Loss of compression
A drop in end-of-compression pressure is a direct measurable indication that the cylinder-piston group has lost its seal. The register contains 68 industrial-test cases; the median compression gain is 13 %.
Causes of compression loss
Compression is determined by the tightness of the enclosed volume above the piston. Its loss results from wear of the mating surfaces rather than from a single defect: the end gap of the piston rings, wear of the cylinder bore surface and loss of ring elasticity act in combination.
Mechanics of tightness loss
- Leakage past the ring seal. Wear of the ring working surface and an increased end gap open a path for the working mixture into the crankcase. Part of the charge bypasses the power stroke, and the oil in the crankcase becomes saturated with combustion products.
- Wear of the cylinder bore surface. Ovality and taper of the liner disrupt ring contact around the full circumference. Sealing ceases to be uniform, and the spread of compression between cylinders grows faster than the mean value.
- Cylinder-to-cylinder non-uniformity. The spread of compression between cylinders is more informative than the mean: it shows that wear is progressing unevenly and allows the residual service life to be assessed without disassembling the unit.
Measurement procedure
Compression is measured on a warmed-up engine cylinder by cylinder, with a standard compression gauge: on petrol engines with the spark plugs removed and the fuel supply cut off; on diesels, through the injector or glow-plug hole. Procedure — appendix “Compression measurement in cylinders.” Values are recorded for each cylinder separately — averaging conceals the very non-uniformity for which the measurement is taken.
Readings for each cylinder separately are available in 64 of 66 cases with a direct measurement: they show how the spread between cylinders changed; the remaining cases have no readings for more than one cylinder.
Treated assemblies
Among the treated components, the cylinder-piston group is listed in 60 of 68 cases; the full list of components is given below.
Equipment types
Test reports
Fifteen test reports with the highest recorded gain out of 66 containing a direct compression measurement. The full list is given in the catalogue.
| ENTERPRISE | YEAR | CHANGE | PARAMETER MEASURED |
|---|---|---|---|
| SE UkrSTC “Energostal”, pilot plant | 2013 | ↑ 35.4% | compression in the fourth cylinder, 6.5 → 8.8 kgf/cm² |
| Gaztekhservis LLC | 2005 | ↑ 33% | compression across cylinders, 8.0–9.2 → 10.5–11.5 atm |
| Taxi Moy Gorod LLC | 2018 | ↑ 27–40% | cylinder compression, 10–11 → 14 kgf/cm² |
| PTP Pavlogradpogruztrans | 2012 | ↑ 25–36% | compression of the worn cylinders, controller position 5, 22–24 → 30 kgf/cm² |
| Seid-Transavto LLC | 2003 | ↑ 25–65.3% | compression per cylinder, 4.9–5.4 → 8.1–8.2 on the UAZ and 8–10 → 12–12.5 on the VAZ |
| Imstalkon JSC | 2001 | ↑ 25–46% | compression across the cylinders, 3.0–5.0 → 4.0–7.0 kgf/cm² |
| Zaporizhzhia Iron Ore Plant | 2006 | ↑ 20–33% | compression across cylinders, 15.5–33 → 27–40 kgf/cm² |
| AKAR LLC | 2002 | ↑ 20–33% | compression pressure across cylinders, 27–31 → 35–40 kgf/cm² |
| SE “Ukrvodput” | 2001 | ↑ 19.3–37.2% | cylinder compression, 22.6–24.8 → 29.0–31.0 kgf/cm² |
| Tri K LLC | 2002 | ↑ 16.7–36.2% | compression across the eight cylinders, 22–24 → 26–32 kgf/cm² |
| SPK Murman fishing collective farm | 2003 | ↑ 16.4% | compression across the cylinders, 9–10 → 11–11.5 |
| Troyanovo-3 mine, Maritsa Iztok | 2004 | ↑ 14–33% | cylinder compression, 18–22 → 24–26 atm |
| Tatneft, Almetyevsk UTT-1 | 2001 | ↑ 14–22% | cylinder compression, 17–27.7 → 22–30 kgf/cm² |
| Ukrzaliznytsia, Shevchenko depot | 2003 | ↑ 13–72% | cylinder compression, 25–28 → 27–31 and 18–20 → 31–35 kgf/cm² |
| JSC “Krasnoarm. Khleb” | 2001 | ↑ 13–65% | compression in worn cylinders, 5.5–7.5 → 8.5–9.1 |
All 68 cases in the catalogue →
Documents of the cases with a direct measurement: plant commission reports — 42, signed test protocols — 20, other documents — 4; the document type is stated on the case card.
Limits of data applicability
- Applicability. The figures relate to the equipment and operating conditions of the enterprises named in the reports; on a different unit the result depends on initial wear, load and oil.
- Source document. A scan of the original report is published under every case; if the card and the scan disagree, the scan is authoritative.