MECH9761 Chap.6 Cylinder Pressure Analysis and Lab Reporting
Cylinder Pressure Analysis and Lab Reporting
This chapter turns laboratory measurements into reproducible engineering evidence. Pressure is aligned with volume calculated from geometry, integrated to obtain work and normalised to IMEP. A first-law model gives apparent heat-release rate, while filtered cumulative release gives start of combustion and CA10, CA50 and CA90. The report must join code, figures, tables and mechanism-based interpretation.
Measurement, processing and inference remain separate layers so every conclusion can be traced to a case, calculation and plotted feature.
What this chapter covers
- 01
Sensor zeroing and aligned pressure-crank-angle data
- 02
Cylinder volume from engine geometry
- 03
Pressure-volume integration and IMEP
- 04
Apparent heat-release rate under a constant gamma assumption
- 05
Start of combustion and cumulative heat-release phasing
- 06
Spark-timing and diesel-load comparisons
- 07
Figure, table, code and discussion requirements
Worked example · free
Calculate IMEP from integrated work
- +1Convert displacement: 0.50 L = 0.00050 cubic metres.
- +1IMEP = work divided by displacement volume.
- +1IMEP = 420/0.00050 = 840,000 Pa.
- +1Convert to bar: 840,000/100,000 = 8.40 bar.
Key terms
- Pressure Transducer
- A fast sensor that converts in-cylinder pressure into an electrical measurement.
- Apparent Heat Release
- Heat release inferred from pressure and volume under a simplified first-law model.
- Start Of Combustion
- The first crank angle meeting the specified filtered heat-release threshold within the search window.
- CA50
- The crank angle at which cumulative apparent heat release reaches half its selected total.
- Gross Indicated Work
- Pressure-volume work from the compression and expansion loop before gas-exchange work.
- Net Indicated Work
- Pressure-volume work including compression, expansion, intake and exhaust portions of the cycle.
- Combustion Duration
- A crank-angle interval such as CA10 to CA90 describing the main cumulative heat-release period.
- Crank-Angle Alignment
- Matching pressure, volume and derived signals at corresponding rotational positions.
- Pumping Mean Pressure
- Gas-exchange work expressed as an equivalent mean pressure under a stated sign convention.
Cylinder Pressure Analysis and Lab Reporting FAQ
Why is apparent heat release called apparent?
It is inferred from pressure and volume through a simplified first-law model. Wall heat transfer, crevice flows, mass exchange, variable properties and sensor effects are not all resolved directly, so the calculated curve represents an apparent release under stated assumptions rather than a direct chemical measurement.
What do CA10, CA50 and CA90 show?
They locate the crank angles where cumulative apparent heat release reaches 10%, 50% and 90% of the selected total. Together they describe burn start, midpoint and duration. Their meaning depends on the chosen start, end, filtering and normalisation procedures, which must stay consistent across cases.
What makes a strong laboratory discussion?
A strong paragraph identifies a plotted or tabled change, quantifies it with units, explains a physical mechanism and states the inference boundary. It distinguishes what the pressure or heat-release data directly show from causes that would require additional measurements. Completed code and correctly labelled figures remain part of the evidence.
Why are the measurement channels zeroed?
A sensor offset can appear as false pressure, torque or flow throughout a run and bias every derived result. Zeroing establishes the baseline before acquisition. The report should still preserve calibration, units and processing choices so another reader can distinguish a corrected offset from a physical change.
How do gross and net work differ?
Gross indicated work uses the compression and expansion loop. Net indicated work also includes intake and exhaust, so gas-exchange work changes the result. Naming the integration boundary is essential because two calculations from the same pressure trace can answer different performance questions.
Why can filtering change combustion timing?
Heat-release calculations use pressure and volume derivatives, which amplify noise. Filtering stabilises the signal but can smooth peaks or shift detected thresholds if its settings change. Apply one declared method and search window across compared cases, then report those choices with CA10, CA50 and CA90.
What belongs in a defensible figure discussion?
Name the cases, quantify the change with units, identify the plotted feature and then explain a physical mechanism. State whether the conclusion is measured, calculated or inferred. Number the figure, label axes and refer to it explicitly so the evidence remains connected to the written claim.
Exam move
Rehearse the analysis as a pipeline: zero and align measurements, calculate volume, integrate pressure-volume work, normalise to IMEP, calculate apparent heat release, filter, locate combustion start and extract CA10, CA50 and CA90. For every comparison, use one sentence for observation and a separate sentence for mechanism.
Audit the report against required cases, code, tables, figure numbering, axis units and explicit references in the discussion. Hand-check one work calculation from array units through joules and pascals before processing the sweep. Mark gross and net integration boundaries on the pressure-volume plot. For heat release, write the filter, search window and cumulative normalisation beside each timing result.
Finally, compare one spark pair and one diesel-load pair using the sequence observation, quantity, mechanism and limitation.
