|View single post by Mark Rosenbaum|
|Posted: 04-02-2005 04:48 pm||
Originally posted August 2004.
My recent water pump studies have led me to a brief study of the JH cooling system, and I've a few comments which may be of interest. As always, I welcome any additional information and/or corrections.
Cooling systems that will handle all the waste heat from a high-power engine are large, heavy, and expensive, so most cars are constructed with systems that can handle only smaller amounts of heat. This works only because few cars are called upon to produce their maximum power output for more than a few seconds at a time. Since most engines normally operate at a temperature considerably below their coolant's boiling point, a fair amount of excess heat can be 'stored' in the coolant for a little while (raising its temperature, of course) and dumped at some later time, when the engine's heat output is not exceeding the cooling system's capacity.
Gasoline engines are not particularly efficient. As a rough rule of thumb, for every horsepower an engine produces, it dumps a horsepower's worth of heat into its cooling system and a horsepower's worth of heat out the exhaust. This being so, a JH engine at full throttle makes enough waste heat to warm its entire coolant supply by 7^F a second. If this engine normally runs at 180^F and the coolant boils at 230^F when pressurized, then it could run at full throttle for no more than (50^F)/(7^F/sec) or a mere 7 seconds before its coolant turned to steam. In practice, of course, the waste heat warms more than just the coolant, and the cooling system transfers much of the heat to the environment, so the time to boilover would be considerably greater.
But for prolonged full throttle operation, boilover is inevitable unless ALL the engine's waste heat can be transferred to the environment. This means more effective heat transfer inside the engine, and more effective heat transfer from the radiator to the air.
Assume that the engine is properly filled with coolant. When the engine runs, rotation of the water pump forces coolant present there into the block, and fresh coolant from the lower radiator hose and upper heater hose flow into the void thus created. The coolant moves through the engine and head, then flows (a) into the heater core through the lower heater hose, and (b) through the intake manifold hose back to the water pump.
When the engine is cold, the upper section of the dual-action thermostat is closed and prevents the return of coolant to the radiator via the thermostat cover and upper radiator hose. The lower section of the thermostat, essentially a spring-loaded disk, is connected to the upper section, and under these circumstances does not block the bypass passage that feeds the water pump's input. This arrangement avoids hotspots, ensures even engine warmup, and provides at least some heat to warm the cabin when that is desired.
The coolant temperature increases as the engine continues to run. When the coolant reaches about 82^C (for the standard thermostat), the upper section of the thermostat starts moving downward, allowing coolant to flow into the radiator. Simultaneously, the disk at the bottom of the thermostat begins move downward and restrict coolant flow through the bypass passage. By the time the upper section has opened about 0.4", the bypass passage has been closed off; further motion of the upper section of the thermostat merely increases the force applied to the disk. Coolant flow through the heater core is probably somewhat reduced as well, but I know of no data to prove or disprove this.
It should now be clear that if the proper dual-action thermostat is not used, or if the thermostat's operation is faulty, then the system will not work as intended and most likely the engine will overheat. In an emergency, though, one could use a conventional thermostat -- or none at all -- provided that the bypass passage were blocked.
Other than some of the passages between block and head, the most restrictive parts of the cooling system appear to be the passage between water pump and engine, and the water pump spigots for the various hoses. I've made a few measurements and calculations for these.
Pump to engine passage: .25" x 1.5" (rough), 0.37 square inches.
Upper radiator hose spigot on thermostat housing: 0.93" ID, 0.68 square inches.
Manifold hose spigot on pump: 1.06" ID, 0.88 square inches.
Lower radiator hose spigot on pump: 1.13" ID, 1.00 square inches.
All of the water pump's output flows through the passage between pump and engine. In the examples I've seen, this passage is quite irregular with uneven edges and appears to have been chiseled open (literally!). Enlarging and smoothing this passage, in much the same way as one would port a cylinder head, should significantly increase coolant flow while reducing stress on the water pump. To my eye, the various spigots also appear small enough to be restrictive. System performance might be improved by modifying these passages for improved flow.
The only ferrous parts exposed to the coolant are the water pump shaft and impeller, and the cylinder liners. A rusty water pump impeller makes an inefficient pump, and rusty cylinder liners are poor conductors of heat -- both conditions promote overheating. Since the 50-50 antifreeze/water mix suggested by the factory is a fairly good rust preventative, if kept fresh, _any_ rust in the coolant should be taken as a sign of impending problems.
Hope this provides food for thought.