How Environmental Conditions Affect Engine Emissions Testing

Engineers and regulators both take a close interest in emissions testing. For the engineers, the levels and proportions of NOx, SOx, carbon oxides (CO and CO₂), lead and residual hydrocarbons indicate how effective and complete combustion is in the engine. That, in turn, gives them insight into measures of efficiency and, ultimately, engine performance. For the regulators, the emissions are the story. Why they are the way they are is less important than the simple fact that the engine is releasing them at those levels.

Emissions testing can determine whether a prototype engine progresses to the next stage of development or whether a class of engines is certified for production

Measuring the key parameters accurately is essential and instrumentation errors can, therefore, have a significant impact financially, contractually or legally

Humidity and hydrocarbon combustion

Humidity has a relatively simple, direct relationship with the different byproducts an engine emits. In the ideal systems we all learn about in school (and never encounter in the real world) an engine will take in dense, dry air. If the optimal quantities of clean, dry air and fuel were the only things entering an engine, then every molecule of fuel could be burned and all oxygen could be consumed. This is known as stoichiometric combustion and would result in negligible emissions.

Water vapor represents a variable proportion of the air. In humid environments, the ratio of water vapor to all other gases (most significantly oxygen) increases, in dry environments it decreases. This affects the quantity of fuel required for ideal combustion.

This variability can reduce the engine's efficiency if not correctly measured and accounted for. During powertrain testing, unexpected levels of emissions could point to an inaccurate humidity sensor.

Influence of air density on relative humidity

For most measures of engine performance, it is the relationship between humidity, temperature and pressure that has most influence.

Talking about an increase in humidity is analogous to talking about a decrease in dry air density. However, the capacity of the air to hold water vapor depends on air temperature. At higher temperatures, higher relative humidity will have an even greater effect on engine efficiency and emissions because there will be a higher absolute level of water vapor entering the engine. There's a compounding effect, then, of a rise in temperature on engine output: the air is less dense, which in itself will decrease output; but there's also the potential for more water vapor, which will further decrease output.

Altitude compounds this dynamic. While the "thinner air" at higher altitudes has less carrying capacity for water vapor, and therefore is drier, whatever humidity is present in the air will have a greater effect because it is interfering with an already lower density of air molecules. 

HEADING: Chilled mirror hygrometers provide high repeatability and accuracy

Environmental conditions are less important than the characteristics of the air brought into the engine. To the extent engineers can control the temperature, pressure and humidity of the air that is brought into the engine, they can tune the powertrain for optimal real-world performance. Setting these parameters and programming the inlet air controllers requires accurate monitoring of the testing environment.

Chilled mirror hygrometers make two direct measurements: the temperature at the mirror and the drop in reflected light caused by condensation. Because there are no derived measures, the instruments experience minimal drift.

Chilled mirror hygrometers have the highest repeatability and accuracy of humidity measurement technology, which is why they are the de facto choice for calibration reference standards.

The Michell 501 Optidew measures dew point within ± 0.15 °C and temperature within ± 0.1 °C. These ranges remain constant across the sensing range of -40 to 120 °C and over the life of the instrument. Whether you are doing a series of tests on a single engine, certifying emissions before delivering an order to a client or taking multiple measurements over a span of years, chilled mirror hygrometers ensure that each test aligns with all the others.

Make emissions test results more reliable

Instrumentation errors during powertrain emissions testing are costly for your company's reputation and bottom line, and to the environment. They are also avoidable.

We have been delivering accurate, reliable and repeatable data to test engineers and operators for decades. Get in touch with any questions you might have about how our chilled mirror hygrometers can enhance your engine test processes.