Many of our Forum members are aware of the linear relationship between intake air temperature and HP output. Considering only these two variables, HP varies essentially as the square root of the change in ABSOLUTE temperature. Because of this relationship, it becomes a matter of interest for anyone involved in the search for increased power output to determine the temperature of the intake charge. This applies to both NA and FI engines. For a more complete explanation, go to:
http://www.wihandyman.com/forum/showthread.php?t=995
Some time ago I installed a digital ambient air temperature gauge in my car with the sending unit mounted in the air box. This gives me the means to record intake air temperatures under varying operating conditions. The gauge is made by Cyberdyne and has a range of -22 to 158 deg F. Gauges are also available with SI readouts in a range of -30 to 70 deg C. For more details, check out:
http://www.wihandyman.com/forum/showthread.php?t=47841
This thread is intended to provide you with some of the observations I've been able to make and some of the conclusions I've drawn based on how intake air temperature varies as a function of different driving and ambient air temperature conditions.
To begin, it's necessary to briefly describe the characteristics of my air intake system. It's a modified OEM system and the two salient features for the purposes of this discussion are that, (1) the inlet is located in a high pressure area sealed off from the engine compartment such that it ONLY ingests ambient temperature air, and (2) with the exception of the inlet, the entire system is sealed up to the air box accomplished by blocking the six drain holes located in the snorkel, the Helmholtz resonator and the air box, thus insuring no underhood temperature air can be drawn into the system. For a more complete description of my system, you can access the details at:
http://www.wihandyman.com/forum/showthread.php?t=1977
It's also worth noting that a digital temperature gauge responds much more rapidly to temperature changes than an analog instrument, as well its readout is precise requiring no interpolation as is often required in the case of the latter.
Determining the Baseline Temperature
It’s important to determine the baseline temperature in the air box before running a test so we’ll know how much variation takes place, but just how accurate is the baseline temperature? This depends on the ambient temperature and whether the car is parked in sun or shade. Below a certain temperature, the baseline can be considered the ambient if the car is dead cold. However, at higher temps and especially when parked in direct sunlight, heat absorption occurs under the hood (and in the air box) and even when starting from 'cold' the baseline is actually above the ambient. This is obvious when, after brief idling or placing the car into motion, the gauge reading falls to a level lower than the initial one. This factor has to be taken into account when making comparative temperature readings. That’s why, in the warmer weather, I always obtain a NWS temperature reading in the area from which the test will begin and use that as the baseline. Incidentally, at start-up, if the temperature read by the sensor is 37 degrees F or less, the gauge display flashes ICE for one minute and then returns to normal operation and displays the numeric temperature.
Ideally, I’d like to have a second switchable sender to measure OAT but if the sender is mounted in the bumper per Cyberdyne’s recommendation, it too could be subject to heat soak. I’ve noticed in my wife’s 3 that in warm weather the OAT gauge reading also falls somewhat after the car is put into motion, before it stabilizes.
We do what we can!
Cruise, High-Speed and Highway Running
As long as the vehicle remains in motion, the air box temperature seems to fairly closely track the ambient, remaining not too far above it. As speed increases to 40 mph air box temperature tends to decrease but there seems to be little, if any, temperature difference in the range between 40 and 85 mph in terms of change. At steady cruise on the Interstate between 60 and 85 mph for very long periods of time (assuming no change in the ambient), the gauge either holds steady or changes by no more than 1-2 deg F.
Idling, Slow-Speed, In-Town, and Stop-and-Go Driving
Here, the situation is completely different and illustrates the pronounced effect the driving mode has on temperature change. At slow speeds and especially at traffic light stops, the temperature rapidly climbs well above the ambient. The longer the stop, the higher the recorded temperature. Stop-and-Go driving has a similar effect. The higher the ambient and baseline temperatures to begin with, the more rapid the climb and the higher the end temperature. Idling produces both the most rapid rise in temperatures over time and yields the highest recorded temperatures. However, once a return to a more fluid driving mode is possible, the temperature fairly quickly falls to more normal levels.
After Shutdown
One interesting phenomenon I've observed is that after engine shutdown, the temperature in the air box climbs to levels much higher than were seen during actual running. Checking the air box temperature one hour after shutdown by restarting the engine, I typically find the gauge reads at least 30 deg F above that recorded at shutdown. Furthermore, after just parking the car for a few minutes, at restart the gauge reads significantly higher than it did at shutdown.
Conclusion
Based on the above observations, I think we can draw two conclusions. The first is that due to the air intake modifications, the system succeeded in providing a relatively cool intake charge to the engine as illustrated by the temperature gauge readings in which the air box temperature (remember, this is where the IAT sensor is situated) remained only slightly above the ambient in driving modes involving cruise, high speed and highway running.
The second is the way in which heat absorption profoundly increases intake air temperatures during idling, slow-speed, in-town, and stop-and-go-driving.
The Effect of Heat Absorption on Intake Air Temperature
The way in which heat absorption markedly increases intake air temperatures in the induction system has been discussed by AEM. Unfortunately, for whatever reason AEM has removed this discussion from its website and the reference is no longer available. Nevertheless, because of its clear, easily understandable exposition of this factor, I’ll still include it here.
Justifying their use of aluminum tubing over plastic even though aluminum is subject to greater heat soak, AEM asserts that the thermal conductivity of the duct material has little effect on engine power. Instead, the rate at which air travels through the intake duct under full throttle demand for maximum power negates the effect of material heat soak, regardless of the material. The article goes on to say that at small throttle openings (including idle), the air speed and airflow in the inlet system are relatively low. The high residence time of air in the inlet at low-throttle settings will increase inlet charge temperatures as heat is easily absorbed from the duct by the slow-moving air. However, when the throttle is fully opened, airspeed and air flow increase considerably. Now, the rapidly moving air has little opportunity to take on heat from the surrounding duct and the intake charge temperature drops.
Further confirmation that idle and slow-speed running increases intake air temperature due to the fact that, under these conditions, the slower moving air has more opportunity to pick up heat as it traverses the intake duct and manifold, is provided by Pulkrabek. According to the author, "All intake systems are hotter than the surrounding air temperature and will consequently heat the incoming air. This lowers the density of the air, which reduces volumetric efficiency. ... At lower engine speeds, the air flow rate is slower and the air remains in the intake system for a longer time. It thus gets heated to higher temperatures at low speeds, which lowers the volumetric efficiency curve at the low-speed end."
Reference:
Engineering Fundamentals of the Internal Combustion Engine, Second Edition, by Willard W. Pulkrabek; Chapter 5: Air and Fuel Induction, Section 5.2, page 193.
From this point I intend to post actual temperature readings as a function of various driving modes, routes and ambient temperatures to give a clearer picture of how these elements are interwoven together and related to intake air temperatures.
I look forward to your comments, additions and suggestions. Please remember, I'm not touting my intake system as the best or denigrating anyone else's efforts. My system is the best for me and the kind of driving I do - only this and nothing more.
Happy Motoring!
http://www.wihandyman.com/forum/showthread.php?t=995
Some time ago I installed a digital ambient air temperature gauge in my car with the sending unit mounted in the air box. This gives me the means to record intake air temperatures under varying operating conditions. The gauge is made by Cyberdyne and has a range of -22 to 158 deg F. Gauges are also available with SI readouts in a range of -30 to 70 deg C. For more details, check out:
http://www.wihandyman.com/forum/showthread.php?t=47841
This thread is intended to provide you with some of the observations I've been able to make and some of the conclusions I've drawn based on how intake air temperature varies as a function of different driving and ambient air temperature conditions.
To begin, it's necessary to briefly describe the characteristics of my air intake system. It's a modified OEM system and the two salient features for the purposes of this discussion are that, (1) the inlet is located in a high pressure area sealed off from the engine compartment such that it ONLY ingests ambient temperature air, and (2) with the exception of the inlet, the entire system is sealed up to the air box accomplished by blocking the six drain holes located in the snorkel, the Helmholtz resonator and the air box, thus insuring no underhood temperature air can be drawn into the system. For a more complete description of my system, you can access the details at:
http://www.wihandyman.com/forum/showthread.php?t=1977
It's also worth noting that a digital temperature gauge responds much more rapidly to temperature changes than an analog instrument, as well its readout is precise requiring no interpolation as is often required in the case of the latter.
Determining the Baseline Temperature
It’s important to determine the baseline temperature in the air box before running a test so we’ll know how much variation takes place, but just how accurate is the baseline temperature? This depends on the ambient temperature and whether the car is parked in sun or shade. Below a certain temperature, the baseline can be considered the ambient if the car is dead cold. However, at higher temps and especially when parked in direct sunlight, heat absorption occurs under the hood (and in the air box) and even when starting from 'cold' the baseline is actually above the ambient. This is obvious when, after brief idling or placing the car into motion, the gauge reading falls to a level lower than the initial one. This factor has to be taken into account when making comparative temperature readings. That’s why, in the warmer weather, I always obtain a NWS temperature reading in the area from which the test will begin and use that as the baseline. Incidentally, at start-up, if the temperature read by the sensor is 37 degrees F or less, the gauge display flashes ICE for one minute and then returns to normal operation and displays the numeric temperature.
Ideally, I’d like to have a second switchable sender to measure OAT but if the sender is mounted in the bumper per Cyberdyne’s recommendation, it too could be subject to heat soak. I’ve noticed in my wife’s 3 that in warm weather the OAT gauge reading also falls somewhat after the car is put into motion, before it stabilizes.
We do what we can!
Cruise, High-Speed and Highway Running
As long as the vehicle remains in motion, the air box temperature seems to fairly closely track the ambient, remaining not too far above it. As speed increases to 40 mph air box temperature tends to decrease but there seems to be little, if any, temperature difference in the range between 40 and 85 mph in terms of change. At steady cruise on the Interstate between 60 and 85 mph for very long periods of time (assuming no change in the ambient), the gauge either holds steady or changes by no more than 1-2 deg F.
Idling, Slow-Speed, In-Town, and Stop-and-Go Driving
Here, the situation is completely different and illustrates the pronounced effect the driving mode has on temperature change. At slow speeds and especially at traffic light stops, the temperature rapidly climbs well above the ambient. The longer the stop, the higher the recorded temperature. Stop-and-Go driving has a similar effect. The higher the ambient and baseline temperatures to begin with, the more rapid the climb and the higher the end temperature. Idling produces both the most rapid rise in temperatures over time and yields the highest recorded temperatures. However, once a return to a more fluid driving mode is possible, the temperature fairly quickly falls to more normal levels.
After Shutdown
One interesting phenomenon I've observed is that after engine shutdown, the temperature in the air box climbs to levels much higher than were seen during actual running. Checking the air box temperature one hour after shutdown by restarting the engine, I typically find the gauge reads at least 30 deg F above that recorded at shutdown. Furthermore, after just parking the car for a few minutes, at restart the gauge reads significantly higher than it did at shutdown.
Conclusion
Based on the above observations, I think we can draw two conclusions. The first is that due to the air intake modifications, the system succeeded in providing a relatively cool intake charge to the engine as illustrated by the temperature gauge readings in which the air box temperature (remember, this is where the IAT sensor is situated) remained only slightly above the ambient in driving modes involving cruise, high speed and highway running.
The second is the way in which heat absorption profoundly increases intake air temperatures during idling, slow-speed, in-town, and stop-and-go-driving.
The Effect of Heat Absorption on Intake Air Temperature
The way in which heat absorption markedly increases intake air temperatures in the induction system has been discussed by AEM. Unfortunately, for whatever reason AEM has removed this discussion from its website and the reference is no longer available. Nevertheless, because of its clear, easily understandable exposition of this factor, I’ll still include it here.
Justifying their use of aluminum tubing over plastic even though aluminum is subject to greater heat soak, AEM asserts that the thermal conductivity of the duct material has little effect on engine power. Instead, the rate at which air travels through the intake duct under full throttle demand for maximum power negates the effect of material heat soak, regardless of the material. The article goes on to say that at small throttle openings (including idle), the air speed and airflow in the inlet system are relatively low. The high residence time of air in the inlet at low-throttle settings will increase inlet charge temperatures as heat is easily absorbed from the duct by the slow-moving air. However, when the throttle is fully opened, airspeed and air flow increase considerably. Now, the rapidly moving air has little opportunity to take on heat from the surrounding duct and the intake charge temperature drops.
Further confirmation that idle and slow-speed running increases intake air temperature due to the fact that, under these conditions, the slower moving air has more opportunity to pick up heat as it traverses the intake duct and manifold, is provided by Pulkrabek. According to the author, "All intake systems are hotter than the surrounding air temperature and will consequently heat the incoming air. This lowers the density of the air, which reduces volumetric efficiency. ... At lower engine speeds, the air flow rate is slower and the air remains in the intake system for a longer time. It thus gets heated to higher temperatures at low speeds, which lowers the volumetric efficiency curve at the low-speed end."
Reference:
Engineering Fundamentals of the Internal Combustion Engine, Second Edition, by Willard W. Pulkrabek; Chapter 5: Air and Fuel Induction, Section 5.2, page 193.
From this point I intend to post actual temperature readings as a function of various driving modes, routes and ambient temperatures to give a clearer picture of how these elements are interwoven together and related to intake air temperatures.
I look forward to your comments, additions and suggestions. Please remember, I'm not touting my intake system as the best or denigrating anyone else's efforts. My system is the best for me and the kind of driving I do - only this and nothing more.
Happy Motoring!
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