Clear Vision Automatic Windshield Defogging System

Clear Vision Automatic Windshield Defogging System

ABSTRACT

The present paper describes the system design for the Clear Vision auto defog system and the improvements made to the Integrated Dew Point and Glass Temperature (IDGT) sensor. The Clear Vision auto defog system has been implemented on a 2000 Cadillac DeVille. Preliminary validation tests demonstrate satisfactory performance.

INTRODUCTION

Vehicle windshield fogging is a recognized safety concern. The presence of fog on the windshield glass reduces or blocks the field of vision for the driver. A distracted or blinded driver endangers both himself and other drivers sharing the road.

Windshield fogging is also a frequent occurring problem. J. D. Power ranked “Windows Fog Up A Lot” as the second highest in Problems Per 100 Vehicles (PP100) in its 2001 quality survey for the HVAC system [1]. It recorded an industry average of 2.0 PP100. For 2002, the Initial Quality Survey indicated an industry average of 2.2 PP100, the highest among HVAC problems [2]. In fact, fogging has persistently remained one of the “Top 10 Problems” of the automotive HVAC industry.

2003 Allison Fisher Survey [3] indicated that end-user interest in “Automatic Defog Control” at the cost of $75 level is ranked as highly desired, similar to the interest level in engine remote start and steering wheel radio control. This interest level is maintained across all the vehicle segments.

Fogging on the windshield occurs when the glass temperature falls below the dew point of air. This can take place either due to the rise of humidity in the passenger compartment, which increases the dew point, or due to the fall of windshield glass temperature. The following are some of the common fogging scenarios found in a vehicle:

• Fogging that occurs before the start of engine due to trapped moisture in the passenger compartment and external temperature drop.

• Flash fogging at the start of engine due to pre-existing evaporator condensate from previous AC operation.

• Fogging that occurs after engine start due to excess moisture introduction by the passengers who have just showered, got rained on, stepped through snow, etc.

• Sudden windshield temperature drop during driving, such as caused by a heavy rainfall during a warm day.

• Fogging due to steady vapor stream generation by passengers of the vehicle during driving in cold ambient conditions.

• Highly humid tropical conditions with air saturated at 100% humidity.

With the increased use of air recirculation, fogging is becoming more of a concern. Air recirculation prevents the discharging of moisture out of the passenger compartment and accentuates fogging through the accumulation of moisture from various sources such as perspiration, respiration, wet clothing, melting of deposited snow on floor mats, etc. Japanese vehicles are particularly vulnerable to fogging due to their heavy reliance on air recirculation in the HVAC system design. A new design variation for Japanese HVAC systems is to allow increased control over the use of air recirculation by the driver. For example, 2003 Toyota Matrix and Pontiac Vibe allow drivers to select “Recirc” even in the Defrost mode. One pitfall of this new design direction is that the limited knowledge of the driving public in terms of their understanding of the intended use of the various HVAC modes could worsen an already significant industry problem.

New, advanced HVAC system designs such as those optimized for energy efficiency and air quality also rely on the increased use of cabin air recirculation [4, 5]. In the case of energy efficient AC systems, the increased energy efficiency is gained through the reduction of reheat in the HVAC module and increased use of recirculated air. Reduction of reheat is achieved through set point changes for the AC system to operate at elevated evaporation pressure. The companion side effect is the partial loss of dehumidification, bearing negatively on windshield fogging. HVAC systems optimized for air quality maintenance do so by keepingoutside odor from the passenger compartment throughclosing off the air inlet door and going into recirculation mode.

Previous works on vehicle windshield fogging include that of Peters [6] in 1972 who investigated the effects of such factors as humidity of in-car air, window glasstemperature, the presence of passengers, vehicle body leakage, vehicle velocity, etc. In 1998, J. Cole, et al [7]reported an experimental study on automotive glass demisting with streams of air at different velocity and temperature. Davis et al [8] described an anti-fog strategy by using an in-car temperature sensor and a humidity sensor along with an ambient temperaturesensor. Kitada et al [9] performed a CFD study in 2002 on the process of defogging. Significantly, Urbank et al reported in 2001 an Integrated Dew Point and Glass Temperature (IDGT) Sensor for automatic control of windshield fogging [4]. US Patent 6508408 by Kelly and Sangwan [10] claimed a method of automatic de-fogging based on a fog factor derived from the temperature difference between the dew point of air and the measured glass temperature.

The present work describes the design considerations of the auto defog system and the general system design. Design improvements to the IDGT sensor are reviewedin some detail. The performance validation tests are summarized for the auto defog system as implemented on the 2000 Cadillac DeVille.

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