DirtRider . Net MX, SX, Arena Cross, Off-Road Community
Dirt Rider . Net Text Version Home
Dirt Bike Dirt Bike Dirt Bike Dirt Bike

This is the text version of DirtRider.Net
Click Here for the Full Version


Pages: 1

Squish velocity Vs Ignition advance

(Click here to view the original thread with full colors/images)

Posted by: 2strok4fun

I would like to open this topic up for discussion to help my learning process.

My thinking on squish velocity vs. timeing is this.

increasing squish velocity by either tighter or larger squish band. This would lead to faster flame front, raising the pressure sooner. This would allow for slightly less ignition advance and consequently more DETO resistant by reducing the time deto could take place. I dont think that there would be a loss of snap or performance *if* the peak pressure could be timed at the correct crank angle.

I am think specifically for large bore 2-stroke running under contant load IE sand drags, PWC, snowmobile

I would like to see what those experienced at building/tuning have to say.

I hope this is interesting, or is it fodder for the R.I.C.H. program.


Posted by: David Trustrum

Ok well no one is biting so I’ll venture this. Specific MSV calculation will keep you in the safe range to avoid too high (possible detonation & high rpm probs) or too low (rough performance at low revs).

TSR does a program to calculate this. One thing you may wish to try on a larger bore is a twin sparkplug conversion or putting a bit more angle in a specific area of the squish band to lower the MSV at that point (ie: over the exhaust side).

The detonation resistance is primarily a heat vs pressure problem not so much a time to occur issue (although as a result of timing this can induce this).


Posted by: nephron

Quote:
This would allow for slightly less ignition advance and consequently more DETO resistant by reducing the time deto could take place


No expert here, but ...the way I understand it, improving combustion efficiency allows timing retard, which then allows for combustion events at a later stage of piston rise (crank angle closer to zero). This then results in reduced 'negative' work 'against' the combustion cycle and improves BMEP and HP.


Posted by: 2strok4fun

Thanx for the responses.

While I can calculate MSV I do not have TSR program.

nephron, that is what I am thinking, but I dont have any idea of what a optimum squish velocity would be for an application. I do know that many factory 2-strokes are not near optimum.

I am trying to understand how too much squish velocity causes DETO. My thinkin is that it raises cylinder pressure too quickly and the pressure and associated temp raise beyond a fuels ability not to auto ignite. This assuming a design has exceeded a optimum MSV. Does this sound right?


Posted by: David Trustrum

15-25 m/s general ballpark. 15-19 for high revving power. Higher probably better for reasonable size MX bikes for torque.


Posted by: Sean Hilbert

2strok4fun,

First of all, you're headed down the right track when you say that squish increases turbulence. Squish creates small scale eddies that in turn create a wrinkled flame front, and this allows combustion to occur faster. Faster combustion is typically more efficient since there is less time for thermal (and other) losses. Higher levels of turbulence (and quicker burn duration) also allow one to advance spark timing closer to TDC.

Faster burn durations also decrease the chance of detonation. Detonation is a time and temperature dependent thing...the longer hot gasses 'cook' inside the combustion chamber, the more likely they are to auto-ignite, therefore, more squish actually decreases the chance of detonation. Faster burn will create higher peak cylinder pressures, however, and the engine can become noticeably rougher (i.e. like a diesel).

The optimal squish trade-off balances two main things: 1) Burn duration – the higher the squish velocity the faster the flame front (for a given operating condition and air/fuel ratio). Faster burn durations are typically more efficient and can wring more work out of a given air/fuel charge, 2) Heat transfer – higher gas velocities create more heat transfer and therefore reduce overall combustion efficiency. There are lots of other secondary factors as well such as Squish %, combustion chamber shape, etc…


Posted by: nephron

Quote:
allow one to advance spark timing closer to TDC.


Sean, do you mean retard?

Also, can you explain the term 'gas velocity" (heat transfer point)?

Thanks


Posted by: Sean Hilbert

Nephron,

Yes...I was backward...higher squish velocitites allow more spark retard...not more advance.

As to your other question...heat transfer between a moving gas and a solid (i.e. convective heat transfer) is directly proportional to the velocity of the moving gasses across the surface. Depending on which is hotter (the gasses or the walls), heat will transfer more efficiently one way or the other if gas velocities are higher. Note that this is true both prior to combustion and after combustion.

Another factor to consider is surface to volume ratio...for a given volume, a sphere has the most efficient S/V, therefore the more 'flat' surfaces that are in a combustion chamber (i.e. squish band), the more heat losses there will be from combustion, thus resulting in lower efficiency (power).


Posted by: MikeS

OK remove my thread on combustion chamber shape because this thread seems to be heading towards what I was researching.

Squish band effects, angle, Combustion chamber shape.

It seems that there is a fine balancing act that must be maintained. I understand some but I am still trying to grasp the whole picture

You mentioned a sphere shape.... That would mean the combustion chamber that is bell shaped will be better due to the surface area ????

Is the sguish angle related to the piston dome or a completely different measurement ?


Posted by: 2strok4fun

This is great. Thanx Sean for the info.

I think I grasp the time temp deal on Deto, and your statement on Squish velocity / turbulance / wrinkled flame front / burn duration relationship certainly backs up what I have been reading. My sticking point now in my brain is, when is there too much velocity? I guess that is where the convection comes into play. Too much heat xfer via convection to the head or piston crown, now would this be a loss of thermal energy to the combustion process or a heating of parts issue or both? Or am I off track? Is the convection directly related to the kinetic energy imparted by squish velocity?

Please excuse my regurgitation, but I am not a engineer and I rehash to try and help myself understand.


Posted by: 2strok4fun

Quote:
Originally posted by Sean Hilbert
Faster burn will create higher peak cylinder pressures, however, and the engine can become noticeably rougher (i.e. like a diesel).


Ok, I get that. Would reducing the ignition advance help with the rougher running by moving the time of the peak cylinder pressure closer to optimum?

I guess the bottom line is I need to do more reading, and there probably isnt a math equasion (short of Blairs 2-stroke software) that will keep me from sticking a piston while learning.


Posted by: nephron

Quote:
heat transfer between a moving gas and a solid (i.e. convective heat transfer) is directly proportional to the velocity of the moving gasses across the surface.


Sean, I've never even considered the effect of whole gas motion velocity on combustion chamber/crown temp (before). Does it really affect convection that much? In relative terms, how much? Or is there a relationship that can be quantified (formula)?

Of course, it makes sense from a chemistry/physics standpoint alone, since increasing the temperature of a gas by applying heat energy increases (molecular) speed distribution function and therefore, kinetic energy. Macroscopically, this is known to us in the ideal gas law, obviously. But clearly, there is no allowing for any discussion of 'gas' velocity per se in the ideal gas law. And theories that discuss associations b/t molecular kinetic energy/speed distribution function (like Boltzmann) are talking @ the molecular level, and relate only to probabilities.

Surely droplet behavior, of which I know nothing, are not governed by the same relationships b/t temp/velocity/energy?


Posted by: MikeS

Nephron

You seem to have spent some time reading

I am not that technical in chemistry and physics but I am brushing up in my spare time.

Great info

Mike


Posted by: Sean Hilbert

Quote:
Would reducing the ignition advance help with the rougher running by moving the time of the peak cylinder pressure closer to optimum?


Reducing advance would reduce peak cylinder pressures and smooth out the engine, however it would do this at the expense of optimum timing. What would be gained in a smoother operating engine would be lost in efficiency.


One of the really cool things about two-strokes (IMHO) is that there are a ton of engineering tradeoffs to be made to make an engine run optimally, and there are very few constraints (like valves) to get in the way. That means that there is a lot of 'design space' out there to find an optimum configuration in. I mentioned a few trade-offs in a previous post...here are some more (and this is just around chamber design!):


More squish area = more turbulence at the time of spark but it also means larger S/V of the combustion chamber (more surface area for heat loss)

More squish area = more turbulence at the time of spark but it also means that for a given compression ratio that the actual combustion chamber must be deeper...this can lead to poor scavenging performance.

Less squish clearance can lead to higher squish velocities (and more turbulence), but this can greatly increase the cost of the engine since tolerance stack-ups get pretty ugly (production engines only).

More squish area = more turbulence at the time of spark, but this also means that the heat of combustion is concentrated on a smaller area of the piston crown since the combustion chamber is more compact. This can lead to durability issues.

More squish can be a great thing for low-load combustion stability. The squish flow helps the incoming fresh charge and the left-over exhaust gas (that wasn't scavenged from the previous cycle) mix so that no 'dead spots' are present at the time of spark (possibly causing a mis-fire). This same amount of squish at higher loads, however, can cause high cylinder pressures and a rough running engine.

Those are all of the trade-offs that I can think of now...can anyone think of one (or more!) that I missed?


Posted by: nephron

Sean, how much does squish velocity contribute to temp, in general terms? Are we talking 5% or less? 15%? I can't imagine it being more than that.


Posted by: Sean Hilbert

Nephron,

What temperatures are you talking about??? Pre-combustion gas temp, coolant temp? Piston crown temp?

Gas temperatures have the potential to be affected the most since the amount of energy required to change their temp is less than that that of metal or water. I don't have any modeling or actual data to back this up, but I would guess that local gas temperature could be affected by 5-10 degrees C depending on squish design. Note that this can be in the hotter or cooler direction depending on operating conditions (speed, load, etc...). While that might not seem like a large change, small changes in gas temperature before spark can have a large effect on detonation.


Posted by: 2strok4fun

Sean-

going off your above statement, which scenario would be more DETO resistant for 2 engines of the same corrected CR; 1 has squish velocity high enough to raise pre-combustion gas temp. Does the reduced combustion time outweigh the added temp?

I am assuming same operating perameters and available octane rating.


Posted by: nephron

Crown, but your response answered the question. Thanks.


Posted by: Sean Hilbert

Simple answer is...I have no idea.

As with many of these kinds of trade-offs, it's worthwhile making a hypothesis, and then testing to see if your idea made any sense. All of these answers are dependent on many other things as well...including engine displacement, jetting, bore-to-stroke ratio, etc...

Think, design, cut, test...and do it over again...


Posted by: 2strok4fun

Quote:
Originally posted by Sean Hilbert
Think, design, cut, test...and do it over again...


somehow I knew it would boil down to this.

Thank you Sean and everyone for the input.


Posted by: bgeisick

[QUOTE=2strok4fun] While I can calculate MSV I do not have TSR program.


How are you calculating MSV? I recently changed my motor quite a bit (stroke, porting, and head) and am having trouble at high rpm's. I am wondering if it has to do with my squish and MSV.

Just wanted to do a quick check and see if I was in the 10-25 range.


Posted by: 2strok4fun

bgesick-

I have some 2-stroke software (2 different ones) that calculate MSV, but not TSR. Both programs come to essentially the same velocities given the same parameters.

The input needed to calculate:
head type (usually central, on most bikes)
Average squish clearance (often the squish clearance grows toward the center of piston)
Squish area as percentage of bore area.
stroke
rod length
exhaust port close in degrees BTDC


Posted by: bgeisick

If you would have time, I would appreciate it if you could run the software using my numbers.

head type: central dome
avg. squish clearance = .035"
sqish area is about 50% of bore
61mm stroke, 115mm rod
the exhaust port closes at 91 deg. BTDC

Thanks,
I appreciate it.


Posted by: 2strok4fun

oops, I forgot bore and trapped compression ratio.


Posted by: bgeisick

Bore is 65mm
Trapped compression ratio is 9.57:1


Posted by: 2strok4fun

I used 8000 as the rpm:

MSV= 29.5 m/sec @ 9* BTDC

Is this a stroked banshee? Running race gas I hope.


Posted by: bgeisick

Yes, this is a 7mm stroked banshee motor (cheetah Cub cylinders). I am having problems where the cylinders begin to cut out (sounds like slow/late combustion) out and not want to rev out. I have tried to fix it with jetting (from rich to lean) with no improvement. The only thing that seems to help is advancing the timing. What worries me is that I have to go +10 to +11 deg advanced above the stock timing curve. This seems extreme to me (luckily, I haven't seen signs of detonation). I was wondering if my MSV was too high at the stock spark timing, and by advancing the timing it allows the spark to fire before the MSV gets to high. I live at about 4500 ft in colorado, and ride in the sand dunes about 8000-9000 ft. altitude. I mix 110 octane 50% with premium pump gas.

Isn't the suggested range of MSV 15-25? What happens when MSV gets too high? Could this explain the poor performance without the +10 degrees spark advance? Also, this motor is supposed to produce peak power in the range of 10,000-11,000 rpm, I assume this only pushes the MSV up even higher.

I am guessing that I am probably loosing power from starting combustion so early BTDC.

If 25 m/s MSV is the suggested upper limit, and I am running at 29+, how can I reduce it? Larger squish gap? Smaller squish band? Won't both of these reduce the detonation protection offered by having squish?


Posted by: David Trustrum

Stroked motor huh? So were the heads modified? With the higher comp from increased capacity, & such a low exhaust port, no power valve I guess (comparing to my old RZ350 engine which is of course a close relation).

You can remove squish area to reduce the MSV or even angle some of it.


But just backing off. Those are quite high revs to be pulling (haven’t bothered to read all the threads as most are very old). The RZ350s used to peak power at 8700 & the ignition used to kill it pretty quick thereafter. I found putting a CDI from the 250 model produced the same curve but let it rev a little bit more.

Maybe this is why your ignition is a little touchy. Not sure on your use, but that is a lot of revs, if you lengthen your pipe you will remove a bit of the problem & likely gain some mid. When you did the stroking mod did you use some pretty good rods/ bearings? Maybe it will be safe at these revs, in which case I’d investigate ignitions & maybe raise the exhaust which if it really is only 91BTDC (178 degrees open, probably due to stroking, you might want to raise the whole barrel) which would lower MSV & suit those revs better.

Ok so maybe I don't know the revs banshees pull, so take what I say with a pinch of salt:

Hmm, thinking. . . Oh yeah, on the old RZ350 I tried a bunch of different reeds & with a lightly ported but stock revs type of engine the std steel ones were best.

Elevating the revs to that extent would likely over power the reeds which would cause flutter & produce a power plateau. Try closing the reedstops a little & see if it helps or making some piggyback reeds.


Posted by: bgeisick

It is a stroked motor, but the cylinders are aftermarket, which were cast and ported specifically to accomidate the longer stroke and rods. The heads are aftermarket as well, they are similar to the cool head design that allows you to switch out the domes (allows you to change compression ratio).

As for the exhaust port timing, i had a typo in my post. It is actually 81 deg. BTDC with an exhaust port duration of 198 deg.

Prior to building this motor, I had a stock cylinder banshee 350 with mild porting, and Toomey t-5 pipes that would make it's peak power around 10,500-11,000 rpm. So I don't think it is unusual to see those types of engine speeds on the banshees. With that configuration, my stock CDI had no problems with those speeds. That is why I believe it is something that has changed either with the stroke, or the porting of the new cylinders. By the way, these new cylinders have a lot different port layout than the stock cylinders. They have 6 transfers (compared to 4 on stock ones) and also has two auxillary exhaust ports. The static compression of the stock engine was about 170 psi, while the aftermarket, stroked motor is about 165 psi. (keep in mind that these were tested at 4500 ft. altitude)

I also tried an aftermarket CDI (dyna FS). The stock curve showed no improvement, but when I selected any of their aftermarket curves it cleared it up. Unfortunately I have no idea what kind of timing the aftermarket curves are set to. This says to me that it is not a spark energy problem, but more of a timing problem.

Also, this problem was not nearly as noticable at high altitude (8000 ft.), and was barely audible. Then when I went back home to 4500 ft. it was VERY audible in the exhaust, and the engine wouldn't rev out no matter what jetting I tried. Unfortunately I didn't get to try the aftermarket CDI at home, just at 8000 ft.

I have carbon fiber reeds now, and have also tried stock reeds. There was no difference between the two.


Posted by: David Trustrum

ok ex height is more believable. Hmm, might try blending angle on the head, but the different ignition does point to some improvement there.


Posted by: 2strok4fun

RPM sound right, my old Toomey piped RZ-350 really woke up around 11,500.

maybe the high squish is blowing out the spark, sorry I heard that on another sight and thought Id pass it along.

What about plug gap? It sounds as if ignition is pointing to somthing, maybe poor scavenge at high rpm causing miss fire from too much charge /exhaust gas dilution? Higher elevation will reduce compression and might make it easier to light off. Just a thought????



This is the text version of DirtRider.Net
Click Here for the Full Version

Text Version Home





vBulletin Copyright ©2000 - 2008, Jelsoft Enterprises Limited.
vB Easy Archive Final ©2000 - 2008 - Created by Stefan "Xenon" Kaeser