Non-Point Source Polluting Lawn Mower

Benjamin GrahamAndrew SmithMichael Smith

University of Maine

ABSTRACT

Lawn mowers and other lawn care equipment are some of the biggest causes of pollution on the market. This project strives to find a suitable power alternative for a lawn mower which will have satisfactory performance without emitting the large amount of pollution produced by a gas driven mower. A new mower has been produced which includes an efficient motor, a removable battery pack which holds high capacity batteries, and a new blade with reduced power losses due to a “fan effect”.

INTRODUCTION

In the present era, with petroleum usage skyrocketing, global warming has become a major crisis that needs to be dealt with. Petroleum is being used as the major power source for many applications such as automobiles, heating, and the study of this project, lawn care.

“Some older lawn mowers are bigger polluters than the family car,” said James M. Lents, executive officer of Air Quality Management District in Orange County California. “A dirty mower operated for 20 hours a year produces the same amount of smog-forming volatile organic compound pollution as a 1996 passenger car driven for 26,000 miles -- more than most people drive in a year,” Lents said.1

With this being the case, it can be seen that in order to reduce pollution emissions, an alternative needs to be found to power lawn mowers. Whether this is done by using a new technology such as a fuel cell, or by utilizing battery or solar power, this project aims to cut the point-source pollution output by the use of these machines.

OBJECTIVES

Currently, there are models of lawn mowers that are on the commercial market which do not

require the use of gasoline for a power source. Most of these are battery powered and do not have enough power to cut more than a quarter of an acre. For a typical Maine resident, a mower that will only cut a quarter of an acre is not sufficient. The full scope of this project seeks to construct a non-polluting lawn mower which will mow a typically sized Maine lawn before running out of power.

The objectives we are striving to meet are:

Substantial reduction in pollution emissions

Suitable power supply to cut a typical Maine lawn

PROJECT RESULTS

POWER REQUIREMENT

The first thing we needed to determine to create a suitable non-polluting lawn mower was the power requirement needed to cut the average Maine lawn. In order to do this, we performed a test with the stock system. Originally, we were unable to get the lawn mower to run. We dismantled the lawn mower and rewired it with both a voltmeter and an ammeter to check the voltage and current running to the electric motor. We found that there was an initial spike at start up. This spike hit 100 amps before declining. With the mower running without cutting any grass, we found that the amperage needed to spin the blade was 10 amps. Since the stock mower runs on two twelve volt lead acid batteries, this gave us a power requirement without a load of 240 watts. When we took the mower outside, we found that the max amperage needed to run the mower through thick, wet grass was 70 amps. This gave us a max power of 1680 watts.

POWER SOURCE

When seeking to find a suitable power replacement to a gas powered combustion engine to run a mower, we found four possible solutions: a fuel cell, solar power, use of biofuel instead of gasoline, or battery power. There were benefits and disadvantages to each alternative. A design matrix of all four selections is shown in Figure 1.

Fuel Cell

Fuel cells are still in the early stages of development. While fuel cells would provide more than enough power to mow an entire lawn, they are still very expensive. Hydrogen is not yet widely available to consumers so this would also present a problem to the average user, along with the addition of a cumbersome storage tank

for the gas. There are fuel cells that use other substances such as methanol to produce the hydrogen needed, but that causes the production of pollutants in the reformer.

Solar Power

Solar energy is by far the cleanest and most abundant source of energy that could be used. The problem presented with a solar powered lawn mower is obtaining enough energy to mow a typical Maine lawn. Large panels would be needed to harvest this much energy. Also, to create a suitable mower using solar energy, a battery would also be necessary to retain charge collected by the solar panels.

Power Source Weight Fuel Cell Solar Battery Biofuel

Initial Cost 7 1 2 3 4

Point Source Pollutants 10 3 4 4 1

Power 5 3 1 2 4

Weight 3 1 2 3 4

Mow Time 9 3 1 2 4

Fuel Storage 2 1 3 4 2

Fuel Accessibility 6 1 3 4 2

Total 90 98 130 122 Figure 1

Biofuel

Biofuel is created using 85% vegetable oil, and therefore is a renewable source of energy. But while this would reduce emissions, this form of energy does not completely eliminate point-source pollutants from a lawn mower.

Battery

Batteries are widely used and widely available to all. A battery powered dc motor is lighter than a typical combustion engine and much quieter as well. Batteries have been around for a long time and therefore most battery technologies are well developed. One question is whether or not a battery will be able to provide the needed power.

Conclusion

After considering the four alternative power sources we had and utilizing our design matrix, we determined the best power source for that could be used for a lawn mower was an electric battery. Use of a battery will keep the initial cost of the mower down while completely eliminating point-source pollutants and making recharging very easy for consumers.

This choice of course led to the question of which type of battery we should use to power the mower. In order to determine this, we looked into multiple battery chemistries and created a whole other set of design matrices.

BATTERY SELECTION

In order to decide which battery chemistry to use with our lawn mower, we looked at five of the most widely available and widely used battery types. Those types were: nickel-cadmium, nickel-metal-hydride, lithium ion, lithium polymer, and lead acid. Battery selection matrices can be seen in Figures 2 and 3 on the

next two pages. Figure 2 is a quantitative matrix while Figure 3 is a qualitative matrix.

Nickel-Cadmium

Nickel-cadmium batteries are fairly cheap and perform well under heavy work loads. It has a long cycle life, but loses performance if it does not have a charge/discharge cycle at least once every month and contains toxic materials.

Nickel-Metal-Hydride

Nickel-metal hydride batteries are a little more expensive than nickel-cadmium. They have an average cycle life but the life is shortened by heavy loads. It is environmentally friendly and needs a charge/discharge cycle every three months to retain performance.

Lithium Ion

Lithium ion batteries have a very high energy density and are very low maintenance. They are very expensive though, and seem to lose performance quickly, even after one year of use.

Lithium Polymer

Lithium polymer batteries are expensive and have a high energy density, although not as high as lithium ion. They have an average cycle life, but have no memory and are environmentally friendly.

Lead Acid

Lead acid batteries are tried and true. They have a lower cycle life then the others, but are the most inexpensive battery, no memory, a high overcharge tolerance, and the lowest self-discharge. They cannot be stored in a discharged condition and a full discharge can rob the battery of some performance.

Battery Nickel Cadmium Nickel Metal Hydride Lithium Ion Lithium Ion Polymer Lead Acid

Life Expectancy 1000+ cycles 300 - 500 cycles 300 - 500 cycles 300 - 500 cycles 200 - 300 cycles

Charge Time 1 hour 2 - hours 2 - 4 hours 2 - 4 hours 8 - 16 hours

Cost (7.2 V) $50 $60 $100 $100 $25 (6V)

Energy Density 45 - 80 Wh/kg 60 - 120 Wh/kg 110 - 160 Wh/kg 100 - 130 Wh/kg 30 - 50 Wh/kg

Toxicity High Low Low Low High

Memory Yes (CD/month) Yes (CD/3 months) No No No

Overcharge Tolerance Mild Low Very Low Low High

Self - Discharge/month 20% 30% 10% 10% 5%

Additional CommentsPrefers High Discharge, Heavy Loads Heavy Load Shortens Life Deterioration after 1 year Can’t be stored discharged

Good performance at low temps

High temp storage shortens life

Store at 40% charge, low temp to prolong life

Full discharge robs battery capacity

Loses performance without CD/month

Figure 22,3

Battery WeightNickel

CadmiumNickel Metal

Hydride Lithium IonLithium Ion

Polymer Lead Acid

Life Expectancy 9 5 3 3 3 2

Charge Time 7 5 4 4 4 2

Cost (7.2 V) 10 3 2 1 1 5

Energy Density 2 2 3 5 4 1

Toxicity 4 1 3 5 5 1

Memory 8 2 1 5 5 5

Overcharge Tolerance 5 3 2 1 2 5

Self - Discharge/month 3 2 1 4 4 5

Additional Performance 6 5 2 1 1 4

Total 185 126 158 161 192

Figure 3

Conclusion

After weighting the benefits and disadvantages of each battery chemistry and using our battery selection matrices, we determined that either a nickel-cadmium or lead acid battery would work well with our lawn mower. We purchased lead acid batteries to run our lawn mower, and that is what we would choose to sell the mower with were it ever to reach the retail market. Since nickel cadmium seems like a very good choice for batteries as well, we decided that we would market nickel cadmiums as replacement batteries as well as lead acid should a consumer prefer nickel cadmium over lead acid.

DC MOTOR

When performing the preliminary tests on the stock system we were provided with, we found that the mower had a difficult time handling thick or wet grass. The life of the mower was severely reduced when presented with these mowing conditions. Even the owner of the stock mower told us that the mower ran fine with dry

short grass, but could not cut the entire lawn when the conditions were not perfect. After some research to determine what the cause of this was, we found that a typical DC motor runs with a good efficiency when a low torque is applied, but as the torque applied to the system increases, the efficiency of the motor decreases. We concluded that one way to increase the battery life of the mower would be to replace the stock motor with a higher efficiency motor.

Our search led us to a new technology in DC motor design known as a lynch motor. Standard DC motors use stacks of steel laminations which are then wrapped with copper wire and an armature. The lynch motor uses stamped, bent, and coated copper bus bars arranged in a disk as an armature. The disk is then placed within an air gap from permanent neodymium magnets, which are about three times as strong as a ferrite magnet. The result is what’s technically known as a wave wound axial gap brushed DC motor. We found two versions of this type of motor and created a design matrix of lynch and standard dc motors. This can be seen in Figure 4.

Motor Power Price Voltage RPM Weight Amps Efficiency

General Electric Model 5BT1344B133 2HP $200 24V 1050 149 72 82%Briggs and Stratton ETEK 15HP $350 12-50V 3500 20.8 180 90%PERM PMG132 34.3HP $650 24-72V 3500 24.8 110 88%PERM PMG080 3.95HP $465 12-24V 6880 7.5 78 83.20%AST-789-6 Military Surplus 5HP $200 28V 4500 53 140 82%Jack & Heintz AST-639-6 10HP $360 30V 4000 80 ? ?Leeson AST-9112-01 1.5HP $240 24V 1800 32 62 ?

Figure 44,5,6,7

This matrix led to the conclusion that the best motor for our mower would be the Briggs and Stratton ETEK. This motor is light weight, outputs high power, and has a very good efficiency, even at high torques. Figure 5 shows an efficiency versus torque graph and a motor speed versus torque graph for both the ETEK motor and a typical dc motor. As can be seen, the as the torque increases, the efficiency and motor speed of a typical dc motor drastically fall while that of the ETEK motor remains almost constant. This high efficiency will allow the battery to run for a longer period of time on thick, wet grass.

Figure 58

BLADE CONNECTION

One problem we encountered when we purchased the ETEK motor was that the output shaft was designed significantly differently than that of the stock motor we had. The stock motor output shaft was merely a cylinder that was threaded at the bottom. The ETEK output shaft

was a hollow cylinder that was threaded on the inside and had a key on the outside. This forced us to consider new ways of attaching a blade to the motor. The final design we came up with can be seen in Figure 6. Our new blade connection consists of a sleeve that fits on the outside of the output shaft. Connected to this with 4 screws is rectangular piece which holds the mower blade in place as it rotates. A bolt with a washer runs through the whole apparatus to secure it all to the motor.

Figure 6

SYSTEM VOLTAGE

After having purchased the Briggs and Stratton ETEK motor, we needed to decide what voltage to run the mower at. The ETEK motor had a voltage constant of 72 rpms per volt. A typical gas lawn mower runs at a speed of around 3000 rpm, so our engine speed needed to be close to that. With that information, we were constrained to either 36 volts, which would give us 2600 rpm, or 48 volts, which would give us 3500 rpm. We purchased four 12 volt lead acid

batteries to test the motor with. We made sure to obtain batteries with a high capacity: 21 amp-hours. This is an increase of about 4 amp-hours over the stock model batteries. These batteries would give us a longer mowing time then those provided with the stock system. A picture of the batteries can be seen in Figure 7 and a discharge versus time graph of the batteries can be seen in Figure 8. We tested the ETEK motor at 36 and 48 volts to see how the motor performed at both voltages. At 36 volts, the motor performed well

Figure 7

Figure 89

with satisfactory results. At 48 volts, the motor performance was suitable, but the running mower was louder than what we were expecting, and the power of the system seemed to be too much. The front wheels of the mower would actually jump to the right on start up at this voltage. Due to this effect, we decided to run the motor at 36 volts, using three of the 12

volt lead acid batteries in series to reach this voltage.

REMOVABLE BATTERY BOX

One of the major goals of this project once it had been determined that battery power would be used was to create a removable battery box. This would allow consumers who had considerably large lawns to easily replace drained batteries with fresh ones so the lawn could be finished without needing to wait for drained batteries to recharge. With it now decided that we would be using three 12 volt lead acid batteries to power the lawn mower, we looked at our options for creating a removable battery box. Originally we had planned on fabricating our own box to contain the batteries, but in our searches, we came across an existing battery box. This box was created for use in marine settings, but we determined that it would be suitable for our purposes. A picture of the battery box can be seen in Figure 9. Along with

the battery box, we needed to find a simple circuit connector that would allow for easy disconnection and removal of the battery box. What we found was a North American style battery charging cable connector. A picture of a North American style connector is shown in Figure 10. The two wires from the circuit are soldered and crimped onto two metal prongs that fit snuggly into the connector. Two of these are used, one on the battery side of the circuit,

the other on the opposing side. The connector on the battery side of the circuit hangs outside of the battery box, making it accessible for disconnection. To remove the battery box, simply pull apart the connectors and lift the box out of the lawn mower. To run the lawn mower again, replace the battery box and reconnect the two connectors.

Were this lawn mower to go into production, we would market two different kinds of battery boxes to supplement the lawn mower. One would be the same as the one offered with the lawn mower itself: a 36 volt power source comprised of multiple lead acid batteries. The other would be a 36 volt power source comprised of multiple nickel cadmium batteries. Since both types of batteries were found to be relatively similar when used in this type of application from our battery matrix, the offering of both types of battery boxes would allow the consumer to determine which battery chemistry they preferred to use and have as a backup for their lawn mower.

BATTERY CHARGER

The stock model lawn mower we first began with had an internal battery charger. Since we had a removable battery box, we determined an external battery charger would be better suited for the new mower. We purchased a 36 volt battery charger which would be sold with the lawn mower. The battery charger had a 3 prong female tip which was incompatible with any of the connections we had at the time. To solve this problem, we found a 3 prong male tip / North American connection adapter. The adapter would come standard with the lawn mower. To

charge the batteries, the consumer only needs to disconnect and remove the battery box from the mower and plug in the charger into the same connection used to connect the batteries to the lawn mower.

BLADE SELECTION

Over the course of the first semester, we had an experienced design engineer overlook the project. He brought to out attention the fan effect of the blade. The fan created by the blade while cutting the grass leads to a loss in power. To measure this effect we ran a test of the motor without the blade attached, to determine the amount of power lost due to just the blade. From the previous power requirement test, we had that the initial spike was 100 amps and the running amperage was 10 amps. When the blade was removed, we measured the initial spike at 15 amps, while the running amperage was only 1.5 amps. This leads to the conclusion that the power lost to the fan effect of the blade was more than 200 watts. This seems like a huge loss of power simply to rotate the blade.

In order to lessen the amount of power loss due to the fan effect of the blade, we first decided to make a lawn mower that only mulches. The stock model was designed to throw grass clippings out the back into a bag. This is one reason why the fan effect of the stock model is so high. A mulching mower repeatedly cuts grass clippings to miniscule pieces to be left on the lawn. This would reduce the fan effect loses of the blade. To find the best blade for this we decided to run an experiment to test varying blade designs and the losses associated with each design. Because we could not produce our own blade, we purchased multiple mulching blades with differing designs for testing.

We spoke with Professor Bhaganagar about how we could test and find the blade with the least amount of fan associated losses. What she told us was that we needed to find the pressure

variation along the length of each blade. The blade with the smallest pressure differences would be the blade with the least amount of fan related losses.

To run an experiment that measured this, we took one of the stock models that we had and drilled 8 holes along the bottom as pressure taps. We ran each blade at 36 volts and took pressure readings at each tap. This gave us the pressure profile of each blade. For the full lab report and results associated with this experiment, refer to Appendix A.

The results of this experiment showed us that blade 3379 had the smallest pressure difference associated with it. This would be the blade that would give us the smallest amount of power loss due to a “fan effect”.

STOCK PARTS

Although we improved upon many aspects of the Black & Decker mower we began with, we found that there were pieces that we could reuse in order to complete our finalized lawn mower. We reused the body of the stock model on our own mower. The design of the body could satisfactorily accommodate the changes we made to the system with only a few modifications: remounting the new dc motor we purchased, creating a level surface for placement of the battery box, and making room for the battery box in the upper portion of the chassis. We also applied a new coat of paint to the body. In addition to this, we felt the height adjustment on the stock model body was very well designed and made for very quick and easy height adjustment. Along with the body, we reused the start and safety switch that was used on the stock model. The start switch was simply a push lever. The safety switch was just like the safety switches commonly seen on most lawn mowers: a secondary bar that needed to be held against the handle bar to start and run, which would stop the lawn mower when released. The

only change we made to these was the position of the circuit board. The removable battery box took up the room where the circuit board had original been placed. We moved the board to an area underneath where the battery box stood, an area originally used to shed grass clippings which was now no longer being used.

FURTHER PRODUCTION WORK

If we were planning on actually marketing the lawn mower, there would be a couple things we would make changes to before selling the model. One is the position of the circuit board. We would like to place the circuit board at the front of the lawn mower where there is plenty of open room and it could easily be housed. It is not placed there now because since we decided to use the stock model start and safety switch, we were constrained by the length of the cable on the switch. The stock model was designed with the circuit board towards the back of the body and the cable would not reach any further than that distance. The other change we would make is the design of the underside of the body where the blade is positioned. The stock model was designed with a discharge bag for clippings and the aerodynamics of the underside was designed to throw clippings out the back. This can be seen in the pressure profiles found in our experiment. Since we decided to create a mower that only mulches, the stock design of the underside leads to losses that wouldn’t be present in a new design created only for mulching grass.

REFERENCES

1. http://www.aqmd.gov/news/scrap1.html

2. http://www.batteryuniversity.com/index.htm

3. Linden, David, ed. Handbook of batteries. 2nd ed. New York: McGraw-Hill, 1995.

4. www.surpluscenter.com

5. www.ebay.com

6. www.enigmaindustries.com

7. 3rivers.net/~cmac

8. http://www.robotmarketplace.com/ marketplace_etek.html

9. http://www.power-sonic.com/psh-12180.pdf

10. http://www.marinegeneral.com/acatalog/ Online_Catalog_Battery_Boxes__Trays ___Accessories_306.html

11. http://www.mcmaster.com/

Appendix A

Pressure Variation Along a Lawn Mower Blade

04/03/2006Crosby Laboratory

University of Maine

By:Benjamin Graham

Andrew SmithMichael Smith

Assisted By:Professor Bhaganagar

Blade Pressure Variation Page 2

Object

A test of the pressure variation along multiple lawn mower blades was conducted in order to find the lawn mower blade with the smallest pressure differences. This would give us the information needed to find the blade with the smallest amount of losses due to a “fan effect”. The following results were determined:

Pressure values along the length of each blade Blade with the minimum pressure difference

Apparatus, Equipment, and Instrumentation

Figure 11, below, shows a bird’s eye view of the placement of the pressure taps. Figure 12 shows the entire experimental setup.

Test Item Identification

1. Pressure Transducer – Sentra Model 239, Range 0-15 WC, Excitation 22-30V2. Multimeter – Micronta Model 22-195A3. DC Power Supply – Aglient E3617A, 0-60V, 0-1A

Blade Pressure Variation Page 3

Results

Table 1 shows the voltage readings obtained at each pressure tap for each blade tested.

Blade # P1(mV) P2 P3 P4 P5 P6 P7 P86294 270 185 225 200 -4 -6.7 183 1726309 200 210 210 205 -6 -5.1 190 1703361 245 215 240 220 -5.4 -11 210 1853370 290 210 255 210 -6.5 -7 210 1903379 130 140 185 147 -16 -6 158 125

stock 155 175 205 220 -15 -5.1 130 130Table 1

Table 2 shows the calculated pressure reading at each pressure tap for each blade tested.

Blade # P1(in_H2O) P2 P3 P4 P5 P6 P7 P86294 0.4361 0.397 0.4154 0.4039 0.31006 0.308818 0.39608 0.391026309 0.4039 0.4085 0.4085 0.4062 0.30914 0.309554 0.3993 0.39013361 0.4246 0.4108 0.4223 0.4131 0.309416 0.30684 0.4085 0.3973370 0.4453 0.4085 0.4292 0.4085 0.30891 0.30868 0.4085 0.39933379 0.3717 0.3763 0.397 0.37952 0.30454 0.30914 0.38458 0.3694

stock 0.3832 0.3924 0.4062 0.4131 0.305 0.309554 0.3717 0.3717Table 2

A graph of the pressure values along the length of each blade is shown in Figure 13 in Appendix B. A graph of the maximum pressure difference for each blade can be seen in Figure 14 in Appendix B.

Conclusion

After running this experiment it was determined that Blade 3379 would be the blade that would have the least amount of pressure associated losses. Each blade that was tested has a similar pressure profile along the blade, but blade 3379 has the smallest maximum pressure difference meaning the other blades have greater losses because of the higher pressure differences. It is interesting to note that any of the purchased blades would be an improvement over the stock model blade. This is due to the intended designs of the blades. The stock blade was designed to help throw clippings out the back of the mower while the purchased blades were all designed to be simply mulching blades.

Appendix B

Figure 13

Figure 14