The Kneeslider

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STIHL 4-Mix – Fuel Lubricated 4 Stroke Engine

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Stihl 4-Mix fuel lubricated 4 stroke engine
Stihl 4Mix fuel lubricated 4 stroke engine

Since we’ve been looking at some interesting engines lately that take the road less traveled, a comment by rafe03 on the Ellwood Hybrid story pointed to this hybrid that needs no oil in the crankcase and no oil pump either, but it operates as a 4 stroke. The fuel mix lubricates the engine like a 2 stroke.

The fuel mix is drawn in through the crankcase and, by a very roundabout way, makes its way to the intake port, lubricating everything along the way. There’s an animation on the Stihl site but I’m having a little problem understanding exactly what’s going on.

Their engine is pretty small and used on weed trimmer type equipment, I have no idea whether this would scale up and produce decent power but it is interesting.

Life used to be so simple, 2 stroke, 4 stroke, diesel. Now we have 2 and 4 stroke hybrids, 6 strokes, external combustion engines …, and turbines and electrics and hybrids. Kinda cool.

Link: Stihl

Comments

  1. so is this just for simplicity and weight reduction?

    do you think there’s a power benefit here in any way?

  2. Looks to me like the crankcase supercharged principle from a few days ago, but I doubt it is for supercharging. More for lub without oil so it will survive upside down, or whatever position.
    But it looks like it needs a reed valve on the intake to prevent blow back through the carb.
    It may get a bit of supercharging so the ports can be made small. This is a string trimer engine, or a chainsaw engine.

  3. This is done to skirt the EPA. The EPA wants to ban the import of two-storke motors, no matter how clean you make it, end of story. However, the emissions allowance for yard care equipment is higher than what a well designed, pre-mix, two-stroke engine can regularly achieve. Burning pre-mix in this engine must still fall within the emissions allowance for a four-stroke engine. The benefits of not having an oil sump is being able to turn the motor upside-down or on end or whatever position is most comfortable to trim a hedge or lawn.

    -todd

  4. problen with 4 stoke hand held gas powered lawn equipment is oiling. they oiling system has to be designed so that no matter the orientation of the motor there is enough oil to the system to adequitely lube everything. adds weight and costs. this seems to fix that. mechanically actuated valves of a 4 stroke prevent the poluting blowbye of unburned gas and oil that plagued 2 smokes.

  5. How do oiling systems in aircraft piston engines work when turned upside down, or suffer negative G’s? Surely they have it figured out by now, and it must be scalable to some degree. This seems like the wrong way to fix the emissions problem, IMHO.

  6. @Mike –

    I’m not too familiar with relatively new piston aircraft engines but many old WW2 birds and aircraft from that era will quit if upside down or in negative g’s for too long. The announcers at airshows will comment in a sarcastic manner on pilots that start killing their engines while performing, “Don’t worry folks – that’s just the sound of his engine stalling!”

  7. they have obviously overcome the problem in aircraft. never said they can’t or don’t make 4 stroke weed wackers. i just said there is an engineering problem that needs to be overcome when they do. which adds weight and costs.
    as far as emissions, smiplified oiling through fuel mixture running through the crank that’s light and less expensive to produce and maintain and at the same time stop the unburned fuel from escaping the combustion chamber. win-win in my book.
    this would be really cool on some race bikes. start with MX lights. could shave many pounds instead of mear ounces. The guys from Husaberg and Vertimati have always pioneered the ever decreasing size of serious 4 strokes. i hope they get wind of this and start exploiting it.
    come to think of it there’s a powered surf-board that might have done this already.

  8. There are 4-stroke strimmer engines that have “inverted” oil systems. It’s been figured out.

    I believe that this fuel lubricated 4-stroke design was patented by a fellow in the Pacific northwest back in the late nineties. I designed and prototyped a 2-stroke with asymmetric port-timing right around the same time. The focus of my effort was to close off the exhaust port, in unison with the transfer ports, to reduce short-cycling and the resultant ultra high raw hydrocarbon emissions that are characteristic with simple 2-strokes. My design approach utilized internal changes only, and preserved virtually all the existing tooling in a typical model, along with carburetion, exhaust, etc. I used the displacement angle between the con-rod and the piston to induce piston oscillation. The crown and ports were “modified” to achieve the bias that was required…and the damned thing worked! Just think…twice as many power pulses and half as many parts.

    I love this stuff.

  11. im pretty sure aircraft engines utilize dry sump oiling so that they can operate in any orientation while still getting lubrication. nothing new/special, just like what is used in race cars, motorcycles and the like.
    this is pretty cool, although i dont see it catching on since the only difference(from what i can tell) between this and a dry sump is the lack of oil pump(assuming that this lubrication system is based on a dry sump of course, and not a modified wet sump of sorts). i think im understanding this thing correctly anyways.

  12. I’ve seen a number of radial engines that will smoke a bike upon start up. I think part of the start-up procedure is to run it through a few revolutions with the ignition off to clear it out.

    -todd

  13. I guess that they need to have twostroke oil mixture in the fuel?
    Does it meen that we have the pollution of a twostroke and the weaker power from a fourstroke?

  14. Todd radial engines accumulate oil in the bottom cylinders when they set for a period of time . Next time you go to an air show look at one oif those old timers you might see oil leaking out of the exhaust of the bottom cylinders . An old joke was if a radial was not leaking it was out of oil . Recips are not dry sumps .

  15. Just be clear recips are not dry sumps and radials are . Back to the subject . I have a 4 cycle weed wacker as long as I don’t go past the horizontal it won’t smoke . I like the idea one gas can for all my lawn equipment .

  16. Kneeslider speculates on scaling this design for motorcycle use. I think the biggest issue with this design is that it pumps its fuel oil mix through the valve gear twice. That’s got to make this motor the most inefficient air pump going, Not big deal for a weed whacker, but can you imagine trying to polish the intake track on one of these?

  17. One sure way to see if it will scale up!

    Seemed like there might have been a reed valve missing on the animation

    rafe03

  18. Ecoforce in Australia has had an engine with the same concept running for the last couple of years. The difference being that, as well as the fuel charge via the crankcases (to take advantage of the primary compression) it is also has self charging pneumatically operated valves. If you go to their website ecoforce.com.au you can read reports describing it as one of the worlds most efficient engines. As mentioned in other posts this type of engine can be used in all manner of applications. With the Ecoforce engine it can easily be scaled down to around 30cc and can be built in ‘boxer’ formation with an unlimited number of cylinders.

  20. yes it has a lot more power than your basic 2 stroke. with a 2 stroke you loose a small bit of fuel when the exaust is being pushed out we refer to this as scavanger loss but with the new 4 mix you retain a lot more fuel and it has better emissions to keep wahington boys off of stihl’s back i promise if you try one you will definatly tell the difference.