The first time you hear about it, oil blending sounds like a simple, good idea. Start with motor oil drained from the crankcase for a regular oil change, filter to remove the small particles of metal and other contaminants, blend with diesel fuel, and pour the mixture back into the fuel tank so the engine burns it.
You save a gallon of diesel fuel for each gallon of oil blended, and there is no cost for disposing of the waste oil. And as we all know, diesel engines are built like bricks and can burn vegetable oil, or standard diesel fuel, or other exotic stuff. Look on YouTube and you can find videos of people advocating this oil blending strategy for the 1970s vintage diesel cars they keep in the backyard. There are even commercially made machines — primarily advertised for use by the military — which automate the whole process from filtration to blending.
“In years past that was a practice used by a lot of trucking folks in North America, a lot of fleets, and you could do that quite easily with Cummins engines,” says Lou Wenzler, technical sales support director for Cummins, Inc.
But as the saying goes: That was then, and this is now.
David McKenna, director of power train sales and marketing for Mack Trucks, says, “You’ve got to understand this – this is a bad idea.”
Wenzler agrees, as does Andre Boehman, professor of mechanical engineering at the University of Michigan.
Oil blending may work for the military, but it is exempt from air pollution rules, and that makes sense because military engines must run anywhere in the world and on whatever fuel is available, McKenna says.
Modern commercial truck engines operating on U.S. highways are different. In particular, they must now comply with new air pollution limits, which took effect in 2007. The drive to reduce emissions of chemical compounds such as sulfur dioxides and nitrogen oxides, and to reduce the emissions of small particles — soot — led engine manufacturers to add a number of systems to the exhaust flow. There are catalysts, which chemically break down pollutants, and ceramic filters, which remove soot from the exhaust flow. These systems are detrimentally affected by oil blending.
As the industry began to grapple with how to meet the federal government’s 2007 pollution regulations, says Boehman, an industry group found that if sulfur was removed or eliminated from fuel, a large array of new pollution-control technologies could be brought to bear on the problem. That is the reason for today’s use of ultra-low-sulfur fuel. Introduce motor oil into the fuel mix, and everything changes.
Effects begin upstream of the cylinders, in the fuel system. Modern diesel engines are designed to work most efficiently with a fuel of a certain density and viscosity, says Boehman. This is also true of fuel injectors, which must create a very fine spray so fuel droplets burn rapidly and completely. Some of the chemical byproducts in used motor oil are more likely to form deposits in the injectors. A fuel-oil mix is also likely to change the spray pattern of the injector nozzle. This will create large droplets of unburned fuel that go into the exhaust system and increase soot and other emissions.
More than oil
Lubricating oil is not just oil. About 80 to 90 percent of it is oil. The remainder consists of additives to reduce friction, resist chemical breakdown of the oil by oxidation, keep dirt and metal particles suspended in the oil, and help the oil spread over the surfaces it is lubricating. In these additives are inorganic chemicals (meaning they don’t contain carbon), which will not burn to release energy in the cylinder but go right through in the exhaust as ash.
Low-ash oils were another innovation from the industry to allow filters to trap fine particles in diesel exhaust, Wenzler says. Every 300,000 miles, a technician must remove and clean the particulate filter on the Cummins ISX15 engine. Blend motor oil into diesel fuel, and you increase the ash content of the exhaust, and this will plug the filter prematurely. That means a truck will need more frequent maintenance, or the engine efficiency will drop because the plugged filter will increase backpressure in the exhaust manifold.
Burned lubricating oil, both the inorganic ash and the chemicals in it, also affect the catalysts intended to rid exhaust of contaminants through chemical reactions. It does this in two ways.
One is by creating fine particulates that mechanically interfere with the catalyst. Pollutants cannot reach the surface of the catalyst. “It would be like dust on your coffee table,” Wenzler says.
The other way is chemically. Compounds in the oil can poison the catalysts, Boehman says. They react with the catalyst and prevent it from reacting with pollutants.
Because the detrimental effects of oil blending are known, doing this on a modern vehicle could be considered as interfering with the emissions systems, which is illegal, McKenna says. But that’s a side point.
“As a technical guy, and being in the industry for 40 years, that would be the least of my problems. I would just worry about ruining the engine,” he says.
A diesel engine burning blended oil may not sustain critical damage right away, but it will, McKenna says. No one knows when such an engine will malfunction, so the question for operators comes down to this: Is it worth risking a $35,000 engine to save a few dollars on diesel fuel and waste oil disposal.
“What I do know is the risk-reward is not worth it,” McKenna says. “I thought that it was not a bright idea 25 years ago, and I think it’s even less bright today.”
Better engines ahead
Until now, you might say the effects of air pollution laws have not been very concrete. Benefits are seen only over a very large area in terms of reduced smog. The next step in air quality controls will have a direct and measurable effect on people using commercial diesel engines.
Beginning in 2014, and again in 2017, the U.S. Environmental Protection Agency will start enforcing rules about the amount of carbon dioxide released from on-highway engines, Wenzler says. Carbon dioxide is of course one of the primary greenhouse gases linked to climate change.
It is also one of the basic products of burning any fuel. The industry will tackle this challenge by pushing improvements in engine efficiency. Burn less fuel, and the amount of carbon dioxide falls proportionately.
One way this could be achieved is by reducing the amount of ancillary loads on engines, Wenzler says. For example, power the fuel pump, water pump, or radiator fan with electric motors, and the engine can devote more power to moving the truck. That translates into burning less fuel.
Cummins will meet the EPA’s 2014 standard one year early with its 2013 engines. That is the result of plain old hard work in improving engine efficiency, he says. Future changes will have the dual benefit of being better for the environment and reducing the amount of fuel engines need to burn and operators need to buy.
The experts also stress the sophistication of modern diesel truck engines. They are not the old diesels you remember from childhood, engines which chug along happily on whatever you choose to feed them. They are highly engineered and very complex. And they need particular care: specific oil change intervals, particulate filter service, certain types of fuel and lubricating oil, and so on.
“Customers really need to follow the maintenance practices as prescribed by the manufacture,” Wenzler says.