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Old 03-May-2004, 05:21 PM
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Cooling System Overheating

Here comes summer. So I expect that there will be a lot of forum questions (again) regarding overheating Ducati's so I thought I'd share my thoughts with you.


Motorcycle Cooling System Design

A motorcycle cooling system is designed to keep the engine coolant temperature at a specific operating point, usually 180F, when moving at speed on a hot day (90F air is usually used in the calculations). This is the operating condition that is used to size the radiator, water pump and hoses. The overall design is based on the expected and recommended 50/50 mixture of ethylene glycol and distilled water. If you use any other coolant or mixture percentage you change the operating temperature.

Coolant

The optimum coolant combination is a 50/50 mixture of antifreeze and water. Any greater concentration of antifreeze does not significantly increase the efficiency of the coolant. Actually, the more antifreeze you add above 50 percent, the lower your freeze protection becomes. An ethylene glycol/water 50/50 mixture will give you protection down to -34F, and boil-over protection up to 250F.

Although regular water will work, the filling of the system should be done with distilled water. The extra expense of distilled water holds a worthwhile benefit. Distilled water doesn't contain any minerals which can dissolve at higher temperatures and turn the coolant mixture into a corrosive compound.

If you use a coolant in other than a 50/50 water/antifreeze mix you can reduce the design operating temperature. Under moderate engine loads, each initial percent of glycol removed (and water increase) from the coolant mixture reduces cylinder head operating temperatures by about 1F. There's a law of diminishing returns (maxing-out for a total reduction of about 15F. The point is that using a higher percentage of water will significantly reduce coolant temperatures. However, the use of 100% distilled water as a coolant is inadvisable because corrosion inhibitors and water pump seal lubricants are still needed, even if freeze protection isn't.

Temperature Effects on Performance

An engine should be at its design operating temperature (usually mid-gauge) to make good power. Operate at too low a temperature and the engine is less efficient and makes less power. Higher temperatures are more thermodynamically efficient, but run at too high a temperature and you exceed the thermal expansion design basis of critical components and raise fuel octane requirements. Modern fuel injected motorcycles use engine management computers incorporating a coolant temperature sensor to retard the engine ignition timing to compensate for any increase in octane requirement when operating temperatures increase.

Most stock-engined sportbikes are designed to produce their best power when coolant temperatures are close to 200F.

Coolant Temperature Sensor

Engines need more fuel when for a cold start as there is no heat in the ports and chambers to keep the fuel atomized as vapor, so it condenses. Fuel as a liquid burns very badly in the combustion chamber, so throwing more at it ensures enough stays as vapor for some sort of combustion. Injected engines get a very nice fuel spray from the injectors and this is why injected engines behave better when cold.

A sensor monitors the engine coolant temperature so the computer can supply additional fuel while the engine is warming up, and retard ignition timing when the engine gets too hot. The sensor tells the ECU to stop warm-up fuel enrichment at 180. So from a fuel correction standpoint, the proper operating temperature is above 180.

Overheating

The engine coolant temperature is controlled during low speed and spirited-riding conditions using auxiliary air flow from a cooling fan. Although the temperature set-points are somewhat different for each bike, here's generally how it's supposed to work.

If the temperature goes above 200F, the cooling fan starts, and continues to run until the coolant temperature drops to 190F. However, temperatures above 200F are not considered outside the range of normal operating conditions. Unpressurized plain water boils at 212F, so coolant systems are designed with a pressure relief valve in the radiator cap in order that even higher temperatures can be reached before coolant is released from the coolant reservoir tank. Some bikes also have a coolant overflow tank to catch and prevent the slippery coolant from reaching the road surface.

So, unless you're discharging coolant, you're not overheating.

Overcooling

When you first start your cold engine, the thermostat is shut so no coolant can flow through the radiator. No coolant being sent to the radiator means you warm up faster. This is good because, as you know, a cool engine often doesn't run very well. Eventually, as the engine coolant warms up, it also warms the thermostat. The thermostat has a bimetallic strip that opens a flow valve, starting around 160F degrees and being fully open around 190F, your normal operating temperature at speed.

In cooler weather, lower air temperatures make your radiator more effective at removing heat so your engine may not even reach it's design operating temperature and performance will suffer somewhat. Sometimes in this case, the coolant doesn't even get hot enough to fully open the thermostat. In this case, you may want to block-off part of your radiator to improve performance. Also, a common failure is a thermostat that is stuck open. This will prevent the coolant from reaching a high-enough temperature.

Temperature Gauge

The temperature gauge displays the range of expected operating temperatures. The midpoint is selected to correspond to the nominal design operating temperature. If your temperature is below this point you're not operating efficiently and you're down on power. If you're at speed on a warm day you should expect to see temperatures between the gauge midpoint and three-quarter point, usually 212F. The maximum gauge reading indicates the boiling point of a pressurized 50/50 mix coolant, the point where the pressure relief cap will likely discharge coolant to the overflow tank.

When you get stuck in stop-and-go traffic, the temperature will begin to rise because now there is reduced airflow through the radiator. At around the three-quarter point on the gauge, the radiator fan kicks-in to provide the needed airflow. The fan stays on above this temperature. With the fan operating, when the temperature drops to just above the gauge midpoint, the fan stops running.

During these traffic conditions, temperatures in the upper quarter of the gauge display should be expected and not necessarily be a cause for alarm or an indication of cooling system problems. Modern fuel injected engine management computers quickly adjust the engine ignition timing so as to run well at this higher temperature.

You should also expect that the temperature will change often and fairly quickly because, unlike a car, a motorcycle only holds about 3-4 liters of coolant. So there's not a lot of heat stored there.

Coolant System Modifications

If you've modified your engine, or your normal operating conditions doesn't match the factory design condition, then you may want to modify the cooling system to avoid chronic overheating or overcooling. For example, larger radiators that have larger heat transfer surface areas are often used for track operating conditions. Unfortunately, a larger radiator might result in overcooling when used on the street, but again, covering a portion of the radiator in cooler or less demanding operating environments will bring operating temperatures back up to the gauge midpoint.

Radiator Damage

One thing that's often overlooked on Ducati superbikes is that both oil and coolant radiator fin damage occurs from road debris. Without protection, the fins get bent over, air flow through the radiator is reduced, so coolant and oil temperatures rise over time.

After carefully straightening the bent fins, place aluminum window screening over the radiators to prevent future damage. Screen material with larger openings won't stop small pebbles. Don't worry, the inexpensive window screen material won't significantly reduce airflow.

Additional Fans

A second radiator fan, standard on some models, will provide additional cooling capability during low speed operation, high ambient air temperatures, and track operation by increasing the heat removal rate. However an additional 60 W fan electrical load not an prudent option for bike models that historically been shown to have marginal charging capacity or voltage regulator failures.

Fan Switch: Lower Temperature Actuation

One technique being tried is to change*the stock thermostatically controlled fan switch to a different switch that completes the electrical contact at a several-degree lower coolant temperature so as to start augmented air flow earlier. This causes the radiator cooling fan to run longer and cycle more often. At shutdown, the fans will run for a more prolonged period in an effort to drop the temperature further. Again, this approach not recommended for model years with weak charging systems since voltage regulators are sensitive to these prolonged 60 W fan loads and they are stressed more on startup as they try to replenish the battery.

Coolant Change Intervals

Coolants should be changed every two years because the corrosion inhibitors they contain, deplete and become ineffective. The resulting circulating corrosion products are particularly abrasive to water pump seals and the low activity levels of old inhibitors can allow pitting of aluminum radiator surfaces and general corrosion of metals in contact with the coolant.

Recycling Coolant

Water treatment plants use biological processes and even highly diluted ethylene glycol is toxic, so don't put it down the drain or storm sewers. Also, apparently it has a sweet taste and will kill pets if left in puddles nearby. Ethylene glycol is easily recycled so keep it with your used motor oil and ask your local repair shop if they will take it, or how to safely dispose of it.

Coolant Alternatives

You reduce operating temperatures when you increase the percentage of water in a water/anti-freeze mixture. Plain distilled water has twice the heat transfer (cooling) capability compared to glycol-based coolant mixes, but shouldn't be used alone (100%) as a coolant. It lacks corrosion inhibitors and water pump seal lubricating properties. So, even though water is the best choice for transferring heat, cooling systems are designed using 50/50 ethylene glycol because water alone freezes at 32 F.

Engine Ice, Liquid Performance, and Evans NPG are coolants formulated with propylene glycol that is less toxic, and consequently environmentally more friendly than ethylene glycol installed at the factory by most manufacturers. Engine Ice is simply propylene glycol premixed 50/50 with de-ionized water. There have been anecdotal reports of accelerated wear of water pump seals on engines cooled with Engine Ice, but this seems unlikely to be caused by the coolant itself.

Even though propylene glycol has a higher boiling point than ethylene glycol, when mixed with water it is less effective in both removing heat from your engine and transferring it to your radiator. So, it seems that the only logic for using it is to minimize coolant discharge to racetracks, not for reducing operating temperatures.

Evans NPG is non-aqueous propylene glycol that is not mixed with water. It has a higher boiling point of 370F that is said to reduce vapor blanketing at engine hot spots for more efficient heat transfer. Also, since it doesn't contains any water, it should be safe to use in bikes having magnesium cylinder gaskets, body parts and wheels.

Dex-Cool is an ethylene glycol based coolant that contains corrosion inhibitors that are said to be longer-lasting and less abrasive to water pump seals than additives used in other products. Probably a good product for owners who will never think to change their coolant. This is a new technology that hasn't seen much use in motorcycles mainly because they see lower annual mileages than automobiles.

Prestone 50/50 ethelene glycol/distilled water premix (or similar products) that are silicate-free are safe to use with aluminum radiators. A gallon of premix is a quantity more than you'll need for a motorcycle, saves you the hassle of getting distilled water, and is proportionally priced compared with the straight Prestone gallon container. Your bike's cooling system was designed to work best with this coolant type and mix proportion.

If you regularly experience high temperatures at speed, have a heated garage, or live in a place where you don't see freezing temperatures, consider using Red Line WaterWetter, especially for track use (where it is usually mandated.)

WaterWetter is often used because it reduces the surface tension of water (the property that makes it bead-up) thereby improving further water's superior heat transfer ability while also adding the necessary lubricants and corrosion inhibitors. Lowering the cylinder head temperatures in a high compression race engine lowers the already-high fuel octane requirements. A WaterWetter/water mix has a slightly lower boiling point than a 50/50 glycol mix and alone provides no antifreeze protection. It can be used in combination with antifreeze.

Most important, WaterWetter will reduce coolant temperatures under all operating conditions. It's easy to see its advantage in modified engines having increased heat loads, and under high-load, high-rpm track conditions. It's important to note, however, that for normal street riding in cooler and moderate weather it also can prevent the coolant from reaching optimum temperatures. Across-the-board temperature reductions of 15F under all riding conditions are commonly experienced using WaterWetter.

WaterWetter is often used as an alternative to glycol/water mixes, primarily in racing applications. Ethylene glycol based coolants are illegal in road racing because they make the road surface slippery when spilled. Propylene glycol based coolants, however, are allowed in AMA, CCS and FUSA road racing. NESBA (advanced group) and WERA (although it was legal prior to 2002) do not allow propylene glycol use.

As a reminder, may sure you use an anti-freeze that contains rust inhibitors that are free of silicates that can damage water pump seals. Modern water pumps use a ceramic seal that only needs a fluid to cool it. Coolants containing silicates (finely ground quartz) are too abrasive and prematurely wear-out a ceramic seal.

Postscript

I removed the water plus Water Wetter mixture from my 916's cooling system. On a day with 70 - 80F ambient air temperatures, coolant temperatures would only reach 140 - 170F (60 - 75C.) I tried blocking off a 4 x 11 inch section of radiator, but temperatures still stayed below 175F (80C), the gauge midpoint. At these temperatures, the ECU is still adding extra fuel for warm-up conditions (below 175F) by adding more fuel. The tailpipes were black and gas mileage was down.

I replaced it with a 50/50 Dex-Cool (compliant)/water ethylene glycol based coolant that has corrosion inhibitors that are longer-lasting and said to be less abrasive to water pump seals than silicate additives used in other products.
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