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.
* 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.
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. :-)
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%.
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:
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?