The chassis dynamometer is a device for applying specific loads to a vehicle to determine if the vehicle will perform to published specifications and to permit a physical inspection for leaks of any kind. It is an excellent method for detecting improper tune-up, misfiring injectors, low compression, and other malfunctions.
Chassis Dynamometer Room Ventilation Recommendations
For safe and accurate dynamometer readings, the chassis dynamometer room must be properly ventilated.
|To avoid injury before starting and running the engine, ensure the vehicle is parked on a level surface, parking brake is set, and the wheels are blocked.|
|ENGINE EXHAUST |
|To avoid injury from inhaling engine exhaust, always operate the engine in a well-ventilated area. Engine exhaust is toxic.|
If a vehicle is tested on a dynamometer located in an area without proper ventilation, the engine will be subject to high ambient air temperatures. High air inlet temperatures can result in false low power readings on the dynamometer.
To help ensure accurate horsepower readings, the dynamometer room should have a ceiling-mounted fan with a 850 to 1133 m3 /min (30,000 to 40,000 ft3 /min) capacity. This will provide proper ventilation of exhaust gases and heat radiated by the operating engine.
For direct engine cooling, Detroit Diesel recommends the use of a 368 m3 /min (13,000 ft3 /min) or greater capacity barrel-type ram air fan. This should be portable so that it can be conveniently placed three to five feet (approximately one to two meters) in front of the truck and aimed directly at the radiator/charge air cooler package.
Chassis Dynamometer Test and Run-in Procedure
The function of the dynamometer is to absorb and measure the engine output after it has been transmitted through the vehicle transmission and driveline to the drive tires.
The vehicle is connected to dynamometer through the roller absorption unit. The load on the vehicle may be varied from zero to maximum by decreasing or increasing the resistance in the unit. The amount of power absorbed in a water brake type dynamometer, as an example, is governed by the volume of fluid within the working system. The fluid offers resistance to a rotating motion. By controlling the volume of water in the absorption unit, the load may be increased or decreased as required.
The following are some tips to keep in mind in preparing the vehicle before the dynamometer run and during the actual testing:
- Follow all applicable safety procedures from the chassis dynamometer manufacturer.
- Observe the air intake duct and charge air fan shrouding to identify misalignments. Correct as necessary.
- Make sure the differential lock is “on.”
- Make sure the radiator/charge air cooler system fan(s) are locked on.
- If the vehicle is equipped with anti-lock brakes (ABS), disable the ABS controller by unplugging it or removing its power fuse/breaker.
- Instrument the engine for fuel pressure, air inlet restriction, exhaust backpressure, and crankcase pressure, and note these readings during the dynamometer run.
- Make sure the vehicle hood is down and locked into its normal position.
- Make sure the engine is fully warmed up before placing the vehicle on the dynamometer. Both oil and coolant temperature should be at least 180° F (82° C).
- When loading the vehicle on the dynamometer, make sure the vehicle is positioned onto the rollers as straight as possible.
- For direct engine cooling, Detroit Diesel recommends the use of a 368 m3 /min (13,000 ft3 /min) or greater capacity barrel-type ram air fan. This should be portable so that it can be conveniently placed three to five feet (approximately one to two meters) in front of the truck and aimed directly at the charge air cooler. This is especially important at high altitudes.
- During high ambient temperature conditions (90+° F, 32+° C), it may also be necessary to use a water spray mist fan in front of the vehicle to prevent overheating.
- Running the dynamometer in “manual” mode instead of “automatic” mode allows for maximum control of the test process and may result in more consistent test results.
- During the horsepower test, make sure “percent engine load” and “torque limiting factor” are 100% by monitoring them with a computer equipped with DDDL 7.0.
- Select a transmission gear with a 1:1 gear ratio during the horsepower test.
- Take horsepower reading at 1500 rpm and 1750 rpm. Stabilize at rated speed for at least one minute before taking a final horsepower reading. Compare the recorded horsepower to the appropriate minimum horsepower tables below for particular ratings.
- Note that the following charts are for vehicles with manual transmissions. Vehicles with automatic transmissions (i.e. equipped with torque converters) will have slightly lower minimum horsepower values then listed in Table “Minimum Acceptable Wheel HP” .
|Rated HP||Wheel HP @ 1800 RPM||Wheel HP @ 1500 RPM|
|455/1550||374 @ 1800||367 @ 1500|
|475/1550||390 @ 1800||367 @ 1500|
|455/1650||374 @ 1800||377 @ 1500|
|475/1650||390 @ 1800||390 @ 1500|
|505/1650||415 @ 1800||391 @ 1500|
|455/1750||374 @ 1800||377 @ 1500|
|475/1750||390 @ 1800||394 @ 1500|
|515/1750||423 @ 1800||416 @ 1500|
|475/1850||390 @ 1800||394 @ 1500|
|500/1850||411 @ 1800||415 @ 1500|
|530/1850||436 @ 1800||436 @ 1500|
|560/1850||460 @ 1800||440 @ 1500|
Note: Note: This is wheel HP with the fan locked on, 17.8% parasitic loss @1800 rpm, & 15.5% parasitic loss @ 1500 rpm