Let's Clear the Air: Preserving CNG System Integrity and Operation June 2012

Saturday, June 30th, 2012 Technical Articles
Let's Clear the Air: Preserving CNG System Integrity and Operation June 2012

Let’s Clear the Air: Preserving CNG System Integrity and Operation June 2012

By Stan Martineau, ASE Master Auto, Truck, Advanced specialist, Alternative Fuel CNG specialist Utah State University/College of Eastern Utah, an NAFTC National Training Center   As a “CNG person,” I have watched compressed natural gas (CNG) system integrity and operation over an extended period of time with interest.. In the area where I live, many vehicles are driven until they die. The truck I drive is a 1999 with 310,000 miles, my work car has 227,000 miles, and our minivan had more than 350,000 miles when our college-age daughter bought her own vehicle, so we sold it. During a recent engine performance class for beginning CNG technicians, our discussion turned to system degradation over time. We brought in two older CNG dual-fuel vehicles that have racked up a lot of miles. The following is a brief description of what we found. First off, let me share how we ran some of the tests. We used a chassis dynamometer that enabled us to operate the vehicle at a steady speed and then apply a load to the vehicle. This was done for the same vehicles operating on gasoline and CNG in order to compare their respective emissions. Using three different five gas analyzers, we verified the emissions results. The five gas analyzer sample lines were placed in the vehicles tailpipe exit (see Figure 1). These analyzers measured emissions of hydrocarbons (HC), carbon monoxide (CO), carbon dioxide (CO2), oxygen (O2), and oxides of nitrogen (NOx).  

Figure 1

  The first vehicle was a 1998 model year with 184,000 miles. The check engine light was off, and there were no codes in the computer. During our pre-stress inspection, we found numerous vacuum leaks with a smoke machine— the spark plugs had been changed only once! The plug we pulled had .065 inch gap and was worn out. The air filter was dirty and very little maintenance had ever been done on this vehicle. We ran the vehicle at 50 miles per hour (mph) with a 10 percent load in fourth gear.   The measured concentrations were high when the vehicle was operated on gasoline. When operated on gasoline the exhaust gas concentration of HC was 650 parts per million (ppm) while the CO2 concentration was just over 7percent. When we switched the vehicle to natural gas, the HC concentration went up to more than 1,100 PPM and CO2 was more than 9 percent.   The second vehicle tested was a 1993 truck with 135,000 miles. This truck had been well maintained with new filters, regular maintenance, and a little care. Don’t get me wrong, this is a three-fourths-ton work vehicle that is used fairly hard, but along with the use, it receives regular maintenance.   The test conditions of the second vehicle can be seen in the screen shot of Figure 2.  The tested vehicle speed was 48 mph, with a 20 percent load. The engine torque and horsepower were 225 lb ft and 68 hp, respectively.  

Figure 2

  The exhaust gas concentrations of HC, CO, CO2 and O2 are shown in Figure 3 as the vehicle operated on gasoline. The concentrations for this 19-year-old work truck were surprisingly low.  

Figure 3

  Next we changed the fuel to CNG and ran the test again at the same operating points. There were less than two minutes between the two tests. The exhaust gas concentrations of HC, CO, CO2 and O2 are shown in Figure 4 as the vehicle operated on natural gas. It is seen that HC and CO emissions were lower while running on natural gas while O2 and CO2 concentrations were higher.  

Figure 4

  I find that with CNG, maintenance is key. CNG has a higher firing KV (voltage required by the spark plug to ignite the fuel), which will accentuate any secondary problems. We have seen numerous customer concerns in which the vehicle seems to run properly on gasoline, but they notice a performance problem while operating on natural gas. This maintenance issue can be seen by comparing the concentrations of the harmful and regulated emissions of HC and CO for the vehicle operating on gasoline and natural gas. Our job as technicians is to provide quality maintenance on alternative fuel vehicles in order to maintain the integrity of the system. As demonstrated by the first vehicle, emissions are higher and drivability problems are accentuated when maintenance is neglected on CNG vehicles.   Disclaimer – Stan Martineau is an ASE Master Auto, Truck, Advanced specialist, Alternative Fuel CNG specialist at Utah State University/College of Eastern Utah, an NAFTC National Training Center. Martineau is the guest commentator for this column. His content and opinions do not necessarily reflect that of the NAFTC.