Non-Point Source Polluting Lawn Mower

Benjamin Graham Andrew Smith Michael Smith University of Maine

ABSTRACT the point-source pollution output by the use of these machines. Lawn mowers and other lawn care equipment are some of the biggest causes of pollution on OBJECTIVES the market. This project strives to find a suitable power alternative for a lawn mower which will Currently, there are models of lawn mowers that have satisfactory performance without emitting are on the commercial market which do not the large amount of pollution produced by a gas require the use of gasoline for a power source. driven mower. A new mower has been produced Most of these are battery powered and do not which includes an efficient motor, a removable have enough power to cut more than a quarter of battery pack which holds high capacity batteries, an acre. For a typical Maine resident, a mower and a new blade with reduced power losses due that will only cut a quarter of an acre is not to a “fan effect”. sufficient. The full scope of this project seeks to construct a non-polluting lawn mower which INTRODUCTION will mow a typically sized Maine lawn before running out of power. In the present era, with petroleum usage skyrocketing, global warming has become a The objectives we are striving to meet are: major crisis that needs to be dealt with. Petroleum is being used as the major power  Substantial reduction in pollution source for many applications such as emissions automobiles, heating, and the study of this  Suitable power supply to cut a typical project, lawn care. Maine lawn

“Some older lawn mowers are bigger polluters PROJECT RESULTS than the family car,” said James M. Lents, executive officer of Air Quality Management POWER REQUIREMENT District in Orange County California. “A dirty mower operated for 20 hours a year produces The first thing we needed to determine to create the same amount of smog-forming volatile a suitable non-polluting lawn mower was the organic compound pollution as a 1996 power requirement needed to cut the average passenger car driven for 26,000 miles -- more Maine lawn. In order to do this, we performed a than most people drive in a year,” Lents said.1 test with the stock system. Originally, we were With this being the case, it can be seen that in unable to get the lawn mower to run. We order to reduce pollution emissions, an dismantled the lawn mower and rewired it with alternative needs to be found to power lawn both a voltmeter and an ammeter to check the mowers. Whether this is done by using a new voltage and current running to the electric technology such as a fuel cell, or by utilizing motor. We found that there was an initial spike battery or solar power, this project aims to cut at start up. This spike hit 100 amps before declining. With the mower running without Fuel cells are still in the early stages of cutting any grass, we found that the amperage development. While fuel cells would provide needed to spin the blade was 10 amps. Since the more than enough power to mow an entire lawn, stock mower runs on two twelve volt lead acid they are still very expensive. Hydrogen is not batteries, this gave us a power requirement yet widely available to consumers so this would without a load of 240 watts. When we took the also present a problem to the average user, along mower outside, we found that the max amperage with the addition of a cumbersome storage tank needed to run the mower through thick, wet for the gas. There are fuel cells that use other grass was 70 amps. This gave us a max power of substances such as methanol to produce the 1680 watts. hydrogen needed, but that causes the production of pollutants in the reformer. POWER SOURCE Solar Power When seeking to find a suitable power replacement to a gas powered combustion Solar energy is by far the cleanest and most engine to run a mower, we found four possible abundant source of energy that could be used. solutions: a fuel cell, solar power, use of biofuel The problem presented with a solar powered instead of gasoline, or battery power. There lawn mower is obtaining enough energy to mow were benefits and disadvantages to each a typical Maine lawn. Large panels would be alternative. A design matrix of all four needed to harvest this much energy. Also, to selections is shown in Figure 1. create a suitable mower using solar energy, a battery would also be necessary to retain charge Fuel Cell 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 next two pages. Figure 2 is a quantitative matrix while Figure 3 is a qualitative matrix. Biofuel is created using 85% vegetable oil, and therefore is a renewable source of energy. But Nickel-Cadmium while this would reduce emissions, this form of energy does not completely eliminate point- Nickel-cadmium batteries are fairly cheap and source pollutants from a lawn mower. perform well under heavy work loads. It has a long cycle life, but loses performance if it does Battery not have a charge/discharge cycle at least once every month and contains toxic materials. Batteries are widely used and widely available to all. A battery powered dc motor is lighter Nickel-Metal-Hydride than a typical combustion engine and much quieter as well. Batteries have been around for a Nickel-metal hydride batteries are a little more long time and therefore most battery expensive than nickel-cadmium. They have an technologies are well developed. One question average cycle life but the life is shortened by is whether or not a battery will be able to heavy loads. It is environmentally friendly and provide the needed power. needs a charge/discharge cycle every three months to retain performance. Conclusion Lithium Ion After considering the four alternative power sources we had and utilizing our design matrix, Lithium ion batteries have a very high energy we determined the best power source for that density and are very low maintenance. They are could be used for a lawn mower was an electric very expensive though, and seem to lose battery. Use of a battery will keep the initial cost performance quickly, even after one year of use. of the mower down while completely eliminating point-source pollutants and making Lithium Polymer recharging very easy for consumers. Lithium polymer batteries are expensive and This choice of course led to the question of have a high energy density, although not as high which type of battery we should use to power as lithium ion. They have an average cycle life, the mower. In order to determine this, we but have no memory and are environmentally looked into multiple battery chemistries and friendly. created a whole other set of design matrices. Lead Acid BATTERY SELECTION Lead acid batteries are tried and true. They have In order to decide which battery chemistry to a lower cycle life then the others, but are the use with our lawn mower, we looked at five of most inexpensive battery, no memory, a high the most widely available and widely used overcharge tolerance, and the lowest self- battery types. Those types were: nickel- discharge. They cannot be stored in a discharged cadmium, nickel-metal-hydride, lithium ion, condition and a full discharge can rob the lithium polymer, and lead acid. Battery selection battery of some performance. matrices can be seen in Figures 2 and 3 on the 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%

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

Good performance at low High temp storage Store at 40% charge, low Full discharge robs battery temps shortens life temp to prolong life capacity

Loses performance without CD/month

Figure 22,3 Nickel Nickel Metal Lithium Ion Battery Weight Cadmium Hydride Lithium 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 short grass, but could not cut the entire lawn when the conditions were not perfect. After After weighting the benefits and disadvantages some research to determine what the cause of of each battery chemistry and using our battery this was, we found that a typical DC motor runs selection matrices, we determined that either a with a good efficiency when a low torque is nickel-cadmium or lead acid battery would work applied, but as the torque applied to the system well with our lawn mower. We purchased lead increases, the efficiency of the motor decreases. acid batteries to run our lawn mower, and that is We concluded that one way to increase the what we would choose to sell the mower with battery life of the mower would be to replace the were it ever to reach the retail market. Since stock motor with a higher efficiency motor. nickel cadmium seems like a very good choice for batteries as well, we decided that we would Our search led us to a new technology in DC market nickel cadmiums as replacement motor design known as a lynch motor. Standard batteries as well as lead acid should a consumer DC motors use stacks of steel laminations which prefer nickel cadmium over lead acid. are then wrapped with copper wire and an armature. The lynch motor uses stamped, bent, DC MOTOR and coated copper bus bars arranged in a disk as an armature. The disk is then placed within an When performing the preliminary tests on the air gap from permanent neodymium magnets, stock system we were provided with, we found which are about three times as strong as a ferrite that the mower had a difficult time handling magnet. The result is what’s technically known thick or wet grass. The life of the mower was as a wave wound axial gap brushed DC motor. severely reduced when presented with these We found two versions of this type of motor and mowing conditions. Even the owner of the stock created a design matrix of lynch and standard dc mower told us that the mower ran fine with dry 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 threaded at the bottom. The ETEK output shaft motor for our mower would be the Briggs and was a hollow cylinder that was threaded on the Stratton ETEK. This motor is light weight, inside and had a key on the outside. This forced outputs high power, and has a very good us to consider new ways of attaching a blade to efficiency, even at high torques. Figure 5 shows the motor. The final design we came up with an efficiency versus torque graph and a motor can be seen in Figure 6. Our new blade speed versus torque graph for both the ETEK connection consists of a sleeve that fits on the motor and a typical dc motor. As can be seen, outside of the output shaft. Connected to this the as the torque increases, the efficiency and with 4 screws is rectangular piece which holds motor speed of a typical dc motor drastically fall the mower blade in place as it rotates. A bolt while that of the ETEK motor remains almost with a washer runs through the whole apparatus constant. This high efficiency will allow the to secure it all to the motor. battery to run for a longer period of time on thick, wet grass.

Figure 6

SYSTEM VOLTAGE Figure 58 After having purchased the Briggs and Stratton ETEK motor, we needed to decide what voltage BLADE CONNECTION to run the mower at. The ETEK motor had a voltage constant of 72 rpms per volt. A typical One problem we encountered when we gas lawn mower runs at a speed of around 3000 purchased the ETEK motor was that the output rpm, so our engine speed needed to be close to shaft was designed significantly differently than that. With that information, we were constrained that of the stock motor we had. The stock motor to either 36 volts, which would give us 2600 output shaft was merely a cylinder that was rpm, or 48 volts, which would give us 3500 rpm. We purchased four 12 volt lead acid run the motor at 36 volts, using three of the 12 batteries to test the motor with. We made sure to volt lead acid batteries in series to reach this obtain batteries with a high capacity: 21 amp- voltage. hours. This is an increase of about 4 amp-hours over the stock model batteries. These batteries REMOVABLE BATTERY BOX would give us a longer mowing time then those provided with the stock system. A picture of the One of the major goals of this project once it batteries can be seen in Figure 7 and a discharge had been determined that battery power would versus time graph of the batteries can be seen in be used was to create a removable battery box. Figure 8. We tested the ETEK motor at 36 and This would allow consumers who had 48 volts to see how the motor performed at both considerably large lawns to easily replace voltages. At 36 volts, the motor performed well 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

Figure 7

Figure 89 the battery box, we needed to find a simple circuit connector that would allow for easy with satisfactory results. At 48 volts, the motor disconnection and removal of the battery box. performance was suitable, but the running What we found was a North American style mower was louder than what we were battery charging cable connector. A picture of a expecting, and the power of the system seemed North American style connector is shown in to be too much. The front wheels of the mower Figure 10. The two wires from the circuit are would actually jump to the right on start up at soldered and crimped onto two metal prongs this voltage. Due to this effect, we decided to that fit snuggly into the connector. Two of these 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 are used, one on the battery side of the circuit, the other on the opposing side. The connector on Over the course of the first semester, we had an the battery side of the circuit hangs outside of experienced design engineer overlook the the battery box, making it accessible for project. He brought to out attention the fan disconnection. To remove the battery box, effect of the blade. The fan created by the blade simply pull apart the connectors and lift the box while cutting the grass leads to a loss in power. out of the lawn mower. To run the lawn mower To measure this effect we ran a test of the motor again, replace the battery box and reconnect the without the blade attached, to determine the two connectors. amount of power lost due to just the blade. From the previous power requirement test, we had that Were this lawn mower to go into production, we the initial spike was 100 amps and the running would market two different kinds of battery amperage was 10 amps. When the blade was boxes to supplement the lawn mower. One removed, we measured the initial spike at 15 would be the same as the one offered with the amps, while the running amperage was only 1.5 lawn mower itself: a 36 volt power source amps. This leads to the conclusion that the comprised of multiple lead acid batteries. The power lost to the fan effect of the blade was other would be a 36 volt power source more than 200 watts. This seems like a huge comprised of multiple nickel cadmium batteries. loss of power simply to rotate the blade. Since both types of batteries were found to be relatively similar when used in this type of In order to lessen the amount of power loss due application from our battery matrix, the offering to the fan effect of the blade, we first decided to of both types of battery boxes would allow the make a lawn mower that only mulches. The consumer to determine which battery chemistry stock model was designed to throw grass they preferred to use and have as a backup for clippings out the back into a bag. This is one their lawn mower. reason why the fan effect of the stock model is so high. A mulching mower repeatedly cuts BATTERY CHARGER grass clippings to miniscule pieces to be left on the lawn. This would reduce the fan effect loses The stock model lawn mower we first began of the blade. To find the best blade for this we with had an internal battery charger. Since we decided to run an experiment to test varying had a removable battery box, we determined an blade designs and the losses associated with external battery charger would be better suited each design. Because we could not produce our for the new mower. We purchased a 36 volt own blade, we purchased multiple mulching battery charger which would be sold with the blades with differing designs for testing. lawn mower. The battery charger had a 3 prong female tip which was incompatible with any of We spoke with Professor Bhaganagar about how the connections we had at the time. To solve this we could test and find the blade with the least problem, we found a 3 prong male tip / North amount of fan associated losses. What she told against the handle bar to start and run, which us was that we needed to find the pressure would stop the lawn mower when released. The variation along the length of each blade. The only change we made to these was the position blade with the smallest pressure differences of the circuit board. The removable battery box would be the blade with the least amount of fan took up the room where the circuit board had related losses. original been placed. We moved the board to an area underneath where the battery box stood, an To run an experiment that measured this, we area originally used to shed grass clippings took one of the stock models that we had and which was now no longer being used. drilled 8 holes along the bottom as pressure taps. We ran each blade at 36 volts and took pressure FURTHER PRODUCTION WORK readings at each tap. This gave us the pressure profile of each blade. For the full lab report and If we were planning on actually marketing the results associated with this experiment, refer to lawn mower, there would be a couple things we Appendix A. would make changes to before selling the model. One is the position of the circuit board. The results of this experiment showed us that We would like to place the circuit board at the blade 3379 had the smallest pressure difference front of the lawn mower where there is plenty of associated with it. This would be the blade that open room and it could easily be housed. It is would give us the smallest amount of power loss not placed there now because since we decided due to a “fan effect”. to use the stock model start and safety switch, we were constrained by the length of the cable STOCK PARTS on the switch. The stock model was designed with the circuit board towards the back of the Although we improved upon many aspects of body and the cable would not reach any further the Black & Decker mower we began with, we than that distance. The other change we would found that there were pieces that we could reuse make is the design of the underside of the body in order to complete our finalized lawn mower. where the blade is positioned. The stock model We reused the body of the stock model on our was designed with a discharge bag for clippings own mower. The design of the body could and the aerodynamics of the underside was satisfactorily accommodate the changes we designed to throw clippings out the back. This made to the system with only a few can be seen in the pressure profiles found in our modifications: remounting the new dc motor we experiment. Since we decided to create a mower purchased, creating a level surface for that only mulches, the stock design of the placement of the battery box, and making room underside leads to losses that wouldn’t be for the battery box in the upper portion of the present in a new design created only for chassis. We also applied a new coat of paint to mulching grass. the body. In addition to this, we felt the height adjustment on the stock model body was very REFERENCES well designed and made for very quick and easy height adjustment. Along with the body, we 1. http://www.aqmd.gov/news/scrap1.html reused the start and safety switch that was used on the stock model. The start switch was simply 2. http://www.batteryuniversity.com/index.htm a push lever. The safety switch was just like the safety switches commonly seen on most lawn 3. Linden, David, ed. Handbook of batteries. mowers: a secondary bar that needed to be held 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

04/03/2006 Crosby Laboratory University of Maine

By: Benjamin Graham Andrew Smith Michael 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 22-30V 2. Multimeter – Micronta Model 22-195A 3. 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 P8 6294 270 185 225 200 -4 -6.7 183 172 6309 200 210 210 205 -6 -5.1 190 170 3361 245 215 240 220 -5.4 -11 210 185 3370 290 210 255 210 -6.5 -7 210 190 3379 130 140 185 147 -16 -6 158 125 stock 155 175 205 220 -15 -5.1 130 130 Table 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 P8 6294 0.4361 0.397 0.4154 0.4039 0.31006 0.308818 0.39608 0.39102 6309 0.4039 0.4085 0.4085 0.4062 0.30914 0.309554 0.3993 0.3901 3361 0.4246 0.4108 0.4223 0.4131 0.309416 0.30684 0.4085 0.397 3370 0.4453 0.4085 0.4292 0.4085 0.30891 0.30868 0.4085 0.3993 3379 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 0.3717 Table 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