Bench Talk

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Rising global temperatures and increasing population have led countries to create guidance for fuel economy and emissions standards across the world. According to the United States Environmental Protection Agency, transportation comprises the largest fraction of greenhouse gas emissions (29 percent), of which light-duty passenger vehicles contribute 59 percent. Current Corporate Average Fuel Economy (CAFE) standards require passenger vehicles to reach an average of 23km/L (54.5m/g) by 2025 for all of a company’s light-duty vehicle offerings. It is worth noting that this standard is an average over the entire fleet and does not require each light-duty vehicle to reach this level of fuel economy.

This target, enacted to push the industry to make considerable strides in automotive development to aggressively curb greenhouse gas emissions, caused shockwaves of development at Original Equipment Manufacturers (OEMs) in two areas. Some are heavily investing in Electric Vehicles (EVs), while others are focusing on improving Internal Combustion Engines (ICEs) with waste heat recovery or other incremental engine improvements. The excitement around EVs and autonomous vehicles (AV) fueled market penetration by new OEMs. Still, there are inherent challenges to the mass commercialization of EVs and AVs, creating an opportunity for ICE technology.

OEMs turned to Micro-Electro-Mechanical Systems (MEMS) sensors to begin to address incremental improvements to ICE fuel economy. This blog examines how engineers are using MEMS sensors to improve fuel economy and emissions.

Engineers are developing MEMS sensors to improve fuel economy. The sensors have been used for automatic start/stop, stopping the combustion reaction while the vehicle is at a standstill. Historically, this feature has led only to incremental improvements—around 10 percent—in city traffic. This outcome means where the vehicle operates has a strong effect on performance.

The most significant benefit that MEMS sensors offer for fuel economy improvement is the automatic adjustment of the air/fuel ratio. This application is driving demand for MEMS sensors in the ICE automotive market. A MEMS sensor records airflow to the engine and passively adjusts the air/fuel mixture to target the stoichiometric (complete) composition, roughly 14:1 air to fuel by mass. Harmful emissions are inversely related to complete combustion of the fuel; the more efficient the combustion, the fewer undesired emissions the vehicle exudes.

Another sub-application of MEMS sensors used in engine performance is Nitrogen Oxide (NOx) control. MEMS sensors can control NOx, an undesired incomplete combustion product whose concentration increases with temperature but monitoring and regulating the pressure (Figure 1). Pressure and temperature are also directly related. As a result, when the pressure sensor reads a level that would cause the combustion chamber temperature to exceed the NOx formation level (1371°C), it sends a signal to the engine to reduce the pressure. Regulating air/fuel brings the combustion chamber temperature back down to desired levels. NOx typically forms in lean fuel mixtures since the reaction has plenty of air to combust the carbon chains completely. This condition prevents thermal energy consumption by the heat of combustion, leaving the excess air to heat the chamber.

Figure 1: NOx is a chemical compound of oxygen and nitrogen formed by a reaction during combustion at high temperatures, mainly combustion of fuel such as oil, diesel, gas, and organic matter. (Source: kvsan/shutterstock.com)

Engineers have used MEMS sensors to monitor and indicate engine conditions, but they are also augmenting those applications to address fuel economy and emissions. While EVs are driving market demand, the desire to continuously improve traditional ICE technology is pulling MEMS innovation in the near term.