Quench distance vs. piston position? 1

[ejit]

Quench and squish are not the same thing, and are not produced to the same degree by the same conditions.
Squish: gasses trapped between the piston dome and head are ejected across the chamber at high speed by the near-collision of the piston dome and head, causing turbulence and mixture homogenization. Squish occurs whenever 2 parallel surfaces approach each other closely at or near TDC. Too close = pumping loss. Too far: low squish velocity, less turbulence. The speed of the approach is a function of stroke length (which controls the absolute distance; longer stroke = higher F/S @ constant RPM) and rod ratio (which controls the relative speed change; low ratio = faster).
Quench: lowers the temperature of end gasses trapped between the piston dome and head by radiation and conduction to prevent a second flame front from igniting mix prematurely due to thermal shock, etc.
For motors with 3.5-4.5” bore, a quench distance of .035-.040” appears to work well (measured dry, cold and static with steel rods). This will result in almost .000” clearance (hot, wet and running) due to thermal expansion, rod stretch, piston rock-over etc.
Why are certain motors “safe” with higher quench distances?
The original intent was to have only relatively cool gas present in the quench band during the “at risk” period, which begins (after ignition) @ TDC and beyond to 14° (the location of peak pressure), since the entire mix is not burning until at least 20° (?) ATDC. A motor with 4” stroke and 6.2” rods (n = 1.55) has moved about 4% of its stroke @ 20° ATDC, or .160” When the motor is running, the quench area is as wide as .160” during the critical flame propagation period, in which secondary ignition will cause knocking. This means that in a motor built to .040” clearance (cold), almost .000” (hot & running) STILL has .160” or more quench distance during its gas pressure rise period, but the trapped gas burns very slowly (if at all), and the flame does not spread to the main chamber to cause knocking.
Doesn't this mean that the stroke and rod ratio affect the width of the quench area during the high pressure period immediately after TDC up to 14° ATDC? A motor with a very long stroke and very short rod will have big movement (5” stroke, 1.5 ratio = .099”) by 14° ATDC; how are these motors safe? Is there a point where the large piston movement makes the quench area so big during the high pressure period that quench doesn't work?
Can a motor with very short stroke and long rods (where the quench area is still very small @ 14° ATDC) get away with looser quench?
Comments, please?

 

[franzh]

Ejit:

Please don’t misunderstand the lack of response as a lack of interest. Your comments are valid with the exception of the absolute numbers you included. Each separate design of engine introduces many variables including the rod and stroke ratio, combustion chamber shape and location, piston dome configuration, camshaft profile, spark plug location and depth of the insulator, and so on. Things such as cylinder finish, piston and combustion chamber surface textures will also have some relevance to the flame propagations.
One item often overlooked is the effect of the upper ring land area with trapped HC, and the effect that has on incoming mixtures. Research into cooling systems and the effect of overcooling the cylinder walls has shown that too has proven to increase the unburnt fuel resulting in increased quench.

Franz

 

[richdubbya]

All my work is with racing engines, so i dont know how this applies to emissions or passenger/commercial vehicles but since you are talking about zero running clearance I assume you are talking about racing engines.

Squish and quench are different things but are closely related and both happen at or very near TDC. During the intake cycle there is a lot of fuel/air seperation and a lot of this is concentrated in the outer edges of the cylinder. There are pockets of puddled fuel along with pockets of very lean mixtures. These lean pockets when subjected to heat and pressure are the principal reason for detonation.

During the squish at TDC these fuel puddles and lean pockets are expelled into the main combustion chamber and in the process the piston and head are wetted by the fuel puddles and shielded from the expanding heat in the chamber. This results in a lower temp in the quench part of the head and piston compared to a non quench design. As the piston moves away from TDC the cooler surfaces keep the mixture drawn back between them cooler. This combined with the drawn back mixture being much more evenly mixed because of the turbulence and heat in the main chamber results in a much lower chance for detonation.

The quench happens at TDC but the effects are still present when peak cylinder pressure is reached.

Another thing to consider is different rod/stroke ratios and cylinder/piston offsets call for different peak pressure points.