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About Cylinder Pressure

About Cylinder Pressure

About Cylinder Pressure

Hi all,

I am working on engine bearing testing systems. I have problem with to determine engine bearing specific loads in the SI and CI ICE.
I would like to learn when the internal combustion engines should give maximum cylinder pressure value? During Maximum Torque or During Maximum Power? Which conditions will give more cyclic stress - load upon the conrod bearing? This is very important for me to make real test our engine bearings. Are there anybody offer me practical methods to determine peak cylinder pressure and maximum specific load on the conrod engine bearings?

happy new year,

RE: About Cylinder Pressure

Peak cylinder pressure is typically going to be highest near maximum torque because that's also closely coinciding with peak volumetric efficiency (highest mass of trapped charge in the cylinder). Usually if the ignition/injection timing is in the ballpark it's going to be highest somewhere near 15 degrees after TDC on the power stroke (because combustion is ongoing as the piston is starting to go down). But this will vary depending on many factors including how well the engine is tuned and whether the operator is feeding it gasoline of the correct octane rating. Preignition or detonation can do lots of damage.

Don't forget that the inertial loads are very significant and those go up with the square of RPM. The peak tensile load on the connecting rod generally happens near TDC between the exhaust and intake strokes. At TDC between compression and power strokes (during combustion) the inertial load on the connecting rod is tensile and the cylinder pressure load is compressive. Which one is greater than the other ... will vary.

RE: About Cylinder Pressure

ATDC between 12* and 16* degrees but this can change by engines.
But the most mechanical stress occurs when the fuel ignited not after it burned.
never ignite the engine less than 8 degrees BTDC, after that it starts to erodes the conrod bearings

RE: About Cylinder Pressure

"never ignite the engine less than 8 degrees BTDC, after that it starts to erodes the conrod bearings"


Typical MBT ignition timing on an automotive gasoline engine running under load (when it matters) is 20 - 35 degrees BTDC depending on how good or bad the combustion chamber is. (A good fast-burn chamber will need less ignition timing to achieve best torque.)

Here is a randomly selected ignition timing map. RPM is increasing across the top, engine load increases as you go down because the chart is done in terms of trapped air mass per stroke.


24 degrees BTDC, or pretty close, ignition timing at full load at 2800 rpm and up.

RE: About Cylinder Pressure

The 1966 Chrysler plot clearly shows that bearings need to be much more concerned with loads created by rod/piston inertia than by combustion events; at least for "modern high-speed" engines.

RE: About Cylinder Pressure


I said "never ignite the engine less than 8 degrees BTDC" means (ingition <8 ) I generally tune up more than x>8

RE: About Cylinder Pressure

Your last post makes no sense at all. What do you mean by "ignite the engine"? Do you mean "ignition timing"? Or, possibly, do you mean START the engine? Ignition timing well before TDC during cranking is of course inadvisable.

RE: About Cylinder Pressure

Thanks to All,

I enlightened your given valuable informations

RE: About Cylinder Pressure

Inertial loads >> combustion loads for non-boosted engines, right?
Overrev = fracture, due to some combination of tension and bending.
Some highly turboed engines have had rods shorten and/or buckle.

Jay Maechtlen

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