Compression ratios

[edisto]

I've seen a lot of posts about increasing compression ratios in saws. I know that it is a necessary byproduct of reducing squish clearance when the combustion chamber is fixed to the cylinder, but it seems like some do it simply as a matter of course, especially when modding saws, which seems like a bad idea.

On a 2-stroke there are benefits to increasing compression at low rpms because there is more mixing of the fuel charge and the exhaust, but at higher rpms, or with more efficient scavenging, the benefits seem to go down as the costs go up.

From what I gather, the losses due to increased friction from increased side thrust on the piston, and from a lower mixture mass in the cylinder due to increased heating (and other, more obvious heat-related problems) can cut quite a bit into any benefits from raising compression for 2-stroke engines running at high rpms. So if scavenging efficiency is increased, it seems one would want to avoid increasing the compression ratio by too much to avoid being hard on the bearings, avoid overheating components, and avoid power losses associated with heating the mixture.

Are chainsaw engines typically inefficient enough at scavenging that increasing compression ratios always helps?

Would a good porting job reduce the need for increasing compression, or would it take an extreme porting job (boost ports and the like) to bring the scavenging efficiency to the point where increased compression would cause power losses?

 

[parrisw]

The way I think about it is. When you port a saw, your letting more fuel/air in, without enough compression your not taking full advantage of all that fuel/air, if you don't have enough compression it wont burn it all, hindering your power gains. So why port in the first place??

 

[edisto]

The way I think about it is. When you port a saw, your letting more fuel/air in, without enough compression your not taking full advantage of all that fuel/air

You might let a larger volume of air fuel mixture in (by having less residual exhaust gas), but (unlike a 4-stroke) with the increase in piston (and overall engine) temperature that accompanies increased compression, that volume will hold a smaller mass of fuel because the air has expanded.

The point of porting is, to some extent, to get more exhaust gas out, thereby allowing more air-fuel mix in for the same cylinder volume. You don't need a higher compression to burn that volume, but you need a higher compression to compensate for less fuel mass when scavenging is poor.

 

[Nitroman]

I've seen a lot of posts about increasing compression ratios in saws. I know that it is a necessary byproduct of reducing squish clearance when the combustion chamber is fixed to the cylinder, but it seems like some do it simply as a matter of course, especially when modding saws, which seems like a bad idea.

On a 2-stroke there are benefits to increasing compression at low rpms because there is more mixing of the fuel charge and the exhaust, but at higher rpms, or with more efficient scavenging, the benefits seem to go down as the costs go up.

From what I gather, the losses due to increased friction from increased side thrust on the piston, and from a lower mixture mass in the cylinder due to increased heating (and other, more obvious heat-related problems) can cut quite a bit into any benefits from raising compression for 2-stroke engines running at high rpms. So if scavenging efficiency is increased, it seems one would want to avoid increasing the compression ratio by too much to avoid being hard on the bearings, avoid overheating components, and avoid power losses associated with heating the mixture.

Are chainsaw engines typically inefficient enough at scavenging that increasing compression ratios always helps?

Would a good porting job reduce the need for increasing compression, or would it take an extreme porting job (boost ports and the like) to bring the scavenging efficiency to the point where increased compression would cause power losses?

To accomplish what you are stating, the mix must be heated and expand as it enters the motor, pushing back on the rest of the incoming mix, to reduce the overall quantity of mix that can enter the chamber.
The time necessary to heat the incoming mix is so short, so little heat transfer will take place, the expansion of the mix is negligible.

 

[edisto]

To accomplish what you are stating, the mix must be heated and expand as it enters the motor, pushing back on the rest of the incoming mix, to reduce the overall quantity of mix that can enter the chamber.
The time necessary to heat the incoming mix is so short, so little heat transfer will take place, the expansion of the mix is negligible.

Pushing back??

As you increase the temperature of a gas, the volume of that gas expands. The exact same amount of air fuel mix will take up more space at the same pressure when it is warmer, so for a fixed cylinder volume, if the charge is warmer, there will be less fuel in the cylinder.

The increase in temperature with increased compression ratios is nonlinear, and climbs very steeply, so you get big increases in temp with small increases in compression. In motorcycle 2-strokes, they found that this can cause power losses that aren't worth the increased risk of pre-ignition and detonation, the richer mixtures, increased bearing wear, etc.

My question is not whether the results of increasing compression actually happen, my question is whether or not scavenging is inefficient enough in a 2-stroke that the benefits of increasing compression outweigh those costs, and whether this balance changes with a well-ported cylinder.

 

[Nitroman]

I am aware of what happens to gases when heated and cooled. I was addressing the following statement:

"a lower mixture mass in the cylinder due to increased heating"

Pardon me if I was mistaken.

 

[edisto]

I am aware of what happens to gases when heated and cooled. I was addressing the following statement:

"a lower mixture mass in the cylinder due to increased heating"

Pardon me if I was mistaken.

I always lost points in math class for not showing my work, and I sometimes skip a few steps in my explanations...sorry.

Because the same mass takes up more volume when heated, and the cylinder volume when the transfers close doesn't vary (much), a volume of warmer gas has less mass than the same volume of cooler gas...so if the charge is heated more in the crankcase, you will end up with a lower mass in the cylinder unless you have an expansion chamber pushing charge back into the cylinder under pressure.

Sorry for the confusion.

 

[Nitroman]

Well that is what I thought you were saying.

I do not know if the overall engine temperature would be significantly higher in two different engines: one modded and with high comp, the other modded but with more emphasis on porting and transfers; with no increase in compression.

The real way to consider this would be to determine how long the mix would be in the crankcase, and traveling up the transfers. Then use the transfer coefficient for aluminum to air (it should be possible to find one for fuel air mix), and calculate just how much heat is transferred, and thus, your increase in volume/reduction in charge volume.

My assertion is the quickness of the mix's passage through the engine precludes an increase in volume significant enough to affect engine performance. That heat transfer is occurring is not questioned, only the quantity of heat.

 

[edisto]

My assertion is the quickness of the mix's passage through the engine precludes an increase in volume significant enough to affect engine performance. That heat transfer is occurring is not questioned, only the quantity of heat.

It affects it in motorcycles...that isn't the question (and there are other losses to consider as well).

In a situation where there is a relatively large proportion of exhaust gasses remaining in the cylinder (poor scavenging or low rpm) the losses are low, so the benefits of higher compression are greater. As rpm increases (or scavenging improves) the losses get greater (in spite of the charge moving faster).

Motorcycles certainly have more efficient exhausts...so the question is:

Is the efficiency of chainsaw engines low enough that there typically is a benefit to increasing compression, or does port work increase the efficiency enough that compression is best left alone unless squish is too high?

In other words...has anyone done port work on a saw, measured performance, and then increased compression ratio and measured the performance difference?

 

[Nitroman]

Dude:

"From what I gather, the losses due to increased friction from increased side thrust on the piston, and from a lower mixture mass in the cylinder due to increased heating (and other, more obvious heat-related problems) can cut quite a bit into any benefits from raising compression for 2-stroke engines running at high rpms".

I am totally fascinated by your evolving explanations of your original question that have no bearing whatsoever on your original question. I am going to keep replying to see just how many variations you can come up with.

 

[parrisw]

Just curious, do you have any experience in modding saws??? Or just theorizing everything?

 

[Crofter]

Some key differences between saws and bikes. Reed induction vs piston porting. bikes almost always piped saws not, bikes need torque and subjected to loading at lower rpms. If you are going to pipe a saw you do not need to go after high static compression ratios since the dynamic effects of the pipe on both induction and compression of the charge make it unnecessary. Heating from higher compression only occurs after the charge is trapped. You are not suggesting that heat from higher base compression is a product of higher compression ratios, are you.
At higher rpms after modding, where horsepower rises even if torque starts to drop somewhat, unless you increase venturi area, charge density starts to drop.
I guess you could say higher compression ratios offset this but you have to get straight whether you are talking piped or not and how the particular engine is being loaded. I think you are trying to extrapolate too much from bike conditions onto chainsaw. Remember the difference in the bore to stroke ratios and the significance.
A variable volume head could show the point of diminishing returns from compression increase under different load conditions. Maybe some of the engine builders are wasting their efforts. Ya think?

 

[Work Saw Collector]

Interesting thread, but I can't keep up with it. I'm not getting it, is making compression higher worth doing or is it a more complacted answer than I am going to get.

 

[Tzed250]

Other than Franks post, there is way to much conjecture in this thread...

 

[edisto]

I am totally fascinated by your evolving explanations of your original question that have no bearing whatsoever on your original question. I am going to keep replying to see just how many variations you can come up with.

It's not evolving...if you fill the same cylinder with hot gas and with cold gas, you will have a lower mass of the hot gas.

Just curious, do you have any experience in modding saws??? Or just theorizing everything?

I've "blueprinted", but not modded...that's why I'm asking the question. All of the theory is for motorcycles, and I'm trying to determine what fits and what doesn't. I need to hear from someone that has done port work, measured performance, and then raised compression and tested again. It'll be a month or 2 before I can afford my new P&C, and I will do the experiment then.

I guess you could say higher compression ratios offset this but you have to get straight whether you are talking piped or not and how the particular engine is being loaded. I think you are trying to extrapolate too much from bike conditions onto chainsaw.

What I tried to convey (unsuccessfully in the first post) was that I've been reading about mods on motorcycle engines, and am trying to figure out what will fit and what won't. Certainly, I'm not talking about anything piped, and the only guidelines I have (apart from this forum) is that the needs of a chainsaw more closely approximate a road bike that is designed to run at WOT.

So...let me start again. The theorizing is NOT mine, it's out of Bell and Jennings. I know they are dealing with tuned pipes for the most part, so you have to sort out what matters and what doesn't, and I know chainsaws are more "square" in terms of bore and stroke.

To keep from having one excessively long post, I'll post some excerpts from Bell and Jennings, then I'll post my interpretation of them and restate my question (hopefully more clearly).

 

[edisto]

Bell

"I agree that the compression ratio should be made as high as practicable, but often the manufacturer has already found the limit and built his engines accordingly."

"Something you must always remember when dealing with 2-stroke engines is that increasing the compression ratio will not give a power gain equivalent to that which you would pick up with a 4-stroke engine. Heat is the enemy of the 2-stroke engines and pushing the compression ratio to give an expected 6% power increase will possibly result in a 1-2% power rise at the most; the rest will be lost in heat energy and pumping losses. However, at lower engine speeds, the cylinder will not be completely filled with fuel/air mixture and the power may jump by 3-4% because there is not such a heat loss. This is, in fact, the real benefit of raising the compression ratio, not to increase maximum power but to pick up midrange power and possibly widen the band."

"In times past when porting was less efficient than it is today, and when expansion chambers were less well understood, which compromised their ability to evacuate the cylinder of exhaust gas and recharge it with fuel/air mixture, compression ratios could be pushed quite high without resulting engine damage. This was possible because not only was the cylinder not completely filled, but what fuel mixture was drawn in was diluted by a large volume of exhaust gas which dramatically slowed the rate of combustion."

 

[edisto]

Jennings

"But higher compression ratios can also bring about a mechanical disaster: improvements in power gained in this manner are purchased at a disproportionate cost in peak cylinder pressure, leading to reduced bearing life and sometimes to an outright failure of a connecting rod or crankpin. Moreover, because the higher pressures are reflected in a proportionately greater side thrust at the piston, frictional losses are such that net power gains are always less than the improvement one would expect from the calculated air standard efficiency. Finally, heat flow from the combustion gases into the surrounding vessel (piston crown, cylinder head, and cylinder walls) rises increasingly sharply with compression ratio, so that a number of thermal-related problems intrude into the already complicated relationship between compression ratio and power."

"The worst of these problems is the overheating of the piston crown. A too high compression ratio will raise piston crown temperatures to the point where heating of the mixture below the piston, in the crankcase, reduces the weight of the charge ultimately trapped in the cylinder during the compression stroke to such extent that net power suffers -no matter what Mr. Otto's air standard efficiency formula may say. And if the compression ratio is high enough, heat input into the piston may raise the crown temperature to the point where detonation and then pre-ignition occur. These phenomena will, in turn, very quickly further raise piston crown temperature to such extent that the piston material loses enough of its strength to yield to the gas pressure above – the piston crown then becoming either concave (which drops the compression ratio to a tolerable level) or develops a large hole (and that reduces the compression ratio to zero:zero)."

"Many people have encountered this last effect, and the tuner's one-time favorite ploy of “milling the head” has fallen into disrepute. But it also is possible to encounter trouble without recognizing it: There is a delicate balance between gains from increased compression ratios and losses due to increased temperatures -which appear not only at the piston's interior, but also throughout the crankcase, crankshaft, rod and all the rest of the engine's interior contacted by the air/fuel mixture. When these parts are hotter, the mixture's temperature is also raised, along with its free volume. Thus, the mixture's temperature-induced efforts to expand inevitably force part of it out the exhaust port, and as power is related very closely to the weight of the charge captured in the cylinder, this heating shows up as a power loss."

 

[edisto]

Sooo...

That's where the costs I described for increasing compression ratio come from.

From the relationships outlined by Bell, the balance between cost and benefit depend on the efficiency: the greater the efficiency, the less benefit and greater cost from increasing the compression ratio.

It seems as though people have had a great deal of success with increasing compression ratios in chainsaw engines, which suggests that without an expansion chamber, the level of efficiency will not get high enough for increased compression to be too big of a detriment.

That is my question: is there a point where you can port a saw to be efficient enough that (without a tuned pipe) higher compression becomes a bad thing?

edit: "but it seems like some do it simply as a matter of course, especially when modding saws, which seems like a bad idea."

In rereading my original post, I can see why it looks like I'm preaching, and not asking. It wasn't my intent.

 

[Nitroman]

It's not evolving...if you fill the same cylinder with hot gas and with cold gas, you will have a lower mass of the hot gas.

Correct. Because the density is less. And my assertion is: the mix isn't in the crankcase/transfers long enough to absorb enough heat to expand to an extent that would cause a performance difference.

You can calculate the time the mix is in the engine, use a transfer coefficient to determine how much heat is absorbed. You can then calculate how much your mix expanded, and determine the density of the mix in the cylinder. Compare it to the surrounding air temp. Bingo.

If you really want to do this, drill a hole in the transfer from the exterior and install a thermocouple available from Westach. Cut a big log lengthwise so the saw will get thoroughly hot, make sure you try to keep the rpms in the same range for 5 minutes or so. Have someone calling the time and someone recording the temp so you have a record of the temp over time.
Now raise the static compression of the saw (remove base gasket). Do the experiment again. Is there an increase in the temperature of the mix that is about to enter the combustion chamber?

 

[edisto]

Correct. Because the density is less. And my assertion is: the mix isn't in the crankcase/transfers long enough to absorb enough heat to expand to an extent that would cause a performance difference.

You can calculate the time the mix is in the engine, use a transfer coefficient to determine how much heat is absorbed. You can then calculate how much your mix expanded, and determine the density of the mix in the cylinder. Compare it to the surrounding air temp. Bingo.

If you really want to do this, drill a hole in the transfer from the exterior and install a thermocouple available from Westach. Cut a big log lengthwise so the saw will get thoroughly hot, make sure you try to keep the rpms in the same range for 5 minutes or so. Have someone calling the time and someone recording the temp so you have a record of the temp over time.
Now raise the static compression of the saw (remove base gasket). Do the experiment again. Is there an increase in the temperature of the mix that is about to enter the combustion chamber?

I understood your assertion, but it seems to be a real phenomenon in 2-stroke motorcycle engines (ceramic coating of the ports also has been shown to help, a cooler crankcase is one of the reported benefits of coating the piston crown). At any rate, it is one of the costs of increased compression, and I'm not interested in isolating that effect, I'm interested in what happens in the cut.

So...once I finish porting the engine and modifying the exhaust, keeping the compression ratio the same (unless the squish is too high), I'll do some timed cuts (and monitor temperatures), and then perhaps try a pop-up to increase compression, and see what happens to the cut time.

I'm more familiar with 4-strokes, so raising the compression seemed like a "universal good" to me until I read a little more.

I'm just asking if anyone has already done the experiment.