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| Chassis - Radiator |
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| A real Lamborghini |
Endre (Andy) Bujtas - Here are a couple of pictures that will show how the radiators are hooked-up on the Diablo. As you can see, Lambo uses a parallel system, and it is quite short to boot. |
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I'm writing to correct the parallel resistance equation I set forth in my last email. I quickly wrote up the email and presented the equivalent resistance equation from my poor memory. The equation is not (R1 + R2) / ( R1 * R2) But (R2 * R1) / (R1 + R2) or for equal radiators R/2 The series equation is still R1 + R2 or for equal radiators 2R Therefore, going back to the DC model example (which by the way we use Kirchoffs Voltage Law V=IR in modeling fluid systems in real-time dynamic simulators) If R1 = R2 = 2 (assuming both radiators have the same resistance) Then in the series system the overall resistance will be 4, To clarify the model further, the voltage (V) would be the pressure source
- in this case the water pump The equation shows (assuming equal resistance between the radiators) that the resistance to flow in a series system is always 4 times that of a parallel system with the same resistance (equal radiators), which will significantly impact the coolant flow rate and thus, the heat transfer rate. |
| Andy
Bujtas -
Water Pumps Belt-driven water pumps are generally all alike. The only differences between different pumps (for the same engine) are casing size and impeller design (for higher flow rates or reduced cavitation), aluminum vs. steel (for weight), long vs. short necks (to fit specific vehicles). The basic difference between an electric pump and a belt-driven pump (beside the obvious difference in prime mover) is the pump speed. Electric pumps run at a constant speed, while the speed of belt-driven pumps are dependent on the engine speed. Therefore, at low engine speeds, the belt pump is pushing less water than an electric pump. In addition, electric pumps can provide greater flow since they can be designed/fabricated with larger casings since their overall footprint (area they occupy) is smaller than for a belt-driven pump - except for one of the Moroso models. Generally, belt-driven water pumps put out around 18-20 gpm, while there are electric pumps that claim 55 gpm. Remember that the faster the coolant flow, the greater the heat transfer rate (q'=m'cDT). The reason why racers go with an electric pump is to squeeze more power from the engine. The fact that the engine must turn a water pump, as well as the rear wheels and other things, requires a certain amount of horsepower, which could have been used elsewhere - like moving the car. Replacing the belt pump with an electric pump could add as much as 20 more HP can can be directed to the rear wheels. A reduction in flow rate also occurs because of the length the water must travel as well. Builders with the radiators in the rear have another issue. The water pump must suck the water from 2 radiators (those in series) all the way from the engine coolant outlet port. Because hoses and pipes are not perfectly smooth, the flow rate is reduced as a result of friction between the water and the plumbing. The longer the water must travel, the greater the frictional losses, the less the flow rate and pressure. So, for such systems an electric water pump would be better since they can "push" more coolant to overcome the losses. For such systems, an electric water pump with a rating of 35 gpm would be better. Again, remember q'=m'cDT, where m' is the mass flow rate, DT is the differential temperature, q' is the heat transfer rate and c is the heat capacity of water. This applies to both the water circulating around the cylinders as well as through the radiators. In the case of the radiators you would use q'=UADT. In this formula, A is the heat transfer area - like the total area of the coolant channels in the radiator core (e.g. 4-core transfers more heat than 2-core, etc., times the number of channels + the number of fins.) Besides the flow rate, the location of the pump is just as important. Ideally, you want the pump to be as close to the engine as possible and as low as possible relative to the radiator or suction point. With a pump close to the engine coolant inlet port means little or no flow losses. Having the pump as low as possible will minimize pump cavitation. Pump cavitation occurs when steam bubbles form during coolant compression. The mechanism is this: the suction of a centrifugal pump is in the center of the impeller. This is a low pressure area and steam bubbles can be formed because of the reduction in boiling temperature due to the reduction in pressure. As the steam-fluid mixture is compressed (centrifugally moved through the curved vanes of the impeller) the steam bubbles will collapse (pop) at the ends of the vane. This reduces pump efficiency as well as causing physical damage to the impeller. Cavitation can be minimized by placing the suction of the pump below the radiator outlet at the bottom. This would result in a higher suction pressure because the height of the water column - now the full height of the radiator - has increased. Normally, cars have their water pumps around the center of the radiator. So, the pump must "pull" the water from the radiator instead of the water being "pushed" into the pump by gravity with the pump below the radiator. An increase in suction pressure will reduce the formation of steam bubbles and will require less effort on the part of the pump to push the water. Considering this, the location of the pump for rear-radiator series systems would best be closer to and below the rear radiators. A builder should examine the system that Rick Page developed for his build. That is a good location for the pump as long as the exhaust headers are not too close. The builder should look at using a pump with a rating of around 35 gpm. But you don't want to over pump the system with an electric pump that has a rating of 55 gpm. Understand that the water is NOT constantly flowing through the engine block - except a little if your heating system is not valved like the original Fiero. The thermostat, located in the engine block coolant outlet port, blocks coolant flow through the engine allowing the coolant to sit virtually idle in the block for a short time. This is done to keep the engine at its optimal temperature - around 160-200 F. Therefore, while the flow is being blocked by the thermostat, the pump is still trying to push the water. A very high flow pump will heat-up faster while in this condition - further reducing its life. Understand that the coolant flowing through the pump is also cooling the pump itself, as well as the engine. The other consideration for using an electric water pump is to ensure that the electrical system of the car can handle the added load. These pump can draw up to 9 continuous amps from the system. Therefore, the builder should ad this load to his/her list of electrical loads to ensure proper sizing of the alternator. Remember, an electric fan can draw up to 22 amps (depending on size and power, but generally around 7-9 amps) multiplied by the number of radiators that have such fans. A good 80-90 amp alternator should be considered - especially if you plan on building your car with a stereo sound system using some of these new mega amplifiers. Andy |
| The following information (in the Red outline) is from the Diablo Support Forum. |
| http://www.kitcentral.com/ubb/Forum6/HTML/001338.html |
| Author Topic: Rear Radiators J.C. Hamlin Member posted 28 April 2003 05:22 PM -------------------------------------------------------------------------------- For those of you who have done rear radiators: which way does the airflow go through the radiator? I think it seems obvious that the air would move in a natural direction, in the big scoops on the top -- forced in by high pressure building up in front of the scoop, through the radiators, and out the back into the vaccum created by the moving car. Is this correct? The reason I ask is that someone that claims to know the aerodynamics of the Diablo actually told me the airflow through the rear radiators is backwards (in the back, and out the top). That can't be right, is it? Anyone ever checked? I supposed I can't immediately discount it, but I would guess that since the air intakes are right in front of the radiator intakes, if the airflow were naturally backwards through the rear radiators, that would cause problems with the air intake location as well having a tenancy to suck air out of the engine instead of blowing air into it. Thanks! -J.C. |
| lambo1 Member posted 28 April 2003 05:32 PM -------------------------------------------------------------------------------- does this thread help any? http://www.kitcentral.com/ubb/Forum6/HTML/000223.html |
| wrkitcars Member posted 28 April 2003 05:48 PM -------------------------------------------------------------------------------- If you think about it, the fan trying to push the air out the top scoop, while the car is moving and catching the air with the scoop and trying to shove it down, doesn't make any sense. You are right J.C. That scoop is there to catch air and shove it down through the radiators and out the back. Luis |
| RickPage Member posted 29 April 2003 10:57 AM -------------------------------------------------------------------------------- Since oil performs a huge cooling function, perhaps a fan on your oil cooler would help. Insuring that the air passing throught the oil cooler is ambient too would be good. I've heard oil performs 60% of the cooling in an engine. |
| Wilson Klassen Member posted 29 April 2003 12:32 PM -------------------------------------------------------------------------------- I think that part of the confusion can probably be best explained with an analogy. Did you ever see a pick up truck or a van where the window is replaced by a piece of plastic duct taped to the car? As you drive, the plastic is being pushed into the vehicle. This is because of the wake of the vehicle being pushed through the air is being filled in pushing on the plastic. Even though the air still wants to do the same thing with the Diablo, the air coming down the scoop will be of a greater pressure, and push its way through the radiator and out the back end of the car. |
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