers have turned to aluminum for reducing engine weight, with many retaining iron sleeves or liners for simplicity and durability. With the downsizing of engines and addition of forced induction, the push for ever increasing efficiency has led to problems with thermal optimization of engines. As engine output increases, the center to center distances between cylinders when using iron sleeves has to be increased to provide for sufficient liner thickness, maintain strength and rigidity between the bores, control temperatures, and ensure head gasket sealing on an aluminum engine block fitted with iron sleeves. Engine block castings are also subject to residual stresses with potential functional drawbacks in long-term operation that can lead to bore geometry issues like ovality and taper.11 This can be further compounded by dissimilar expansion rates between cast iron and aluminum alloys, leading to deformation and local heat transfer issues. To make lighter, smaller, higher output engines, another solution other than fitting iron sleeves would be needed to allow cylinder inter-distances to be reduced to less than 5mm for both packaging and thermal considerations.12 Although Porsche used aluminum cylinders in the 1960s with Chromal or Ferral cylinder coatings, the first mass-produced all-aluminum watercooled engine without iron sleeves and uncoated aluminum cylinder bores was in 1971 with the Chevrolet Vega. It utilized Reynolds A390, more commonly known as KS Kolbenschmidt Alusil with iron-clad pistons.13 Eliminating iron sleeves overcomes the problem of using different materials caused by dissimilar Young’s modulus and expansion rates.14 Mahle’s Nikasil and Kolbenschmidt’s Galnikal cylinder bore electro-platings have also been used for coating bores in aluminum blocks successfully for decades by Porsche, BMW, Jaguar, Ferrari, and others. The resulting nickel silicon carbide coating provides an extremely durable wear surface. It has 3 to 10 times less wear than cast iron cylinder bore surfaces due to its extreme hardness and oleophilic nature.15 However, due to its high cost, complexity, and environmental concerns over hazardous chemicals used in its production, alternatives to these electro-platings are needed. 9 Improving Heat Transfer and Reducing Mass In a Gasoline Piston Using Additive Manufacturing. Reyes Belmonte, M. et al. SAE International 2015-01-0505. 10 Reconditioning of Aluminum Blocks. KS Aluminum-Technologie GmbH. 11 Reconditioning of Aluminum Blocks. KS Aluminum-Technologie GmbH. 12 The Aluminum Automotive Manual. Version 2011. European Aluminium Association. 13 New Honing Options For Hypereutectic Aluminum Bores. Tim Meara. Modern Machine Shop Online. 9/18/2008. 14 Metal Matrix Composites in Industry: An Introduction and a Survey, Volume 1. Alexander Evans, Christopher San Marchi, Andreas Mortensen. 15 Nikasil and Alusil. John Goodman. Engine Professional Oct-Dec 2008. LN Engineering Technical Library. Copyright ©2019 LN Engineering LLC