Squish, mystery or not?

[sicivicdude]

SO we all have these recommendations about "squish" or "quench" areas but there seems to be quite a lot of speculation or guesstimation going on about what's the "right" amount and why.

The rule of thumb is as follows: on a trail motor .050" between the piston crown and TDC and the surface of the head squish area is "about right" but why?

First, lets get into MSV. What we're talking about is Maximum Squish Velocity and it equates to the fastest you can push fuel laden air before you excite it enough to explode on its own. Think about it, fuel and air mixed is a bomb waiting for a hair trigger (the spark plug usually) but pushing it fast enough causes enough molecules to slam together for the ignition to start on its own from friction.

The latest and greatest head design (designed around MSV) results in a small hemispherical combustion chamber centered around the are you WANT to detonate (the spark plug) and tight enough tolerances out away from that to dampen the fire but not SO tight that it ignites the fire itself.

When I say dampen, I mean that the "squish" or "quench" area is designed NOT to burn so that the charge is centered inside the combustion chamber and right around the spark plug.

So let's walk through a combustion cycle in a MSV designed head..... It all starts when the transfer ports close (piston coming up) and the volume inside the cylinder is either rushing back in the exhaust port or trapped back inside the combustion chamber already. As the exhaust wave pulse from the tuned pipe does it's job, it rams the charge that has escaped out through the exhaust port back into the port right as the piston closes the exhaust port. This has a mild super charging effect in that the pressure inside the chamber before any piston led compression has taken place is already above ambient (14.7psi at sea level).

As the piston rises (in the area above the exhaust port) the charge is being compressed. We have homogenized fuel and air (mixed) that is being pushed upwards as the piston rises. This is creating local hot spots (the reason why carbon on the crown is important!) but overall the charge is only gaining pressure and heat as a square of one another.

As the piston gets near the top of the stroke, the fuel and air around the outside spikes in temperature and pressure locally as it's pushed between the piston and quench area. This action is the beginning of the REALLY hot charge as the speed of the engine and size of the squish area determine exactly how fast the charge gets pushed in this part of the stroke. It's VERY important not to push the charge too fast during this part, Remember, that charge is a powderkeg at this point and ANYTHING could set it off.

Now here's where the magic of MSV heads really comes to shine.... at a certain point (assuming the charge was never pushed "over the edge" and predetonated) the piston rises close enough to the head surface that combustion actually CANNOT take place. This is the reason why this head design has two different names, squish and quench. One describes the action of pushing the charge towards the spark plug, the other describes what happens at TDC (and the spark event more or less). Once the clearance between the head and piston gets close enough, (generally that sweet spot of .050") the charge is pushed out hard enough that it becomes non uniform inside the chamber. The areas around the outside are no longer suitable for combustion. Also, because there is only a thin layer of charge between two pieces of relatively cool aluminum, more heat is drawn out of the charge (cooling by the head and piston) than is being put in (pushing upwards by the piston). Once that occurs, it's called "dampening" of the fire out in the squish area.

Then it brings us to the ignition event. What we all came for.... The spark plug fires and all that fresh charge that was just pushed in towards the spark plug all explodes with the fireball growing out away from the electrode of the spark plug as a flame front.

This flame front pushes out and down as the pressure rises exponentially pushing down on the piston and crank (and out eventually towards the wheels!). As the flame reaches the edge of the hemispherical bowl shaped combustion chamber, it has to "roll" out around the edge of the squish area. This is why the "break" or edge angle is actually fairly important.....

 

[motoman159]

nice job, sticky?

 

[blasterdemon7]

Good white up there.

 

[nboldman]

Nice write up and illustration. Have some green!

 

[sicivicdude]

I told blasterbogger earlier that I would explain how squish area works. I'm not sure I hit all of the information he was looking for though. I may have to go back to paint and make another thread explaining how squish angle and diameter, and combustion chamber depth, radius, and diameter all play together to determine cranking compression (cylinder pressure on a compression gauge) and knock potential....

Who needs me to cover those points?

 

[Larry's Shee]

To
much
info,
head
ready
to
xplode

Rep given, you're on a roll , might as well finish the lesson B) I:I

 

[2smoker85]

that was a good explanation of it.

 

[sicivicdude]

I can cover squish in this thread. I've already got a good start up there ^^^^

We'll start off with too much squish (too much area between the piston and head at TDC). This isn't a particularly dangerous situation and it's how the stock yamahe head is setup. Basically, the only issue is that you are leaving power on the table.

If you take a given octane rating gasoline at a perfect stoich mixture rate (14.7 for standard non-oxygenated gasoline) and heat it to a given point JUST below the point that it will spontaneously combust, the resulting explosion when it does go off will carry more energy with it (push the piston harder) and more completely use up all of the fuel (combustion efficiency). The magic point when you are exciting the mixture of hydrocarbon molecules TOO hard is impossible to measure exactly without destroying the sample (burning it ) which is done using a variable compression engine (usually by government agencies charged with making sure that gas stations aren't cheating).

What you can do is generalize about fuel quality by using the MON number (octane rating) at the pump and build the engine around that. What we call a "pump gas engine" is built differently (specifically in the head) than a "race gas" engine because race gas is less likely to detonate on its own when it's pushed very fast.

Too large a squish area won't push that fuel/air mixture fast enough to heat it to just below the spontaneous combustion temperature and the resulting fireball during the ignition event won't carry as much energy.

Too small a squish area has the opposite effect but MUCH worse results.... As the piston is rising and nearing the top of the stroke, the pressure inside the mixture around the outside of the cylinder is spiking. Because pressure and temperature are absolutely tied together, (See: Ideal Gas Law!) the termperature in that area also spikes. Once the temperature in that area spikes to above the auto-ignition temperature, a flame breaks out. Because the fuel in this area is superheated and unstable, a small torus (think, doughnut) of high pressure high temperature fuel/air mixture begins to overpressure the area inside the squish area. The rise in pressure is, unfortunately, confined rather well by the squish area and pushes outwards onto the cylinder walls, head, and down onto the piston. The last is the MOST devastating as the piston is still moving upwards towards the heat and pressure, all of that force is directed into the crown (believe it or not, slight plastic deforming the piston crown!) of the piston, wrist pin bearing, wrist pin, down the rod, big end bearing, crankshaft, main bearings, and finally into the cases. Enough "knocking" will eventually chew all of the bearings up or worse, ruin the main bearing seating areas and that set of cases forever (suzuki LT's are known for this!)

 

[sicivicdude]

Now, there are several programs out there to handle the calculations (not that this sort of thing is "normal" for a rider to know) but the jist of the situation is this:

Engine rpm, squish band "depth", squish band angle, and squish band height (above the piston crown) all go into figuring how "hot" the charge is going to get as you push it outwards.

The faster the engine is turning (the upper most point that the pipe/porting/carb will ever allow the engine to run) the faster the piston is coming up and the more pressure it can generate faster. Since temperature rises with pressure, the faster you generate a pressure wave, the hotter the gas will get as the piston pushes it.

If you have an area of equally distributed fuel and air, the farther you have to push it, the more you are going to push against and the more pressure is going to be generated by a given squish clearance. There are functions to the efficiency gains (and losses) about making the squish area more of the total bore but not as "tight" or less of the bore but "closer". The gains basically are tied to the ability of the squish area to quench the fire out around the edges of the cylinder at TDC versus pushing the charge too fast. The "magic number" is actually a simple calculation of about 50% of the bore size (in area) being just about right.

Obviously, in order for the quench area to do its job, the layer of gas inside the squish area at TDC needs to have the maximum contact area with the "cool" head and piston crown. If you have the squish area at an angle that's too great, the pressure wave created during the squish phase is too weak to excite the charge properly and the squish zone towards the middle of the combustion chamber isn't tight enough to dampen out the flame. (THIS IS THE STOCK YAMAHA BLASTER HEAD DESIGN!)

If the squish area is angled too shallow, the pressure wave is actually squished into a narrower area as the piston rises with the pressure skyrocketing and the likelylihood of predetonation going right along with it. The "sweet spot" of squish angle is just a tiny but more than the piston crown. Blaster pistons along the outside 50% of the piston crown have a nominal angle of 10 degrees. Most squish areas are cut to about 11 degrees to avoid any chances of the squish area converging and creating a hot spot.

Squish band height is what I've already covered really..... the distance from the piston crown to the cylinder head around the outside 50% of the cylinder. Already covered why too tight and too loose is bad.

 

[sicivicdude]

There are three methods to measure the squish area. One is the "recommended" way but it has a drawback...

You can cut two pieces of thick plumbing solder and insert them into the spark plug hole with the engine assembled and rotate the engine around so that the piston squishes the solder flat and then remove the solder and measure it. The disadvantage is that the solder that is large enough diameter to be used successfully for squish testing is generally acid cored. Squishing that is going to squirt acid out onto the piston, cylinder, and head. Not a problem if you plan on running it the same day. Kind of an issue if you are going to leave it for a while!

The second method is to insert some heavy modellers clay onto the top of the piston, install the head, and then roll the motor over. This squishes the clay flatter around the outside. The disadvantage is that it requires removal of the head after the squishing has taken place to measure the clay thickness around the outside edges and the clay has to be handled VERY carefully in order to measure.

The third method is purely mathematical. You measure how far down (or up) the piston is inside the cylinder with a squished head gasket installed. Then you measure the "lip" (or lack there of) of the head and add the two together. The distance between the piston crown at TDC and head is the squish thickness.

All three methods have some drawbacks, obviously but they all can work equally well if you pay attention to what you're doing and take your time!

 

[Triple_B]

 

[dksix]

WOW, awesome explanation. Just so I'm sure I'm getting this. So the area of the squish band should be about equal to 50% of the area of the piston top? Would I need to calculate the area of the piston dome or would the area of the diameter be close enough? The angle of the squish band should be slightly greater than the angle of the piston dome (10* vs 11*)? What about a radius at the edge of the squish band? Should there be a radius there to better direct the pressure of detonation down and better direct the fuel charge coming up? Would there be any advantage to having a piston more fitted to the squish band and combustion chamber? Like dome to flat (piston domed up to the inside edge of the squish band but flat on top directly under the combustion chamber)? I have a couple more questions about heads but I don't want to get off subject here. If you see them, please check them out. I'm sure you can shed some light on my questions. Thanks again for taking the time to write and illustrate this.

 

[sicivicdude]

The 50% figure is just a general figure. The programs that do the calculations for squish velocity allow the adjusting of this figure (along with all of the other parameters of head design) but basically, you only need to calculate roughly 50% of the area of the cylinder bore and then "run the numbers" from there.

The radius of the piston crown is actually parabolic (not constant). Instead of making a circle if it were continued, it would make an oval. The 10 degrees figure is actually an average of the angle of the crown around the outer 50% of the bore area. The average works well for being able to actually cut that angle into a head surface. Without a CNC lathe, it would be quite difficult to cut the complicated parabolic shape into the head surface.

 

[yamaha28]

Well that was some educational stuff! I smell something sticky

 

[dksix]

I was talking to someone last week about helping me do a fixture to hold a head so I could turn it. He suggested doing it on a mill, this information comes at a good time. I've got a good idea on the starting point for the squish band area, what about the combustion chamber area? Do you have any suggestions on size and shape for that part of a head?

 

[sicivicdude]

Using the 11 degree fixed angle figure for the squish is certainly good enough for the distance that you're needing to turn (~13mm on a 265cc big bore kit) as not to vary off the piston crown curve too much. In other words, you don't NEED to turn it on a mill, a manual lathe will work more than satisfactorily.

The final amount of squish band area needed it determined by calculating the speed of the charge for a given exhaust port height, bore, stroke, trapped volume, and rpm limit and comparing that to the fuel you intend to use. It's all a quite complicated process and I let a computer program do the figuring...

Once the squish thickness has been set, the combustion chamber volume needs to be calculated to keep the overall compression ratio low enough for the given fuel. Shooting for a specific cranking compression pressure with a given squish area will give you the head design that puts out the most amount of power for a given fuel.

 

[Surfrjag]

Too large a squish area won't push that fuel/air mixture fast enough to heat it to just below the spontaneous combustion temperature and the resulting fireball during the ignition event won't carry as much energy.

Too small a squish area has the opposite effect but MUCH worse results....

this confuses me. i would think a larger squish area would push the fuel/air mixture faster than a smaller squish area. :-/

good write up civic!! always something to learn here I:I

 

[sicivicdude]

this confuses me. i would think a larger squish area would push the fuel/air mixture faster than a smaller squish area. :-/

good write up civic!! always something to learn here I:I

I mispoke and I am sorry if it caused any confusion!

I meant larger squish "thickness" not square area. A larger squish area WILL push the charger harder (the charge will actually move the same speed, but it will encounter more molecules along it's path therefore heating more) than a small area.

 

[blasterbogger]

Im sorta understanding this , now lets talk numbers . What squish gives me what compression , and what head milling gives me what squish.

 

[slickerthanyou]

I don't understand all of it , But nice write up james!