# Someone please explain a 4 stroke engine needing 2 stroke oil mix?



## Adirondack (Dec 2, 2011)

I think they are called hybrid. Anyway I was looking at the Stihl backpack blowers and it seemed like it was 4 stroke but unless I misread the info it seems you have to use two stroke oil??????? If that is correct please explain? Thanks :bang:


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## SawGarage (Dec 2, 2011)

You read correctly..

the "4-mix" as they were called (not sure if Stihl STILL uses that nomenclature, but they did...)

requires a 2 cycle mix, but it actually a 4-stroke engine, complete with camshafts, pushrods, valves, rockers and valve springs...

it does NOT have a crankcase for oil-lube, so lube must go through the engine. 


With today's 50:1 and thinner mixes, it's quite feasible to work well.. (and they do, as long as the user ADDS THE OIL!!!)

the KM (kombi-motors, IE: powerbroom, polesaw, etc... km-101, 110, 130, 131's ) are all '4-mix'


hope this helps..


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## madhatte (Dec 2, 2011)

Simple: no oil pan. Mixed fuel provides lubrication which would otherwise require an entire separate system. 4-mix engines aren't quite the power monsters that 2-strokes are, but they are better than straight 4-strokes. Stihl makes 'em because they're "cleaner", probably mostly because they don't rev as high and can't emit as much NOx per time. I've run one of the 4-mix brushcutters (I forget the model) and liked the power well enough but could definitely feel the weight.


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## Captain Crunch (Dec 2, 2011)

I've run a 4 mix trimmer, love the torque and the dirt bike thumper sound.


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## CTYank (Dec 3, 2011)

madhatte said:


> Simple: no oil pan. Mixed fuel provides lubrication which would otherwise require an entire separate system. 4-mix engines aren't quite the power monsters that 2-strokes are, but they are better than straight 4-strokes. Stihl makes 'em because they're "cleaner", probably mostly because they don't rev as high and can't emit as much NOx per time. I've run one of the 4-mix brushcutters (I forget the model) and liked the power well enough but could definitely feel the weight.



Reportedly, it's because the "4-mix" engines are not subject to short-circuiting of normal 2-stroke, where fuel & air entering cyl from transfers can exit the exhaust un-burnt. Huge emissions challenge. Thus strato-scavenging to avoid that, and the need for all the mechanical gubbinses of a 4-stroke.


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## Adirondack (Dec 3, 2011)

Thanks for all the posts. I think I understand. As far as gas consumption how do the new "green" two strokes compare to these hybrid 4 strokes? I assume the new 2 stroke like the x-torq are not going to be as fuel efficient as the hybrid 4 stroke?


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## BloodOnTheIce (Dec 3, 2011)

SawGarage said:


> You read correctly..
> 
> 
> With today's 50:1 and thinner mixes, it's quite feasible to work well.. (and they do, as long as the user ADDS THE OIL!!!)



I've had a couple units run on up to 5 gallons of straight gas and were still running, they don't seize like a normal two stroke.
On these 4 mix motors the piston and cylinder is free of damage but, all of them had lower end damage, or atleast bearing noise.

In general these motor take abuse of landscapers much better than their conventional two stroke cousins. 

2 weeks ago we had our 2nd catastrophic failure of a 4 mix engine, it was a 2005 BR550 that had thousands of hours
of commercial use, and broke the connecting rod and after having 2 gallons of straight gas run through it.


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## Chris-PA (Dec 3, 2011)

2 strokes are inherently nasty polluting little buggers. When your pushing some of the air/fuel charge out the exhaust port how can it be otherwise? Another consequence of pushing some of your fuel out unburned is the waste of precious fuel energy too. They'll push the technology as far as they can with stratified charge and better mixture control, but there are limits. I expect these 4-mix designs will be the way to go. The downside is more weight and complexity.


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## K&L Landscaping (Dec 3, 2011)

In my business, the crews run STIHL FS130R trimmers and STIHL BR600 Magnum blowers that are equipped with the 4-mix engines. All I can say is...these things seem to be bullet proof! These units are run hard 5-6 days a week from late March to early November and no failures yet. As you can imagine, the crews don't "baby" equipment.

I have nothing but praise for the 4-mix engines!


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## c5rulz (Dec 3, 2011)

My neighbors Stihl pole saw has a 4 mix motor. The thing is amazing how well it will idle for long periods of time without loading up.


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## RES (Dec 3, 2011)

There seems to alot talk around that you must use Stihl Ultra oil with the 4-mix engines or else the valves get carbon build up. I don't now if anyone can substantiate this or not.


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## K&L Landscaping (Dec 3, 2011)

RES said:


> There seems to alot talk around that you must use Stihl Ultra oil with the 4-mix engines or else the valves get carbon build up. I don't now if anyone can substantiate this or not.



100% true! All my 4-mix units are run with HP Ultra. Those valves are tiny in there and are very sensitive to carbon buildup.


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## sunfish (Dec 3, 2011)

Adirondack said:


> Thanks for all the posts. I think I understand. As far as gas consumption how do the new "green" two strokes compare to these hybrid 4 strokes? I assume the new 2 stroke like the x-torq are not going to be as fuel efficient as the hybrid 4 stroke?



The new 'strato' XT etc are very fuel efficient and produce more torque than the standard 2 stroke.

As for the '4-mix' stuff, I have a Stihl FS110R and love the thing.


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## sps3172 (Dec 3, 2011)

RES said:


> There seems to alot talk around that you must use Stihl Ultra oil with the 4-mix engines or else the valves get carbon build up. I don't now if anyone can substantiate this or not.



Stihl wants you to use 'Stihl Ultra'. The real message, as I understand it, is to use a fully synthetic oil. As others have stated, the valve train is sensitive to pre-mix related carbon buildup.

As an interesting side note, this 'technology' (pre-mix in a 4 stroke) has been used by model engine manufacturers for over a decade. If you're up for some light reading, check out a company called 'YS engines'. YS has developed a method to harness the positive pressure under the piston during the down stroke (typically wasted on 4-stroke engines...vented to atmosphere) to provide positive manifold pressure (boost). It seems that Shindaiwa is attempting something simliar (although they use reed valves instead of the diskvalve in the YS) with their '4-hybrid' engines.

All that being said, the model engines in question are usually run on methanol/nitro based fuels which typically run much cleaner than gasoline.


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## stihl86 (Dec 3, 2011)

sps3172 said:


> Stihl wants you to use 'Stihl Ultra'. The real message, as I understand it, is to use a fully synthetic oil. As others have stated, the valve train is sensitive to pre-mix related carbon buildup.
> 
> As an interesting side note, this 'technology' (pre-mix in a 4 stroke) has been used by model engine manufacturers for over a decade. If you're up for some light reading, check out a company called 'YS engines'. YS has developed a method to harness the positive pressure under the piston during the down stroke (typically wasted on 4-stroke engines...vented to atmosphere) to provide positive manifold pressure (boost). It seems that Shindaiwa is attempting something simliar (although they use reed valves instead of the diskvalve in the YS) with their '4-hybrid' engines.
> 
> All that being said, the model engines in question are usually run on methanol/nitro based fuels which typically run much cleaner than gasoline.



Stihl uses the same principle. + and - crankcase pressures that is.


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## sps3172 (Dec 3, 2011)

stihl86 said:


> Stihl uses the same principle. + and - crankcase pressures that is.



I've actually spent quite a bit of time with the 36.3cc Stihl 4-mix (found in KM130, FS130, etc.). I was testing it for use in a UAV project. That being said (and I'm only speaking about the specific 36.3cc engine I tested...other sizes may vary in architecture but I highly doubt it) Stihl does not appear to be utilizing the positive pressure under the piston to provide manifold boost....at least not across the entire RPM range, like the YS and Shindaiwa solutions do. 

The 36.3cc 4-mix draws mixture though a small passage above the intake valve, through the rocker box, and down the push rod tubes to the bottom end. However, when positive pressure in the crankcase exists, the mixture is forced back through the same path. When the intake valve is open, this returning mixture could just as easily flow backwards out the carb as enter the combustion chamber. Stihl uses a fairly sophisticated 'air box' that provides some intake tract tuning to combat this spit back. However, they do not seem to positively 'capture' the mixture and 'force' it through the intake valve. The engines I tested would not run (with factory carb) at any speed above 'idle' with this airbox removed from the carb.


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## SawGarage (Dec 3, 2011)

BloodOnTheIce said:


> I've had a couple units run on up to 5 gallons of straight gas and were still running, they don't seize like a normal two stroke.
> On these 4 mix motors the piston and cylinder is free of damage but, all of them had lower end damage, or atleast bearing noise.
> 
> In general these motor take abuse of landscapers much better than their conventional two stroke cousins.
> ...




I work on these ALL WEEK long @ work... we have over 40 kombi-motored handheld brooms in the field... I have seen a couple failures where the bottom end was GOOD, but they lack compression.... to the point of NOT running....



The get worked HOT... they get to the point of BACKFILLING holes with them; thus heating the top ends up GOOD... lol

I will say that carbon build-up can be an issue as well..

BUT, for what they go through, they are QUITE durable...


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## Adirondack (Dec 3, 2011)

I was under the impression the new two strokes only inject air until the exhaust port is covered and then the fuel mixture is added. In theory the engine would be as fuel effective as the 4 strokes. However still retaining the power of the two stroke. If this is correct then is the Strato and x-torque the same thing? One stihl and one husky? Thanks.


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