Sensors for Combustion Control

Surface Acoustic Wave Sensors

Low-cost surface acoustic wave (SAW) sensors absorb target gases on a piezoelectric crystal coated with a chemically selective material. The degree of change in the velocity or the resonance frequency of surface waves identifies and quantifies the target chemicals. Argonne is testing various coatings to detect hydrocarbons.

Design of a typical SAW chemical sensor

Frequency shifts of the SAW sensor with carbowax coating for a flow rate of
50 cc/min of nitrogen mixed with 50 cc/min of 300 ppm hydrocarbons

Ion Mobility Sensors

Projected to cost less than $15 per sensor, these spark-plug-sized devices work by ionizing nitrogen oxides (NOx) to form both positive NO and negative NO₂ ions that drift to a current detector through a uniform electrical field. The sensor can be placed in the exhaust line to monitor the performance of a catalytic converter and quantify the NOx emissions.

Basic design of a corona discharge ion-mobility spectrometry sensor

Ion mobility sensor peak amplitude versus NO₂ concentration measured with dry nitrogen and 2nA/V in sensitivity

Ultrasonic Sensors

These low-cost, fast-response ultrasonic sensors measure air/fuel ratio to a combustion engine and detect hydrogen leaks in a fuel cell process. The sensor uses an air-coupled, high-frequency transducer that transmits and receives ultrasonic pulses traveling through a narrow flow channel. Change in speed of sound quantifies the fuel ratio or detects the hydrogen concentration.    

Design of the acoustic sensor and associated control electronics and processing system

Changes of speed of sound in different hydrogen concentrations in air 

Microwave Cavity Sensors

These compact dual-cavity sensors determine polar gas concentrations in engine-out or tailpipe gas stream by measuring their absorption as a function of microwave frequency. The sharp resonance provided by the cavity structure yields detection sensitivities in parts-per-million range.

Diagram of microwave cavity built at Argonne

Sensitivity test using 102 ppm formaldehyde.
(N₂ reference signal subtracted from gas data)