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Combustion Analysis

Combustion Analysis is a term that is still foreign to many motor enthusiasts. Many have not heard of it; those who have might not understand its meaning or function, or the performance benefits when applied to an engine.

Perhaps a better definition would be “constant speed in-cylinder pressure measurement”; because that is exactly what we are doing. We are measuring the pressure in each cylinder and recording it every one half degree of crankshaft rotation at any desired rpm. Cylinder pressure is measured, and recorded, by cycles (720 degrees being one combustion cycle); usually 50 or more depending on the test objective. This insures uniform data.

Special pressure transducers are used, very sensitive and accurate. There are two basic types. One is for programs requiring continuous development on the same, or similar engines. These are usually mounted in the cylinder head or close to it. The second is a special spark plug transducer that is substituted after warm-up.

The purpose of combustion analysis is to measure the performance imbalance between cylinders: the difference of output between cylinders. That’s right -  all engines have an imbalance from cylinder to cylinder. The reasons for these differences vary; resulting from air/fuel mixture, timing differences, temperature variation, air flow (air flow in a port of a running engine is different than constant flow on a flow bench), and intake manifold, just to mention a few.

With the application of special analysis software, the recorded data of each cylinder identifies peak pressure, location peak pressure, rate of rise of peak pressure and CA50 (CA50 is where 50% of trapped mixture is burned) These are but a few of the many performance parameters recorded and identified by crankshaft location (degrees usually before or after TDC).

Fig. 1 is a series of five charts generated by one of two software programs used at iSystems. This particular program is used immediately after a test is completed to compare averages of each cylinder for Peak Cylinder Pressure, Peak Cylinder Pressure Location, CA50 and Exhaust Absolute Pressure. As shown, each parameter is compared with each other, with the exception of Absolute Exhaust Absolute Pressure. AEAP for a performance engine should be in the range 13 to 15 psi. More or less could be due to less than desirable exhaust valve timing or port configuration. Perhaps most important is the comparison of IMEP, Indicated Mean Effective Pressure, to the other parameters. IMEP is directly proportional to brake torque and can vary with Peak Pressure and Peak Pressure location. CA50 can be changed by altering cylinder ignition timing and it will affect cylinder pressure and IMEP. Experience has shown that a CA50 around 7 to 8 degrees ATDC is close to optimum for basically all engines. By optimizing cylinder pressure and location, performance, efficiency and reliability will be improved.


Fig. 2 is a graphic illustration of a pressure-volume loop (P/V). It gives an approximation of valve activity and piston location; “approximation” because valve activity will vary with cam design.

Fig. 3 is an actual recorded P/V loop. First, notice the area of “blow down”; a decrease in volume, then pressure and volume increases. This is an excellent indication of a less-than-adequate exhaust lobe profile and/or an exhaust port that is restrictive. Either one is causing a pumping loss and loss in performance. Pumping loss is the additional force required for the piston to push burnt gasses out of the cylinder. With excess gasses in the cylinder the chance of diluting the incoming mixture is increased during the overlap period. Not good. Obviously we would like the “volume” portion to be much smaller.

The red circle is in the vicinity of the intake valve closing. We would expect all cylinder traces to be smooth and equal; but we see quite the opposite here, an indication of pressure activity, or valve bounce. Loss of cylinder pressure = loss of torque!

These are only a few of the many performance parameters identified and recorded in combustion analysis. These are perhaps the easiest to address so we can make corrections to the engine for performance improvements. Remember, a dynamometer is an important tool, but it only gives a total for the complete engine. It cannot tell how each cylinder is performing at operating rpm.

Combustion analysis should be considered after basic performance preparations have been completed. For those interested in more detailed information I suggest reading the publications in this web site.

iSystems utilizes the most accurate instrumentation from AND Technology, AVL, and Kistler Instrumentation. We also have atmospheric control from sea level to 6000’.

If you have questions or would like to discuss a particular application please do not hesitate to contact us.