CHAPTER-1

WHAT IS DYNAMOMETER?

1.1 Introduction:



In today’s modern world the I.C. engines or any rime movers are tested for its performance such as power and torque developed. To test the performance of these prime movers dynamometers and associated instrumentation is used. The Installations of Dynamometers have become the important part of the automotive industry. The subject of dynamometer seems to cause more concern, many misunderstandings and notions. There are three methods of testing an automotive engine in general, and they are:



1. Testing an engine using dynamometer.

2. Testing a vehicle with engine under consideration on chassis dynamometer.

3. Or actually running the vehicle on test track.



1.2 Definition of the dynamometer:

1. A device for measuring the torque, force, or power available from a rotating shaft. The shaft speed is measured with a tachometer, while the turning force or torque of the shaft is measured with a scale or by another method. Power may be read from the instrumentation or calculated from shaft speed and torque.



2. It is an apparatus for measuring force or power, esp. one for measuring mechanical power, as of an engine.



3. DYNAMOMETER an instrument for measuring force exerted by men, animals and machines. The name has been applied generally to all kinds of instruments used in the measurement of a force, as for example electric dynamometers, but the term specially denotes apparatus used in connection with the measurement of work, or in the measurement of the horse-power of engines and motors



The most common use of the dynamometer is in determining the power of an electric motor or engine of a car, truck, or other vehicle. A dynamometer that connects to the engine crankshaft is an engine dynamometer. One that has rollers turned by the vehicle drive wheels is a chassis dynamometer; this type is widely used in the automotive industry for mileage accumulation, emissions, fuel economy, and performance testing of cars and trucks.



In this book our discussions are mainly concentrated on Engine dynamometer used for engine /motor testing.



1.3 History of the Dynamometer.

The dynamometers are being used to measure the power since long. During the eighteenth century, James Watt introduced a unit of power to compare the power of his steam engines with a more familiar source of work. This unit of power became known as Horsepower. It was defined as the amount of power required to move a 550 pound weight one foot in one second.

Figure 1.1 work done

The first device used probably date back when Gaspard de Prony invented the Prony brake in circa 1821. The de Prony brake (or Prony brake) is considered to be one of the earliest dynamometers. Over the next 200 years, the Prony Brake Dynamometer and variations of same were developed to measure engine horsepower. Modern day versions of these brake dynamometers are still in use today.

1.4 Dynamometer- A Chronology of Innovation:

1. Gaspard de Prony invented the de Prony brake in 1821 in Paris. The de Prony brake (or Prony brake) is considered to be one of the earliest dynamometers.

2. 1838 Charles Babbage, known to historians as the Father of the Computer, introduces a dynamometer car to measure the pulling power of English railroad locomotives

3. William Froude with the invention of the hydraulic dynamometer in 1877 and first commercial dynamometers were produced in 1881 by their predecessor company, Heenan & Froude.

4. 4.1921 Professor E.V. Collins of Iowa State College develops a draft horse dynamometer, used to measure a horse's capability to pull the era's heavy metal farm implements.

5. In 1928, the German company "Carl Schenck Eisengießerei & Waagenfabrik" built the first vehicle dynamometers for brake tests with the basic design of the today's vehicle test stands.

6. 1930 Using designs pioneered through collaboration with Rudolph Diesel, John Taylor forms the Taylor Dynamometer and Machine Company to produce engine dynamometers.

7. The eddy current dynamometer was invented by Martin and Anthony Winther in about 1931.

At that time, DC Motor/generator dynamometers had been in use for many years. A company founded by the Winthers, Dynamatic Corporation, manufactured dynamometers in Kenosha, Wisconsin until 2002. Dynamatic was part of Eaton Corporation from 1946 to 1995.

8. 2002, Dyne Systems of Jackson, Wisconsin acquired the Dynamatic dynamometer product line. Starting in 1938, Heenan and Froude manufactured eddy current dynamometers for many years under license from Dynamatic and Eaton.

9. The first popular, true high speed, computer controlled, eddy current chassis dyne systems produced were motorcycle systems that were produced by Factory Pro Dynamometer of San Rafael, CA, USA in 1990.

1. 5 The word Dynamometer

The word dynamometer is derived from a Greek word dunamis meaning power and meter means measure i.e. dunamis + metron = dynamometer

1.6 Essential features of dynamometer.

If we think about a good dynamometer which serves the purpose of the engine testing then the following four essential features are important.

 Means of controlling torque

 Means of measuring torque

 Means for measuring speed

 Means for dissipating power

1.7 Speed Measurement of Dynamometer.

The speed in dynamometer is measured by either a mechanical tachometer or by an electronic device. In case of an electronic device, the speed measurement consists of a magnetic pulse sensor working in conjunction with a geared wheel generally having 60 teeth. The pulses generated are processed and displayed by Electronic digital indicator. In some cases where accuracy is of utmost importance an optical encoder is used.

1.8 Torque Measurement.

The concept of Torque is important enough to be clarified. Actually it is the direct result of the load of the spring or weight. Its distance from the axis of rotation is also responsible for determining the torque. In reality dynamometers are used to calculate the production of torque by an engine.

1.9 Torque – turning force

This is also called Moment of a Force, in physics, the tendency of a force to rotate the body to which it is applied. The torque, specified with regard to the axis of rotation, is equal to the magnitude of the component of the force vector lying in the plane perpendicular to the axis, multiplied by the shortest distance between the axis and the direction of the force component, regardless of its orientation. The following figure shows how the torque is understood.

Dynamometers, a.k.a “dynos,” are brakes used to measure the power of an engine at a given speed. The torque of an engine is determined by a complex measuring mechanism and reaction transferred by the dynamometer to measuring mechanism. Dynamometer manufacturers construct their products using basic components: frame, Trunion bearings, absorption unit, and torque measuring device.

1.10 Dynamometer Constant

As discussed in earlier in this chapter by definition torque is derived quantity. It is a product of force applied and distance from the center to the point of application of the force. Generally if the force is applied at the circumference of the rotating disk then the distance between the center of disk and the point of application is radius of the disk under consideration.

Thus Torque T = Force (f) x Distance (l)

Or Torque T= Force(f) x Radius(r)

The constant ‘K’ in the equation is known as Dynamometer constant.

1.11 Mounting of Dynamometer.

The main casing of dynamometer consists of a power absorbing unit. Incase of hydraulic a dynamometer it will be rotor and pair of stator and controlling mechanism and incase of Eddy current dynamometer it is a rotor, Excitation coil and cooling chamber. This main chamber or casing is also called as cradle.

1.11.1 Trunion bearing:

A pedestal with Trunion bearings is either bolted to base plate or they are integral part of base plate as per the proprietary designs of different manufacturers. The dynamometer cradle is mounted between the pair of the Trunion bearings. This gives the freedom to oscillate when a reaction force acts on cradle as virtue of absorbed power.

Trunion is nothing but pivot forming one of a pair on which something is supported, here in this case a dynamometer carcass is supported which is free to oscillate and transfer the reaction force to the measuring mechanism. The movement is limited by torque arm connected to the side of housing and connected to the torque measuring system. The advantage of Trunion bearing is that it is the simplest type of cradle mounting with freedom of movement for the carcass

Trunion bearings located between the ends of the dynamometer housing, or carcass, and a set of pedestals do not rotate. They do, however, allow the carcass to rotate slightly for torque measurements. Since bearings and lubricant directly affect performance and accuracy of dynamometer, Trunion bearings should be inspected and rotate frequently. Grease lubricated Trunion bearings do not require periodic lubrication. However, if grease becomes dry or lumpy, it should be flushed and replaced.

Figure 1.2:- Trunion mounting

The disadvantage of the Trunion bearing is that after a long use they try to be sticky. This is mainly because of their minimal movement. This is called the Brinelling effect

Some specially designed dynamometers consist of hydrostatic trunnion bearings, referred to as lift trunnion bearings. They are oil pressure lift type sleeve bearings used to reduce trunnion bearing friction to a negligible value. This in turn typically improves system accuracy. Bearings of this type are oil lubricated with high pressure oil piping system to circulate oil through the bearings and support the carcass on a film of oil as long as the high pressure oil is supplied. Carcass floats during the operation.


1.11.2 Flexural support:

The dynamometer housing is cradle-mounted in a flexure support on the frames. The flexure support guarantees absolute maintenance-free operation, optimal measurement accuracy and minimal hysteresis between loading and unloading. The advantage of the flexure support is simple construction and no maintenance required. The construction with flexure mount in place of Trunion bearing gives a big advantage of reduced overall weight of dynamometer.

Figure 1.3 : Flexure mounted support

Flexure supports replace the heavy pedestals and large trunion bearings. However a little insight will tell us that while dynamometer is at work the flexural deflection may move the dynamometer center. However, this movement, whatsoever is negligible.

1.11.3 Fixed Mounting

The fixed mounting is simplest type of mounting and it also eliminates the oscillating cradle assembly. However this arrangement employs in line torque transducer for the measurement of the torque.

Figure 1.4:- Foot mounted dynamometer with torque transducer

1.12 Scientific Accuracy of Dynamometers

The accuracy of dynamometer is of prime impotence and needs to be evaluated. Many a times doubts are raised about the absorption and transmission of stator (cradle) reaction to measuring mechanism. The issues raised are about frictional losses in Trunion bearing and measuring mechanism such as load cell mounting swivel joints. This is how it works. The net frictional force will have its reaction force which will act on cradle and ultimately considered in measuring force. Thus the dynamometer transmits measured reaction to measuring chain faithfully. Hence dynamometer is considered the 100% accurate machine.

1.13 BHP Concepts.

Work done is product of force times distance moved in one scathes power (energy) is defined as rate of doing work. In FPS system it is defined as if 550 lb weight is moved one foot in one second then the work done is 1 hp or 33000 lbs/min.

Figure 1.4: Horse power explained.

It was originated by James Watt, (1736-1819) the inventor of the steam engine and the man whose name has been immortalized by the definition of Watt as a unit of power. If F is the force applied at flywheel of radius R then the work don in on revolution is
Power in F.P.S. system
Work done = Force x Distance moved (F= force in lbs and r= feet)

Power in M.K.S. system

Power is 1 hp if work is done at the rate of 75 kg-m/sec. if a weight of 75 kg is moved to 1 meter distance in one sec then the work done is 75 kg-m/sec= 1 hp.

Work done = Force x Distance moved

Power in S. I. System.

Power is one watt if work done is 1 joule per second. It is worth recalling here from physics that 1 joule/sec = 1 Newton-Meter/sec

Work done = Force x Distance moved

(1 Kw =1000 watts)

Closure

The dynamometers are basic apparatus used to measure and load the engine or the unit under test. The modern dynamometers, however, are equipped with more sophisticated and complicated instrumentation to suit the today’s engine testing requirement. There are various types of dynamometers and modern instrumentation which will be dealt in forthcoming chapters.

Ref:

1. Timeline of Dynamometer history article publishd by laurel peters on http://automotive-articles.blogspot.com/2008/03/dynamometer-hjistory-timeline-of.html

2. http://en.wikipedia.org/wiki/Dynamometer#Hsitory