Defogger
Encyclopedia
A defogger, demister, or defroster is a system to clear condensation
and thaw frost
from the windshield
, backglass, and/or side windows of a motor vehicle
. For primary defogging, heat is generally provided by the vehicle's engine coolant via the heater core
; fresh air is blown through the heater core and then ducted to and distributed over the interior surface of the windshield by a blower
. This air is in many cases first dehumidified
by passing it through the vehicle's operating air conditioning evaporator. Such dehumidification makes the defogger more effective and faster, for the dried air has a greater capacity to absorb water from the glass at which it is directed. Secondary defoggers, such as those used on a vehicle's backglass and/or side view mirrors
, often consist of a series of parallel linear resistive conductors in or on the glass. When power is applied, these conductors heat up
, thawing ice and evaporating condensation from the glass. These conductors may be composed of a silver-ceramic material printed and baked onto the interior surface of the glass, or may be a series of very fine wires embedded within the glass. The surface-printed variety is prone to damage by abrasion
, but can be repaired easily with a conductive paint
material. Resistive-heat defoggers are usually equipped with an automatic timer to operate for a set time period of 10 to 15 minutes before switching off. This is because most defogging is achieved within that timeframe, after which the vehicle's heater has usually brought the interior of the vehicle to a warm enough temperature that the fog does not recur. However, if this is not the case, the driver may activate the system again once it has timed out. There is usually a telltale
on the vehicle dashboard
, often on the defogger switch, to let the driver know the system is active.
Condensation
Condensation is the change of the physical state of matter from gaseous phase into liquid phase, and is the reverse of vaporization. When the transition happens from the gaseous phase into the solid phase directly, the change is called deposition....
and thaw frost
Frost
Frost is the solid deposition of water vapor from saturated air. It is formed when solid surfaces are cooled to below the dew point of the adjacent air as well as below the freezing point of water. Frost crystals' size differ depending on time and water vapour available. Frost is also usually...
from the windshield
Windshield
The windshield or windscreen of an aircraft, car, bus, motorbike or tram is the front window. Modern windshields are generally made of laminated safety glass, a type of treated glass, which consists of two curved sheets of glass with a plastic layer laminated between them for safety, and are glued...
, backglass, and/or side windows of a motor vehicle
Motor vehicle
A motor vehicle or road vehicle is a self-propelled wheeled vehicle that does not operate on rails, such as trains or trolleys. The vehicle propulsion is provided by an engine or motor, usually by an internal combustion engine, or an electric motor, or some combination of the two, such as hybrid...
. For primary defogging, heat is generally provided by the vehicle's engine coolant via the heater core
Heater Core
A heater core is a radiator-like device used in heating the cabin of a vehicle. Hot coolant from the vehicle's engine is passed through a winding tube of the core, a heat exchanger between coolant and cabin air...
; fresh air is blown through the heater core and then ducted to and distributed over the interior surface of the windshield by a blower
Centrifugal fan
A centrifugal fan is a mechanical device for moving air or other gases. It has a fan wheel composed of a number of fan blades, or ribs, mounted around a hub. As shown in Figure 1, the hub turns on a driveshaft that passes through the fan housing...
. This air is in many cases first dehumidified
Dehumidifier
A dehumidifier is typically a household appliance that reduces the level of humidity in the air, usually for health reasons. Humid air can cause mold and mildew to grow inside homes, which pose various health risks. Very humid climates or air make some people extremely uncomfortable, causing...
by passing it through the vehicle's operating air conditioning evaporator. Such dehumidification makes the defogger more effective and faster, for the dried air has a greater capacity to absorb water from the glass at which it is directed. Secondary defoggers, such as those used on a vehicle's backglass and/or side view mirrors
Wing mirror
A wing mirror is a mirror found on the exterior of motor vehicles for the purposes of helping the driver see areas behind and to the sides of the vehicle, outside of the driver's peripheral vision .Although almost all modern cars mount their side mirrors on the doors, normally at the "A" pillar,...
, often consist of a series of parallel linear resistive conductors in or on the glass. When power is applied, these conductors heat up
Joule heating
Joule heating, also known as ohmic heating and resistive heating, is the process by which the passage of an electric current through a conductor releases heat. It was first studied by James Prescott Joule in 1841. Joule immersed a length of wire in a fixed mass of water and measured the temperature...
, thawing ice and evaporating condensation from the glass. These conductors may be composed of a silver-ceramic material printed and baked onto the interior surface of the glass, or may be a series of very fine wires embedded within the glass. The surface-printed variety is prone to damage by abrasion
Abrasion
In dermatology, an abrasion is a wound caused by superficial damage to the skin, no deeper than the epidermis. It is less severe than a laceration, and bleeding, if present, is minimal. Mild abrasions, also known as grazes or scrapes, do not scar or bleed, but deep abrasions may lead to the...
, but can be repaired easily with a conductive paint
Paint
Paint is any liquid, liquefiable, or mastic composition which after application to a substrate in a thin layer is converted to an opaque solid film. One may also consider the digital mimicry thereof...
material. Resistive-heat defoggers are usually equipped with an automatic timer to operate for a set time period of 10 to 15 minutes before switching off. This is because most defogging is achieved within that timeframe, after which the vehicle's heater has usually brought the interior of the vehicle to a warm enough temperature that the fog does not recur. However, if this is not the case, the driver may activate the system again once it has timed out. There is usually a telltale
Idiot light
A tell-tale is an indicator of the status or malfunction of a system within a motor vehicle. Tell-tales consist of a light bulb or an LED which lights up a symbol or text legend...
on the vehicle dashboard
Dashboard
A dashboard is a control panel placed in front of the driver of an automobile, housing instrumentation and controls for operation of the vehicle....
, often on the defogger switch, to let the driver know the system is active.