POWER OUTPUT OF AN  IC ENGINE

TEAM: IC ENGINEERING

ME 415W

Spring '02

I. Overview

II. Introduction

III. Problem Statement

IV. Design Specs

V. Alternative Solutions

VI. Plan of Manufacturing

VII. Budget

IX. Manufacturers

Overview

A standardized method for measuring power output of a 2 horsepower, one-cylinder test engine is needed for a student laboratory.  Currently, a DC cradle dynamometer system is being employed, but not effectively.  Test data revealed that the power output measured approximately 10% above the capable limits of the engine.  Attempts had been made to correct this error through calibration but were unsuccessful.  Furthermore, the system is in use currently and it would not be feasible to try to fix it.

           Through engineering methods, the main logical solutions are to:

1.      Purchase an entire dynamometer system.

2.      Take a risk and attempt to fix the current system.

3.      Design and build a cradle dynamometer from scratch.

4.      Employ the accuracy and simplicity of a torque transducer/motor combination.

After a thorough investigation of each alternative through a series of weighted criterion, we have decided to design a torque transducer system.  This system consists of the transducer, a regenerative DC motor of 2 Hp output, and two couplings.  The motor, couplings, and transducer are connected in series to the test engine.  The motor will initially be used to start the engine, and be set at a specific constant speed.  This will effectively break the output power of the engine, and result in a proportional torque through the transducer.  From the speed measurement through a tachometer, the power of the engine can be calculated as: Power = Torque x angular speed.

The main reasons for choosing this direction are cost, simplicity, and accuracy.  A preliminary cost analysis proved that this method would be significantly cheaper than purchasing a dynamometer.  Also, the transducer is marketed to be accurate within .2%.  This is more than a factor of ten less than our required accuracy of 4%.

ii.   Introduction

The main portion of this project will be the research stage.  The chief issue of our project is the determination of which system to use.  A dynamometer is the general term for a device, which has the ability to apply a load to a rotating machine and measure the power output of a rotating machine component.  Considering power is the product of torque and rotational speed, a dynamometer must consist of components to measure both.

Angular velocity is measured through a device known as a tachometer.  There are numerous types of tachometers.  One such device is a small electric generator.  The voltage created is related linearly to the shaft speed and can be calibrated.  Another useful tachometer is a magnetic pickup.  In this case, a permanents magnet is oriented beside a shaft.  If a piece of iron or other magnetic material is placed at some point on the shaft, a voltage peak will be created as the magnetic piece approaches the permanent magnet.  As the piece passes, a negative voltage peak will occur.  The frequency of peaks, therefore, is directly related to the rotational speed of the shaft.

Other forms of tachometers are the stroboscope and photoelectric tachometer.  Both are generally hand-held.  A stroboscope flashes a pulse of light calibrated to an rpm setting.  The shaft speed can be determined by matching the light flashes to a mark on the shaft.  Equivalence is achieved when the shaft “appears” to be stationery and only one mark can be seen.  A photoelectric tachometer senses light reflected.  If a piece of reflecting tape is placed on the shaft, speed can be determined by correlating pulses.

The other aspect of a dynamometer is torque measurement.  In most cases, a controlled torque is applied to the rotating shaft.  Therefore, a dynamometer must provide a method to apply a load and a component to measure the torque. This is accomplished by a brake or absorbing force

                 One common example of a brake is the hysteresis model.  This brake is able to provide precise resistance independent of speed.  This brake works by the creation of magnetic flux in the air gap between the rotor and the pole structure as shown.  It has an effective speed range of approximately 1000-30000 rpm with an accuracy of +/- .5%

Figure #1: A hysteresis brake is shown with all components labeled.  The torque is produced by magnetic flux between the magnetic pole structure and the rotor (Courtesy of MAGTROL).

Simpler brake models include a friction belt.  A friction belt may not be as accurate however, because the force of friction is a function of rotational velocity.  This would result in the need to calibrate whenever it is desired to run the engine at different speeds.

A DC motor or generator can also be used as a brake.  An electric motor, controlled at a preset speed will act to dissipate the power of engine if the engine is pending a higher speed.  This “absorbed” power can be dissipated through a bank of resistors.  The motor thus acts as a generator and puts a torque on the shaft.  A generator would perform the same function.

          In addition to a braking component, there must be a means for calibration and measurement of the applied torque.  A dial weight dynamometer is of the simplest type and generally low-cost.  It is usually used for applications of low torque.  This dynamometer may the hysteresis brake as discussed earlier or a DC motor.  Consider such a device, suspended on bearings.  When breaking force is applied, the device will tend to rotate in reaction.  Now consider an arm connected to the device with a test weight at some radial constant distance.  The mass-arm system creates a torque opposing the motion of the device.  Furthermore, the torque created is the cross product of the weight and the moment arm:

Torque = F x r

As a result, the torque will increase and peak when the weight is directly horizontal to the ground.  If the proper weight is used, the device will reach an equilibrium point and cease movement.  This is where the torque of the weight equals the torque created by the device.  Simply knowing the mass of the weight and the angular displacement, a calibration can be developed to determine the torque.

Figure #2: Eddy-Current Dynamometer

Figure #3: Setup of the dial weight dynamometer.  Various weights can be used for calibration at different operating speeds (Courtesy of MAGTROL)

Hysteresis dynamometers measure torque by means of a brake.  The brake is able to provide precise resistance independent of speed.  This brake works by the creation of magnetic flux in the air gap between the rotor and the pole structure as shown in Figure #4.  It has an effective speed range of approximately 1000 to 30000 RPM with an accuracy of +/- 0.5%

        Figure #4: Model HD-500 Hysteresis Dynamometer

It may be noted that the dial-weight assembly is bulky, and proper weights must be used at different speeds.  One alternative to this would be a rotary torque transducer.  This is an electronic component which is coupled to the DC motor or generator and to the engine.  The transducer is basically a test segment of shaft with known stiffness.  A pair of strain gauges accurately measures the angular deflection of the shaft segment.  From the electronic measurement of the deflection, the torque can be determined as follows:

T = ΘJG/l

Where Θ = deflection, T = Torque applied, l  = length,

J = polar area moment of inertia, G  = modulus of rigidity

Note that the moment of inertia and the modulus of rigidity (stiffness) are both constant material properties. The length is a constant obtained from the specifications of the transducer. 

Electric components such as motors and transducers must be controlled and powered.  These costs must be taken into consideration.  In summation, there is a wide variety of possible arrangements.  The search for the most efficient and cost-effective will be our greatest task, considering that underlying function of dynamometers is not complicated.

For student demonstrations, Professor Horatio Perez-Blanco runs a two horsepower Wisconsin Robin Model W1-080 one-cylinder spark ignited engine.  The Professor wishes to use this engine to illustrate the spark ignition cycle and the effects of timing on power output. 

Previously, a motorized THRIGE-SCOTT 1.5 kW capacity, shunt LAK 100-A dynamometer was employed for this purpose.  This dynamometer was inaccurate and difficult to calibrate.

The pressure-Volume Diagram in the Appendix demonstrates first, the textbook-like pressure-volume curves, which illustrate the success of the pressure transducer system. The following Figure #6shows the comparison between power readings taken from the current dynamometer system versus values calculated from the pressure-volume curves.

Figure #6: This figure compares the power output of the original dynamometer to the power calculated from the previous p-V curve.  The results show that the dynamometer output is significantly larger than reference.

Unfortunately, the current dynamometer is in use therefore it must remain in tact while a new system is under construction.

Our objective is to accurately measure power output of a standard two horsepower, one-cylinder test engine.  The final product must be easy and safe for students to operate and collect data from.

Design Specifications

A.     The purchase of a new dynamometer.

a.       Two types of new dynamometers were considered

                                                                         i.      Eddy-Current dynamometer           

                                                                         ii.  Dial Weight Dynamometer

b.      Why this approach was avoided

                                                                           i.The purchase of a new dynamometer exceeds budget limitations.

                                                                         ii.      The sponsor did not prefer this approach.

B.     Repair of existing dynamometer

a.       Calibration consists of hanging several weights on a specified moment arm. 

b.      Why this approach was not used

                                                                           i.      Numerous attempts at fixing this dynamometer have been made to the point of deeming it not viable. 

                                                                         ii.      The demonstration is in use currently and it would not be possible to take it apart to work on it.

                                                                        iii.      In a real-world situation, our time is costly. If we for some reason are not able to fix the dynamometer, the entire time we spent on it is wasted.

C.     Build DC Cradle Dynamometer from scratch

a.       Very likely to be more expensive than purchased as a whole

b.      Accuracy may vary depending on workmanship

IV.              Justification

The process of justifying our choice can be summed up in the following design    matrix.  We chose the following criteria to measure the effectiveness of each individual alternative.  Each category is weighted appropriately. Cost, as can be seen, is weighted the highest at .25.  This is because we have a limited budget, and cannot attempt projects that may be out of our financial reach.   Customer preference and accuracy are also weighted high, because the system must absolutely function within the allotted accuracy range, if anything.  Also, no one understands the needs of the customer better than the customer himself.  Low on the list are simplicity, ease of use, and weight/size.  This is not a racecar.  It does not necessarily need to satisfy weight requirements.  The only conceivable constraint would be portability.  The following figure illustrates this comparison.