ISO 789-4:1986

Overview

ISO 789-4:1986 - "Agricultural tractors - Test procedures - Part 4: Measurement of exhaust smoke" (second edition, 1986) specifies a standardized method for measuring exhaust smoke (opacity) from tractor engines running at steady speed. It is the fourth part of an eight‑part series on tractor test procedures and replaces the 1982 edition. The standard defines laboratory conditions, engine setup, measurement procedure, reporting and the required testing equipment.

Key topics and technical requirements

• Test scope and conditions: Measurements are taken with the engine at steady speed under 80% of maximum load, using manufacturer engine settings and installed equipment per ISO 789/1. Laboratory temperature and pressure must keep the correction factor F within 0.98–1.02.
• Measurement points: Smoke opacity is measured at six engine speeds spaced across a defined interval (including maximum‑power speed and lower limits).
• Opacimeter requirements (Annex A):
  • Two scales: absolute light‑absorption (m‑1, 0 to ∞) and linear 0–100 scale.
  • Light source: incandescent lamp (colour temperature ~2800–3250 K).
  • Photoelectric receiver: spectral response similar to human eye (peak ~550–570 nm).
  • Effective light path length L must be determined and stated; accuracy and zero/span checks are specified.
• Installation and sampling (Annex B):
  • Probe geometry and sampling line design to avoid dilution and soot accumulation.
  • Probe placement in a straight exhaust section (distances expressed as multiples of pipe diameter).
  • Expansion tank/cooler and pipe routing guidance to preserve sample integrity.
• Reference fuel (Annex C): Specifications for a reference diesel fuel (e.g., density 0.830 ±0.005 g/ml at 15 °C, cetane index ~54 ±3, viscosity ~3 ±0.5 cSt at 37.8 °C) for approval and conformity checks.
• Reporting (Annex D): Required test report items include manufacturer, tractor/engine ID, measured absorption values, opacimeter type and fuel details.

Practical applications and users

ISO 789-4:1986 is used by:

• Tractor and engine manufacturers for development, type‑approval testing and quality control.
• Accredited test laboratories performing emissions measurement and certification.
• Regulatory authorities and conformity assessment bodies that need a consistent method to compare tractor exhaust smoke.
• R&D teams and emissions engineers evaluating combustion tuning, turbocharger effects, and after‑treatment influence on visible smoke.

Related standards

• ISO 789/1 and other parts of ISO 789 series (power tests, p.t.o., centre of gravity, etc.)
• ISO 2288 (engine bench test code, net power) referenced for engine test context.

Keywords: ISO 789-4:1986, agricultural tractors, exhaust smoke measurement, opacimeter, tractor emission testing, reference fuel, test procedures.