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?

 

[Greenlight]

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.

 

[patprimmer]

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

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Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[mattsooty]

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

 

[al1]

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

 

[automotivebreath]

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 sr 1.jpg

 

[SMOKEY44211]

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

 

[automotivebreath]

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 350 r3_edited.jpg

 

[SMOKEY44211]

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

 

[patprimmer]

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.

 

[automotivebreath]

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?

 

[SMOKEY44211]

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

 

[Fabrico]

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.

 

[automotivebreath]

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.

 

[patprimmer]

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.

 

[automotivebreath]

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?

 

[automotivebreath]

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.

 

[cleverlever]

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

 

[automotivebreath]

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?

 

[cleverlever]

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.