Guest Blogger: Street Rod 101
A dynamometer is a device that is used to load down an engine. Putting a load on an
engine is like pulling a trailer up a hill, in which case more horsepower is needed. Dynamometers are used to simulate a load on an engine for several reasons, including:
1. To check emissions under load,
2. To tune the engine under load in real time,
3. To determine the best speed to create a specific torque on the engine for shifting
and drag racing,
4. To calibrate engine management controls in conjunction with the on-board
5. To investigate combustion behavior under load,
6. To check friction, lubrication, and wear of moving surfaces under load
7. To determine maximum torque and horsepower ratings at different engine
8. To determine the effects of engine load on the air/fuel ratios using an oxygen
There are several types of dynamometers. The engine dynamometer is attached directly to the back of the engine. The chassis dynamometer is attached to the drive wheels of a vehicle. In this case the friction from the drive train (transmission, U joints, drive shaft, differential, axles, bearings and tires) consumes some of the engine horsepower, generally between 15-20%. Thus, engine power will be always more than the power at the drive wheels. There are also stationary as well as portable chassis dynamometers. Often at street rod shows, portable chassis dynamometers are used to test maximum horsepower of a specific street rod.
The illustration below shows the parts and operation of a typical
have a knurled surface to reduce slippage during testing. The vehicle is then held in place with hold down straps in the front and rear of the vehicle. Chocks are used on the front wheels to keep the vehicle stationary. A computerized control console sends data from the engine, in particular, the rpm of the engine. As the vehicle is accelerated, the back wheels spin the drive rollers. The drive rollers are connected to the power absorption unit by gears, cog belts, or other mechanical means. The
computerized control module also reads data from a transducer on the dynamometer
used to read the torque that is being produced. In the drawing, the transducer is shown sensing torque off of the power absorption unit. The get the most accurate torque reading it should be taken off of the drive rollers.
On the control console there is a load control. As the load control is increased, more load is placed on the power absorption unit. Depending upon the type of
dynamometer, the load is applied to the power absorption unit in one of several
methods. The most popular systems include water braking, mechanical friction braking, hydraulic braking, and electromechanical braking. For example, if a water brake is used, as the load control is increased, more water is sent through the power absorption unit, causing it to slow down. The more water pumped into the power absorption unit, the greater the load. As the load is increased, the torque arm and torque/load sensor send data to the control console to be read on the torque meter.
Note that only torque, read in ft. lbs. and rpm, read in revolutions per minute are sensed. The horsepower is then mathematically determined using a constant, and
read out on the HP meter. The formula for determining HP is:
Horsepower = torque (ft. lbs.) X rpm / 5252
There are many types of chassis dynamometer tests that can be done. For example, tests can be designed as follows:
1. A constant vehicle speed (or engine rpm) is established for a specific period of
time with a variable load trying to load down the engine to reduce the vehicle
2. A variable vehicle speed (or engine rpm) is tested under a constant load, and the
speed is changed with increasing increments.
3. A variable vehicle speed (or engine rpm) and variable load is used to determine
maximum torque at various increments.
Chassis dynamometers are also used to determine maximum horsepower characteristics. Horsepower and torque charts often are used to determine maximum power at all rpm and/or vehicle speed. See the charts shown in this website entitle Engine Performance.