Among the biggest challenges facing electric vehicle users are battery life and range. One way to increase both is to improve the efficiency of the entire vehicle. However, there are major consumers in the car standing in the way. One of them is the air conditioning system. The electric powertrain has less power loss than an internal combustion engine and therefore has less waste heat that can be used to heat the interior. This means that in electric vehicles, additional electric heating is required to achieve or maintain the desired temperature.
Ways to create an efficient air conditioning system
One way to increase the efficiency of the air conditioning system is to reuse the air in the interior (recirculation). In the winter, heated air is reheated, while in the summer, conditioned air is recooled and fed back into the interior. Since only a smaller temperature difference needs to be bridged, less energy is required. A major disadvantage of this method is that no fresh air is supplied to the interior. If the used air is not renewed, CO2 levels increase and the air quality inside the vehicle gradually deteriorates. This may result in headaches, fatigue, and a less-than-ideal driving experience. This represents a potential source of danger in road traffic, as ventilation measures are necessary above CO2 levels of 1000 ppm. One solution is to use CO2 sensors for control purposes. When integrated into the vehicle’s air conditioning system, they monitor the air quality inside the vehicle. If CO2 values are too high, a warning can be output or fresh air can be added directly to maintain healthy air quality. Choosing the appropriate sensor for electric vehicle applications depends on several factors. These include the size of the vehicle, the desired measurement range, and the type of measurement. Furthermore, the dimensions, performance, and cost of the sensors are crucial.
Small and precise CO2 sensor
A CO2 sensor with a particularly small form factor (14 mm × 13.8 mm × 7.5 mm) is the Xensiv PAS from Infineon (Fig. 1). It reduces the space requirement by more than 75 percent compared to commercially available CO2 sensors. At the same time, it offers precise CO2 measurements based on MEMS technology. For example, on a printed circuit board the Xensiv PAS CO2 sensor integrates a photoacoustic converter, including a detector, an infrared source and an optical filter, a microcontroller for signal processing and algorithms, and a MOSFET chip to drive the infrared source. The integrated microcontroller performs ppm calculations as well as advanced compensation and configuration algorithms. The result is the true CO2 content and not just a correlation. In addition, various configuration options (e.g. measurement frequency, baseline calibration) and interfaces (UART, I2C, PWM interface) are available. The spectrum for the CO2 measurement covers a range from 0 ppm to 32,000 ppm. The accuracy is ±30 ppm ±3 percent of the read measured value. The supplier guarantees it for the measurement range of 400 to 5000 ppm, which is perfectly adequate for this range of applications. This is because a typical atmosphere has a CO2 content of 400 ppm; the value inside the vehicle is typically higher.
Further advantages for customers are to be found in the production process. Infineon claims to offer the first SMD-capable CO2 sensor (SMD package, available on tape & reel) to comply with the international JEDEC standard for lead-free surface-mount reflow – for easy assembly as well as system integration even at high production volumes. The Xensiv PAS CO2 sensor also offers a high degree of flexibility thanks to a wide range of configuration options, enabling a fast time to market. An evaluation kit consisting of the Xensiv PAS and a microcontroller of the PSoC-4100S family for data evaluation are also available. The features and capabilities of the Xensiv PAS make the sensor the ideal choice for intelligent control of air conditioning systems in e-cars. On the one hand, it ensures optimum air quality, contributing to passenger safety. On the other hand, it increases efficiency for improved battery life and range.
Photoacoustic spectroscopy (PAS)
The PAS method is based on the photoacoustic effect (Figure): Gas molecules absorb light of a certain wavelength, causing them to expand. In the case of carbon dioxide, it is the 4.2 μm wavelength. Light pulses are emitted in rapid succession by an infrared source. Only light with a 4.2 µm wavelength enters the sensor chamber via an optical filter that is specially adapted to CO2 molecules. The CO2 molecules in the sensor chamber absorb the energy. Rapid heating and cooling causes thermal expansion and contraction. This produces a change in pressure, which is detected by the highly sensitive MEMS detector. The higher the CO2 concentration in the chamber, the stronger the signal. The signal is processed by an integrated microcontroller, which outputs the result in real time as ppm (parts per million). For the most accurate results, the acoustic detector is optimized for low frequencies and the absorption chamber is acoustically shielded from external noises.
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