Turbulence Induces for increasing burn rate 4

[automotivebreath]

Without turbulence in the combustion chamber we would burn the mixture at the laminar burning rate which is ten to twenty times slower than the turbulent rate. Numerous designs beyond common squish areas have been developed to shorten burn times. Does anyone have experience with turbulence inducers and knowledge of the potential impact on the combustion process?

 

[automotivebreath]

cleverlever,
Thanks, Ill read the patent and reply back. I’m confused because what you are saying goes against what I’m doing. I’m increasing turbulence to reduce detonation. This is allowing a reduction in ignition timing and increased compression.

 

[cleverlever]

I understand that this doesn't sound logical. When I first stumbled into this scenario we were didn't believe the data.
We put the baseline cam back in the engine and another late close intake cam and we couldn't even get close to the same low speed numbers we got with the early close.

I assure you that their are attributes of the Atkinson cycle engine that go way beyond the relation ship of VE and compression

 

[mattsooty]

clevelever,

I totally disagree!!! (Unless you are talking about a CI engine)

"The best way I can say it is at low speed you can greatly reduce the burn rate at high load with earlier than normal intake closing which allows higher compression and higher VE without incurring detonation"

How on earth does reducing the burn rate stop detonation?

Detonation occurs because of the initial heat release of the flame kernel raising the temp & pressure of that portion of the charge that is furthest away (the so-called end gases) to a point whereby autoignition occurs.

It is a balancing act of the burn initiation (spark advance), flame speed (homogenuity of the charge/mixture prep.) and distance of the spark plug to the edge of the combustion chamber (or the fire face in a squish zone).

This is why high reving racing engines do not knock at high engine speed (and therefore gas speed), the burn is so short that the charge is consumed before autoignition can occur.

In high load, low speed (with a corresponding low KE of the charge) what is required is a very fast burn. With a totally homogenous charge (ie better atomization) this can be achieved by excellent mixture preparation, usually anti-tumble.

This is because the gas velocities will be slow enough that swirl does dont add much and the piston speed will be slow enough that squish rate is also very low. This is the ver reason that OEMs add swirl control valves to throttle one of the pair of intake ports in a 4 valve per cylinder head - to increase the gas velocity & mixing.

If at low speed/high load your burn rate is slow then that gives ample opportunity for the endgases to be heated up and more chance for autoignition - even more so with bad mixture preparation.

Another point to note is that of the twin spark plug designed cylinder head - what purpose do you think the second plug serves???

Also how on earth does closing the intake valve earlier lead to a higher VE? what you are infact doing is throttling the combustion chamber with the intake valve.

MS

 

[cleverlever]

As politely as possible I will suggest you have it all wrong

Does high octane fuel burn slower than low octane fuel? Which fuel resist detonation better at low speed heavy load conditions?

Earlier than normal intake valve closing will increase low speed VE because in a fixed event camshaft intake valve closing is a compromise between high speed and low speed. Earlier than normal intake closing can give more than 100% VE at slow speed if used in conjunction with variable length intake runners

Most engines with more than one spark plug disable one plug at low speed heavy load to avoid the rapid presure rise that causes detonation.

Please read my patent and the SAE paper references to understand this situation better.

 

[automotivebreath]

As politely as possible I will suggest you have it all wrong

Mattsooty may have it all wrong but his thinking on low RPM combustion, burn rates and detonation are consistent with my beliefs and with what is being taught in universities around the world.

I’m a very open minded person and I will educate myself on the Modified Atkinson Cycle Engine and what it has to offer. I fully understand the benefits of reduced valve openings at low RPM. I’ll reserve my opinions on how this relates to the quality of combustion.

 

[automotivebreath]

In high load, low speed (with a corresponding low KE of the charge) what is required is a very fast burn. With a totally homogenous charge (ie better atomization) this can be achieved by excellent mixture preparation, usually anti-tumble.

Can you explain anti-tumble?

 

[mattsooty]

Cleverlever

A high octane fuel resists detonation due to its chemical properties, not its speed of combustion.

There is very little if no change in burn rate between high octane fuels & low octane fuels - only a change in the fuels propensity to autoignite.

I disagree that most twin plug engines turn off 1 spark plug at low speed/high load conditions. This is exactly the speed/load area where 2 plugs per cylinder are required!

Maybe you are getting confused with a CI engine here, in which rapid pressure rise does indeed cause 'knock'. However the combustion system of a CI engine is vastly different to an SI engine. Not least becuase an SI engine has a laminar/turbulent combustion whereas a CI has diffusion combustion.

There are myriad papers written on the phenomena of detonation and it is a subject that I have personally had a fair bit of involvement. To say that your thinking is 'radical' is an understatement. It goes against all modern learning on the subject and all empircal data that I and others have seen. Do a google search and find out for yourself!

Automotivebreath

Anti tumble is where the charge is swept up into the plug area as opposed to tumbled down on to the piston crown. It is the best dynamic for steady idle and consistent combustion at low engine speed.

MS

 

[al1]

There are so many, many variables in the combustion time or mass faction burn rates i.e. low load and/or low RPM that lacks efficient savaging included EGR dilution, lean or rich air/fuel mixtures, hot or cold operation conditions, high or low octane, early or late intake valve closing, combustion chamber shape etc. etc. However, what has not been address here is that as a general statement, the mass factional burn rate is relatively constant and that one of the key variables is that the piston moves further in proportion to the combustion time as the RPM increase. This increase in displacement is what really changes the tendency for less to no detonation at higher RPM. At low RPM the piston swish area induces swiral/tubulance is more effective at the piston stays closer to the combustion chamber during the combustion process. However, as the RPM increase the piston actually reverses itself before the combustion process is completed and can if fact reverse the swirl when is move away from the combustion chamber before the combustion is completed. In a typical 15 degree ATDC peak pressure calibration, the piston has travel far enough to increase combustion chamber displacement by around 25%. So with this additional combustion chamber volume, do you count the expansion ratios from TDC or do you count them from peak pressure or where do you start to count them? The changing RPM is a true constant but acts like a variable (oxymoron) but is not addressed here. 40 degrees of combustion time at 1,000 RPM is 6.66 Milliseconds; at 5,000 RPM it is 1.33. So if you have a 2-millisecond combustion time the crank travel and/or piston travel (depends of the bore to stroke ratio) increases the displacement which controls the rate of the cylinder pressure build up and heavily influences the combustion rate and/or detonation. It’s a game of compromising the best combination and I’m not sure if there will ever be a absolute --- one size fits all --- answer when is comes to engine design, as the learning curve is far from over and each engine has it’s own calibration idiosyncrasy due to the incredible number and combination of the above variables.
al1

 

[thundair]

I am only adding my two cents worth to have it challenged and get a better understanding of the magic moment of ignition.

I always understood that lead and now chemicals were added (high octane) to fuel to slow the burn rate...is that now wrong?

Cheers

 

[cleverlever]

You aren't going to get me to challenge that post

 

[mattsooty]

Thundair,

It is a common misconception but one that is ill founded that octane refers to burn rate. In actual fact higher octane has never, ever meant slower burnrate.

It simply refers to a fuels propensity to autoignite or 'explode'. Basically the lower the octane number the lower the temp of the fuels autoignition point.

What this means in real terms is that instead of the flame front that has been initiated by the spark plug consuming the charge, portions of the charge ahead of this flame front begin to combust through 'self ignition'. This is because the bulk temp of the combustion chambers contents is increasing in temp/pressure because of the burning charge.

Auto ignition is a similar sort of combustion as in a diesel, except in this case the compression is more produced by the burning charge as opposed to the high compression ratio of the engine.

MS

 

[automotivebreath]

In high load, low speed (with a corresponding low KE of the charge) what is required is a very fast burn. With a totally homogenous charge (ie better atomization) this can be achieved by excellent mixture preparation, usually anti-tumble.

This is because the gas velocities will be slow enough that swirl does don’t add much and the piston speed will be slow enough that squish rate is also very low. This is the very reason that OEMs add swirl control valves to throttle one of the pair of intake ports in a 4 valve per cylinder head - to increase the gas velocity & mixing.

If at low speed/high load your burn rate is slow then that gives ample opportunity for the end gases to be heated up and more chance for auto ignition - even more so with bad mixture preparation.

as a general statement, the mass factional burn rate is relatively constant and that one of the key variables is that the piston moves further in proportion to the combustion time as the RPM increase.

Isn’t this a contradiction, or am I missing something, how could the burn rate remain constant with variable RPM?

 

[WilliamH]

< TIME >

Time is the biggest contributor to the phonomonon refered to as spark knock or "detonation". It takes time for the end gases in the chamber to spontaneously ignite. Reduce the available time and there will be no detonation.

As stated by prior posts, swirl, tumble, squish, etc., all help the mixture burn FASTER. Less spark lead can be used resulting in a better torque curve. Two plugs per cylinder make the chamber burn FASTER and with even less spark lead.

Higher engine speed allows less TIME for detonation as well as less chamber pressure. Additionally, higher engine speeds induce more chamber turbulence and a faster burn rate.

The chemical engineer who works for us concocted up a blend of chemicals that we tested in an engine several years ago. This "fuel" had a burn speed almost twice as fast as the previously fast "gasoline" we had tested. We could not get this engine to detonate at even 18:1 static CR. Motor octane number? Who cares. It did not detonate. There was no TIME for detonation. All the mix was consumed----oxidized.

As to lead and other additives resulting in slower burn speeds I would assume this: The chemistry results in a slower burn speed, but adds additional knock resistance. The INTENT was not to slow down burn rate.

 

[mattsooty]

Hmmmmm, good point!

I personally do not think that mass fraction burn is a constant, if it were then surely the position of MBT (optimum spark advcance for best torque, which is normally around 8degCA) would also be constant for a given load (mass air/stroke) at lambda=1

If this were the case surely it would then suggest that there would be no need for the speed columns in an optimised spark advance map - No?


MS

 

[cleverlever]

Isn't this basically an arguement about the chicken and the egg? I would argue that heat is what causes detonation. Heat is certainly impacted by how long the mixture is in the cylinder.

My limited experience with a dyno has demonstrated that an engine will make more power at high speed on low octane than high octane and that a bi fuel engine that could vary the portion of low octane and high octane could make more power at both ends of the torque curve.

High octane will definitely give you less fuel economy at light load. How can that be unless the fuel burns slower?

 

[mattsooty]

WilliamH

You have hit the nail right upon the obvious....

MS

 

[mattsooty]

Cleverlever,

I think that your statement "limited experience with a dyno" explains why you are getting confused about these things.

As WilliamH has said, as I have said and as everybody else says the whole point of good mixing & rapid combustion is to ensure the charge is consumed BEFORE the end gases get chance to be heated. If it has already been burnt then it cannot detonate!!!!

The reason why lower octane fuel will make more power when not knock limited is because the additives that increase the octane number actually decrease the calorific value of the fuel. However using an inappropiately lower octane fuel on a WOT accel would soon destroy your engine.

MS

 

[WilliamH]

mattsooty,

I totally agree with you. Mass burn rate is not a constant. Variables include, rich/lean, temperatures, turbulence, and other variables.

 

[automotivebreath]

I totally agree with you. Mass burn rate is not a constant. Variables include, rich/lean, temperatures, turbulence, and other variables.

Agreed, if you go back to the original post, thats how this whole thing got started:

Without turbulence in the combustion chamber we would burn the mixture at the laminar burning rate which is ten to twenty times slower than the turbulent rate.

 

[al1]

I’m sorry that when I said relatively constant, I didn’t mean from idle to full power. I meant at a fixed throttle position and fixed volumetric efficiency wherever you select to test it at. The point I was trying to make was that piston speed increase proportionality to RPM but the burn time stays relatively the same under these conditions. See Heywood’s book or other tech book to support that. As a result, the displacement changes quicker in proportion to the cylinder pressure build up time and therefore affect all the other variables as well. Most of the other variables were being covered and I thought that this area is very much over looked.
al1