What are the Causes and Countermeasure of Engine Oil Loss Via Piston Ring?

When engine oil enters the combustion chamber of a 4-stroke engine, it spits out white smoke from the exhaust system, just like a 2-stroke engine. There are two possible reasons for engine oil to enter the combustion chamber that should not be there: "engine oil loss via the piston ring" and "engine oil loss via valve guide". When oil enters the combustion chamber through the gap between the piston and the cylinder, this is called "engine oil loss through the piston ring". If you don't have any problems with damage to the pistons and cylinders, the suspected cause of the oil rise is often due to problems with the piston rings.

Engine oil is essential for lubrication, but you don't want it in the combustion chamber.

Piston rings in a 4-stroke engine. From left to right: top ring, second ring, oil ring, side rails, and oil ring. The oil ring, which is thinner than the top and second rings, sits in the ring groove of the piston and functions between the corrugated side rails.

Valve stem seal. The valve stem goes through the center hole and scrapes off the engine oil (which lubricates the camshaft and rocker arm) on the stem seal; on a two-valve engine, there is one seal on each intake and exhaust valve. In the image, the two on the left are new and the two on the right, which are covered in oil, are used.

The engine oil of a 4-stroke engine is pumped from the oil pan underneath the engine to lubricate each part of the engine (in the case of a wet sump, the oil is pumped from the oil tank on dry sump models such as the Yamaha SR400). On dry-sump models such as the Yamaha SR400, the oil is pumped from the oil tank). Oil lubrication is essential as the metals rotate at high speeds and rub against each other with great force.

On the other hand, no engine oil is needed in the combustion chamber, which consists of the top of the piston and the inside of the cylinder head, where explosive combustion occurs when the mixture of air and gasoline is ignited by an electric spark from the spark plug, and the force of the spark plug pushes the piston down. It works completely differently than a four-stroke because it mixes engine oil, lubricates the inside of the crankcase, then enters the combustion chamber and burns with the mixture.

However, since the area around the combustion chamber of a 4-stroke engine is always lubricated with engine oil, a system is needed to prevent that oil from getting into the combustion chamber. That's the "valve stem seal" of the intake and exhaust valves and the "piston ring" of the piston.

The shaft part of the intake and exhaust valves, which inhales the air mixture and discharges the exhaust gas, must be lubricated to prevent it from burning. However, if oil flows into the combustion chamber from the stem, it burns with the air mixture and causes white smoke.

On the other hand, the piston ring attached to the piston is responsible for scraping off the engine oil that lubricates the gap between the piston and the cylinder. As with the valve stem, oil is absolutely necessary because if there is no oil film in the piston and cylinder gap, it will seize up. However, if the oil in this gap enters the combustion chamber, it will still burn with the mixture as it burns and spits out white smoke. Remaining engine oil can also become carbon deposits on the top of the piston and the walls of the combustion chamber.

Either way, it's important that the valve stem seals and piston rings are healthy to prevent engine oil from getting into the combustion chambers while ensuring that the engine is lubricated to prevent seizure.

Oil drops are often caused by stem seals and oil rises are often caused by piston rings.

When you open the throttle, it exhales white smoke and smells like oil mixed in with the exhaust fumes. It's more severe when accelerating. It's not impossible to get this kind of symptom when the oil goes down, but it's rare, so it's safe to assume it's a case of high oil prices.

If you are unfortunate enough to have a 4-stroke engine spitting out white smoke while there is engine oil above and below the combustion chamber, you can assume the cause by observing when the white smoke is emitted.

1) When white smoke appears immediately after engine start-up or at idle

Immediately after starting and at idle, the engine RPM is low and the throttle should not be open. When the throttle valve is closed, the pistons move back and forth with less air (mixture) available to be sucked in, thus increasing the negative pressure generated in the intake system.

Then, when the intake valve opens, the gap between the valve stem and the valve guide that supports the stem is also negatively pressured, and the engine oil used to lubricate the stem and the guide tries to be sucked into the combustion chamber.

Normally, this oil should be scraped off by the valve stem seal, but if the rubber in the stem seal hardens with age, the adhesion to the stem may decrease and the oil may be sucked in. When engine oil enters the combustion chamber in this way, it is called "oil drop" because it falls through the valve stem.

2) If white smoke appears while accelerating and driving with the throttle open

As the throttle valve opens and the engine speed increases, the velocity of the mixture in the intake manifold increases, but the negative pressure is reduced. The conditions that cause oil drop from the valve stem seal are then mitigated.

The piston ring is pressed against the inner wall of the cylinder by the tension of the steel material as it moves back and forth to obtain the engine oil needed for lubrication while scraping off excess oil from entering the combustion chamber.

If the engine is spitting white smoke here, the piston ring may not be scraping off the oil on the inside wall of the cylinder even though the engine is rotating at high speed = the piston is moving back and forth at high speed. This condition is called "oil up" because the oil rises through the gap between the cylinder and piston.

That's the kind of mechanism why I wrote that a 4-stroke engine spitting white smoke is a trouble in itself, but knowing that it can be categorized into two patterns will help you to envision the cause.

Low tension in the oil ring was the cause of the white smoke.

When you try to insert a piston with a piston ring set into a cylinder, the ring will stick out of the cylinder bore diameter and try to pop out, but the oil ring on a white smoke engine is still stuck in the ring groove of the piston. This would be insufficient tension and less able to scrape the oil out.

The top and second rings maintained a strong tension against the cylinders and appeared to be airtight enough not to leak combustion pressure.

Now that we've explained the reasons for white smoke and the specific causes of white smoke, here's a real-life example.

It was less than 10,000km from new when the 125cc Indonesian spec business motorcycle's exhaust system began to belch white smoke in a big way. The engine on this motorcycle had been bored up, but it was still a few thousand kilometers.

If you look at the white smoke pattern, it's not a big deal at idle, but the higher the engine rpm and speed, the more white smoke is emitted. In the previous example, this is a pattern of rising oil. The valve stem seal can be replaced as well, as I removed the cylinder head and cylinder, but the cause was clear when I removed the piston from the cylinder.

I mentioned earlier that the piston ring is pressed against the cylinder by the tension of the ring itself, but the oil ring assembled on this piston is still in the ring groove and has no outward tension left at all.

The 4-stroke engine piston ring consists of a top ring, a second ring, and an oil ring, with the top and second rings sealed to prevent high-pressure gases from escaping through the piston and cylinder gaps in the event of an explosive combustion mixture and the oil ring, is responsible for scraping the oil off the cylinder wall.

If you drive in the rain with deteriorated wiper rubber, the wiping residue may remain on the windows after the wipers move, but the oil left behind burns in the combustion chamber because the pistons move back and forth while the tension in the oil ring is reduced.

I don't know if this is because of the quality of the piston ring that was set in the external piston for the bore up or something else, but when I set a new oil ring in the piston, it adhered to the cylinder with strong tension. It stopped. In other words, I was correct in my assessment of the oil rise.

Due to the bore up, the manufacturer's stock piston ring cannot be used, so I got an external piston ring based on the bore diameter. Since there is no proof that the models are identical, it's possible that this ring is thicker than the ring groove width of the piston ring, in which case, the ring can't be installed in the piston.

Fortunately, the rings were the same thickness, so I figured I could use them to fit the pistons. The oil ring protrudes beyond the piston diameter and it takes a lot of effort to push it back into the ring groove.

This engine was a single-cylinder so it only needed one set of piston ring replacements, but a 4 cylinder engine would require 4 times as many parts and work. Also, if the problem is low oil levels in a 4 cylinder engine, a 2 valve engine would require 8 valve stem seals per cylinder, or 16 valve stem seals per cylinder for a 4 valve engine, which translates directly into repair costs.

If you're prepared to replace both the valve stem seals and piston rings with the best of intentions when the exhaust system spits out white smoke, there's no need to assume that the oil is down or up before disassembly. But if you're trying to get the most out of your engine with the least amount of cost and effort, it's not a waste of time to assume before disassembling your engine whether the white smoke will be at idle or at a certain speed.

There was no evidence of scratches or hard hits to the pistons or cylinders, but I inserted a thickness gauge into the gap between the two and measured the clearance very briefly. Even if the ring is fine, if the clearance is excessive and the piston shakes, oil up can occur.

When a four-cylinder engine blows white smoke, the first step is to identify the faulty cylinder and then determine if it is high or low oil.