material with a view to having the valves made in a certain homogenous metallic alloy.

If the parts of the valve are studied in isolation, we might choose to make some parts in different materials to achieve optimum performance. To an extent, through the use of coatings, we are already able to place a different material at the surface of the valve for tribological reasons, and it has been possible for many years to provide hard-wearing valve tips using materials such as ’stellite’.

There are manufacturing techniques applied to valves that can offer different properties in the head and stem. Production methods such as stem extrusion and head forging (upsetting) provide different levels of work and grain orientation in the stem and head. However, through the use of techniques such as friction welding, we can produce the head and stem in different alloys of a given type of material. Indeed, this technique is often used to provide hard-wearing valve tips.

The friction welding process is widely used in industry, but it is generally for ease of manufacture rather than a method for joining dissimilar material grades. There are instances where the technique is used for aerospace and nuclear applications to join materials that differ widely in nature, such as aluminium and steel. The technique is clearly very versatile and can also be used to produce excellent bonds between similar materials, such as different alloys of a given basic material or some more widely differing material pairs. Titanium is an obvious application for such a technique, where a ’stem alloy’ could be joined to a ‘head alloy’ with ease.

Most of the deflection in a valve is in the stem, so naturally we might choose to use a material with the highest possible modulus here, and pay less attention to the density. The head of the valve could be made from an alloy selected for a combination of low density and other properties such as temperature resistance.

While there isn’t a great deal of difference between many of the suitable titanium alloys in terms of the properties that we might choose to optimise for our valve, there is the opportunity to join titanium to other materials. We might for example, if the rules pertaining to our race series allow, join a titanium valve head to a titanium aluminide stem (or vice versa). At the moment, titanium aluminide is still a very expensive material, and the manufacture of whole valves from this material would be comparatively costly. However, most of the gain might be realised at significantly lower cost if only part of the valve were to be made from it.

The technique is cost-effective enough for it to be used for the manufacture of some steel racing valves, with a higher-grade material being used for the valve head.

Fig. 1 - Friction welding is used to provide economical valves by using expensive materials only where they are really needed

Written by Wayne Ward

Stanton Racing fields both Mopar and Toyota Midgets in USAC competition. The company took over the build and rebuild procedures for California-based Toyota Racing Development about three years ago, and since then it has reworked the engine and focused on longevity as well as power.

“A good example of our valvetrain in one of our Toyota engines are the results for Tracy Hines, who went 18 nights on the same set of valve springs,” Milholland says. “They came back to us and were down only about 10 lb.” Stanton Racing used to have to change the springs after four to six nights due to breakage, and were losing upwards of 30-40 lb, but changed its valvetrain package after the 2009 season. “We fell in with a company that offers a good overall spring - and it all works. They found the magic bullet,” Milholland says.

Of course, the mandated 8700 rpm limit for all USAC pushrod Midget engines has also helped keep the valvetrain happy. “We were running 9300 and 9400 rpm, and that was really twisting the springs, because there’s a huge difference in harmonics from 8700,” he says. “At the same time, we’ve got customers who get into the rev limiter so hard and for so long that they were getting deflection and other issues. It seems the valve springs are staying a lot more consistent with the lower rpm, more than anything else.”

Milholland points out that his company - as with every other National Midget engine builder - is somewhat limited by the outside and inside diameters on the springs. “It all depends on the size and shape of the cylinder head,” he explains. At the moment, Stanton Racing uses a double beehive spring with inner damper; it has an outside diameter of 1.500 and inside diameter of 0.790, with coil outside diameter of 0.210 and insider diameter of 0.150. Weight on the valve spring is 145 g, and Stanton is using the same spring for intake and exhaust.

This steel product - steel retainers and seats are also specified - has been through a nitriding process that allows for the kind of wear and fatigue in typical USAC National Midget races, which are held on both asphalt and dirt-track circuits of varying lengths and preparation. “We rely on the manufacturer for consistency of the material,” Milholland says. “We don’t have a good way of analysing material consistency here; our focus is on spring tension.”

When a typical USAC National Midget engine is built, spring tension starts at about 270 lb of pressure on the seat and may soften by about 10 lb before rebuild, after ten to 14 races. “It’s really inconsequential,” Milholland says. “We use an install height of around 2 in installed at 270 lb open; our camshaft package will be open at around 800 lb, actually in the area of 795-805 lb.”

Stanton Racing finishes the ends of the valve springs itself. “We’ve got a little procedure we go through as far as trimming the ends off them. We use a high finish sanding cone, maybe 400 grit or so, to finish them before insertion,” Milholland says.

Cooling is accomplished through spray procedures. “It’s a bit different for the Mopar and the Toyota designs,” he says. “Some of the Toyota designs have oilers through the valve covers, and others come through the rocker arms. We’re spraying them, one way or the other, but it’s a different design philosophy for the Toyota and Mopar engines.”

Before Stanton Racing came up with its current package, it was breaking springs and needed to find products that would last. For the past couple of years, none of those problems have been evident.

“We used to worry about softening of our springs - that’s the only type of wear we experience, aside from breakage - which we haven’t had since we put our new package together,” Milholland says.

At the final race of the USAC National Midget season, the 71st annual Turkey Night Midget Grand Prix at Toyota Speedway in Irwindale, California, Midget engines prepared by Stanton Engines held the night. Its Toyota engines set fast times with a new track record and won the race. And no, there were no valve spring issues.

Fig. 1 - Stanton Racing uses the same valve springs for intake and exhaust on its National Midget engines (Photo: Erik Milholland)

Written by Anne Proffit

Magnesium:
Magnesium has long been a favoured material for gearbox casings, thanks to its useful mechanical properties. Magnesium alloys are about 35% lighter than aluminium alloys, and certain alloys can be heat-treated to UTS values approaching 43 ksi, making them attractive because of their high strength:weight ratio.

The stiffness of magnesium is generally only about 63% of aluminium alloys, however, so components being switched from aluminium to magnesium will need larger cross-sections and section moduli to achieve the same stiffness as the aluminium part, but can still give a weight saving of 20-25% depending on the design. Magnesium can also absorb shock loadings better than aluminium, making it ideal where durability is need. However, in recent years the emergence of new casting techniques for aluminium has toppled magnesium from its position as the material of choice.

Aluminium:
In the past, the major factor governing the weight of aluminium gearbox casings was the casting wall thickness. Traditional casting methods did not allow thin wall sections to be cast reliably, thus limiting weight savings irrespective of the actual component strength. However, new casting methods that allow for very thin wall sections to be created mean engineers can now create casings that are marginally lighter than a magnesium counterpart, while still retaining the same strength characteristics.

As a result of these new casting methods, combined with advances in FEA simulation, the benefits of an aluminium casing now outweigh those of a magnesium unit, although the production costs are considerably higher. As a point of note, the manufacturer of the aluminium gearbox used by the HRT and Virgin Racing teams quotes the weight of the unit as being “in the region of 40 kg”.

Titanium:
The first titanium gearbox to appear in Formula One was produced by Ferrari in 1997, and was fabricated as opposed to cast. Although it was exceptionally lightweight compared to a magnesium or aluminium unit, the difficulties of fabricating titanium meant the production costs were astronomical.

It was not until the Minardi team began experimenting with a cast titanium unit in 2000 that the material became a viable option for casing construction. The use of a rapid casting process, with the extensive use of rapid prototyped patterns (created using a selective laser sintering process), meant that not only did the production cost of the transmission fall considerably, but the entire design could be optimised to a greater degree. This was thanks to the complex patterns that could be created through rapid prototyping, allowing for internal structures that were not possible with traditional casting methods.

The end result in the case of the Minardi gearbox was a weight saving of 25% and a reduction in package size of 20% over the previous magnesium casing. Titanium is still used extensively in Formula One transmissions, although the current trend is to combine it with composite components.

Composites:
The first carbon fibre Formula One gearboxes appeared in the late 1990s, but considerable problems were experienced in relation to the heat rejection properties of the material. Today, the most common use of carbon in transmissions is in conjunction with metallic - usually titanium - structural components.

One of the first teams to take this approach was Renault with its R23 car in 2003, which used a casing with a titanium lower section and carbon fibre upper section. However, the design was dropped the following year in favour of a 100% titanium unit. The current trend is now for the structural ‘bulkheads’ of the gearbox to be fabricated from titanium, with the rest of the structure moulded from composite material, providing a very light yet exceptionally stiff structure.

Fig. 1 - The fully cast titanium gearbox produced for the Minardi team provided significant weight savings

Written by Lawrence Butcher

The process itself is generally carried out either by immersion or spraying, with the parts being treated using a combination of phosphoric acid and various phosphate salts. There are variations on the coatings, with iron phosphate, zinc phosphate and manganese phosphate being used.

The zinc and manganese variants are most commonly used where improved lubricity is sought. There is the possibility, however, of hydrogen embrittlement through the use of phosphate conversion coatings: the metal-acid reaction liberates hydrogen at the surface, which can be absorbed by the surface, causing it to become brittle and lowering the fatigue limit. The problem is more serious in electroplating processes, and should be borne in mind if specifying phosphate coatings on high-strength steels, which are known to be most affected by hydrogen embrittlement. This hydrogen embrittlement can be minimised by adding an oxidising inhibitor to the solution, which converts any hydrogen to water.

While often specified for corrosion resistance, the phosphate conversion of the surface itself offers only a small improvement, as the phosphate layer is porous. The coating is therefore sealed using oil, which fills in the pores. Both manganese and zinc phosphate treatments are used where corrosion resistance is required.

The more useful property of phosphate treatments to engineers involved in race powertrains is the tendency to improve lubricity and prevent galling during periods of oil starvation, especially at first build and start-up. Again, some of this lubricity comes from the fact that the phosphate coating is sealed with a lubricant, which can be oils or greases.

Where this property of lubricity is required, manganese phosphate coatings are generally specified. The treatment is used on cam followers as an aid to break-in, and is also used on pistons for the same reason. Other notable engine applications are camshafts and piston rings, although gears and cylinder liners have also been treated with success. Owing to the fact that the coatings are porous and hold oil, even as they wear, they present a lubricating surface because there is oil present throughout the structure.

Both manganese phosphate and zinc phosphate treatments can show a range of colours, between black and grey, with the manganese salts showing a generally darker colour.

As mentioned above, one point to be wary of with the phosphating of high-strength steels is the possibility of hydrogen embrittlement. Normally associated with metallic plating processes (most especially with electroplating), this problem occurs where hydrogen is formed at the surface of a steel part. In the case of phosphating, this can happen during ‘pre-cleaning’ processes, where any oxide layer is removed by acid before phosphating, and during the phosphate process itself. Both involve the interaction of acids and metals; from school we might remember that salts and hydrogen gas are formed. For high-strength steel components, some standards recommend a ‘baking’ treatment as used with electroplated parts.

Fig. 1 - Phosphating is widely used to treat engine components, including piston rings

Written by Wayne Ward

One of the simplest approaches has to be the straightforward flange. Simple to machine, with its surface at right angles to the cylinder centre line, the mating flange of the liner is clamped against it by the reaction of the cylinder head against its retaining bolts. In the past, sealing has been achieved using what is essentially a large, flat copper washer placed around the liner. Trapped between liner and block, this type of seal can be highly effective, rather like a ‘fire’ ring only midway down the liner and sealing the lower gap.

Unfortunately though, pure copper begins to soften at not much beyond 150 C, and the ensuing creep can very quickly destroy the clamping load at the cylinder head. When such simple seal arrangements are still used, copper has invariably been replaced by medium strength, anaerobic methacrylate sealants, which are not only cheaper but more forgiving and much easier to assemble to retain the correct amount of liner ’stand-off’.

However, the most common method these days is to use O-ring technology which, although comparatively expensive in terms of machining costs, has proven to be far more reliable, if a little fiddly at the assembly stage. Modern O-ring elastomers are also far less prone to the effects of aggressive oil basestock technology or hardening at the temperatures experienced, but despite the levels of reliability generally achieved, manufactures still often opt to put two or even three rings where arguably one should suffice. Whether the retaining grooves are machined into the block or the liner is down to the designer’s preference, but in most race applications the former is more common, and since the whole of the liner is not loaded in compression, its thickness can often be reduced.

At one time I well remember these separate O-rings being replaced by a two-pack fluid silicone elastomer. Injected into a hole or gallery in the side of the block that led into the O-ring groove, the thixotropic mixture (one that becomes less viscous when subjected to an applied stress) flowed around the outside of the liner and into another gallery on the opposite side of the block. A close fitting between the block and liner at this position ensured that little or no elastomer escaped, and that when the sealant appeared at the other side a continuous sealing ring was assumed to be in place. Once the elastomer had set (after about 20 minutes), however, removal of the liner was difficult since, rather than relying on contact pressure - as in the case of an O-ring - the seal was a result of a bond between the elastomer and the materials surrounding it.

While the trials with this approach were successful for the most part, new rig test equipment and techniques had to be developed to seek out any weakness well away from the engine test cell. For, as we know, once inside a running engine, finding the root of any leak becomes so much harder!

It may often be the smallest of seals in the most obvious of places, but finding out what is truly happening if things go wrong is never that simple.

Fig. 1 - Lower liner sealing

Written by John Coxon

All steels of course contain some level of nitrogen within them, particularly when in the molten state. With a maximum solubility of around 450 ppm, which drops to nearer 10 ppm once the material has cooled to ambient temperature, nitrogen can have a beneficial effect on steel if it remains in solid solution or precipitates out as a fine coherent nitride at the gain boundary. The presence of molybdenum, chromium or vanadium in the mix can also help the material form metal nitrides. However, as the amount of nitrogen increases, its degree of formability decreases, so it is often better to make the component first, introducing nitrogen into the surface layer at a later time.

Such a process is called nitriding, and is the accepted practise of forcing nitrogen into the surface of a steel component to produce a much harder layer which is more wear resistant. Typically somewhere around 0.001 in ( 0.025 mm) thick, this layer is comparable with other forms of case hardening, and gives a hard outer layer around a softer but tougher central core. Since lower temperatures are generally used, less distortion of the final product is to be expected. Processed after heat treatment and subsequent tempering, in some cases final grinding may be necessary to remove a hard ‘white’ layer before finishing to size.

More commonly associated with the case hardening of crankshafts and camshafts, there are principally three methods in use. The first is a salt bath that uses cyanide-based salts, and because of that it is no longer in general use. Of the other two methods - gas nitriding and ion or plasma nitriding - both are of potential interest to piston ring manufacturers. Gas nitriding, whereby components are placed in an atmosphere of nitrogen-enriched ammonia gas in an air-tight furnace, will produce all-over surface benefits. Plasma nitriding, however, as a result of its better targeting, can be confined to particular surfaces.

Used in diesel and other automotive OEM engines to replace the toxicity of chromium plating methods, nitrided rings can be used on their own against just about any Nikasil or iron bore surface and in the aggressive environments offered by exhaust gas recirculation. At around 110 Vickers harness (50% greater than the base steel) it has also been shown to offer improved durability over chrome. For competition use, nitrided rings have been shown to produce a higher level of performance over the traditional moly-coated alternative, since the nitriding layer is integral with the base steel and not an ‘add-on’ coating. In some circumstances molybdenum coatings have been known to flake off as a result of thermal shock experienced in heavily turbocharged engines.

Mindful of the strength benefits that nitriding gives to the base ring material, in some cases manufacturers offer traditional PVD coatings on top of the nitrided layer - a sort of ‘belt and braces’ approach that produces even higher hardness numbers, with 1400-2200 Vickers being quoted.

Nitrogen may be all around and even within us, but when introduced to the surface layer of a steel piston ring its effects may be more noticeable.

Fig. 1 - A PVD-coated ring after nitriding

Written by John Coxon

This means that the relationship between the tip of the valve, the roller tip and rocker pivot are as designed. Not only does a badly set-up mechanism deviate away from the designed valve lift profile, but the relationship of the contact between the valve tip and roller compared to the valve axis is wrong. This can lead to unintended bending stresses in the valve, and this in turn can lead to problems such as valve guide wear.

There are three major variables in the equation here, each of which can be modified independently by component selection. First, we have a variable-length valve, by virtue of having a range of lash caps at our disposal of varying thickness. Second, we can order pushrods of almost any length we might desire. The third variable is the height of the rocker stand axis.

Essentially, for a given combination of rocker geometry, valve geometry and valve lift, there will be an ideal height from the top of the lash cap to the axis of the rocker. While people can have different ideas about which point in the valve lift cycle the roller axis and valve axis should intersect, most would deliberately avoid putting excessive bending into the valve stem. For teams running the rollerless rocker designs that have been used by some of the Sprint Cup teams in recent years, the problem of bending is likely to be more of a concern, with a greater coefficient of friction being present between lash cap and rocker.

The suppliers of pushrods and associated components are pretty clear that you need to have this relationship between valve tip and rocker pivot axis correct, and only then specify the pushrod length. People who simply try to accommodate a collection of good parts they have to hand are inviting trouble.

A number of suppliers manufacture checking tools that are simple to use and ensure that an engine builder always gets the correct geometry. Of course, if you are running something unusual and special to your engine, you will need to design your own jig or modify an existing one.

Other suppliers specify a procedure during which you check by eye that the roller contact moves across the top of the valve, with the extremes of travel being equidistant from the valve axis. If the centre height of the rocker pivot is too high compared to the valve tip, the options are to fit a longer valve or a thicker lash cap, or machine the base of the rocker stand or the seat area on the cylinder head. If the pivot height is too low, the options include fitting shorter valves, thinner lash caps or shims under the rocker stands. The procedure, whether using a bespoke tool or checking the ’sweep’ of the rocker over the valve tip by eye, may involve building the components onto the head a few times.

In the case that the rocker stands incorporate mountings for a number of rockers, they cannot be shimmed to each individual valve, and adjustment of lash caps is likely to be required.

Fig. 1 - The height of rocker stands carrying individual rockers can be adjusted to suit the individual valve they operate

Written by Wayne Ward

Erik Milholland manages Stanton’s racing division, and travels with the USAC circuit, handling any powertrain issues its customers might have.

The Toyota and Mopar engines have differing cylinder heads that necessitate a choice of dome shapes on its forged pistons. “With our current cylinder head-camshaft combination,” Milholland says, “the lower piston dome of .020 on both our Toyota and Mopar engines performs better because of less disruption of airflow.”

The 2618 forged aluminium piston has three rings - the gapless top and second rings are 0.032-and-a-half while the oil ring is 2 mm. The Toyota piston weighs 465 g; the Mopar is 450 g.

“We use a shorter dome because we wanted to flow more air through the combustion chamber. That’s our basic priority,” Milholland says. “We can do that with smaller combustion chambers on our cylinder heads and still keep our compression up where we want it to be, even as we minimise the dome shape.”

The compression on these national USAC Midget engines is 15:1, and the Toyota and Mopar inline four-cylinder engines make upwards of 390 hp on the dyno and about 365-370 hp at the wheel, which is standard for the breed.

With their emphasis on low-end torque for racers competing on differing track sizes and specifications - both asphalt and dirt - engine builders in the USAC National Midget Series also have to deal with a limit of 8700 rpm for pushrod engines like those produced by Stanton.

The shorter piston dome shape came about because Stanton had cracking and breakage problems with its previous manufacturer’s design. “Our new partner helped us work on our own piston and ring package designs so that we’ve been able to eliminate those problems,” Milholland says.

In the Toyota and Mopar engines that Stanton produces, he says, “The shapes are different because the cylinder head shapes are a bit different. Still, overall, the pistons - as far as the forgings, coatings and ring packages go - are all primarily the same.”

Skirt profile is determined by “trial and error on our end,” Milholland confesses. “We’ve run piston clearances as low as three or four thousandths and as big as eight or eight-and-a-half thousandths. Over the last year-and-a-half or two years, analysing skirt profile, we came up with the height clearance and skirt profile we were looking to achieve.”

To accomplish piston cooling, he says, “We run a twin oiler system squirting two jet oilers on the piston and pin at all times with the Toyota, whereas we only run a single oiler on each cylinder with the Mopar. The Toyota is a purpose-built engine specific to the Midget racers, and they did a really good job of building up the oiling system. We have a little machine process we go to with the Mopar engines, so we can go to the main oil gallery and feed that single oiler to the piston.”

Wrist pins, which come with pistons from the manufacturer, are Casidiam coated and according to the engine being prepped. “The Toyota was developed with 0.866 pins, and the rods are fitted for that dimension,” Milholland says. “The Mopar engines run the standard 0.927-diameter pins everyone else uses.” Pin retention is by round wireclip.

Stanton Racing likes to check the pistons after four to six nights of racing and may, depending on the client’s budget, re-use them. “We make sure the clearances, pin bores and rings are all within tolerances. We recommend ten to 14 nights on an engine, but that depends on the size and length of the track ,and the type of circuit. We have more wear with dirt, but dirt always gets into these engines,” Milholland says.

He reckons 90% of the engines get new pistons each time they’re rebuilt. “If they’re coming back for a rebuild, you might as well spend another $1000 and get new pistons. A standard rebuild runs to $5000-7000; fresh engines cost around $33,000,” he says.

Because the USAC National Midget Series isn’t in the best of health - and because this is, after all, a business - Stanton Racing has gone to a heavier piston for its inline fours. “We’ve been helped out by the rpm rule, and lighter is not always better. We’re still able to pull the rpm with our heavier piston, and we’ve got our longevity. By adding a bit of weight we’re able to get our engines to live longer, and we’ve doubled or even tripled the life expectancy of our engines,” Milholland says.

“With the added piston weight, your biggest thing is taking the bob weight, the rotating assembly balance and crankshaft balance into consideration. We’re probably giving up 15-20 g - a total of maybe 80 g overall - but we’re getting much more life out of it. You’ve got to keep the cars going and keep the customer happy.”

At the Turkey Night Midget Grand Prix on the half-mile Toyota Speedway at Irwindale on Thanksgiving night, Stanton Racing’s engines set fast times, a new lap record and won the race. That’s keeping the customer happy.

Fig. 1 - Toyota (left) and Mopar pistons have coatings on all but the short dome and in the ring grooves (Photo: Erik Milholland)

Written by Anne Proffit

Running wet sumps has caused problems for many years, as one engineer from a notable engine manufacturer explains, “We’ve said for a long time that a dry-sump system would be cheaper, more reliable and easier for everyone. Big parts of the budgets are routinely spent trying to make a wet-sump system with the standard production sump pan work in a racing environment. It’s been a nightmare for every project we’ve been involved with, right back to the Sierra Cosworth days.”

So what can engineers do to optimise what is essentially a flawed system? Here are some of the approaches used to mitigate the disadvantages of running a wet sump.

Baffles:
Baffles or windage trays are one of the most commonly used systems to control oil in a wet sump. They often consist of horizontal and vertical plates within the sump, and often feature one-way gates to prevent oil surging away from the pick-up. In the case of the S2000 regulations, the cars have to retain the standard sump pan, so controlling the slosh of oil within is vitally important.

However, baffles need to be carefully designed to ensure they do not in fact restrict oil flow towards the pick-up, or slow the return of oil from the upper half of the engine. With powerful CFD simulation packages now becoming more accessible outside the upper echelons of motorsport, engineers have a powerful tool to assess the effectiveness of baffling. Whereas previously a system could only be tested by running the car and logging the oil pressure traces, designers can now get far closer to an optimum solution without making large numbers of prototypes.

Movable oil pick-ups:
One other solution to preventing oil starvation is a pivoting oil pick-up pipe. Instead of trying to keep the oil near the pick-up, the pick-up is able to move around in the sump, using the same force vectors that affect the oil.

However, to be effective, a pivoting pick-up will often require the use of a dedicated sump pan, in order to allow the pick-up to move freely. This means that if the standard manufacturer’s sump needs to be retained, the effectiveness of any pick-up design is severely limited.

External oil reservoirs:
If it still proves impossible to maintain reliable oil pressure using either of the above methods, another option is the use of a pressurised, external oil reservoir. In recent years this has been the route adopted by a number of teams competing in the British Touring Car Championship.

The reservoirs are pressurised up to normal oil pressure and then the release of oil is controlled by a pressure-sensitive switch. If the pressure in the engine oil system drops below a preset level, the valve opens, allowing extra oil into the engine to maintain pressure.

Although it’s effective, there are a number of disadvantages with the system, notably additional weight and complexity. It also does not give complete insurance against pressure loss - as the author discovered, at the cost of a crankshaft and set of rods, after a pressurised system could not handle the oil surge in a boxer engine!

Fig. 1 - A wet-sump system with built-in pivoting oil pick-up, designed specifically for off-road racing applications

Written by Lawrence Butcher

Open-deck cylinder blocks, where the engine coolant is in close proximity to the liner top, are unpopular in race engines. Insufficient lateral location invariably creates cylinder head sealing issues. While ingenious solutions for increasing the wetted surface area have been used successfully, as heat fluxes increase then attention surely has to be paid to the cooling medium used, with the liner and heat transport fluid surrounding it being viewed as a system.

For most applications, water is the perfect heat transport fluid. Good specific heat characteristics, low viscosity - and, above all, its easy availability - have made it the most obvious candidate. Various additives will reduce its tendency to corrode while safeguarding against freezing and boosting its boiling temperature; ethylene glycol, for example, can be introduced generally at concentrations up to 50%. But the presence of water around the top of the liner can have both good and bad consequences.

In the cylinder head and around the top of the cylinder liner where large heat fluxes occur, the process of nucleate or instantaneous boiling next to the hot surfaces helps to conduct the heat into the body of the fluid. Heat is removed from the surface using the latent heat effect, and the fluid later condenses and offers up that heat when away from the liner wall. So long as there is a constant stream of coolant, the process will continue efficiently.

However, if the flow becomes stagnant for any reason, the steam produced will eventually act as a thermal barrier, resulting in local overheating. Known as film boiling, even if it doesn’t harm the contact surface then the instantaneous pressure perturbations may cause fatigue damage and consequent pitting, eventually leading to liner failure. The presence of water in a high-performance cooling system, where heat fluxes are high and the flow uncertain, is surely therefore questionable.

Pressurising the system raises the boiling point of pure water to 121 C at 15 psig, but increasing the pressure brings an invisible cost - that of requiring better sealing, stronger hoses and a greater risk of cooling system failure at some time. The alternative of a 50% mix of ethylene glycol and water is only a partial cure because any boiling of water in the mix can still cause film boiling at critical parts. But since ethylene glycol is toxic, surely this is undesirable anyway.

One solution alternative to using ethylene glycol (C2H6O2) is to use a close cousin, propylene glycol (C3H8O2). It has a very similar boiling point, at around 180 C (at atmospheric pressure), and although the specific heat is around two-thirds that of water, when used as a heat transfer fluid it gives more consistent cooling at high heat flux and virtually eliminates film boiling.

Since propylene-based fluids have much greater viscosity, engines will have to be preheated, but since this is common practice anyway, it shouldn’t cause any extra concern.

Above all, however, the non-toxic nature of propylene glycol means that any accidental spills are not particularly hazardous - and the workshop cat, useful in the keeping the mice under control, will be spared serious harm should it accidentally drink any.

Fig. 1 - Nucleate and film boiling

Written by John Coxon