Slippin' and Slidin'
Most often, the difference between failure and success is how close one comes to doing something absolutely right. There's a fine line dividing almost right and 100% wrong, and crossing that line requires little more than an understanding of some basic principles. Many come so close to success they say they can taste it, but those who know success are the ones who have gained the understanding and are applying it in their everyday lives.
One of the few disadvantages is that the two-cycle engine it does not yield useful power until relatively high rpm. Another is the heat buildup caused by twice the number of power strokes, compared to it's for stroke big brother, things are complicated by the heat retention due to the engine design's lack of a full exhaust and intake stroke. Piston cooling takes time. In a four-cycle engine, the piston cools during approximately 1-3/4 crankshaft revolutions for each power stroke. The cooling time in the two-cycle is only 3/4 of a revolution per power stroke. This equates to the two-cycle having roughly 42% of the cooling capacity (time) of a four-stroke.
On the positive side, horsepower is equal to engine speed multiplied by the torque. In the two-cycle, peak torque can be moved to any rpm range with intake and exhaust tuning. Therefore, horsepower is directly proportional to rpm, and more rpm = more horsepower.
High rpm creates severe stresses on mechanical engine parts and high stresses generate even more heat. Heat fries oil. Hence the need for an understanding of potential problems resulting from the use of lubricants in the two-stroke fuel.
WHAT'S BEEN GOING ON?
In the early '60s, when two-stroke engines began their quest for recognition in the performance world, piston temperatures began to rise. Lubricating properties of oil up until this time played a rather insignificant role. A relatively large quantity of just about any oily substance was mixed with the gasoline and performed what was thought to be an adequate job.
In the '70's, planet earth's air quality began to experience some serious scrutiny. Al machines found to be clouding our oxygen supply were deemed to need improvement by one or more "Clean Air Acts". The oil in a two-cycle moves through the engine and is emitted as blue, gray, or white smoke as part of the exhaust. All the "Acts" cited smoke as BAD! If the two-stroke was to survive at all, smoke had to be drastically reduced. The oil we put in the fuel causes the smoke, so the first logical step was to reduce the quantity of oil.
Both engine and oil manufacturers made strides toward improvement. Redesigned engines requiring less lubrication and high-tech lubricants affording superior protection at much lower concentrations began a journey toward, hopefully, smokeless exhaust emissions
As a beneficial sidelight, these improvements increased power production in our favorite little friend. A typical 1950's 125cc engine operated at compression ratios around 9:1 and produced only 12.0 HP (10cc/Hp). With the dawning of the high-tech, clean air age, cc/horsepower were cut by a third. More horsepower created more heat. Today, a 125cc engine with compression ratios up to 16:1 and producing 40 horsepower is not untypical.
High horsepower is a direct result of increased rpm. Careful engine design and ever improving lubricants to handle the high rpm stresses helped in this development. Racing machines operate at the upper limits of torque and rpm, demanding the best lubrication possible. Any departure from high quality lubricants leads to serious power loss or complete dysfunction. There is absolutely no space left for mistakes. I'm amazed at the number of people who will not hesitate to spend 2 to 4 grand on engine modifications and then settle for the cheapest lubricant money can buy. We need detailed information regarding the result of using lubricants having various lubricating and cleaning properties, their limits and intended applications
I HOW IT WORKS
No one would use transmission oil in a four-stroke crankcase or a heavy differential lubricant in an automatic transmission, would they? I'd like to think not. When selecting a two-stroke lubricant, one soon realizes that there is very little distinction between the numerous brands' advertising and label claims.
How do you as a tuner, engine builder, or owner protect your or your customers' investment? The only rational thing to do is to gather information, then make a decision. Find out from people who have good records at race tracks, contact oil manufacturers, and demand information in the form of laboratory and field test results. If you are a dealer, be certain your lube supplier understands your customers' needs and is willing to back you up. Whenever possible, test the stuff on your personal machine, or if finances permit, build your own test stand. If you're made of money, get your product analyzed by a laboratory expert in performing lubricity, ring sticking, and deposit testing.
I can't emphasize enough that it is important for you to gain a personal understanding. Call your engine and oil OEMs. Ask to speak to sales people, test engineers, even chemical formulators. These people are wrapped up all day in million-dollar sales, full-time testing, and little tiny molecular chains. Almost every one I've ever gotten in touch with was overjoyed to find out that there was a real person out there who actually cared about what makes the whole thing work. If you want to have your product tested professionally, ask these people to steer you toward the laboratories they use. You'll gather indispensable information without which an intelligent understanding cannot be achieved.
Success or failure of lubricants occurs at the top of a piston where all the heat and extreme pressures reside. Examination of used or tested parts shows piston ring sealing effectiveness. Without going into detail, fuel bums above the piston, producing extreme heat. This heat release causes tremendous gas expansion, which elevates the pressure in the confined space above the piston. The increased pressure pushes the piston down (at the correct degree of rotation) and makes the crankshaft turn. The piston ring must keep the pressure above the piston until the top of the piston passes the top of the exhaust port. If the ring doesn't seal effectively, the high-pressure, high-temperature gasses leak past the ring and cause heating of the ring, lands, and skirt (far beyond the piston's design limitations). Ring leakage occurs from one or more of three causes:
A. Improper physical conformity (bent or twisted ring, cylinder out of round, etc.)
B. Scuffing.
C. Deposit-induced ring sticking. For the sake of this exercise, we'll assume that the engine was set up with optimum conformity and, 'A" can be ruled out.
Some forms of scuffing, and all deposit-induced ring sticking can be attributed to lubricant failures. I say "some" forms of scuffing, meaning those not caused by the "hot dogger" that starts up an ice cold engine and immediately takes it to full power.
(Above) These pistons came from the same engine, using different compression, ignition, fuel, and lubrication setups (all show about 20 hours of running time). From left to right, they show: A clean top and clean rings mean no blow-by on the piston's sides, the highest degree of efficiency. Next, hot and cold spots all over the piston show that the combustion pressure was off, giving an engine that will last but will have less than optimal efficiency. Third, the oil deposits on the top and sides show that there is a lubrication problem that will cause problems if the engine has to endure heavy loading for prolonged periods. Finally, the worst case is a heated, melted, and destroyed piston that has more or less self-destructed as a result of improper tuning.
(Above) Big problems begin with little spots like this one. The black deposit under the second ring signals that either the rings, the piston, or the dome needs some attention. Also, there may be too much heat in the combustion chamber due to over-compression or improper squish velocities. Then again, this could show improper lubrication or too-low-grade fuel. In any event, this is a symptom of a problem waiting to happen. Only frequent engine inspection will reveal problems this early. (Above center) This four-corner seizure is due to improper engine cooling or lubrication. The left side shows blow-by and carbon deposits about an inch below the second ring. (Above right) A seizure like this shows not only problems in cooling and lubrication, but an engine builder who is less than adequate. Damage is evident all the way around the piston, and if you ever see damage on the intake side of the piston, you know it's a result of something you or your mechanic has done - this just doesn't happen by itself.
Lubricant related scuffing due to inadequate lubricity usually occurs as soon as certain conditions are met. It can happen in a brand new machine, or one which has many hours on it but suddenly is subject to a fuel with inadequate lubricant. It is evidenced by a general lack of carbon and varnish deposits in and around the ring groove with major damage occurring on the thrust and anti-thrust sides of the piston. The thrust side is usually on the intake side of the cylinder. (Envision the angle of your connecting rod during the power stroke and if it were an arrow with the point at the top, it would be pointing toward the thrust side.) This type of failure can be alleviated by increasing the amount of oil in the fuel or by using a more slippery oil.
Deposit induced lubricant failure usually takes some time. It is usually indicated when varnish and carbon deposits in the grooves and on the lands and skirt are of such magnitude that they decrease ring side clearance and cause the ring to stick. I've seen deposits which seem to have been confined to the ring groove and others which seem to have originated above and below the ring on the lands and skirt, propagating into the groove and eventually immobilizing the ring. In either case, the ring, because of normal piston rocking, becomes firmly stuck deep in the groove and can no longer contact the cylinder wall. Immediately, compression is lost, combustion pressure blows by the side of the piston instead of pushing down on it, and the net result initially is a massive loss of power. Engines which continue to operate under these conditions usually experience total seizure. I'm not sure which came first, the chicken or the egg, nor am I sure of this but here's a couple of gut feelings I have:
A. After deposit ring sticking, as the hot combustion gasses blow by the ring, the piston skirt is heated higher than its designed tolerance. Because the confined aluminum skirt expands at a rate much greater than the water or air cooled cylinder, critical piston to cylinder clearance is reduced below minimum design limits. Any film of lubricant present between the piston and cylinder wall is forced out and the piston skirt makes physical contact with the cylinder. The piston skirt, being the softer of the two materials, sticks to the wall but the piston doesn't stop immediately. Some aluminum is ripped off the skirt and left on the cylinder. The next time or two the piston moves up and down, more aluminum is hogged out. As the compression ring crosses the accumulation of aluminum, wiping type damage occurring adjacent to the ring causes more ring sticking in an area not even related to the original deposits which induced this whole episode. More blow-by occurs, more skirt heating and eventually, SCREECH!
B. If the deposits are of the skirt and land variety (even if they haven't yet caused deposit ring sticking), the finite thickness of the deposit may decrease the piston-to-cylinder clearance beyond its limit, and a similar scuff and "scuff-induced ring sticking" may occur. I've seen this on several pistons where the skirt varnish or carbon appears burnished as though it were acting as a bearing area. Except in extreme cases of seizure, scuffing occurs in areas away from the deposits. Deposits form when oil gets cooked. Have you seen the TV commercial where two crankcase oils are superheated on a kitchen range? One leaves a brown while you can't see anything from the other. The marketing implication is that some oils bum cleaner while others leave residue. In an engine, the absolute temperature and the time the oil is exposed to that temperature are the real determinants of deposit formation. The color of residue left in a fry pan means little to nothing.
Excessively high combustion temperatures play a major role in the failure of lubricants, the concentration of oil in the fuel is another and, far from being least important, is the suitability of the additive or chemical package built into the oil that's purchased off the shelf. The additives are what make today's oils "high-tech". In an environment producing nearly enough heat and pressure to produce zircons, today's additive components must be capable of keeping cooked oil in a fluid state allowing it to be easily washed out by the next incoming charge of fuel and oil.
Additive manufacturers are usually separate from oil companies. Both are "big business". Oil companies get crude oil out of the ground, refine it into a seemingly endless variety of different products. One of those products is lubricating oil. Additive manufacturers put together packages to be "blended" with the oil company's lubricating oil and the result, after a hundred thousand dollars or so of testing, is an oil which is packaged and sold commercially for use in internal combustion engines. I often wonder if "Joe Blow's, Private Blend, Do All Ring-Ding Oil" went through all this. Buyer beware.
As engine designs change, as fuel quality changes, as oil is depleted from wells which used to be considered "pure" and as oils from various regions of the world enter the American market, the only sure thing is that yesterday's additive package probably won't work today. Products undergo reformulation and reblending as situations require. Testing is ongoing.
Laboratory testing procedures employ various methods to evaluate not only lubricating properties and their deposit prevention performance, but also their impact on the environment. Today's oils are formulated to meet the demands of both lubrication and environmentalists. This is a compromise, however, and in striving to keep the environmentalists from outlawing two-strokes, it is feasible that the lubrication side may be suffering.
One of the best natural components in refined oil before the additives are blended in, is "bright stock". Unfortunately, bright stock is one of the elements producing the highest exhaust smoke. Additive manufacturers are in a constant state of research and development trying to replace bright stock with "cleaner" synthetic components. Smokeless synthetics are coming. As oils become more highly synthesized, the price goes up. Builders and users both want longevity. Bright stock works, new synthetic technologies reduce smoke; either way, there's a price to be paid.
The components of any two-cycle oil is a delicate balance of the refined base oil, the solvents used for dilution (to make it easier to mix with gasoline), and the additives blended in.
One of the toughest tests was developed by Yamaha International Corporation, which was an enhancement of the then current BIA (Boating Industries of America) test. Heavy load runs employing 24:1, 50:1 and 100:1 fuel-oil ratios were tested. Product evaluation was weighted heavily on piston ring zone deposits and ring sticking. The test consisted of 10 one-hour cycles which started with a five-minute idle warm-up followed by a 55-minute load run using a centrifugal water pump for load. At 7000 rpm and a fuel-oil ratio of 24:1, the test was so severe that mechanical component failures rendered it useless. The second configuration consisted of the same warm-up, 25 minutes at 6000 rpm at half throttle, and back to idle for five minutes. This was followed by a minimum one-hour shutdown with the entire cycle repeated until two test hours were reached.
The engine was disassembled and rated for wear and deposits. Ring sticking, scuffing, crown, undercrown, land and skirt deposits were all reported. It was realized that combustion temperatures played an extremely important role in this test. Temperatures of 1000 degrees Fahrenheit seemed to be okay, while increases to 1300 degrees F or more resulted in very severe ring sticking and scuffing. Some interesting observations were made at the 24:1 fuel-oil ratio. Ring sticking was much lower here than at 50:1; more oil means less potential for ring sticking. Further tests revealed that it was also possible to reduce ring sticking using special additives even at 50 or 100:1, but because of the decrease in the concentration of the lubricant, piston scuffing reared its ugly head. Further experiments indicated that mixing products from two different manufacturers could be disastrous. This philosophy is best adhered to today also. The last thing we want to find is a fuel system plugged with bubble gum because we thought that oil was oil. Find a product that suits your needs and stick with it.
Oil blended for specific applications should be used as intended. Four-cycle crankcase oils won't work, gear oils won't work. Marine oils contain special additives to combat moisture and other harmful effects which may not occur in air-cooled motocross machines. Most tuners do not have access to elaborate test instruments to test for hot spots in combustion chambers. The use of infrared and exhaust gas analyzers is a luxury usually afforded to laboratories, but you might be able to afford a less expensive and easily understandable instrument such as a spark plug gasket temperature probe. As stated earlier, tests have shown that combustion temperature around 1000 degrees F is a safe zone in which to operate. Also, it was found that plug gasket temperatures over 400 degrees F resulted in piston scuffing.
Here is some good information: All-out racers should begin to pay attention when plug gasket temperatures rise to around 350 degrees. My own 650 and 750cc R&D work showed me at temperatures below 225 F meant the power was way down but getting the temperatures up in excess of 375 F invoked strong protests in the form of piston tightening. I've never personally experienced a power reduction due to a cable slipping. When the engine starts slowing down, it means trouble. If you ever feel the power falling, get off the throttle and let everything cool down a little.
A special note for experienced investigators: Make sure that both cylinders have the same cylinder water temperature (about 10 degrees F from each other is as far as I go). Further deviations, especially on a single carbureted engine, will make one cylinder much leaner than the other. Always set up a single carburetor to satisfy the leaner cylinder. One fat cylinder will outlive 100 lean ones. Twin carb setups make it possible to individually adjust the fuel requirement for each cylinder. (Did you ever think that your twin is really two engines sharing a common crankshaft? If you have two carbs and two exhaust systems attached to two sealed crankcases each with a piston, what else could it be?)
The laboratory tests have shown that at 1100 degrees F combustion temperature, the top ring remained free but the second ring stuck; at 1350 degrees F top rings also became solidly stuck. Sustained operation under rich air/fuel ratios will never allow combustion temperatures to rise to dangerous points. Conversely, a few seconds under lean AFRs under wide-open throttle conditions usually means SCREECH!
When you get to the point of reviewing test reports, it's important to understand that ring sticking and deposits are usually reported on a 10-point merit scale with 10 being best. Scuffing, scoring, and scratching is reported as a percentage of the total area which could have been affected, and wear is expressed as a decimal figure indicating the difference between new dimensions and post-test dimensions. Look for oils which produced lows on ring sticking and deposits, zeroes in the scuff categories, and wear decimals with at least three zeros between the decimal point and the first integer. If you are evaluating product using your own machinery, remember that from a lubricant standpoint, deposits are usually predecessors to problems. The faster a deposit appears, the sooner you can anticipate failures.
In today's world, the push is in the direction of reducing the oil used in two-cycle engines. Obviously, a reduction of fuel-oil ratio from 50 to 100:1 will cut world two-cycle-produced smoke in half. What does that mean? Consider that Japan alone has over 9,000,000 two-cycles in use every day. To that, add the rest of the world and it becomes easy to appreciate that two-cycles make a lot of smoke.
We as engine builders, tuners, and enthusiasts are oftentimes at the forefront of technology. We find things today that take the bureaucrats years to uncover. I've seen reports that ratios up to 400:1 will work without scuffing. That's remarkable. I am a firm believer in the "Clean Air Acts". I think we have a responsibility to future generations to "Keep It Clean". I'm also an engine tuner, and from a performance standpoint, less oil in the induction charge means higher peak temperatures and to me that is a potential for more power. Consider that in two-stroke engines, pollution remains only a small fraction of that of a big four-stroke brother. Another very important factor is that two-stroke engines respond very nicely toward environmental control of emissions, much better than four-strokes. The potential of two stroke engines is great in fuel economy and reduction of environmental pollution, providing that it will reach a high recognition point and approval in the public's opinion.
George Grabowski HPT Sport USA