The combustion seal: seal it up good!August 17, 2020
Intake, compression, power, and exhaust are the four strokes of an Otto cycle engine design. Within the engineering community, the third stroke that laypeople identify as power is referenced to as expansion.
That is what the combustible mixture does as it burns —- it expands and works against the piston driving it down in the bore. The expansion stroke is responsible for the conversion in energy from a chemical to a mechanical state.
During expansion, the pressure in the cylinder goes extremely high. While responsible for allowing the engine to produce the power, it also creates a significant obstacle — keeping the pressure contained in the cylinder bore and not escaping into the oil pan.
Any engine is only as good as its combustion seal, and this will be the topic of discussion.
Creating the combustion seal
The thermal efficiency of an engine is linked to its ability to use as much of the energy in Btu from the expanding fuel and air charge as possible.
It can be thought of in simple terms of trying to heat a house with many air leaks. Contrary to what many believe, a draft in a building when it is cold outside is the warmer air escaping and not the cold air coming in. Hot always wants to go to cold.
In like fashion, a high-pressure region above the piston evoked by the expansion event is desperately looking to escape. Pressure either fluid or gaseous is always seeking the path of least resistance — it would sooner escape instead of performing any work.
Thus, as the flame in the bore of an engine expands, the pressure needs to be contained, or the engine would not run. The job of keeping the low and high -pressure regions separated then becomes a synergistic approach involving many design factors.
Running engine cylinder pressure resulting from combustion is often confused with the engine’s compression ratio.
The compression ratio is a mathematical calculation in the change in volume with the piston at bottom dead center and then again at top dead center. In simple terms, an engine with a 10:1 compression ratio has ten times less volume in the bore at the top dead center then when the piston is at the bottom of its stroke.
The compression ratio does have a direct correlation on thermal efficiency. The most thermally efficient engines, such as diesel, have a high compression ratio. Regardless of the compression ratio, it is of paramount importance to have the least amount of energy from the combustion event escape. The cylinder needs to be sealed and sealed tightly.
The other benefit of a good combustion seal is exposed during the engine’s intake stroke.
In the vernacular, we say that the engine pumps air. Still, it creates a low-pressure region (vacuum), and nature through atmospheric pressure is doing the work. It is the piston’s task to create a differential in strength and nature’s job to push charge into the cylinder. Thus, the combustion seal is just as crucial during the intake stroke as when expansion begins.
The first area of attention that needs to be addressed is a seal between the cylinder head and the engine block. This is accomplished using a head gasket.
Over the years, the design of a head gasket has gone through many changes, usually based on the material it is made from.
Early engines employed either a steel or copper shim as a gasket. At the same time, some air-cooled designs did not even use a physical seal. In these applications, since there was no water (coolant) traveling between the cylinder head and the engine block, a perfectly machined surface on both the deck of the head and that of the block was enough to seal the bore.
A modern engine still employs a head gasket; it is usually made from a composite or graphite-based material, but its job of containing the bore’s pressure is always the same.
The piston rings are crucial in creating the low-pressure region during the intake stroke and the cylinder seal during expansion.
The ring set must be able to seal while rubbing against the cylinder wall in both directions. The bias is one way as the piston goes up and then the other direction when it is going down.
It needs to be remembered that on the intake stroke, the ring needs to seal to create the low-pressure region in the bore. Then on the compression stroke, it must not allow the charge to escape into the oil pan as the bore volume is greatly diminished (compression ratio). Then the real heavy lifting occurs during expansion when both the cylinder pressure and temperature in the bore are extraordinarily high and must be contained. When that is complete, the ring then needs to seal the cylinder during the exhaust or pumping loop. The unburned gases leave the bore first through a differential in pressure. Then once that equalizes, via the action of the piston.
The piston rings have their work cut out for them, sealing both a low and high-pressure condition.
The piston ring can only accomplish its job with the cooperation of the cylinder wall and the finish on it.
Identified as a crosshatch, the cylinder wall’s surface is necessary to allow the lip of the piston ring to seal against it. A smooth, shiny cylinder wall will not allow for a leak-free fit of the piston ring against it, and both low and high-pressure performance will suffer.
The piston ring is designed to work with a specific crosshatch. It is an application-specific to the ring material and manufacturing specifications.
The finish on the cylinder wall is created with a hone using a specific grit stone in conjunction with the number of strokes up and down. The crosshatch can be confirmed with an instrument called a profilometer that reads in root mean square (RMS).
As an engine accumulates use, it has the tendency to glaze over and remove the crosshatch from the cylinder wall. Ring seal is degraded in lockstep.
The last possible area for the combustion gases to escape is past both the intake and exhaust valves. When both valves are closed, the valve head rests against the valve seat. This can be either via extra material cast in on an iron head or by a ring or insert that is made from steel. A cast-iron cylinder head can employ either an integral or pressed in the seat. In contrast, an aluminum cylinder head must use a steel insert.
If the valve angles are not appropriately machined or end up degrading over time from pounding and thermal cycling, the seal at the combustion chamber becomes compromised.
When this occurs, and the cylinder pressure is high, some of the expanding gasses are pushed up the intake path or into the exhaust system. In most cases, this occurs over a period and not at once unless there is a catastrophic failure. Thus, the engine loses power slowly and is often not felt by the operator until it becomes glaring enough to be known.
As an aside, a valve that does not seal properly is an excellent candidate for failing. The expanding combustion gases will erode it. Within the industry, this is known as the valve being flame cut and usually presents with a small V-shaped cut in the (valve) head and possibly the seat.
An engine will suffer a loss in power and thermal efficiency regardless of where the leak is. For this reason, a simple test can be performed in the farm shop to determine the engine’s ability to seal. It is called a leak down test.
To assign a number in percentage to cylinder leakage in any region, the leak down tester uses air from a shop compressor to fill it. The test pressure is traditionally 100 pounds per square inch.
The tool reads the amount of pressure retained in the bore as a percentage. Good values on an in-service engine is below ten percent leakage. A well-built and maintained engine will come in at about five percent. If the leakage on the tool is more than 10 to 15 percent, the problem area can usually be identified with your ear.
If the dipstick is removed and you can hear air escaping into the oil pan, the piston rings/cylinder wall will be the culprits. Air escaping through the carburetor is an intake valve, and through the exhaust is an exhaust valve.
A good mechanic always does both compression and leak down tests on an engine to determine its true health. A compression test may often not show any real issue, but it will be found with a leak down test.
Cylinder pressure is often compared to making money — what you keep is the only thing that counts!