Dynamometer

A dynamometer is a load device which is generally used for measuring the power output of an engine. Several kinds of dynamometers are common, some of them being referred to as “brakes” or “brake dynamometers”: dry friction brake dynamometers, hydraulic or water brake dynamometers and eddy current Dynamometers.

Dry friction dynamometers are the oldest kind, and consist of some sort of mechanical braking device, often a belt or frictional “shoe” which rubs a rotating hub or shaft. The hub or shaft is spun by the engine. Increasing tension in the belt, or force of the shoe against the hub increases the load on the engine. Hydraulic dynamometers are basically hydraulic pumps where the impeller is spun by the engine. Load on the engine is varied by opening or closing a valve, which changes back pressure on the hydraulic pump. Eddy current dynamometers are electromagnetic load devices. The engine being tested spins a disk in the dynamometer. Electrical current passes through coils surrounding the disk, and induce a magnetic resistance to the motion of the disk. Varying the current varies the load on the engine. The dynamometer applies a resistance to the rotation of the engine. If the dynamometer is connected to the engine’s output shaft it is referred to as an Engine Dynamometer. When the dynamometer is connected to the vehicles drive wheels it is called a Chassis Dynamometer. The force exerted on the dynamometer housing is resisted by a strain measuring device.

Several components are typically packaged together in a dynamometer: the shaft with bearings, the resistance surface, the resistance mechanism in a “free” rotating housing, a strain guage, and a speed sensor . Generally some method of cooling is also required, and this may require either a heat exchanger or air or water circulation. The entire assembly is typically mounted on a stout frame, which is mechanically linked to the frame of the engine being tested.

The force signal (F) from the strain gauge may be converted into a torque (T) by multiplying by the distance from the center of the shaft to the pivot point of the strain gauge : T = R x F If the units are in Newton-meters and shaft speed (S) is measured in radians per second, then the shaft power or brake power P of the engine can be calculated by multiplying the speed and the torque: P = T x S Obviously if you are using different units than you will need to use the appropriate conversion factors.

DYNAMOMETER CONTROLLERS

In order to test the engine it is generally necessary to use a dynamometer controller. This is usually an electronic unit which has the capability of controlling the load on the dynamometer and can measure or sense the load and speed. Dynamometer controllers generally operate in two modes: Speed Controlled operation or Load Controlled operation. In Speed Controlled mode a set speed is given to the controller. If the measured speed of the shaft is less than that of the set speed, the load is decreased. If the measured speed of the shaft is greater than that of the set speed, then the load is increased. Assuming the engine has sufficient torque to attain the set speed, this will maintain a constant speed.

In Load Controlled mode a set load is given to the controller. If the measured load on the dynamometer is greater than that of the set load, the load is decreased. If the measured load on the dynamometer is less than that of the set load, then the load is increased. Assuming the engine has sufficient torque to attain the set load, this will maintain a constant load while the speed varies . The dynamometer is a device used to measure the torque being exerted along a rotating shaft so as to determine the shaft power. Dynamometers are generally classified into:

1. Absorption dynamometers (i.e. Prony brakes, hydraulic or fluid friction brakes, fan brake and eddy current dynamometers)
2. Transmission dynamometers (i.e. Torsion and belt dynamometers, and strain gauge dynamometer)
3. Driving dynamometers (i.e. Electric cradled dynamometer)

ROPE BRAKE DYNAMOMETERS:

A rope brake dynamometers consists of one or more ropes wrapped around the fly wheel of an engine whose power is to be measured. The ropes are spaced evenly across the width of the rim by flywheel. The upward ends of the rope are connected together and attached to a spring balance, and the downward ends are kept in place by a dead weight. The rotation of flywheel produces frictional force and the rope tightens. Consequently a force is induced in the spring balance.

Effective radius of the brake R = (D+ d)/2 Brake load or net load = (W-S) in Newton Braking torque T = (W-S) R in Nm. Braking torque =2πN* T/60 in N-m/s = 2πN * (W-S)R/60* 1000 in

kW D = dia. Of drum d = rope dia. S = spring balance reading

APPLICATIONS:-

i) For torque measurement. ii) For power measurement.