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Combustion Analysis


     Combustion analysis (CAS), or in-cylinder pressure measurement, was first used in the late 1800s. Obviously there have been vast improvements. Dr. Andrew L. Randolph has been given credit for introducing it into motorsports around 1994 (see “Articles”). It is currently used in all forms of motorsports, F1, endurance racing, NASCAR, GP and drag racing, to mention a few.

Why should you be interested?

    All multi-cylinder engines have varying pressure from cylinder to cylinder. No matter how hard you work to ensure combustion chamber volume, piston dome volume, ignition timing, fuel mixture, and so on, there will be differences from one cylinder to the next, and some can be substantial. This is a FACT that has been proven many times (see “Articles”). A cylinder with a higher or lower pressure can reduce the performance and speed of other cylinders, resulting in less torque and horsepower. Equalizing cylinder pressure not only can provide performance gains but increases reliability as well. In a basic engine evaluation, peak pressure, pressure location, location of where 50% of the mixture is burned, IMEP and more, are identified by individual cylinder and located by crankshaft degrees. Is it a surprise no one wants to talk about the number-one engine evaluation and tuning tool?

Below are graphs of measured data taken from different engines thought to be built and tuned to maximum performance. These are only two of many that are used to evaluate an engine's condition.

peak pressure by cylinder and cycleFig. 1 is a graph of peak pressure by cylinder and cycle. As you can see, the sample rate was 40 cycles. The engine being tested was 305 cu. in., 4 valves, that produced 594 hp at 7000 rpm. Impressive, but look at the cylinder and cycle pressure variation, which is not unusual: a maximum of 1255.6 psi and a minimum of 1168.7 psi; almost a 90 psi difference (click to enlarge). Information such as this, where peak pressure occurred, and where 50% of the mixture was burned, all referenced by crankshaft degrees, contributes to balancing cylinder pressure. More pressure, more torque and horsepower. It is impossible to evaluate and correct this on a dynamometer alone.


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.


recorded pressure volume (P/V) loop

Fig. 3 is a recorded pressure volume loop (P/V) from a small vintage race engine, 175 cu. in., 6 cylinders, 2 valves, producing 281 hp. Notice the exhaust traces drop below the intake traces after BDC and remain there until just before TDC, indicating a partial vacuum. This could have a detrimental effect on the incoming charge if all exhaust gasses have not been scavenged. The desired form would be the exhaust traces above, parallel and in close proximity to the intake traces. This form justifies a conversation with the cam grinder.

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 would like to know more, email us and we will call you to discuss further benefits of individual cylinder pressure measurement.