Squish

[drmiller100]

squish means different things to different people, but in general squish is the narrow band between the top of the piston and the head.

if you have too wide of a squish, then you are leaving "free" horsepower on the table.
If you get too narrow of squish, the piston can hit the head, and BAD THINGS HAPPEN (BTH).

Stock saws have fairly wide squish and relatively low compression. Joe homeowner somehow expects to run 3 year old 84 octane gasoline in his chainsaw and expects it to not blow up.

if you are RELIGIOUS about using fresh premium gas in your saw, you can cut the squish down. the easiest way is to measure the squish and then decide if you can simply pull the basegasket out and use silicon as a base gasket.

This will raise the compression ratio, giving you more snap in the mid range and ultimatly slightly more top end horsepower.

If you now use junk low octane gasoline in your saw expect BTH.

 

[drmiller100]

engine designers in 2 stroke engines that are more high performance then saws further refine squish. examples of engines wiht this technology are bikes in the past 20 years and snowmobiles in the past 5 years.

Detonation is a BTH event. Basically, as the piston compresses the air/fuel mixture, pressure and heat build to where the mixture "self ignites." On the flip side, the higher the compression ratio the engine can run, the more power it will make.

One way to prevent detonation is to keep the mixture swirrling around in the chamber as the piston is coming up. We could put a fan in there, but it wouldn't last long.

The basic issue is there are hot spots in the combustion chamber. spark plug threads, microscopic bits of superheated carbon, sharp edges, and the metal around the exhaust port are all areas that will be hotter, and lead to more detonation.

So, if a volume of the mixture is allowed to hang out in these hot areas they might cook off and start burning.

Again, the fan. What advanced engine builders do is use the space between head and piston to create almost a bellows type event. They are squeezing the mixture, and using it to blow the mixture out into the main combustion chamber. This keeps mixture flowing past the hot spots, delaying detonation.

They do this by designing the angle of the head and piston to be at a specific angle for a given width. think of the dome of the piston, then think of the angle of the inner dome of the head. those two will together make a specific angle that squeezes the mixture towards the center of the combustion chamber.

I don't think saws are that sophisticated. I think guys who crank the compression up run good gas and still don't have detonation issues.

Ultimately, I think chainsaws are all pretty low horsepower machines for the cc displacement and you can't hardly help but make horseower if you change things.

 

[timberwolf]

I think the term you are using for width is a bit confusing. Might be clearer to speak of height when we are refering to the clearance between the piston and head, and width when we are refering to how wide the band is as it extends from the cylinder wall to the combustion chamber dome.

Yes, overly high clearance squish leaves power on the table, but going tight, you don't need to have the piston hit the head to have problems. I have tried to run under .010, and the piston was quickly erroded away under the squish band, looked like it was bead blasted and struck by tiny meteors. Go too tight and the boundry layer gets stripped away and the piston is no longer protected from momentary temps over 2000 deg C or 3600 f. Interesting, alluminium piston will start to fail structurally at only 700 f to 900 f and will melt not much over 1200 F. As soon as the boundry layer is stripped away and the piston crown is exposed to direct flame impingement, it will absorb more heat and quickly expand the crown as well as start melting and erroding away the surface. This crown expansion may sieze the engine, and also if the piston crown temp heads up towards 320 C or about 600 f it gets into autoignition potential, so rather than the tight squish preventing detonation it can be the source of it.

Here is a simple squish calculator I found, It does not cover angled squish bands or off set heads, but maybe useful to someone. http://www.torqsoft.net/squish-velocity.html

Here also a link to piston position calculator http://www.torqsoft.net/piston-position.html

 

[Crofter]

Doug, I think that analogy of the squish turbulence preventing the fuel from hanging around hot spots might perhaps more to do with preventing Preignition. That occurs with the heat of compression combined with the cylinder hotspots. Preignition would usually occur much before maximum squish velocity. Detonation is usually defined as occurring some time after normal ignition by the spark plug, not prior during compression. I am not knocking you at all, because I think squish is an important consideration and has lots of different things affecting it.
The squish turbulence does increase the flame front propagation rate and prevents the time related decay of combustion gases into elements that will detonate. I agree that squish is like fanning a fire. With the squish band area feeding the fuel charge in toward the spark plug as fast as the flame is propagating outward, the combustion is more efficiently concentrated and less heat is lost into the whole of the available combustion surfaces (fewer hot spots.) I think there is a point where extremely high squish velocity besides blowing off the boundary layer as TW mentions, can over fan the fire and be on the virge of blowing it out. I am sure that the width, height and angle of the squish band in relation to the shape and position of the individual combustion chamber does have an effect on the ultimate output without detonation. I think squish characteristics and how it affects combusion plus its affects on the amount of unburnt end gases, also has a considerable effect on polution aspects. Manufacturers do seem to be tightening it up more than they used to.
I wonder if the reduced output of saws per CC. in comparison to bikes etc might have something to do with compliance with vibration levels. There is a lot more pressure on them from workmans compensation organizations to reduce the payouts for Vibratory White Hand syndrome. Maybe RPM limiters factor in here too but this is almost topic for another thread.

 

[timberwolf]

I think a big part of keeping the squish tight from the EPA prespective is unburnt HC and CO emmissions.

The more space in the squish the greater the quantity of end gas that either does not burn or burns incompletely.

Here is a graph of squish velocity looking at the velocity accross the piston from the wall to the combustion chamber. Three different angles, straight accross, 2.1 deg (.020 at wall and .030 at chamber) and 4.2 deg (.020 at wall and .040 at chamber). Shows how much velocity is reduced by having a larger squish height where the squish band meets the combustion chamber.

 

[timberwolf]

Here is a 2D model of a 56 mm bore with 8mm squish band offset 4mm so its 12mm wide over the exhaust and 4mm wide over the intake. Pretty big effect on the velocity with the squish velocity up near 60m/s on the widest part of the band and under 20 m/s on the narrow part.

 

[drmiller100]

here comes the rub though.

if you try to get the velocity too high, you get detonation, evidenced by hen pecking the edges of the piston, broken plugs, broken bearings, and crankshafts.

so, a goal is to have the velocity consistent all around the combustion chamber. i do not know what the target velocity is, but do know you want it consistent.

so if you did desire it to be consistent, you could machine the long side to increase the volume, which makes things even more complicated to calculate.

when all done, i think the goal is to have decent velocity to prevent detonation, and then focus on other things to take advantage of it.

like increasing transfer effectiveness to improve absolute pressure at tdc, and to chase the exhaust gasses out of the combustion chamber at bdc.

 

[wood4heat]

Speaking of snowmobiles and jet skis they used to make swirled dome inserts for cyl heads. It was supposed to swirl the incoming charge and prevent preignition but I never understood how it would do that. Looking at one it had a lot of ridges ending in sharp points that I thought would become hot spots and a source of preignition.

 

[timberwolf]

I don't think the goal is really to have even squish velocity all the way arround, look at the sucess of bathtub heads. Squish is needed on the exhaust port side to give turbulance and experiments with chamber offset back to the intake side have shown messed up scavenging flow with the charge going right up the back of the cylinder like it is supposed to do and but then flushing the chamber out. However very little squish is need on the sides as this is often a fairly cool part of the piston and quite a detonation resistant area. You don't want extra squish band where you don't need it, at it eats up the charge volume that power can be netted from, if it was not for detonation a true full hemi head with no squish band would work great with no wated end gas and full exposure of the combustion cloud to push down on the piston in the first few deg about TDC.

30 m/s is absolutely fine, I have run work saws into the 40s no problem and race saws over 50 m/s, no signs of detonation. Something to keep in mind is squish is only one dimention to the detonation issue, RPM, and engine scale are also factor in allowing saws to run compression and squish numbers that would hole a bike or sled piston. Also if you look at a stock saw with small carb and restricted can muffler, it will not be packing in anywhere near the same charge as a piped bike or sled motor and in that way the saw will be able to stand more compression.

I have seen stuff too on this turbo head stuff where grooves are cut into squish band, Looks pretty questionable, not much third party testing, lots of claims few results. Like you say potential hot spots and increased heat transfer are significant down sides to such an idea.

 

[drmiller100]

hmmmm.

i don't think i'm explaining myself very well.

you can for sure find measurable horsepower by upping the compression ratio, which the easiest way to do this is to remove the base gasket or mill the jug which tightens the squish distance.

you are not going to find significant or measurable amounts of horsepower by just playing with squish angles or shapes or even distance (height) assuming the compression ratio remains the same.

if you figure out ways to get more fresh charge into the combustion chamber, and start making more horsepower, Eventually we will have detonation issues.

When you have detonation issues, and are reluctant to lower the compression ratio and run into limits for your fuel, optimizing squish can for sure allow you to keep your higher compression ratio.

after this weekend, I think time is much better spent on transfers and ignition timing then on squish angles. We are a long ways from having efficient chainsaw feeding systems.

-doug

 

[timberwolf]

Doug, you are missing something, read about end gas and chamber turbulance, whatever is trapped in the squish band does not make power. I am talking even given keeping the compression the same.

Say detonation was a non issue, three heads all the same compression ratio, one with a .040 squish 10 mm wide, another with a .020 squish 5mm wide and a hemi with no squish at all. In that case the hemi loses almost no charge to wasted end gas, the tight squish some and the wide squish more.

Yes were talking only a few percent, take it or leave it.

Here is a read, I think it explains end gas and squish pretty well.

http://www.motorcycle.com/how-to/wrenching-with-robchemical-soup-the-mystery-of-detonation-3420.html?page=2

Rather, the biggest benefit is obtained by reducing the squish band to it's safe minimum (about 0.020-0.040 in, depending on the particular engine used). This will have a far greater effect on increasing the turbulence in the combustion chamber than any other modification.

It is important to realize the two important functions of reducing the squish band clearance: (a) to enhance turbulence due to rapid ingestion of gas into the combustion chamber, hence increasing the burning rate of the mixture and (b) to reduce the volume of the unburned gas in the boundary layer of cool gas near the piston top and cylinder head surfaces. Typically, gas trapped in the squish area doesn't burn, even if the squish band clearance is relatively large.

 

[drmiller100]

my motorcycle makes 200 horsepower per liter.

my chain saw makes 100 horsepower per liter.

i'm not looking for a few percentage points. I'm looking for the other 100 percent.

 

[timberwolf]

Apples and oranges.

Bike = pipe
Saw = can muffler
Bike making 200 hp/L = liquid cooled most likely
Saw = lower torque due to short stroke used to keep weight and size down
Saw = primative carb
Saw = machiene designed for work
Bike = machiene designed for performance and racing

So for sure a bike is going to have a higher specific output.

However, take a newer bike and there is very little room for improvement, on the otherhand a saw can be improved a lot. Here is a 262 I finished up tonight.

Stock its close to 7 seconds to cut through a 13.5 block of hard dry wood.
http://www.youtube.com/watch?v=b1tZUkHE_Fg

Ported as a strong worksaw its down at about 3.25 seconds. Exact same chain, bar and block of wood.
http://www.youtube.com/watch?v=7JhU54qrWcM

It's first principals, if you do the work in less than half the time, it going to take more than twice the power to start and then some to make up for increased friction and inefficiencies.

I have no doubt a 100 cc saw can make 20 plus HP with a pipe, sure it can't do it for long due to the lack of cooling, but if it had the same cooling as a sled or bike i'm sure it would hang in there.

 

[drmiller100]

a shame u couldn't make it to the pnw get together.

we sure had a great time there, and learned a lot.

 

[youngs24]

a shame u couldn't make it to the pnw get together.

we sure had a great time there, and learned a lot.

So who had the best saw and who was the biggest disapointment there they are for the most part a lot of fun.:greenchainsaw:

 

[Mr.]

Doug,

I'm not sure you know it, but you pulled the nugget out of this discussion. Ignition timing.

The OPTIMUM ignition timing will change when squish velocities change. There is "free" horsepower to be had with increased squish velocity, but it must be coupled with the ignition.

Fred

 

[drmiller100]

So who had the best saw and who was the biggest disapointment there they are for the most part a lot of fun.:greenchainsaw:

there were pair of 5 cube piped saws on alky.

ian has a husky built by ed heard that was really fast.

after that adam clarke's 681 and my pair of 7900's were all really close.

dean's saws were shiny and started really well.

 

[wood4heat]

there were pair of 5 cube piped saws on alky.

ian has a husky built by ed heard that was really fast.

after that adam clarke's 681 and my pair of 7900's were all really close.

dean's saws were shiny and started really well.

Did you run Bookerdogs Dean modified 390xp?

 

[youngs24]

there were pair of 5 cube piped saws on alky.

ian has a husky built by ed heard that was really fast.

after that adam clarke's 681 and my pair of 7900's were all really close.

dean's saws were shiny and started really well.

So Doug do you remeber what kind of 5 cube pipe saws was they?

You said Deans were shiny and started really well. D:greenchainsaw: id they run as good as everybody says his saws run? I bet they were hard to beat.

 

[wood4heat]

So Doug do you remeber what kind of 5 cube pipe saws was they?

You said Deans were shiny and started really well. D:greenchainsaw: id they run as good as everybody says his saws run? I bet they were hard to beat.

There are pics of the pipe saws in the Pacific NW get together thread. One was a 7900 and the other a Stihl but I'm not sure what model, 440 or 460 I would guess. As for deans saws the 390xp was impressive but the 346xp was a shocker! That little thing TORE through the wood!