The myth of high compression in 2-strokes

[wcorey]

At least some of the reason relatively high octane gas is recommended by many outdoor power equipment manufacturers is to compensate for users storing fuel for extended periods and the resulting loss of octane.
Dealers then recommend it just because the manufacturer does without understanding the reasoning behind it and when questioned will pull a bs answer from their behind...

 

[Thomas Lilli]

Ah man, I wanted a 748 in the worse way. 2002. I almost bought one a couple years ago.

 

[Ted Jenkins]

At least some of the reason relatively high octane gas is recommended by many outdoor power equipment manufacturers is to compensate for users storing fuel for extended periods and the resulting loss of octane.
Dealers then recommend it just because the manufacturer does without understanding the reasoning behind it and when questioned will pull a bs answer from their behind...

WC you are correct in that octane dissipates over time and especially in plastic containers. With fresh gas it has never been a problem. I live at 6,000 feet, but most of my work is at 4,500 feet. My saws are not modified much and have always used 87 octane for fifty years. When I am cutting I go through about three gallons a day or at least one gallon a week when not cutting seriously. Thanks

 

[CR888]

I don't get why folks think octane is so important in gas for saws. Its far more of a concern to run old stale gas that's lost its volatility. Reed vapour pressure is what concerns me, I buy the lowest octane I can. Fresh gas is my goal, I try to keep saw mix fresh nothing over 2 weeks old & store it indoors away from sunlight frost etc. I don't like the way hi octane pump gas runs in my saws, much prefer regular.

 

[Thomas Lilli]

WC you are correct in that octane dissipates over time and especially in plastic containers. With fresh gas it has never been a problem. I live at 6,000 feet, but most of my work is at 4,500 feet. My saws are not modified much and have always used 87 octane for fifty years. When I am cutting I go through about three gallons a day or at least one gallon a week when not cutting seriously. Thanks

Can‘t beat experience!
Im strictly a homeowner firewood cutting enthusiast that also maintains the farm. I will say with all honesty that I’ve only recently, after 40 years, experienced the horrors of bad fuel and it was in our standby generator Of all things. It had premium fuel but hadn’t run for six months. I was away when the power went out. Wife couldn’t get the the generator started. The fuel have gelled up the line to the carb. The starter battery died and wife was without power until the neighbor came to save her. Other than that, I’ve had power equipment run flawlessly with gas that I never managed. I guess I was just extremely lucky until that luck ran out with the generator. Now I’m a fresh fuel maniac. Additives, temperature, no ethanol supplier.

 

[Ted Jenkins]

There are systems to prevent the problems you describe. For chain saw use I mix about two half gallon clear bottles for an average day. almost all the time I will use one bottle up in two days. If the second bottle sits around more than a few days then I just dump it. Its the easiest cheapest thing to do. If I have a heavy week of cutting coming I will mix several bottles of mixed gas to start out the day. Clear bottles tell you if there is contaminates in the fuel or if moisture is present. For other equipment the situation is different. Here three out of every household here has a generator, at least half are programmed to start up every few days and run for fifteen minutes. For all others a petcock is installed with a drain hose. About once every week or two the owner starts up generator and runs it for fifteen minutes. The owner never lets the fuel go below a half a tank. After the fuel is three months old fuel is drained with new fuel added. Here we have fifty outages a year with the longest being about three weeks at a time. Then we will go five years with only one or two outages knowing the loss of power will happen. Thanks

 

[Thomas Lilli]

There are systems to prevent the problems you describe. For chain saw use I mix about two half gallon clear bottles for an average day. almost all the time I will use one bottle up in two days. If the second bottle sits around more than a few days then I just dump it. Its the easiest cheapest thing to do. If I have a heavy week of cutting coming I will mix several bottles of mixed gas to start out the day. Clear bottles tell you if there is contaminates in the fuel or if moisture is present. For other equipment the situation is different. Here three out of every household here has a generator, at least half are programmed to start up every few days and run for fifteen minutes. For all others a petcock is installed with a drain hose. About once every week or two the owner starts up generator and runs it for fifteen minutes. The owner never lets the fuel go below a half a tank. After the fuel is three months old fuel is drained with new fuel added. Here we have fifty outages a year with the longest being about three weeks at a time. Then we will go five years with only one or two outages knowing the loss of power will happen. Thanks

I lived “in town” for 20 years. Never had an outage. I moved onto the farm about 18 years ago and have been without power ever year for about a week at a time. Like most, I’ve been terribly under funded so, in ths last few years, finally got a portable generator and am catching up with best practices. Thanks. I learned a lot about fuel and storage now that I have equipment that I rely on. I did get a generator about six years ago. I ran it about 5 hours. It sat for a year and the exhaust valve locked up on it. Got a new one that had the fuel fouling. I run it every three or four months now. Thanks.

 

[PakProtector]

I am going to have to weigh in and call trapper's reasoning flawed. Show me the numbers. Heck, show me the equations those numbers are generated with first. Some is right, but there is a severe disconnect going on; there is nothing special about a 2 stroke that would have it liking only moderate compression ratios, whilst 4 strokes are quite happy getting more than half way to triple digits by the time forced induction is counted.

I can see a high pressure attempt not running as well, and that the pressure is what caused the lack of power...but take the example of high compression pressure wrecking the spark delivery, and put a real ignition system on it and that will go away.

The pipe is more like a Ramcharger's intake. A musical, resonance-based instrument...and completely off topic with the thesis that high compression is wrecking the power output because of pumping loss. Maintain the combustion chamber proportions, and don't exceed 5000 fpm average piston speed and spark advance requirements don't change until nearly 30,000 rpm for a 4 stroke. The efficiency envelope is governed by compression ratio; the higher the CR, the higher the potential efficiency. RPM is *NOT* in that equation. Neither is 2-stroke/4-stroke...

Now what would actually be interesting is to determine just what is costing the high compression engines their power...and then fix it. Better that than having to live with it because you believe him.

cheers,
Douglas

 

[trappermike]

Well if my numbers are flawed,show me any factory racing engine that has 180 plus compression,from the 70's to now.
4-stroke engines only make compression every 2 strokes,their compression losses are halve as much. Show me any factory high performance 2-stroke where their compression is 180 plus. Any hot motor from 1970 to now. with 2-strokes the limiting factor was how hot you could get the piston before it melts. That's why some latest hot 2-strokes had an electrinic ignition that retarded timing at high rpms to prevent that. Show me some specs from modern hot 2-strokes that have that high cranking pressure from the factory. In the last 4 decades I have not seen any.
As for flaring an intake port outwards from the carb that means you are slowing intake velocity.
As for timing advance on 4-strokes same to 30,000??? All the ones I worked on were about 8 degrees to 38 degrees,and very sensitive to rpm,barometric pressure,mixture etc.
Show me any factory hi-po 2-stroke engine specs with that compression...

 

[PakProtector]

Well if my numbers are flawed,show me any factory racing engine that has 180 plus compression,from the 70's to now.
4-stroke engines only make compression every 2 strokes,their compression losses are halve as much.
*The angle of crankshaft rotation is the same. The loss for compression pressure will therefore be the same. That the 4-stroke has to complete an additional rotation v the 2-st on a per firing stroke is absolutely irrelevant. Actually for something with a turbocharger, the exhaust stroke is also a load. The additional air volume to compress adds an additional load...and yet the compression pressure is as high as design octane ratings allow. Or for CI engines, it is an order of magnitude higher( 60 psi boost, plus 18:1 CR for a relatively tame street engine).

Show me any factory high performance 2-stroke where their compression is 180 plus. Any hot motor from 1970 to now. with 2-strokes the limiting factor was how hot you could get the piston before it melts.
*Just because it has not been done does not mean your thesis is correct. It means there is something limiting, but it is not pumping loss due to high compression pressure. And now you bring piston temperature into the argument? I thought you said it was pumping loss?

That's why some latest hot 2-strokes had an electrinic ignition that retarded timing at high rpms to prevent that. Show me some specs from modern hot 2-strokes that have that high cranking pressure from the factory. In the last 4 decades I have not seen any.
As for flaring an intake port outwards from the carb that means you are slowing intake velocity.
*And this has what to do with your thesis?
As for timing advance on 4-strokes same to 30,000???
*Read the paper. The short answer is YES. It was written by some Honda engineers, who were building 4-stroke 250's to compete with 2-stroke GP bikes in the early 60's. Ignition advance was not effected by RPM until about 30,000 rpm. There were conditions on this, as you will see after reading it. In a nutshell, it is why there are 8 rods for the NS750 V4...and 32 valves, and two spark plugs per 'cylinder'. Average piston speed limits demand a short stroke, short stroke means large bore...which gives a thin, disc shaped combustion chamber( since in this class the No. of cylinders is dictated by the rules). Disc shaped chambers are poor for flame front propagation, so make them other than circular...surely you are aware of the reasons behind the oval piston tech Honda developed? maybe? In any case, if the combustion chamber was not turned into a disc, the spark advance requirements did not require additional advance as rpm climbed past 10,000 rpm, or double...but right around triple more advance was required.

All the ones I worked on were about 8 degrees to 38 degrees,and very sensitive to rpm,barometric pressure,mixture etc.
Show me any factory hi-po 2-stroke engine specs with that compression...
*One more time, show me that it is pumping losses that dictate that...you started to cast your net wider when you brought up piston temperature....but that has zip to do with pumping losses due to compression pressure.
cheers,
Douglas

 

[trappermike]

Wow,your math is incredable,how does it equate into real engines today? How many racing engines have you designed,I mean with all your math they should beat any engines today. You've outfoxxed anyone making a 2-stroke engine today,have thrown out all timing and compression values used in today's racing engines. You should be up there with Elon Musk,why waste your time here? Or do you have a 2-stroke engine to take us to Mars?
Fact is it's all theory,I give you credit for being smart. But trying to spank someone who's being doing it long before you're born,not so smart. Pick on someone else,or meet one of my saws in a real Loggers Sports competition like I'm used to..
If everything I'm saying is wrong ,prove it.I have never read one word of what you say,that makes any real sense,in modern times as a mechanic. Don't try to "outfox" me as a mechanic or engineer with your long-winded formulas,meet me on raceday with a hotsaw-
Well you're pretty young and smart,find another hobby.
"When the hammer drops,the ******** stops".

 

[trappermike]

Well you touched on 2-stroke racing pipes(exspansion chambers),what can you tell us about that?
"as flaring stuff outwards means a loss of velocity."

 

[PakProtector]

You made a statement way back there. It is wrong. You have got nothing. I thank you for admitting it.

Why,because the more the compression ratio the more horsepower it takes from the engine to crank the engine against that high compression,at some rpm the HP taken to crank that high compression takes more HP than it makes,and then the engine starts to lose HP. Thats why a factory motor making 60 HP at 8.5-1 compression,makes 100HP at 7.5-1 comp. ratio.

Your original lead off just in case you have forgotten.

It is of course, fantasy. And then you bring up melting pistons. and other sputterings of somebody who can't back up their thesis. The melting pistons 'thing ought to be obvious...getting heat out of a 2-stroke is more difficult, and heat gets put in twice as often. Pumping loss is not the reason for building 2-strokes with low compression ratios and low( comparitively ) compression pressure.

Thanks also for showing how to be a jerk when your argument goes south...
Douglas

 

[Leafy]

Are there any spark ignition engines that have more than 190psi on a compression tester? Even monster 14:1 static compression super bike motors don't do that. You know why? Static compression only matters in its effect on dynamic compression. For a given fuel in a naturally aspirated motor there's an optimal amount of dynamic compression. If you want to run more duration to get more cylinder filing you need to run more static compression to maintain the same, optimal, dynamic compression ratio.

 

[ironman_gq]

Are there any spark ignition engines that have more than 190psi on a compression tester? Even monster 14:1 static compression super bike motors don't do that. You know why? Static compression only matters in its effect on dynamic compression. For a given fuel in a naturally aspirated motor there's an optimal amount of dynamic compression. If you want to run more duration to get more cylinder filing you need to run more static compression to maintain the same, optimal, dynamic compression ratio.

Factory motors, probably not many if any. I've seen plenty of alcohol motors in the 2-230psi range, they run hard and fast and don't last long. Static compression is dictated by a lot more than compression ratio, valve timing (port timing) has the most influence and dynamic compression is almost completely unrelated to compression ratio when you start to factor in valve timing, ignition timing, burning characteristics of the fuel, volumetric efficiency and any other variables you can think of. Many modern engines vary all of these based on operating condition (variable valve timing, asymmetric valve timing, variable ignition timing, multipath intakes/exhausts).

 

[Leafy]

I

Factory motors, probably not many if any. I've seen plenty of alcohol motors in the 2-230psi range, they run hard and fast and don't last long. Static compression is dictated by a lot more than compression ratio, valve timing (port timing) has the most influence and dynamic compression is almost completely unrelated to compression ratio when you start to factor in valve timing, ignition timing, burning characteristics of the fuel, volumetric efficiency and any other variables you can think of. Many modern engines vary all of these based on operating condition (variable valve timing, asymmetric valve timing, variable ignition timing, multipath intakes/exhausts).

I think you're taking about cylinder pressure. Dynamic compression ratio can be calculated based on the timing overlap, inlet valve pressure (on a 2 stroke for this case the inlet valve is the transfer port and the inlet pressure is the average case pressure while the transfer port is open), and the static compression ratio. Peak and average Cylinder pressure adds in those extra variables you're talking about.

 

[ironman_gq]

I

I think you're taking about cylinder pressure. Dynamic compression ratio can be calculated based on the timing overlap, inlet valve pressure (on a 2 stroke for this case the inlet valve is the transfer port and the inlet pressure is the average case pressure while the transfer port is open), and the static compression ratio. Peak and average Cylinder pressure adds in those extra variables you're talking about.

I believe you are correct

 

[Dennbb]

I want to put to bed the myth of high compression in chainsaws,having worked on 2 stroke racing engines since the 70's I want to lay some straight facts down. High compression was first made popular during the 60's with big v-8 racing engines(4-stroke) and compression ratios went up to 13-1 or more. But the best 2-stroke racing engines actually reduced the compression ratio from the stock engines. Why,because the more the compression ratio the more horsepower it takes from the engine to crank the engine against that high compression,at some rpm the HP taken to crank that high compression takes more HP than it makes,and then the engine starts to lose HP. Thats why a factory motor making 60 HP at 8.5-1 compression,makes 100HP at 7.5-1 comp. ratio.
Now higher compression does make more power at low and mid range power for sure,but not many saws run in that range. so if you want more low end or mid range power compression is good,but if you need high rpm race power high compression will simply cost you Hp. As my instructor told me in 1975 "high compression fights high rpms",he was right.
So more more compression can help you,but not at high rpm's.
When you yank the cyl. gasket out of a saw you do gain some compression,BUT you also lower the exhaust and transfer ports down,reducing their duration,and reducing higher rpm power. I would much prefer to raise ports than compression for power,much more can be gained.
In many engines I would much prefer to raise the exhaust port and lose compression,I know I will make much more top end power.
Removing the cyl. gasket and lowering the transfer ports is really bad,you reduce their duration and reduce their open time from tens of thousanths of a second to even less!
I checked a new Poulan 3400 compression and was surprised to see it was barely 110 psi,so yes I yanked the cyl. gasket and it was better.
So in my mind port timing is far more important than high compression. All 2-stroke race engine designers agree,port timing makes big power,not high compression.

Thanks. I have been trying to tell people that for decades.

 

[Maintenance supervisor]

"Is that a Hemi? In there?"

 

[PakProtector]

It is not the pumping losses that limit power. Second, increasing compression does not reduce power...within the bounds of keeping the ignition advance due to detonation. Some of y'all are rolling fuel octane, and durability into this, and a few have even mentioned heat transfer out of the piston...
cheers,
Douglas