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SAE J690: Understanding Net Horsepower Certification for Motor Trucks and Truck Tractors
This article details the now-cancelled SAE J690 standard, which defined the methodology for certifying maximum net horsepower in motor trucks and truck tractors. First issued in 1941, it provided a consistent framework for determining net brake horsepower delivered to the clutch, accounting for all standard accessories. While superseded, its definitions and testing principles remain foundational in the heavy-duty vehicle industry.
⚠️ Note: SAE J690 was cancelled in November 1998. This article is provided for historical reference and understanding of the standard’s concepts.
Gross vs. Net Horsepower: Key Definitions
Under SAE J690, two distinct power measurements were established:
| Aspect | Gross Horsepower | Net Horsepower |
|---|---|---|
| Definition | Brake horsepower of the stripped engine—only those accessories necessary for the engine to function during the test. | Brake horsepower delivered to the clutch (or its equivalent) with all standard chassis accessories functioning, including exhaust pipe, muffler, tail pipe, etc. |
| Accessories | Only those required for engine operation (e.g., water pump, oil pump, ignition). | All regular equipment as installed in the specific chassis model (e.g., fan, air filter, generator, governor). |
| Determination Method | Direct dynamometer test of the stripped engine per SAE J816. | Either by subtracting accessory power consumption from gross horsepower, or by direct test with all accessories installed and functioning, per SAE J816. |
The standard specified that net horsepower should be used as the index of the potential ability of a motor truck or combination.
Engineering Design Insight: Accounting for Accessory Power Absorption 🛠️
A critical engineering insight from SAE J690 is the systematic treatment of power-absorbing accessories. The standard provided Table 2—Power Absorbing Accessories, Equipment, and Attachments—which required manufacturers to list all items functioning during both gross and net horsepower tests. Items like the fan, air filter, muffler, and generator each consume a portion of the engine’s output. By subtracting these parasitic losses from the gross figure, engineers arrive at a realistic net horsepower rating that reflects actual on-road performance. This attention to accessory power loss is essential for accurate vehicle capability ratings, enabling fleet operators to match power to payload and grade requirements.
🔍 Design Insight: When evaluating engine performance, always consider the cumulative effect of accessories. A high gross horsepower number may be misleading if the net figure—after accounting for fan, exhaust, and other losses—is significantly lower. SAE J690 formalized this accounting process, ensuring consistency across truck manufacturers.
Frequently Asked Questions
What is the difference between gross and net horsepower as defined by SAE J690?
Gross horsepower is measured on a stripped engine with only essential accessories for engine function. Net horsepower is measured with all standard chassis accessories functioning, including the exhaust system, fan, generator, and other equipment typically installed in the vehicle. Net horsepower represents the power available at the clutch for propulsion.
What accessories must be considered when determining net horsepower?
All accessories that are standard or regular equipment on the engine as installed in the particular chassis model must be included. This includes, but is not limited to, oil pump, water pump (if water cooled), fan, exhaust pipe, muffler, tail pipe, fuel pump, ignition distributor or magneto, air filter, governor, generator, vacuum pump, and compressor. Each of these consumes power and must be accounted for either by subtraction or by testing with them installed.
Can net horsepower be determined without testing with all accessories installed?
Yes, the manufacturer may determine net horsepower by subtracting the accessory power consumption from gross brake horsepower, provided the power consumption of each accessory is known and accurately measured. Alternatively, direct testing with all accessories installed and functioning is permissible per SAE J816.
Why was SAE J690 cancelled, and what replaced it?
SAE J690 was cancelled in November 1998. It was superseded by updated standards and practices, likely incorporated into broader engine test codes or vehicle rating guidelines. However, its core principles regarding net horsepower determination and accessory accounting remain influential in modern powertrain evaluation.
This article provides a summary of SAE J690 for informational purposes. For current industry standards, refer to the latest SAE publications.
