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Diesel Engine Oils: Today and Tomorrow
Diesel fuel is very dense – that’s one of the reasons diesel engines give better fuel economy (and power) per unit volume than almost any other hydrocarbon fuel (nitromethane is the winner). However, it is much more difficult to completely combust a dense fuel charge in an engine in a limited period of time.
By John Martin
Why are diesel engine oils different than any other engine oils? The Lubrizol Corporation, the world’s largest oil chemistry supplier, just created an excellent website (www.hddeo.com
) that explains diesel engine oil composition very well.
Diesel fuel combustion (or incomplete combustion) is accompanied by the production of soot (unburned carbon). Prior to exhaust emissions legislation this soot was allowed to pass into the atmosphere through the exhaust system. With the advent of emissions legislation, engine OEMs were required to find methods to either, 1) reduce the amount of soot formed or 2) contain as much soot as possible in the engine’s crankcase to be removed later with each oil change.
Soot reduction was accomplished by improving injector spray patterns (finer), combustion chamber design (reentrant bowls), diesel fuel refining (minimizes unburnable heavy materials), and retarding fuel injection timing. Retarded fuel injection timing serves to burn soot by keeping the exhaust manifolds hot, thereby allowing more time to complete the combustion process as the unburned carbon passes through the heated exhaust system.
Engine OEMs and oil additive manufacturers then developed diesel engine oils that could suspend a greater amount of soot (as much as 10% volume) in the engine oil rather than depositing it on engine components where it could cause premature engine failures. This was primarily accomplished by adding more detergents to the oil to better neutralize acidic byproducts of combustion and more dispersants to suspend soot particles in the oil. Current diesel engine oils contain about twice as much detergent and dispersant as gasoline engine oils.
From the advent of American Petroleum Institute (API) CD oils in the ’60s to API CI-4 Plus oils in the early 2000s, detergent and dispersant levels have increased significantly to improve diesel oil performance and lengthen oil change intervals.
New environmental laws set in 2007 have forced emissions levels so low that catalytic converters and particulate traps are necessary to pass exhaust emissions tests. Engine builders, additive manufacturers and oil marketers got together to develop API CJ-4 oils for these engines. CJ-4 oils have reduced ZDDP levels to minimize phosphorus “poisoning” of catalytic converters and particulate traps. Exhaust sulfate emissions (remember, sulfates plus water equal sulfuric acid) were also reduced by placing a limit on the detergent content of the oils.
These new API CJ-4 plus oils offer improvements in all areas of diesel engine oil performance except one. If soot loading isn’t excessive, diesel engine oil change intervals are effectively determined by the detergent content of the oil. Detergents neutralize acidic byproducts of combustion, and when the detergent is depleted, acids build up in the engine oil and increase engine wear rates (In modern diesels corrosive wear is more prevalent than abrasive wear). Fleet operators learned long ago to utilize used oil analysis to determine when the oil’s detergency is depleted. At that point the engine oil should be changed.
Don’t misunderstand me, API CJ-4 oils offer outstanding performance. They just can’t be run as long in your diesel engine as most API CI-4 Plus oils. Remember diesels love detergents.
Let me take a little side trip here and explain something about diesel engines and soot particles. Modern diesel engine oils utilize detergents and dispersants to disperse soot particles evenly throughout the engine. These detergents and dispersants also minimize the size of soot particles to less than 1 micron in diameter. Why is this important? The larger the soot particles are, the more destructive they can be. Large soot particles can act like valve grinding compound and significantly increase wear throughout the engine. Never, and I mean never, run a non-detergent oil in your diesel engine!
So, what do you use if you have a high performance diesel engine? Do you need better oils? The short answer is probably not, but let me explain why I say this.
In a gasoline engine, most extreme pressure (EP) loading is in the valve train area. Likewise diesel engines have high EP loads in the fuel injector area, but most diesel engines utilize roller cam followers to mechanically reduce the effects of high EP loads on injector valve train durability. Only those older engine designs utilizing an old-style distributor fuel pump have high EP loads. Modern distributor-type fuel pumps have had mechanical improvements made to enhance fuel pump durability.
Where will diesel engine oils be headed in the future? Our good old legislators (is it merely a coincidence that “legislator” and “liar” both start with the letter “l”?) recently required both diesel and gasoline engines to achieve significantly better fuel economy in the next few years. As a result, diesel engine manufacturers recently requested a new API diesel engine oil performance category to help them meet their 2017 model year fuel economy requirements.
The use of friction modifiers in diesel engine oils could provide some benefit (most current diesel engine oils don’t use friction modifiers), but the consensus is that thinner diesel oils will be the most commonly utilized solution. Remember, thinner oils require less power to pump. The Engine Manufacturer’s Association (EMA) has requested that the new diesel engine oil category provide a scuffing wear test and improved high temperature shear tests to protect vital engine components from increased wear caused by the use of excessively low viscosity oils. The EMA has also asked for improved compatibility with biodiesel fuel blends. The EMA is requesting prototype oils for testing prior to January 2016 to aid them with their 2017 model year engine development efforts.
Thinner oils make great sense for fuel economy (and horsepower) improvements. Switching from SAE 15W-40 grade to SAE 5W-30 grade oils can improve fuel economy as much as 3%. However, be certain your engine can maintain at least 20 psi oil pressure at hot idle with thinner oils to prevent excessive component wear. And be very careful where you measure the oil pressure on your engine. Take the oil pressure measurement at the point farthest away from the oil pump to be safe. Let me explain a little something about oil pressure.
Oil pressure is primarily determined by the sum of the internal oil leaks in your engine. Your oil pump puts out a given amount of pressure and flow (volume), and then each component (particularly bottom-end bearings) allows a little bit of flow and pressure loss as the oil is pumped through the part. Rod side clearance (or cheek clearance) and piston undercrown oilers (P tubes) can bleed off significant oil pressure.
Most engines have oil pressure gauges in locations convenient to the instrument designer engineers. I once lost the bottom end in a big-block Chevy because I was experimenting with very thin oils, and I hadn’t thought about where I was measuring oil pressure (at the oil filter boss). When I disassembled the bottom end, the main rod bearings at the rear of the crank looked fine. But as I inspected the front of the crank, the bearings looked progressively worse. Guess which bearing failed? The #1 rod bearing. I learned my lesson and relocated the oil pressure sensor to the front of the engine. Be careful when experimenting with less viscous engine oils and/or low volume oil pumps.
Also monitor oil pressure carefully when utilizing low viscosity engine oils and biodiesel-containing fuels. Biodiesel fuels are usually not as volatile as diesel fuel, so biodiesel doesn’t evaporate out of the crankcase as rapidly as diesel fuel. Many engines utilizing higher concentrations of biodiesel have higher than typical levels of fuel dilution in the oil. Since biodiesel is also thinner (less viscous) than diesel oil, excessive concentrations in the oil can also lead to excessive component wear if the oil is very thin.
When you stop and think about it, diesel oils are some amazing products with the highest additive content of all engine oils. In fact, I use diesel engine oils in my motorcycles and stock passenger cars. I only use hot rod or racing oils in my modified engines. If you want to learn more about Diesel Engine Oils, don’t forget the Lubrizol Corporation’s excellent website (www.hddeo.com
) for useful information on current diesel engine oils and future trends.