The usage of Compressed Natural Gas (CNG) engines is increasing as requirements for cleaner emissions are required by state and federal agencies such as C.A.R.B. and E.P.A. Also, to further reduce emission levels, tighter air/fuel ratio control is required.
There are many ways to control air/fuel ratio on a CNG engine. It can be performed in a feedforward method, a feedback method or a combination of both. CNG fuel can be introduced to the engine via single-point injection, multi-point injection or with an air/gas mixer. Mixer-type and single-point injection are good candidates for the application of a gas flow sensor for accurate air/fuel ratio measurement. Reduction of valve hysteresis can also be achieved. Fuel delivery and control systems cost can be kept low compared to using multi-point injection where high flow injectors are required for each cylinder.
A gas flow sensor is placed in the CNG stream to monitor mass gas flow rate. The gas flow sensor is based on the hot wire anemometry concept, currently used in mass air flow measurement.
Topics regarding CNG composition effects on sensor output, temperature effects, pressure effects, high pressure noise management and oil contamination test results are important to the operation of the gas flow sensor and will be described in this paper.
A new 10 mm sensor concept is also presented for cost reduction. The manufacturing process is also simplified.