Two stroke four stroke.

We have all these stihl weed wackers and they take mixed gas but they have valves. Kinda strange. It's like a 2 stroke, 4 stroke hybrid.

20200312_095838.jpg

Re: Two stroke four stroke.

2 heads are better than one, and 2 strokes are better than 4. Probably just lacks good enough lubrication to the cylinder. pretty neat. alot of early 4 strokes have the offset valves with pushrods. like he intake-over-exhaust early designs (also known as F-head, not to be confused with certain posters on this site hehe)

Re: Two stroke four stroke.

^ HA! :D

Re: Two stroke four stroke.

Pushrod Fifty /

Its a plain 4 stroke engine.

Re: Two stroke four stroke.

> Pushrod Fifty Wrote:

> -------------------------------------------------------

> Its a plain 4 stroke engine.

No it is not. Takes mixed gas. No crank case oil.

Re: Two stroke four stroke.

I use a few Stihl 4Mix engined things at work. They don't have the instant acceleration a 2 stroke has, especially the pole saw I use. You pull the trigger and there is a noticable delay before anything happens. They sound funny too.

Re: Two stroke four stroke.

Yeah weird little buggers. I got to find one that is complete junk and tear into it to see what its guts contain.

Re: Two stroke four stroke.

yeah its just a four stroke that doesn't have a wet crankcase. I was pretty skeptical of them at first, they had a few issues, but they seem to be holding up pretty well. They said the big brushcutter they make the torque is way better now.

Hard to say...

I'm gonna keep running the saws and stuff that i got until electric gets good/cheap enough. Its pretty close, about 4x better battery capacity and it would be over.

Re: Two stroke four stroke.

Pushrod Fifty /

Now I know, interesting.

Re: Two stroke four stroke.

Pushrod Fifty /

From the Stihl website:

THE FOUR STROKES OF THE 4-MIX™ ENGINE ARE:

Induction

Compression

Power

Exhaust

Unlike other four-stroke engines, the 4-MIX™ engine is lubricated by the gasoline-oil mixture. As such, it must be run on a mixture of gasoline and engine oil. This allows a compact construction without a separate oil reservoir, oil pump and oil filter.

Re: Two stroke four stroke.

Rick Bergsma /

Those engines are very finicky about the valve clearance , fortunately it's easy to set .

Re: Two stroke four stroke.

> Pushrod Fifty Wrote:

> -------------------------------------------------------

> Now I know, interesting.

Haha, didn't mean to sound like a dick. It's a four stroke that takes two stroke mixed gas. We use amsoil sabre only mixed at 50:1 and have had very good results with these trimmers. The problem is the workers that use them don't clean the filters very often and they get clogged up with shit and makes them run super rich and get super oily and gross. We go through them this time of year and change plugs, filters and clean off all the debris and they keep on trucking. Awsome trimmers. The mowing crew uses them every day all day in the summer and they last for years.

Re: Two stroke four stroke.

I have yet to try a 4Mix grass strimmer. As I said, the 4Mix pole saw I use at work has a delay when you throttle it, so have wondered was a grass strimmer would be like. Although, the pole saw is 3 years old now, so maybe there have been improvements.

Re: Two stroke four stroke.

Pushrod Fifty /

How is the valvetrain lubricated?

Re: Two stroke four stroke.

> Pushrod Fifty Wrote:

> -------------------------------------------------------

> How is the valvetrain lubricated?

From the mixed fuel.

Re: Two stroke four stroke.

yea i'm also surprised that it gets enough oil up to the valves rods and rockers, esepcially since it's intimately on the head getting roasty up there

Re: Two stroke four stroke.

Pushrod Fifty /

With all of that dead space above the head, how would the intake be consistent? Have to scout the s rap bins for one to take a peek inside.

Re: Two stroke four stroke.

I dont understnad that question, theres no more dead space than typical, and less compared to the vertical offset valve type engines

Re: Two stroke four stroke.

Pushrod Fifty /

Ill see for myself someday.

Re: Two stroke four stroke.

this might help, it uses the crankcase like a supercharger kinda, but not really because there isn't any valving to let it build intake pressure. its pretty wild. the important thing is that it 'breathes' the gas/oil mix through the bottom end and valve train and keeps everything awash in loob.

yeah i used to have a lawn mowing business- in fact, thats kinda what got me into engines in the first place, its hard for most people to fathom how much maintenance it takes to keep power equipment running 30+ hours a week. Something like a trimmer can get hundreds of starts and stops each week.

definitely makes the battery stuff look appealing from that perspective as well.

Re: Two stroke four stroke.

Pushrod Fifty /

I can imagine the maintenance involved. The build quality of power equipment isnt so great but the Stink machines I have owned seem like the best.

Re: Two stroke four stroke.

Yeah it's actually pretty amazing how well they hold up, a couple of my saws probably have over a hundred hours at WOT close to 10k rpm. It's hard enough keeping Moped engines together for that, and these are built way cheaply with self tapping screws and plastic parts everywhere

Re: Two stroke four stroke.

> Graham Motzing Wrote:

> -------------------------------------------------------

> this might help, it uses the crankcase like a supercharger kinda, but

> not really because there isn't any valving to let it build intake

> pressure. its pretty wild. the important thing is that it 'breathes'

> the gas/oil mix through the bottom end and valve train and keeps

> everything awash in loob.

>

>

Graham FTW. thanks!

Re: Two stroke four stroke.

Blaine- The artist formerly known as Plumber Crack "(OFMC)" /

So what’s the point of it all? I thought going to a 4 stroke was to get away from having oil in the fuel, like an EPA thing. Are all four stroke trimmers this way? I haven’t bothered with gas trimmers since they went cordless.

Re: Two stroke four stroke.

This is a example of a type of engine called the "Otto" engine, which gets its name from the acronym of the process which burns gasoline "Octain Tenacity Torrification Operation". The "Otto" engine has several extra parts, namely a camshaft, valve train, ignition system, and spark plugs. It is a very complex engine where the camshaft has eccentric lobes which are acted upon by the movement of the valves. The valves are placed in the intake and exhaust ports and the movement of the intake and exhause gasses push on the valves which spin the cam shaft. The cam shaft is attached to a ignition distribution system that will fire a spark plug in each cylinder to ignite the gas. The spark comes at the end of the exhaust stroke, adding additional heat to the last portions of the spent implosion gases in the combustion chamber. Most of these have booster springs on the valves to increase horsepower. Unfortunately this complex system only replaces the function of the transfer ports which are used to store up energy in the 2 stroke engine which is delivered directly to the muffler bearings in the expansion chamber. Without the springs on this engine the expansion chamber would expand too much and might rupture.

This engine has many extra complicated components, each of which is required for the proper functioning of the whole. When the crankshaft spins, the offset lobes carrying the journals set up harmonic vibrations in the cases and cylinders. These vibrations cause air to be sucked out of the combustions chambers, past the rings which act like one-way valves in much the same manner as the reed valve in a diaphragm pump. The air is then stored in the transfer ports which act as a energy reservoir and can hold tremendous pressure. Because there are at least two transfer ports and reservoirs this also provides and equalized balance to the engine. These ports are opened up when an engine is blueprinted and balanced and sometimes more are added for added boost.

Once the transfer ports are fully charged the engine will create a vacuum in the combustion chamber. The resulting vacuum causes the intake and exhaust ports to open (due to the pressure differential) allowing mixture to be drawn into the crankcase on the carb side, and spent combustion fumes on the exhaust side. The momentum of the inrushing gas pulls the pistons down from underneath, in what is known as the "intake stroke". The pulling effect is so strong that the piston needs to have a rod to hold it in the engine called a connecting rod. The connecting rod prevents the pistons from falling out of the cylinders and into the cases, or from hitting the combustion chamber sides. Turbo engine are usually equipped with more then one connecting rod because of the increase in piston vacuum, often two or three rods per piston.

After the chamber is full, the pressure differential holding open the ports is relieved, and the ports shut. The mixture begins to cool slightly, producing a reverse pressure gradient, owing to the large amount of air pumped into the crankcase. The reverse gradient along with the temperature differential allows the energy stored in the transfer port to transfer down the passage called the crankshaft. Along with the vibratory energy supplied by the crankshaft, the pistons begin to move back towards the tops of the cylinders in what is known as the "compression stroke". As the density of the cylinder gases increases, the air and gasoline molecules are forced together more and more.

Because they are already quite hot from spent gases which were taken in during the intake cycle, their kinetic energy is high, and many collisions occur. At a critical point, the air and gas molecules suddenly combine violently. The kinetic energy from tis violent collision is focussed in the magneto because of the spinning magnets attracting the electrons towards the spark plug. When the energy contained in the magneto becomes too great the points are made to open which work like a circuit breaker and cause a spark for combustion.

The "combustion" causes the mixture to burn off all of the gasoline in the mixture which creates a void as the fuel is depleted. The mixture starts to implode, and cool rapidly as the kinetic and thermal energy is absorbed. This makes the gas pressure drop, and the pistons are sucked strongly towards the top of the cylinders. The resulting unbalanced acceleration produces a driving force which is resonant with the vibration of the crankshaft, reinforcing its motion.

Because of the momentum imparted to the pistons during the implosion of the fuel/air mixture, they are now moving quite rapidly. As they are constrained by the connecting rods, they rebound at the top of the compression stroke, before hitting the cylinder head. The wrist pins and rod bearings are made of very hard metal, so the collision is very nearly elastic, and no energy is lost. At each end of the connecting rod are bearings, the spinning of the wrist pin bearing is transferred through the connecting rod and is amplified by the crank bearing. The magnitude of amplification is based on the difference in size between the top end and bottom end bearing, usually about 4:1. The rotational energy is then transferred to the main bearings which are bigger still then the bottom end bearings and they are in pairs. The difference between the initial bearing in the top end and the main bearings is usually about 9:1. This ratio is called the compression ratio because the wrist pin bearing has all of the energy compressed into it and the main bearings are releasing the energy. The spinning of the main bearing is what is transferred through a transmission and finally to the PTO or plenum turning output shaft which delivers the engine power.

In the two over four stroke engine the power is created twice in each stroke, first in the compression stroke as mentioned above and then in the power stroke. When the pistons rebound, and begin traveling downward with an equal, but opposite velocity. The cylinder volumes thus begin to increase, and the gas pressure drops even lower than it was after implosion. This is called the "power stroke" because the motion of the pistons at this point is more powerful than it is at any other time in the cycle.

This extreme cylinder vacuum causes the intake ports to open, because the intake ports are maintained at atmospheric pressure while the transfer ports have now spent the energy stored which causes a differential. The exhaust ports, however, stay closed because of a vacuum which has developed in the headers. The cause of this vacuum is several fold: First, a rarefaction was caused during the intake stroke when hot exhaust gases were sucked into the cylinders. Second, the heat sinking effect of the long mufflers (whose job it is to cool the exhaust in order to most efficiently extract spent implosion products) has caused the exhaust to cool during the intake/compression cycle. Third, the acoustical resonance of the pipes reinforces the vacuum pulses at certain frequencies.

When the intake ports open, fresh mixture rushes into the cylinders, immediately neutralizing the vacuum due to implosion. This allows the pistons to move full speed to the bottoms of the cylinders where they again rebound off the connecting rods and begin traveling upwards. The gas is squeezed and pressure rises, creating a superheated condition in the spent gases. The pressure is relieved by the piston rings and vented into the crankcase, where the gases are safely relieved to the exhaust pipe by the crankcase breather and exhaust gas recirculation system. This is called the "exhaust stroke" because this will exhaust all of the stored energy. When the kill switch is pressed on a engine this is where the engine will always stop. To get more power from the engine you need to add on an expansion chamber to the exhaust system. An expansion chamber allows the exhause recirculation system to go through a pressure inversion because the pipe is always bigger then the engine itself. The inversion will make the piston become sucked down faster by the power stroke vacuum which will spin the wrist pin faster and generate more power. As the sides of the expansion chamber expand and contract it sets up a harmonic frequency which further strengthens the acoustical resonance of the intake/compression cycles.

They actually will work well on a moped, I have heard from a friend in New jersey who reports he has been able to GPS his bike at 72mph on a 50cc going uphill with the wind against him with a friend on the back holding two cases of beers.

Re: Two stroke four stroke.

"“I like this stuff. I really get it, People are surprised that I understand it. Every one of these (MECHANICS) say, ‘How do you know so much about this?’ Maybe I have a natural ability. Maybe I should’ve done that instead of running for president.”

Re: Two stroke four stroke.

Blaine- The artist formerly known as Plumber Crack "(OFMC)" /

Thanks Rebel. I had no idea that these new fangled string trimmers were so technologically complex. It sounds like they’ve made quite a few changes to them since i’ve torn one down. Good to know.

« Go to Topic — end of thread

Want to post in this forum? We'd love to have you join the discussion, but first:

Login or Create Account