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SAE J902-2019: Design and Testing of Windshield Demisting and Defrosting Systems
Scope and Essential Definitions
SAE J902-2019 establishes minimum performance standards and uniform test procedures for windshield demisting and defrosting systems in passenger cars, light trucks, and multipurpose vehicles with a gross vehicle weight (GVW) of 4 500 kg (10 000 lb) or less, as well as M1 category vehicles. The standard applies to vehicles with internal combustion engines (IC), hybrids (HD), and electric vehicles (EV), including those running in pure electric mode.
Key definitions differentiate between defrosting (removal of frost or ice on the exterior surface) and demisting (removal of condensate from the interior surface). The vision area required for both systems is defined using the 95th percentile tangential cutoff ellipse per SAE J941, with the driver’s seat in the rearmost position as specified in SAE J903.
Note: The clearing area requirements are directly tied to the driver’s eye location ellipse and the wiper pattern defined by SAE J903. Always coordinate defroster outlet design and wiper performance early in the development process.
Test Procedures and Performance Requirements
The standard details specific test conditions for both defrosting and demisting. Vehicles must be tested in a cold chamber with the following tolerances:
| Parameter | Defrosting | Demisting |
|---|---|---|
| Chamber temperature | −18 °C +3/−10 °C (i.e., between −15 °C and −28 °C) | −3 °C ±1 °C |
| Application target | External frost/ice | Internal condensate |
| Spray/steam characteristics | Nozzle Ø1.7 mm, 350 ±20 kPa, 395 mL/min, cone Ø300 mm at 200 mm | Steam generator: 2.25 L water, ≤75 W heat loss at boiling, 0.07–0.10 m³/min air at 50 Pa |
| Data collection | Two observers (tracing or photographic) or fixed camera (perpendicular to glass) | One or two observers outside vehicle (tracing/photographic) or fixed camera |
A chassis dynamometer simulates road load during defrosting tests. The air resistance force is calculated using a typical drag coefficient (Cd = 0.35) and the vehicle’s overall width (W117) and height (H101):
F = 0.00275 × A (where A is the cross-sectional area in cm²); then horsepower = (F × km/h) / (3.6 × 746).
Rolling friction from non‑driving wheels is omitted because the dynamometer’s driving‑wheel friction compensates sufficiently. 🛠️
⚠️ Common Mistakes: Not allowing enough soak time, using an incorrect ellipse (not per SAE J941), miscalculating the dynamometer load, and positioning the camera off‑perpendicular are frequent errors that can invalidate test results. Always follow the specified tolerances and ensure the battery is fully charged before soak.
Design Insights and Frequently Asked Questions
Engineering design insight: The required clearing area corresponds to the oval defined by the driver’s 95th percentile eye ellipse and the wiper coverage area. This means that the defroster outlet placement, airflow volume, and temperature must be optimized to match the swept area of the wiper blades. For hybrids and EVs, the propulsion mode must be assessed—if the vehicle can operate in pure electric mode throughout the test, it should be classified as an EV, simplifying the test conditions.
Data collection methods offer flexibility, but cameras must be fixed and perpendicular to the glass to avoid parallax. Manual tracing by trained observers is still acceptable, provided the vehicle’s glass is traced with velum and the traces are digitized correctly. 🔍
- FAQ: How is a hybrid vehicle classified for demisting/defrosting tests?
- Under SAE J902-2019, a hybrid (internal combustion plus electric propulsion) is designated “HD” unless it runs in pure electric propulsion throughout the entire test. If the vehicle operates exclusively on electric power during the test, it is then called “EV”.
- FAQ: What are the critical factors for dynamometer load calculation?
- The load formula assumes a typical Cd of 0.35 and uses the product of overall width and height to approximate the frontal area. The absence of rolling friction from the non‑driving wheels is intentional because the dynamometer’s driving‑wheel friction already accounts for that portion.
- FAQ: Can cameras replace manual tracing for data collection?
- Yes, a fixed camera mounted perpendicular to the windshield surface is permitted. The camera must not be handheld, and the imagery must be processed to eliminate distortion. Manual tracing by two observers inside the vehicle is still a valid method, especially for defogging tests where outside observers may be used.
- FAQ: How long must the vehicle soak before the defrosting test begins?
- The standard requires that the vehicle soak in the cold chamber until all temperatures have stabilized at the target temperature. Typically this means several hours, and the battery should be fully charged (and may remain on charge) during the soak. The passenger cabin must not be preconditioned.
By adhering to the clear guidelines in SAE J902‑2019, engineers can design and validate windshield demisting and defrosting systems that meet both regulatory and customer expectations. The standard’s harmonized test procedures and flexible data‑collection options make it a practical tool for global vehicle programs.
