Abstract: This paper describes the hazards of windshield fogging and introduces the concept of the most advanced sensor-based intelligent anti-fogging system for automobiles. It presents a leading international theory of fully automatic intelligent anti-fogging based on dew point and humidity measurement. The paper explores the causes of windshield fogging, analyzes the specific theories of intelligent anti-fogging systems, and discusses the contribution of intelligent anti-fogging systems to fuel efficiency. The Importance of Windshield Fogging Prevention With the rapid development of China's economy in recent years, owning a car is no longer an unattainable dream for ordinary people. Statistics show that in 2008, China's car sales exceeded 8.7 million vehicles. However, while the rapid development of China's automotive industry has brought convenience and joy to people, hidden dangers have also emerged. In 2007, my country recorded 327,209 road traffic accidents, resulting in 81,649 deaths, 380,442 injuries, and direct property losses of 1.2 billion yuan. This road accident rate far exceeds the average level of developed Western countries. Among the causes of these road accidents, a significant number occurred in winter or on rainy days when fogging of windshields obscured the driver's vision, leading to accidents. Many car models today are equipped with a "defogger" button. For customers, using this button to defog seems simple, but many drivers are unaware of when defogging is needed and when it's no longer necessary. Often, drivers activate the defogger at inappropriate times or leave it on even when it's not needed, wasting energy. This is where an intelligent automatic defogging system comes in, helping to determine the necessity and timing of defogging. An automatic defogging system not only protects the safety of passengers but also saves fuel. In the past, scholars and organizations worldwide have conducted research on windshield fogging, a common problem. Professor JD in the US proposed that solving the windshield fogging problem is the second most important criterion for judging the quality of modern automotive HVAC systems. [1] In 2003, scholar Allison studied that consumers were quite interested in "intelligent automatic anti-fog systems" and surveys showed that most consumers were willing to accept the additional $75 cost required for the anti-fog system. Intelligent automatic anti-fog systems, engine control, and wireless monitoring of car wheels have become the three most interesting topics for consumers today. [2] Nowadays, many new cars have begun to complete the research and development and application of this new HVAC anti-fog control technology. For example, the 2007 Acura MDX; Volvo S80; Audi A8, S8, BMW 3, 5, 6 and other models have begun to use this mature sensor-based car windshield anti-fog system. [3] Analysis of the causes of windshield fogging Before explaining the causes of windshield fogging, let's start with the concepts of water vapor, relative humidity and dew point in the air. After all, these are the most basic reasons for windshield fogging. At a given temperature, the pressure of water vapor that air can withstand is constant. The higher the temperature, the greater the pressure the atmosphere can withstand, and the less likely water vapor is to reach saturation pressure. Conversely, the lower the temperature, the easier it is for water vapor to reach saturation, and the easier it is to form dew. Relative humidity refers to the percentage of the actual water vapor density in the air to the saturation water vapor density at the same temperature. The relative humidity level is related to how close the air is to saturation with water vapor, but not directly to the absolute amount of water vapor in the air. When relative humidity reaches 100%, we say it has reached saturation. If more water is added, the air will become supersaturated, thus forming water droplets. Dew point temperature refers to the temperature at which air, without changing its water vapor content or air pressure, cools to saturation. In simpler terms, it's the temperature at which water vapor in the air turns into dew. Therefore, when the moisture in the atmosphere reaches saturation, it forms water droplets or fog when it adheres to dust, dew when it adheres to grass and plants in the early morning, and fog when it adheres to glass. This is because the dew point temperature in the air is very high, and the humidity in the air reaches saturation when it encounters cold solids. The main reason for fogging on the windshield is that the temperature of the windshield drops below the dew point temperature of the air inside the car. This phenomenon can occur due to increased humidity inside the car, which raises the dew point temperature, or due to the continuous drop in windshield temperature, causing fogging. Here are some examples of the causes of windshield fogging: 1. In the early morning, there is some moisture inside the car, and with the outside temperature quite low, the windshield temperature is lower than the dew point temperature inside the car, causing fogging. 2. After the car starts, the relative humidity inside the passenger compartment increases due to passengers' breathing, sweating, or after showering or entering the compartment from rain or snow. This raises the dew point temperature inside the compartment, making it higher than the windshield temperature, causing the windshield to fog up. 3. In environments with relatively high ambient temperatures, a sudden downpour lowers the windshield temperature, causing it to drop below the dew point temperature inside the compartment, resulting in fogging. 4. During the process of passengers generating stable water vapor through respiration, if the car enters a cold cave environment, the windshield temperature drops below the dew point temperature inside the compartment, causing fogging. 5. In subtropical or high-humidity environments, the relatively high humidity in the air raises the dew point temperature inside the compartment, making it higher than the windshield temperature, resulting in fogging. In the past, there have been many studies on windshield defogging, including automatic monitoring of windshield fog[4]; comparison of intelligent automatic defogging systems and infrared fog monitoring systems[5]; and Kelly and Sangwan’s method of calculating the fogging coefficient by comparing the temperature of the glass and the dew point[6]. All of the above studies have demonstrated the efforts made in automatic defogging and the importance and practicality of defogging. Automatic Defogging System As mentioned above, the main reason for the formation of fog on the windshield is that the temperature of the windshield drops and is lower than the dew point temperature of the air inside the car. Therefore, the two main parameters we measure and calculate are the temperature of the windshield and the dew point temperature inside the car. In general, the automatic defogging system includes three main parts, including parameter measurement input, parameter calculation and analysis, and defogging control. Parameter Measurement Input In order to analyze the conditions for fogging, three parameters need to be measured: the temperature of the windshield, the relative humidity and temperature at the same point in the car, and the temperature of the dew point in the car. Some mature sensor modules are now available, which are attached to the windshield (Figure 1). [7] The module contains a sensor that can measure the temperature of the windshield and a sensor that can measure the temperature and humidity at the same point in the car. Through these sensors, the three physical quantities that cause fogging are monitored in real time and then transmitted to the air conditioning MCU system for analysis and calculation. Figure 1: Schematic diagram of sensor module measurement Parameter Calculation and Analysis After collecting the measured data of the windshield temperature, relative humidity and temperature at the same point in the car, the air conditioning central controller will analyze and compare the data. First, the relative humidity and temperature at the same point in the car are used to calculate the temperature of the dew point in the car through a series of formulas. Then, the dew point temperature and the windshield temperature are compared to determine whether the windshield is fogged. We use Td to represent the temperature of the dew point and Tws to represent the temperature of the windshield. Δ represents the safety factor judgment: Td + ΔTd + Δ => Tws. The windshield may fog up (in a high-risk fogging zone, defogging control starts). Figure 2: Analysis of the relationship between dew point temperature and windshield temperature. As can be seen from the experiment in Figure 2, when using the intelligent automatic defogging system, the windshield temperature in the passenger compartment is relatively low at the beginning (1-2 minutes). Furthermore, due to the breathing of several people inside the passenger compartment, the relative humidity continuously increases, thus raising the dew point temperature. When the dew point temperature gradually approaches the windshield temperature, we can see fog gradually appearing on the windshield. At this point, if no measures are taken, the fog on the windshield will increase, completely obstructing the driver's vision, which is extremely dangerous on the road. At this point, we need to automatically turn on our defogging system. After the defogging system is turned on automatically, we can see from the time period of 3 to 5 minutes in Figure 2 that the temperature of the windshield begins to rise, and the dew point temperature inside the car gradually changes from rising to falling, and gets further and further away from the temperature of the windshield. At this time, the fog on the windshield will also disappear quickly. Because we need to activate our defogging system before the windshield fogs up, we set a safety factor Δ. When the dew point temperature approaches the windshield temperature (Td + Δ => Tws), the windshield is in a high-risk fogging zone. At this point, the defogging control system is activated to begin defogging. When the temperature difference between the dew point and the windshield reaches a certain value (Td + Δ < Tws, within the 6-8 minute timeframe shown in Figure 2), the system determines that the vehicle is in a low-risk fogging zone, and the defogging system can be turned off. This allows for real-time control of the defogging system through measurement, achieving intelligent defogging without any fog on the windshield. The automatic defogging system is more efficient, saving energy consumption. After the air conditioning MCU system detects the need for defogging, the following methods are commonly used to remove fog from the windshield. 1. When the relative humidity inside the car is high, maintain good ventilation so that the dry air outside the car and the humid air inside the car can be exchanged, thereby reducing the relative humidity inside the car and thus reducing the dew point temperature inside the car. 2. Directly heat the windshield of the car, as shown in Figure 3, so that the temperature of the windshield of the car rises and is higher than the dew point temperature inside the car, or accelerate the air flow in front of the windshield, [7] which can appropriately increase the temperature of the windshield and the exchange of air in front of the windshield. This reduces the relative humidity in front of the windshield. Figure 3: Schematic diagram of defogging control 3. Turn on the HVAC air conditioning system in the car, set the air conditioning system to external circulation, and the compressor will turn on, so that the relative humidity inside the car drops rapidly. At this time, the dew point temperature inside the car will drop and be lower than the temperature of the windshield of the car. Comparison of automatic antifog system and fog detection system The automatic antifog system requires a module that integrates humidity and temperature sensors, while the fog detection system is mainly composed of an infrared sensor. Table 1: Comparison of Automatic Anti-Fog Systems and Fog Detection Systems The Contribution of Automatic Anti-Fog Systems to Fuel Consumption Since last year, oil prices have been soaring. With the rapid development of the global economy, energy and environmental issues have become increasingly prominent. Saving energy and protecting the environment have become major challenges facing all countries in the world. Currently, China's average fuel consumption is 50% higher than that of developed countries. China's average fuel consumption is 10 to 15 liters, while that of developed countries is 5 to 10 liters. Therefore, how to improve efficiency by all means is an urgent issue for modern automobiles. In Europe, when the temperature is below 12 degrees Celsius and the relative humidity inside the car is above 80%, the car's air conditioning system should begin defogging. Statistics show that hundreds of millions of tons of fuel are used annually for air conditioning defogging. For example, if a typical car's air conditioning system has an engine power of 8-10 kilowatts, and the car's engine has a power of 100 kilowatts, consuming 10 liters of fuel per 100 kilometers, then the air conditioning system consumes approximately 1 liter of fuel. If we can effectively utilize the air conditioning defogging system, we can save 1 liter of fuel for every hour of effective use. Therefore, an automatic defogging system can more efficiently and fully utilize the defogging function of a car's HVAC system, intelligently defogging only when truly necessary, greatly improving vehicle functionality and saving energy waste and consumption. Conclusion By measuring the windshield temperature and cabin humidity, an automatic defogging system intelligently notifies the car's air conditioning system and the driver before the windshield fogs up, controlling the defogging system to automatically begin defogging. This prevents passengers from being in a dangerous environment, and automatically shuts off the defogging system when the cabin is in a fog-free environment, significantly improving the efficiency of the car's air conditioning system and saving energy consumption. References: 1. JD. Power Preliminary Vehicle Quality Study, 2002 2. Allison Advanced Vehicle Characteristics Study, 2002 3. New Climate Control Systems, October, 2006 4. CHUEY, Mark, D., Moisture Sensor and Windshield Fogging Measurement, 2004 5. Wang, Thomas, Karma, Automatic Windshield Defogging System, 2004 6. Kelly, SangWan, Automatic Defogging Method for Air Conditioning Systems, January, 2003 7. Eismann, Cedric, Defogging System Research, 2007