Efficiency of Electric Mower vs. Gas Mower

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My lawn requires about 45 minutes to mow.  With a gas mower, I consume about 0.2 gallons of gasoline.  The energy released by 0.2 gallons of gasoline is 33.705 kWh/gal * 0.2 gal = 6.7 kWh.  According to the specifications, the EGO self-propelled lawn mower should run for about 45 min.  The energy capacity of the 7.5 Ah battery is rated at 420 Wh.  Does a gas mower really expend 6.7 / 0.42 = 16 times as much as the EGO?  That's a lot of wasted energy.  For an EV vs gasoline powered car, the ratio is closer to 3.   

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larryh

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Posted 3 years ago

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Dave .

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Think of all the heat generated by an internal combustion engine.   I think therein lies your answer.  I switched to all electric garden equipment because I got tired of breathing carbon monoxide, not to mention the noise.
(Edited)
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Sharon Beasley

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All I can say is - your knowledge of such information is way above mine.  All I know is I like my EGO and I don't have to mess with gas or oil, plus Ego is quieter.  
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Brad Christie

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If it helps the math, my battery lasts a full hour. Also:

* I'm not breathing in exhaust,
* I'm not worring about combustible [liquids] in garage (introducing a battery is a trade-off),
* I'm leveraging a renewable energy source,
* I'm saving my hearing, and
* I'm not making trips to the gas station.

I can't confirm your math on efficiency, but given the other factors I call it a win.
(Edited)
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Julio Villanueva

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Man, some people do have free time in their lives
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larryh

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This is very relevant.  I am attempting to determine the operating cost for a gas powered mower vs. the EGO electric motor.  To completely charge the battery requires about 0.5 kWh of electricity at a cost of $0.06 assuming a rate of $0.12/kWh.  EGO claims the mower will run for 45 minutes at medium load which should be enough to complete the lawn.  The gasoline powered mower I have is rated at 1.1 liter/hour.  That will require about 0.22 gallons of gas for a cost of $0.46 assuming gas is $2.10/gallon.  So an electric mower is much cheaper to operate.  The primary reason is because the electric mower is so much more efficient.   But it seems hard to believe that there is such a big difference in efficiency. 

Of course, there are additional expenses associated with a gas mower such as air filters, oil, spark plugs, etc.   There are also additional costs associated with the EGO mower such as replacing batteries. 

(Edited)
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Brad Christie

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Though I can appreciate the amount of effort you've put into this, my primary reasons for getting this mower were already listed. Whether it costs me more/less to operate wasnt a driving factor. Though given the climate I'm generally expecting a higher cost of ownership for electric (due to high initial cost, lack of infrastructure, and maintenance fees). On the flip side, I can rest easier knowing I'm decreasing my carbon footprint marginally.
(Edited)
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larryh

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I am looking at this from a PHEV perspective.  For my PHEV car, I spent $455 on fuel and electricity per year to operate my car for the past three years.  I drive approximately 11,500 miles per year.  I too am concerned with greenhouse gases.  But it is nice to know that you don't have to pay a high price to reduce them.  People are going to want to know what it will cost them to go green. 

(Edited)
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Blue Angel, Champion

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Larry, when referring to the efficiency of a gas lawnmower engine I always use car engines as a starting point.

The most efficient ICE (Internal Combustion Engine) in a car is in the new 2016 Prius.  It was all over the news as having achieved 40% efficiency.  To my knowledge, the average new car engine with Direct Injection, cam phasing and all the other common tricks is in the low-to-mid 30% range.

Now consider that those numbers are peak ratings, meaning they are only that efficient over a narrow region of their operating range (load/RPM).  Away from that range their BSFC (Brake Specific Fuel Consumption) increases (efficiency drops).  This is why we see cars coming out with more and more gears in their transmissions, as well as CVTs (Constantly Variable Transmissions), which allow the engine to stay closer to peak efficiency.

So let's assume that the average modern car engine is operating at somewhere between 20-30% most of the time.

Enter the lawn mower engine, who's last big technological leap (in most cases) happened with the Over-Head Valve (OHV).  Compared to anything resembling a modern car engine it's a dinosaur.  I wouldn't be surprised if most mower engines were only 1/2-2/3 as efficient as a modern car engine, but that's purely speculation.

As far as Ego goes, the 7.5Ah battery actually has about 378Wh maximum capacity (average 50.4V x 7.5Ah), and somewhat less than that in reality based on how much of the 2500mAh capacity they use in their charging program.  And, as you pointed out in your other post, the efficiency of the charging process must be taken into account as well.

So far that's just looking at the motor and battery.  Another HUGE piece of the efficiency pie is the cutting system, and the biggest part of that is the blade.  Cordless electric lawn mowers use much more aerodynamically efficient blade designs than typical gas mowers do.  The average 3-5Hp (2200-3800W) gas mower uses considerably more power just moving the blade through the air, which is probably why a 1000W Ego can feel just as powerful or more than some gas mowers.

Not only that, but the Ego 21" mowers have a variable speed motor controller that steps up motor speed in heavy grass and slows it back down under lighter loads.  Most people throttle their gas mowers up to full speed or close to it and just leave them there the whole time they're mowing.

So not much in the way of hard numbers, just a bunch of hopefully accurate speculation. :-)
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Willy Bee Too

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I like your last line,,, 
What kind of governor is on the modern gas mower? Maybe that is where some of the inefficiency comes from. With an electric motor we have CEMF ( counter Electro Motive Force )   As an electric motor is slowed down the resistance in the wire in the windings is decreased. And that allows more current to flow, which increases the magnetic field, and that increases the speed of the motor.
All of this is to say, that the ego does not use any more electricity than it has to .  
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Blue Angel, Champion

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According to this link:

https://www.briggsandstratton.com/na/...

There are two types of governor used on small engines, mechanical and pneumatic, and they pretty much do the same thing; maintain your pre-set engine speed under varying loads. So as the engine senses increasing load, the throttle opens up to compensate, and closes as the load decreases.

The Ego mowers do the same sort of thing electronically, but the new 21" mower goes a step further and actually increases the speed of the blade in dense turf. This allows spinning at a lower and more efficient speed when there's less dense grass to cut, and a higher speed when the grass thickens up.
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larryh

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That's another rather surprising difference between gas and electric lawn mowers.  The rated power of my lawn mower is 6.5 HP or 4.8 kW.  The power rating of the EGO lawn mower is only 1 kW.  The claim is that the EGO lawn mower has the power of a gas lawn mower.  If the cutting ability of the EGO lawn mower matches that of a gas powered mower, then the gas powered mower must be very inefficient in transmitting its power to the blades.  I haven't had a chance to try it out to verify that it matches the cutting ability of my gas powered mower or that I will be able to mow my entire lawn on one charge using less than 387 Wh of energy (I just mowed it a couple of days ago with the old mower.)   Without a dramatic increase in efficiency over gas lawn mowers, electric mowers would not be practical.  From a energy/power physics perspective, the feasibility of an electric mower seems rather suspect.  I will get a chance to verify these claims later this week.  If someone came up with a new car that gets 17 times the MPG of current cars, I think most people would be rather skeptical and wonder how it was done. 

Many gas powered lawn mowers claim a torque of 7.5 ft-lbs.  I wonder what the torque of the EGO lawn mower is. 

 Note that I have measured the efficiency of the ICE in my PHEV to be 37% over a rather broad range of RPM/Load--far better than the estimated ~10% efficiency of a lawn mower engine.  The efficiency of the electric motor in the PHEV is approximately 90%.

(Edited)
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Blue Angel, Champion

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Ego claims "the torque of gas" which is much different than saying "the power of gas". A power rating includes an operating speed, and at 1000W they wouldn't be fooling too many people into thinking that their mower was actually producing more power than gas mowers.

Having said that, I have not seen a torque rating for the Ego.

As far as your car's engine goes, how did you measure its efficiency? 37% is not an unreasonable peak number, but I would fairly confidently bet against it having that efficiency over a broad range of rpm and load.

Most ICEs have a BSFC graph that looks something like this:



That particular engine is a 1.9L Saturn, not a hallmark of efficiency but not bad, but it's a very typical looking graph. In this case the engine is generating somewhere in the 40hp range just to be in the peak efficiency zone. Since normal driving doesn't take anywhere near 40hp, that engine will be operating at a much lower overall efficiency much of the time.

I got that pic from this thread over at Ecomodder:

http://ecomodder.com/forum/showthread...

Your PHEV undoubtedly has much more intelligence baked in than most cars do as far as being efficient is concerned, and uses an electric generator to charge the battery as well as power the vehicle. Knowing this, it surely keeps the engine operating very efficiently much of the time. How did you measure the efficiency?
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larryh

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I used Torque Pro to record data from an OBDLink MX adapter connected to the car's OBD II port.  There are PIDs for engine torque and rpms as well as fuel consumption rate.  You can easily compute the power output of the engine from torque and rpms.  Using fuel consumption, you can then compute efficiency.  You could also take a video of the same information on the car's displays and then replay it back later--but that would be a lot of work.

Perhaps my wording could have been better.  Efficiency is 37% over much of the torque/rpms at which the car actually operates when driving.  I can't determine the car's efficiency for most rpm/torque combinations since the engine never operates there.  The car is programmed to operate the engine only in its most efficient regions.  Any power surplus or deficit goes to/comes from the battery.

I wonder what a similar diagram would look like for a gas powered lawn mower engine.

If the maximum power of the EGO lawn mower is 1 kW and the normal RPMs is 2800, then that implies torque is 2.5 lb-ft at normal operating speeds.  That is about 1/2 or 1/3 of a gas powered lawn mower. 








 

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larryh

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This chart is similar to BSCF map for an ICE, but for a permanent-magnet synchronous motor.  I'm not sure what type of motor EGO uses.  The solid lines are efficiency contour lines.  Maximum torque occurs at low rpms as seen by the straight portion of the dashed line at the top left.  Once rpms exceeds a threshold, torque decreases as observed by the curved portion of the dashed line.  The power output of the motor is now limited by the maximum power output of the battery.  Prior to the rpm threshold, power output is limited by maximum torque of the motor. 

A torque rating for an electric lawn mower won't make much sense--it varies greatly with rpm.  Only power makes sense for an electric lawn mower. 


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(Edited)
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Blue Angel, Champion

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My understanding is that the flat line indicating maximum torque is determined by the current limit imposed by the weakest link in the chain, be it the battery, controller or motor windings.  In theory, the more current you supply the higher that line goes.

The end of that line to the right indicates the system hitting peak power output, where current is still being supplied at its limit but system voltage has been reached (or duty cycle hits 100%), and from there on as speed increases power output remains constant (varying with the motor's efficiency/speed) as torque output drops.

Agreed, rating a battery mower with the same units as a gas mower would not only look bad, but also confuse consumers trying to relate the two.
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larryh

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I mowed the lawn yesterday.  The SoC of the battery was between 15% and 25% when I completed.  That means it took about 0.29 kWh of energy to mow the lawn.  I have an old gas powered lawn mower with a 1.6 quart tank.  Last time I mowed, it took almost an entire tank of gas (approximately 0.27 gallons of gas, i.e. 9.1 kWh).  The gas powered mower required 32 times the energy of the EGO mower.  However, the old gas powered mower is not working very well (hence the new EGO mower).  It doesn't have much power anymore.  It dies very easily cutting tall grass.  I have an impossible time getting the oil level right--it is very sensitive.  Either too high or too low and it makes a lot of noise and simply dies when it warms up.  Without significantly improved efficiency, an battery powered mower would not be feasible.  It would take a 300-400 pound battery to provide the 9.1 kWh of energy of the gas mower.  The battery was still relatively cool after I completed the lawn. 
(Edited)
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Blue Angel, Champion

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Sounds like your old mower might be down on compression, and the oil level thing might be oil getting past the rings and fouling the plug if it's high enough to run into the cylinder and get by th piston? Just speculating.

A fair comparison would be for someone to take a relatively new mower that's running well and do the same. My bet is the 32:1 would fall quite a bit, but the Ego will still win by a mile. Efficiency is cool. :-)

I find the hottest part of my battery after a mow is the black end cap where the power indicator is, and I have a feeling it's hot only because it's exposed to the sun under that clear cover. The body of the battery where the cells are is usually barely even warm after a cut, and that even goes for the small 2Ah batteries. I've considered painting the inside of my battery cover just to keep the sun light out.
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