Tuesday, September 6, 2011

Even More About Piston Rings

One of the most vital components of the internal combustion engine is the piston ring. It's probably safe to say that without the invention of a long lasting, pressure tight piston ring, we would still be driving coal fired steam engines.

The Lost Motorcyclist has a previous blog about piston rings. http://lostmotorcycles.blogspot.com/2010/03/everything-you-need-to-know-about.html

The piston ring these days is an often ignored part, because the rings are so good that we never change them any more, and they rarely fail. But when you think about it, the only part of a car that cannot be engineered to last longer than a typical 15 year life span is the piston ring. It's likely that the piston ring is the weakest link in the chain of parts that contribute to the 15 year life span of the car. I would guess almost any other cart part could be engineered to last 20-25 years if we wanted to, but what's the point if the piston rings can't last as long.

When the piston rings fail, the car starts smoking, power drops off, it's hard to start and the engine loses oil rapidly. In other words, it's time to scrap the car. Although back in the nineteen thirties, this only meant it was time for new piston rings.

So what is a piston ring? First we must know what a piston is. The piston is a round part that moves up and down in the cylinder, and drives the crank that eventually turns the wheels. Hopefully I don't need to explain the piston any more than that. The piston is designed to fit tightly into the cylinder, so that when the gasoline burns above the piston, all the high pressure created by the burning gas is trapped and cannot escape between the wall of the cylinder and the piston.

Now the tricky part of the design of the internal combustion engine: It is impossible to design a piston tight enough in the cylinder to prevent loss of pressure when the engine is running. Why? Because the engine runs at widely different temperatures, and it is impossible to make the piston exactly match the shape of the cylinder at every temperature. If the piston fits tight at high temperature, it will be too loose at low temperatures, and if it is tight at low temperatures, then when it heats up it will stick tight in the cylinder and the engine will stop turning.

Long ago, engineers gave up on getting an exact match between the piston and the cylinder, and invented "piston rings".

The piston ring is a springy ring of steel with a small opening on one side. The piston ring can be designed to almost exactly match the size of the cylinder at every temperature, because it depends on the springiness of the ring to hold the ring tight in the cylinder. No matter what temperature the ring is, it will hold tight in the cylinder, and it will never get too tight because of the gap on one side. The only purpose of the piston ring is to fit tight in the cylinder.

So the piston by itself cannot seal the cylinder properly. But neither can a piston ring, because is is not solid. But if you find a way into lock the ring to the piston, the two can work as a unit to convert the pressure of the burning gas into downward movement, while at the same time preventing any pressure from escaping.

To make the piston work with the piston ring, a groove is cut all the way around the piston, and the ring is fit loosely into the groove. So when the piston is in the cylinder, the piston ring can spring out to press against the cylinder wall, while being held inside the groove on the piston.

Believe me, piston rings are refined even though they look very simple. The exact shape, the materials, the lubrication have all been refined over many years of expensive research by the anyone interested in internal combustion engines. And there is a lot of interest because of the money to be made.

I should mention a further refinement, that is not too difficult to understand. The pressure from the burning gas is actually routed behind the ring to help force the ring out even tighter against the cylinder wall.

What can go wrong with the rings? They can wear down, by rubbing on the cylinder walls. Remember at 3,000 rpm how many times the piston and rings move up and down the cylinder walls, pressed tight against the cylinder. The walls cannot be well lubricated because of the exposure to flames of burning gasses. And that's 3000 up/downs in just one minute. How many times would you do that in the life of a car? OK I just did it roughly for you, 360 million times that the piston would move top to bottom of the cylinder.

Even when not being driven at 3000 rpm, the rings can get stuck in the grooves, which means they can't spring out to press against the wall. This happens when too much oil is being burned in the cylinder, and it gets turned to gummy carbon in the piston groove, and eventually hardens to trap the piston ring.


  1. Yes, it's certainly a good thing that metallurgy and ring composition have come such a long way over the past half-century.

    I can't recall the last time I heard anyone complaining that they had to do a 'ring job' on their engine next weekend. New rings, of course, are a standard part of a rebuild, but no longer (thank goodness) a maintenance issue.

    You mention the problem of metal-to-metal tolerances changing with the temperature of the engine, and this is one of the reasons (along with noise and emissions) that water-cooling (and oil/air cooling) has become so prevalent, especially on larger displacement motorcycle engines.

    Heck, even the venerable Porsche 911 was eventually converted to water cooling after 34 years of production to squeeze additional performance from the engine (admittedly to meet emission and noise requirements, too).

    People even believed the April Fool's article that Harley was switching to water-cooled engines.

  2. I wonder if they also believed that the imaginary radiators would be made in Japan, as it said in the article about Harley Davidson water cooled engines?

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