Husqvarna versus Hummvee. The one on the left is the true environmental bad guy? Really? Who says so? And how do they know?
This post is the second in a series tracking down the origins of this generic statement:
- One hour of mowing your lawn using some gas-powered lawn device
- produces as much something-something-something
- as 200 or 300 or 350 miles of driving something.
The information you requested is unavailable.
A good overview of this issue can be found on The Straight Dope. Note how old that is — that posting dates to 2010. Clearly, many variants on this one-mower-hour-equals-300-miles theme already existed at that time. That posting specifically notes that all of the information behind that claim was made obsolete in 2012, when the EPA issued new standards for pollution from small engines.
And yet, we still see that exact same language today. It was one hour equals 300 miles (say) 20 years ago. And it’s the same today. Despite the 2011/2012 EPA regulations limiting small engine pollution (Source: EPA). And despite two decades of changes in passenger vehicle technology and mix of vehicle types.
I get my first clue about the loosey-goosiness of this mower-versus-car statement from the California Air Resources Board (CARB), which currently says (emphasis mine):
Today, operating a commercial lawn mower for one hour emits as much smog-forming pollution as driving a new light-duty passenger car about 300 miles
Based on that critical word — commercial — maybe the commonly-cited one-hour-equals-300-miles statement has little to do with my 21″ Husqvarna mower with its Honda GVC-160 engine. It was based on … something else.
And as I dig deeper, I’m beginning to understand why I couldn’t find any details on what these lawn-mower-versus-car statements actually mean. After some hours of internet search, I can find authorities such as CARB that make those statements. But I can find absolutely nothing on the details behind those statements.
In short, I know what CARB said, but I still know nothing. I have no idea what CARB means by:
- “a commercial lawn mower”, or
- “a new light-duty passenger car”, or
- “smog-forming pollution”.
As far as I can tell, CARB provides no details whatsoever. Or, at least, none that I can find on their website. Do they mean operating the equipment in a typical use-case, or do they mean running it full throttle, flat-out? Do they mean a new piece of equipment, or the average mower currently in use. How big is a commercial mower? Does “passenger car” include SUVs or not? Did they use a specific passenger car, such as a Prius?
And I still don’t know how to scale it down to my actual, as-used 21″ lawn mower.
Even worse, after looking into the regulations, I may never know how much pollution that mower emits. That’s because U.S. regulations appear to be stated in terms of maximum limits, when the engine is run through a pre-defined duty cycle. As far as I have been able to tell, nobody publishes the actual as-measured data on actual engine use.
Can you derive that statement from the regulations?
Now things get really nuts. Even if I can’t find data on actual emissions, I ought to be able to find information on emissions limits for small engines and cars, and compare them.
And I can do that. The only problem is, if I do that accurately, with modern emissions limits, that makes small engines appear vastly worse than the one-hour-equals-300-mile meme suggests.
Let me start with my lawn mower, with a Honda GCV-160 engine, displacement of 160 CC or 9.8 cubic inches, rated for 4.4 horsepower or 3.3 KW. All of that is per Honda.
Next, the EPA standard for “Class I” small portable engines is 10 grams of NOx and exhaust hydrocarbons per engine KWH per hour. So, for the Honda engine rated at 3.3 KW, the EPA would appear to allow 33 gram per hour, combined NOx and exhaust hydrocarbons, under its mandated duty-cycle testing.
But the EPA standard for cars (shown here) works out to new-fleet average of just 0.03 grams of NOx and exhaust hydrocarbons per mile, for all passenger vehicles.
When I put those together, the exact statement appears to be that for one hour of running my Honda-powered lawn mower, the EPA allows that mower to release as much N0x and unburned hydrocarbons as (33/0.03 = ) 1100 miles of driving, by the average new gasoline-powered passenger vehicle.
I think I understand why I get such an extreme answer. I used the modern (Tier 3) emissions standards for cars. Those only went into place around 2017 or so. Whereas these statements about mowers-versus-cars originated much earlier. If I track down the Tier 2 standards for cars, and use the cleanest “bin”, the standard calls for no more than 0.125 grams NOx and unburned hydrocarbons per mile. For that standard, the maximum allowable NOx and unburned hydrocarbon emissions from one hour of mowing equal the allowable emissions for (33/.125 =) ~250 miles of driving a typical passenger car at the maximum allowable Tier 2 emissions.
So it appears plausible that the one-hour-equals-300-miles statements derive from comparing maximum allowable levels of smog-producing exhaust emissions. And that if anyone bothered to update those old statements to the current (Tier 3) car standards, they could make an even more extreme statement.
But. But those are the upper limits on what is allowed. They aren’t the actual emissions.
And none of that squares with the current CARB statement cited above. For CARB to make that one-hour-equals-300-miles, they had to specify a commercial (presumably, large) lawn mower.
So I now think I understand how you could come up with that statement. But I’m still not quite sure whether that statement reflects the real-world outputs of those pollutants.
Still, it remains plausible that a small lawn mower engine really is that “dirty”, by modern car standards. The EPA estimates that modern vehicles produce about 2% of the smog-forming pollutants that (say) 1960s-era vehicles did. And, basically, lawn mowers are still back in the 1960s in terms of pollution controls. Catalytic converters, sealed fuel systems, exhaust-gas regeneration — all of those pollution controls are standard on cars, and unheard-of on lawn mowers.
In any case, I’m going to keep digging. Somewhere, somebody should be able to show actual measurements of emissions of a modern lawn mower, in a form comparable to emissions measured for a modern car.
