Two-stroke oil premix ratios: Is less more? Is more less? Is more more? Is less less?

Fabian

Well-Known Member
Joined
Sep 8, 2009
Messages
4,521
Now here is a man who knows what he is talking about - wise advise and carries a good knowledge base of 2-stroke engines and his data from dyno testing backs up my own experiences; consistant with my posts on the subject in previous threads.

Quote Spoom: Our tests were performed in the rpm range of 2500 to 9000 rpm... We started at 10:1, and went to 100:1. Our results showed that a two-stroke engine makes its best power at 18:1 ... The loss from 18:1 to 50:1 was nearly 9 percent... The loss from 18:1 to 100:1 was nearly 18 percent. The reason for the difference in output is simple. More oil provides a better seal between the ring and the cylinder wall.

With the high oil concentrations that I use, I tend to get far more life from my cranks and rings than most of my friends that run leaner oil ratios. The high oil content also produces better ring sealing, so more of the combustion pressure is retained. One small point. No one ever broke an engine by using too much oil.


My reply in kind: people who are using 2-stroke air cooled engines must understand that the piston to cylinder wall clearances are larger than water cooled engines to account for the extra expansion of metals under different ambient temperatures and running conditions.
This has in part the effect of reducing the stability of bore concentricity in relation to the piston rings, as a result of thermal variation in the cylinder barrel.
In a perfect world with an engine made to blueprint tollerances, piston rings would have a perfect surface to seal against.
If these factors were to be true, and lubrication were not required, the fuel (be it gasoline, petrol, whatever you may call it) would provide a fluid film to create a perfect seal against the cylinder bore.

The reality is actually very different - these Chinese engines are made with low quality metals and quality control that leaves much to be desired.
Bore distortion is a real problem under operating conditions and the piston rings have a seriously hard time trying to create a leak free compression stroke.
There is a solution though as explained by "spoom": The loss from 18:1 to 50:1 was nearly 9 percent... The loss from 18:1 to 100:1 was nearly 18 percent. The reason for the difference in output is simple. More oil provides a better seal between the ring and the cylinder wall.

Not only does the extra oil provide a far better hydrodynamic seal but it also has the ability to absorb heat and help minimise excessive bore distortion by drawing excess heat away from the hot spots along the exhaust side of the cylinder.

I think that comment should end all argument, unless using castor oil, but thats a whole new scenario and worth of a topic of discussion on it's own.

With high quality Japanese water cooled 2-stroke engines, it's a different scenario again but it all comes down to the quality of metalurgy and water cooling to keep bore distortion to absolute minimums, providing good ring seal with minimal oil quantity.
Having said that, even Japanese engines will make more power with more oil in the fuel, even though the extra oil is reducing the octane rating and ever so slightly leaning out the mixture.

It's all about the hydrodynamic aspects of piston rings working against an inconsistant bore surface.

Fabian
 


arceeguy

Active Member
Joined
May 20, 2008
Messages
1,157
......In a perfect world with an engine made to blueprint tollerances, piston rings would have a perfect surface to seal against.
If these factors were to be true, and lubrication were not required, the fuel (be it gasoline, petrol, whatever you may call it) would provide a fluid film to create a perfect seal against the cylinder bore.

The reality is actually very different - these Chinese engines are made with low quality metals and quality control that leaves much to be desired.
Bore distortion is a real problem under operating conditions and the piston rings have a seriously hard time trying to create a leak free compression stroke......

I have to disagree with you. While the external finish of the cylinder looks rough because of the sand casting process, the piston and bore are finished to tolerances typical of a small air cooled engine. A consumer grade weed trimmer or blower has the piston riding in a plain aluminum bore. These bicycle engines have a hard chrome plated bore for longer life, but they need (In my estimation) about 10 hours of running time for the rings to seat and produce maximum compression. Some engines now feature a steel cylinder bore for even greater durability.

I use a 24:1 for break-in, and 32:1 for running.
 

arceeguy

Active Member
Joined
May 20, 2008
Messages
1,157
More oil may get you more horsepower (a few percent) but it will cause more carbon deposits. For guys who race dirt bikes and service their engines often, this is not a problem.

IMO, proper carburetor adjustment is more important than what ratio you are pre-mixing. If your carburetor is running too lean, you'll toast your engine running 18:1 or 50:1. Lean is lean, no matter how much oil you have.
 

artmaker

Active Member
Joined
Jun 12, 2008
Messages
334
How do you know if the carb is adjusted properly?

My bike seems to run just fine. Looses a little power going up hills, but then again, it's hauling ME! There is a lot of me so I never gave that much thought.
 

arceeguy

Active Member
Joined
May 20, 2008
Messages
1,157
Your main concern is "lean bog" - which will happen when you are near top speed. Your engine will run stronger (and sound stronger) at part throttle than it will at full throttle. If this happens, you need to adjust the jet needle (with the c-clip) and possibly replace the main jet.

As the weather gets colder and the air gets more dense, your engine will need more fuel to compensate. Lean mixtures cause combustions temps to skyrocket. This breaks down the lubrication film, causes pre-ignition, and severe engine damage.
 

Fabian

Well-Known Member
Joined
Sep 8, 2009
Messages
4,521
Hi Arceeguy

I agree with your comment (this one): These bicycle engines have a hard chrome plated bore for longer life

That is a correct and true statement.
I am not judging the quality of the metalurgy on the finished casting appearance, but judging the quality of the metalurgy on the purchase price to the consumer and these motorised bicycle kits are far, far too inexpensive for high quality metals to be used.
You get what you pay for - that's the only way it works, trying to reason otherwise would be foolish.

Onto the subject of disimilar metals with disimilar rates of expansion with the internal combustion process.
What logic would suggest that disimilar expansion rates would be preferable for a device that's being heat cycled.
Weight aside, there is a good reason why high performance engines use similar metals for containing the combustion process.
Aluminium expands at twice the rate of steel (or thereabouts) necessitating larger piston to cylinder bore clearances, particulary when heat cycling from a cold condition (startup) where the piston is rapidly expanding at faster rates than the steel liner.

From memory and feel free to correct any false or erroneous information, chrome plating was introduced to try and eliminate cast iron sleeves but the chrome was prone to flaking. The plating was generally only a few thousands of an inch thick. If it was more than about ten thousands of an inch thick it was prone to chipping or flaking because of heat expansion and contraction. In the early seventies Electrofusion became popular. Electrofusion was different from electroplating in as much as it was a fusion process instead of electroplating. Next came ceramic composites and Boron. All of these perform in about the same way as far as wear and durability. Then came Nikasil. Nikasil was a new plating formula of nickel-silicon-carbide. It was harder than the others that came before it and adheared well to aluminium allowing a super hard surface but with microscopic level of porosity that allowed oil to reside in the small pockets in the material, thereby reducing total oil content which was nescessary for ever tightening emission controls.

Chrome plating is old technology, it's cheap (great for the western consumer) but environmentally polluting - perfect for a country like China which is on an all out drive to boost it's economy at the expense of it's own environment and population.

If given a choice, i would take a well plated aluminium cylinder with sound metalurgy over a steel sleeve (less piston to wall clearance, less rattley and a better ring seal through the warmup stage and less blowby contaminating the fresh charge below the piston), but if given the choice of a poorly plated aluminium cylinder, i would naturally take a steel sleeved version.

One of the better thermally stable metals is brass (now i remember something about that, being a microscope repair technician at one stage) and it's also noteworthy that small model aeroplane motors use an A.B.C construction for their cylinder bores. "A"luminium block with a "B"rass liner that's been "C"hrome plated - very thermally stable and wear resistant bore structure.

Having said that, the newer model aeroplane motors use Nikasil for their bore plating requirement, so do high performance automotive engines and it's filtering down to regular production engines - tell me what type of chainsaw you can by with a steel sleeve - those things are little power houses for their engine size.

Now, back to the requirements of our cheap and nasty Chinese engines: built to a price; far too low a price for what we are demanding and naturally, the consumer gets what he/she pays for.

If you don't have an engine with a steel sleeve, use 25:1 oil and fuel ratio as the sleeve holds better concentricity particularly in engines that are run lean.
Cast iron sleeves also have another handy feature, they are a relatively soft metal and have a little graphite added to the mix which almost works like a friction modifier.
If you have an engine with a plated aluminium cylinder use 20:1 oil and fuel ratio.

Basic rule of thumb for a reasonable amount of engine hours is 20:1 oil and fuel ratio unless stepping up to better quality lubricants with much better thermal absorption qualities, like castor oil.

Fabian
 

Fabian

Well-Known Member
Joined
Sep 8, 2009
Messages
4,521
Correction: If you have an engine with a steel sleeve, use 25:1 oil and fuel ratio - that's what it should have read
 

arceeguy

Active Member
Joined
May 20, 2008
Messages
1,157
Castor oil? Talk about ancient technology! For an all out high perfomance, high revving two stroke that gets torn down frequently, castor oil has its benefits. For just about any other two stroke, it is not a benefit but a trouble maker. (carbon deposits, stuck rings, etc)

In any case, I don't want to make my life more complicated so all of my two stroke equipment (lawn mower, leaf blower, trimmer, motorized bike, generator, etc.) all get their fuel from the same can of 32:1 pre-mix. A lot of you guys seem to think that these engines are bored using a hand drill and plated in a bathtub of a rice farmer. LOL

Chrome plated bores are still very common in commercial duty two stroke engines. Chrome plating gives a long life wear surface, at a more economical cost than a nikasil plated bore. Cast iron sleeves are also used in heavy duty applications where weight is not a concern.
 
Last edited:
Top