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Changes in Motorsport Regulations Are Prompting Fresh Developments in Engine Valve Technology, As Wayne Ward Reports

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Changes in Motorsport Regulations Are Prompting Fresh Developments in Engine Valve Technology, As Wayne Ward Reports Breathing Changes in motorsport regulations are prompting fresh developments in engine valve technology, as Wayne Ward reports our-stroke engines are now dominant in road transport What all reciprocating engines have in common is some means of applications, as they are in most forms of racing, often because ‘timing’ the entry of the fuel-air mixture (or simply air in the case of of regulation that mandates their use. Granted, there are niche diesel or gasoline direct-injection engines) and the exit of the products racing series that remain the preserve of the simpler two- of combustion. In reciprocating four-stroke engines, one method Fstroke engine, while gas turbines are powerplants of choice for some predominates, the poppet valve, which is controlled by camshafts offshore powerboat racing, but unless there are huge advances in these and springs. The fact that it remains so without serious challenge or rotary engine technologies, four-strokes will probably continue to is something we have to thank (or curse) the governing motorsport dominate motor racing for some time to come. bodies for. The scope for developing an alternative to the poppet valve Various racing valves. Note the different machining details close to the seat area; some have multiple angles (Courtesy of Manley) 28 FOCUS : VALVES isnew closed in many motorsport series where there are budgets to do so. life Forging of valve heads allows valves to be made in a It is well documented (1, 2, 3, 4, 5 and 6) that a lot of development single piece from a small-diameter bar (Courtesy of KPMI) has gone into such systems for a long time. The rotary valve for example, which represents the most serious challenger to the poppet valve, has undergone sporadic development for well over 50 years. The design, manufacture and engineering of poppet valves and their associated components is well understood after a century of continuous development, and it is unlikely that we will see them replaced to any great extent in the next 20 years or more. That is not to say that development of the poppet valve has stopped or even slowed appreciably. It is this continuing development, and the pace at which it happens, that makes it difficult for any new technology to replace the poppet valve. The current and rapid change in the architecture of passenger car engines, from large-capacity naturally aspirated engines to much smaller boosted engines, has been remarkable, but we are only part of the way through this process. Car manufacturers who are known for large-capacity performance cars are looking to replace their engines with boosted units with half or, in some cases, less than half the capacity of their naturally aspirated forebears. However, customers dynamics (CFD). Exhaust ports are also important: should they have won’t want to lose any car performance, so the levels of boost will be insufficient flow capacity, the amount of exhaust ‘residuals’ in the high compared to turbocharged engines from only a few years ago. combustion chamber when the inlet valves close will be higher than With high boost pressures will come high combustion temperatures, we want, again harming the output of the engine. so valve materials will have to change to cope with their new Combined with our optimised ports, the devices that control the operating environment. Racing has not been immune from this entry and exit of fluids to or from the combustion chamber – namely trend. For example, IndyCar has switched from naturally aspirated the valves – need to be opened and closed in an optimal manner. V8 engines to 2.2 litre turbocharged V6 units, and 2013 will be their So, the performance development engineer also spends a great deal second season of competition, while in 2014, Formula One will wave of effort in optimising valve lift profiles. He, or she, is limited in their goodbye to its 18,000 rpm V8 engines and usher in a new powertrain ambitions by a number of engineering realities, such as the proximity combining a 1.5 litre V6 turbocharged engine with high-tech energy of other moving components, considerations of stress and the mass of recovery systems. the reciprocating valvetrain components. The mass of the reciprocating valvetrain components is limiting in Considerations of mass a number of respects and at different points in the valve lift curve. Engineers with responsibility for engine performance tend to focus During the initial part of the curve, the valve and its associated much of their attention on the ability of the race engine to ‘breathe’ components are subject to high accelerations. As we know, for a given – that is, to inhale as much air as possible – and, having taken in mass, force is directly proportional to acceleration. The development this air, to trap the maximum amount possible in the combustion engineer would like the scope to be able to use high acceleration, so chamber. There is a great deal of attention paid to the design of the that the valve opens at the fastest possible rate, opening the port and ports so that the air (or air-fuel mixture) can enter with the least loss allowing the maximum flow through it. However, the forces involved of pressure, taking other factors into consideration. For example, a can be so high as to restrict acceleration through considerations of large port combined with a large valve may well cause a reduction in stress: the cam lobes or followers may become scuffed or pitted if the performance, despite having displayed high flow rates when analysed contact stress is too high. with a flow bench or when simulated using computational fluid In order to allow higher accelerations, we can look to the use t 29 FOCUS : VALVES of problems. First, the stress in the stem increases, as does the stress concentration factor at the junction of the stem and head, leading to significantly higher stresses. These can be mitigated to an extent though by increasing the radius at the transition between head and stem, but this takes away some of the advantage of reducing the mass. The second problem becomes clear if we consider the valve as an elastic system, with the head effectively acting as a mass and the stem acting as a spring. As we try to open the valve quickly, some of the intended motion imparted to the valve This thermal simulation shows the temperature throughout a section of the head and valves. The exhaust is sodium-cooled and shows good temperature distribution throughout the head from the camshaft is lost in compressing the stem. Of course, this is and stem (Courtesy of MW Racing) recovered soon after, but we have set an oscillating system in motion. Much of the complexity of valvetrain simulation comes from having of better materials than those currently employed, but in some to consider the influence of all components and their mountings as a applications the budget is sufficient to use the best materials as a collection of masses, springs and dampers. The amount of lost motion matter of course, and this avenue of allowing higher acceleration rates and the frequency of oscillation are affected as we change the mass offers no scope for a ‘quick fix’. The other way to reduce force and of the valve head and the stiffness of the valve stem. The further away stress is to reduce the mass of the valvetrain components, and there we go from the head, if we divide the stem lengthwise into small are various ways to do this, but here we also need to exercise restraint. elements, each element of the valve is compressed to a greater degree Later in the valve lift cycle, close to the point of maximum valve due to the inertia of the head and the part of the valve between the lift, we obviously need to bring the valve to a halt, so that it can begin element under consideration and the valve head. It is not a simple its return journey to close the port at the optimum time. Here, when spring-mass system; we also have to consider the mass of the spring, or developing a valve lift profile, we will often want to use the highest in this instance the valve stem. rate of acceleration again, but this time in the opposite direction. This Reducing stem mass by minimising cross-sectional area also acceleration is the reason why we need to have a method to maintain influences the valve’s bending stiffness. For example, if we have a the valvetrain in the motion that the development engineer has planned. 6 mm valve stem and wish to reduce its mass per unit length by 30%, In the vast majority of cases, a return spring is used. This might we can reduce its diameter to 5 mm or we can make it hollow by be the familiar physical spring or an ‘air spring’, of the type used in drilling it to 3.29 mm bore – that is, reducing the cross-section by t MotoGP or Formula One. In motorcycle racing and on its road-going bikes, Ducati ploughs its own furrow with ‘desmodromic’ valve actuation, which has no valve return spring but uses a separate cam Typical racing valves. profile to close the valve. At the point of maximum valve lift, the Note the use of dishing, different surface treatments, spring force required is proportional to the mass of the valvetrain finishing processes and reciprocating components, including a proportion of the collet groove styles (Courtesy of Supertech) spring mass. Again we see a case for the reduction of valve mass. A poppet valve is a very simple component, being a flat disc at the end of a slender column, and opportunities for reducing mass simply involve doing so at one or both ends.
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