Part Four: Compression

[Cerberus]

OH I've finally got the guts to mention this publicly.... Why can't we fine-tune compression by simply protruding the decomp-plug into the chamber a lil? Turbulence in-chamber isn't problematic, it's rare a plug is "smooth" on the chamber ceiling anyways and, the way they're threaded, 10 threads = 1cc....so it'd make measurements real easy if, say, fine-tuning the ~10cc chamber of a 660 (after the build, after deciding on pop-up and deck height and all that)

The big-bore 660 jugs, awesomely, use 54mm sized chambers, and it ends them with chamber-ceiling wall thickness that is way more than usual (also, awesome!), but it leaves the spark plug & decomp plug recessed so you have to 'hack'/mcguyver both and, when doing decomp, I realized how easy it'd be to simply add 1/3rd CC of decomp into the chamber, given I'm deciding how long to cut the XL plug I got from the hardware store, anyway yeah someone better than I at calculus could help relay hard#'s but pop-up's work and they're hardly 20thou thick in many instances, you're definitely adding significant mass by just 3-5 threads' intrusion of the decomp-plug into the chamber!

Cool trick, right? Or dumb idea? LOL I think it's an awesome trick but this is 1st time I've run it by anyone...have heard of "just weld a spot/dab of MIG or TIG in the chamber" in terms of displacement-reduction in the chamber and it's the same idea, heck it's basically same idea as a pop-up unless there's some "turbulence/velocity" aspects to pop-up's I'm unaware of....but again the protruding plug would help w/ turbulence anyways....it's also a steel plug so maybe it'd get hot and be pre-igniting, although same could probably be said for body of spark plug...

 

[Shibby]

A protruding plug or bolt end may affect burn pattern, whether that matters or not is a different question.

 

[drf256]

Intricate questions.

Compression merely adds potential energy to the charge in the form of heat. Compressing something in gaseous state makes it warmer and expansion makes it cooler-all have experienced with our own air compressors. The tanks get hot when they fill and the air coming out cools and gets condensation when you let it out. It’s also the basis of how refrigeration works.

So the closer you get the charge’s atoms to the point where they want to oxidize, the more complete burn will occur giving one the highest cylinder pressure possible.

The spark only adds the last bit of energy needed to trip the chemical reaction over. If you compress the charge too much and there is already enough heat in the chamber, the charge can ignite before the spark (pre ignition/dieseling). Spark timing needs to to be earlier in a chamber with a charge that’s not as pre-excited to burn and later in one that’s perfectly on the edge. On a Stihl 361, the saw like timing advance stock. Once I add compression, open the muffler and add an o44 carb, I actually retard the timing to stop preignition which is heard as an audible “pop” when the saw is running.

Then, there is chamber geometry. You also want a homogenous charge so that the entire mass is equally pre-excited to burn. Swirl helps here, and a smooth chamber with little to no asymmetric areas seems to work the best. You get swirl with less squish because you get more velocity with tumbling on the charge in the chamber. A pop-up can accomplish both if it’s done correctly-but it’s difficult to accomplish. For it to be right, the squish needs to be equal on the flat of the piston as well as the sides of the pop-up where it goes up into the chamber. They should match the entire way.

In addition, whatever charge is left in the squishband area doesn’t contribute to the combustion pressure, so you lose that bit of charge volume.

Popups also help because the small elevated central area can help deflect the initial charge coming out of the transfers up into the chamber for better loop scavenging.

I personally prefer a cut squish band for compression and charge swirl. The only time I will use a popup is if just a band cut will change the port timing too much or cause free porting. An example for me is the echo 620. Cutting a popup and cleaning the band up a bit leaves the transfers at a manageable height. If I just cut the band, I’d have to cut the rough the roof of the transfers to get the height I needed.

Clear as mud, huh?

 

[Mastermind]

Intricate questions.

Compression merely adds potential energy to the charge in the form of heat. Compressing something in gaseous state makes it warmer and expansion makes it cooler-all have experienced with our own air compressors. The tanks get hot when they fill and the air coming out cools and gets condensation when you let it out. It’s also the basis of how refrigeration works.

So the closer you get the charge’s atoms to the point where they want to oxidize, the more complete burn will occur giving one the highest cylinder pressure possible.

The spark only adds the last bit of energy needed to trip the chemical reaction over. If you compress the charge too much and there is already enough heat in the chamber, the charge can ignite before the spark (pre ignition/dieseling). Spark timing needs to to be earlier in a chamber with a charge that’s not as pre-excited to burn and later in one that’s perfectly on the edge. On a Stihl 361, the saw like timing advance stock. Once I add compression, open the muffler and add an o44 carb, I actually retard the timing to stop preignition which is heard as an audible “pop” when the saw is running.

Then, there is chamber geometry. You also want a homogenous charge so that the entire mass is equally pre-excited to burn. Swirl helps here, and a smooth chamber with little to no asymmetric areas seems to work the best. You get swirl with less squish because you get more velocity with tumbling on the charge in the chamber. A pop-up can accomplish both if it’s done correctly-but it’s difficult to accomplish. For it to be right, the squish needs to be equal on the flat of the piston as well as the sides of the pop-up where it goes up into the chamber. They should match the entire way.

In addition, whatever charge is left in the squishband area doesn’t contribute to the combustion pressure, so you lose that bit of charge volume.

Popups also help because the small elevated central area can help deflect the initial charge coming out of the transfers up into the chamber for better loop scavenging.

I personally prefer a cut squish band for compression and charge swirl. The only time I will use a popup is if just a band cut will change the port timing too much or cause free porting. An example for me is the echo 620. Cutting a popup and cleaning the band up a bit leaves the transfers at a manageable height. If I just cut the band, I’d have to cut the rough the roof of the transfers to get the height I needed.

Clear as mud, huh?

Good stuff Al.

 

[Sawrain]

Intricate questions.

Compression merely adds potential energy to the charge in the form of heat. Compressing something in gaseous state makes it warmer and expansion makes it cooler-all have experienced with our own air compressors. The tanks get hot when they fill and the air coming out cools and gets condensation when you let it out. It’s also the basis of how refrigeration works.

So the closer you get the charge’s atoms to the point where they want to oxidize, the more complete burn will occur giving one the highest cylinder pressure possible.

The spark only adds the last bit of energy needed to trip the chemical reaction over. If you compress the charge too much and there is already enough heat in the chamber, the charge can ignite before the spark (pre ignition/dieseling). Spark timing needs to to be earlier in a chamber with a charge that’s not as pre-excited to burn and later in one that’s perfectly on the edge. On a Stihl 361, the saw like timing advance stock. Once I add compression, open the muffler and add an o44 carb, I actually retard the timing to stop preignition which is heard as an audible “pop” when the saw is running.

I will offer an alternative explanation as to why higher compression ratios (within limits) increase power.

Compression ratios are a, if not the fundamental factor in regards to an internal combustion engines maximum thermal efficiency.

Higher thermal efficiencies extract more power from the same fuel charge per cycle.

Simple explanation being that the higher the compression ratio an engine has the smaller the volume you combust the charge in, this results in higher combustion pressures being created.

To be clear, not just an increased pressure because of the higher initial compression pressure, but relatively higher, because you are releasing the same energy in a smaller space, hence a larger jump occurs from peak compression pressure to peak combustion pressure, followed by a higher mean effective pressure.

Higher combustion pressures (relative to compression pressures) mean more torque, this equals more power from the same amount of fuel consumed.

Anyone in the steam or gas turbine scene can probably see the relationship, and hopefully confirm, can’t escape the PV, TS diagram.

 

[Ketchup]

OH I've finally got the guts to mention this publicly.... Why can't we fine-tune compression by simply protruding the decomp-plug into the chamber a lil? Turbulence in-chamber isn't problematic, it's rare a plug is "smooth" on the chamber ceiling anyways and, the way they're threaded, 10 threads = 1cc....so it'd make measurements real easy if, say, fine-tuning the ~10cc chamber of a 660 (after the build, after deciding on pop-up and deck height and all that)

The big-bore 660 jugs, awesomely, use 54mm sized chambers, and it ends them with chamber-ceiling wall thickness that is way more than usual (also, awesome!), but it leaves the spark plug & decomp plug recessed so you have to 'hack'/mcguyver both and, when doing decomp, I realized how easy it'd be to simply add 1/3rd CC of decomp into the chamber, given I'm deciding how long to cut the XL plug I got from the hardware store, anyway yeah someone better than I at calculus could help relay hard#'s but pop-up's work and they're hardly 20thou thick in many instances, you're definitely adding significant mass by just 3-5 threads' intrusion of the decomp-plug into the chamber!

Cool trick, right? Or dumb idea? LOL I think it's an awesome trick but this is 1st time I've run it by anyone...have heard of "just weld a spot/dab of MIG or TIG in the chamber" in terms of displacement-reduction in the chamber and it's the same idea, heck it's basically same idea as a pop-up unless there's some "turbulence/velocity" aspects to pop-up's I'm unaware of....but again the protruding plug would help w/ turbulence anyways....it's also a steel plug so maybe it'd get hot and be pre-igniting, although same could probably be said for body of spark plug...

Do it and see how you like it.

You can build a saw with a lot of compression but beware heat and detonation.

 

[drf256]

I will offer an alternative explanation as to why higher compression ratios (within limits) increase power.

Compression ratios are a, if not the fundamental factor in regards to an internal combustion engines maximum thermal efficiency.

Higher thermal efficiencies extract more power from the same fuel charge per cycle.

Simple explanation being that the higher the compression ratio an engine has the smaller the volume you combust the charge in, this results in higher combustion pressures being created.

To be clear, not just an increased pressure because of the higher initial compression pressure, but relatively higher, because you are releasing the same energy in a smaller space, hence a larger jump occurs from peak compression pressure to peak combustion pressure, followed by a higher mean effective pressure.

Higher combustion pressures (relative to compression pressures) mean more torque, this equals more power from the same amount of fuel consumed.

Anyone in the steam or gas turbine scene can probably see the relationship, and hopefully confirm, can’t escape the PV, TS diagram.

Not sure how alternative our statements are. I’d say that they are more additive than alternative.

All comes down to the Law of Conservation of Energy, it cannot be created or destroyed, it can only change forms.

E=MC2 and all that other crap.

But I see what you’re saying. In 4 stroke car engines, aluminum heads can tolerate more compression because they transfer heat away better and avoid detonation. Even though they wick the heat away, the increased compression ratio allows for more power. I believe it’s because of what you are saying.

 

[Sawrain]

Not sure how alternative our statements are. I’d say that they are more additive than alternative.

All comes down to the Law of Conservation of Energy, it cannot be created or destroyed, it can only change forms.

E=MC2 and all that other crap.

But I see what you’re saying. In 4 stroke car engines, aluminum heads can tolerate more compression because they transfer heat away better and avoid detonation. Even though they wick the heat away, the increased compression ratio allows for more power. I believe it’s because of what you are saying.

There are other ways to solve combustion instability problems though, better design, higher octane fuel, water/meth injection, mbt timing and so on.

I am suggesting a direct link between compression ratio and power, one you cannot really work around, well apart from with forced induction.

In agreement on the law of conservation of energy, lets consider the husky 261 & 262, same cylinder with a dished piston on the 261 for reduced compression ratio right? through what mechanism does reduced compression ratio like this reduce power?

Consider both saws have the same trapped mass of air/fuel mix when compression starts.
~25°BTDC ignition fires, 262 has a higher compression pressure from compression ratio alone (a larger loss), still the same mass of air/fuel mix trapped for both saws.
~10°ATDC now the 262 has an even higher percentage of pressure than the 261 as combustion pressure is peaking, and maintains that lead through the power stroke, why? mostly because the same fuel charge was ignited in a smaller space = higher temperature/pressure = more force on piston = more power by virtue of increased thermal efficiency.

The same amount of heat energy was released, the 262 trapped it in a smaller volume, higher pressure was created, pushed the piston with more force, extracted more energy from the same air/fuel charge.

I'm not saying more is always better, not even trying to insinuate I know what compression is best for any saws.

But for a quick simple answer to how does a little increase to compression ratio help a stock stock saw, I believe the above to be the major factor.

There is math to back this up, it also explains why Diesel engines are more efficient.

Skip to 8 minutes in, same type of equipment I was using on large marine engines for fault finding and performance monitoring, let’s you keep a good eye on what’s going on with each cylinder.

 

[Moparmyway]

Consider both saws have the same trapped mass of air/fuel mix when compression starts.

Ummmmmmm ..............................
But they don't, no ??

 

[Ronie]

Ummmmmmm ..............................
But they don't, no ??

I don't know about the other stuff but was thinking the one with the dished piston would have more trapped mix, right?

 

[Maintenance Chief]

Combustion chamber could be altered to dictate volume.

 

[Ketchup]

I don't know about the other stuff but was thinking the one with the dished piston would have more trapped mix, right?

I don’t think so. Same swept volume = same quantity of mix transfered. The dish piston just doesn’t squish it as hard.

 

[Sawrain]

Ummmmmmm ..............................
But they don't, no ??

I will draw attention to my use of the word consider, it was easiest for this discussion.

I 100% agree the the 261 cylinder has a larger volume at the beginning of compression, but at the moment of being trapped by port closure it was still fed from the same crankcase volume/shot, this is why I mentioned air/fuel mix mass was the same, not total mass trapped.

I have no idea how much of the extra volume is more useful air/fuel charge or just unscavenged spent exhaust gasses wasting space during compression, decided it was easier to leave it from discussion.

If there is more trapped air/fuel mix in a lower compression chainsaw engine, that means low compression 2T engines flow more air?

Even if they do, it still holds true with my original statement, more air air/fuel consumed for lower power, = low compression = low engine efficiency

 

[Moparmyway]

I don't know about the other stuff but was thinking the one with the dished piston would have more trapped mix, right?

Probably, since it holds more volume. The space in the dished piston creates this extra volume. However, what does that extra volume do for the combustion cycle and how does it affect the motor as it runs ?

Combustion chamber could be altered to dictate volume.

It could, but for the sake of this rabbit hole, it’s not

I don’t think so. Same swept volume = same quantity of mix transfered. The dish piston just doesn’t squish it as hard.

This, I believe, might not be as accurate as one might believe. More volume within the cylinder, which isn’t compressed as much, which doesn’t push as hard, probably doesn’t exit as efficiently, will tend to suck in less volume on it’s intake stroke. Remember, if it doesn’t compress as hard, it shouldn’t suck as hard either.

but at the moment of being trapped by port closure it was still fed from the same crankcase volume/shot, this is why I mentioned air/fuel mix mass was the same

This is where I don’t believe that the masses are the same

I have no idea how much of the extra volume is more useful air/fuel charge or just unscavenged spent exhaust gasses wasting space during compression, decided it was easier to leave it from discussion.

But isn’t it true, based on everything else within the cycle being the same (port timing, port shape, etc) that this is the main difference ?

 

[Moparmyway]

Higher combustion pressures (relative to compression pressures) mean more torque, this equals more power from the same amount of fuel consumed.

But, after a bump in compression alone, more fuel is needed. I’ve shaved cylinder heads and reduced head gasket thickness to increase compression. Every single time, I’ve needed to increase fuel flow.

While I agree that an increase in compression will increase the thermal efficiency of an engine, that’s not the only thing happening with the increase of compression. The motor also has a significant increase of intake vacuum, and correspondingly, an increase in fuel is needed

 

[Sawrain]

But isn’t it true, based on everything else within the cycle being the same (port timing, port shape, etc) that this is the main difference ?

drf256 and myself were talking about the direct benefits of compression itself, how does squeezing your charge tighter create more power? I never meant to venture off to how does compression ratio effect cylinder fill.

What is your take on how the dish Vs non dished piston effects the volumetric efficiency in regards to 261 vs 262?

My statement is a similar air/fuel mix being be ignited in a smaller space, leads directly to higher pressures, to more power.

But, after a bump in compression alone, more fuel is needed. I’ve shaved cylinder heads and reduced head gasket thickness to increase compression. Every single time, I’ve needed to increase fuel flow.

While I agree that an increase in compression will increase the thermal efficiency of an engine, that’s not the only thing happening with the increase of compression. The motor also has a significant increase of intake vacuum, and correspondingly, an increase in fuel is needed

Maybe I should have said per unit fuel used, are you talking four stroke now? where yes I agree a higher CR can increase VE by virtue of stronger vacuum, but two stroke, I've not considered it that important a factor, figured that is from under the piston, happy to reconsider though.

But again everyone knows the more air and fuel you can utilise per cycle the more power you can make, nothing new here.

But what directly is the benefit of increasing compression ratio?

 

[drf256]

There are other ways to solve combustion instability problems though, better design, higher octane fuel, water/meth injection, mbt timing and so on.

I am suggesting a direct link between compression ratio and power, one you cannot really work around, well apart from with forced induction.

In agreement on the law of conservation of energy, lets consider the husky 261 & 262, same cylinder with a dished piston on the 261 for reduced compression ratio right? through what mechanism does reduced compression ratio like this reduce power?

Consider both saws have the same trapped mass of air/fuel mix when compression starts.
~25°BTDC ignition fires, 262 has a higher compression pressure from compression ratio alone (a larger loss), still the same mass of air/fuel mix trapped for both saws.
~10°ATDC now the 262 has an even higher percentage of pressure than the 261 as combustion pressure is peaking, and maintains that lead through the power stroke, why? mostly because the same fuel charge was ignited in a smaller space = higher temperature/pressure = more force on piston = more power by virtue of increased thermal efficiency.

Interesting stuff for sure. But the situation in an engine is dynamic.

The 261 has more swept volume than the 262, but it’s compressed into a larger space. The dished piston is responsible for both of these volume increases in the 261. It’s a ratio, not a constant.

Not claiming to be an expert here at all, enjoying the conversation and hoping it’s not construed as an argument.

I see 2 major things here with the increased compression.

Firstly, it allows more complete and therefore productive combustion and more cylinder pressure. Increased cylinder pressure from a faster more complete burn has to play a major role in pressure production. Higher peak pressure, as you state, has to play a role. Similar to theory on longer connecting rods increasing dwell time at TDC and creating more overall power.

Secondly, it’s dynamic. So at 14,000 rpm the swept volume is decreased in both saws and the dynamic compression ratio is reduced. There isn’t enough time to fill the cylinder, so there is only partial charge compression. The 262 has more parasitic loss from the increased compression but can still have better combustion at the higher rpm. I’ve actually not seen much a reduction in rpm from increasing compression in saw in the real world.

You mention forced induction. As you know, the reason static CR is reduced in a forced induction engine is because the effective CR is much higher than the static one. The cylinder starts out filled with more volume under pressure already.

In 4 stroke engines, static CR increase are needed for higher output engines for multiple reasons. One big one that often overlooked in intake valve closing time. You need the higher static CR in order to have enough compression to create power in an under filled cylinder at lower rpm. As rpm rises, there is better volumetric efficiency but also not a fully filled cylinder because of less time. It’s all a balancing act.

 

[drf256]

So, why does a saw go from 4 stoking to 2 stroking when a load is placed on the engine?

Why does a hot saw, at the end of a long cut, need to be richened up to stay in tune when the entire engine is already hotter?

 

[drf256]

Let’s also not forget how these chainsaw engines are scavenged.

A dished piston adds to poor scavenging because the transfers can’t sweep the top of the piston as cleanly if it’s dished.

So the 261’s dished piston is theoretically holding onto more spent charge than the 262’s flat top. If the 261 is burning less fresh charge it’s making less power as well.

 

[Sawrain]

Enjoying the conversation and hoping it’s not construed as an argument.

I was thinking the same, good comment.

So, why does a saw go from 4 stoking to 2 stroking when a load is placed on the engine?

Guess I need to state what I think 4 stroking is first, something like a combination of reduced scavenging effieicinay not clearing enough exhaust gasses and our simple carbs making an overly rich condition at high rpms, until it gets so bad that there is more exhaust gas than fresh charge and the mix wont even burn, needing a second attempt at scavenging to have enough fresh charge to be an ignitable mixture.

The explanation I gave myself was that when you load the saw rpms drops, as rpm keep dropping eventfully scavenging efficiency returns enough to clear the cylinder each cycle, so as to fire on every stroke, this idea is of course 100% non founded guess work.

Other reasons?

Why does a hot saw, at the end of a long cut, need to be richened up to stay in tune when the entire engine is already hotter?

Don't know, hot less dense fuel?