The Shift to CNG-Powered Trucks

The refuse industry is experiencing a significant shift to CNG-powered trucks as haulers realize the benefits in comparison with conventional diesel-powered vehicles. Engineered for the extreme environment of the waste industry, CNG-powered trucks provide lower fuel costs and a greener image, among other benefits. “Fuel cost is the number-one advantage,” states Curtis Dorwart, refuse segment manager for Mack Trucks, which has been manufacturing the TerraPro Series since 2009, and the cabover since 2010. Because of that, he says sales have been up every year. [text_ad] With more CNG trucks on trash routes, attention must be directed to maintenance.

CNG Versus Diesel: Difficulty

Servicing a CNG-powered trash truck isn’t considered any more difficult than servicing a diesel-powered trash truck, but it is different. “You must understand that it is vastly different than your typical diesel injections system,” says Ken Shurboff, national technical communicator for McNeilus Companies, which is part of Oshkosh Corp. The most common CNG fuel system on trucks does not include injectors. “Cummins/Westport utilizes a mixing chamber that closely resembles a carburetor on a gasoline engine,” continues Shurboff. The mixing chamber is a precision-designed component that interacts with the computer on the engine, constantly adjusting the fuel-air mixture for optimal performance. “We have found that water or oil suspended in the CNG can affect the mixing chamber in adverse, undesirable ways, so proper maintenance of the fuel supply may be considered by some as more intense. I tend to disagree with the evaluation of more intense. You have to keep water out of the diesel fuel; that requires filters designed to remove water, so filter change is, in my opinion, equal.” Natural gas contains both water and methane, along with a few other byproducts. “When natural gas is compressed at the station, before it is transferred to a vehicle, it naturally becomes tainted by compressor oil known as ‘drip gas,’” explains Trevor Milton, of dHybrid Systems. Drip gas is extremely harmful to the engine and can quickly ruin a spark-ignited engine. For this reason, there is a filter that operates on the high-pressure side of the fuel system that filters it. This filter needs to be monitored and changed regularly to ensure no drip gas reaches the engine cylinders. Tim Hooker, with Fontaine Modification Co., considers CNG and diesel simply two different types of technologies. A high-pressure CNG fuel storage system stores fuel at 3,600 psi, regulated down to the low-100s psi and delivered to the engine, as opposed to diesel fuel stored at minimum pressure as a liquid that is delivered by high-pressure direct injection (HPDI) to the engine. “The key is to deliver fuel in [the] cylinder at the proper flow and pressure rates that the engine requires to meet proper horsepower and torque demands.” The biggest problem McNeilus customers have had, Shurboff says, results from the oil introduced by the CNG compressors that fill the tanks. “Too many of the filling units I have been around do not adequately remove the lubricating oil used in the compressors. At the pressures the natural gas is being compressed, there is quite a bit of heat being generated and in these conditions, the oil becomes suspended in the CNG.” In this state it transfers into the storage tanks on the trucks, where it cools and precipitates out of the CNG, settling into the storage tanks. This oil will work its way back into the delivery system, emulsifying with any moisture that is naturally suspended in the gas. This emulsified water/oil mixture tends to plug up filters intended to remove moisture, causing a lean-burn situation or giving the effect of running out of fuel. If not properly dealt with through more frequent filter changes, it can enter regulators, solenoids, senders, and the mixing chamber, all of which can be expensive to fix. Because of this problem, many companies have installed their own fueling stations to address this issue. While it sounds onerous, the filter and tank inspections are the only regular service intervals you have with a CNG fuel system, Milton says. To the daily inspection of fuel filters, Dorwart adds ignition coils and differential oil. Spark plugs should be replaced every six months. “Other than that, there’s not a lot of difference from a diesel truck, except for the fuel injectors.” Spark plugs are a consideration not typically found on diesel machines, Shurboff confirms. “Unlike a car with spark plugs lasting 70,000 miles, CNG engines need to have the spark plugs [replaced] more often.” He estimates they need to be changed somewhere between 15,000 to 30,000 miles. “Since CNG is a unique fuel requiring an air-fuel mixture ratio between 5% and 15% with an octane rating of 117, the engine manufacturer is running the engine with a 16:1 compression ratio, also requiring the spark plugs to fire longer and hotter to get the fuel to burn.” [text_ad use_post='27767'] McNeilus’ exclusive CNG fuel management system features a control box that simplifies the system by incorporating a new manifold block that reduces the number of fitting connections and increases reliability. The fuel storage pods are equipped with boss-mounted, lightweight DOT Type 4 tanks and integrated three-sixteenth-inch steel guards for tank protection. CNC-formed high-pressure lines, O-ring face seal fittings, and high-pressure hoses with a working pressure of 5,000 PSI are also part of the system. A unique aspect of CNG is that because natural gas is half as dense as air, any residue left in changing filters will quickly dissipate. Therefore, Shurboff points out, there is no diesel fuel spill to clean up, no hazardous waste to deal with.

Managing municipal solid waste is more than landfilling: publicity, education, engineering, long-term planning, and landfill gas waste-to-energy are specialties needed in today’s complex environment. We’ve created a handy infographic featuring 6 tips to improve landfill management and achieve excellence in operations. 6 Tips for Excellence in Landfill Operations. Download it now!