Heated Exchange

TDC

Heated exchange

Kevin Cameron

HEAT Is TRANSFERRED IN ROUGH PRO portion to the temperature difference between hotter and cooler materials. If there is no temperature difference, no heat is transferred.

One of the disagreeable results of this is that the heat of combustion is transferred to an engine's pistons, valves and combustion chambers. If this heat is not removed as fast as combustion adds it, the parts in qües tion will get hotter and hotter until they fail. This, ultimately, is why en gines need cooling systems. The cool ing system is designed to remove heat at a rate that will keep hot parts tem peratures at levels where they can function and survive. Because none of the parts in the engine can withstand full-flame temperature, cooling has to be a vigorous process. Aluminum pis tons or cylinder heads melt at 1300 degrees F, and they soften seriously at even lower temperature. Even exhaust valves-made as they are from heat-re sisting alloys of nickel-erode or melt far below the 4000-odd degrees of peak flame temperature.

The process of cooling is. ultimately, one of transferring the heat to the sur rounding air-directly through fins on hot engine parts, or indirectly through the medium of a liquid coolant and ra diator. As noted above, this process of heat transfer is driven by the tempera ture difference between the hot object-an engine cooling fin or a radi ator secondary surface-and the air streaming past it. Direct air-cooling is in this sense more efficient, because the fins are hotter than the usual liq uid-coolant temperature. It is for this reason that direct air cooling via fins on hot engine parts requires less fin area than does liquid cooling via radia tor. Fins can be small because they are very hot: radiators must be larger be cause they are less hot.

This brings us to a curious truth about modern racing engines. Tuners and engineers have found that engines give more power as coolant tempera ture is reduced-provided that the fuel being used will continue to vaporize and form a proper mixture at the lower temperature. If the fuel no longer va porizes, owing to the engine~s nol being hot enough to accomplish this, there will be classic symptoms of not being warmed-up: poor throttle re sponse, backfiring and lean operation as only part of the fuel vaporizes.

Why do engines give more po~~er at lower temperatures? This refers hack to the first paragraph. about heat transfi~r from hotter objects to cooler ones. As air is drawn into a hot engine. that air is heated by contact with the hot intake port and valve, and thereby loses den sity. Because engine power is propor tional to air mass flow, this density loss is also a power loss.

Now you may ohiect-and rightly so-that an engine can be overcooled to the point that its cool combustion chambers `suck" heat out of combus tion gases, cooling them, reducing their pressure. and reducing their abili ty to push pistons. This is also true. But the compromise between the two truths favors the former over the latter-reducing coolant temperature raises power more by cooling the in take air than it lowers it by overcooling the combustion chamber, so there is a net gain. Today~s GP two-stroke race engine coolant runs at about 1 25 de grees F-pretty tepid compared with the 180-190 degrees of auto coolant. And four-stroke Superbike race engines only run a little hotter 1 50 degrees F.

Now comes the dilemma. As coolant temperature has been reduced, it has required more radiator surface area to dissipate the same amount of heat. And the bigger the radiator, the more power it takes to push it through the air. Making more power means making more heat, which in turn means a big ger radiator again. It would be nice to

step off this vicious circle.

John Britten, the New Zealand racin~z engineer who created the Britten \`1000 Twin, is able to use a smaller radi ator because he has increased the airflow velocity through it. Rads in the usual position pick up air from the con fused region behind the front wheel, then discharge it against the front of the engine-and the result is indifferent (or even reversed) radiator airflow. Britten's rad is under the seat. Air is delivered to it by ducts from a high-pressure region under the fairing nose. and is discharged from it into the low-pressure region under and behind the seat. The lar~er cooling air pressure difference across the radiator in such a ducted system in creases airflow-and cooling.

Moto Guzzi's Dr. John \Vittner and I discussed the cooling problem at Day tona. Yond like to run the hottest pos sible engine coolant because that allows you to cut radiator size, but still somehow limit the heat trailsferred by hot parts to the intake air going to the engine's cylinders. One way to do this would be to insulate the entire intake tract from engine heat. Another way would be to run two sep arate cooling systems-one hot, to effi cientlv cool combustion chambers and exhaust side, allowing use of a small er radiator; the other cool, to hold the intake side of the cylinder head to the t~, `rqti irr'

Because it's easier to just add even more extravagant amounts of drag-pro ducing radiator surf~ice. that's what's being (lone. If new thinking isn't adopted soon. racehikes will come to resemble sailing ships ever more close ly. The main radiator is the mainsail, the secondary rad is the topsail, the tnangular oil cooler is the royal, and supplementary coolers tacked-on here and there resemble studding-sails. Surely someone is presently planning to put little rads in the faming nose~ these will be the spritsails. .

Fortunately, none of these measures is needed on machines that operate at highway speeds. Power requirement (and heat output) rises much faster than does radiator airflow as a vehicle OVCS faster. That means that more and more cooling area is needed as speed rises. Conversely, this also means that at modcrate speeds. radiators smaller than the side of a house work just fine.