Oil and the Four-Stroke
LUBRICATION IS NOT a simple task. Nor is lubrication an oil’s only purpose in a four-stroke engine. These statements may or may not be surprising. One thing is sure: oil companies have not made it easy or clear for the consumer to understand the uses, applications and virtues of their product.
Following are some questions and answers which will give a basic understanding of oil as it relates to the four-stroke engine, explaining how it is graded, how it should be used, and at what point oil fails as a lubricant.
Some Questions And Answers
HOW IS OIL CLASSIFIED?
Oils are classified by the American Petroleum Institute Service Classification System. Look at an oil can sometime. If it is from a reputable oil company, it should carry a phrase similar to: “For Service ML, MM, MS, or DG.” These letters stand
for light service, moderate service, severe service and light diesel service respectively. Oils with markings DM and DS are for use in diesel engines.
In addition, oils are divided according to a viscosity number.
WHAT IS VISCOSITY?
Viscosity is defined as the property of a fluid that resists the force tending to cause the fluid to flow.
In other words grease, a thick substance which does not readily flow, is highly viscous. Conversely, SAE
5W oil has low viscosity.
The letter W following an SAH number means that these oils are suitable for use in temperatures freezing or below. Higher numbers without the W, such as SAE 30, denote thicker oils designed to withstand higher engine temperatures.
HOW IS VISCOSITY DETERMINED?
One of the most common methods of testing viscosity is the Saybolt Universal Seconds method. Oils suitable for use in winter (followed by the letter W) are tested at O degrees Fahrenheit. A known quantity of oil is allowed to pass through a hole with a known diameter. The longer it takes the oil to pass through the hole, the higher the viscosity.
Thicker oils are tested in the same manner at 210 degrees Fahrenheit. But because oil behaves differently at different temperatures, an oil Viscosity Index was developed which gives a clearer indication of an oil's overall characteristics. As one might expect, an oil with a high viscosity index shows relatively little change over a wide range of temperatures.
Multi-grade oils also have a high viscosity index classification because they are light enough for easy starting and good low-temperature lubrication, but also perform satisfactorily at higher temperatures. Because these oils meet the requirements of two or more singlegrade oils, they are labeled accordingly (for example, SAE 10W-30).
HOW IS MULTI-GRADE OIL MADE?
Modern multi-grade oils generally use SAE 10W oil as a base, with “shear-stable polymers” added in the “correct” proportion.
These polymers resist heat-induced molecular breakdown and produce a considerable increase in the oil’s viscosity index.
And if you want proof, the Ford which wa's driven to second place in this year’s Indianapolis 500 used a 20W-50 oil-and only one quart was added during the entire race!
SO WHY DO THEY STILL MAKE SINGLE-GRADE OILS?
Even though more manufacturers are recommending the use of multi-grade oils in their products, some (notably BMW and Porsche) still maintain that a single-grade is better for summer use. Both of these companies, however, recommend multi-grade oils for use in winter.
WHAT DOES OIL ACTUALLY DO?
Oil does much more than just lubricate. It protects against rust
and corrosion, cleans, seals, and cools the engine.
In order to lubricate properly, oil must establish and maintain a complete, unbroken film between the moving parts in the engine. This basic condition is called “full-film lubrication” and relies on a relatively thick film of oil to separate moving parts completely. Crankshaft bearings, connecting rods and the camshaft usually operate under such conditions.
“Boundary lubrication” occurs when the load is only partially carried by the oil film and there is intermittent metal-to-metal contact between moving parts. 'Phis is common during starting, stopping, and during the breaking-in period when rough surfaces exist within the engine.
The top piston ring also gets boundary lubrication because its oil supply is more limited and the temperatures are higher.
At points where friction between two surfaces can generate enough heat ‘ to cause metal to weld itself together, an “extreme pressure” condition exists. In modern four-cycle engines, the valve system-cams, lifters, push rods, valve stem tips and rocker arms -all operate under this condition.
Some causes of extreme pressure are lack of lubricant, inadequate clearance between moving parts, extreme heat, highly loaded parts, or the use of the wrong type of oil.
CORROSION AND RUST
The prevention of corrosion and rust is another important function of oil. Actually, the acids formed by the products of combustion are responsible for most of this form of wear. For example, for every 10 gal. of gasoline burned, the following items are found in addition to the exhaust gases:
Nine-tenths to 1.2 gal. of water.
Between 0.2 gal. and 1 gal. of unburned gasoline.
Up to 0.2 lb. of soot.
Up to 0.1 lb. of resins and varnishes.
Up to 0.25 lb. of nitrogen and sulfer acids.
Up to 1 oz. of insoluble lead salts.
Up to 0.2 oz. of hydrochloric and hydrobromic acids.
From the standpoint of engine wear, it would be best for these contaminants to go out the exhaust pipe, but such is not the case. Many blow past the piston and settle in the oil, but modern additives have a tendency to neutralize harmful acids and detergents tend to slow down the formation of varnish and sludge in the engine.
Another function of detergents is
to hold these contaminants in suspension instead of allowing them to be deposited on the engine’s parts.
But even with an oil filter, these impurities can only be removed by an oil change..
COMBUSTION CHAMBER DEPOSITS
Reducing combustion chamber and piston top deposits is another important function of motor oil.
Because oil is needed to lubricate the pistons and cylinder walls, some gets into the combustion chamber where it is burned.
A good motor oil leaves only small deposits of carbon when burned, and detergent action helps keep the piston rings and ring grooves clean and free (piston rings have a tendency to stick in their grooves).
COOLING AND SEALING
Most people think that the air moving past the cylinder and cylinder head fins on a motorcycle does most of the cooling. But only about 60 percent of an engine’s cooling is by air flow. In fact, the crankshaft, main and connecting rod bearings, the camshaft and its bearings, timing gears or chains, pistons and, in some engines, many transmission and clutch components are cooled entirely by the oil flow.
And all of these parts have temperature limits which should not be exceeded. Even though some parts are able to withstand high temperatures, others, such as plain and connecting rod bearings, will fail rapidly if their temperature limits are exceeded.
In addition, oil seals the combustion gases against leakage past the piston rings. But it must not be viscous enough to cause too much drag between the rubbing surfaces.
HOW HOT DOES AN ENGINE GET?
Internal temperatures are high: combustion temperatures often reach more than 2000 degrees F. Exhaust valves attain temperatures of 1000 to 1200 degrees F. Pistons often reach 1000 degrees F, and this heat is conducted down the connecting rod to the rod bearings.
Cooling in this area of the engine is extremely important since tin and lead are commonly used materials in bearings. These metals become very soft at temperatures around 350 degrees F and melt (in their pure forms) at 450 and 620 degrees F respectively.
After warm-up, oil reaches a theoretically ideal temperature range of 175 to 210 degrees F, and is supplied to the bearings at this temperature. As the oil cools the engine (by removing heat from the
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rubbing surfaces) it becomes hotter.
For this cooling process to work satisfactorily, large volumes of oil must be constantly circulated through the engine. If this process is interrupted or inadequate, “burned out” bearings and seizure are sure to occur. And while only a small quantity of oil is needed for lubrication, proper cooling requires a larger amount.
WHAT'S TOO HOT?
When the oil’s temperature becomes too high (approximately 250 degrees F for 30W and 300 degrees F for 50W), lower viscosity may cause the oil film to break down. Thermal damage to the bearings and/or “high temperature sludge,” caused by oxidation of the oil, may result.
CAN AN ENGINE RUN TOO COLD?
Of course, too low a temperature is just as dangerous, because unburned fuel will contaminate the oil. And this causes a build-up of sludge in the
crankcase. Fortunately, this occurs only during short-trip, start-andstop riding in cold weather.
But, the viscosity of a lubricating oil increases drastically as the temperature is lowered, and this increases the oil drag between moving parts.
What is often overlooked is that the viscosity scale is logarithmic.
For example, when a typical SAE 30 oil is reduced in temperature from 210 to 175 degrees F, the viscosity doubles from 10 to 20 centistokes (a stoke is a kinematic measure of viscosity).
An additional 35-degree temperature drop (from 175 to 140 degrees F) again doubles the viscosity, and so on.
Multi-grade oils also behave in this manner, but to a lesser degree.
Fortunately, most air-cooled engines warm up readily, and too low an oil temperature is not a problem.
GAUGES TELL THE STORY
As an experiment, we chose a Honda CB750 and outfitted it with Stewart Warner direct-reading oil temperature and pressure gauges.
The thermocouple for the oil temperature gauge was installed near the bottom of the oil tank, making an accurate temperature reading on the oil returning to the engine possible.
The oil pressure gauge was installed in a dual mount next to the oil temperature gauge on the handlebars.
The pressure-sending end was installed in the oil passage behind the cylinders on the right side of the machine (where the factory oil pressure gauge found in most Honda service shops is installed).
According to Honda, “an extremely high oil temperature” is 244 degrees F (118 degrees C). But riding the Honda Four around town rarely produced an oil temperature of 1 90 degrees F.
Out on the highway, with the ambient air temperature approximately 80 degrees, the oil temperature stabilized at 210 degrees F and cooled down to 180/190 degrees F when riding in downtown traffic.
The highest temperature recorded was on the San Diego Freeway near Hollywood at approximately 70 mph. Ambient air temperature was nearly 100 degrees and the oil temperature reached 230 degrees, still below Honda’s recommended maximum.
It should be noted that this test was conducted under normal driving conditions on a stock machine. If an engine is modified to achieve a higher power output, additional cooling may be required. And in AMA class C racing, oil coolers have been cropping up on some machines. [Ö]