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The thermocouple data logger was zip-tied in the engine bay next to the air filter. |
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The temperature measurements were taken at a sample rate of 1hz. I data logged everything in degrees Celsius but showed the data here in degrees Fahrenheit. |
I took the data from the logger, plugged it into Excel, and created this little graph. At 700 seconds, the car is sitting still and you can see the temperature at the air filter hit 170F degrees! Hence the reason why the air box on practically every car made is plastic (poor thermal conductor) and has a snorkel grabbing air from somewhere at the front of the engine bay.
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This is the engine bay of a BMW with the turbo inline-6 engine. Yup, plastic airbox sitting on top of the engine with a big air snorkel grabbing cool air from the front of the car. |
As soon as the car started moving, the temperature at the filter dropped. Note however that the temperature at the air filter was about 30F hotter than the ambient track temperature which was at a toasty 115F. The temperature of the charge air going into the intercooler hit almost 400F and was cooled down to the 160F-170F temperature range. My calculated intercooler efficiency was around 80%-85%.
Remember, there are a number of errors introduced to our intercooler efficiency calculation. On the hot charge air side, the temperature on the outside of the pipe is a little cooler than the actual air inside due to the thermal resistance of the aluminum pipe. However, the temperature at the location measured was probably hotter than at the actual intercooler inlet. On the outlet measurement side, we have the error due to the thermal resistance of the pipe again along with the hot air coming of the radiator maybe heating the air up a bit. In the grand scheme of things though, the temperature of the air going into the engine is roughly 50F hotter than ambient.
There is room for improvement in this however along with a lesson in test setup. When setting up a test or experiment, one critical thing to check is if your devices for taking measurements has an impact on the test. In this case, I don’t think the temperature at the air filter is quite as bad as we measured. Why not?
Well, the data logger did a pretty good job of blocking a hole from the fender to the engine bay. I’d guess some cooling air would have made it through the hole resulting in cooler air getting to the air filter. But because the data logger blocked off most of the hole, the temperature at the air filter was probably hotter than usual. So it just goes to show, your test setup can alter the measurements you’re trying to get. With that said, Novak and I have already come up with a few concepts for getting cooler air to the air filter and by extension the turbocharger.
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Remember these handy dandy equations from “Compressor Efficiency and More?” Let’s put them to use. |
When running a test, it’s a good idea to do a sanity check to see if the numbers make sense. Using the measured temperatures at the air filter and hot side charge pipe along with assuming a value for compressor efficiency, let’s see how realistic our measured values are. The whole idea for the article “Compressor Efficiency and More” was to demonstrate the importance of getting cool air to the turbocharger. The testing here is a good case study on the opportunity to improve performance. It’s also a good study in test setup. So we’re going to calculate the performance difference between getting the turbocharger air at ambient temperature vs. the 30F higher temperature we measured in testing (yeah, partly high due to our placement of the data logger).
