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SAE J35 Diesel Smoke Measurement Procedure: Engineering Insights and Test Guidelines
The SAE J35 standard (cancelled in 2002) remains a foundational reference for measuring transient smoke emissions from vehicular diesel engines. It defines a dynamometer-based test procedure, equipment requirements, and data analysis methods. This article distills key engineering takeaways from the standard, including the critical role of fast-responding opacimeters, strict environmental controls, and the correlation between in-line and end-of-line instruments.
Scope and Purpose of SAE J35
SAE J35 applies to the dynamometer test procedure used to assess smoke emission characteristics of vehicular diesel engines. It describes a smoke test cycle, equipment and instrumentation, instrument checks, chart reading, and calculation methods for evaluating transient smoke emissions. The procedure is similar to the US regulatory method but includes additional recommendations to improve test precision. It was cancelled in 2002 but continues to serve as a reference for engine development and evaluation. Note: As the standard states: “Whenever this procedure is cited, any additional or modified test conditions must be clearly reported.” 🛠️ This recommendation highlights the importance of documenting any deviations to ensure reproducibility across tests.
Essential Test Conditions and Fuel Specifications
To ensure consistent and comparable results, the standard specifies tight control over environmental conditions and fuel properties. Key parameters include:
- Intake air temperature: 25 °C ± 5 °C
- Fuel pump inlet temperature: 38 °C ± 5 °C
- Barometric pressure: 725 to 785 mm Hg (96.7 to 104.7 kPa)
- Intake air restriction and exhaust back pressure: Must be set per manufacturer specifications (for turbocharged engines at rated speed and full load; for naturally aspirated engines at high idle).
The standard also defines speeds (rated, peak torque, intermediate, idle) and loads critical to the test cycle. Fuel specification is vital for smoke measurement reproducibility. The recommended test fuel should comply with the current Code of Federal Regulations, Title 40, Part 86. The table below summarizes the fuel specifications from the standard (for two ranges). Any deviations must be reported.
| Property | Range No. 1 | Range No. 2 |
|---|---|---|
| Cetane Number | 48–54 | 40–48 |
| Distillation IBP (°C) | 166–199 | 171–204 |
| Distillation 50% (°C) | 210–249 | 243–282 |
| Distillation EP (°C) | 260–293 | 321–366 |
| Sulfur (%) | 0.03–0.05 | 0.03–0.05 |
| Gravity (°API) | 40–44 | 32–37 |
| Aromatics (min %) | 8 | 27 |
| Flash Point (°C min) | 49 | 54 |
| Viscosity (cSt at 40°C) | 1.6–2.0 | 2.0–3.2 |
⚠️ Common mistake: failing to control intake air temperature and fuel pump inlet temperature can significantly affect smoke emissions and reduce test repeatability. Always monitor and record these conditions.
Opacimeter Requirements and Data Analysis
Because the smoke test is transient, the opacimeter must be a fast-responding full-flow type. The standard explicitly warns: “Slow responding or sampling, or both, type instruments must not be used.” This is a critical engineering design insight: the meter must capture rapid changes in opacity during acceleration and lugging modes.
- Response time: The opacimeter must have a fast response (typically better than 0.5 s for full-scale step change). The standard references SAE J255 for instrument specifications.
- Mounting: The opacimeter must be installed in the exhaust stream without disturbing the natural flow. In-line (IL) and end-of-line (EOL) types are both acceptable, but a correlation procedure must be established between them to maintain consistency.
- Instrument checks: Before and after the test cycle, zero and span must be set, and linearity verified.
Data is recorded continuously: chart speed at least 25 mm/min during idle and at least 200 mm/min during active modes; for automatic acquisition, at least 0.1 Hz at idle and 10 Hz during other modes. The cycle includes a preconditioning period at rated power for 10 minutes.
🔍 Engineering design insight: For consistent transient smoke measurement, use a full-flow opacimeter with a response time under 0.5 seconds. Sampling-type instruments introduce delays that distort the opacity signal and should never be used for this test.
Frequently Asked Questions
1. How do I correlate in-line and end-of-line opacimeter readings?
The standard provides a correlation procedure (Section 13) that involves simultaneous measurements with both instruments and a least squares regression (LSR) analysis. You must establish a correlation for your specific setup and document it. This ensures that smoke data can be compared across test cells or equipment.
2. What fuel should I use for the SAE J35 smoke test?
Use a fuel that meets the current US Code of Federal Regulations, Title 40, Part 86 fuel specifications. The standard gives two ranges (No.1 and No.2) covering cetane, distillation, sulfur, aromatic content, etc. If you cannot exactly match these specs, report the actual fuel properties along with the test results.
3. What are the acceptable environmental conditions for the test?
Intake air should be controlled to 25 °C ± 5 °C, fuel pump inlet to 38 °C ± 5 °C, and the dry barometric pressure should be between 725 and 785 mm Hg. Testing outside these limits may affect smoke emissions and should be avoided or noted.
4. Why was SAE J35 cancelled and is it still useful?
The standard was cancelled in October 2002, likely because newer regulations and standards replaced it. However, it remains a valuable reference for understanding transient smoke measurement principles, especially the use of full-flow opacimeters and the importance of test conditions. Many of its recommendations are still applied in engine development and testing.
🛠️ Always remember: the SAE J35 test procedure is for transient smoke measurement. If you are conducting steady-state smoke tests, refer to other applicable standards.
