Do cars burn fuel so much more effeciently than lawn mowers that the difference in noxious emissions is that great?
Isn’t there X amount of crap in a measurement of gasoline that can be released total?
6 cars traveling 40 miles per hour, assuming 20 mpg each, will burn 12 gallons of fuel total in a one hour period.
1 Briggs & Stratton, going through about a pint in 20 minutes (per the original question) will burn about 3 pints per hour (this seems about right to me, it takes me about an hour to mow my lawn and I usually go through a tank and a half on average).
That’s a 16:1 ratio of fuel being burned in the car versus fuel being burned in the lawn mower; and that’s at the lower end of the numbers cited in the article.
Is an automobile really 16 times more efficent than a lawn mower? Is there really enough crud in equal amounts of gasoline to justify that ratio?
For the amount of power put out-yes. Try to imagine how many lawnmower engines it would take to run a standard mid-sized car down the freeway at 60mph, and what the total emissions would then be. Next, try to imagine how large and powerful a lawnmower you could make if you used a V6(or even a V4) engine.
There’s also the related matter of emission controls. Lawn mower engines don’t come with the plethora of recirculation, monitoring, mixture tweaking, and converters that any car after about 1975 has come with. I don’t have any numbers, but I suspect that those control devices are a significant part of the reason that automotive emission standards can be higher.
That said, I do my part for Mother Earth by mowing the home quarter acre when the wildlife starts threatening to snatch pets and small children in the night. And watering when God turns on the sprinkler system in the sky.
Also, among the test conditions cited in the article is the lawn mower engine running “at half throttle”. Maybe it’s because I’m mowing whenever the grass gets above knee-height, but 1/2 throttle equals “restart the engine again because it stalled out.” Besides, at 1/2 throttle, most mowers will neither bag nor mulch grass well, guaranteeing huge wads of masticated cut grass falling gracelessly out of the back of the cutting deck as you advance. Plus even more likelihood of stalling. So I’m dubious of the real-world applicability of that part of the test conditions.
So it would take 28 B&S lawn mower engines (at 3hp each) to equal my Saturn station wagon’s V4 at 85hp.
1 hour at 60mph / 20 mpg equals 3 gallons of gas consumed by the Saturn.
1 hour at 3 pints per hour times 28 mowers equals 21 gallons of gas consumed by the horse power equivalent array of lawn mowers.
So we have a 7:1 consumption ratio versus a 16:1 emissions ratio.
Recirculating and catylizing the unspent HCs reduces the NO+HC emissions, but the number still seems high* to me considering CO is CO.
Athough the thoughts of putting a Mitsubishi 2.8L DOHC on top of a 28" mower deck does intrigue me a little; should help eliminate some of my mowing time :).
Actually, I’m really Fierra, and Fierra is really Stan Grabasski, a 6’10" love machine from Scranton, Pennsylvania. Oddly, I was Pitted once by someone who believed Fierra was my sockpuppet.
Serious answer: Cecil lives in Chicago. I talk to Cecil on the phone. I mail Cecil and he mails me. Cecil sent me a holiday card once. Cecil is Cecil.
If I appear frequently in the columns over the last few years it’s because I’ve had time and interest to help out, I’m not averse to spending hours in research libraries chasing down strange and obscure subjects, and I like doing experiments like baking alcoholic cupcakes, trying to cook eggs with my stereo, or mixing up storm glasses.
Others help out Cecil of course, both on and off-board, and he’s credited them as well. We do this because we believe that the Straight Dope is a good thing, a worthy effort which delivers a damn lot of value to folks for absolutely free. I think the millions of page hits on the main column every week (as well as untold numbers of readers of the print version) validate that belief to some extent.
I always believed that that it wasn’t the engine of the lawnmower which created so much pollution, it was the cut grass itself.
This opinion came about from listening to lectures at Uni, although with no independent research of my own.
15 seconds of Googling comes up with papers such as:
Small engine woes
Haberer, Kym | Stauffer, Julie
Alternatives. Vol. 24, no. 4, pp. 3-3. 0000
No, that’s not true. Both automobile engines and modern four-stroke lawn mower engines operate on the same cycle, with the same theoretical efficiency at the same throttle position. The smaller lawnmower engines suffer in comparison because of scale effects (increased heat transfer losses) and higher relative friction when they have fewer cylinders. So they do certainly wind up less efficient than the larger automobile engines (at the same throttle), but not by a factor of sixteen.
Actual engine efficiency at any one time is a function of how it’s being used (the speed and load). For an automobile, that’s highly variable; for a push-mower, not so much. I suspect, although I don’t know, that the push-mower engine might actually be more efficient on average than the automobile because it’s used in a narrower band at higher throttle.
The difference in emissions, however, is more attributable to emission controls and aftertreatment, as gnoitall suggests, than to base engine efficiency.
I’m not quite sure of your intent here, but the cited paper doesn’t support the idea that “the cut grass itself” created pollution.
The article raises an interesting question on fatalities, which is more dangerous to operate, the vehicle or the lawn mower? Based on the numbers in the article, and the lower ownership rates compared to cars, I’m betting the lawn mower is more dangerous to operate than a motor vehicle. Anothe reason not to cut the grass!
Personally, when I eventually get my own place with a yard, I plan to get an old-fashioned mower that’s powered by the user pushing it. Getting a little exercise just seems more convenient to me than having to refill a tiny gas tank and fuss about with a motor, or worrying about accidentally running over a 50-foot extension cord draped across the lawn.
Yeah, that’s what I thought I’d do. I have a tiny lawn and I believe in reducing pollution and taking responsibility for my own surroundings. But you know what? Push mowers suck. They really do. It was impossible to get the lawn cut. For that and other reasons I finally got a guy to come mow n blow every couple weeks.
Push reel mowers used to be much better than they are today. Just like ratchet screwdrivers and crank drills, they’ve been pushed down to the bottom of the market, and are consequently crap now, with gears, etc., machined to a tolerance of about 1/8" and little or nothing in the way of bearings. It was easier to push my father’s old reel fifty-odd years ago than it is to push one in the store, today, even though the new one is half the weight and factory-clean.
This. where toxic emissions are concerned, a modern automobile has at least three things going that a small engine doesn’t:
Exhaust gas recirculation. Seems weird, but feeding 10-15% of the exhaust gas back into the intake actually reduces peak combustion temperatures, greatly limiting the formation of NOx.
3-way catalytic converter. This takes engine-out exhaust gas and reduces HC, CO, and NOx by a huge factor; NOx in particular is reduced by something like 90+%.
closed-loop fuel injection. This lets the computer control fuel quantities so as to very precisely control air-to-fuel ratio within a very narrow band that minimizes the engine-out levels of all three criteria pollutants, and even deliberately wiggles that ratio back and forth ever so slightly a few times per second to make best use of the catalytic converter.
The result is that lawn mowers are indeed filthy little bastards, even if their overall thermal efficiency isn’t dramatically lower than that of a car cruising down the highway.
There is X amount of hydrogen and carbon in a gallon of gasoline. The ideal would be to turn it all into water and carbon dioxide; the reality is that some carbon monoxide, oxides of nitrogen, and unburned hydrocarbons are inevitable.
A gallon of gasoline weighs about 6 pounds (2724 grams), so if your car is getting 30 miles per gallon, you’re burning about 90 grams of gasoline per mile, and making a combined 406 grams-per-mile of water (H2O) and CO2. (the 406 doesn’t include the atmospheric nitrogen passing through the engine.)
0.25 grams per mile of HC
0.4 grams per mile of NOx
3.4 grams per mile of CO
Given that the car is spitting out 400+ grams of exhaust per mile, and the nasty stuff is measured in single-digit grams per mile, it becomes easier to see how an engine could emit many times the regulated level while still not suffering a major drop in overall thermal efficiency. This is particularly true of NOx, which doesn’t require fuel for its formation - just heat and excess oxygen. turn up the peak temperatures add in a non-homogeneous mixture that has lean areas with excess oxygen, and NOx output goes through the roof.
As it happens that same web page mentions how bad cars used to be in the 1950’s:
13 grams per mile of HC (52X a modern car)
0.4 grams per mile of NOx (9X a modern car)
3.4 grams per mile of CO (26X a modern car)
Bottom line is that yes, modern cars are very, very good at burning gasoline cleanly.