Stihl 4-Mix Engine

[velvetfoot]

Anybody heard anything about this motor? I'm shopping for a brushcutter and most of the Stihls have them. It is a 4 stroke motor that burns a 50:1 ratio. It has no separate lube oil system. The guy at the store said it's about 3 years old and has been good so far.

Any other brushcutter recommendations?

 

[Eric Johnson]

I've heard good things about the Stihl four-cycle engines.

 

[velvetfoot]

Thanks Eric. Stihl seems to be the main brand sold by full service places around here. I guess I'll go with one.

 

[Corie]

I've used one before in the form of a hedge-trimmer. Felt lower vibration to me than a standard two-stroke hedge trimmer and it ran great. Started first pull everytime and idled very smoothly. I'd recommend it although I cannot comment on longevity, because I only used the machine for one summer.

 

[Eric Johnson]

A good four-cycle engine ought to outlast a two-stroke, I would think.

 

[velvetfoot]

I would think an advantage of this design (no sump) would allow more positions without losing lubing.

 

[velvetfoot]

I got the Stihl FS 90 (handlebars) http://www.stihlusa.com/trimmers/FS90R.html.
They seem to characterize it as low-end commercial.
It sounded pretty quiet in the shop.
It uses the same Stihl oil as my chainsaw.
The guy claimed the valves really won't need adjusting.
Seems like good balance with the shoulder strap, even though 'they' say it isn't terribly light, it didn't seem that way to me, but I have no frame of reference.
Got a blade too: there's quite a selection. I wasn't sure which one to get http://www.stihlusa.com/trimmers/blades.html. I got one (not pictured) with many teeth that's supposed to cut down 3" trees . It was also 10 bucks cheaper than others, and also made by Stihl.
Won't be able to try the unit for a couple of days or so.

 

[Corie]

I have to say that the hedge trimmers I used did seem heavier than the two stroke pair I had used. Makes sense when you consider the addition of a valvetrain I suppose. Ultimately though it was smoothier both in the vibrations sense and the powerband sense imho.

 

[velvetfoot]

I have an update.
It was "glugging" at full throttle.
Turns out the 4Mix engine has a rev limiter!
The whole reason for the 4Mix is pollution.
Coming soon to a chain saw near you.....

 

[Corie]

Glugging?


Ohh, you mean like bouncing off the rev limiter like a car would do? Yeah, I knew the reason for the 4-mix was pollution but you have to admit its quite a bit smoother, isn't it? I think a 4-mix chain saw, if producing the same power and torque as the 2-stroke counter part would be awesome! You can put me on the list for sure.

 

[velvetfoot]

I used it today again with a different blade: a three-segment blade not really meant for woody stuff. I didn't really notice it banging against the rev limiter today. I might get another blade for the additional remaining woody stuff-I think the first fine-tooth blade wasn't that good.

It is pretty smooth and quiet.

I tell you what though, you DEFINATELY want to wear the full-face hat with the ear protection, especially along the roadside. I had gravel pinging me every once in a while and it's not that much fun.

 

[Gooserider]

Well I know the motorcycle industry has been building 4 cycle race bikes for some of the dirt classes that are competetive with the 2 strokes, so it certainly seems possible to get similar power levels out of a 4-stroke package that a 2-stroke is noted for and less pollution. However the 4-stroke bike engines have motor oil in the crank cases, and burn plain gas just like a car engine... I don't quite see how you are supposed to reduce pollution with a 4-stroke engine burning 2-stroke premix. Yes you get some unburned hydrocarbons in a two-stroke due to charge overlap, but that's fairly well controlled these days, and as I understand it the big issue was that the oil in the mix doesn't get burned in the combustion chamber so it comes out in the exhaust. How does a 4-stroke solve this?

Gooserider

 

[TMonter]

Because of the way a 2-stroke works inevitably some of the fuel/air mixture gets sucked out of the combustion chamber because of the way it is filled/emptied. 2 Strokes use ports and a "reed" valve instead of valves, it simpler and you can rev higher, but it's also not as clean burning because the combustion chamber does not completely seal for the power stroke.

 

[Andre B.]

Figured I would come to the defense of the lowly 2 stroke.

The largest and most efficient internal combustion engines on the planet are in fact 2 stroke.
http://people.bath.ac.uk/ccsshb/12cyl/

Not sure where you could find a bar and chain to turn one into a saw.

 

[Corie]

Does engine efficiency scale with size?

I know its true for steam turbine power generation, that the small steam turbines simply can't be as efficient as the large scale ones. I think it has something to do with entropy or enthalpy, but to be honest, I don't remember enough of thermo to recall.

 

[Andre B.]

Does engine efficiency scale with size?

I know its true for steam turbine power generation, that the small steam turbines simply can't be as efficient as the large scale ones. I think it has something to do with entropy or enthalpy, but to be honest, I don't remember enough of thermo to recall.

Yes.
A big part of it is the combustion chamber volume to surface area ratio. Which results in less heat loss during the burning and expansion cycles. If you don't know it already you soon will with your new job, that much the same thing applies to wood stoves, why it is so much easier to get a long consistent burn in a large fire box then in a small one.

Another way to boost efficiency is to speed up the burn and expand cycles, the less time the hot gases are exposed to the relatively cold cylinder walls the less heat is lost. Why small engines tend to develop toward higher RPMs, not long ago a saw turning at 8000 rpm was fast now that's just a bit over idle.