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.
12.12.1
Chassis Dynamometer Room Ventilation Recommendations
For safe and accurate dynamometer readings, the chassis dynamometer room must be properly
ventilated.
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.
12.12.2
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 12-4.
