Rather like tuning a piano, reducing exhaust emissions from a heavy diesel engine combines an artistic talent for adjusting the machinery with complex engineering. In general, fleet operators are the beneficiaries of this artistry and have little to do except run and maintain new engines according to the manufacturer's recommendations.
Many fleets have taken advantage of new products from major oil companies that have allowed them to stretch oil change intervals significantly. These changes have the potential for huge maintenance savings. Using an extremely conservative example, a fleet with 100 tractors that run an average of 140,000 miles annually would need nine oil changes per year if oil were changed at a 15,000-mile interval. Stretching the oil change interval to 35,000 miles would result in only four oil changes per tractor per year.
Depending on fleet location and labor rates, parts and labor for an oil and filter change costs $100 to $150, with the higher cost being more nearly average. Eliminating five oil changes per year in a 100-tractor fleet could save the operator almost $75,000 per year plus increasing revenue per tractor by keeping equipment out of the shop and on the road.
New Emission Regulations
However, new exhaust emission regulations may make achieving these savings more difficult. For instance, changes in emission regulations in the mid-1990s increased fuel consumption slightly. Now, the new regulations for 2002 require engine makers to utilize new technology to reduce particulates and oxides of nitrogen in the exhaust stream. The two best technology candidates for reducing oxides of nitrogen are severely retarding the injection timing or combining cooled exhaust gas with intake air. However, retarded injection timing reduces fuel economy, so most manufacturers chose to use exhaust gas recirculation, which allows roughly 10% better fuel economy than retarding fuel injection.
These new emission controls replace up to 15% of intake air with cooled exhaust gas to absorb heat during combustion and reduce peak flame temperature during engine firing. The result is rejection of 25% to 35% more heat to the engine coolant and higher oil temperature. The higher coolant temperature results from using the engine cooling system to reduce exhaust gas temperature before it is returned to the intake manifold.
Depending on engine model, higher oil temperature can result in increased piston ring and bearing wear and sludge formation in engine oil. The new emission controls may also lead to increased soot and acidity in engine oil. Higher soot levels in engine oil are the result of lower combustion temperature.
Why EGR Works
Although exhaust gas contains unburned hydrocarbons, it acts like an inert material when used as part of the air charge in a diesel engine. While a precise explanation of why exhaust gas recirculation reduces NOx levels is not known, three possibilities seem most likely. The first suggests that inert gas in the air charge retards ignition with basically the same result as retarded injection timing. The second possibility suggests that adding exhaust gas increases the heat capacity of the non-reactive components — everything except the fuel and oxygen — of the cylinder charge, which lowers the peak combustion temperature. The third suggestion is that diluting the cylinder charge with exhaust gas reduces the flame temperature upon combustion. Essentially all these suggestions say that adding an extra component to the cylinder charge soaks up energy from combustion and reduces oxides of nitrogen formation.
Using recirculated exhaust gas to control NOx formation requires substantial modification to engine intake systems. In the first place, exhaust gas is hot — much hotter than intake air even after it has been compressed by a turbocharger powered by the engine exhaust stream. Both compressing the intake air and running it through a turbocharger heat it. Diesel engines use a heat exchanger between the turbocharger and the intake manifold to extract heat from the compressed air. With these new emission regulations, heat must be extracted from the recirculated exhaust gas as well.
The engine intercooler typically reduces compressed intake air temperature from about 400°F to about 110°F. This can be done by using a heat exchanger bathed by engine coolant, but is more commonly done with an air-to-air heat exchanger mounted in combination with the cooling system radiator.
Higher Coolant Temperature
Cooling the exhaust stream presents a much bigger problem, because it leaves the combustion chamber at 1200°F or higher. For best performance, the exhaust gas needs to be cooled to about 250°F. This is done using engine coolant in a stainless steel heat exchanger. Recirculated exhaust gas temperature must be regulated within fairly tight tolerances, because getting it too cool would allow acid in the gas to condense, causing corrosion of the aluminum intake manifold. An engine ingesting the result of such corrosion would suffer from abrasive wear on the piston rings.
Adding the burden of exhaust gas heat to the engine cooling system will raise total coolant temperature, because most tractor chassis cannot accommodate substantially larger radiators. Hotter coolant will raise engine oil temperature, because the coolant also is used in the engine oil cooler.
Higher turbocharger boost pressure is required also, because exhaust gas replaces some of the oxygen that would make up the cylinder charge if only air were injected. A larger total mass of air must be forced into the engine along with the exhaust gas if it is to burn the same quantity of fuel and retain the same power as an engine without exhaust gas recirculation.
Some manufacturers will use variable geometry turbochargers to reach the higher boost pressures. Mack, for instance, has tested an engine with two turbochargers lined up in series to increase boost pressure. Of the major engine manufacturers, Cummins, Detroit Diesel, and Mack have announced the use of exhaust gas recirculation to control emissions. Caterpillar will use proprietary technology known as ACERT that uses advanced injection controls.
Higher Acid Levels
All these emission control techniques have a direct effect on engine oil and oil change intervals. For instance, modern low-sulfur diesel fuel produces so little sulfurous and sulfuric acid that they present little problem in terms of oil life. However, adding exhaust gas containing vapors of these acids to the cylinder charge increases the total acid in the engine. Motor oil must be formulated with detergents to neutralize these acids, which are particularly problematic at low engine speeds such as at startup and at idle. Exhaust gas recirculation also increases the level of nitric acid in engine cylinders. This acid also gets picked up by the engine oil. In addition, lower flame temperature increases the amount of soot in motor oil.
Controlling soot is critical, because oils that do not can allow valve bridge and overhead system wear. Poorly controlled soot also can result in sludge formation and filter plugging. In addition, engines can suffer bearing failure and other reduced component life along with loss of fuel economy. One of the major problems associated with extended oil drain intervals is the buildup of soot in the oil.
To meet these challenges, the American Petroleum Institute has issued API CI-4, a new oil specification to ensure that engine oil can perform in engines with exhaust gas recirculation. This is the fourth new oil standard set by the Institute in the past 10 years. The name of the standard is “C” for commercial followed by an alphabetical designator for the category. In this scheme, the previous oil standard was CH-4. The numeral indicates that the oil is intended for four-cycle engines.
Rapidly Evolving Standards
Oil standards have evolved rapidly in recent years after remaining stable for a long time. The standard CE-4 was set in 1984, and CG-4 hit the books in 1994. Prior to CE-4, the standards remained the same for 27 years, going back to CD-4 in 1955. Before a new standard is announced, new oils are known as PC- for proposed category. Today's CI-4 was PC-9 in its early days. A new category, PC-10, is already under development for engines that will follow emission standards proposed for 2007.
Major refiners have oils ready for CI-4. ChevronTexaco, ExxonMobil, and Shell all say that they have oil ready to meet the new standard. These oils also are compatible with current and previous engines.
For instance, ExxonMobil says its Mobil Delvac 1300 Super 15W-40 oil uses patented Trimer technology to provide high oil performance in all heavy-duty applications. The oil is said to disperse soot efficiently to protect against deposits with resulting wear. It also effectively neutralizes acid throughout the drain interval, the company says. According to ExxonMobil publications, the oil should provide drain interval extension capability, fewer filter changes, and extended engine overhaul intervals.
ChevronTexaco says that its Chevron Delo 400 Multigrade has met the requirements for CI-4 without reformulation since 1998. The company leaves the question of extended drain intervals to engine manufacturers, saying that extensions are possible and should be a cooperative effort among ChevronTexaco, fleet personnel, and the engine manufacturer. Existing maintenance practices may need to be evaluated and adjusted.
Some changes are anticipated in the results of oil analysis used to determine oil change intervals. Analysis may show changed values in the reports of TBN, TAN, soot, and calcium content, ChevronTexaco says. In addition, oil viscosity may increase in response to an oil's attempt to control higher soot loads, heat, and oxidation. The current formulation of Delo 400 has been commercially available for three years and has proved to perform well at extended drain intervals, the company says.
In general, fleets should expect slightly lower fuel economy from new engines with exhaust recirculation and caution from engine manufacturers about extended drain intervals. In fact, the move toward extended intervals will probably progress in much the same way as the last cycle of drain interval extensions. Engine manufacturers will work with individual fleets to audit mileage and vehicle applications in recommending drain intervals. In all likelihood, drain intervals for the new oils will increase in small increments as experience with engines and oils develops.
