Recent environmental standards and global demands for lower emissions and fuel consumption on gasoline and diesel engines require innovative combustion strategies, whereby air intake humidity allows for closed-loop monitoring. Humidity, pressure, and temperature can all be measured from a single transducer, packaged to survive the harsh conditions common to commercial vehicles.
Air intake humidity measurement is a key parameter for engine management and it allows commercial vehicle manufacturers to find a compromise in real time between pollutant emissions, fuel consumption, and engine power. Humidity management in the air intake has demonstrated how the humidity ratio in the air is conversely proportional to max cylinder pressure, engine torque, and nitrogen oxide (NOx) emissions. Emission standards have become more severe over the years. For example, today, the NOx emission from a diesel engine must be less than 80 mg/km. Air intake humidity control is one of the key technologies that enables engines to optimize the coordination of an air/fuel mixture and reduce exhaust gas emissions.
To achieve these goals, sensors able to monitor the multiple parameters critical to vehicle engine management are critical. TE Connectivity, for instance, has developed the TRICAN pressure/humidity/temperature sensor (Figure 1) that can monitor all three parameters, while being rugged enough to withstand harsh vehicle environments.
Digital combination sensors are used in different applications where humidity, pressure, and temperature need to be monitored with high accuracy and with a fast time response. For engine management in diesel and gasoline engines, humidity is known to affect air intake. The exhaust gas recirculation (EGR) loop adds humidity to the air intake. The humidity, temperature, and pressure sensors provide global performance enhancements and fuel consumption optimization. The measurement of air intake properties can offer injection and ignition timing adaptability, prevent corrosion and enhance cylinder lifetime by monitoring ERG condensation, reduce NOx emissions, and optimize ERG loop control.
Natural gas (NG) engines can also benefit from the addition of sensors to the air intake. In NG engines, maximum achievable power is a function of air intake humidity. An accurate air/fuel ratio in lean-burn engines is essential. Excess air reduces combustion temperature; thus, NOx emissions are reduced by half compared to a conventional NG engine. With sufficient oxygen, the combustion is more efficient because more power is produced from the same amount of fuel. Lean-burn limit is a function of humidity and must be adapted in real time to improve efficiency and reduce NOx emissions, knock, and misfire.
Humidity Impact on Combustion Engines
Air intake humidity affects engine efficiency in terms of max cylinder pressure, torque, and pollutant emissions. The following section refers to an experimental study on Renault K4M-700 four-cylinder gasoline engines, where specific humidity influence on engine torque and emission gases is highlighted.
Humidity Impact on Engine Torque
Due to combustion speed reduction, cylinder max pressure decreases when specific humidity increases. Engine torque decreases by 5.5 percent when specific humidity increases from 10 to 40 g/kg due to max pressure reduction. For a particular condition where specific humidity is 15 g/kg, if the measurement varies by 5 g/kg, engine torque will drop by 1 percent.
Humidity Impact on Emissions
Hydrocarbon emissions rise from unburned particles due to the wall extinction phenomenon, and these emissions also increase when air humidity increases. Carbon monoxide and nitrogen oxide decrease when air humidity increases.
For a particular condition where specific humidity is 15 g/kg, if the measurement varies by 5 g/kg, the emissions impact is: 1.5 percent more hydrocarbons, 7.2 percent more nitrogen oxide, and 5.4 percent more carbon monoxide. Adiabatic end-of-combustion temperature affects the amount of heat released during combustion. This influences the work done by the piston which affects engine power.
Specific humidity monitoring is a key factor for engine management and fuel cell operation. Several advantages of a humidity sensor at the engine intake have been demonstrated. It helps provide accurate closed-loop control. This accuracy is mandatory to meet emissions regulations over the operating temperature range of the engine. Air intake humidity affects burned gas composition and pollutant emissions. NOx and carbon oxide emissions can be decreased by accurate monitoring of specific humidity.
Combination humidity sensors typically employ a signal conditioning circuit and microcontroller to process the humidity, pressure, and temperature signals to provide a digital output. Self-diagnostic capabilities provide indications of a short circuit, open circuit, or out-of-range operation.
Temperature and pressure measurement can increase overall system accuracy and performance. The TRICAN sensor incorporates a negative thermal coefficient (NTC) temperature sensor operating for -40 to +125°C and accuracy of ±0.5°C. The pressure sensor is designed specifically for the transducer to measure air intake pressures up to 250 kPa from -40 to +125°C.
The sensor can support two-way communication for system-level verification with other external sensors. Its digital output conforms to J1939, CAN2.0 and can be configured to customer needs (CAN Frame). CAN bus signals were originally developed in the 1980s, with CAN 2.0 published in 1991. It is a common platform for sensor output signals in automotive and commercial vehicles.
Virtual NOx estimation allows for major cost reductions due to the removal of the upstream NOx sensor. The TRICAN accuracy is high even at cold start, where 50 percent of a driving cycle’s emissions are generated. During cold start, the NOx sensor is not efficient for at least 20 minutes. In these specific conditions, emissions are the highest and require a specific strategy. In addition, temperature and pressure sensors at the intake can be replaced by the TRICAN allowing additional cost reduction. It can also be used in combination with the upstream NOx sensor to offer system diagnostic capability and to monitor NOx accuracy over a lifetime (Figure 2).
In fuel cell applications, water content in the proton exchange membrane (PEM) is critical to ensure proton permeability, which provides optimal efficiency over the stack lifetime. The PEM must be kept damp through humidification of the reaction gases. Pressure is also a key parameter for power density control. It has a quick response time and demonstrates fast recovery after water condensation. The sensing elements are protected against chemical contamination.
A humidity sensor located in the engine requires rugged construction to tolerate the harsh underhood environment. The TRICAN digital combination sensor is constructed using a plastic housing, four-pin connector, and printed circuit board assembly (PCBA). The humidity cell accesses the air flow through a PCBA membrane (PTFE), allowing the humid air to enter the cell but protect it from liquid water and dust. It also includes enhanced protection against pollution and quick recovery time after condensation due to the placement of a heater near the humidity sensing element. A thin dielectric polymer sandwiched between an upper and lower electrode allows for a quick recovery after condensation and very good protection against pollutants at the same time.
The sensor is designed and calibrated for three standard supply voltage options: 5 V, 12 V, and 24 V. Various types of humidity can be provided on request with measurement for m0 to 100 percent and operation from -40 to 105°C.