Phantom Posted January 2, 2012 Share Posted January 2, 2012 Cooling 101 In order to properly troubleshoot engine cooling problems we must first identify each of the components in the cooling system. They include: The coolant The engine cooling jacket The water pump The radiator The thermostat The cooling fan and shroud Each of these plays an important role in ensuring that an engine warms up properly and doesn’t either freeze or overheat. I’m going to take them one at a time and address specific features/functions of each. The coolant: Most vehicles run a blend of ethylene glycol and water for coolant. There are other anti-freezes on the market but they function in much the same way as “the green stuffâ€. Depending on location, the mixture would range from 25% to 75% ethylene glycol with the balance water. The most common blend is 50/50 which is probably the best except where it is exceptionally warm or exceptionally cold. Although ethylene glycol is mostly thought of as an antifreeze it also extends the boiling temperature and provides additional boil-over protection for engines. “Antifreeze†products also contain corrosion inhibitors and lubricants to keep the cooling system in good operating order. Thus it is important that the coolant be periodically replaced to ensure the mix is fresh and still effective. The engine cooling jacket: This is the collection of channels and cavities cast into an engine block that allows the coolant to circulate around the cylinder walls and then out to the radiator and back. They are designed for a water flow (GPM) that ensures turbulent flow in the channels when maximum cooling is required. The capacity of the cooling channels, water pump and radiator must all be matched. Turbulent flow of a coolant results in more effective cooling than laminar flow which occurs when the velocity of the coolant drops below a certain level in any tube/cavity. The water pump: It is what it says it is. It is a centrifugal pump that pumps water. It has a maximum pressure that it can develop at a given RPM but it’s flow is determined by the pressure drop (resistance to flow) in the system. The lower the pressure drop the higher the flow. The radiator: This is a tube and fin heat exchanger. The fins are there to increase surface area for heat to radiate into the air stream passing through them. If they are badly bent or plugged the efficiency of the unit is significantly reduced. The tubes are carefully selected to handle the maximum flow that can pass through the coolant jacket of the engine and have a velocity high enough for turbulent flow (most efficient heat transfer). If there are too many tubes or they are too large the velocity drops and the radiator actually loses ability to remove heat from the coolant. The Thermostat: This device is essentially a water flow control valve that opens and closes in response to temperature changes in the coolant. The valve opens as a result of water pressure developed by the water pump and heat in the coolant that affects the wax in the center cylinder of the thermostat. As the wax heats it expands which works with the water pressure to overcome the spring and open the thermostat. The wax is specifically selected to have the thermostat open to a specific point at a specific temperature that has been determined to be the optimal operating temperature of the engine. It can open wider when coolant temperature starts to rise and close as it becomes cooler. A thermostat also usually has a small diameter hole in it that allows a bit of water flow at all times. This provides a small amount of water flow through the water pump at all times to ensure it’s lubricated and cooled. The cooling fan and shroud: The cooling fans purpose in life is to ensure adequate air flow through the radiator when the car is moving slower than the optimal air flow for cooling. The fan then provides additional airflow to make up the deficiency. The shroud encloses the fan and seals to the radiator to ensure air is pulled across the entire face of the radiator rather than just where the fan blades are. If the shroud is removed from a fan/radiator combination it is possible that the engine will overheat. If the fan does not come on or operates mores slowly than it should the engine may overheat. Troubleshooting: Please note that the guide below assumes a properly set up cooling system to start with. Incorrectly sized components add another level of complexity. Overheats always: If the engine comes up to temperature normally but then overheats whether it’s idling or running at a steady speed check the following: 1) Is it full of coolant? If not, fill it and then check for leaks. That fluid went somewhere and hopefully it didn’t go past a blown head gasket or cracked block. Steam in the exhaust and/or a rough running condition would indicate these problems. 2) Look inside the radiator, check the condition of the coolant. Do either or both look fouled? If so, drain the coolant and backflush the system. It may require chemical cleaning or even having the radiator “rodded†out if it’s really bad. 3) Check the thermostat. It may not be opening. The quickest way is to put it in a bowl of water and bring it to a boil. If it doesn’t open before the water boils, it’s bad and needs to be replaced. 4) Check the water pump. If there is coolant running out of the drain hole in its bottom – it’s bad. Start the engine when it’s cold with the radiator cap off and watch to see what the water does when it reaches operating temperature. If you see good movement, it’s probably not the water pump. If you see little to no water movement, you probably have a bad water pump. Overheats at idle (in stop and go traffic): This generally can be attributed to the cooling fan and/or shroud not providing enough air flow when the car is not moving. This could be due to: 1) A failed clutch in a centrifugal clutch fan 2) A failed control relay or burned out motor on an electric fan 3) A failed coolant temperature sensor on an electric fan system. 4) A problem in the ECU on the electric fan system 5) A missing, improperly sized, or badly damaged fan shroud that is allowing the fan to pull air from behind the radiator instead of through it. Runs Cold: There is really only one thing that could cause an engine to run cold all the time – besides living in Antarctica in winter. 1) The thermostat is frozen in the fully open position or there is no thermostat. The engine will be very slow to warm up at all and probably will not ever come to full operating temperature unless operating in extremely hot conditions or running hard at the track. Slow to warm up but then tends to run hot: This is most likely caused by a thermostat that is frozen in a partly open position. Thus it initially allows too much water flow and then engine warms up to slowly. Then, because it is only partly open, it doesn’t allow enough water flow and the engine overheats. This can be masked a bit if running at a constant speed around 50-55 when the motor load is low yet plenty of air is going through the radiator. It would then tend to overheat at either higher or lower speeds. Quote Link to comment Share on other sites More sharing options...
neotech84 Posted January 2, 2012 Share Posted January 2, 2012 Very good write up. Thanks for the time put into this. Sticky??? Quote Link to comment Share on other sites More sharing options...
bjhines Posted January 2, 2012 Share Posted January 2, 2012 I appreciate the interest, but we need some engineering math to deal with the issues that come up with cooling modified engines and cooling systems. Considering the discussions regarding,... 1. Radiator size and design of the tubes, fins and tanks. 2. radiator baffles(covering part of the radiator). Cooling shock/temperature fluctuations due to over-cooling water in the radiator. 3. Airflow through the front end, cowl induction scoops, and additional inter-coolers and oil-coolers that have caused problems for many people on this forum. 4. water pump size, operating RPMs, and electric WP control systems. 5. coolant bypass in the head, block, and thermostat housing. 6. mechanical and electric cooling fans and control systems. There is a small book's worth of theory, part number tables, historical data, and electrical diagrams just to cover DATSUN 240Zs. Quote Link to comment Share on other sites More sharing options...
Phantom Posted January 3, 2012 Author Share Posted January 3, 2012 (edited) bjhines - You are completely correct. That's why I wrote it as a "101", Freshman level. Some of the items you discussed are at the 501, graduate level. My intent was to get the basic concepts out so folks with no knowledge would have a starting point. Once we get beyond that it gets very case specific because of the way people configure their cars. An example would be radiator sizing. A radiator needs to be sized to handle the flow volume that the water pump can deliver AND have turbulent flow within the tubes to maximize available cooling. If the tube diameter or number of tube is too large and the velocity slows enough to get laminar flow then cooling effectiveness is reduced and could end up in an overheating situation. Or - if the tubes are properly sized for turbulent flow but are too long then an overcooling situation can exist. This is not nearly as frequent an issue, however, because of the physical underhood space limitations. The simplest way to approach sizing is to determine the volume that the water pump is capable of actually pumping through the block and then size the radiator accordingly. The assumption is that the engineers that designed the pump for the engine specified one thatcan flow enough water to remove the heat from the engine. There is also the theories on fin density and size which is dependent on tube size and numbers of tubes. Then all we have to do is ensure the heat is moved from the radiator to the air flow. After the radiator is sized then ambient air temperatures have to be evaluated and we need to ensue that sufficient air can flow across the radiator to remove the heat. Remember, the fan should only be doing this in stop and go traffic, not cruising down the highway. Then we size the fans to also ensure this flow and make sure that they are properly shrouded. Remember - if the fan is not shrouded it can as easily pull it's air from behind the radiator as through it. Not a good thing when you're trying to remove heat from the radiator. Remeber, if you know your engine, and how much heat it needs to reject, you can go to a company that designs radiators, who have the engineers on staff with the formulas, and have them properly design the radiator. They do it every day. We don't. The problem is most people are trying to find the cheap way out and then wonder why it doesn't work. Edited January 3, 2012 by Phantom Quote Link to comment Share on other sites More sharing options...
Tony D Posted January 4, 2012 Share Posted January 4, 2012 (edited) Thermal mass is overlooked continually in these discussions, and leads to some vehemently defending theories which seem to make sense when looked at superficially, but lead to very poor design changes in practice. It goes hand in hand with the fact that water isn't the sole coolant in any engine. Edited January 4, 2012 by Tony D Quote Link to comment Share on other sites More sharing options...
Phantom Posted January 5, 2012 Author Share Posted January 5, 2012 Ahh - you're correct - it's a combination of water and an ethylene glycol derivative or substitute that extends the effective range of the water. Oil, as well as lubricating, also acts somewhat as a coolant using it's residence time in the oil pan to dissipate the heat that it absorbs. That is not it's primary function, however, or all engines would come with thermostats and radiators on their oil systems too. Bottom line - to have thermal balance the heat in has to equal the heat out. Intitially the engine mass needs to be brought up to "operating temperature" so the initial heat in is less than the heat out. Once shut off that heat will then radiate from the engine and we're back in balance again. Does the cooling system remove all the heat - no. Is it the cooling system that we look to when we have an overheating situation - you bet. The reason for that is that it's the 800 lb gorilla in the game. The rest of them are just little badgers - and for this discussion - "we don't need no stinking badgers"! Quote Link to comment Share on other sites More sharing options...
Tony D Posted January 5, 2012 Share Posted January 5, 2012 Badgers...huh huh huh Quote Link to comment Share on other sites More sharing options...
260DET Posted January 6, 2012 Share Posted January 6, 2012 Thermal mass is overlooked continually in these discussions, and leads to some vehemently defending theories which seem to make sense when looked at superficially, but lead to very poor design changes in practice. It goes hand in hand with the fact that water isn't the sole coolant in any engine. Tagging yours Tony because it is the shortest From a practical perspective what is constantly and continually neglected is airflow and basic physics, which becomes particularly important when there is a hot hi-po engine involved. No apologies for the shouting, CONCENTRATING ON GETTING AIR OUT OF THE ENGINE BAY IS MORE IMPORTANT THAN GETTING IT IN, via the radiator or otherwise, via vents or whatever. I have banged on about this before, so for all the ignorers/doubters/nonbelievers which seems to be most who seem obsessed with getting air in, try this. Unlatch your rear opening bonnet/hood and in a safe environment, get up some speed and see what happens. I tried this with a fairly light fiberglass bonnet and at ~80kph the back of the bonnet lifted up, lots. Which shows how much air gets trapped in the engine bay under pressure BEHIND THE RADIATOR so obstructing air flow through the radiator, so.......... There is more but the audience is intelligent so.......... End of CAPS Quote Link to comment Share on other sites More sharing options...
Tony D Posted January 6, 2012 Share Posted January 6, 2012 Been a vented fenderwell fan for decades. Very common in Japan in the 70's and 80's. Only recently finding favour in the US and elsewhere... Quote Link to comment Share on other sites More sharing options...
260DET Posted January 6, 2012 Share Posted January 6, 2012 With no other changes, optimumising the air exit flow capacity of the stock bonnet/hood vents on my 280ZXT cured a engine overheating problem. There is still air pressure in the engine bay though so for aero reasons I will be looking at gaining more air exit capacity, not sure how yet. Quote Link to comment Share on other sites More sharing options...
Tony D Posted January 7, 2012 Share Posted January 7, 2012 Check the aero thread/forum, likely you can greatly reduce potential inflow with proper ducting and exit management. The Euro Turbo cars had a 100% ducted radiator on the S130, plastic duct attached to the front of the radiator and a small flexible duct attached to it and the air entrance point in the front lower fascia. Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.