Tartilla wrote: ↑Thu Sep 22, 2022 4:01 pm
At high RPM, a larger quench deck may not be able to expel all the air in time, and may create localized high pressure zones. Essentially, how fast can you push a gas? Like pushing a rope up the hill.
That's a good question. What I know from at least 3 racing series that the design of the squish height and area is done to the extreme. Biggest possible area and lowest operated squisch height of almost 0.0 mm. DTM, STW, BTCC, NASCAR (according the Video) and Formula 1 NA engine (e.g. BMW P80) run very low squish height's which would calculate in enormous squish velocities.
If you look into scientific papers to that topic you see a lot of ongoing in Diesel and Otto lean combustion engines as well as some motorcycle engines. Only a few of those match the conditions and chambers of the mentioned extreme squish approaches. Here the tenor is like, squish is added to tumble or swirl as squish is a short time but very intense supporter to get down the ignition delay reduced and the 0-15 % mass fraction burned time down, which is a good control lever arm of cyclic variations. That is important in emission related engine at low, mid and high load and low engine speeds. Here the Authors report of limits, even distinguishing effects of ignition initiation.
A high revving engine is not comparable to that application, as inertia of the mixture is accounting for some loss as well as tumble, swirl and hybrids of this get of less efficiency as I wrote in a previous post. So we need to take everything we can take to keep TKE adopted to the necessary flame speed (e.g. 10,000 rpm, 86 mm bore, 0.7 ms of time gives roughly 61 m/s on average). The incoming mixture speed is around 0.2-0.6 Mach, where peak is around peak valve lift. That means a lot of TKE potential lays in the incoming impulse. Therefore intake runner, port and valve seat design have a huge effect on what is coming as well as valve timing (reverse flow). To preserve this energy and to dissolve it into micro turbulence (= TKE) is one of the tasks of the combustion process designer. Effects from tumble, swirl and squish are way less in potential, one cause why critical pressure drop over the intake port throat at boosted engines (= Mach 1 possible at overcritical pressure ratio) don't really count on those supporters as the most TKE comes with port flow at even lower engine speeds. Of course much of it get lost during compression, only a few will build up TKE, therefore spherical chambers can have advantages on boosted engines.
NA engines are less blessed with intake impulse, keeping it high is essential for combustion process design. Every supporter is necessary, but as impulse is lower not every is useful in terms of efficiency. Squish is the cheapest, but also the shortest. What it does at least on all engine intake concepts is to push mixture, especially fuel, to the mid, which is very important when fuel separation is an issue on a mixture process. Once I've tested the ID injectors (5° fuel beam) and lost immediately 7 hp at best EOI of it compared to an OEM double spray pattern with single cones of 30° and a 6 whole each pattern. Lambda went greatly lean with the ID's. It just meant a greater portion of fuel run just through the engine as mixture homogenization efficiency went down significantly. Squish didn't help here. Obviously all systems have to be optimized, none has potential laying around and come up with it just when there is a demand. I can't say higher squish velocity would have helped here to get again more fuel into oxidation action, maybe you guys have experienced differences on that.
Regarding the acceleration inertia of air at e.g. 10,000 rpm, we have 2nd Newton's law. To keep it simple I've assumed the squish velocity must be double of the average speed at squish inner ring as max. happens around 12° BTDC and must reach flame at a proper progress, about 50 % mass fraction burned, assuming combustion start is around the same as max happens. The force to accelerate the total ring mass is around 83 N on a 86 mm bore, 4 mm depth squish ring and a proper squish height mixture to model the continuous ongoing squish flow from 30 to 0 degree BTDC. The available force on the crank is significantly bigger and FMEP (that force is lost on crank not on PMEP) won't see much of it. The bigger part of force is created by compressing the inner ring mixture. Here my model is simplificated most, as here calculated it only at 2 mm squish height, assuming it to be a compromise of the integral'd force by the moving piston. Finally 187 N, which also comes from the crank. So about a 270 N/piston, which is reducing the negative acceleration force during piston slow down into TDC position, and therefore increasing a bit FMEP. If combustion is slower and IGT more advanced the force of course increases, so this represents an thumb value for the best case condition. What I conclude from that is the force is available to get this done, but it is clear the TKE need to be increased at a relevant radius from spark plug center. So higher is better in terms of squish velocity to get a higher "throwing distance" for the relevant TKE effect as the impulses in the main center of the chamber are also not from bad adults. From that standpoint of time window demand the squish design may be pushed to the extreme as knock doesn't play that role, as time is quite short on rational IGT, also here retarded IGT's but optimal 50 % MFB (= shorter delay) helps to improve FMEP as well as IMEP.
If squish can support this, why not pushing it to the extreme?!