Thinking out loud about squish
Turbo Mini Forums

Hi there,

I was in a really boring meeting this morning so I had time to think.

And I was thinking about squish.

You know, squish, the area of the chamber that pushes air in the direction of the plug when the piston comes up.

My thinking was you don't want a squish band on the plug side because all it will do is locate the "heavier" gas (the one with fuel dropplets) towards the middle of the chamber, away from the plug.

Does this makes sense ?

Anybody cut a head along this concept ?

Rusty by nature

according to Vizard (I watched ALL the videos) if the distance the piston from the head at the squish band is around 0.100" at 14 degrees ATDC, the engine is more prone to detonation. The theory being that a smaller gap (< 0.100" at 14 degrees ATDC) will cool the un burnt charge with the piston and head surface helping it to resist spontanious combustion or by quenching the flame. Visard recons if the only thing done to the engine is skim the block to bring the pistons to the top of the block, thus actually increasing compression, the detonation problem normally goes away.

The more I think about it, the more obvious it is.

The 14 degree ATDC number given as the point of detonation is nothing Visard has discovered or invented, this number is quite a common number within the industry, as I have heard it come up in conversations I have had with those who run engine tests for a living for a very large petro chemical company.

I thought MED did wht was called a V wedge piston that pretty much does what you are suggesting. It pushed the charge towards the plug side of the chamber

Edited by Sprocket on 9th Nov, 2010.

I was just thinking of the same thing ! I intend to have the block skimmed to flush those pistons that I had from you,Colin, to raise the CR a little, as they have such a deep dish.I dont want to skim the head.
According to my calcs I should get about 8.8:1 which will do me, as I am expecting to run 15psi with the GT17.

ATDC meaning when the piston goes down ?

If so, I don't quite understand the cooling concept ( think I can figure out the flame quenching).

My idea was more simple than that really.
I looked at the cylinder from the side and thought that if ther is a gap (between block and head obviously) then the compressing air is pushed away from the side of the head whre the plug is.

Rusty by nature

this 'squishing' effect promotes turbulence in the charge giving a better mix and promoting a better burn. i'm not sure if removing the squish band in one area is of any benefit.

Like I said, the MED V wedge piston had a wedge shaped dish that pushed all the charge towards the plug.

After Top Dead Center.

The spark event is Before Top Dead Center, and according to the boffins, detonation usually occurs at 14 Degrees ATDC.

The charge will be cooled by the cooler piston, head and the exposed cylinder surfaces. The spontanious combustion that is detonation, is as a direct result of the increase in pressure and temperature, so if the temperature is lower, it is less likely to spontaniously ignite. a smaller volume of charge in a narrow area will cool quicker, or not heat up as quick, depending on how you look at it. That at least is how I have understood it.

Edited by Sprocket on 9th Nov, 2010.

http://www.davidandjemma.com/mazda/FAQ/quench.htm

I found this an interesting read.

I don't think you can control the point where the mixture is pushed in this kind of heads. The chamber in the head is much smaller than the piston's surface, it's very compact (which is good).
All you can do is improving turbulences to break fuel droplets before ignition occurs. Which means keeping squish.
Skimming the block is a dangerous game as if you skim too much you'll create a small cavity which will be source of knock. This is what happens around the piston's crown, this is the most common place where knock starts.
You can pay attention to how you screw spark plugs, in order spark is well exposed to the mixture. This is an old trick everybody seems to have forgotten.

Edited by alpa on 14th Nov, 2010.

std 998 A+, g295, MD266, RHF4, 109hp @0.8bar/5400rpm

I came to understand that a squish that is the thickness of the head gasket prevents from knock (and I am now trying to understand if it is what is reffered as quench).

More is not good => just enough space for knock

Less is risky => rod stretching at high RPM mean the piston can hit the head and they both dislike it

Am I understanding right ?

Rusty by nature

std 998 A+, g295, MD266, RHF4, 109hp @0.8bar/5400rpm

Funny then that the last 1,2 from VW has parrallel valves and shrouding much alike the A-series.
I know because I saw the paperwork they send to their mechanics.
Being in the automotive business, I have access to these informations.

Anyway, pistons flush with the top of the block is a validated way to avoid knock on the A-Series.

The way I understand it, it is because the volume between flat head and piston is small enough to prevent knock.

Thinking about it, I saw those flat places of the head on several head from modern engines I have stripped.
Renault D4F 1,2 16V has a roof shaped chamber that is smaller than the bore hence some flat on head facing a flat top piston.
Suzuki Freewind 650 single has the same layout.

In fact, if the roof shape was the size of the bore, then we would start to have those small cavities you are talking about.

Actually the article Sprocket pointed is a good read even if I am not too sure I understoood the quench concept.

Rusty by nature