Natural gas fuel is making big inroads in the transportation and construction industries for a simple reason. It’s $1.50 to $2 cheaper per diesel gallon equivalent than traditional diesel fuel.
There’s a lot to pencil out before you invest in a natural gas driven truck. You have to calculate how much you’ll save on fuel and weigh that against the additional cost of the truck (most of which goes into the cost of the fuel storage tanks) and then come up with a reasonable ROI.
You should also investigate the maintenance side of things, to make sure you and your shop is ready to work on these trucks. These engines are actually a little less complex than today’s emissions compliant diesel engines. But they are different, and that includes maintenance.
We talked with Chip House, director of customer support for natural gas products at Cummins-Westport, to find out more about how you deal with these engines in the shop.
There are significant differences in the combustion byproducts of a diesel engine vs. a natural gas engine, says House. As a result, there is a separate lube oil spec.
“It’s based around sulfated ash content,” House says. “All engines burn some oil so the ash content in the oil is a factor. The higher the ash content the more susceptible the engine is to ash deposits on the top of pistons, sparkplugs and valves. For our specification, the oil cannot have an ash content higher than 0.6 percent,” he says. By contrast diesel engine oils can contain up to 1.85 percent ash.
There’s no industry-wide American Petroleum Institute spec or label for this oil, House says. It’s simply referred to as gas engine oil, or GEO, he says.
Natural gas engines produce very little soot, compared to diesel, and natural gas creates other combustion byproducts, so the additive package on the oils is slightly different.
The oil change intervals on natural gas engines are very close if not the same as comparable diesels, House says.
“The ISX 12 G (natural gas burning) is the same as the ISX 12 diesel, 400 hours,” House says. “The basic engine is the same. Same block, same crankshaft, same rods and rod bearings. The surfaces that need to be lubricated are the same.” Oil filters are also the same as used in the diesel engine, he says.
Assuming the fueling station is configured correctly, a fuel-water separator is not necessary, since natural gas fuel is dry, House says. There is a fuel filter that should be drained daily, he says, but this is for other contaminants. The fuel filter also protects against the possibility of oil contamination from the compressor in the fuel station, House says.
As for oil analysis, look for the same things as you would in a diesel lube oil analysis, House says, including wear metals, total base number, total acid number, and contamination.
Natural gas engines meet the same exhaust emissions standards as diesel engines, but because CNG and LNG burn cleaner, they generate fewer pollutants in the combustion process.
“The engines we produce today use a three-way catalyst. It’s very simple and maintenance free, much like what’s on your automobile, just a little bigger,” House says. “We don’t have a DPF, and we don’t use SCR.” This arrangement is particularly well suited to applications that involve a lot of slow speed operation and frequent starts and stops, he says.
Although Cummins-Westport does not make the pressurized fuel tanks that hold natural gas (that’s done by truck upfitters) there is a requirement for periodic fuel tank inspections, House says. Most companies get their technicians trained and certified to do this. And there is another required tank inspection any time a truck has been in an accident, he says.
Pressure regulators step down the fuel pressure to an average of 150 psi at the engine, House says. “It’s regulated in the on-board fuel system and then we further regulate it as it comes into the engine. There are also shutoff devices that you close when working on the system to prevent your fuel from escaping into the atmosphere.”
Unlike traditional diesel engine and even today’s gasoline engines, the Cummins-Westport design does not have fuel injectors. “We have an on-engine fuel regulator and a fuel control valve. Then there’s intake air that’s controlled by a butterfly throttle. It’s similar to the old carbureted engines,” House says. “Very simple in design and componentry. And you don’t have injectors to adjust.”
Diesel mechanics may take a while to get used to it, but these natural gas engines do have spark plugs and they have to be replaced on a recommended change interval. Spark ignited natural gas engines run a bit hotter even than gasoline engines, so it’s important to stick to the published spark plug change intervals, House says.
Another important maintenance consideration is adjusting the intake and exhaust valve lash. “Sometimes in the diesel world, people get a little lax with that,” House says. “With some of the new technology it seems not as important. But it’s extremely important to do on a natural gas engine, especially for knock control.”