Why do lawnmower and leaf blower engines have to pollute so much?
We always hear about them being restricted on “Smog Days”.
Why can’t their engines be made more efficient? Is it a size constraint? I know it has something to do with 2-cycle vs. 4-cycle, but why can’t they then just use 4-cycle?
Why can’t you put 4-cycle principles into a small engine?
Cost.
Either way, they’re likely going to have direct exhausts. Unless, as Uncle Beer says, you want to pay $500 for a lawnmower with a dangerous catalytic converter.
Most of the lawnmowers I have used all my life have variations of Briggs & Stratton motors that ARE 4-cycle. Granted some manufacturers may still use 2-cycle motors, but that tends to be the exception rather than the rule…at least from my experience.
Why should the engine cost that much more?
Can’t they have 4 cycles and just scale it down? I’m not talking about putting a car engine there, or doubling the current pistons, but using smaller parts with the same ignition pattern, so the piston doesn’t go into exhaust phase of the cycle before the fuel is completely burned.
Why can’t you just use an electric? Mine work perfectly fine for me.
One of the reasons I’ve heard that chainsaws are still 2 stroke is that they have to operate upsidedown. A 4 stoke with an oil pump and sump can’t operate upsidedown, unless you take extreme measures, like with a stunt plane.
Because adding a small valvetrain is very costly. A 2-stroke engine is much simpler. You need to change the cylinder head and cylinders, add valves, springs or other actuating mechanisms, a cam or some sort of device to provide lift and timing, different induction system and exhaust system, etc. Sure, there are always some alternative methods as well to get a four-stroke Otto cycle, but they are all much more costly than the simple, reliable, two-stroke.
Very important - also note that if you keep the cylinders the same, because you now have only half of the power strokes per cycle, you therefore have a less powerful engine - and this may require either making it larger, or adding extra cylinders.
Also, another reason that these devices have a much higher pollution and lower efficiency than a two-stroke is due to their very primitive fuel management system.
What about the Wankel rotary engine? It’s lighter than a conventional four-stroke, has fewer moving parts, and causes less vibration at high speeds; this sounds ideal to me. Though I admit to not being an engineer or a mechanic, so maybe I’m overlooking something important.
Not entirely true. Many model airplane engines are four stroke and use a gas/oil mix. They operate just fine in any orientation. Some of them are even radial or opposed configuration. Just like the “real thing”.
Two strokes offer a much higher power to weight ratio as they don’t need the “extra” parts that Anthracite mentioned.
One thing that makes lawnmower and chainsaw moters dirtier is that they are air cooled, auto engines are water cooled (except old VW’s and Chevy Corvairs).
Air cooled engines have a wider operating tempature range than water cooled and run on a rich fuel-air mix to keep combustion tempature cooler than optimum.
YMMV
Peace
LIONsob
>> Can’t they have 4 cycles and just scale it down?
It don’t work that way. First of all, can you imagine if your leaf blower had a four cycle motor with all the antipollution gear of a car? Sounds like the steam powered dildo mentioned in another thread.
Some things do not scale down. The sensors and electronics are whatever size they are. They are not smaller for a smaller motor.
Also, larger motors are more efficient for several reasons. Take the specs of a 100 Kw transportable electric generator and of a 3 Kw portable generator and you will see the efficiency of the smaller one is pitiful compared to the larger one. Friction is proportionally greater in a smaller engine and there are other factors.
You cannot just “scale down” things.
Also, lawn mowers and leaf blowers are not used daily like cars. Even if they pollute, the cost of reducing their pollution would be prohibitive.
Gary M just said "Many model airplane engines are four stroke and use a gas/oil mix. They operate just fine in any orientation. Some of them are even radial or opposed configuration. Just like the “real thing”.
If they can be scaled to fit into a toy plane and a real plane, they can be scaled to any size inbetween.
Also, they already ban leafblowers in many southern California cities, and are talking about banning gasoline lawnmowers, so there is a lot of incentive to build better ones.
Mine, too. Oh, it still pollutes, but the spent fissionable material will be buried in some other state and won’t bother me!
Seriously, the cord’s a real bummer. I hate it (the cord, not the blower/vacuum). My battery-powered electric lawn mower really rules though!
And speaking of things that suck, I once knew this girl… 