port flow by SMOKEY YUNICK

[grumpyvette]

VALVE FLOW
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In airflow and a poppet-valved four-cycle engine, I can't describe how much that damn valve butts airflow and the problems it causes trying to get the proper amount of working fluid evenly to all cylinders. New high-buck engines have variable-length runners, and varying numbers of valves for various RPM or power modes, and variable cam action in lift and timing. Well, you know the air can't go through the valve, it has to go around it. It's amazing how many experts worked on airflow for 20 years before they included that in their plan of attack.
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To see how air flows around a valve, get a ruined good flowing aluminum cylinder head. Cut a 3 5/8 inch circle off the center of the intake valve, clean through from the head surface to the top of the head. Machine it down to 3 1/2 inches. Epoxy it into a 3 1/2 inche ID clear plastic tube 'bout 1 foot long. Now cut a hole through the plastic tube where the intake runner adjoins the tube. Add a manual screw-type lever to open and close the intake valve with a weak valve spring, 30 pounds on the seat is plenty. OK, when the valve is open to say, 0.5OO inch, you got a circuit. Intake port (what's left of it) to and around the intake valve ending at the bottom of the combustion chamber and into the remaining 6 inches of length of the 3 ½ -inch plastic tube.
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Now let's epoxy a cover over the port orifice. Cut into it and epoxy a garden hose fitting to it. We want to run water pressure through the port and valve and exit into the combustion chamber to see what happens at 0.050 to 0.600 of valve lift. Most water pressure is 30 to 60 PSI, so we need to have a pressure regulator and cut it back to 'bout 5 PSI so we can vary velocity to look for changes. Water will do exactly what air does. You are gonna notice the water comes around the valve from the bottom of intake port where guide ends, to some distance after the valve head in combustion chamber. The shorter the cone the better the flow; the higher the pressure, the more defined the cone is. A perfect 360-degree even cone is affected by shrouding. Anything closer than a 1/2 inch to the edge of the valve slows flow down. And it's possible in heavy shrouding to only flow about 300 degrees around the valve and the other 60 degrees just bubbles, rolls, and twists. You'll also notice, that not a drop of water hits the lower inner area around the valve stem, or for that matter, over a 1/4 inch (at worst) inward of the valve, and you'll notice on the bottom side of the valve combustion-chamber side that it isn't touched by water either.
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So you see, swirl in the port and whatever has been done to the valve or valve exit in the combustion chamber is overridden by the huge pressure drop at the valve seat caused by the tremendous increase in velocity. Remember, before that valve cracks open, the air column's velocity was at zero and pressure was at the maximum. That tremendous flow acceleration, accompanied by the sudden steep pressure drop, overrules all other physics. As the valve opens farther, velocity starts to drop and pressure starts to rise, but the kinetic energy induced by the start of the event remains in total control.
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There is more to it. The intake and exhaust are open together quite awhile and the temperature and kinetic energy of the exhaust gas cycle play a part in the total result, and so does the pressure in the combustion chamber. I agree, it's getting deep now, but again, what has to be considered is whether you want to port knowledgeably or blindly. We've played the game a "blind hog gets an acorn every once in a while" for too long already.
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What's the best combustion chamber? A flat-bottom head with no combustion cavity, no shrouding, and valves inline with pistons. What's the best port angle? Shallow as you can get-the top of the port up against the valve spring seat. Take any port shape, straighten it out all that you can, and for every degree you straighten it out, with no other changes and for every degree you straighten it out, the more you flow. That's great for airflow, but the compression target has to be addressed.
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Piston shape has to be considered. The best piston theory-wise is flat with a minimum surface area. In real life, the best is 0.5-inch around the bore, 0.030 to 0.040 inch clearance to the flat head area, and a concave inner shape.
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But how about valve clearance? Also, it's best to have straighter ports and raise the top of the engine. Can you close the hood? What's best valve seat angle? Probably 45 degrees, and remember, you do better with sharp, short angle changes than with pretty radii, but only when turning over 10 degrees. If the run is essentially straight, radiuses that are aerodynamically correct will increase velocity and drop pressure economically enough to affect a net gain in airflow.
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In production engines there are two concepts argued all the time: Is it better to straighten the bore to valve angle and increase the port angle, or to decrease port angle and spread the valve to piston angle wider? The first is Ford, the second is Chevy. I'll tell you this: The combustion chamber probably still has 50 horsepower left in it when the piston and combustion chamber are maximized. Consider this: When you react the working fluid, all entrances and exits of the chamber are closed. If you start with a 14:1 air/fuel mixture, when you have consumed half of the charge, you have polluted the remaining half of the air/fuel charge with reacted gasses and the mixture is now 7:1 too rich to support combustion. It turns out, reacted working fluid weighs less than it did unreacted. So if the original charge is spun (as in a centrifuge), the reacted and unreacted would never mix, Therefore, you can react the total volume of the working fluid and pick up another 50 horses. I have a patent on this. I have run it, but I'm sure we'll hear more about this later because there is always more than one way to skin a cat.
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Here's a good experiment for a first grade lesson in airflow: Go to a clear creek, stream, or river if you can find an unpolluted one that has a fair flow speed with rocks on the bottom, Notice the rocks are worn in various smooth shapes. Now reach down to the river bed and rotate one of the rocks 90 degrees. Notice how turbulent the water is now, compared to the way it was before you disturbed it? This will show you what happens with clear air, it's the same thing. Therefore, if you experimented with see through parts and added colored smoke to the air when doing flow tests, you could see what's happening as well as you can see what's happening in the water reaction. This would also show you that a better plan for doing aerodynamic studies on a car body would be with a water tunnel rather than a wind tunnel, and that colored smoke is much better than clear air.
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For those of you who know enough about airflow, I'd recommend that you buy a flowbench if you can afford it. I think there are newer ones coming that should put a lot of the present and used equipment on the market, inexpensively. There is a mystery we've been aware of for about 15 years: Even in a very good running engine the cylinder firing pressure varies, even at steady state, RPM, and load. That may or may not turn out to be an airflow problem, but more likely it has to do with cylinder scavenging and the state of homogenization of the working fluid or the ignition system. If you have, make, or buy a flowbench, do your testing at a lift close to the maximum of the cam you intend to use.
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The newest airflow deals that apply to even running engines (such as those I have) give you the opportunity to use gasoline, so there are no compromises in wet flow for safety (in a running engine). I haven't brought this up yet, so let's cover the fact that multi-cylinder engines don't operate with all the fuel vaporized. It turns out that in a V8 manifold of clear plastic, along with the vapor, there are almost always eight little rivers of liquid fuel going into the combustion chambers why? Because vaporization is caused by pressure and temperature drop. There are no means to attain super vaporization (as in an air conditioner), so some vaporized fuel goes back to liquid before it is reacted. Why? Velocity, temperature, and pressure changes caused by the shape, temperature, pressure differences, and unlaminar flow in the length of the conduit.
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To think out a proper starting point for manifold design, do the math. If RPM multiplied by the number of cylinders, multiplied by the cylinder size says you need 810 cubic feet of air per minute in order to fill the cylinders 100% at 7500 RPM, you have 15 psI ambient pressure, and you average 2 inches of manifold depression each, the conduit has to flow approximately 100 cubic feet per minute. Figure the conduit size from that, check it out in a one-cylinder flow model, or draw on your experience from other engines for a place to start.
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Remember, you have a valve that's not wide open all the time. For example, in my flowbench we have to get around 280 cubic feet of air at 0.600-inch lift to be in the ballpark for 8000 RPM for a 45 cubic inch cylinder. I'd advise you not to try to design a head or manifold from a clean piece of paper unless you've spent at least 10 or more years of your life working and studying engine airflow.
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Because most of you are interested in carbureted engines (two or four barrel) as you work in the plenum of the manifold or the port entrances, consider that all two or four air columns must be bent or twisted to fit each individual cylinder entrance. That's how it works. That's the biggest reason for carb spacers. Also, carb bore spacing is critical to how well this can be done. Wide bore spacing carbs are much worse to deal with because of increased turning angles and the long distance from the source of individual columns to serve a given intake port. There is a possibility that chemical milling, such as Extrude Bone, may be very beneficial, particularly in low-buck porting. It's not as good as Kenny Weld's port milling, but it allows you to get into areas where no amount of work can be accomplished by hand. I used a homemade deal on the order of Extrude Hone 40 to 45 years ago and it helped like hell. Extrude Hone has controls I didn't have. I had to really watch that we didn't eat through the water jacket, and sometimes we did. I was involved with the Mexican road race in 1952, and at a jungle town at the race start, I watched an 18-year-old Mexican boy start to straighten a Cadillac front bumperette that had smacked into a mountain. He cut the damn thing in four pieces straightened each out, welded them together again, and then ground, dressed, and plated them. You couldn't tell it from new. He taught me, without saying a word, how to cut a cast-iron intake manifold apart and oven-braze it back together again. After I did the trick to the inside, I could then reach it.
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Well, I'm sure you have questions. Some of you got lost, some of you have specific questions 'cause there was something I didn't cover, or I was unclear. Pick up your pen, or better yet, sit down at your typewriter or word processor and ask your questions. Maybe we can fill in all the blanks. Most of all this is a big topic. Nobody is gonna get it all in one shot. From time to time, CIRCLE TRACK can get other input from guys who are hitting the home runs today

 

[grumpyvette]

Fast Burn Concepts - Enginology
Elements Of The Rapid Burn...
From the July, 2010 issue of Circle Track
By Jim McFarland

Fast Burn Concepts
This chart is a generalization...


Fast Burn Concepts
This chart is a generalization of in-cylinder combustion temperatures showing the effects of improved mixture quality as a function of "mixture motion" that also relates to flame travel. T1 is the combustion space temperature at intake valve closing, T2 is maximum temperature and T3 is the temperature at exhaust valve opening. Temperatures noted in red indicate gains in peak combustion temperatures and reductions in temperature at the time of exhaust valve opening. Not that e.g.t is reduced (at e.v.o.) Increased peak values can translate into higher cylinder pressures and power gains.
In order to extract the greatest amount of heat from a given volume of fuel, several critical elements come into play. Some of these objectives have been discussed in prior Enginology columns. Obviously, efficient mixing of air and fuel is important. Making certain fuel droplet size (certainly during combustion) as small and uniform becomes essential. Providing sufficient time to complete the burn is linked not only to spark timing but relates directly to burn rate. So because it's a vital feature that can contribute to combustion efficiency and net power, we'll focus this month's discussion on factors involved in burn rate.

Since most racing engines operate at fairly high mechanical compression ratios, air/fuel charge density is high, which helps accelerate the burn. However, proper mixture conditioning as provided by some form of turbulence in the combustion space further aids burn rate, whether by some form of swirl, tumble, or combinations of these.

As explored many years ago by Sir Harry Ricardo and continued into the late '70s by Switzerland's Michael May, along with other students of combustion, the fast burn concept is particularly applicable to circle track engines. Ricardo Engineering also developed a "high ratio compact chamber" (HRCC) concept as an outgrowth of Harry Ricardo's investigations in the '40s. So, the notion of benefits derived from increasing turbulence in the combustion space has been around for a time and simply undergoing further exploration and refinement.

Other consequences of an accel-erated burn include a reduction in exhaust gas temperatures. In fact, if the combustion process is more rapid, it stands to reason more heat will be converted into useable power (before the exhaust event begins), compared to engines with a slower burn rate. Most importantly, it's necessary to work toward improving cylinder-to-cylinder air/fuel charge distribution. Failure to address this concern can lead to lean mixture misfire and periodic or sustained detonation that varies among cylinders operating at or near their lean or knock limits. While it's important to be concerned about cylinder-to-cylinder and/or cycle-to-cycle air and fuel distribution under all conditions, it is particularly critical when making engine modifications that increase the rate of combustion.

But as you might expect, with advantages often come disadvantages, although slight and possible to compensate for in this instance. In particular, creating what we'll call "irregular flow" in the inlet stream and combustion space tends to reduce net volumetric efficiency. The trade-off then involves a comparison of potential power lost through reduced v.e. to that obtainable from improved combustion efficiency. Unless the mixture motion created causes a substantial reduction in volumetric efficiency, offsetting gains in power usually dominate. One middle ground between no intention to improve air/fuel charge homogeneity and specific mixture motion generation points to surface texturing of the inlet path and selected areas of the combustion space. Since this has been a prior Enginology topic, we'll stop short of a regurgitation and simply emphasize the importance of considering the potential gains from surface conditioning.

Also previously mentioned but worth bringing to your attention again is how rapid burn rates affect spark ignition timing requirements (or opportunities). By increasing burn rate, spark timing advance can be reduced, leading directly to less negative torque on the crankshaft in the early, pre-TDC stage of the combustion cycle. Bottom line, less spark advance correlates directly to more net (positive) pressure on the crank.

You should also observe a reduction in brake specific fuel consumption. In a sense, this can relate to on-track fuel economy. From a fuel cost standpoint, you'll be making more power from the same amount of fuel with the potential of reducing fuel use for the same amount of power. Results from either perspective can be a plus. So, if you subscribe to the notion that reduced BSFC (on the dyno) equates to improved acceleration rates on the track, the good side of creating increased burn rates makes even more sense.

So how does all this translate into use if you're an engine builder or racer? Let's carve out specific points raised in this discussion and put them to applications. By the way, if you've been following this magazine's Project G.R.E.E.N. since the January issue, we invite you to begin applying some of this column's subject matter to engines using alternative or sustainable fuels. Even though we won't be investigating how the use of sustainable fuels would be affected by using current ways to enhance the use of gasoline, keep this thought in mind. We're still dealing with an internal combustion engine that provides a chemical reaction with a heat-liberating fuel in a way that mechanically converts to horsepower. An engine's crankshaft cares very little about the specific composition of the substance that's creating its rotation.

Now back to the first sentence of the previous paragraph. A major factor involved in creating a rapid combustion process is air/fuel charge conditioning. Of course it's important to achieve high levels of volumetric efficiency and sustain them throughout the engine speed range of most frequent use. That's not meant to be academic. It's a fact. So think about and work on how you choose to do that. Surface texture does matter, from the point of fuel delivery and throughout the combustion space (if you're using a carburetor) and from injector location to the same space, if FI is your choice for induction system. And don't forget to include piston crown surfaces in your considerations. Just because these represent the combustion space floor doesn't exclude them from importance.

Evaluate your fast burn experimentation's on the dyno. As you begin to achieve success, look for reductions in exhaust gas temperatures. If you're fortunate enough to be including exhaust gas analysis equipment as part of your engine (or parts) development regimen, watch for reductions in oxides of nitrogen (NOx). In fact, making this type equipment part of your engine dyno measurement process can be a valuable tool, even though you might not otherwise be concerned about emissions. It'll help you evaluate the combustion process and often point to problems that are masking potential gains in power. Check out the emissions equipment being used in CT's Project G.R.E.E.N. for more information or contact Dave Kalen at Sensors, Inc. at 734/429-2100, ext. 216.

Also, experiment with mixture ratios. You may find that less fuel will make the same amount of power, yet another indicator of an increased burn rate. And don't forget about doing a map of initial ignition spark timing. As a suggestion, make repeated WOT power measurements at peak torque rpm, steady state. Perform a series of runs at spark settings below and above what you would normally run on the dyno, not at the track. If you discover less spark timing is required at peak torque, use that value for a full dyno run. Again, if your modifications to increase burn rate are successful, a trend toward (or actual) power gains will indicate success. At the end of the day, all this can amount to difference between the checkered flag and something less.

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