“Be kind, for everyone you meet is fighting a hard battle” - Often attributed to Plato but likely from Ian McLaren (pseudonym of Reverend John Watson)

Wednesday, December 09, 2009

Push lawnmower?

Tim Garrett, an associate professor of atmospheric sciences at the University of Utah, makes the provocative claim that global warming is unstoppable. It's a very interesting argument and is reminiscent of Jevon's Paradox. I'll have to do more thinking before I post on the crux of Dr. Garrett's main argument, this post is about a minor point in the article linked above.

Garrett opines that conservation and efficiency are useless in the long term with respect to minimizing humanity's primary energy conversion but yet he bicycles to work, line dries his clothes, and uses a push lawnmower. Ah, there's something into which I can sink my meager teeth. Is using a push lawnmower more Earth friendly than an electric or gasoline mower? I'm going to make a guess and then see if I can confirm or refute the guess with numerical estimates. You'll have to trust that the guess is prior to the calculation. My guess is that the electric lawnmower is most Earth friendly, followed by the push mower with the gas mower bringing up the rear.

First, I'll have to state my definition of Earth friendly. I'm going to go with primary energy consumption for all inputs, i.e., for the electric mower it will be the electrical energy used by the mower plus the energy required to generate and transmit the electricity. The gasoline will be the gas used by the lawnmower plus all energy used in extracting, refining, and distributing the gasoline. I won't include gas for the car to go to the gas station since that trip will be added to a fuel trip for the car. For the push mower, it will be the energy used by the person pushing plus the energy used in planting, fertilizing, harvesting (or slaughtering, etc.), and distributing the food providing the energy. I won't include the embedded energy in the lawnmowers, though that may hurt the push mower. Needless to say, there's ample room for error in this analysis but damn the torpedoes, full speed ahead.

Let's set the lawn as follows: 1,500 meters^2 (a little over 16,000 feet^2 or about 0.37 acres). Starting with the gas mower, I'll look at the Honda HRX217K2HMA. It has a cutting width of 21" but I'll use 20", there must be some overlap. Its maximum self-propelled speed is 4 m.p.h., I'll go with 3 as an average. Cutting 1500 meters at 20" requires 1.84 miles of mowing, at 3 m.p.h., that will take 0.613 hours. Sounds about right. The mower uses the Honda GCV190 engine, which uses 1.21 quarts/hour of gasoline. Therefore, it will use about 0.742 quarts or about 0.186 gallons to mow the lawn.

Using the estimates from my previous post, the 0.186 gallons will require a total on the order of 2.84*10^7 joules of primary energy from an oil well. To this I must add the energy required to walk 1.84 miles and using other figures from the same post, this will require 181 kilocalories or 6.4*10^6 joules of primary energy to produce. This is yields a total of 3.48*10^7 joules (Big hat tip to Chris for pointing out the factor of 100 error in my original post).

For the electric mower, I'll use a rechargeable, the Earthwise 20 in Cordless Electric Lawn Mower. I'll reduce the cutting width by an inch as for the gas mower and find that I'll travel 1.93 miles. I'll figure that I'm going a little more slowly, say 2 m.p.h., since it's not self-propelled. Thus, I'll need 0.96 hours or 58 minutes. The specification says that a charge is good for 45 minutes (give or take) so I'll need 1.29 charges. This is for a 24 volt, 17.2 ampere-hour battery that I'll assume we recharge when it's 80% discharged. Thus, a charge uses 1.19*10^6 joules and the lawn takes 1.54*10^6 joules of electricity from the wall socket. Again using figures from my earlier post linked above and assuming some fossil fuel (coal, natural gas) is used, this quantity of energy will require about 5.27*10^6 joules of primary energy to provide the battery charge.

Since the electric isn't self-propelled, I'll be working harder to push it, so I'll estimate about half again the caloric input as for the gas mower, or 9.6*10^6 joules of fossil fuel input to push it for a total of 1.49*10^7 joules total. Note that, by my estimate, it takes more fossil fuel to have me push the lawn mower than to power the swirling blades.

Finally, let's look at the manual, reel, or push mower. I'll use the Brill Razorcut 38 Push Reel Lawn Mower. The cutting width of this mower is 15.2", I'll reduce it by an inch as above and find that I have to walk 2.58 miles. Since I'm doing all the work, I'll assume that I can move at about 1.75 m.p.h. and thus I'll mow for 1.47 hours. This handy page says that, at 180 pounds, I'll burn 447 kilocalories/hour for a total of 659 kilocalories. Again going back to the earlier post linked above, producing the food to power me through this walk will take about 2.34*10^7 joules of fossil fuel energy.

It would seem that my instinct was correct. In increasing order of energetic impact, it's the electric, followed by the manual, and finally, bringing up the rear, the gas powered mower. My original analysis was off by a factor of 100 with respect to the energy contained in the gas mower's fuel burn (blush) so it isn't as bad as I'd first calculated.

Finally, take a look at what is, in my opinion, the best solution. Here's the Epic Cordless Electric Solar Mower Model EP21H. It features 45 minutes on a charge, has a 21" wide cut, and the optional solar panel will recharge it in about three sunny days. This will leave the 9.6*10^6 joules of food energy as the only fossil fuel consumption to mow your lawn.


Anonymous said...

You can save another 75% of mowing effort by switching to eco-lawn, which.

King of the Road said...

I hadn't heard of Eco-Lawn but the reduced mowing requirements is among the many claimed benefits. Here's a link to a page with information on this grass. Assuming it meets the claims, it will reduce mowing by about 50% through slower growth rate. It's also claimed to be drought tolerant and thus require less water. Further, it's claimed not to require fertilization and pest control.

Chris Rusbridge said...

I was very surprised at the factor 170 difference when I read this, and it bothered me. But is it possible you have made a factor 100 transcription error between the two blog posts? It turns out that both use approximately 0.18 (US) gallons of fuel; in the supermarket post this is 29*10^6 joules, in the lawnmower case it is 2.8*10^9 joules. If so, a factor 170 difference becomes a much more credible factor 1.7 difference.

King of the Road said...


Most embarrassingly, you're absolutely right. I've repaired the post to correct the error. Thanks for taking the time to read and point it out.

crf said...

Your analysis contains another error.

For the honda mower and electric mower, you calculate the extra energy, above that of homeostasis, used by a person to walk behind the mower, as calculated in your "going to the store" post.

For the reel mower, you use the energy required to push the mower AND (I presume) all the metabolic energy used to maintain homeostasis, taken from the peoplepoweredmachines faq website (the website's table shows energy use of a person mowing compared to that of a person watching (or, at rest)).

I think you should remove the energy used to maintain homeostasis, and use only the extra energy needed to push the reel mower. From the peopolepoweredmachines page this would be ( 147 C/h - 122 C/h ) = 25 C/h, for a 180 lb person. (You're a mammal, for goodness sakes!)

This will greatly affect your analysis.

I like your blog :-)

Honestly, my feeling is that the reel mower will be much more efficient that either the electric or gas mowers. This is due to how the reel mower works. In the gas and electric mowers, energy is used to spin a heavy rotating cutting blade extremely fast. This creates an upwards lifting force of the air around the blade which holds the blade vertical, and a speeding knife edge to cut the blade of grass cleanly (the blade has to be fast to avoid simply pushing the grass blade against the air (which has little resistance) and out of the way).

That is a hugely energy-inefficient way of cutting something, even if grass is light!

By contrast, a reel mower has a much more efficient way of cutting the grass: the metal reel scissors the grass blade against a stationary cutting blade. The stationary blade slices through the grass blade, which is caught, stable and unmoving in a mechanical vice until the cutting action is finished, just like a pair of scissors: this takes virtually no significant force at all. No extra energy is used to create hurricane force wirlwinds just to keep a light blade of grass vertical! Most the energy used in the reel mower is to maintain the motion of the reels, and to overcome friction between the reels, grass and cutting blades, and to push grass out of the way of the travelling wheels. If you keep your grass short, those forces are small. If the grass is lengthy, those forces can seem overwhelming.

crf said...

Sorry, I misread the graphic on the people powered website,

The calculation is:

447-122 = 325 C/h

is the energy only used to push the mower per hour, and therefore

325 C/h * 1.47h = 477.75 C

is the energy used to mow your lawn. Multiplying by the 8.5 C fossil input per Calorie usable human food, and 4182 J/C we get about 1.7 * 10^7 J .

Anonymous said...

You seem to be assuming that in switching from a push mower to a power mower I would reduce my caloric intake. I wonder if this is actually true of people who are likely to waste time and energy maintaining a lawn?

I suspect that those with the time and energy to maintain a lawn are mostly urbanites with largely sedentary lifestyles, in which case the energy to power the mower likely comes from a pool already reserved for non-productive, "recreational" exercise necessary for the maintenance of physical health. My guess is that for every calorie spent mowing lawns, roughly a calorie less is expended in other forms of recreational exercise.

The question of how much energy is spent operating the mower therefore would seem to me to be moot, making the push mower the most fuel-efficient choice.

Anonymous said...
This comment has been removed by a blog administrator.
RobF said...

Absolutely. If you finish mowing your lawn with the electric mower, and then drive to the gym to work out, you haven't saved much.
Ignoring the materials and manufacture of each machine is most unfair as well.
3rd - there are electric reel mowers available...

Raymond Luxury-Yacht said...

I just want to say that your estimate of a speed 1.75 mph for someone using a push mower is, in my experience, much too slow. I have used a relatively heavy push mower: not a reel mower but one with a circling blade that had probably about a 20" diameter.

I found that my speed when using this mower was generally faster than it had been when using a self-propelled gas mower, because I was not slowed by the weight of that mower. I was definitely going at least as fast as average walking speed, probably closer to a marching or jogging pace. Wikipedia gives average walking speed at 3mph.

I'm not in particularly impressive shape, probably about average. I may have less than average patience for tedious tasks like mowing lawns.

ISAWHIM said...

There are additional factors, which are left-out. (Well, infinity is hard to summarize.)

Manual scissor mowers have six rotary blades and one fixed blade. These are expensive to manufacture and upkeep. They rarely last more than a year, in efficient condition. The degraded blades require additional effort and re-cutting. Also, the wheels and extra pushing forces will compact and destroy the lawn. That will require re-sodding or plugs or reseeding and additional fertilizer and demand aeration. With a two-year viable life-span, it is either suffer with shoddy lawns for the next five years or full tool replacement.

Electric mowers, out of the box, will run at those specifications. However, they also degrade rather fast. They are also nearly impossible to repair, and thus require landfill and waste to be added to that calculation. Batteries are heavy, loose power fast, drain more than you calculated while they charge, and still require pushing which leads to lawn-compacting. The battery drains all week, and refills all week, even when you are not using it. The life of a battery is roughly two to three years, if you use it or not. Though, they require less pushing power.

Gasoline engines have a life expectancy of seven to twelve years, even if poorly maintained. (Though that is part of the issue too.) Yes, cheap abused and neglected mowers live about four to five years, but that is irrelevant to this topic. The pulling power is less stressful on the grass, because you are also subconsciously pushing it too. The faster RPM blades with more power, give a more consistent cut over an electric mower. Obviously being faster is not a question here.

However, you also failed to calculate "time = money", even when you do it. That is time not available for rest. Time not spent making money. Time not spent doing more important things. Time in the sun. Time under stress. Time spewing out CO2 and consuming O2 as a person. Time draining your body of H2O. Time for more recovery.

Time is highly relevant in that factor. Food, medical issues, water, money, and other things which are the hidden costs. Spend 0.5 hours in the sun, chasing a self-propelled mower is obviously the winner over the 2.0 hours that a manual mower will offer. (Don't forget that sharpening seven blades takes more time than sharpening only one in the other mowers.)


ISAWHIM said...


There are exceptions, obviously...

"The right tool, for the right job."

Using those formulas, and factoring in some other missing data, you can safely determine when each item is best. If your yard is only 1000 square feet, a manual mower is the best option. If your yard is 1000 to 3000 square feet, a battery powered mower might be good, but an electric-cord mower will absolutely be better. If your lawn is 3000 to 10000 square feet, a gas powered self-propelled is the best choice. If your yard is larger than 10000 square feet, you should use a riding mower and one of the three mentioned above.

There is one riding mower which can cut grass at nearly 15 MPH, and costs under $3000. Mowing a 10000 square foot yard in nearly 4.75 minutes, without looking like a herd of sheep visited the yard. (It is made by "Dixie Chopper".)

Now, for all realistic purposes, you should reduce your results to a value per square foot. Each plotted, as time and yard-size is the curve of the plot, against the joules and carbon-footprint. (Environmental clean-up is another missing factor. It takes one McChicken to fuel me on a riding mower, but four McChickens to fuel me to push a manual-mower across an acre of grass.)

I would place the order of mowers as...

Fuel-powered, Electric, Human.

A majority of the power we use, on average, is dirty fossil fuel, dirty natural gas, and dirty nuclear. Soon, hopefully, there will be a 50/50 split of dirty versus clean energy. Pollution will not reduce, it will just not get as bad. They won't stop dirty power, just build less when we want more. Now the illusion is going to force us to want/use more. We just moved the pollution into the neighbors yards, as usual. That is where it appears greener, in the neighbors yard, where it actually is not greener.

huin said...

My father is saying me to purchase Honda lawn mowers Marietta, but I am some obscure about tits fuel power consumption. I want some instructions for its future use from those ones, who are currently using it. Please share.

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