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outback
07-14-2014, 08:10 PM
An old of mine was telling me that 2 cycle outboard engines had a compression ration of 5:1 or 6:1 therefore premium grade gasoline is not required.

To me a 6:1 compression engine would make a pretty wimpy engine and certainly would not need
high octane fuel. Seems like 8:1 would be a minimum compression ratio for a performance engine?

I purchased new a 1991 115HP Evinrude outboard and always used alcohol free premium gasoline if I could find it. Have I been wasting money buying premium grade gasoline for the past 23 years. The motor still runs like new after replacing several water pumps and the ignition stator two times. Always used OMC carbon guard in the gas as well.

I asked my local Evinrude engine dealer about this and he knew nothing about compression ratio. He Only knew about maximum compression in each cylinder and said 87 octane (alcohol free) should be fine in my motor.

Thanks,
Jim

Black_Moons
07-14-2014, 08:38 PM
No, These days, all 'regular' fuels contain alcohol and that generally plays hell on all small engines, Especially those stored for long times like.. All small engines.
You can likely run on regular fuel with nothing more then needing to do carb rebuilds more often (the alcohol likes to eat the plastics faster but most modern carbs have rather resistant plastics), but at the very least fill it up with premium fuel before you store it for extended periods, or better yet drain it and the carb dry.

As its a boat engine, I wouldn't risk being stuck and would just use alcohol free, and that generally means premium as I can't find any regular alcohol free fuel these days.

flylo
07-14-2014, 08:57 PM
Old aircraft had 6.5 comp ratio & used 80 octane fuel. Ethanal is added to the tanker or at the station & all stations don't use it. Our local shell didn,t use it because we agreed to buy it for aircraft use if he didn't. You can test for it by putting water in a jar marking the water level with a marker, add fuel & shake, let settle & if the line is in the same place where the water meets fuel no ethanal. If you have airports nearby see if they sell Mogas which is car gas with no ethanal.

shawnspeed
07-14-2014, 09:06 PM
Compression ratio's on 2 strokes can be tricky..it depends on port timing..also there is a static & dynamic compression ratio on 2 stroke....I am not totally sure on how they are all computed...but I run premium in most of my 2 stroke engines as the ethanol content seems to be less...Shawn

outback
07-14-2014, 10:06 PM
The compression ratio should be the bore & stroke after the ports are closed and the volume of the combustion chamber in the head. In other words, if 8 parts of air and fuel are squeezed into 1 part the ratio is 8:1. Hope I said that right.

I remember seeing pistons back in the 1960's with domes on top to occupy volume in the combustion chamber thus increasing compression ratio.

CarlByrns
07-14-2014, 10:19 PM
6:1 static is about right but don't forget that there is a supercharging effect that will vary with port timing, engine speed, and airflow through the cylinder, so dynamic compression ratio will be higher.

Two cycle engines are a lot like turbines: they have been called gas-dynamic engines. The flow of air through the engine is complex and changes with speed and load.

You've spent your money wisely: when I was into boating, I always used the best gas and oil available and never suffered a fuel-related breakdown either.

Willy
07-15-2014, 01:04 AM
6:1 static is about right but don't forget that there is a supercharging effect that will vary with port timing, engine speed, and airflow through the cylinder, so dynamic compression ratio will be higher.

Two cycle engines are a lot like turbines: they have been called gas-dynamic engines. The flow of air through the engine is complex and changes with speed and load.

You've spent your money wisely: when I was into boating, I always used the best gas and oil available and never suffered a fuel-related breakdown either.


This description of how most 2 cycle engines function pretty well nails it. Shawnspeed also mentions the important difference of dynamic and static compression ratio. Two cycle engines are designed around this concept of high volumetric efficiency
Think of your engine as a supercharged engine, which by the way also has a much lower compression ratio than a natural aspirated engine in order to compensate for the increased airflow and volume under actual operating conditions.
The static compression ratio figure is not as meaningful when referring to a 2 cycle engine as it is in reference to a 4 cycle naturally aspirated engine.
Don't forget your engine is producing well over 1 HP per cubic inch, there is a very high degree of volumetric efficiency at work here. A static 6:1 or 7:1 compression would not suggest that type of efficiency or power output.


I purchased new a 1991 115HP Evinrude outboard and always used alcohol free premium gasoline if I could find it. Have I been wasting money buying premium grade gasoline for the past 23 years. The motor still runs like new after replacing several water pumps and the ignition stator two times. Always used OMC carbon guard in the gas as well.



Sounds like your current operational regime is working very well for you...why change it?
Also don't forget that when a manufacture recommends an 87 octane fuel, one must remember this is a minimum standard requirement in order for the engine to function correctly, this does not mean to suggest that one should not exceed the minimum requirement. As your experience has demonstrated exceeding minimum fuel quality standards has allowed you to reap the benefits of a motor that still runs like new after all these years.


I remember seeing pistons back in the 1960's with domes on top to occupy volume in the combustion chamber thus increasing compression ratio.



I have several of these pistons from older designed 2 strokes kicking around the shop as well.
More than a means of increasing compression ratio the domes on these pistons were used more as an air deflector.
When the intake port opened the fresh intake charge would hit this "dam" and be deflected up toward the top of the combustion chamber. Without this dome the intake charge would mostly slide right across the piston and escape out of the open exhaust port.
The illustration below depicts this phenomena.


http://upload.wikimedia.org/wikipedia/commons/thumb/2/2c/Two-stroke_deflector_piston_%28Autocar_Handbook,_13th_ ed,_1935%29.jpg/170px-Two-stroke_deflector_piston_%28Autocar_Handbook,_13th_ ed,_1935%29.jpg

macona
07-15-2014, 02:30 AM
You can buy aviation fuel at the local airport. You dont have the issues you do with alcohol doped fuels like you find at the gas station. You do pay more.

Yow Ling
07-15-2014, 03:58 AM
If you measure the compression like a 4 stroke (clearance volume + swept volume)/clearance volume you will get a number between 12 and 16 ish

the Japanese method is the same except the swept volume isn't measured from bdc but from when the exhaust port is closed by the piston this is when you get the 6 or 7 numbers

Avgas is a bit of a waste of time if you havent got loads of compression and loads of advance

A.K. Boomer
07-15-2014, 09:56 AM
Great write ups by both Willy and CarlB, it is a type of supercharging due to the volume of the piston on the crankcase side having the full stroke advantage,,, therefore it's displacement is more than that of the top chamber measured from the exhaust port up, and flow dynamics really end up playing a huge role in this area - but at least with the old style two stokes there's only so much "stacking" you can do before things get counter-productive and you start throwing the baby out with the bathwater,,,

although - I suspect things are about to change ( or already have ) drastically for the high performance two stroke as there is no real reason direct injection gas cannot be adopted into the "mix" ...

both exhaust and intake ports can be lowered and more of the stroke can be used for producing work than having to get rid of and introduce charges, this will not only equate to more efficiencies it will also make the engine quieter...

guess what im saying is turbo charging and increased efficiencies could drastically alter the two stroke more so than ever if direct injection gas is used - the diesel version has already been out for quite sometime now and has done very well:)




Don't forget your engine is producing well over 1 HP per cubic inch, there is a very high degree of volumetric efficiency at work here. A static 6:1 or 7:1 compression would not suggest that type of efficiency or power output.




This is a tricky statement - and while I have to agree with the first part (for the most part) using the word "efficiency" and Power output" on there own to suggest that all is good and up to par with the two stoke combustion cycle is really missing some important facts about how these engines function, The fact is is the two stroke combustion cycle is not very efficient - and they are in fact very dirty because of it, the other fact is - is that compared to a four stroke the two stroke combustion cycle is very weak and anemic - in fact the two stroke combustion cycle is very close to 1/2 to 2/3rds (at best) of what a 4 stroke can produce... NA,

But ------------ there's one other very important fact - as lame as the combustion stroke is - there's twice as many of them in the same amount of time frame (RPM)
and while this has nothing to do with the efficiency of how much work an engine can produce for every gallon of fuel it consumes, it has quite a bit to do with the amount of power it can produce in a set time frame (RPM's) and RPM's along with Torque are the two things that make up horse-power... (ahhh torque - the achilles heel of the two stroke - the other half of what makes up horsepower, two strokes rob Peter to pay Paul, and then for the most part realize that Paul is a wasteful - ungrateful slob who never changes his underwear...)

So - not to nit-pic Willy, but it's that last sentence of;

"A static 6:1 or 7:1 compression would not suggest that type of efficiency or power output."

For one if you break it down the efficiency your talking about is actually pretty poor, and using power production as a gauge in comparison to other types of engines in reference to thermal efficiencies (which has to do with compression ratio's and effective compression ratio's) is really not a good comparison ,,,



but the reason they both "appear" to be good at first glance is due to the first part of your statement holding some merit,

"there is a very high degree of volumetric efficiency at work here."

Yup

but even that statement is a slippery slope with the typical almighty two stroke as part of that volume is moot, not even being burnt inside the combustion chamber - and the other part that is? it's a big compromise due to it having to prematurely exit the chamber so soon, when optimum piston speed and rod angle are just begging for more...

Now enter once again the low torque output of the two stroke, they (as far as the type of 2stroke were talking about) actually have two very serious things going against them - one is they generally have to throw some of the baby out with the bathwater and two is even when they get the baby in the tub they don't have enough time to scrub it's balls and wash it's crackerbean....

all part of doing business with engines of great compromise... just is what it is until you drastically change the design and there are some examples of that... but that's a whole nuther topic....

MTNGUN
07-15-2014, 10:54 AM
8:1 is common. Most of my chainsaws have been modded to have about 10:1. I use nothing but regular ethanol gas (all gas in my area has ethanol). It's hard on carbs and fuel lines but otherwise no issues.

Max practical compression ratio has as much to do with the chamber design and squish as with the compression ratio.

Willy
07-15-2014, 11:56 AM
I agree completely AK with everything you said. When I mentioned efficiency I was comparing apples to apples. Efficiency is a relative term, I'm not comparing 2 strokes to 4, completely different ball game I quite agree.


Max practical compression ratio has as much to do with the chamber design and squish as with the compression ratio

MTNGUN brings up a very good point as well. Modern techniques in squish areas and combustion chamber design have allowed compression ratios to climb using regular gasoline to what was unheard of of years ago.

Modern refinement of the two cycle engines continues as they do have their advantages. Chrysler was set to introduce a direct injection two stroke in 1997 in the Neon that actually had a 10% fuel efficiency advantage of conventional engines of similar size but they could not meet nitrous oxide emission standards.

http://www.allpar.com/neon/stroke.html

A.K. Boomer
07-15-2014, 12:13 PM
I know what you were talking about, thanks for not taking it the wrong way but thought it might need clarification for some that might be interested in the topic...

Engines really are amazing and I don't want to also sound like im bagging on the two stroke as they have their purpose for sure,,,

To me they are a harder to dial-in/predict type of motor due to the two different types of flow dynamics going on, one in the crankcase and the other in and out of the chamber, some of that is why they can be so "pipey" they can really hit a sweet spot when it's "all systems go" and both chambers are working in harmony --- along with the intake and exhaust system, crazy for tuning and really takes allot of know how...

flylo
07-15-2014, 04:00 PM
But when they're"on the pipe" or "dialed in" you can't beat the weight to HP & many brands are reliable. When I was young I raced Can Am motocross bikes with Rotax engines mostly at Redbud. I still use Rotax in ultralights. Hard to beat, back then I went a whole season on a top end while my buddy raced a Yamaha YZ & the pistons came in 6 packs LOL!

oxford
07-15-2014, 04:33 PM
Another big reason for no ethanol in boats(and anything else that has one) is the fiberglass fuel tanks. There is very few resins that will withstand the alcohol, and they weren't being used up until recently(no reason to).

CarlByrns
07-15-2014, 04:49 PM
To me they are a harder to dial-in/predict type of motor due to the two different types of flow dynamics going on, one in the crankcase and the other in and out of the chamber, some of that is why they can be so "pipey" they can really hit a sweet spot when it's "all systems go" and both chambers are working in harmony --- along with the intake and exhaust system, crazy for tuning and really takes allot of know how...

You should see (and hear!) what model aircraft wizards can do to an itsy-bitsy glow engine. Crazy amounts of power. Long life, not so much.

A.K. Boomer
07-15-2014, 06:25 PM
No doubt, crazy things can happen when the RPM's go off the charts, makes up for the lack of torque in a big way...

atty
07-15-2014, 08:45 PM
Avgas is a bit of a waste of time if you havent got loads of compression and loads of advance

It may be a waste of time from the performance standpoint, but it is worth its weight in gold when it comes to avoiding the dreaded deterioration of input components from ethanol. It is also a gas that you can safely rely on when storing away small engines for extended lengths of time. Sta-Bil and other brand name gas "preservatives" are not necessary.

lakeside53
07-15-2014, 08:52 PM
But.. it has 7X the lead of the old "leaded gas"... What your health or that of others worth?

A.K. Boomer
07-15-2014, 09:21 PM
I think I have seen more damage to two strokes in the form of pre-ignition and detonation than in 4 strokes and aside from the motor cycle service department that I ran iv worked on four strokes most of my life,,,

don't think that for a second high performance two stroke engineers aren't trying to push things to the absolute max either,,,

they are - seen many a piston on the exhaust port side with chunks missing or pitted or flat out melted...


although have to give credit to the honorable mentions like lots of kids jacking with their own jetting on their dirt bikes and such - lean mixes run like a bat out of hell, for a little while...

two strokes need luvin (octane) too...

CalM
07-15-2014, 09:33 PM
The calculation of a two stroke engine "Should be" no different than that of a four stroke. that is, swept volume, plus static volume, divided by the static volume. But there are two conventions that have been popular. Mechanical, and effective CR, as has been well explained.
The result obtained when calculations are made at the point of port closing by the piston position*(effective CR) is DIRECTLY equal to doing the same calculation with regard to valve closing events in a four stroke engine. Something that is NEVER DONE. I am not sure why the disparity, but it's there! (* NOTE, two stroke engines are frequently described as "piston port" designs).

Also, the extended piston crown of the Schnuerle loop charge is quite an old design, and has been mostly replaced by low profile dome piston crowns, leaving the scavenging and "efficiency" of combustion chamber filling to the design of the transfer ports. Looking down the bore often shows the entire circumference of the cylinder perferated by intake, transfer or exhast port openings. And with carburation directly into the crancase via reed valves, leaving the cylinder wall area free from that requirement, transfer ports dominate.

With that, out board marine applications have relied heavily on the Schnuerle design. "Loop Charged" used to be a graphic on some of the engine cowls IIRC> (Johnson Evenrud)

And with that! and a return to the original topic. In two cycle engine performance, the onset of detonation is usually driven by Temperature, Lean fuel air mixes (often caused by too much oil!) incorrect spark plug heat range, or even just a carbon build up can melt a piston top faster than you can say OH oH! And octane won't give a tinkers damn ........

By the way, What is the octane of the fuel with a 32:1 oil mix in it? What about when oil injection systems are present? Moving target? Yes, I think so ;-)

I would love to see a revival of the two stroke for personal transportation application in keeping with the wonders of GDI, (Gasoline Direct Injection)

cheers

lakeside53
07-15-2014, 09:42 PM
Tuning of 2 stokes is an art. I can tune by ear pretty well, and feel the effects under power, but I've had a lot of time doing it.

It's easier on the likes of a blower where max hp is being developed at max throttle, less so on a chainsaw or bike where you are actually required to do develop the power. The usual technique for a blower is to tune to max rpm (say 7200 rpm), then enrichen the mix until the rpm drop by 100. Tricky, so I err on the "drop more" side and make sure it's still able to develop the max loaded rpm with smooth sound. On a chainsaw I do something like that by setting the mixture to get the published unloaded rpm (say 12-14,000), then by listening for the burble often referred to as 4-stroking" to smooth out under load. All a bit subjective and easy to get wrong if you are not "one with the saw". For chainsaw milling (one of the more brutal uses of a saw I mount a tiny tach on the saw, listen and and watch what happens under load.

An exhaust Gas temperature probe (like you find on an small plane engine) takes a LOT of the guess work out of the problem of finding "slightly rich of peak" under power. Simply lean the mixture (under power) to max temperature, then back off 25 degrees. it doesn't work for beans unless you are developing rated hp though as the temperatures are way down.


A lot of 2 stoke tuning will depend on what engine life you expect. Quantity pro chainsaws from Stihl or Husky, conservatively tuned, will live for 2000-4000 hours between rebuilds (not when milling); If leaned way out for "more power" (DON'T)... a heck of a lot less. A racing bike? lol... can you even get to 100?

Yow Ling
07-16-2014, 03:30 AM
But.. it has 7X the lead of the old "leaded gas"... What your health or that of others worth?

Thats the funniest post Ive red in a while !

vpt
07-16-2014, 09:01 AM
1939 waterwitch outboards have the piston crown.

http://i.imgur.com/JPPFIiC.jpg

http://i.imgur.com/cNn8cKr.jpg

EVguru
07-17-2014, 12:23 PM
The result obtained when calculations are made at the point of port closing by the piston position*(effective CR) is DIRECTLY equal to doing the same calculation with regard to valve closing events in a four stroke engine. Something that is NEVER DONE.

It's been discussed in every good four stroke tuning book I've ever read.