Cooling Clarifications

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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/)

Readers raised a number of interesting questions in response to my recent story “Cooler Heads Prevail,” about ways that engine waste heat can work against the builder.

Side-Mounted Radiators

TK4 wanted to know what had happened to the idea of side-mounted radiators. Stu Hilborn noted side radiators had been used on the first build of Honda’s NR500 oval piston four-stroke 500 GP bike. (You can see an example on display in the Honda Collection Hall.)

Side-mounted radiators are susceptible to crash damage and increase drag. (Cycle World Archives/)

The first reason not to do it this way is crashing, which is almost certain to result in coolant loss and DNF. With the rad in the traditional place between front wheel and engine, crash protection is fairly good. Ducati discovered another reason: Routing hot, low-energy rad air out the sides of the fairing makes it less likely that airflow will remain attached, thereby increasing drag.

Ducted-Radiator Designs

Motorsyklist asks, “Didn’t John Britten come up with a decent solution for cooling?” The answer is that he did, and so did the builders of the Triumph-powered “Over” bike seen some years ago at Daytona. The idea is the ducted-radiator concept. I think the KR race team also played with the design but never adopted it. The plan was to duct ram air from the fairing nose back to a rad mounted almost horizontally under the rider’s seat. That was more practicable with Britten’s V-twin than it would be with today’s V-4 or inline-four engines, since their top-mounted intake airboxes constitute a barrier to locating such a front-to-rear cooling duct.

When I look at the giant radiators and oil coolers on modern racebikes I naturally think of the sails of a full-rigged ship. Surely this can’t be the last word on how to combine good aero with lowered cooling drag?

High-Temperature Systems

MonkeyButt describes being on a thousand-mile trip with a friend, himself on a ZX-10R and the friend on a 2020 Ducati Panigale V-twin. The Ducati, he notes, consistently ran 20–30 degrees higher coolant temperature than his Kawasaki.

One of the measures Ducati has taken to reduce cooling drag in racing is the same basic idea that the US Army Air Corps hoped would be a winner in the 1930s: pushing coolant temperature up so that engines needed a smaller volume of cooling air. The Air Corps tried running pure ethylene glycol (“Prestone,” as they then described it) as coolant at temperatures up to 300 degrees Fahrenheit. They were ultimately defeated by the difficulty of containing that fluid.

The idea remains attractive.

Compression Ratio and Engine Temperature

NoahKatz questions whether higher compression increases engine temperature, correctly noting that, “extracting more mechanical work from the same amount of fuel…should leave less waste heat to dispose of.”

In effect, a higher compression ratio is actually a higher expansion ratio. The rule of thumb is that given a modern standard of cylinder-filling and combustion efficiency, peak combustion pressure is roughly 100 times the compression ratio. The higher the compression ratio (subject to the limits set by detonation), the higher the peak pressure; and the greater the expansion ratio, the more energy you are taking as mechanical work out of the combustion gas.

Anyone who is familiar with diesel vehicles knows that diesel exhaust is relatively cool-—so cool, in fact, that winter passenger-cabin heating can sometimes be a problem. Diesels typically have compression ratios in the range of 16 or 17:1. Such high compression is possible because diesel’s combustion does not produce a flame front capable of leading to detonation. Their fuel is injected directly into hot, highly compressed air, igniting as soon as 1) the cooling effect of fuel-droplet evaporation is overcome, and 2) fuel vapor evaporating from the droplets has had time to diffuse outward to mix with oxygen in the air charge.

Temperature and Detonation

In spark-ignition engines, the small flame kernel produced between the spark plug’s electrodes expands to form a turbulent flame front, shredding, mixing, and advancing at between 50 and 200 feet per second. In the process, unburned mixture out at the edge of the combustion chamber is steadily heated as it is compressed by expanding hot combustion gas. This heating drives chemical changes in the fuel, converting tiny volumes of it into a sensitive explosive. If the process continues long enough, those tiny volumes auto-ignite and burn at the local speed of sound—several thousand feet per second. Despite the small volumes of mixture involved, the resulting shock waves can be highly destructive. First, they scour away the layer of gas next to metal surfaces—the so-called “boundary layer” that forms a natural insulation which normally protects the piston-crown and combustion-chamber surfaces from high heat flow.

Those surfaces now heat up rapidly, especially the poorly cooled piston, whose aluminum material loses strength as its temperature rises. The top ring land softens and is forced down by repeated shockwave impacts, trapping the ring in its groove and destroying its seal. The same process now attacks the second ring, and so on.

Detonation erodes a piston’s ring land as shockwaves pound softened aluminum material. (Jeff Allen/)

In the meantime, those shockwave impacts begin to erode material from the hottest surfaces, initially giving the piston’s edges a rough, sand-blasted look.

When the above process puts the second piston ring out of action, hot combustion gas can jet inward through the oil scraper ring’s oil return holes. Very quickly the piston develops an eroding flame channel into the crankcase and complete failure occurs.

This is why modern engines are equipped with accelerometers that detect detonation and signal the ECU to retard the ignition timing enough to cause detonation to cease.

But back in the later 1930s when AJS was developing its initially air-cooled 500cc V-4, there were no ECUs and no electronic detonation counters. The problem that caused engineers to reduce compression ratio on the rear cylinders was not overheating per se, as a result of their being cooled by hot air that had just cooled the front cylinders. It was the detonation that resulted from that overheating, destroying the pistons in those rear cylinders.

Everything that can raise the temperature of the last bits of mixture to burn also makes detonation more likely:

1. Higher intake air temperature<br/>
2. Excessive heating of mixture on its way to the cylinders<br/>
3. Early ignition timing (which provides more time for heating)<br/>
4. Compression ratio too high for the fuel being used (compressing a gas heats it)<br/>
5. Excessive cylinder head and/or piston temperature<br/>
6. Engine operation at low rpm and high throttle (“lugging”)<br/>
7. Making power beyond the capacity of the cooling system (This is why engine designers eventually adopt liquid-cooling.)<br/>

Because fuels vary in their resistance to detonation (measured as Octane Number, or ON) the fuel used must provide adequate ON for the application.

Air-Cooled Aircraft Engines

In air-cooled aircraft piston engines, all cylinders and heads have equal access to cooling air entering the nacelle from the front; no cylinder is ever “cooled” by air that has been heated by passing through another cylinder’s hot fins. Close-fitting sheet-metal baffles clamped to cylinders and heads ensure this, forcing all air taken into the nacelle to pass only through fin space. A similar baffle system can be seen on Porsche’s air-cooled flat auto engines, with the difference that a cooling blower, rather than ram pressure resulting from an aircraft’s high speed, provides the pressure required to push the necessary volume of cooling air through fin space.

AJS’s last try with its V-4—liquid-cooling adopted for 1939—did achieve equal temperatures on all cylinders, and with all safely producing good, detonation-free power, top speed became satisfactory.

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