Cylinder Pressure

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What is Cylinder Pressure?

Cylinder Pressure - Graph of Cylinder Pressure from Engine Analyzer Pro

Cylinder Pressure is the pressure in the engine cylinder during the 4 strokes of engine operation (intake, compression, combustion and expansion, and exhaust). You could argue that pressure during expansion is the most important, because that is the cylinder pressure pushing on the piston to produce power. But the cylinder pressure during all 4 strokes is necessary for accurate engine performance prediction.

Intake Stroke

First, the definition of the pressure used for engine simulation is Absolute Pressure. On earth, there is always air pressure present, called barometric pressure. You've probably heard of 29 or 30 inches of (mercury) barometer. Well, 30" of barometer is also about 15 PSI of pressure in the PSI units you are probably most familiar with.

During the intake stroke, the intake valve opens and the piston moves down in the cylinder. This increasing cylinder volume creates a cylinder pressure which is lower than atmospheric pressure to draw air into the cylinder. You probably think the piston is "sucking" the air in, but actually the barometric pressure is able to push the air in because there is less than atmospheric pressure in the cylinder.

The less restrictive the heads, runners and throttle, the more easily the air can fill the cylinder and the higher the cylinder pressure during the intake stroke. For example, at idle with the throttles closed, the cylinder pressure is very low, which means less air and fuel is in the cylinder when the intake valve closes and less power is produced. As you open the throttle, intake cylinder pressure goes up and more air and fuel are ingested and more power is made.

Also, the higher the cylinder pressure during the intake stroke, the less work the engine has to do to "suck in" the intake air and fuel. This will be discussed later.

Compression Stroke

After the intake stroke, the intake valve closes to trap in the air and fuel. (The exhaust valve was already closed during the intake stroke.) Now the piston starts it's upwards stroke, compressing the air/fuel mixture to pressures of 100 to 200 psi or more. The actual pressure depends on when the intake valve closes, the pressure in the intake manifold (MAP or Manifold Absolute Pressure), Compression Ratio, and other details.

The compression of the air and fuel creates heat and somewhat helps vaporize the fuel. It also takes even more work from the engine to do this compression. So far, the engine is doing all the work and getting no power from the air/fuel mixture.

Combustion / Expansion Stroke

This stroke is where the spark plug ignites the air/fuel mixture, creating very high cylinder pressure which rise very quickly. Peak cylinder pressures near TDC (where spark occurs) will be in the range of 300 psi for engine's at light loads, to 1000 psi for production engines at full power to 1500 psi or greater for race engines. This is where the engine's power comes from, as it forces the piston down. As the piston goes down, the cylinder volume increases which reduces the cylinder pressure. When the piston gets to the bottom on the cylinder (BDC) there may only be 100 to 500 psi in the cylinder.

Exhaust Stroke

Near BDC, the exhaust valve opens, releasing any cylinder pressure which is higher than atmospheric pressure. This is called "blow down". Depending on the cylinder pressure and how quickly the valve opens, a loud or sonic pulse can be generated. That is where the exhaust noise comes from, and why engines make more noise at full power than at idle.

The piston then moves back to TDC up the cylinder, forcing all the remaining burnt air/fuel (exhaust) out. Again, depending on how restrictive the exhaust valves, ports, mufflers, etc are, the cylinder pressure in the cylinder will be somewhat higher than atmospheric. How high the cylinder pressure is during the exhaust stroke determines how hard the engine must work to expel the burnt exhaust.

Summary

So, cylinder pressure is the constantly changing pressure inside the cylinder for all 4 strokes. There are certain characteristics of this pressure which people may want identified, like peak (maximum) cylinder pressure near TDC, or peak cranking compression pressure (pressure at TDC without combustion), or average cylinder pressure (IMEP, indicated mean effective pressure). They are all just measures of cylinder pressure at certain times (or averaged over certain times) in the 4 strokes.

It should be noted that for 3 of the 4 strokes, the engine is putting work into the process, and that only during Combustion/Expansion stroke is any work being output from the process. The work during the Combustion/Expansion stroke must make up for the other 3 strokes and still have enough left over to produce a net power. Luckily it works out that there is LOTS of work (or energy released) produced during Combustion/Expansion stroke.

Programs that take Cylinder Pressure into account

We have 3 programs that work with Cylinder Pressure when conducting Engine Simulation: Engine Analyzer v3.4, Engine Analyzer Plus v3.4. And Engine Analyzer Pro v3.9. However, only the Engine Analyzer Pro will calculate Cylinder Pressure degree by degree and graph if for you.

And our Compression Ratio Calculator v2.3 also takes Cylinder Pressure into account when making calculations.