Truck Checks: Choosing the right motor oil
Choosing the right motor oil
December 7, 2007 By Don Henry
Which motor oil do you think is most effective for fire department pumping apparatus use? The traditional motor oil grade for diesel engine use is 15w-40. This oil has served the fire service very well. There are now, however, newer, more expensive synthetic motor oils available. But are they worth the added expense and will they protect as well in hot weather? The new 0W-40 motor oils for heavy duty (HD) diesel engines do, in fact, protect fire department engines better in hot weather and high temperatures than does the industry standard, 15W-40. To understand this higher performance level it is necessary to examine the development of motor oils for HD diesel engines.
For many years the oil of choice for HD diesel engines was a straight weight motor oil. That choice was usually 40-weight oil. This oil was very thick in cold weather but when brought up to operating temperature in the engine, it had the correct viscosity to protect the engine parts. It became possible during the early ’70s to formulate at reasonable cost a multi-grade oil (15W-40) that would last in a four-cycle diesel engine (at the end of this text is a discussion of two-cycle diesel oils). This oil would flow in cold weather like 15-weight oil and would also thicken in high temperatures to act as 40-weight oil. How was this possible? It was achieved with the use of polymers.
The best way to describe polymers is to think of them as looking like very small octopuses. In cold temperatures these polymers shrink (think of the arms of the octopus closing up to the body and thus reducing the overall size of the octopus.) The oil containing these polymers is able to flow very easily into the engine oil pump in cold weather and then on to the engine bearings. At high temperatures these polymers stretch out and are able to form longer chains of polymers. As these chains form, the oil viscosity increases and the oil flows as if it were now thicker. Motor oils are composed of approximately 80 per cent base-stock and approximately 20 per cent additives. The polymers are found in these additives. But even with the best base-stock crude oil and an excellent additive package it is only possible to obtain about a 20- to 25-point spread, i.e. a 10w-30, 0w-20, 15w-40 in a multi-grade oil. To be able to get a wider spread it would be necessary to completely break down the crude oil at the molecular level and reassemble it to perform as needed. That process creates an oil that is called a synthetic. This has now been achieved, but the process of making these synthetic oils is expensive. It does, however, create a very pure oil with uniform properties. Now with the correct additive package it is possible to create synthetic multi-grade oil with tremendously low temperature performance. Such formulations as 0w-30, 0W-40 and 5W-50 are now available. It has even been possible to produce these oils without using 100 per cent synthetic base-stock; these oils are called semi-synthetic or part synthetic.
But can oil that is made to pump in an engine at -40 ºC also protect an engine at very high temperatures? The short answer is “yes” and here is why: all motor oils and, in fact, all hydraulic oils have a viscosity measurement. The measure of oil’s ability to flow is called “kinematic viscosity” and is expressed in centistokes (cSt), measured at both 40 ºC or 100 ºC. It is derived from the measurement of the time it takes for a fixed volume of oil to flow through a standard capillary tube. This measurement is done in a laboratory but this information can be found for any reputable oil in the oil company’s product information handbook (available for your asking from the oil company or with some companies on their website).The numbers used in the following example come from the Imperial Esso handbook. The numbers from other companies will be much the same.
Example: Let’s start with 15W-40 XD-3 Extra diesel motor oil; this oil is rated by the American Petroleum Institute as a CH-4 oil for severe service applications with turbocharged engines. The Esso handbook says that this oil has a kinematic viscosity at 40 ºC of 118 cSt and at 100 ºC of 15 cSt. These numbers can be plotted on an oil viscosity logarithmic chart. (Refer to the back page of the handbook for the completed chart, reproduced on next page.) The viscosity numbers are on the vertical axis and the temperatures are on the horizontal axis. When you join the two kinematic viscosity numbers you will get a line that runs from the upper left hand corner of the chart to the lower right hand corner. Thus, for any given temperature it is possible to determine the kinematic viscosity, hence, how well this oil will flow.
Now plot the lines for the same oil but this time use the 0w-40-grade oil. At 40 ºC use 80 cSt and at 100 ºC use 15.6 cSt. You will notice that the gradient of this line is less than the gradient for the 15W-40 oil, and the two lines intersect just above the 15cSt mark. The ultimate multi-grade oil would be one that displayed a horizontal line on this chart with no change in viscosity with temperature but that has not been invented yet. What becomes apparent is that the 0w-40 oil is less viscous (flows easier) at low temperatures and is more viscous (provides better protection) at high temperatures than the 15W-40-weight oil. This is contrary to what a lot of people might think when they see the low number in the 0W-40 weight oil; they usually assume that it is an oil for extreme cold temperature use only.
From the chart you will notice that the difference between the viscosities of the two oils at 150 ºC is greater than one cSt point, which can make a significant difference on the wear of engine components in an older vehicle. What makes 0W-40 oil thicker than the 15W-40 oil at high temperatures? It is the polymers chaining together that make the oil thicker. Thus it is the same technology that makes the oil flow so well at cold temperatures that ensures it protects the engine parts at high temperatures.
From the previous information you can conclude three points: first, 0w-40 is an excellent cold weather oil for extreme cold weather start-up such as volunteer fire fighters would encounter when attempting to start their personal vehicles when responding to a fire call from their home. Second, that the 0w-40 protects and flows better to critical engine parts during a start-up in the fire hall. This is important because fire departments don’t have the luxury of long engine warm-up time before responding. Finally third, that 0w-40 oil is a better high temperature motor oil and will not thin out at the abnormally high engine oil temperatures encountered by fire apparatus during stationary pumping.
Two-cycle diesel engine oils
To understand why multi-grade oils have not be recommended for two-cycle diesel engines, it is necessary to understand the operation of the two-cycle diesel and the differences between it and the four-cycle diesel. The cycles of the four-cycle engines are intake, compression, power and exhaust. It is during the intake cycle that there is very little, if any, pressure on the piston rings. As the piston moves down on this intake cycle, the engine piston rings can wipe oil from the cylinder walls and recharge their supply of lubricating oil. The piston of a two-cycle engine never does have an intake cycle; it only has a power cycle and a compression cycle. Its top piston rings are always under load and, therefore, have extreme pressure between the ring, piston and cylinder walls that shear the oil polymer. The camshaft of a four-cycle engine rotates at only one-half of the engine crankshaft speed while the camshaft of a two-cycle engine turns the same speed as the crankshaft. This faster rotation of the camshaft gears and camshaft followers break down conventional polymers. This breakdown is called shear. When the polymers are sheared, they no longer can expand or retract to changes in the oil temperature.
Remember, 15w-40 oil is really a base stock of 15-weight oil that uses polymers to make it perform like 40-weight oil when at high temperature. If the polymers have been sheared (destroyed) because of these higher temperatures and shearing contact between the camshaft and its followers, the oil will revert back to its base stock of 15-weight.
This 15-weight oil would be far too thin to protect engine components and increased engine wear would result. This is the reason only straight weight motor oils have been recommended, i.e. 40-weight or 50-weight. The manufacturer of a two-cycle diesel, therefore, has not recommended multi-grade oils. Use of multi-grade oil in a two-cycle engine has caused increased piston ring wear, increased camshaft lobe and follower wear. While this engine is no longer available on new fire apparatus, your fleet will most likely contain both two-cycle engines and four-cycle engines. There have been some recent developments with polymer design and some oil manufacturers are now producing motor oils for these mixed engine fleets. Be careful and obtain in writing from the oil manufacturer a guarantee of performance.
Don Henry teaches in the Automotive Service Technician and Heavy Equipment Technician programs at Lakeland College in Vermilion, Alta., where he has been a faculty member for 19 years. Henry also works closely with fire etc. in a partnership between the two institutions, and has co-developed and delivers Canada's only post-secondary level fire apparatus maintenance program.
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