Winter Approaching: Reverse Throttle Body Bypass???
Originally Posted by JohnnyC
I was tired of hearing everyone talk about the "laws of physics", so I decided to actually post some of them.
Here is a simplified (believe it or not) calc showing the effect of a 200 degree throttle body on intake air temperature:
Assumptions:
1) Outside air temp (Tin) = 70°F (21°C, 294°K)
2) Throttle Body is maintained at a temperature of 200°F (93°C, 366.5°K), which maintains the average TB to air temperature difference (Tave) at 72°C (345.2°K, conservative).
3) The throttle body flows 100 CFM of air at idle.
4) Heat Transfer Coefficient (h) of 6.81 W/ M^2-K is used for air cooling steel.
The following equation describes internal flow heat transfer through a thin-walled tube with a constant surface temperature:
h = (mfr) (Cp) (Tout – Tin)
( 3.14 ) (Dia) (Length) (Tave)
Where:
h = Heat Transfer Coefficient (6.81 W/ M^2-K is used for air cooling steel).
mfr = mass flow rate of air, calculated using an air density of 1.16 kg / m^3
Cp = specific heat of air at 21°C and atmospheric pressure = 1007 J / Kg-K
Dia = throttle body diameter
Length = throttle body length
Tave = average temperature difference between the TB and the air
mfr = (air volumetric flow rate) x (air density)
mfr = 100 CFM = 100 ft^3 / min = 2.83m^3 / min
mfr = [2.83m^3 / min] x [1.16 kg / m^3] x [1 min / 60 seconds] = 0.055 kg air / sec
By rearranging the equation above we can solve for Tout and finally determine just how much the hot throttle body heats up the air going to the engine:
Tout = Tin +
(h) ( 3.14 ) (Dia) (Length) (Tave)
(mfr) (Cp)
Tout = 294.3°K +
(6.81 W/ m^2-K) ( 3.14) (0.076 m) (0.102 m) (345.2°K)
(0.055 kg air / sec) (1007 J / Kg-K )
Tout = 295 °K = 71.7°F
To recap; that’s an increase of just 1.7 degrees at the worst-case, low-flow idle condition.
At a higher airflow of 500 CFM the temperature only rises 0.4 degrees to 70.4°F!
Hopefully this will help people see how little of an effect that heating the throttle body has on the intake air temperature.
Here is a simplified (believe it or not) calc showing the effect of a 200 degree throttle body on intake air temperature:
Assumptions:
1) Outside air temp (Tin) = 70°F (21°C, 294°K)
2) Throttle Body is maintained at a temperature of 200°F (93°C, 366.5°K), which maintains the average TB to air temperature difference (Tave) at 72°C (345.2°K, conservative).
3) The throttle body flows 100 CFM of air at idle.
4) Heat Transfer Coefficient (h) of 6.81 W/ M^2-K is used for air cooling steel.
The following equation describes internal flow heat transfer through a thin-walled tube with a constant surface temperature:
h = (mfr) (Cp) (Tout – Tin)
( 3.14 ) (Dia) (Length) (Tave)
Where:
h = Heat Transfer Coefficient (6.81 W/ M^2-K is used for air cooling steel).
mfr = mass flow rate of air, calculated using an air density of 1.16 kg / m^3
Cp = specific heat of air at 21°C and atmospheric pressure = 1007 J / Kg-K
Dia = throttle body diameter
Length = throttle body length
Tave = average temperature difference between the TB and the air
mfr = (air volumetric flow rate) x (air density)
mfr = 100 CFM = 100 ft^3 / min = 2.83m^3 / min
mfr = [2.83m^3 / min] x [1.16 kg / m^3] x [1 min / 60 seconds] = 0.055 kg air / sec
By rearranging the equation above we can solve for Tout and finally determine just how much the hot throttle body heats up the air going to the engine:
Tout = Tin +
(h) ( 3.14 ) (Dia) (Length) (Tave)
(mfr) (Cp)
Tout = 294.3°K +
(6.81 W/ m^2-K) ( 3.14) (0.076 m) (0.102 m) (345.2°K)
(0.055 kg air / sec) (1007 J / Kg-K )
Tout = 295 °K = 71.7°F
To recap; that’s an increase of just 1.7 degrees at the worst-case, low-flow idle condition.
At a higher airflow of 500 CFM the temperature only rises 0.4 degrees to 70.4°F!
Hopefully this will help people see how little of an effect that heating the throttle body has on the intake air temperature.
hmmmm
