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Cold Start and Driveability Evaluation: Mastering SAE J1635 Procedure
SAE J1635 provides a standardized method for subjectively evaluating a vehicle’s starting behavior and driveability after a prescribed soak at a set ambient temperature. This recommended practice helps engineers and technicians assess cold start reliability and identify driveability defects such as surge, hesitation, and bucking. The following guide distills the key elements of the procedure, from test conditions to vehicle preparation and the rating system.
🛠️ Test Conditions: Setting the Stage for Reliable Cold Start Evaluation
The test environment greatly influences results. SAE J1635 specifies that the test road should be paved, level, smooth, and dry. Public roads are acceptable if traffic is light. Ambient temperature can vary, but average winds should not exceed 25 km/h (15 mph) and gusts should stay under 40 km/h (25 mph), as wind masks defects like surge and hesitation. Precipitation or fog must be avoided because they affect traction and can conceal driveability issues.
The soak period is critical. The vehicle must be soaked at ambient temperature for a specified duration depending on the type of start test:
| Type of Start Test | Soak Period |
|---|---|
| Cold | 8 to 36 h (12 h nominal) |
| Partial cooldown | 1.5 to 4 h |
| Hot | 5 to 30 min |
🔍 Design Insight: Consistent soak conditions are essential for repeatable driveability assessments. Variations in soak time or temperature can skew results, making comparisons between tests unreliable.
⚠️ Vehicle Preparation: The Foundation of Consistent Results
Proper vehicle preparation is vital to ensure accurate and repeatable cold start tests. The vehicle must be equipped, maintained, and adjusted for ambient temperature operation. Key preparation steps include:
- Use the manufacturer-recommended fuel (at least ¼ tank).
- Ensure correct engine oil weight and viscosity.
- Verify coolant composition, thermostat, spark plugs, and battery condition.
- Set tire pressure using the recommended cold inflation, adjusted for large temperature differences between prep and test areas using the formula: ΔP = k × (T_prep – T_test), where k = 1 kPa/°C (or 1 psi/13 °F).
- Load the vehicle to 136 kg ± 11.4 kg (300 lb ± 25 lb) above curb weight, including driver and equipment.
⚠️ Common Mistake: Failing to adjust tire pressure when ambient temperature differs by more than 14 °C (26 °F) between preparation and test locations can alter vehicle dynamics and affect driveability evaluation.
After preparation, the engine is brought to stabilized coolant temperature by driving at least 16 km (10 miles) at 70‑90 km/h (45‑55 mph). For ambient temperatures below −18 °C (0 °F), the drive should be at least 32 km (20 miles). Record idle MAP and RPM after warm‑up.
🔍 Evaluating Driveability: Defects and Rating System
Once the vehicle has soaked for the prescribed period, the cold start procedure begins. For diesel engines, record the “WAIT” light on‑time. The subjective evaluation focuses on identifying driveability defects such as surge, hesitation, bucking, stumble, and stall. The standard defines a rating system to quantitatively score the severity of each defect, enabling engineers to compare different vehicles or calibration changes.
The acceleration schedule during the test is defined by manifold pressure and pedal travel. For example, light acceleration is set at ¼ pedal travel with a target MAP of 40.4 + 0.60 × idle kPa. This ensures consistent loading during evaluation.
🛠️ Design Insight: The rating system helps quantify subjective driveability issues for comparison. By standardizing defect definitions and severity scales, engineers can make objective decisions even from subjective observations.
Frequently Asked Questions
How does ambient temperature affect cold start performance?
Lower ambient temperatures increase fuel viscosity and reduce battery capacity, making cranking harder and potentially extending start time. The rating system captures these variations under controlled soak conditions.
Why is proper tire pressure adjustment important?
If the test temperature differs from the preparation temperature by more than 14 °C (26 °F), tire pressure must be compensated using the factor 1 kPa/°C (1 psi/13 °F). Incorrect tire pressure alters rolling resistance and vehicle response, masking or exacerbating driveability defects.
What fuels are recommended for testing?
The vehicle should be evaluated with all fuels recommended by the manufacturer, including reformulated gasoline, ethanol/gasoline and methanol/gasoline blends, and diesel #1 and #2.
Can tests be conducted in the wind?
Winds up to 25 km/h average and gusts up to 40 km/h are allowed, but stronger winds can mask defects such as surge and hesitation. Precipitation and fog must be avoided for consistent traction and visibility.
By adhering to the SAE J1635 procedure, engineers can obtain reliable, repeatable cold start and driveability data that supports vehicle development and quality assurance. 🚗
