In this article we shall begin to look at other valve materials, beginning with those materials used for supercharged or turbocharged applications. Highly charged engines extract impressive bmep and high ‘power density’ by forcing much greater amounts of air into the combustion chamber than is possible with even the most highly optimised naturally aspirated engines. To this greater quantity of air is added fuel in the correct ratio and much greater energy is released in a combustion chamber which is not significantly larger than found in a naturally aspirated engine. Consequently combustion pressures and temperatures are very high and the exhaust valve in particular is required to run at extremely high temperatures, having high-speed burned gases flowing over it at high velocity, with the consequent high levels of heat transfer to the valve head and stem.

Owing to the temperatures involved in such applications, those materials normally used for the manufacture of valves, such as steels or titanium are often not sufficiently strong, or may offer insufficient durability. For the majority of those racing every weekend, the option of rebuilding engines and replacing valves after each race is not an appealing one. Some very highly optimised turbocharged racing engines have successfully been able to exploit titanium valves, but these items have a short service life and are extremely carefully designed and managed.

The materials that valve manufacturers and engine designers turn to for these applications are those developed specifically for high temperature use and which find wide application in the gas turbine engine sector. They have been developed for their ability to retain high levels of strength at high service temperatures and also to resist creep. Creep is the phenomenon whereby materials will continue to stretch under constant load over time, and this is particularly apparent with increasing temperature. Interested readers will find that the particularly dry book Stress-Rupture Parameters by Conway, contains much data on the subject.

The materials which maintain high strength at high service temperatures and which are creep resistant are often expensive, as they contain large amounts of expensive elements such as nickel and cobalt. They are often described as ’superalloys’.

One of the most popular classes of materials for poppet valves on turbocharged or supercharged engines are the ‘Nimonic’ alloys, which are based on Nickel, but which also contain significant percentages of other expensive alloying elements. A typical material often used for valves is described as having a service temperature of around 800 degrees C, or 1500 degrees F, and which maintains around 90% of its room temperature strength at 600 degrees C (1100 degrees F). The valves shown in the accompanying picture are Nimonic parts for a motorcycle application.

Another suitable class of alloy are known as Inconel alloys. These alloys are again based on Nickel, but are not as widely used as the Nimonic alloys.

Fig. 1 - Nimonic exhaust valves.

Written by Wayne Ward

“Our biggest challenge with valve springs is making them survive with more lift and extra cam work, trying to make them survive. We are constantly working on different designs and lots of testing to improve the valve spring,” Currier told me. “It is basically a straight spring with flat tops; any taper would be unnoticeable to the human eye.”

Because NASCAR rules determine valve spring material of simple steel spring wire, Currier can’t work with exotic materials for his engine’s needs. “We do a lot of FEA analysis and we drive development on our valve springs because we end up doing most of the testing and most of the analysis.”

TRD drives development of its valve springs, but their suppliers have a lot of input into it. “For example a stiffer spring design might be more driven by them than by us: they’ll say a reason for doing it and we’ll say okay, we’ll try it. Or it will be integrated into something we are doing, so it’s kind of a mutual thing most of the time.”

Although it’s not always necessary, TRD does exchange its valve springs after every Cup race, which is a life cycle of about 700 miles on a standard race weekend. “We don’t necessarily trickle those down even though they could last a bit longer,” Currier said.

The configuration for an intake valve spring could be different from that of an exhaust and TRD makes changes “because the dynamics of the intakes versus the exhausts are a little bit different. The demands on each side are a little bit different; because of the interaction between them sometimes it’s helpful to have them be a little bit different,” he noted.

For TRD, specification updates on Cup valve springs is part of a whole package of things. “We run the changes about the same period for all of our internal parts and we might have three different valve spring options per year. We might use up some of our specs. For example, at some of the short tracks where top end power is not so critical, when we go to a new valve spring spec, we may use up some of the engines of the older spec at those particular races where it’s not so critical or doesn’t matter as much.

“Even though we’re on and off the pedal a lot at that type of short track, it affects the wear of the spring somewhat, but when we qualify an open spec package, it’s generally either for a particular track like a high-speed track (such as Atlanta or Fontana) and then it’s only going to be for that. Or it’s qualified for both the short tracks and fast tracks, which makes it less of a problem to switch over between one or the other.”

The difference between Plate and Open springs is due to the duty cycle. At a Plate race, “It’s such a constant speed; the RPM is lower and the throttle is always open, so the stresses are reduced and we can take advantage of that and do more with the spring,” Currier said. “We can try and reduce friction or make it stressed more - put in more lift or a more aggressive cam - because it’s not using that amount of stress capability, so we can lean on it a little harder. But then, when you lean on it harder, it usually fatigues and breaks and we end up with a milkshake,” he laughed.

Fig. 1 - TRD prefers a double valve spring without insert.

Written by Anne Proffit

Photo courtesy of Toyota Racing Development.

Why should this be so, and how do we then determine what we need?

Essentially we seek to transform rotational motion and energy at the engine flywheel into linear motion and kinetic energy at the vehicle tyre contact patch. To do so we require a mechanical connection between the two. Consider something as simple as a slot car. This does not employ a gearbox, but a single pair of gears in mesh, one on the motor shaft and the other on the solid axle to which the driven wheels are rigidly attached. Depending upon the characteristics of the electric motor there will be a variation on the number of teeth on each gear - the gear ratio - but this will be fixed. Speed control is achieved completely by the hand held rheostat operating on the electric motor. We often call this hand controller the ‘throttle’ although the electric motor does not breath air to produce power as an internal combustion engine does.

In theory and with a sufficiently powerful motor there is no reason why this type of control cannot be used on a full size electric vehicle. Multiple ratio gearboxes are only required where the torque and power characteristics of the motor unit are such that, if geared directly through a fixed ratio to the road wheels, it cannot provide sufficient torque throughout its speed range to impart the required in line tractive effort throughout the vehicles operational speed range.

With an internal combustion engine this is almost universally the case. It’s characteristic power curve rises to a peak from zero before falling sharply away again, which imparts a ‘torque curve’ that will also eventually fall with increasing speed . The phrase ‘running out of steam’ comes from an earlier era and a different type of powerplant with different characteristics again, but is still used to describe an internal combustion engine that can no longer sustain acceleration, or cope with an incline.

We use gearing to obtain twin objectives. On the one hand, it is intuitive that if we ‘gear down’ from the motor at a ratio of 2:1, then the geared axle will revolve at half engine speed. Our road speed will then depend upon the diameter of the driven tyres. At the same time, we will have increased the torque acting through the driven axle by a ratio of 2:1 when compared with the torque at the crankshaft of the engine. Conversely, when we ‘gear up’ at 2:1 we double speed and halve torque in comparison to the motor. On a typical car, we seldom find ourselves ‘gearing up’ except in the highest gear, which in a road car is referred to as an ‘overdrive’, and, in any case we put all of the torque through the gear train itself through one ‘final drive’ which reduces speed at something in the order of 3:1 and increases the torque to the driveshafts by 3:1.

In effect then, we are gearing down in all ratios between crankshaft and road wheel. Our lower gears, with the highest numerical ratio, give us the highest torque multiplication so that they can provide high tractive effort from a standing start, or to get out of a slow corner, but the vehicle will be speed limited in them. The mid range gears provide a balance between tractive effort and speed, whilst at the top of the range, we must sacrifice force for velocity.
To visualise this process and the estimate what ratios we might require, we plot the tractive effort in each gear against the road speed. Tractive effort is obtained by multiplying engine torque and the overall gear ratio (gear and final drive), and dividing by tyre radius. Road speed in each gear is obtained from engine rotational speed using the same factors, and so in effect we obtain scaled versions of the engine torque curve in each gear, plotted against the road speed range that we obtain through the engine rev range in that gear. In the lower gears these will appear to be scaled up along the vertical axis (which represents Tractive Effort) and down along the horizontal, or speed axis. In effect they looked ’squashed’ horizontally and stretched vertically. In the higher gears the opposite occurs. A typical series of tractive effort curves is shown in the accompanying illustration. This type of graph, when used with a conventional gear ratio chart, can tell us a great deal about performance, and this is something we will develop next time.

Fig. 1 - Traction effort graph.

Written by Peter Elleray

In terms of endurance, we know from reading Race Engine Technology, other magazines, learned papers and textbooks but probably more from experience, that poor attention to surface finish can cause a part to break. The textbooks will help us quantify the effect of surface finish on the fatigue strength of materials by giving us de-rating factors which depend on the process by which the surface is prepared and the level of finish that it achieved. We know that a part with a machined finish should be more durable than one which has a cast finish, that a ground finish should be more durable still, and that a component with a polished surface should prove to have the greatest fatigue resistance of all of these surfaces. A polished bar, such as that shown in the accompanying picture, will have a higher fatigue limit than a machined bar of the same material and dimensions. There are, of course, various levels of ground finish, or machined finish, and often the subtleties of the method of finishing are important, such as the alignment of any machining marks compared to the load path in the part. So, in this case, surface finish has a delaying effect in the initiation of surface cracks.

It is not difficult to imagine or demonstrate the effect of surface finish on wear. One only has to imagine the difference between rubbing one’s hand against a smooth piece of metal and a cheese-grater or a file to see that surface finish on a smaller scale can cause wear. This wear mechanism is abrasive wear and the ploughing effect of hard asperities (peaks in the small-scale surface finish of the part) is the main mechanism by which material removal is effected. By providing a smooth surface finish we can reduce wear by limiting the ploughing action of asperities, and by finding it easier to establish a protective lubricant film. Wear not only leads to harmful wear particles being carried to parts of the engine which are vulnerable to damage from such debris, but to loss of precision and early failure. Also, if wear debris remains within the contact area it can cause further damage.

In terms of friction, improvements in the surface finish can lead to lower frictional losses and therefore improved engine performance. This is especially true where lubrication is concerned. When the ‘combined’ surface finish of the two surfaces in contact is less than the established film thickness of the lubricant, then the coefficient of friction in the contact is determined solely by the action of oil shear. When there is no lubricant present, we have the coefficient of dynamic friction acting and at any point in between these two points we have boundary, or mixed lubrication, where the coefficient of friction is some intermediate value. By having a smooth surface, we need establish only a thin oil film in order to minimise friction and prevent damage.

In the next article we shall begin to consider some processes by which we can achieve a really good surface finish and look at some of the possible applications.

Fig. 1 - Polished bar.

Written by Wayne Ward

That modern engine lubricants are a complex cocktail of chemicals, required to perform a range of functions, (lubrication, cooling, cleaning, friction reduction, protection, and so on) is most likely well understood. And while the base oil will constitute between 60 to 90 % or more of the overall volume, the remaining part, that of the additive pack, can have as many issues associated with it as the choice of base oil itself.

Base oils can introduce swelling or shrinkage to a seal according to the type of oil and the elastomer used but it is the additive pack that can sometimes have the most unintended of effects. One of the most common historically was the cross linking of nitrile rubbers by lubricants. The active sulphur, part of the extreme anti-wear agent, zinc dialkyldithiophosphate, can react with the double bonds present in nitrile rubber and other elastomers to appreciably increase this cross linking density. The result is hardening of the rubber and subsequent cracking.

Molybdenum disulphide, a common solid lubricant included in many greases cannot be introduced into traditional engine lubricants because of its insolubility. However, soluble versions of this product, again often used as extreme pressure agents, in the form of molybdenum dithiocarbamates or dithiophosphates can also cause cross linking in nitrile rubbers. In the case of dithiophosphates this can also lead to de-polymerisation of silicone rubbers.

Perhaps the most concerning in recent times, is the effect on fluoroelastomers by free amines present in modern day lubricant dispersants and some friction modifiers. These dispersants are present in the oil to prevent the agglomeration of degradation products caused by oxidation or thermal stressing of the lube, which might otherwise cause harmful deposits. Because of their thermal stability and durability, fluoroelastomers have become a desirable seal material in engines but even a small amount of basic amine present in the oil in the form of a dispersant can result in dehyrofluorination leading to premature failure. Base resistant fluoroelastomers have been developed to offset this issue, but in formulating oils and balancing the needs of long term stability, soot handling capability and overall engine cleanliness, taking into account both new and older seal materials, this is a task not to be taken lightly.

In many cases it has been up to the oil industry to develop solutions to these sealing issues but, in the case of RTVs or Room Temperature Vulcanising sealants, similar to those used commonly in engine build, the converse is true. First generation sealants of this type produced the by-product acetic acid, which leached out into the oil after curing. This reduced the base reserve in the oil designed initially to ‘mop up’ all the weak acids caused during the combustion process. RTVs developed for use in engines have now addressed this problem. However a secondary issue, the leaching of low molecular weight silicone plasticizers from the sealant into the oil is causing major concern. In the oil industry high molecular weight silicones are used to inhibit foaming in engine oil. Insoluble in the base stock in order to work effectively, they are dispersed through the oil by clever formulation. Silicone plasticizers however are soluble in the base oil and rather than working to prevent the formation of foam, actually stabilise it. In any engine particularly competition engines, the presence of foam because of these silicones can have potentially catastrophic consequences.

Fig. 1 - Highly Saturated Nitrile Rubber seal.

Written by John Coxon

Forming a seal between the piston and cylinder bore the goal is to prevent the passage of combustion gases between combustion chamber and crankcase on the one hand, and at the same time minimise the passage of oil from the crankcase up into the combustion chamber on the other. During the compression and power strokes the compression rings seal the combustion gases and control the exhaust blow-by while on the downward strokes, the excess oil thrown up onto the cylinder walls is scraped off and returned eventually back to the sump, leaving only a small amount of oil on the cylinder wall. Since the majority of parasitic losses occur in the ring pack in the form of friction, all this has to be done using the minimum of side force between ring and bore throughout the cycle.

The Dykes ring is therefore a lightweight ‘L’-shaped ring with only a very low spring tension on the cylinder wall in the static condition. During the intake and exhaust strokes the arrangement will exert little in the way of side force on the cylinder wall but on the power stroke combustion gas can flow down past the increased gap around the top land and through the convoluted space between ring and piston and force the ring out against the bore. In this way, or so the theory goes, friction is only apparent during the combustion stroke. Furthermore, the reduced mass of the ring because of the ultra thin section minimises the chances of ring ‘flutter’ caused by acceleration and deceleration at high engine speeds. Additionally, machining the grooves into the piston required much more care than other arrangements.

Needing a special piston with groves to match, their asymmetric profile can make them hard to seat leading to higher bore wear. Replaced by more effective designs, other than in certain specialist applications, Dykes rings have now been consigned to history.

Fig. 1 - Cross-section of the Dykes ring.

Written by John Coxon

Although the service life of intake and exhaust pushrods is, conceivably, longer than the 700 miles that make up each Sprint Cup race weekend, TRD elects to replace them after each race. “Generally they are in a condition where we probably could reuse them, but we just don’t,” Currier told me. “We’ll use them again for something else, like in development or performance engines or in dyno engines for testing other components, because they are in good shape.”

The pushrods used by TRD are part of the TP (Trend Performance) line of double-tapered rods, using 7/16-inch diameter and a 165 wall. The manufacturer places a 100-thousandth wall down the centre, which is, according to Trend’s John Williams, a standard hole. Cases are hardened to Rc 60 and there is a black oxide finish with laser-etched length. The pushrod, 7/16-inch at its widest point, tapers to 3/8-inch. Lengths on both intake and exhaust pushrods runs in the 7-8-inch range, Currier said.

The material for TRD’s pushrods is 4130 chrome moly and the 5/16-inch ball end is CNC-lathe machined onto the rod during manufacturing; it is a single-piece item. “There used to be some tip wear issues, but we don’t have any of those issues anymore,” Currier said.

“Configurations are changed when we do something different to the valve train; then we change the length of the pushrod; sometimes we’ll do a little other work with it, but it just fits into the process. We might change the diameter or something like that,” Currier explained.

The challenge, he noted, “is to fit them into the head so it’s a little bit tricky because we have to work to make it fit. We’re trying to get the biggest pushrod we can fit into the head and to accommodate the bigger rod depends on what kind of machining we can do around the head for clearance where the pushrod goes. This depends on what we’re doing with the rocker arm geometry. That can affect where the rod is placed, relative to the head.”

Of course there are times when you can’t compromise on placement, so Currier might need to go with a different size pushrod in order to get it to work. “That is where we have to compromise,” he shrugged. “Sometimes we might have to go with a less hefty pushrod for fitment purposes, but we try to go with a stiffer, more responsive pushrod in all instances.”

Fig. 1 - Toyota Racing Development uses double-tapered pushrods from Trend Performance.

Written by Anne Proffit

Photo courtesy of Toyota Racing Development.

Still, according to David Currier, TRD’s vice president of engine engineering, the specs change on these pistons about three times each season for open configurations and about twice annually on the plate engines. “We change our specifications fairly frequently because if we change our head geometry, that changes the crown shape; we change chamber shapes from time to time, as well as valve lift, which changes pocket depth. Of course we’re always working on friction, slip skirt shape and rings so we’re continually evolving those things.”

Currier admits there are more challenges with the open engine - used far more often than plate configurations - but the pistons, he noted, aren’t that difficult to deal with because of the mandated 400-gram weight for all configurations. No matter the configuration, TRD uses its pistons for a single use of one race weekend. “Structurally speaking, there aren’t any big problems with the pistons; certainly, wear relative to skirts and optimizing the complex three-cam shape is a continual battle in adjustment. But as the engine changes we change the crown shape and that changes some of the structure so it will flex differently,” he told me.

Working together with their supplier, TRD has verbal discussions of “take a little off this area” and occasionally makes a proposal based on their post-race teardowns. “It just depends on who’s got the experience and what it is we’re seeing,” in regard to wear. “If it’s something they are better at than we are, then we let them use their expertise. If it’s something we feel we have more knowledge about - or a stronger opinion about - then we’ll dictate it more.”

Currier emphasized that changes to the piston skirt design has to be a balance between cost and logistics, because there are races nearly every weekend between February and November on the NASCAR trail. “We try to do fewer steps but bigger steps” when a piston redesign is called for. “Just because of the costs,” Currier said. “Trying to control our costs and get our budgets further refined and further down as they tend to be going. If we change every weekend, then we have spare engines that need to be changed and there is a lot of cost with that.”

When TRD does change piston design, “It might be due to other things that cause the piston to be changed, like a combustion chamber change, or the head’s a bit different and the valves are different. Those things usually go as part of a package.”

Fig. 1 - TRD’s piston specifications change a few times each season.

Written by Anne Proffit

Photo courtesy of Toyota Racing Development.

Now before you start to think that I’m some sort of crackpot with a grudge against the filter manufacturers, I’ve done my homework. Once many years ago I had occasion to run a series of tests on an engine with the oil filter removed. The tests, part of a research project into engine wear using radioisotopes, were conducted in an engine dynamometer test cell, monitoring the build up of radioactivity in the oil due to the minute quantities of wear that was taking place in the top ring reversal point on one of the cylinders. At first we ran a traditional set up monitoring the build up of radioactivity in both the oil and filter, but for some reason, the exact details of which have long been forgotten, it was necessary to remove the filter and continue testing purely monitoring the condition of the oil.

Testing continued apace with a series of tests each lasting up to ten hours of wide open throttle running on a proprietary 1600cc gasoline engine. After each test the oil was carefully drained, flushed and replenished with a new, slightly different test lube. Altogether we must have completed some 15 or more tests with very strong and repeatable radioactive wear signals coming from the oils. Altogether with flushing cycles and warm up cycles, the engine must have covered 200 or more hours of quite arduous testing, but it was at the cessation of testing when we received our greatest shock. Once the engine had cooled down and had been carefully stripped adopting all the usual techniques and safety precautions necessary for handling (only very slightly) radioactive engine components, there was no visible evidence of wear of any sort on any of the critical parts. Wear there most obviously had been since we had monitored it during the tests and confirmed it using more sensitive off-line radioactive measurements. And the wear we had monitored in the oil signal was greater than we had measured in the oil during earlier tests when the filter was still in the line.

The conclusion to all this was while the filter was obviously trapping some wear debris, when removed, that debris was retained in the oil no doubt held in suspension by the detergents but it didn’t do any harm to the engine overall. The tests were short (less than 10 hours) and the oil changes were made under ideal conditions, so why can’t we do the same with our sprint car engines and dispense with the oil filter? Discuss.

Fig. 1 - A typical oil filter.

Written by John Coxon

However, inevitably when the glory days are over and the unit is discarded, perhaps left on its own in damp storage or bolted into the back of some historic racing car somewhere, another process is slowly taking over, more silently or deadly than the occasional missed gear change - that of corrosion. And while this can (and does) occur in many parts of the engine, the main area of concern is at the back of the cylinder liner inside the cooling water jacket.

Euphemistically now referred to as environmental stability, the causes of corrosion can be many and varied. We can have general surface corrosion, pitting corrosion, crevice corrosion, stress corrosion, fretting corrosion and some, maybe even all may be found inside a water cooling jacket of an engine. The common link between them all however, is water, but some materials are more prone to this type of activity than others. In the case of that most common of cylinder liner materials, cast iron, this process can be highly sensitive to the presence of water while others, aluminium for instance, apparently not so. The governing factor in all this is normally the position of the metal in the electrochemical series, but this is not always the case. For example, with its position higher in this table you would expect aluminium to be more readily corroded. This is not always so. When exposed to the air, aluminium rapidly reacts with the oxygen in it to produce a thin layer of aluminium oxide. Aluminium oxide has good adherence properties and is by and large impermeable to water and therefore will form a protective coat around the metal. The standard electrode potential of -1.66 volts being greater than that of iron at -0.44 volts therefore has little influence.

Perhaps the most common form of corrosion in the water jacket however, when the cylinder liner is cast iron or steel, is rust. Formed in the presence of both oxygen from the air and water, the iron atoms of the liner outer surface give up their free electrons and become positively charged (Fe2+). Electrons liberated so then migrate through the metal and into the water where they can combine with any dissolved oxygen to create negative hydroxyl (OH-) ions. These negative hydroxyl ions then combine with the positive metal ions to produce hydrated iron oxide or Fe(OH)2. This oxide, referred to as rust is somewhat powdery and easily crumbled and unlike aluminium oxide when coating aluminium, does not protect the metal from attack. If anything it actually promotes it by retaining the water close to the metal surface. In cast irons however consisting of a mixture of iron and carbon in the form of graphite, this effect can be much worse since the graphite can act as the cathode and the iron as the anode. In the presence of water the pair act as a galvanic cell and the iron corrodes producing rust and a phenomenon sometimes referred to as graphitisation.

In general however, the easy way to prevent or certainly reduce the effects of corrosion, is to use the correct inhibitor or anti-freeze solution if ambient conditions are likely to fall to freezing.

Fig. 1 - Corrosion in a cast iron block - the deadliest of enemies.

Written by John Coxon