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Turbulence Induces for increasing burn rate

Turbulence Induces for increasing burn rate

(OP)
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?

RE: Turbulence Induces for increasing burn rate

The shape of the intake port, the shape of the combustion chamber, the shape of the piston top, and "fins" in the intake port are common ways to increase "swirl" and "tumble" in the cylinder.

Many cylinder head flow benches are equiped with "swirl" and/or "tumble" meters that measure the effect of the combustion chamber shape, fins, and port shape.

On 4 valve engines, some manufacturers open one intake valve before the other to increase swirl in the cylinder.

When there is "good" swirl/tumble in the cylinder the ignition timing can be reduced, adding to the overall efficiency of the engine.

RE: Turbulence Induces for increasing burn rate

When you consider all the swirl or tumble you can induce into the chamber via the ports on the induction stroke then see how much remains at TDC vs how much rolls out of the squish area just after ignition as the piston almost touches a substantial area of the cylinder head, I would think the latter is by far the most effective, and if attention is paid to piston top shape and fit into chamber to take advantage of the gas velocity across the piston top, I would expect that is the area where most gain can be achieved, although I do like the idea of different valve timing to create sheer between the two columns of air.



Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

RE: Turbulence Induces for increasing burn rate

I have used both throttled inlet ports for effecting swirl and also the fancy variable valve lift both in a GDI engine.

Both were effective in what they achieved but only really useful at lowerspeed/part load. At higher engine speeds the action of squish does indeed become more important.

Having said that the most important factors for lightload/idle is infact antitumble.

All of which are compromises, Im afraid...

MS

RE: Turbulence Induces for increasing burn rate

Like in most all things, too much turbulence is not good either, as high turbulence will centrifuge the fuel out --- a kind of reverse entrainment of the fuel and air.  This is especially true in rich fuel mixtures and is one of the reasons why carbureted race engines need to run rich air/fuel mixture.  If the fuel is already vaporized it is not as much of a issue since it too is a gaseous form like the air (even though it is still heavier), however there is typical a low % of non vaporized fuel in most fuel deliver system and it is venerable to being centrifuged out of suspension just like any other situation of when the heaver mass pulls away from the lighter ones in a centrifuge that can be caused by too much turbulence.
al1

RE: Turbulence Induces for increasing burn rate

(OP)
I have found that swirl and tumble inducers dampen volumetric efficiency. In addition turbulent flows that are created before the intake valve closes are dampened by the viscosity of the fluid to some extent. This has led me to altering squish action to enhance the conversion of kinetic energy to turbulent flow. As patprimmer suggest; generating turbulence at the time it is needed will likely be most effective.

Al1 describes a form of swirl or vortex turbulence that has benefits and limitations. Is it too much turbulence that is bad? Or rather too much swirl? A large swirl may have a centrifuge effect, but not all turbulence is created equal. This turns my attention to developing small scale eddies rather than large areas of circulation.  

I have studied various methods of generating in chamber turbulence of people like Michel May, Somender Singh & Larry Widmer. Looking for simplicity I’m currently involved with the grooved turbulence generator of Singh. Find a link to one of my current projects. All comments welcome.   

http://members.cox.net/dwynne7/bart%20sr%201.jpg

RE: Turbulence Induces for increasing burn rate

automotivebreath: Interesting method. Have you done any back to back testing? Any change in timing and or octane requirements? My thinking is that by the time the piston gets close enough to the head the combustion process has already consumed most of the air/fuel mix. I think a single groove aimed at the spark plug may accelerate the consumption of any remaining unburned air/fuel at TDC. I may give it a try on next engine build. Two things I have learned is there is a lot left on the plate if every effort is not made to bring the piston as close to the head as possible without making contact. The difference in optimum timing requirements from .080" clearance down to zero changes about 1 deg. per .010" reduction in piston to head distance. Measurable power increase and corresponding reduction in BSFC #'s. Additional benifit is the octane requirements either remain the same or go down. On a typical SB Chev. with OEM rods .027" piston to head cleance is the closest I've been able to get without signs of contact.------Phil

RE: Turbulence Induces for increasing burn rate

(OP)
Phil, Thanks for the information. “On a typical SB Chev. with OEM rods .027" piston to head clearance is the closest I've been able to get without signs of contact” What is the maximum RPM at 0.027” piston to head? I normally stay above 0.030” when running high RPM.

Formal testing, no. Informal yes, I have modified more than 20 engines to date, most with a single groove in each chamber. It does reduce octane requirements and fuel consumption. I have found it allows one point higher compression ratio, this provides additional benefits of power and economy.


http://members.cox.net/raunch/stock%20350%20r3_edited.jpg

RE: Turbulence Induces for increasing burn rate

Automotivebreath: Thanks for the visual. I like it. Impressive #'s. The engine I was reffering to was a rules restricted SB that could only get up to 6,700 rpm with the valve springs that were legal. Only one bank of cylinders (I must confess I forget which side) showed signs of contact. I think most of the clearance is taken up due to expansion of rod and piston. I've not heard of anyone who's been able to reduce the clearance with premium rods and fasteners. I know of one engine that used aluminum rods that had to set the cold clearance .060" to avoid signs of contact. Keep up the good work-----Phil

RE: Turbulence Induces for increasing burn rate

Piston to bore and piston skirt design also affects piston to head as the amount of rock can be changed.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

RE: Turbulence Induces for increasing burn rate

(OP)
My efforts so far have brought me to the conclusion that additional turbulence is good. Improving the quality of burn has many advantages and that are relatively easy to realize. My current focus is to develop this idea and bring it to a higher level. The question remains is it possible to create too much turbulence? We discussed the negative effects of large scale swirl and the centrifugal effects it can have separating the fuel from the air. I feel this is different; creating small scale eddies rather than swirl or tumble.

Will my efforts of creating additional turbulence bring me to some limit?

Perhaps the flame velocity will become too fast producing negative effects?

RE: Turbulence Induces for increasing burn rate

In Gordon Blair's book Design and simulation of 4 stroke engines he makes reference to the negative effects of too much turbulance. According to him he states that the detonation threshold is reduced for a given fuel quality if too much turbulance is introduced. However he doesn't state under what circumstances this phenominon occurs. My own personal testing (albiet somewhat limited) does not coincide with that. Perhaps a large squish pad and/or a domed style piston may be contributing factors for him to publish that conclusion.--------Phil

RE: Turbulence Induces for increasing burn rate


Quote:

Formal testing, no. Informal yes, I have modified more than 20 engines to date, most with a single groove in each chamber. It does reduce octane requirements and fuel consumption. I have found it allows one point higher compression ratio, this provides additional benefits of power and economy.

Interesting, but how can you, or anyone, be sure you are not inadvertantly cancelling out or modifying an existing defect?   

How does inducing additional turbulance relate to modern production practice? Engines are more powerful, cleaner, and longer lasting than ever, and pistons are now either flat or dished. Chambers have been opened and simplified for smooth flame travel. This allows higher compression pressures and compression ratios, all with pump gas. Ratios are getting back up near 11 to 1 thanks mainly to smoothed-out combustion chambers.

RE: Turbulence Induces for increasing burn rate

(OP)
Fabrico, You are correct, many of the engines that I have modified utilize combustion chambers designs that are ten+ years old. These inefficient designs have plenty of room for improvement and respond favorably to increased turbulence.

I have modified several late model Chevrolet V8 engines with their current efficient combustion chamber design. These engines were modified for other people and they have not been able to confirm improvements because additional modifications were done.  

I have the same question “How does inducing additional turbulence relate to modern production practice?”  and Will I be able to surpass the current compression ratios with the additional turbulence?

These are answers that I don’t have at this time, additional testing is needed and planned.

RE: Turbulence Induces for increasing burn rate

I would think that a substantial influence on maximum compression ratio vs octane rating is due to modern engine management systems with knock detectors and accurate and programmable spark timing and A/F ratio.

Also modern manufacturing equipment and techniques allows for more precise parts with a better surface finish, and better design to eliminate hot spots.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

RE: Turbulence Induces for increasing burn rate

(OP)
patprimmer, having the ability to detect detonation and retard the ignition accordingly has given the auto manufacturers the ability to eliminate the compression ratio/octane safety factor. The important point here is the engine must detonate for it to function, the system then responds with ignition retard to lower cylinder pressure.

I’m not current on modern fuel injection systems, the key is to be able to control air/fuel mixtures in each cylinder. This eliminates the need to tune to the cylinder causing a specific problem.

In my opinion the advancement that has had the biggest impact is combustion chamber design and as you mention, the quality of the components. Reducing flame burn distance in particular has had the biggest impact on reducing timing requirements allowing the increase in compression ratios.

The question remains, will additional turbulence in the modern combustion chamber improve combustion quality and further reduce detonation tendencies. Furthermore can the lean burn limits be altered with increased turbulence intensity?

RE: Turbulence Induces for increasing burn rate

(OP)
Phil, I have a copy of Gordon Blair's book on the way. I’d like to understand what he is saying and the basis for his thoughts. Thanks for the information.

My thinking is combustion chamber turbulence is inadequate at low RPM when utilizing  mot current four stroke combustion chamber designs.  

RE: Turbulence Induces for increasing burn rate

Turbulence is good for fast burn at light load but at high load turbulence is terrible for detonation. At high load, lower speed, you want less turbulence and uniform mixing without to much atomization.

The best tool to achieve burn control is proper variable control of the intake valves which requires something similiar to Hondas i-VTEC system which I patented in 1990 (4,961,406)
condensed version of patent - http://modifiedatkinsoncycleengine.blogspot.com

RE: Turbulence Induces for increasing burn rate

(OP)

Quote:

Turbulence is good for fast burn at light load but at high load turbulence is terrible for detonation. At high load, lower speed, you want less turbulence and uniform mixing without to much atomization.

Can you explain the reasoning behind this, how does less turbulence and little atomization inhance high load low RPM performance?

RE: Turbulence Induces for increasing burn rate

It is very hard to compress 30 years of research, 12 patent
10 prototype engines two of which were put in cars into a message.  I am not computer skilled to cut and paste reference data in this message but all the reference data is in the patent.

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.

RE: Turbulence Induces for increasing burn rate

(OP)
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.

RE: Turbulence Induces for increasing burn rate

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

RE: Turbulence Induces for increasing burn rate

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

RE: Turbulence Induces for increasing burn rate

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.

RE: Turbulence Induces for increasing burn rate

(OP)

Quote (cleverlever):

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.

RE: Turbulence Induces for increasing burn rate

(OP)

Quote (mattsooty):

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?


RE: Turbulence Induces for increasing burn rate

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

RE: Turbulence Induces for increasing burn rate

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

RE: Turbulence Induces for increasing burn rate

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

RE: Turbulence Induces for increasing burn rate

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

RE: Turbulence Induces for increasing burn rate

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



RE: Turbulence Induces for increasing burn rate

(OP)

Quote (mattsooty):

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.

Quote (al1):

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?

RE: Turbulence Induces for increasing burn rate

< 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.  

   

RE: Turbulence Induces for increasing burn rate

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

RE: Turbulence Induces for increasing burn rate

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?

RE: Turbulence Induces for increasing burn rate

WilliamH

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

MS

RE: Turbulence Induces for increasing burn rate

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

RE: Turbulence Induces for increasing burn rate

mattsooty,

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

RE: Turbulence Induces for increasing burn rate

(OP)

Quote (WilliamH):

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:

Quote (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.

RE: Turbulence Induces for increasing burn rate

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

RE: Turbulence Induces for increasing burn rate


automotivebreath
A consensus won't give you concrete answers. Have you considered using real time combustion pressure analysis equipment like TFX or ?

http://www.tfxengine.com/software7.html

RE: Turbulence Induces for increasing burn rate

(OP)
Al1,
Thanks for the clarification, I understand what you’re saying and its a very good point. I would think the window of opportunity for detonation at high RPM would be very small.

Fabrico,
For the most part this has been good aside for a few distractions, for me it's all educational. My main objective at this point is to try to learn what to expect with increased in cylinder turbulence intensity especially at low RPM. Eventually I will be attempting to reduce burn time at low RPM to some sort of minimum. If known hurdles exist, I’d rather learn about them now. I'm content with my testing methods for the time being.

RE: Turbulence Induces for increasing burn rate

(OP)
To those who may be following this thread, Rob wrote this on another forum. I love the way he simplifies the whole process.

Quote:

Spark ignition (SI) engines all work on the principle of a turbulent flame front (TFF) consuming the air-fuel charge. A normal combustion event would be considered one where the spark ignites the air fuel mixture (a complex process in itself) and the flame propogates throughout the air-fuel mixture with this turbulent flame front. Skip the next paragraph if you know how stuff burns  

I think most people have a pretty good idea of how a TFF works, but lets go through some of the details. First, think of a chamber (say a cube or a disc for simplicty) full of nice calm, still air-fuel mixture. A spark in the middle ignites, and begins to consume the mixture. Ideally, the flame front (in this case a laminar flame front, since the mixture is still) would form a spherical shell, as it progresses. Now, this flame front is propelled by a couple of forces. First, the mixture in the wake of the flame front is obviously heated by the combustion. This heat translates to an increase in pressure. This higher pressure burned gas compresses the mixture ahead of the flame front. Since the volume of the burned gases expands it helps accelerate the flame front. Think of blowing up a ballon. The compression of the end gas also raises its temperature. Flame speeds are higher in higher temperature mixtures.

For a turbulent flame front, consider instead of a calm chamber, a chamber full of turbulent eddies, of all size scales. As the flame front approaches one of these swirling eddies, the flame edge is 'torn' and spun around by the eddy, into fresh mixture. This helps to shred up the flame front, and helps to progress the burn of the mixture. In short, this is really why SI engines work at all, lol.

Now, this burning action is really a race. As you compress the combustible end gas, you get ever closer to the auto-ignition temperature. Auto-ignition is a process by where a series of branching chemical reactions (which mainly all have a very strong dependence on temperature) result in the combustion of a mixture, with no flame front. These reactions begin to oxide the mixture simply due to the thermal energy available from the high temperature. Now don't get me wrong, this is a VERY complex series of chemical reactions, but some of the basics help give a good understanding.

The auto-ignition is very dependent on the time history of the mixture. If you hold the mixture at a low temperature, it may not auto-ignite for a long time. Raise the temperature, and it ignites sooner. So if the TFF takes a long time consuming the mixture, the compression effects of the TFF are present for a longer time, and hence the temp. of the end gas is higher for longer.

If the end gas does auto-ignite before the TFF reaches it, or before the relatively cool cylinder wall quenches the flame, the end gas auto-ignites, or detonates. Now, if one corner of this chamber is the last to receive the flame front, and indeed does detonate, what is the result? The auto-igniting mixture essentially 'explodes' and sends a pressure wave across the chamber at the local speed of sound in the cylinder. This pressure wave is what is heard as 'knock' Oddly enough your ear picks up short pulses of a tone, as a 'knock', and not a ringing sound.

Hopefully this helps a bit, one of the prof's here also has a small program to help you hear how the short duration pulses are heard as knock. I will try and post it soon.

Sorry for the novel, I hope it is somewhat informative

-Rob

RE: Turbulence Induces for increasing burn rate

Cleverlever,

I reviewed the link provided and took a brief look at the patent.  I did not see specific figures for the intake valve closing point BBDC.  If you are at liberty, would you please provide information on just how early you were closing the valve?  I would be interested in fully closed and 0.020 or 0.050" angles.  
Ken

RE: Turbulence Induces for increasing burn rate

Ken

The SAE papers mentioned in the patent give lots of the information  you are looking for.

What you are asking is like asking what the valve timing is for an Atkinson engine.  Its a ratio thing, the more restrictive the VE the higher the expansion ratio that can be tolerated.

The difference is you can get a lot more detonation limited max low speed power from an early close than a late close intake valve closing which is good for max power.  However the slow burn is bad for light load fuel economy so you have to revert to the late close for faster burn at light load, requires a valve timing scenario similiar to i-VTEC.

Some of MS comments about combustion are completely the opposite of what I observed on 3 different dynos.  Fast burn is not desireable at low rpm heavy load conditions because the rapid presure rise will cause an uneven combustion rate which will cause detonation.

Brief periods of detonation are tolerable without destroying an engine. Desireable Presure rise characteristics that impact detonation can be manipulated with twin spark plugs which can fire at the same time ,one at a time or at staggered timing.

RE: Turbulence Induces for increasing burn rate

Cleverlever,

My recommendation to you would be to go away and buy a decent book on IC engines (Heywood would be a start), read it and then try and understand it. Then maybe join the SAE and read some of their papers and try and get a better understanding of these things.

At present your knowledge seems very flawed and the complete opposite to what is a well documented, understood & even modelled mechanism.

Unless of course you think that your experience on a whopping '3 dynos' was somehow groundbreaking and brought fresh understadning to the IC engine community - in which case I suggest you get your paper in quickly!!!

MS

RE: Turbulence Induces for increasing burn rate

Come on guys play nice, otherwise this thread might disappear, and that would be a shame as it contains some very interesting information.

cleverlever.

You are mistaken re octane rating and burn rate.

Octane is a measure of detonation resistance and is independent of burn rate.

Try this link for some well researched data.

http://naca.larc.nasa.gov/

http://blizzard.rwic.und.edu/~nordlie/cars/gasoline.html

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

RE: Turbulence Induces for increasing burn rate

(OP)
My suggestion is to try to understand the stated claims and the reason for the benefits. If statements like these are fact, it would be desirable to understand why.

Quote (cleverlever):

…You can get a lot more detonation limited max low speed power from an early close than a late close intake valve closing which is good for max power…

… for light load fuel economy so you have to revert to the late close…

Assuming these statements are factual:

Why is an engine running under heavy load at low RPM is more detonation resistant with early intake valve close?

Why is it advantageous to revert to late close with an engine running under light load at low RPM?

RE: Turbulence Induces for increasing burn rate

Why am I being repeatedly asked to answer questions that are explained with relevant SAE references in my patent 4,961,406?

Obviously most of you are younger and far more educated than I am. That being said I am the first person who ever applied for a patent on the concept of using a detonation sensor to control intake valve events for the purpose of altering combustion rates. Have any of you read Toyotas patent 6,848,422? Pretty much says the same thing I patented
over a decade before their patent was issued.

Many years ago Champion Spark plug would host seminars and demonstrate the different burn rates of low octane and high octane fuel.  They would use two clear tubes about 50 feet long and put low octane in one tube and high octane in the other.  They would light them both at the same time and you could watch the low octane burn to the end of the tube faster than the high octane.

Maybe somebody could explain to me how this doesn't demonstrate the different burn rates of low octane and high octane fuel?

RE: Turbulence Induces for increasing burn rate

Automotivebreath,

I must confess that I have spent a lot more time modeling and testing CI engines than SI engines, so take my comments with that in mind.  However, as complex a subject as detonation can be, with many variables, I think that some of the variables coincide with NOX generation in the CI engine.  (The most basic variables are time and temperature, as one is working with involved chemical reactions and partial equilibrium of various hydrocarbon species.)  So a consideration of Caterpillars ACERT engines, that use a modified Atkinson cycle to reduce NOX generation, may be instructive.  I think that the dominant effect of early intake valve closing is the expansion and cooling of the charge prior to BDC, thus reducing the temperature profile throughout the compression stroke and initial combustion process.  The cooler profile effectively retards the preparation of the end gases for detonation (or NOX generation in the CI engine).  This may partially answer your first question above.

I also have a question.  Considering all the interesting comments in this string, do any of the authors have experience testing or modeling HCCI engine technology?  Those data may help answer some of the questions.

Vincent

RE: Turbulence Induces for increasing burn rate

Vincent

Bless you for your comments.

Clyde Bryant has several diesel patents that are basically the same thing I am saying about gasoline engines  (6,279,550)

So I would like to know how many of these critics that are saying I don't understand the fundamentals of engine design have ever built an engine from scratch. Modeling is no better than the assumptions you plug into the front end of your model.

I built those engines and have seen the low speed high load detonation problems associated with Intake valve throttling

Moore (4,280,451) built several early close Atkinson cycle engines and his data was exactly the same as mine.  The early close Atkinson cycle engines made more low speed torque than the base line engines.

Moore and I actually exchanged engines and retested them and confirmed each others data.

One thing that nobody talks about in late close Atkinson engines is the additonal heat dumped in the mixture when you over fill the cylinder and then push it back in the intake manifold to be consumed in the next intake stroke.  Not a real bad deal at light load but the extra heat is a detonation disaster at heavy load.

RE: Turbulence Induces for increasing burn rate

(OP)

Quote (turbomotor):

I think that the dominant effect of early intake valve closing is the expansion and cooling of the charge prior to BDC, thus reducing the temperature profile throughout the compression stroke and initial combustion process.  The cooler profile effectively retards the preparation of the end gases for detonation (or NOX generation in the CI engine).
Turbomotor, Thanks for the insight, this makes good sense. The way I look at it is regardless of the timing for intake valve closing; once the valve is closed it is now recent history and all that remains is the charge that has been captured in the cylinder and how the contents and condition of the charge are related to present and future events leading to ignition. This charge has qualities that are related to the induction process as well as the early intake closing. In addition, as you stated, more time is available relative to crankshaft rotation for "expansion and cooling".



Quote (cleverlever):

Why am I being repeatedly asked to answer questions that are explained with relevant SAE references in my patent 4,961,406?
Two reasons I have questions.

1. I am interested in learning more about the advantages of early intake closing and its relationship to the quality of combustion.

2. Some of your reasoning goes against current scientific belief. These people are very knowledgeable and can help us understand what is happening in the diminishing cylinder that contains a charge prepared for combustion with early intake valve closing.


RE: Turbulence Induces for increasing burn rate

"Reasoning against scientific belief"

I have no education but Mr Moore was a college grad.  In his patent with the early close Atkinson cycle it says  "With the sub-atmospheric compression,less than a full charge is used with each cycle.  Thus,a larger engine would have to be used to obtain the same power as would be obtained on a full charge on a smaller engine."

However when he built a prototype engine and put it in a car he experienced an unexplainable increase in low speed torque without a trace of detonation.  So we both, working without knowing about each other thought we could do what Fiala did 4,364,353 did and just limit low speed VE with a detonation sensor hooked to the throttle valve.  That was a detonation disaster.

So what was it we didn't understand that was giving us power gains where we expected power losses?

Tuttle  SAE 820408 had observed "similiar combustion duration" with early intake closing. Hara SAE 850074 observed an "increase in combustion duration" when he studied early intake closing.  Neither tested early closing with an atkinson cycle and variable valve events.

Obviously Hara identified the illusive factor that was giving us the increase in low speed torque. Effectively we were increasing the octane of the fuel at low speed high load conditions with early intake closings.

What amazes me is this info has been known for 25 years and there are still educated people challenging its validity.

Let me close with these words of wisdom from Charles A Amann ex head of engine research , who refused to test my patented technology with an Atkinson expansion ratio  -   "As history is unfolded,it is seen that the experimenter sometimes finds the right answer for the wrong reason" (1985)

 I couldn't have said it better with my own lips.

RE: Turbulence Induces for increasing burn rate

(OP)

Quote (cleverlever):

…Obviously Hara identified the illusive factor that was giving us the increase in low speed torque. Effectively we were increasing the octane of the fuel at low speed high load conditions with early intake closings….


Although it’s not possible for an early intake valve closing event to increase the octane of the fuel, the early intake event may have an effect on the state of the mixture that decreases detonation sensitivity.

As turbomotor stated earlier “dominant effect of early intake valve closing is the expansion and cooling of the charge prior to BDC”. There could be more, the early closing allows for more time for the KE to be absorbed by the mixture to decrease turbulence in the compressed charge, this would effectively increase combustion duration.

RE: Turbulence Induces for increasing burn rate

Turbomotor,

Agree with a lot that you say but: -

The NOx production in a CI engine is a function of the amount of free 02 in the cylinder, the rate at which heat energy is imparted upon it and the max temp the Nitrogen Oxygen mixture attains. Whereas in an SI engine this is offset by the rate at which the O2 is utilized in hydrocarbon reactions, so it acnnot actually react with the N2 because it has already formed H2O, CO2, CO, SOx etc.

Also, wont the cool intake charge actually be heated by the combustion chambers temps more than the 'cooling' effect of expansion can occur with IVC BBDC (I dont know????)

As an aside, in an CI engine with CAT ACERT a lot of the NOx reduction strategy is injection retard, close coupled pilot injection and the 'bumpy camshaft' introducing EGR from the exhaust manifold. That's why this system suffers from terrible 'real world' transient reponse and pretty poor fuel economy.

Although, I totally agree that the propensity of knock is heavily reliant on the preparation of the end gases, ie temp upon combustion initiation. Thats what Octane is all about - the reduction of an engines propensity to knock.


Cleverlever

The seminar that you describe has nothing, in my mind, to do with octane rating - and everything to do with RVP.

To put it this way; liquid gasoline is pretty inert but gasoline vapour is easily combusted.

Take a fuel, such as RFG2 with 9 psi RVP and compare it with a 'lower temp fuel' of lets say 15psi RVP and it's obvious that at room temp there will be more vapour available to burn for the higher RVP fuel, hence the rate at which the liquid fuel vapourizes & combusts at _room temp_ will be much faster. Salesmen will show you anything.

By the way RVP is unrelated to Octane Number.....

As for the push back into the intake manifold that occurs with late IVC; this is a common tecnique in a VVT GDI engine, reducing pumping losses by drastically decreasing the actual compression ratio - hence better part load SFC.

In a PFI engine this results in terrible cycle to cycle repeatability and an awful idle, part of the detonation that you speak of is the result of inaccurate air prediction within the cylinder - spark advance is calculated on the amount of air entering the chamber, not the amount that stays in there.

Both of these are more pronounced with a larger residual gas fraction. Although this 'hot' residual will not increase knock propensity because of the action of the inert working fluid.

MS

RE: Turbulence Induces for increasing burn rate

Aromatic hydrocarbons are often used as octane boosters.

By coincidence, they have relatively high vapour pressures.

Therefore a test of more volatile low octane vs a test with less volatile high octane will show the high octane burning slower.

Try the same test with low octane diesel fuel and high octane methanol and see the result. It will be consistent with vapour pressure and exotherm from the reaction, and inconsistent to octane rating.

Much of the data necessary to work this out and understand how your observations work is contained in the links I provided.

As I understand it, a patent does not need to actually work to be issued in the USA, therefore issue of a patent is not proof of principal.

Also, test data can be misleading if we do not understand every variable that might be influencing the results, like in the Champion burn test.

One purpose of these fora is to question results of tests and established dogma, so as to understand and learn from each other.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

RE: Turbulence Induces for increasing burn rate

(OP)
MS
I am aware of large natural gas fired turbines that use an O2 rich environment to cool combustion temperatures to reduce NOx emissions. H2 is added to the combustion mixture to prevent flame out.

Is this sort of rational possible with the CI engine?

RE: Turbulence Induces for increasing burn rate

(OP)
I meant to ask, is this sort of rational possible with the SI engine.

RE: Turbulence Induces for increasing burn rate

automotivebreath

Basically what you are talking about is a very lean burn engine. I dont have any experience with this sort of application but can imagine that with an SI engine it would be neigh on impossible to initiate combustion, unless as you suggest another 'fuel' were added, in which case things start getting really complicated!!!

This combustion initiation problem is one reason that HCCI look attractive.

MS

RE: Turbulence Induces for increasing burn rate

(OP)
Mattsooty,
What air/fuel ratio would you consider very lean. I'm thinking in the 17:1 range.

RE: Turbulence Induces for increasing burn rate

Automotivebreath

17:1 is about the lean misfire for most stoic. engines - so yes I would agree with you there.

MS

RE: Turbulence Induces for increasing burn rate

Good article in popular hot rodding this month part of it touches on this very subject. D. Vizard states .025" minimum as assembled clearance obtained without signs of contact 7,000 rpm with stock rods and hypertunetic pistons SB chev 350. Also reports 7hp loss per .010" increase in clearance between piston and head. I'd love to see a with/without comparison with that single groove aimed at the plug.--------Phil

RE: Turbulence Induces for increasing burn rate

(OP)
Phil,
Eventually I will be dyno testing the scenario you describe. My experiences to date lead me to believe the results will be very different from D. Vizard's report.

Not long ago I would run tight piston to head clearances, that's no longer the case. My preference now is 0.050" to 0.060" assembled clearance with 35% squish to bore ratio with grooves. I'm find this to be more powerful, reduces HC emissions and lessen detonation tendencies.

RE: Turbulence Induces for increasing burn rate

0.025" piston to head might be OK with every thing brand new and tight piston to bore, but as things wear, I have seen 0.035" hit with SBC 350 TRW forged pistons, stock rods stock steel crank, 6000 rpm.

While the piston to bore was beyond recommended wear limit, the need for a rebuild was due to the ring lands closing down from impact and nipping the rings.

I was about to build a 408" SBC engine at 0.038" clearance with non twist forged crank, 6" long Carrillo rods and SRP forged pistons. I would like to see automotivebreath's results before proceeding.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

RE: Turbulence Induces for increasing burn rate

It is for a boat with raw water cooling and a roller cam with about 235 deg at 0.050 lift duration at 112 deg lobe centres and about 0.600" lift and aftermarket iron heads.

I was hoping to run 10.5:1 with premium unleaded pump fuel with a nominal 98 octane rating.

I don't mean to hijack the thread, but I am guessing what CR I can run with. I would like to run maximum possible CR without need to retard to avoid detonation.

Water temperatures in the head will be about 60 deg C.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

RE: Turbulence Induces for increasing burn rate

(OP)
Pat,
I would like to know more about the fuel you intend to run and the expected cylinder head temperature.

patprimmer said
"I was hoping to run 10.5:1 with premium unleaded pump fuel with a nominal 98 octane rating"

On another note the Vizard’s example utilized hypereutectic pistons. These allow a much tighter piston to bore clearance eliminating piston rock allowing tighter piston to head clearance.

Agreeing with your example, TRW forged pistons require additional clearance, I consider 0.040" piston to head at assembly safe.

RE: Turbulence Induces for increasing burn rate

(OP)
Pat,
What cylinder head do you intend to run and what piston top design. My current thinking is to go with the largest squish to bore ratio available, I use RHS.

I missed your CH water temperature post; I see now 60 deg. C

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