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Student-designed device reduces gas lawnmower air pollution by over 90 percent


July 8, 2014

The UCR NOx-Out device replaces the muffler on an existing gas mower

The UCR NOx-Out device replaces the muffler on an existing gas mower

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Gas-powered lawnmowers are notorious polluters. According to the US Environmental Protection Agency, running a new gas mower for one hour produces as much air pollution as would be generated by 11 typical automobiles being driven for the same amount of time. Switching to an electric or reel mower is certainly one option, but for those applications where it's gotta be gasoline, a team of engineering students from the University of California, Riverside are developing another: an attachment that they claim reduces noxious emissions by over 90 percent.

Known as UCR NOx-Out, the device takes the form of an L-shaped stainless steel pipe, that replaces an existing mower's muffler. It cleans up the exhaust via a three-step process.

First, a stainless steel filter removes the bulk of the particulate matter. Next, a fine spray of urea is released into the exhaust stream. Finally, that urea reacts with a copper zeolite catalyst to convert the exhaust's nitrogen oxide and ammonia content into innocuous nitrogen gas and water, which are released into the air. That catalyst also converts the carbon monoxide into carbon dioxide.

The device with its filter (top left) and catalyst (bottom left)

NOx-Out was initially developed by another team last year, and will likewise be further developed by a third team next year. All of the students have been (or will be) advised by Dr. Kawai Tam, Prof. David Cocker, and Assistant Professor Phillip Christopher.

The previous version of the device was reportedly shown to reduce carbon monoxide content by 87 percent, nitrogen oxides by 67 percent, and particulate matter by 44 percent. In the current version, the figure for particulate matter removal jumps to 93 percent.

Other improvements introduced by the most recent team include the longer-lasting steel filter (it was formerly made of quartz), a honeycomb-structured solid catalyst (which previously took the form of a powder that could get blown out), a less obtrusive one-piece design, and the addition of a noise-reducing muffler.

The device is about to be trialled on the lawnmowers used to maintain the campus grounds. Once commercialized in an estimated four to five years, it is estimated that NOx-Out should be available for about US$30. Dr. Tam tells us that an 8-oz (237-ml) container of urea should be good for approximately 10 to 13 weeks of use, in cases where an average-sized lawn is being mowed for around one hour per week.

Sources: UC Riverside, UCR NOx-Out

About the Author
Ben Coxworth An experienced freelance writer, videographer and television producer, Ben's interest in all forms of innovation is particularly fanatical when it comes to human-powered transportation, film-making gear, environmentally-friendly technologies and anything that's designed to go underwater. He lives in Edmonton, Alberta, where he spends a lot of time going over the handlebars of his mountain bike, hanging out in off-leash parks, and wishing the Pacific Ocean wasn't so far away. All articles by Ben Coxworth

Just don't forget to make them retro-fit capable, and size them for lawn and garden tractors as well.

Glen Jacobsen

They pollute more, because they're 2-stroke, which spews unburned fuel. This seems not to mention that, but this would be the main fact that makes the "11 car" comparison stand up.


Still using the old 2-cycle mower/ 11 car analogy. How many 2-cycle mowers are manufactured these days?

Fairly Reasoner

Affordable AND effective? I'd add that to my mower.

David Finney

So you need a supply of urea for it to work.

Another way is to miniaturize EURO VI technology, but it will not be so affordable, I suppose.


At that price I'd use it!


Yaay! The first practical looking catalytic converter for motor mowers ... License the manufacturing to a big maker and sit back to watch the $$$$ roll in! Seriously though, congratulations to the team who brought this to its present state, absolute genius!

The Skud

Christopher, the percentage of two stroke engines on lawnmowers has never really been very high, and the actual number of them still in operation since the late 1980s is so low as to not even be worth mentioning. But instead of using an exhaust-side solution that requires a chemical additive and plumbing/parts to make it work, why not use a better fuel delivery system to vaporize the gasoline instead of trying to burn droplets? Oh, they can't do that, it would make too much sense!!


Expanded Viewpoint

The motor in the photo appears to be a 4-cycle. Note the yellow cap for the oil fill. The article seems unclear about whether this device works on both 2 and 4-cycle engines. Certainly the 2-cycles are bigger polluters than the 4's. Adding urea would be an inconvenience, but maybe not a big expense.


How much of an improvement would just switching to propane/butane fuel make. It would certainly reduce the amount of spilled fuel.


The only reason we still have gas powered lawn mowers is that the price of those cheaply made engines has remained remarkably within a small range since the 1950's when my brother and I got Dad to buy one on the promise we would pay him back through mowing other people's lawns. That Sears Roebuck 4-stroke rotary cost $60 bucks and was the cheapest 4-stroke they sold. It was a young fortune to us however and took what seemed forever to pay off at 50 cents per lawn. Its equivalent today is less than triple that price, when automobiles cost ten times what they did then. Perhaps if the lobbyists for Sears and the other retailers hadn't managed to exempt them from most EPA regulations we would be paying automobile equivalent prices for mowers that run as clean.

Paul Gracey

The 4 to 5 years is ??? Do the profs limit the student involvement to the new class each semester? Each year we start at 0? Sounds like an old soviet 10 year plan.


How much does it muffle the sound and how much power do you lose to back pressure in the exhaust? Who's going to keep filling the urea supply after the novelty wears off?


It's physically impossible for one small, 4-cycle engine, as used on all lawn mowers the past several decades and most other power yard equipment and motor scooters for about 20 years, to pollute as much as 11 (or choose some other number) of "typical" cars.

The small engine simply does not burn that much fuel or pump that much air through it.

I suppose it's 'possible' if the amount of output per gram of fuel or some other mathematical mummery is used... but the total amount is just impossible BS.

Making small engines cleaner is a good thing, but please stop the phoney-baloney.

What people won't go for on this is having another tank to fill up with the urea (I assume it's DEF, as used by some newer diesel trucks to work with exhaust particulate filters) because it's something else they have to buy.

What would help as much or more would be a micro electronic fuel injection system. That would have a dramatic improvement in the efficiency of small engines over the simple and primitive carburetors they use. Less fuel in, less pollution out!

Gregg Eshelman

@Gregg Eshelman, it's been well established that 1 hour of mowing produces as much HC as a 100 mile car trip. Break it down from there any way you like. No surprise given the effectiveness of catalysts. Certainly regarding the more important CO2 lawn mowers will have little consequence in the big picture.

Paul Axford

It's always great to reduce any pollution; what I can't figure out is why they can't develop a grass plant that only grows to 3 - 3 1/2". No mowing. Ever.

Old J Hawthorne

@ Paul Axford How does a gas that is vital for life, that was at above average concentrations when the thermal maximums (heat ages) ended and below average concentrations when the ice ages ended get to be the important pollutant?


Compulsory retro-fit for all those horrible old Kawasaki bikes out there!

Dirk Scott

EPA regulations have basically banned any lawn mowers from being able to use 2 cycle engines. If the mower has an extremely small engine, under 100cc, it may be able to use a 2-cycle engine, but most mowers are well above 100cc. As you can see in the list here, there is basically no 2 cycle mowers on the market, but they can still be found on string trimmers and other handheld tools.

Bill Mottashed

I like the original suggestion: Go electric (or push), that's what I did. No more noxious fumes (or tune-ups) no running to the local gas station for a gallon of gas, no winterizing.

Ele Truk

@ Old J Hawthorne If you let grass grow to its full height it goes to seed.


I hope this is good for the grass and leaf blowers. I feel sorry for the people who have to run them.

Barbara Baldwin

the internal combustion engine will have an abrupt end. I suspect nano- particle emissions


Emission reduction technologies for small engines such are those used in lawn and garden equipment, ATVs, motorcycles, etc., have existed for many years but do to a LACK of emission regulations they remain IDLE, thanks to the small engine manufacturers.

I'm a designer / developer and patent holder, of the worlds BEST emission reduction technology for small engines. This technology has been tested by Emissions Research and Measurements Division (ERMD), Ottawa, Canada, and Carnot Emission Services, San Antonio, Texas, USA, demonstrating unprecedented emission reductions in carbon monoxide (CO), hydrocarbons (HC's) and oxides of nitrogen (NOx). I've sent hundreds of letters to both governments, Canadian and the United States of America, and the lack of support was truly disappointing to say the least, and yes, CARB, the US-EPA and Environment Canada, are all aware of my technology and it's superior emission reductions.

This specific technology would increase the cost of a basic lawn mower by 20-USD, and maybe 30-USD for a ride on mower, ATV, etc, a cheap price to pay for clean air.

The following explains how small engine manufacturers are beating the system.

Emission regulations for cars and trucks measurers all three pollutants separately, carbon monoxide, hydrocarbons and oxides of nitrogen, each pollutant tested and measured individually, however, emission regulations for small engines are different and actually allowing an increase in oxides of nitrogen levels.

While emission regulations for small engines measures carbon monoxide separately, hydrocarbons PLUS oxides of nitrogen, are measured and combined together for ONE emission value, and therefore, with all these new engines running lean thus increasing temperature while reducing hydrocarbons, the oxides of nitrogen values increases significantly, many time higher then ANY car, truck and or other.

If emission regulations for small engines were the same as those of cars and trucks, small engine manufacturers would be forced to use a catalytic converter and or other emission reduction technology.

Note = the combining of two SMOG forming pollutants for one emission value is truly allowing an increase in either SMOG forming pollutant.

FYI, oxides of nitrogen created by small engines can move or blow from town to town, state to state and country to country, mixing with hydrocarbons created by other sources and still form SMOG, therefore, WE NEED A SIGNIFICANT CHANGE IN SMALL ENGINE EMISSION REGULATIONS.


Trust me, small engines ARE big time polluters, and yes, many times worst then cars or trucks. Don't be misled by the amount of fuel they burn, that really don't mean all that much. One 4 stroke lawn mower operating for one hour will emit more oxides of nitrogen (NOx) versus 200 cars running for the same amount of time. 2 stroke engines are extremely high is carbon monoxide and hydrocarbons while oxides of nitrogen values are extremely low, however, both 2 and 4 stroke engines are significant contributors to the formation of SMOG do to one being rich in hydrocarbons while the other is rich in NOx. If you haven't tested these engines then please don't comment unless you explain with technical date, otherwise, it's better to say nothing.

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