How Critical Engine Parts Are Cooled

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Kevin Cameron has been writing about motorcycles for nearly 50 years, first for <em>Cycle magazine</em> and, since 1992, for <em>Cycle World</em>. (Robert Martin/)

Because heat engines are powered by the chemical energy of a fuel, released through combustion, their parts are in varying degrees exposed to high temperatures. This drives the temperature of those parts (piston crowns, exhaust valves, turbine blades, etc.) upward. If nothing limits this process, the parts may rise to temperatures that destroy their function.

Failure of exhaust valves is now rare, but under difficult conditions they may lose pie-shaped pieces of their heads, or break off where the valve stem begins to flare out to form the valve head. Whirling turbine blades slowly elongate via the process of high temperature creep, until they may scrape against nonrotating parts of the engine. Aluminum piston crowns lose strength rapidly at quite moderate temperatures and may sag or be punched through by combustion pressure.

Some form of cooling is therefore necessary to limit the temperature rise of engine parts to levels their materials can tolerate.

Pistons

Pistons must for long periods withstand the gas pressure forces of combustion (rule of thumb: Peak combustion pressure at peak torque can be as high as 100 times the compression ratio) and the often larger inertia forces of the piston’s stopping and starting at top and bottom dead center. Very high-performance 600s of the sportbike era subjected their pistons to peak accelerations as high as 7,000 times the acceleration of gravity (i.e., 7,000 G). At the peak of Formula 1′s V-10 era, peak piston accelerations of 10,000 G were common, and present MotoGP engines are reaching 11,000 G.

The above stresses are large, driving the process of fatigue, in which steady rearrangement of metal atoms under strain can lead over time to crack formation and failure. Fatigue is strongly temperature-dependent, so the hotter the highly stressed parts of a piston operate, the more quickly fatigue progresses to failure. More effective piston cooling slows this process.

Back when nearly all motorcycle engines were air-cooled, pistons were cooled largely by accident:

1. By contact with the cooler (you hope!) cylinder wall, especially through the piston rings which have the most intimate contact with the wall.
2. By conduction to oil which is naturally flying about inside the crankcase as it is squeezed out of main and con-rod bearings.
3. Don’t expect the air in the crankcase to do much piston cooling—oil is about 600 times denser than air!

When liquid-cooling was adopted for most motorcycle engines, pistons gave a sigh of relief because cylinder walls backed by liquid coolant stay at a lower and more constant temperature. They don’t run hotter in summer or cooler in winter.

As engines were pushed to make the annual power increase that has sold us pushovers so many new motorcycles, lighter pistons were needed to reach higher revs without greatly increasing inertia loads on bearings. That meant lighter, thinner pistons containing less metal to conduct heat rapidly to the cylinder wall. The answer has been piston-cooling oil jets, located down at the crankcase mouth and aimed up at the underside of the piston dome. Most production engines have just one jet for each piston, but race engines can have several, or even “many” such jets in the interest of achieving a more uniform piston temperature.

Remember that heat flow is proportional to the temperature difference between the hot object and whatever is cooling it (called “delta-T” by engineers). Vincent designer Phil Irving noted in his autobiography that the rear cylinder of an air-cooled V-twin runs hotter than the front cylinder for this reason.

Cylinder Head

The combustion chamber’s “other half” is the cylinder head, which receives the same heating that the piston does. Yet experimental work has revealed that half of the heat going into the cylinder head enters through the walls of the exhaust port. The reason for this is the high velocity of exhaust gas, which greatly accelerates heat transfer from gas to metal. Some are surprised by this, for the old idea that “The gas goes through there so fast, there’s no time for heat transfer” dies hard. High gas speed accelerates heat transfer because the turbulent and fast-moving heat greatly thins the boundary layer of stagnant gas that has lost energy from frequent collisions with the surface. This, plus the turbulence and speed, assures that every square millimeter of port surface is constantly in contact with fresh hot stuff, as gas that has cooled is so rapidly swept away.

This is why the exhaust ports on modern engines are made as short as possible, and of minimum possible diameter (minimizing its surface area). A cylinder head from one of MV’s “late fours” (the early ‘70s bikes Phil Read rode) was once handed to me: I was delighted to see that it had carefully shaped steel exhaust port liners, insulated from the head material by a 360-degree air gap. Some turbo engines have been designed with insulating ceramic exhaust-port liners.

Not part of the head but closely related is the top few millimeters of the cylinder bore. Exposure to combustion plus the contact of the hot piston at TDC can threaten the oil film on this critical surface (not only does the oil there lose viscosity, it can just plain evaporate). For this reason, designers take special pains to bring coolant as close as possible to it.

Exhaust Valves

Aren’t both intakes and exhausts exposed to hot combustion gas? They are, but exhaust valves receive intense heating from high-velocity combustion gas leaving the cylinder, heating from both sides. High gas speed increases heat transfer.

Because the head of the valve has considerable area it collects the most heat, but fortunately the valve seat on which it typically rests two-thirds of the time is part of the head, which in the present era is usually liquid-cooled. This contact accomplishes most of the necessary valve cooling. If you have worked on engines yourself or have seen them apart, you will have noticed that the width of the exhaust valve’s seating surface is wider than that of the intake. This is to provide plenty of contact area through which valve heat is transmitted to the seat and cylinder head.

In general, bigger valves have bigger problems, made all the worse if the engine is air-cooled (because cylinder-head temperature is higher with air-cooling, there is less temperature difference between the hot valve and its seat, which reduces heat transmission, resulting in higher valve temperature).

Large exhaust valves such as that of a 500cc two-valve single may need more help. In some cases the head and stem of such exhaust valves have been made hollow and two-thirds filled with sodium metal, which melts at just below boiling water temperature. Valve motion causes internal agitation of this liquid, carrying heat from the hot valve head up the valve stem, which is in contact with the cooler valve guide. In the case of Norton’s 1953 500cc factory roadrace bike, the exhaust valve stem was encircled by a spiral passage through which oil from a small cooler was circulated. In some liquid-cooled designs, the middle portion of the exhaust valve guide is directly exposed to engine coolant.

Engine Bearings

Modern engines have plain bearings rather than the heavier and more fatigue-prone rolling element bearings of former times. As the clearances in plain bearings are measured in thousandths, the volume of oil in them is tiny and in operation would therefore rapidly overheat, lose viscosity, and suffer lubrication breakdown were it not being constantly replaced by fresh, cool, and filtered oil from the oil pump.

Valve Train

Valve train friction is a small fraction of engine friction as a whole. Yet the pressure between the cam lobe and tappet is high. As the rotating cam lobe takes up the clearance to the valve tappet and then begins to accelerate the valve up off its seat, the oil film between lobe and tappet must carry the valve’s inertia and the force of the valve spring(s). In modern designs the camshaft is usually made hollow and is kept filled with oil from the pump. Each cam lobe has a drilled hole to keep it and its tappet supplied with oil adequate for both lubrication and cooling.

Eugene Goodman of Velocette discovered in the mid-1920s that pumped circulating oil systems tend to level the temperatures of engine parts. Hot-running parts are cooled and cool-running parts are warmed.

Electric Motors

Electric motors convert electric power to mechanical power efficiently (94 percent or higher) but losses exist and do generate heat—sometimes enough to require active cooling. The way it works is that power in watts (1 hp = 746 watts) equals voltage times current, or V x I. The biggie in electric motor losses is resistive heating, whose familiar example is the glowing wires in any kitchen toaster. Resistive loss is proportional to I, the current, squared, times the electrical resistance, R. This is why electricity is transmitted over long distances at very high voltage—because this reduces the current to low values, greatly reducing resistive loss. This is also why electric vehicles are generally using the highest voltage that the insulation inside their traction motors can safely withstand (400V was normal but 800V systems are beginning to appear).

Some electric motors are even constructed with liquid-cooling lines wound into their coils. Coolant is circulated through a heat exchanger to prevent wire insulation from reaching temperature at which voltage from the vehicle’s power supply may punch through it, creating a short circuit and failure.

Eddy current loss occurs as magnetization reversals occur in iron magnet poles and is proportional to frequency, squared. Internal windage and bearing losses are also present.

Heat is also generated in the variable frequency AC power supplies of electric vehicles, but efficiency has fallen from 90 percent in early days to recent IGBTs at 95 percent and now to the 97 percent of silicon carbide switching devices.

As laptop users know, batteries generate heat during charge/discharge. To prevent shortening of battery cycle life or other disagreeable outcomes from overtemperaturing, battery cooling by air or liquid may be required.

It all comes down to this: Heat accumulating in power system elements raises their temperature. If temperature rise threatens reliable function, some form of cooling will be required to take away waste heat as fast as the system generates it.

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[Simon Davey]

Sodium metal in valve stems! That must have been quite an expensive production. 

 

What a great read, many thanks.