Piston metallurgy is the type of subject where marketing hyperbole can often override good engineering decisions. Is a hypereutectic piston better than a eutectic piston? And what do the terms mean anyway? Here one of Australia's top automotive engineers takes us on a quick guided tour.
Castings and Forgings
Castings are made by gravity die cast process using 5-piece inner dies, a two piece outer and usually a head chill which can form any cast valve pockets and bowls. There are also core pins to form the pinholes. The shapes are re-entrant (ie curve back on themselves) and the 5-piece inner part of the die collapses to allow withdrawal. On the other hand, forgings are made from a solid billet of aluminium alloy. The alloy is preheated and then pressed into shape in a sequence of two or three stages, each getting closer to the final shape. These shapes are always such that the inner die piece can withdraw (and therefore the piston shape cannot be re-entrant).
Choosing between a cast and a forged piston:
- Cast pistons can be very strong and light. One disadvantage they have is that in a blow-up (say, a dropped valve) a cast piston is more likely to shatter and destroy the rest of the engine than a forged piston. Forgings tend to be more ductile so they resist breakage better in a blow-up.
- Very high output engines, say 75 kW per litre and above, usually need forged pistons due to the inherent strength and ductility of the forged blanks. However forged pistons are very expensive - generally three times as much as cast pistons - and so are beyond the reach and needs of the majority of performance engine owners and builders. Forged pistons can be made from a variety of alloys but the most common in high performance applications are the low (ie 1 to 5%) silicon alloys, though the higher silicon alloys can still be forged.
There are many engines built with forgings because a suitable lightweight cast piston is not available. Incidentally, the material used to make forged pistons varies according to who makes them and where. Traditionally the US sourced forgings are quite low silicon, about 7.5%. This is why a lot of these pistons need a lot of clearance (apart from their relatively crude straight tapered skirts). The Germans came up with 14% silicon alloys for forged pistons because they had to satisfy noise requirements for road engines.
Cast Pistons
The vast majority of passenger car pistons are cast pistons made by the gravity die casting process.
These pistons are made from a variety of aluminium alloys, all of which contain silicon as the main alloying element. Silicon is used to improve wear resistance, reduce the expansion which occurs with increasing temperature, and to improve hardness and strength - so improving wear and scuff resistance. Hardness is also needed to enable the castings to be machined. Used in smaller proportions are other alloying elements, eg copper, nickel, magnesium and manganese. These improve hardenability, improve fluidity for casting and increase piston strength at high temperatures.
While there are many variations, the following are the three major categories adopted by piston manufacturers around the world:
1. Hypoeutectic
This term refers to an alloy of aluminium containing up to 12% silicon. Most hypoeutectic pistons have around 9% silicon. An analogy often used by metallurgists is to say that this alloy has all of its silicon fully mixed with the aluminium, just as a small amount of sugar will fully dissolve in a drink. While the highest level for hypoeutectic is around 12%, this can be influenced by the amount of the other alloying elements. Although a standard industry alloy for many years, this material is now being phased out in favour of the next category.
2. Eutectic
A "eutectic" alloy is one that has the maximum amount of silicon that will remain fully in solution with the aluminium (to continue the analogy, consider the largest amount of sugar you can stir into a cup of tea without some remaining at the bottom). Due to the effect of other alloying elements, most eutectic alloys have an 11-13% silicon range but the eutectic point is actually 12.7%, varying slightly according to the level of other alloying elements.
The most common piston alloy for automotive engine pistons contains around 12.5% silicon. Alloys of this type are used for the vast majority of passenger car automotive and light diesel pistons, including in very high performance engines such as those produced by BMW, Ford, Opel, Holden, Jaguar, Rover, Mercedes Benz, Porsche, Rolls Royce, Honda, Toyota, Mitsubishi, Nissan etc.
3. Hypereutectic
A true hypereutectic alloy is one in which there is a significant amount of free silicon in addition to that which is fully dissolved. Hypereutectic alloys can start as low as 12.7% silicon and in the case of two-stroke engine pistons, they go up to 18% silicon. For passenger car engines, there has been a trend from both directions to use an alloy closer to the eutectic, ie around 11-13% silicon. The lower silicon alloys (9% nominal) are still in use but the greater hardness and superior scuff and seizure resistance of the 11-13% silicon alloy, and its better response to hardening, make eutectic more popular.
An advantage of hypereutectic pistons is that the free silicon reduces piston ring groove wear. In some engines developed for current emission control levels, the top ring is placed higher in the piston than in earlier engines - typically the top land width is reduced from 6 mm to 3 mm. This may not sound a lot but the temperature becomes greater nearer to the top of the piston and there is a tendency for the top ring to weld itself to the bottom face of this groove, and thus cause wear, increased clearance and ultimately loss of oil control and so higher emissions.
The higher percentage of silicon in hypereutectic alloys reduces the tendency for ring/groove welding but has no significant effect on either strength or coefficient of expansion - the alloy is not chosen for either of these reasons. If higher strength is needed, especially at high temperatures, the use of a eutectic alloy containing higher levels of copper is favoured and this is probably going to be the trend for high performance, low emission engines of the future. Also nickel, though very expensive, raises the hot strength of any alloy and is often used in amounts of around 1 to 2%. A typical high copper alloy (say with 5% copper instead of the more usual 1 to 2%), is easier to cast than a hypereutectic, especially in pistons with thin sections. Hypereutectic alloys often require section thickness to be increased, especially the side panels, with a resultant weight penalty.
Comparing Eutectic and Hypereutectic - Strength and Expansion
The differences between a cast eutectic and a cast hypereutectic piston are so small that there is no significant advantage of one over another, unless the pistons have top lands of less than 4mm, which is unusual. The tensile strength of hypereutectic at operating temperature is actually slightly lower than eutectic, though for practical purposes it is so close that it can be regarded as the same. The coefficient of thermal expansion for eutectic is 0.0000214 mm/mm/degree C and for hypereutectic it is 0.0000196.
This means that the difference in actual skirt expansion for a 100mm piston at its probable operating temperature of 100 degrees C is around 0.01 mm and in practical terms this represents a negligible difference, especially in a performance piston where the fitted clearance would probably be around 0.1 mm. In designing a piston for a passenger car application, the same nominal clearance is specified regardless of which alloy is used - though if a moly graphite skirt coating is used both can be fitted at less clearance.
Summary
In the end the most important aspect of a cast piston is its design, not the material it is made from. If a piston is going to break or seize it will do so whether it is made from a 16% hypereutectic or a 12.5% eutectic alloy. Some new pistons under development for low emission engines will have either the high copper or higher silicon hypereutectic alloys due to the short top lands envisaged for these new engines.
*Nigel Tait is Chief Engineer of Automotive Components Limited, Australia's largest manufacturer of original equipment and aftermarket engine parts.
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