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Posted on 09 Jan 2003 at 05:06:30 (UK time)
Sean Brown, Oregon, USA, sevenshop@terragon.com
Subject:   "Go with the flow or get left behind"

Fire Breathing Engines?

      The reciprocating internal combustion engine is often likened to an air pump and to a point this is true. However, the fact an engine runs at all is not so much due to any phenomenal pumping abilities, but rather the presence of the combustion event...Something we must keep foremost in our minds as we consider the following.

Why then does an engine require air?

      Air is one component necessary to support combustion, fuel and spark being the others. The pressure exerted by the combusting gasses (fuel and air) is what pushes the piston and turns the crankshaft. It would thus stand to reason that anything we can do to increase the amount or duration of pressure, will improve the performance and torque output of the engine.
      By increasing the amount of charge (air and fuel mixture) inhaled by the engine at any given speed, we will find that torque output will increase accordingly. This comes about due to the increased volume of gasses acting on the same piston area in the same amount of time. The key here is time and we will later find that increases of inhaled mixture may not produce the desired effects, if the burn rate is made to suffer as a consequence.

So what then is Ve?

      Volumetric efficiency is the term given to the amount of charge inhaled by an engine of fixed displacement, verses the volume of its cylinders, 100% being theoretically optimum. It is practical to note that highly tuned racing engines frequently exceed 100% Ve, and is the reason for their higher specific outputs. However, considering the MG series of engines, and the 'B' in particular, one finds himself hard pressed to achieve 100%, even for full bore racing efforts.
      Some factors affecting Ve include breathing efficiency and pressure wave tuning. Naturally one would assume that, were we to increase the efficiency through which the engine is able to breathe, it will pump in more mixture with less effort. We will find that, while this is true, "breathing efficiency" is a multifaceted component and beyond the following statement, cannot be considered as a whole.
      The other most notable factor would be pressure wave tuning. Using the elastic and compressible qualities of air to our benefit, it is possible to increase the filling of an engine's cylinders up to and beyond 100% Ve. It is important to note that this is a fully developed subject in itself and thus will not be considered in the remainder. It will also be noted as an unfortunate fact that siamese port cylinder heads do not lend themselves well to this form of tuning.

The next step:

      For many years, engine builders have modified cylinder heads in an attempt to increase the performance of the engines they have built. Whatever the focus was on, (an increase in Ve, combustion quality, or both) the underlying goal was performance. For airflow directly, we will consider its elements in a three dimensional context.

Dimension one, airflow Quantity:

      Imagine two inlet manifolds for a single cylinder engine having one inlet port and carburetor (like a lawn-mower engine). In the first example, we will use a straight section of pipe 12" long and of a cross sectional size adequate for the desired output and operating range. In the second example, we will bend the above pipe in two places at 90º angles, effectively forming an acute dog-leg of constant cross section, this time sized to flow the same quantity as in our first example.
      Given both units flow the same quantity of air, it may be surprising to find that in practice (on a running engine), the straight section out-performs the bent one. The reason for this, is that flow quantity, is only one aspect of total "breathing efficiency." It follows then that an engine must have sufficient quantity for it's desired output and operating range, anything more simply detracts from performance within this range.
      Fortunately, the quantity needs of an engine are easiest to calculate. For example, an 1800cc engine running at 7,500 RPM will require a flow rate of 237.75 CFM per revolution. The forgoing example assumes we have obtained 100%Ve, which at that RPM, at least for an MGB, is pretty unlikely.

Dimension two, airflow Efficiency:

      The fact is, a straight pipe will always prove to be more efficient than a curved one, regardless of flow quantity. This is due to losses encountered whenever air is made to turn en-route to a cylinder or other means of relative pressure depression. Because of this, the straight pipe can be made to flow considerably more air at a smaller cross sectional size than its curved counterpart, and increased performance is the result.
      Air flows due to a differential of pressure between two points. An example would be the air at atmospheric pressure flowing into a cylinder at sub atmospheric pressure. Additionally, airflow will always take the shortest route between these two points of pressure differential. I.E. airflow takes the path of least resistance. Therefore, air will "hug" the inside radius of any corner present in it's path from pressures high to low, while attempting to take the straightest route. This in turn causes unequal pressures in the port, with flow sheering and subsequent losses in efficiency the result. These effects could thus be termed as "resistance" to flow motion.
      One way to decrease the "resistance" felt by the engine, is to increase the cross sectional size and thus volume of the manifold. While this does increase the flow quantity potential, it does not increase the flow efficiency, as turbulence remains much the same as before. We already know that smaller cross sections are desirable and we will now explore the reasons why.

Inertia tuning explained:

      Inertia tuning or (ram charging) is a state of tune where-by the incoming gasses to a cylinder are provoked (by their inertia only) to continue flowing despite a lessening, or reversal, of the pressure differential between the atmosphere and cylinder. We learned in physics class that bodies in motion tend to stay in motion and while air is basically invisible to the eye, it none the less possesses significant inertia at speed. To understand how this relates to cross sectional sizing of ports and manifolds, let's carefully consider the following.
      For a given pressure differential, the air must flow at a given velocity, to achieve a given output quantity, through a duct of a given cross sectional size. Therefor, if we decrease the cross section, we decrease the manifold volume and thus the velocity must increase to achieve the same output quantity. Since velocity increases inertia and therefor cylinder filling, one is compelled to size port and manifold cross sections to the smallest that will deliver the necessary volume at a known pressure differential. This results in the maximum cylinder filling with the least resistance to the engine, which is the essence of airflow efficiency.
      Since cylinder heads and manifolds must fit into engine compartments as well as satisfy other practical requirements, most ports are not straight. Therefore the need to increase the efficiency of a curved manifold arises. Knowing how cross section affects velocity, it is possible to use these effects to increase airflow efficiency. Since straight sections can be sized smaller than a turn for the same output quantity, a port or manifold utilizing these characteristics will see an increase in efficiency over one of constant cross sectional area. Additionally, cross sectional shape can be used to enhance efficiency. By increasing the area along the inside radius of a turn, flow sheering and subsequent losses can be reduced. The most efficient manifold or porting arrangement, will likely utilize all of these design aspects.

Dimension three, Airflow Quality:

      When dealing with airflow in the internal combustion engine, it is easy to loose sight of the fact that intake air includes not only air from the outside atmosphere, but atomized droplets of fuel as well. This fact makes the routing of fuel and air particularly troublesome as degradation of mixture quality will have a directly imposing negative effect on the production of power. Remembering back to the beginning of the article, we will recall that the combustion process is critical to engine performance. The "quality," or state of mixture homogeneity, is one factor affecting the combustion process.
      If the air/fuel ratio varies within the combustion chamber, (that is 12:1 in one cc, 16:1 in another and so on) the flame propagation will be impaired. When a situation like the above occurs, the flame front is forced to advance in a stop-start manner which increases the overall length of time necessary to complete the burn. The effects of this can lead to problems such as detonation and other losses, including low fuel efficiency. A simple way to see how mixture quality affects combustion, (and thus power and efficiency) is to change the air/fuel ratio metered by the carburetors. Owners of smog carbureted cars who have changed to non-smog needles will be the first to attest to the performance enhancements obtainable. In this case we changed the overall ratio only; however changes in airflow quality can have equally as pronounced effects. Additionally, this quality is directly affected by certain properties of airflow.

Keeping the fuel suspended:

      Reasons for a non-homogenous mixture having instances of poor fuel atomization, are due to deficiencies in the airflow quality during it's route from the carburetor, to the cylinder. Since fuel is heavier than air, it tends to be "centrifuged" out of suspension whenever a turn is encountered in the flow. This problem can also be caused by the effects of viscous sheering in the air. If the port or manifold has been designed so as to minimize sheering of the airflow, atomized fuel will be less likely to depart company from the bulk of the flow, whenever a turn is encountered. Additionally, an increase in cross sectional area through a turn will slow the air and fuel mix as it makes the turn, thereby reducing the chance for the "centrifuging" problem mentioned previously.
      Although cross sectional changes and flow velocity can be used to our advantage (as seen above), taken to the extreme, these effects can become negative. Fuel "drop out" as it is often referred to, is usually due to sudden cross sectional changes to the flow path. This has the effect of suddenly slowing the air through the change (in this case, from a smaller to larger cross sectional area) and the fuel literally "drops out" of suspension. Properly designed ports and manifolds will not have sudden or radical changes in cross sectional area. Any change needed must me made smoothly or the proceeding problems will result.

Let's finish on this:

      Last on the list of factors affecting airflow quality, is port finish. The fact is, a properly finished port is critical to the performance of the engine, as it is the final word on airflow quality. However, when "port finish" and "high performance" are mentioned in the same sentence, most enthusiast immediately believe that the finer the finish, the better the airflow. This is an easy trap to fall into, as utilizing the "finger test method," one would consider a port which feels smooth, must flow "smooth" as well. Unfortunately, this is not necessarily the case at all.
      The airflow through a port of an internal combustion engine is what is known by fluid dynamists as "fully developed, turbulent flow." This simply means that the flow is not of a laminar state, and instead consists of molecules of air continually flowing at different velocities all the way from the center of the port, out to the edge of the port. For a perfectly straight pipe (as in our first example), the molecules in the very center are flowing the fastest, with those closer to the edges slowing incrementally as we near it. The molecules directly in contact with the edge of the port are in fact, "stopped" for all practical purposes and this is all due to the viscosity of the air itself. Potential problems due to these effects include flow shearing, as more molecules attempt to go faster near ones which are going slower. This tends to help promote fuel drop out and centrifuging, and poses further resistance to flow motion.
      Since we cannot change the viscosity of the air, we must instead work to find a way to keep the flow from adhering to the port walls. One way to do this is to create a rough (textured) surface which in turn will promote what are called "Karman ring vortices." These vortices will "energize" the layer near the surface of the port wall, acting as some head modifiers have stated, "like needle roller bearings." The results of this phenomena act to reduce the adherence of the airflow to the port walls, allowing the bulk of the flow to proceed in a more homogenous, less disrupted fashion.
      Just as cross sectional shape, size and consistency can be used to influence the airflow, port texturing and finish can likewise be used to our benefit. This means that, not only should a port be of a courser overall texture, but this texture should also be used to influence (or at least not degrade) the flow pattern in that specific area. If you haven't gotten all of this, don't worry. What it all boils down to is the fact that a finely polished port will not produce optimum results. We use anywhere from 36 to 80 grit on our manifolds and heads with the latter not being for the intake ports!

      So knowing now how the various aspects of airflow influence the operation of the internal combustion engine, it is easy to see how concentration on one aspect only will generally not produce the intended results. For instance, concentrating on airflow quantities only, to the exclusion of efficiency and quality, may look good on a flow bench but in fact reduce output due to poor cylinder charging and/or fuel atomization, etc.

      Next time we'll deal more with the development issues for a real eye-opener of how a flow bench can be used to reveal the true nature of airflow phenomenon.

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