On Board Diagnostic (OBD) II compliance of natural gas vehicles can be
achieved by demonstrating that the Malfunction Indicator Light (MIL)
will be illuminated when a fault is detected which will cause the
emissions when operating on CNG to exceed 1.5 times the applicable
emissions standard for the vehicle. There are 120 diagnostic trouble
codes and 10 major monitors in a typical OBD II monitoring system, of
which at least 5 of the major monitors are affected by switching to
natural gas operation, including fuel trim, oxygen sensor, misfire, and
catalyst monitors.
The OBD II emissions performance on natural gas for each of the monitors must be shown to comply with fault detection thresholds of 1.5 times the applicable emissions standards. The OBD II monitoring system has, of course, been designed for gasoline operation, and comprises sophisticated monitors which detect adverse behavior in the primary engine management system.
The gasoline monitor fault detection thresholds have been set up by the vehicle manufacturer based on a correlation between the monitoring strategy designed to detect faults, and the emissions performance when the fault is detected by the monitor. That same correlation will not necessarily exist when the vehicle runs on CNG. When the vehicle is operated on CNG using unaltered gasoline thresholds, then when a fault code is set during CNG operation, the emissions may be greater or less than the required value of 1.5 times the emissions standard. For example, when the MIL is illuminated as a result of a fault detected by one of the monitors while the vehicle is running on natural gas, the emissions may be 2.5 times the emissions standard when the MIL is illuminated, and the vehicle will not be in compliance with OBD II regulations.
Ideally, when the vehicle operates on natural gas, the gasoline monitor thresholds should be recalibrated for natural gas operation, but this involves an expensive test program designed so that faults are progressively induced in the CNG system until the emissions reach 1.5 times the emissions standard, at which point the MIL is set to illuminate. Not only is this an expensive undertaking for an aftermarket converter, but he would generally be unable to change the MIL thresholds to comply with the 1.5 times emission malfunction criteria.
A recently completed research project was undertaken to investigate to what extent the emissions would be out of compliance if the gasoline monitor thresholds were used without recalibration. The NGV converter, perhaps, is being forced to carry out an expensive test program without good reason.
Since neither the US Environmental Protection Authority (EPA) or the California Air Resources Board (CARB) had any knowledge of the OBD II performance when a vehicle was converted to run on natural gas and the gasoline monitoring system was simply used without alteration, a major test program was initiated, supported by the California South Coast Air Quality Management District, (SQUAMD), the New York State Energy R & D Authority (NYSERDA), and Natural Resources Canada. The participants included a number of representative aftermarket NGV converters, two test laboratories, and program management was provided by Alex Lawson Associates Inc. as a consultant to the Canadian Natural Gas Vehicle Alliance.
The objective of the program was to develop an understanding of the OBD II performance of typical aftermarket NGV conversions. The OBD II system is designed to detect faults which cause the emissions to exceed 1.5 times the emissions standard. Therefore, the program was designed so that a fault was progressively induced, during CNG operation, for each monitor, until the MIL was illuminated, as determined by the gasoline threshold settings. At that point the emissions were measured to compare them against the required malfunction criteria. For bi-fuel vehicles, threshold performance could be compared directly between CNG and gasoline. Tests were run on bi-fuel vehicles certified on CNG to the same emissions standards as gasoline, and on dedicated CNG vehicles certified on CNG to much lower emissions standards than on gasoline. The study therefore covered different CNG conversion technologies, different OEM vehicles, bi-fuel and dedicated CNG conversions, and different levels of certification stringency.
The results of these tests will be reported in detail in a paper to be presented at NGV2008 3 – 5 June in Rio de Janiero, together with the test methods employed. The following general conclusions were reached by the study:
• Provided that the certification level remains the same as the base gasoline vehicle, the gasoline monitors will generally detect faults with an NGV, at emissions levels below the malfunction criteria set by the OBD regulations.
• When the certification level on CNG is more stringent than the base gasoline vehicle, then the threshold levels may have to be reset from the gasoline settings.
For cases where it is desired to certify an NGV to a more stringent level than gasoline, the technology now exists for some NGV converters to access the OEM computer to make appropriate changes to the original threshold levels in order to reach OBD II compliance. A brief description of this approach will be provided in the paper.
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