Race Engine Technology issue 043 : DECEMBER/JANUARY 2010
47
FOCUS : PISTONS
Wayne Ward reports on the state of the art in piston manufacturing processes and technology, and explains the issues behind piston materials and coatings
Filippo Preziosi: DUCATI GRAND PRIX ENGINE WIZARD
THE COMMUNICATIONS HUB OF THE RACING POWERTRAIN WORLD
PISTON POWER Focus on the state of the art
SPYKER 40 VALVE V8 Alternative 24 hour race technology
the efficiency of that process, and have a major effect on the level of friction losses in the engine.
Background
When we talk in generic terms of two-stroke and four-stroke engines, we often refer to them as ‘reciprocating engines’. It is the piston that gives them this description, where ‘reciprocating’ describes the motion of the piston. In terms of the fundamental crank-connecting rod and piston mechanism, it is only the piston and the parts housed within it that reciprocate. While we attribute a reciprocating mass to a connecting rod when undertaking design calculations, the reality is that no physical part of the rod undergoes truly reciprocating motion. The piston is the part which, by its motion during the expansion process, transfers the energy released during fuel combustion to the connecting rod via the heavily loaded piston pin bearing contact. It has to withstand the pressures and forces of combustion, as well as the variations in rapid changes in surface temperature. The piston also has to provide a bearing housing for the piston pin (gudgeon pin/wrist pin) and grooves for the piston rings, and act efficiently as a bearing when sliding against the cylinder bore. In most cases, the piston also has to house the wire clips that retain the piston pin. Each of these functions and features brings its own risks, which have to be considered in the design and manufacture of these components.
Working environment DEC/JAN 2010
Clearly the piston forms a large part of the combustion chamber, especially close to top dead centre (TDC). As part of the chamber, therefore, it has to withstand very rapid changes in surface temperature. We measure the time elapsed between the cool incoming charge entering the combustion chamber and the point of maximum combustion temperature in milliseconds in most racing engines, and in reasonably recent Formula One history this time has been less than 3 ms in some cases. So the design of the piston affects how efficiently the heat is conducted away from its surface, making this an important factor. The changes in operating stress on the piston are also very rapid and numerous, with large combustion forces and ‘inertia’ loads to cope
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Reciprocating ideas P
istons are among the most important components in any two-stroke or four-stroke racing engine. They have to be able to withstand the pressure and temperature of the combustion process, they play a key role in influencing
with. The maximum ‘inertia’ load due to acceleration always occurs at TDC, but the minimum ‘inertia’ load does not necessarily occur at BDC. Depending on the ratio of crank throw to rod length, the geometry of the crankshaft and connecting rod mechanism can shift the angle at which the maximum ‘BDC’ inertia load occurs by more than 30 crank angle degrees away from BDC itself.
Materials
Until recently there was only one choice of material for racing pistons – aluminium. This has been the material of choice for decades, for both road and race applications, because of its combination of low density and high thermal conductivity. While there have been a number of attempts to improve on aluminium alloys for piston applications, the favourites remain 2618 and 4032. The 2618 alloy is basically RR58, which was developed by Rolls-
Royce for its aero engines and saw service in its Merlin engine which served with distinction in the World War II. Figure One shows a Merlin piston and a NASCAR piston, both having benefited from the excellent properties of RR58.
Despite attempts at improvement, however, RR58 is still generally
the alloy of choice for many of the companies RET spoke to for this article, a number of whom also said they have no plans to try to replace such a well-proven material. There are other promising aluminium alloys being developed by some companies, which should
Fig One - Alloy RR58 was developed for the Rolls-Royce Merlin engine. A Merlin piston is shown here next to a 106mm NASCAR piston (courtesy Omega Pistons)
give better high-temperature properties and greater fatigue strength. We spoke to one company that has developed an alloy specifically for pistons and is now trying to bring this to market for premium applications. The recent ACO rules that govern the Le Mans 24-hour race and form the basis of the rules for other endurance racing series have encouraged the development of diesel engines for racing. It is for this specific application that steel pistons have been developed by one prominent piston manufacturer. Both of the teams in LMP1 operated by motor manufacturers and running diesel engines – current Le Mans champion Peugeot and past champion Audi – are reported to have changed to steel pistons. It has been common to have steel inserts cast into pistons for commercial vehicle applications, but the all-steel piston is a step-change in design. The pistons take maximum advantage of the greatly increased fatigue strength of steel compared with aluminium to reduce wall thickness and to remove material from an area where it would have proven too risky with an aluminium component. The thermal properties of steel in terms of strength, expansion and conductivity are used to good effect in this application, something for which other piston manufacturers are known to be in the process of developing.
In the past, we have seen several attempts to use other materials for pistons, many of which have been unsuccessful. Until the recent tightening of materials regulations and homologated engines, Formula One was the natural arena for piston material development, but this is no longer the case, and the considerable budgets that still exist are not directed at engine materials development. Formula One is often the means by which technologies are quickly brought to a level of maturity to make it possible technically and economically for other racing series to take advantage of them. Some of those questioned for this article said development of new materials is now minimal in comparison to the recent past, and that promising avenues of material development have come to a halt. While there is undoubted merit and skill in designing an engine from ‘non-exotics’, developing new materials is a field that can positively influence engines outside the racing world. Magnesium has been tried by several companies in the past, because of its very low density, although magnesium pistons have not been offered widely for sale. The people we spoke to regarding magnesium pistons told us there are problems with using this material, not only in manufacture but in service too. Magnesium has a low elastic modulus and low thermal conductivity – two reasons why this material is not being developed further. But it may offer an interesting development path in the future, for certain applications. The development and banning of aluminium beryllium was big news a few years ago. Widely regarded as a ‘wonder-material’ for pistons, it has all the traits a piston designer would normally want. It has high thermal conductivity, a high elastic modulus, good fatigue strength and good high-temperature mechanical properties. The publicly stated reasons for its subsequent banning are dubious and, according to some, mainly political. Several people involved in the aluminium-beryllium piston said they felt it was banned at
Fig Two - Metal matrix composite (MMC) piston forging for F1 (courtesy AMC-MMC)
the behest of one manufacturer, who had asked for, but was refused, exclusive rights to use the material supplied by a certain company. Beryllium-only pistons have been considered in the past – Porsche reportedly looked at their development as long ago as the famous and wonderful 12-cylinder 917 sportscar of the late 1960s. But pure beryllium poses serious health risks, as exposure to it and its compounds can with only a single exposure cause the chronic and debilitating lung disease berylliosis, which is currently incurable. Aluminium metal-matrix composites have some applications for
racing engines, and a number of companies offer pistons in these materials, either machined from billet or from forgings. Since being banned in F1, the demand for MMC piston development has fallen, but a number of those questioned for this article have experience of producing pistons using this type of material, and some currently produce MMC pistons for racing applications. Figure Two above shows an MMC piston forging. Titanium was examined in the 1970s in drag racing in the US by
John Farkonas, with some success. In this application a titanium crown was used with an aluminium skirt. The highly advanced Polimotor engine, to which I’ve referred in a number of articles, made novel use of materials for its pistons, which featured an aluminium crown and polymer skirt.
Manufacture
Manufacturing a piston can be quite a complex process, depending on which route is taken. A few racing pistons are machined from solid billets, a production route that offers wide scope to change the design at will and which is used by a good number of those surveyed for this article. Modern manufacturing technology has made this possible, with four- and five-axis machining centres no
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043 contents • INSIGHt:
DucatI MotoGp eNGINe Ian Bamsey discusses Ducati’s MotoGP engine with Filippo Preziosi
• RepoRt:
Race eNGINe oF tHe YeaR Who won the honours in the 2009 Race Engine of the Year Awards?
• DoSSIeR: SpYKeR LM Gt2 v8 Ian Bamsey investigates the 40-valve V8 that excelled in this year’s Le Mans 24 Hour race
• MotoRcYcLe:
DucatI BIG BaNG We may finally be close to understanding what really happens when a MotoGP bike is fitted with a big bang engine, suggests Neil Spalding
• FocuS: pIStoNS Wayne Ward reports on piston manufacturing processes and technology
• INSIGHt: DuaL SWIRL poRt John Stowe gives a progress update on his dual swirl-port Cosworth BDA
• INSIGHt:
KeepING tHe BDa aLIve John Coxon investigates how the Cosworth BDA lives on in motorsport today, highlighting the technology that keeps this classic alive
• FocuS:
tHe INDuctIoN SYSteM John Coxon explains the principles and design issues behind the key components of modern race engine induction systems
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ISSUE 043 race engine TECHNOLOGY DEC/JAN 2010
SPYKER GT2 V8 • PISTONS • COSWORTH BDA • RACE ENGINE OF THE YEAR 2009 • DUCATI MOTOGP • INDUCTION TECH • PMWE
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