Keeping Heat Out of the Cylinder Head

[Hugh Janus]

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/)

Our intuition whispers to us that actual combustion, in which gas temperature quickly jumps up by 2,600 degrees Celsius soon after ignition, must be the biggest source of heat flowing into the cylinder head.

Exhaust Port Heat

But actual experiment shows that half of the cylinder head’s heat inflow is picked up through the walls of the exhaust port. How can this be? The temperature of combustion gas at peak pressure is much higher than that of gas expanded through the power stroke (giving its energy to the piston, which drives the load), then further expanded through the fast-lifting exhaust valve(s). What gives?

It turns out that conditions for heat transfer are very good at the exhaust port walls.

1. Gas temperature in the exhaust port, while lower than in actual combustion, is still quite high.
2. The combustion chamber surface facing combustion is somewhat insulated against hot combustion gas by the presence of a thin layer of gas that has become stagnant by energy-losing collisions with the cooler metal wall. In the exhaust port, any such “boundary layer” is greatly thinned by the very high exhaust velocity, scouring it away. This loss of insulation accelerates heat flow.
3. We know that if the exhaust gas were stationary, it would rapidly cool as it lost heat to the metal it was in contact with. This cooling would reduce the temperature difference (<a href="https://www.cycleworld.com/story/blogs/ask-kevin/motorcycle-engine-cooling-explained/">Delta-T</a>) between the hot gas and the port wall. This would quickly reduce heat flow, for heat flow is proportional to Delta-T. But if the gas is moving, and especially if the flow is turbulent, that motion constantly replaces cooling gas at the port wall with fresh, hot gas, thereby keeping the temperature difference (and the heat flow into the metal) high.

Because of all this, when Harley-Davidson’s “Evo” engine was designed, it was given the shortest possible exhaust ports.

The late Jim Feuling (who created the Harley “V3″) made good money persuading automakers to adopt his smaller, shorter exhaust ports and smaller valves. Why would automakers care how much heat enters their cylinder heads? Just stick on a bigger water pump and radiator, right?

On the other hand, the more heat that pours into the cylinder head, the more careful the designer must be to prevent formation of hot spots where slower-moving coolant boils. Where can we put all these head bolts, ports, and drainbacks? Everything’s in the way! Reduce heat inflow and such problems are solved at lower cost. Feuling’s exhaust ports and valves were conveniently small, yet flowed very well.

Once when visiting the Brooklyn shop of vintage racing impresario and historian Robert Iannucci, he showed me a cylinder head from one of MV’s late fours, the racing engines designed as successors to Giacomo Agostini’s favorite: the MV triple. I saw that thin steel exhaust port liners, fitted with an insulating air gap, were present in each exhaust port.

In the present day, engines designed for turbocharging are sometimes given insulating ceramic port liners for the same reason: to reduce the heat flow into the cylinder head. The less heat that enters the head, the less cooling it requires (airflow through fins, circulating water, or circulating oil) and the less likely it is to distort, possibly causing valve seat deformation (with leakage) or loosening.

There’s a lot of heating happening in the short run after the exhaust valve to the pipe. (Ducati/)

Direct Heating From Combustion

The other source of heat flowing into the cylinder head is combustion. The larger the cylinder bore, the greater the area of the combustion chamber, and the greater the area picking up heat from hot combustion gas.

At one time, short strokes and big bores were the excitement in vehicle engines. Big bores made room for the big valves that could fill cylinders at high rpm, and so make more power. Short strokes reduced piston acceleration at any given speed, helping to make high rpm mechanically safe. In Formula 1, this idea drove design for a long time, resulting in extremes such as 96 x 41.4mm during the V-10 era (a bore/stroke ratio of 2.32).

As the era of controlled emissions and fuel consumption arrived for production vehicles, design went in the opposite direction, toward smaller bores and longer strokes. Why? To reduce energy loss in the form of heat, by reducing cylinder bore and increasing stroke. This is why production auto engines now tend to have strokes greater than bores, and why the new parallel-twin motorcycle engines are moving (albeit more slowly) in that same direction.

Another way to reduce combustion heat loss into cylinder heads is to reduce the number of cylinders, thereby reducing the total surface area inside of engines that is exposed to hot combustion gas. This effect, too, is driving the move from fours to twins or triples.

Why not just coat piston crowns and combustion chamber surfaces with insulating materials such as zirconium oxide? Wouldn’t that satisfy the need for energy conservation, allowing us to continue enjoying big-bore, short stroke four-cylinder engines?

Here Comes the Blanket Analogy

When we lie down to sleep at night, we cover ourselves with a blanket or comforter because its high insulating value allows its outside to remain close to the temperature of room air, while its inside surface, in contact with ourselves rises to nearly our skin temperature, keeping us from rapidly losing heat. The same happens with an insulated piston crown. The bottom of the insulation remains close to piston temperature, while the surface facing combustion warms up to approach hot gas temperature. That hot insulating surface heats the next fresh charge entering the cylinder and being compressed within it, driving the temperature of the last bits of charge to burn toward the detonation threshold. Don’t want!

Fortunately, the new breed of parallel twins delivers a kind of performance never available from the high-revving fours: torque at almost any rpm level, which makes better riders of us by not requiring that we split our attention between the tach and the road ahead.

Source

[YamaHead]

Have ALWAYS loved reading the writings of Mr. Cameron....has an impeccable knack for descriptions.....Learned a ton from this guy.