a low-cost efi engine dynamometer part 1 – design and

A Low-Cost EFI Engine Dynamometer Part 1 – Design and Construction

Andy Moore, C.J. Fisher, Pat Crosby Dr. Wayne Helmer, Dr. Chih-Hao Wu

Mechanical Engineering/Electrical Engineering

Arkansas Tech University Russellville, Arkansas

Abstract

The purpose of this project is to design and fabricate an electronic fuel injection (EFI) engine dynamometer using standard, low-cost components. Energy conversion devices are a main component in any mechanical or electrical engineering department. Energy conversion devices such as internal combustion engines usually require expensive dynamometers to accomplish performance testing. A 5 hp internal combustion engine was modified to run with a Megasquirt programmable EFI system. The engine is coupled to a centrifugal water pump provides the basis of the system. The ideal work produced by the pump can be calculated knowing the pressure drop across the pump and the water flow rate. These two measurements can be performed rather easily with inexpensive equipment. This equipment can be used in upper and lower level engineering courses. The total cost of this test apparatus is less than $2000. Similar commercial systems such as these would cost well over $20,000. This lab equipment was designed to be used by the engineering students in courses ranging from the freshman introductory class through senior level courses. Nomenclature

C = electrical capacitance h = u + pv = enthalpy H = head in feet

HP = horsepower ke = kinetic energy per unit mass

"Proceedings of the 2005 Midwest Section Conference of the American Society for Engineering Education"

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m = mass flow rate pe = potential energy per unit mass p = pressure ΔP = pressure differential in psi Q = heat transfer rate Qf = flow rate in gpm R = electrical resistance u = internal energy per unit mass v = specific volume V = volumetric flowrate W = mechanical work rate done by the control volume τ = R*C = time constant

Introduction

A test facility was constructed to provide an opportunity for engineering students at Arkansas Tech University to study the fundamental components of an electronic fuel injection fuel injection system similar to those used in modern automobile engines [1]. In this system, both the mechanical and electrical components of the system needed to be available for study, including the following: the fuel injector, fuel lines, fuel pump, electronic control module, automotive relays, and multiple sensors.

The specifications for this test station included that it act as a dynamometer by

measuring the generated horsepower from the engine. Also, the station was to allow for different selectable engine speeds. The ultimate purpose for the station was that it be used as an instructional test facility to acquaint students with electronic fuel injection and IC engine performance characteristics. The first phase of this research describes the design and construction of the low-cost EFI dynamometer system using the Megasquirt programmable engine computer [2],[3],[4]. Dynamometer Theory

A dynamometer is a device used to measure engine performance. To develop the performance equation consider the steady state energy equation for an open system written on a rate basis assuming that there is only one flow stream where Qin is positive and Win is positive:

Qin + Wiu + m(hin + ke in + pe in) = m(hout + ke out + pe out) [1]

If the control volume is applied to a pump (or work-input device) assuming that

the temperature change of the fluid is negligible, the control volume is essential adiabatic with small changes in velocity and height then the final equation for the work rate input is

Win = m vΔP = VΔP [2]

"Proceedings of the 2005 Midwest Section Conference of the American Society for

Engineering Education"

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The conversion from mechanical work to pressure energy is not 100% with some of the mechanical energy being converted to heat through friction.. These losses are accounted by having the right hand side of equation 4 be divided by an efficiency[5] . The engine used for this test station was purchased as a 5-hp carburetion engine with a water pump attachment. The water pump was used to create water flow, allowing for horsepower to be determined from the flow rate and pressure differential. The flow rate was measured from a flow meter in gallons per minute (GPM) and the pressure differential was measured from a pressure differential gauge with the low side before the pump and the high side after the pump. The pressure differential was converted to head using Equation 3. H = ΔP x 2.307, [3] The horsepower was determined using Equation 4.

HP = (Qf x H)/3960, [4]

With no known pump efficiency, the actual engine horsepower was

indeterminable, but horsepower from one specific engine/pump does combination should have the similar efficiency characteristics.

The water flow rate through the pump was measured with a calibrated rotameter in the range of 5-50 GPM. Water pressure drop data was taken using a differential pressure meter (5-35 psi).

The horsepower was measured for the engine while carbureted, before any conversions or modifications were made. The data for the horsepower was compared to the data provided from the manufacturer and the measured data was comparable at maximum engine speed. Electronic Fuel Injection Theory

Effectively converting a carbureted engine to an electronic fuel-injected engine requires the input of several sensors and a supply of pressurized fuel. The electronic control module (ECM) receives inputs from the sensors and processes them to determine the best output, that is, fuel quantity supply. The required sensors include the throttle position sensor, oxygen (O2) sensor, ambient air temperature sensor, engine coolant temperature sensor, and tachometer sensor. With the input of these sensors, the ECM can calculate the appropriate time duration for the fuel injector to operate. The fuel injector squirts fuel by activating a solenoid in the injector, which opens a valve through which pressurized fuel is ejected. The fuel is pressurized by an electric fuel pump, increasing the pressure of the fuel to a known value, which is a constant in the ECM algorithm. Most automobile EFI systems have non-programmable engine control strategies in the ECM. The advantage of the Megasquirt system is that many of the control variables can be changed even as the engine is running.



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Since carburetion engines are not typically equipped with the appropriate sensors or electric fuel pumps, these components must be installed on the engine. Once installed and tested for functionality, they also must be calibrated to provide the correct information to the ECM. For instance, automotive temperature sensors (thermisters) determine temperature based on measured resistance; as the temperature increases, the resistance decreases. At least thirty-two different types of thermisters are in production, with thirty-two different characteristic curves for temperature versus resistance. To account for the differences in sensor types, computer programs have been developed to adjust the inputs based on tests of three data points (ice, boiling and room temperatures of water) taken using the thermister. From these data points, the appropriate curve can be created using a general equation for the function of thermisters, sending proper temperature readings to the ECM. Calibrated thermistors were installed on the engine test stand. Other sensors may also need calibration for the ECM to work properly. Once all the sensors are adjusted and the ECM is reading inputs properly, it can send suitable outputs to the fuel injectors. The engine can then be tested to ensure the desired performance is achieved. Engine Speed Sensor Once the preliminary engine tests were completed, the tachometer sensor was designed. The tachometer functionality was essential to the operation of the ECM, therefore, it was to be built and tested on the original engine before dismantling and modifying the engine.

The tachometer design required the provision of a square wave longer than 100 microseconds for the Megasquirt board to detect. This required a pulse to be generated from some part of the motor that triggered for every crankshaft revolution. This pulse then was to be filtered and sent to a multivibrator which would convert it from an analog signal to a digital square-wave signal long enough for the Megasquirt board to detect.

The first step was to determine a viable source to detect crankshaft revolutions. Two main ideas were considered for this: installing a sensor that generated a pulse for each revolution, or capturing a signal from the ignition system to create a pulse for each spark plug firing. Both ideas had advantages and disadvantages. Installing a sensor on the crankshaft would have been more expensive, and involved designing a means to hold the sensor in place. However, the output signal from the crankshaft would be very clean and require minimal filtering before sending to the Megasquirt board. Inducing a pulse from the spark plug wire would involve minimal cost, but more hardware would be required to filter the signal and create a square pulse for the board. This induction coil design was the more advantageous design for this test station based on lower cost and less mechanical modification.



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By wrapping a small coil of wire around the spark plug wire, a current was induced through the wire by means of a magnetic field, thereby creating a pulse. For the first test one end of the wire was connected to engine ground and the other sent to the filter. Figure 1 shows this signal wave form before filtering. The signal period was around 40 nanoseconds (ns), which was too small for detection in the Megasquirt ECM.

A second tachometer design was implemented by disconnecting the coil from

engine ground and connecting one end to the signal filter. Figure 2 shows this signal wave form before filtering. This method increased the period from 40 ns to almost 100 microseconds (μs), which is much more suitable for detection in the ECM. The signal is more functional with the inductor in this arrangement because the current has only one way to flow to engine ground. With both ends of the induced coil connected, the pulse shorted itself because two outlets were available for current flow.

The generated signal from the inductor was then sent to a filtering circuit in order to provide the desired signal to the ECM. Several different filter designs were considered and tested. The preliminary filters considered were half-wave and full-wave rectifiers. These filters responded well to signals from a standard signal generator, but did not respond well to the signal from the induction coil. The full-wave and half-wave rectifier filters failed to provide adequate signal to trigger the chip (see Figures 3-5). However, a simple low-pass filter was designed and implemented (see Figure 6), which correctly processed the signal and send a usable signal to the Megasquirt ECM. The tachometer sensor was created by wrapping 15 coils of 20 gauge wire around the spark plug wire. When the spark plug fired, a current was induced in the coil, resulting in a peak voltage ranging from +50 volts (V) to –86 V (see Figure 2). With these data, values for the resistor and capacitor were chosen for the low-pass filter using the time constant formula presented in Equation 5.

τ = R*C [5] With the peak engine rpm at approximately 3600, the frequency (ƒ) was determined to be 5 kHz, allowing for some leniency in the higher rpm range. Since the time constant is the inverse of the frequency, the time constant was determined to be

ohm-farads (Ω-F). A capacitor value of 0.1 μF was chosen, resulting in a resistor value of 2 kΩ. These calculations are presented in Figure 7. This tachometer and filter design yielded graphical data which is presented in Figure 6.

4102 −×

The signal processing section of the tachometer sensor included a multivibrator which converted the signal from analog to digital. The multivibrator used was model 74LS122, which is a retriggerable multivibrator. This chip model had four inputs; a regular output and a negated output (see Figure 8). The regular output was chosen so that the chip would send a signal from the initial detection of each received pulse. The logic of the chip is presented in Figure 8. The underlined section of the logic table shown here was chosen for this application. The pin connection schematic is provided in



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Figure 9. This chip also had the option of creating an external timing device using a capacitor and resistor. A resistor value of 240 kΩ and capacitor value of 1000 pF yielded a total signal time of 1.4 ms, which was within the allowable constraints of the Megasquirt ECM. A graph of this output is located in Figure 9. The diagram of the Megasquirt ECM external wiring is shown in Figure 10. Throttle Body With the tachometer sensor designed and functional, the engine was then modified by removing the air filter and carburetor. A throttle body was mounted in place of the carburetor using a mounting bracket (see Figure 11). Different mounting designs were considered ranging from a simple metal plate to the final design of an L-shaped pipe arrangement. The final design was chosen based on the ease of assembly and manipulation. The base plate was fitted to the intake manifold of the engine and was shaped to fit into the limited space with an appropriate seal. The mounting plate was also shaped to fit the throttle body more securely and gasket material was cut to make a secure seal. The throttle body was attached to the mounting bracket using two bolts with nuts and washers. A manifold air pressure tap was mounted to the throttle body base. Fuel Pump

An electric automobile fuel pump was mounted to the base of the test apparatus with fuel lines running from the fuel tank to a fuel filter, through the pump and to the intake of the throttle body (see Figure 12). The return fuel line was run from the return fuel port on the throttle body to a hole drilled in the top of the fuel tank. All of the hoses were secured with hose clamps and all copper and steel tubing was connected with ferrule fittings. Oxygen Sensor

A hole was drilled in the exhaust pipe and a mounting bung was welded to the top of the pipe to mount the O2 sensor. The O2 sensor was screwed into the hole and into the exiting exhaust air stream (see Figure 13). Using a one-wire, non-heated O2 sensor required the exhaust pipe to be insulated to keep the temperature of the exiting gases at the desired temperature. The pipe was insulated by wrapping woven insulation fiber around the exposed exhaust pipe. Temperature Sensors

For temperature sensors, both ambient air and coolant temperatures were measured using thermisters. As mentioned earlier, the thermisters were calibrated by measuring the ice, boiling and room temperatures for water and measuring the resistance of the thermisters at each temperature. These three data points were entered into the EasyTherm computer program which adjusted thermister readings for non-standard thermisters. The coolant temperature sensor was mounted to the engine block fins using



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epoxy compound and the ambient air temperature sensor was simply placed in open air in the vicinity of the air intake of the throttle body. The sensors were then connected to a screw terminal and then on to the Megasquirt ECM, which processed and controlled the electronic fuel injection system. The ECM had an output to a laptop display which allowed for interactive control of the fuel injection system, with displays including engine temperature, throttle position, engine speed (in rpm), and input controls including fuel pump pressure, number of fuel injectors, and engine displacement. Once all the sensors were connected to the Megasquirt ECM, the system was debugged. Problems arose in the tachometer circuit as the voltage regulator was improperly grounded and overheated. The regulator was replaced with an 820 ohm resistor which supplied appropriate voltage and current to the chip. Also, the zener diode was placed in parallel with the low-pass filter, which did not create the correct pulse shape. The diode was then placed in front of the filter, creating the desired pulse shape for the ECM. Results

The horsepower was measured for the engine while carbureted before any conversions or modifications were made. The horsepower-pressure drop data compared well with the data provided from the manufacturer. See Figure 14.

With the EFI sensors working as desired, the engine was started. The settings on

the computer were reading rpm two times the actual speed, so the input parameters of the computer were changed to accurately read the engine speed. Once the tachometer input was corrected, the engine was shown to be running at around 3500 rpm. This was near the maximum rpm of the original engine. The engine idled at this 3500 rpm speed, which was undesirable since the speed could not be lowered, yet was at its maximum limit.

The idle speed was lowered by entering different parameters into the ECM. Originally, the program was set for a high impedance injector, yet the injector in the throttle body was low impedance. Correcting for this factor reduced the idle speed to 3000 rpm. Also, the pulse width modulation (PWM) threshold was set to 1.0 ms and the current limit was set to 75%. The required fuel (RF) was also reduced from 3.3 ms open to 1.2 ms open, which reduced the time of each fuel squirt. While the engine was running, the PWM current limit was slowly reduced from 75% to 46%. At this point, the mixture became too lean for the engine to operate. Engine operation at a PWM current limit of 50% appeared to offer optimum engine efficiency. Cost Professional-grade dynamometers typically cost between $20,000 and $80,000, yet this dynamometer test station cost less than $1300. The low cost of such a test facility is beneficial for smaller or more limited institutions to develop a test station



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without the monetary burden. Commercially-purchased dynamometers can produce pressure-volume plots, have interfaces for packages such as Labview and Matlab and offer the possibility for monitoring other exhaust gas emissions. Although this current test station is not as accurate or versatile as the more expensive dynamometers, it is ideal for educational purposes as it demonstrates applications of horsepower, fuel injection, and signal processing. The primary costs for this test facility were the engine, ECM, flow meter, and pressure gauge. Table 1 provides a tabulated cost analysis of the test facility.

Table 1: Cost Analysis of Electronic Fuel Injection Test Facility Item Price ($)

Megasquirt ECM 325.00 Simulator kit 47.00

Briggs and Stratton Engine 250.00 Throttle Body Injector 16.00

Pressure Gauge 110.00 Flow Meter 196.00

Water Storage Tank 49.00 Steel 40.80

Pipe Fittings 73.18 Casters 85.00

Oxygen Sensor 26.35 Welding 16.28

Electronic Connectors 20.00 Insulation 9.50

Hose clamps 10.62 O-rings 5.00 Total 1279.73

Typical Dynamometer Facility At least 20,000

Savings 18,700 Conclusions

The engine conversion was a success and the construction of the electronic fuel injection test facility was completed. Initial horsepower-pressure drop data compared well with the data provided from the manufacturer. The objectives of creating a working, controllable, electronic fuel injection dynamometer test facility were accomplished, while keeping the cost to a reasonable amount. In the next phase of this research program performance data on the Megasquirt EFI dynamometer system will taken and evaluated.



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References

1. A Low-Cost Electronic Fuel Injection Test Facility. Senior Design Project. CJ Fisher, Andy Moore. Pat Crosby. Arkansas Tech University. Russellville Arkansas. May 2005.

2. http://www.bgsoflex.com/megasquirt.html 3. http://www.diyautotune.com/ 4. http://megasquirt.info/ 5. Pumps and Pump Systems. Wen-Yung Chan and Milton Meckler. American

Society of Plumbing Engineers. 1983. p. 3-8. Biographical Information C. J. Fisher is a senior electrical engineering student at Arkansas Tech University. Andrew Moore is a mechanical engineering graduate of Arkansas Tech University. Pat Crosby is a mechanical engineering graduate of Arkansas Tech University and an engineer at Cooling & Applied Technologies in Russellville Arkansas. Dr. Chih-Hao Wu is an assistant professor in the electrical engineering department at Arkansas Tech University. Dr. Wu received his B.S., M.S., and Ph.D. all from University of Texas at Arlington, Arlington, Texas, Department of Electrical of Engineering, at 1995, 1996, 2000, respectively. He joined the faculty of Arkansas Tech University Department of Electrical Engineering in 2003. Dr. Wu is a current member and vice president of IEEE Arkansas River Valley section. Dr. Wayne Helmer is a professor in the mechanical engineering department at Arkansas Tech University and the Affiliate Director for Project Lead the Way in the state of Arkansas. He has performed consulting work for NASA and the USDA Forest Service and has over 30 years of experience in engineering education.



http://www.bgsoflex.com/megasquirt.html

http://www.diyautotune.com/

http://megasquirt.info/

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Figure 1. Induction Coil Pulse

Tachometer pulse (Connected to engine ground)

Schematic

0

To Oscilloscope

Induced Coil

1

2 To Oscilloscope



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Figure 2. Tachometer Pulse (Second Attempt)

Schematic

0

To Oscilloscope

Induced Coil

1

2 To Oscilloscope



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Figure 3. Tach Filter Schematics and Graphs

Full-Wave Rectifier

Schematic

Induced Current

1

2

To Oscilloscope

2D1N914

To Oscilloscope

4D1N9141D1N914

0

3D1N914

R1



13

Figure 4. Full-Wave Rectifier with 5.6 Zener Diode

Schematic

5.6V

D1N750

100k

To Oscilloscope

2D1N914

To Oscilloscope

Induced Current

1

2 4D1N914

0

1D1N914

3D1N914



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Figure 5. Half-Wave Rectifier Schematic:

Induced Coil

1

2D1

D1N914

To Oscilloscope

To Oscilloscope100k

5.6VD1N750

.1uF

Note: Data corrupted on diskette and have not been able to retrieve. However, peak voltage was only 400 mV, so this filter did not work.



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Figure 6. 5 KHz Low-Pass Filter

Schematic

5.6V

D1N750

To Oscilloscope

0.1uFInduced Coil

1

2

To Oscilloscope

2k



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Figure 7. Thermister Value Table and Time Constant Calculations Thermister Values for Ambient Air Temperature and Block Temperature: Temperature (in Fahrenheit) Resistance (in Ohms) 32 226.2 77 77.4 212 10.5 Determining the Time Constant (τ) Equations:

1) τ = R*C 2) ƒ = 1/τ

Frequency Assigned: 3) ƒ = 5 KHz

Equation Worked: 4) τ = 1/ƒ 5) τ = 1/5 KHz 6) τ = 2*10-4 7) R*C = 2*10-4

Capacitor Assigned: 8) C = 0.1 μF

Resistor Solved: 9) R = τ/C 10) R = (2*10-4)/(0.1*10-6) 11) R = 2 kΩ



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Figure 8. 74LS122 Multivibrator (all information provided by Texas Instruments)

Schematic

Truth Table



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Figure 8 (cont.) Recommended Operating Conditions

Proceedings of the 2005 Midwest Section Conference of the American Society for Engineering Education

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Figure 9. 74LS122 Schematic and Graph

Pulse from 74LS122 (Retriggerable Multivibrator)

Schematic

240k

1000pF

From CoilVcc

U1

74LS122

5

68

11

13

24

913

CLR

QQ

C

A1B1

A2B2

RRC

To Megasquirt


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Figure 10. Schematic for Wiring to Megasquirt (Provided by Megasquirt Manual)

Original:

Modified:


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Figure 11. Throttle body and Mounting Bracket

Figure 12. Electric Fuel Pump

Figure 13. O2 Sensor


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Figure 14. Horsepower Performance Data

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

0 10 20 30 40 50

ΔP (psi)

Horsepower (HP)

Manufacturer's Horsepower Data HP at 3800 rpm

HP at 3500 rpm

HP at 3000 rpm

Hp at 2200 rpm


a low-cost efi engine dynamometer part 1 – design and

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