J.Griff
01-11-2005, 02:22 PM
BACKGROUND:
In a nutshell, the intercooler's function is to cool charge air before going into your intake manifold. When your turbocharger has compressed inlet air to force-feed your engine, a by-product of this compression is the generation of heat. By passing this now-heated air through a heat exchanger (the intercooler or I/C for short), the goal is to cool the charge air, increasing its density (to increase power) and reducing the potential for detonation (also to increase power - as it allows higher boost before detonation).
There are really two things to discuss when evaluating the "performance" of an I/C. One is its THERMAL efficiency, and the other is its PRESSURE efficiency. Your goal would be 100% thermal AND pressure efficiency, meaning that the air exiting your I/C is back down to ambient temperature, with no reduction of pressure whatsoever. But limitations of size and cost dictate that your I/C might not get charge air back down to ambient temperature. Additionally, there is some pressure drop across the core, clearly reducing power. It would be sad to gain 15% density from the cooling, only to give back 10% of that free power in pressure drop… resulting in only a 5% power gain for your I/C investment. (And that happens more commonly than you might think.) Occasionally you'll find someone that says, "Well then, just raise the boost to compensate for the pressure loss", but what that really does is require you to compress the air to a higher pressure ratio in the first place. Thus the compressor exit air will be even hotter when it heads into the I/C (reducing the benefit) and, to get this higher compressor exit pressure, must take more work out of the exhaust - you guessed it, more exhaust back-pressure (cutting the benefit again). You can now see the goal: maximize temperature drop, minimize pressure drop!
THERMAL EFFICIENCY:
Let's say that it's an 80 degree day and you send 200 degree hot air into the I/C. If your I/C was 100% thermally efficient, then the air exiting it would match ambient - 80 degrees - which in this example, is a reduction in charge air temperature of 120 degrees. If your I/C was 50% efficient, then you'd only see half this delta, or a 60 degree cooling of the air - resulting in I/C exit air of 140 degrees. Mathematically, the formula is:
Thermal Efficiency (percent)= 100 x (Temp drop across I/C) / (Maximum Possible temp drop)
Or more commonly;
Thermal Efficiency (percent) = 100 x (Tin - Tout) / (Tin - Ambient)
Where of course Tin and Tout are the temperatures of the air into and out of the I/C, and Ambient is the temperature of the air outside the car.
Before I list the testing results, there's one final discussion; efficiency isn't just ONE number to quote, as it changes according to the airflow going through the system. When airflow is high, the temperature drop is reduced - and efficiency is reduced. If you found your intercooler to be 80% thermally efficient on your Volvo and then installed it on a turbocharged 500 cubic inch engine, you'd find that the efficiency is now lower (not to mention the fact that the pressure drop would also be worse!).
THERMAL TEST RESULTS:
I've run a series of tests on my stock '87 745ti. A great tool for this is a Fluke two-channel data-recording thermocouple, as you can record both temperatures while driving and retrieve the numbers later. You can also use this for oil temp, coolant temps, EGT and so on… but I digress.
I've run a series of tests in both Winter and Summer, over boost levels ranging from 5.5 to 11 psi. In the plot below, boost was left at the stock level (7.5 psi), and the ambient temperature was just below 40 degrees F. During steady state driving at 40 mph, I/C thermal efficiency seemed to run around 80%. However, I then slowed to 0 mph and then ran wide-open throttle for 21 seconds. You'll note that the I/C thermal efficiency for that portion started at 69% and dropped down to 63% range during the accel.
http://turbobricks.com/specs/img/ictemp.gif
Given the cold ambient conditions, I figured that the fan clutch was relatively disengaged, so the next morning I did the same test again, but with the mechanical fan totally removed. The results were within 1%!
HOWEVER, in the Summer, I have noted that the fan clutch does become a player, as it increases airflow over the the I/C core. Typically the efficiency levels are a solid 5% to 8% better under all conditions when it's hotter outside.
A quick discussion of temperatures: As you can see, the inlet temps peaked at just over 140 degrees in the plot above, and were cooled to the mid 70's. However, this was a pretty cold morning, especially for Phoenix. Naturally, hotter days and/or raising boost raises the temperatures: For instance, I did an 11 psi test on a 87F day last Fall and the I/C inlet temps stabilized at 256 F!! Exit temps peaked at 139F, which was a minimum efficiency of 69%.
BOTTOM LINE ON I/C THERMAL EFFICIENCY:
I'm quoting two numbers because of the mechanical fan's contribution. If you have no mechanical fan, use the "70 F or less" values.
Ambient temps around 70 F or less; 80% during cruise, 62% to 70% during WOT operation.
Ambient temps above 85 F degrees; 85-87% during cruise, 68% to 77% during WOT operation.
Generally speaking, in-between 70 and 85 F, the data varies between these values.
PRESSURE EFFICIENCY:
Instead of quoting efficiency in percent, most people discuss the pressure drop (in psi) across the I/C. Pressure loss is of course related to airflow through the core, thus pressure drop increases with RPM, throttle position, and of course boost. I have a good delta-P gauge which allows both the inlet and exit pressure to be hooked up to the same gauge, and the pressure difference is simply displayed. This is helpful because the pressure drop is usually pretty small when compared to the pressures being measured, and increases accuracy of the data.
http://turbobricks.com/specs/img/icpressure.gif
Above you'll find a plot of pressure drop vs. RPM for two boost levels; a close-to-stock 7 psi, and also for the higher airflow associated with 10.5 psi of boost.
Of course the goal is zero psi drop - but it is widely accepted that peak values under 0.5 psi are great, and even up to 1 psi is still considered acceptable. When the pressure drop gets above that, everyone agrees that there's a power gain out there for those willing to make improvements.
Looks like the factory intercooler at stock boost has a maximum pressure drop of 1.2 psi. How much power is this costing us? A good rule of thumb on a B230FT is that each additional pound of boost increases engine output approximately 8 HP, thus we could find an additional 10 HP if we could get the pressure drop to completely go away. But you'll never find a 0 psi drop I/C to replace this, thus the gain would be more in the territory of 5-8 HP. If your replacement I/C also had higher thermal efficiency than the factory unit, then you could make squeak out 10, maybe 15 HP from your modifications. As you might guess, this isn't the best bang-for-the-buck modification to make on a stock car. Exhaust work would yield 10 HP for less money.
However with boost up at 10.5 psi, the max pressure drop was 1.4 psi. With 15 to 18 psi of boost and an aftermarket exhaust system also increasing flow, it wouldn't surprise me to find a maximum restriction even up at 1.8 to 2.0 psi. Since a better-breathing engine will respond with more than 8 HP per psi (say 10 for this discussion), we're looking at a pretty solid 20 HP gain if pressure drop can be held to 0.5 psi and thermal efficiency can be increased by 10-15%. The bang-for-the-buck is finally getting reasonable.
Hope this was helpfull
In a nutshell, the intercooler's function is to cool charge air before going into your intake manifold. When your turbocharger has compressed inlet air to force-feed your engine, a by-product of this compression is the generation of heat. By passing this now-heated air through a heat exchanger (the intercooler or I/C for short), the goal is to cool the charge air, increasing its density (to increase power) and reducing the potential for detonation (also to increase power - as it allows higher boost before detonation).
There are really two things to discuss when evaluating the "performance" of an I/C. One is its THERMAL efficiency, and the other is its PRESSURE efficiency. Your goal would be 100% thermal AND pressure efficiency, meaning that the air exiting your I/C is back down to ambient temperature, with no reduction of pressure whatsoever. But limitations of size and cost dictate that your I/C might not get charge air back down to ambient temperature. Additionally, there is some pressure drop across the core, clearly reducing power. It would be sad to gain 15% density from the cooling, only to give back 10% of that free power in pressure drop… resulting in only a 5% power gain for your I/C investment. (And that happens more commonly than you might think.) Occasionally you'll find someone that says, "Well then, just raise the boost to compensate for the pressure loss", but what that really does is require you to compress the air to a higher pressure ratio in the first place. Thus the compressor exit air will be even hotter when it heads into the I/C (reducing the benefit) and, to get this higher compressor exit pressure, must take more work out of the exhaust - you guessed it, more exhaust back-pressure (cutting the benefit again). You can now see the goal: maximize temperature drop, minimize pressure drop!
THERMAL EFFICIENCY:
Let's say that it's an 80 degree day and you send 200 degree hot air into the I/C. If your I/C was 100% thermally efficient, then the air exiting it would match ambient - 80 degrees - which in this example, is a reduction in charge air temperature of 120 degrees. If your I/C was 50% efficient, then you'd only see half this delta, or a 60 degree cooling of the air - resulting in I/C exit air of 140 degrees. Mathematically, the formula is:
Thermal Efficiency (percent)= 100 x (Temp drop across I/C) / (Maximum Possible temp drop)
Or more commonly;
Thermal Efficiency (percent) = 100 x (Tin - Tout) / (Tin - Ambient)
Where of course Tin and Tout are the temperatures of the air into and out of the I/C, and Ambient is the temperature of the air outside the car.
Before I list the testing results, there's one final discussion; efficiency isn't just ONE number to quote, as it changes according to the airflow going through the system. When airflow is high, the temperature drop is reduced - and efficiency is reduced. If you found your intercooler to be 80% thermally efficient on your Volvo and then installed it on a turbocharged 500 cubic inch engine, you'd find that the efficiency is now lower (not to mention the fact that the pressure drop would also be worse!).
THERMAL TEST RESULTS:
I've run a series of tests on my stock '87 745ti. A great tool for this is a Fluke two-channel data-recording thermocouple, as you can record both temperatures while driving and retrieve the numbers later. You can also use this for oil temp, coolant temps, EGT and so on… but I digress.
I've run a series of tests in both Winter and Summer, over boost levels ranging from 5.5 to 11 psi. In the plot below, boost was left at the stock level (7.5 psi), and the ambient temperature was just below 40 degrees F. During steady state driving at 40 mph, I/C thermal efficiency seemed to run around 80%. However, I then slowed to 0 mph and then ran wide-open throttle for 21 seconds. You'll note that the I/C thermal efficiency for that portion started at 69% and dropped down to 63% range during the accel.
http://turbobricks.com/specs/img/ictemp.gif
Given the cold ambient conditions, I figured that the fan clutch was relatively disengaged, so the next morning I did the same test again, but with the mechanical fan totally removed. The results were within 1%!
HOWEVER, in the Summer, I have noted that the fan clutch does become a player, as it increases airflow over the the I/C core. Typically the efficiency levels are a solid 5% to 8% better under all conditions when it's hotter outside.
A quick discussion of temperatures: As you can see, the inlet temps peaked at just over 140 degrees in the plot above, and were cooled to the mid 70's. However, this was a pretty cold morning, especially for Phoenix. Naturally, hotter days and/or raising boost raises the temperatures: For instance, I did an 11 psi test on a 87F day last Fall and the I/C inlet temps stabilized at 256 F!! Exit temps peaked at 139F, which was a minimum efficiency of 69%.
BOTTOM LINE ON I/C THERMAL EFFICIENCY:
I'm quoting two numbers because of the mechanical fan's contribution. If you have no mechanical fan, use the "70 F or less" values.
Ambient temps around 70 F or less; 80% during cruise, 62% to 70% during WOT operation.
Ambient temps above 85 F degrees; 85-87% during cruise, 68% to 77% during WOT operation.
Generally speaking, in-between 70 and 85 F, the data varies between these values.
PRESSURE EFFICIENCY:
Instead of quoting efficiency in percent, most people discuss the pressure drop (in psi) across the I/C. Pressure loss is of course related to airflow through the core, thus pressure drop increases with RPM, throttle position, and of course boost. I have a good delta-P gauge which allows both the inlet and exit pressure to be hooked up to the same gauge, and the pressure difference is simply displayed. This is helpful because the pressure drop is usually pretty small when compared to the pressures being measured, and increases accuracy of the data.
http://turbobricks.com/specs/img/icpressure.gif
Above you'll find a plot of pressure drop vs. RPM for two boost levels; a close-to-stock 7 psi, and also for the higher airflow associated with 10.5 psi of boost.
Of course the goal is zero psi drop - but it is widely accepted that peak values under 0.5 psi are great, and even up to 1 psi is still considered acceptable. When the pressure drop gets above that, everyone agrees that there's a power gain out there for those willing to make improvements.
Looks like the factory intercooler at stock boost has a maximum pressure drop of 1.2 psi. How much power is this costing us? A good rule of thumb on a B230FT is that each additional pound of boost increases engine output approximately 8 HP, thus we could find an additional 10 HP if we could get the pressure drop to completely go away. But you'll never find a 0 psi drop I/C to replace this, thus the gain would be more in the territory of 5-8 HP. If your replacement I/C also had higher thermal efficiency than the factory unit, then you could make squeak out 10, maybe 15 HP from your modifications. As you might guess, this isn't the best bang-for-the-buck modification to make on a stock car. Exhaust work would yield 10 HP for less money.
However with boost up at 10.5 psi, the max pressure drop was 1.4 psi. With 15 to 18 psi of boost and an aftermarket exhaust system also increasing flow, it wouldn't surprise me to find a maximum restriction even up at 1.8 to 2.0 psi. Since a better-breathing engine will respond with more than 8 HP per psi (say 10 for this discussion), we're looking at a pretty solid 20 HP gain if pressure drop can be held to 0.5 psi and thermal efficiency can be increased by 10-15%. The bang-for-the-buck is finally getting reasonable.
Hope this was helpfull