BAROMETRIC PRESSURE, % HUMIDITY, AIR TEMPERATURE, ALTITUDE
Mar 25, 2017 1:38:12 GMT 10
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Post by Deleted on Mar 25, 2017 1:38:12 GMT 10
The weather has a direct and measurable affect on carburetor jetting because of the changes in air density and ability to atomize fuel.
(General term = weather and altitude require fueling adjustments). Adjusting Main Jet and the Needle position is often needed. Rarely will you ever touch the air adjustment once fired and idling (because this circuit relative to the available air at the moment of firing). Smooth idle = proper air mixture (for low speed). All the high speed stuff occurs primarily at the Needle and Main Jet.
This affects Fuel Injection too - but F.I. is managed by your ECU which compensates via A/F calculations/sensors, Piggyback fuelers (Bazzaz.etc..) still require tuning because the values are hard set via their coded maps relative to A/F ratio.
AIR = Nitrogen (N2), Oxygen (O2), Carbon Dioxide (CO2), Water (H2O), and many other minute components.
2 STROKE: are very dependent and sensitive to these changes and must be monitored and adjusted closely and constantly dependent upon the available air/oxygen to fuel and oil lubrication ratios. Not enough A/F/L and you have a seizure (Fuel also cools 2T's very noticeably. (Lean = little fuel & lubrication = loss of power and prone to seizure = grenade)
4 STROKE: are less finicky because they are not dependent upon fuel and oil lubrication delivered in the fuel system. They are reliant upon fuel and air only in carbed models. They may lose power due to the weather changes but are not likely to grenade as result of leaning out and seizing. (lean = not enough fuel = heat and loss of power)
LOW BAROMETRIC PRESSURE / WARM AIR TEMPS:
When the air density decreases, you will need to "RICHEN" the air-fuel mixture
(Larger Main Jet and Needle Position Raised (the clip is raised which lowers the needle)
HIGH BAROMETRIC PRESSURE / COLD/COOL AIR TEMPS:
When the air density increases, you will need to "LEAN" the air-fuel mixture to compensate.
(Smaller Main Jet and Needle Position Lowered (the clip is lowered which raises the needle)
HUMIDITY:
When the percentage of humidity in the air increases, the percentage of oxygen in the air lowers because it is displaces with the water molecules (humidity)
20% Humidity vs 90% Humidity - @ 90% humidity you have more nitrogen and less oxygen. Oxygen is what catalyzes when combined with fuel.
GENERALLY: Increased moisture content reduces the density of moist air - dry air is more dense than moist air.
+ HIGH HUMIDITY:
Will make the air-fuel mixture richer, so you should change to smaller jets.
- LOW HUMIDITY:
Will make the air-fuel mixture leaner, so you should change to larger jets.
ALTITUDE:
In general, the higher the altitude the lower the air density.
At high altitude, you should change to smaller jets and increase the engine's compression ratio to compensate for the lower air density.
but will need to adjust the needle to atomize fuel efficiently.
WANNA GET SUPER NERDY?:
The density of humid air varies with water content and temperature. When the temperature increases a higher molecular motion results in expansion of volume and a decrease of density.
The density of a gas, dry air, water vapor - or a mixture of dry air and water vapor like moist or humid air - can be calculated with the Ideal Gas Law.
Density of Dry Air
The density of dry air can be calculated
Pa = 0.0035 Pa / T
where
Pa = density dry air (kg/m3)
Pa = partial pressure of air (Pa, N/m2)
T = absolute dry bulb temperature (K)
Density of Water Vapor
The density of water vapor can be calculated
Pw = 0.0022 Pw / T
where
Pw = partial pressure of water vapor (Pa, N/m2)
Pw = density of water vapor (kg/m3)
T = absolute dry bulb temperature (K)
Density of Moist Air - an Air Vapor Mixture
The amount of water vapor in air influence density. Water vapor is relatively light compared to diatomic Oxygen and diatomic Nitrogen - the dominant components in air.
When vapor content increases in moist air the amount of Oxygen and Nitrogen are decreased per unit volume and the density of the mix decreases since the mass is decreasing.
Dry air is more dense that humid air!
Based on specific volume of moist air the moist air density can be calculated
:
P = 1 / v
= (P / Ra T) (1 + x) / (1 + x Rw / Ra)
where
v = specific volume of moist air per mass unit of dry air and water vapor (m3/kg)
Ra = 286.9 - individual gas constant air (J/kg K)
Rw = 461.5 - individual gas constant water vapor (J/kg K)
x = humidity ratio (kg/kg)
P = pressure in the humid air (Pa)
Density of dry air can be expressed as:
Pda = p / Ra T
where
Pda = density dry air (kg/m3)
Hows that Gents?
DG
(General term = weather and altitude require fueling adjustments). Adjusting Main Jet and the Needle position is often needed. Rarely will you ever touch the air adjustment once fired and idling (because this circuit relative to the available air at the moment of firing). Smooth idle = proper air mixture (for low speed). All the high speed stuff occurs primarily at the Needle and Main Jet.
This affects Fuel Injection too - but F.I. is managed by your ECU which compensates via A/F calculations/sensors, Piggyback fuelers (Bazzaz.etc..) still require tuning because the values are hard set via their coded maps relative to A/F ratio.
AIR = Nitrogen (N2), Oxygen (O2), Carbon Dioxide (CO2), Water (H2O), and many other minute components.
2 STROKE: are very dependent and sensitive to these changes and must be monitored and adjusted closely and constantly dependent upon the available air/oxygen to fuel and oil lubrication ratios. Not enough A/F/L and you have a seizure (Fuel also cools 2T's very noticeably. (Lean = little fuel & lubrication = loss of power and prone to seizure = grenade)
4 STROKE: are less finicky because they are not dependent upon fuel and oil lubrication delivered in the fuel system. They are reliant upon fuel and air only in carbed models. They may lose power due to the weather changes but are not likely to grenade as result of leaning out and seizing. (lean = not enough fuel = heat and loss of power)
LOW BAROMETRIC PRESSURE / WARM AIR TEMPS:
When the air density decreases, you will need to "RICHEN" the air-fuel mixture
(Larger Main Jet and Needle Position Raised (the clip is raised which lowers the needle)
HIGH BAROMETRIC PRESSURE / COLD/COOL AIR TEMPS:
When the air density increases, you will need to "LEAN" the air-fuel mixture to compensate.
(Smaller Main Jet and Needle Position Lowered (the clip is lowered which raises the needle)
HUMIDITY:
When the percentage of humidity in the air increases, the percentage of oxygen in the air lowers because it is displaces with the water molecules (humidity)
20% Humidity vs 90% Humidity - @ 90% humidity you have more nitrogen and less oxygen. Oxygen is what catalyzes when combined with fuel.
GENERALLY: Increased moisture content reduces the density of moist air - dry air is more dense than moist air.
+ HIGH HUMIDITY:
Will make the air-fuel mixture richer, so you should change to smaller jets.
- LOW HUMIDITY:
Will make the air-fuel mixture leaner, so you should change to larger jets.
ALTITUDE:
In general, the higher the altitude the lower the air density.
At high altitude, you should change to smaller jets and increase the engine's compression ratio to compensate for the lower air density.
but will need to adjust the needle to atomize fuel efficiently.
WANNA GET SUPER NERDY?:
The density of humid air varies with water content and temperature. When the temperature increases a higher molecular motion results in expansion of volume and a decrease of density.
The density of a gas, dry air, water vapor - or a mixture of dry air and water vapor like moist or humid air - can be calculated with the Ideal Gas Law.
Density of Dry Air
The density of dry air can be calculated
Pa = 0.0035 Pa / T
where
Pa = density dry air (kg/m3)
Pa = partial pressure of air (Pa, N/m2)
T = absolute dry bulb temperature (K)
Density of Water Vapor
The density of water vapor can be calculated
Pw = 0.0022 Pw / T
where
Pw = partial pressure of water vapor (Pa, N/m2)
Pw = density of water vapor (kg/m3)
T = absolute dry bulb temperature (K)
Density of Moist Air - an Air Vapor Mixture
The amount of water vapor in air influence density. Water vapor is relatively light compared to diatomic Oxygen and diatomic Nitrogen - the dominant components in air.
When vapor content increases in moist air the amount of Oxygen and Nitrogen are decreased per unit volume and the density of the mix decreases since the mass is decreasing.
Dry air is more dense that humid air!
Based on specific volume of moist air the moist air density can be calculated
:
P = 1 / v
= (P / Ra T) (1 + x) / (1 + x Rw / Ra)
where
v = specific volume of moist air per mass unit of dry air and water vapor (m3/kg)
Ra = 286.9 - individual gas constant air (J/kg K)
Rw = 461.5 - individual gas constant water vapor (J/kg K)
x = humidity ratio (kg/kg)
P = pressure in the humid air (Pa)
Density of dry air can be expressed as:
Pda = p / Ra T
where
Pda = density dry air (kg/m3)
Hows that Gents?
DG
