Time-Area values and Port Timings are crucial to determine the power curve obtained by the engine. The empirically calculated numbers given below express time and area, and the ratio between port-window area and cylinder volume. They represent narrowly defined guidelines for intake, transfer and exhaust port time areas as follows:
For piston-controlled intake ports
|0.00014 to 0.00016 sec-cm2/cm3|
For transfer ports
|0.00008 to 0.00010 sec-cm2/cm3|
For exhaust ports
|0.00014 to 0.00015 sec-cm2/cm3|
To work with any time-area problem we must be able to convert engine's timing, in degrees, into actual time at some given engine speed. To convert the known factors into time we use the following formula:
Where T is time, in seconds
N is engine speed, in rpm
U is port-open period , in degrees
The process of finding a port's "mean" area is somewhat more time-consuming. To find the port area we transfer all of the pertinent dimensions to a sheet of fine-grid paper and work from that. We begin by drawing a vertical line to represent a cylinder axis and then, towards the bottom of that line, we add a circle representing the path followed by the crankpin. With that done, we measure the up from the top of the circle a distance equal to the on-centers length of the connecting rod and add a horizontal line at that point. Repeating this, measuring from the bottom of the circle, and we end up creating the space swept by the piston. The ports are then drawn in, flanking the line that represents the bore axis, with the exhaust and transfer ports down against the bottom of the space and the intake port situated up at the top. Finally, mark the mean port-open points on the circle, using a protractor, and connect these points and the bore-axis line with lines of precisely the length of the connecting rod, center to center. We will find that the connecting points, on the bore-axis line, fall about 70 percent down on the exhaust port, about 75 percent down the transfer ports (of which only one side need be shown) and about 65 percent up on the intake port. Draw horizontal line through the port windows at these points, and we'll then be able to measure the mean open area assuming that we have accurately reproduced all the dimensions.
The most difficult job is to accurately reproduce the port window shapes. Exhaust ports tend to be more nearly rounded than square in high output engines with wide, unbridged ports; intake ports have rounded sides even when they are basically rectangular. Transfer ports often have angled upper edges; and in addition to this we must accurately determine the radii existing at the corners of all ports. So, finding the area of a given port aperture is not simply a matter of multiplying width times height. To overcome this we make a close-fitting paper sleeve, slide it inside the cylinder being checked, tape it in place and then, by rubbing around the edges of the port windows either the side of a pencil tip, we transfer the window shapes to the paper. These shapes may not then be transferred to the graph paper directly on which the bore, connecting rod and crankshaft lines have been drawn. This is because in laying the sleeve flat, the width of the ports is exaggerated and what we need is the true size of the aperture. Instead we make the cylinder out of fairly stiff stock, and after getting port shapes marked on its inner surface to reverse its sides, creating a cylinder with the port window drawn outside where they can conveniently be measured with precise calipers and the true dimensions then transferred to the working drawing.
Having found the ports' mean areas, we are ready to complete the calculations in finding each port's value in sec-cm2/cm3. We do this by dividing the product of time and the area of port by the cylinder volume. As an example, for the exhaust ports the suitable values of sec-cm2/cm3 lies between 0.00014 and 0.00015. In case our calculated sec-cm2/cm3 lies above or below the high and low recommended values respectively, we need to alter i.e. increase the port area effectively increasing the width or the height to bring it within these limits. Increasing the exhaust port duration will increase both time and mean area. That's where the drawing becomes handy as it enables us to chart how much area is gained by increase in timing and to establish what combination of port width, height and timing will yield the specific time-area needed for a particular engine. In most instances we will not be able to obtain the correct values by merely widening ports; a combination of increase in both time and area are required if out engine has a peak power a specific rpm and we want to push is to a slightly higher rpm.
With all the numbers entailed in finding correct combination it is advisable to find short cuts. One of these is to work with specific angle-area numbers instead of time area. One cannot simply substitute angle-area for time-area and ignore the engine speed factor. The chart accompanying gives the relationship between time-area and angle area over a range of engine speeds. Take, for instance, the exhaust port time-area/angle-area chart: we will find that there are two lines marking the exhaust time-area limits of 0.00014 and 0.00015 sec-cm2/cm3 with vertical lines marking the engine speed and horizontal lines for angle-area. We need to correlate the specific angle-area values of our engine at a particular rpm and ensure that these limits lie within the two exhaust lines. This process is repeated for all of any given engine's ports.
These time-area and angle area numbers are enormously useful in planning the porting of two-stroke engine. Many other factors however intrude upon this seemingly uncomplicated picture: Referring once again to the time-area/angle-area charts we observe that there is, in every instance, a range for what constitutes correct values, which reflect the fact that the optimum is influenced somewhat by factors not accounted for and which is beyond our control.
Exhaust system characteristics are very closely related to both exhaust port and transfer port time-areas. In case of a micro-car engine, where the power range is more important than maximum power, we would want an exhaust system that provides a relatively weak but extended duration resonant pulse (which means that it would be effective over a very broad engine speed range) and, relatively speaking, a low exhaust port time-area value in combination with a high transfer port time-area. In fact, we would select a transfer time-area at the very top of the range, and an exhaust time-area established at the bottom. That is to say, a transfer port time-area of 0.00010 sec-cm2/cm3 and an exhaust port time-area of 0.00014 sec-cm2/cm3. This combination of time-areas provide an exhaust timing that borrows minimally from the working stroke, which maximizes broad-range torque, and it gives the negative wave returning from the exhaust system a better opportunity to finding the transfer port still open - meaning that this negative, or scavenging pulse will be able to help pull the fresh charge up from the crankcase. Relatively large transfer port time-areas also give the fresh charge conditions allowing it to make its way up into the cylinder even if there is little or no assist from the exhaust system. The sole disadvantage of the condition described is that it does appear to permit a somewhat greater degree of mixing between residual exhaust gases ant the incoming charge, as well as an enhanced tendency towards short-circuiting.
However in case we desire the engine to be tuned for maximum power, the power range becomes a secondary consideration. In this case we try to obtain the maximum value for exhaust port time-area and minimum transfer port time-area. Also scavenging is aided by high crankcase pressures resulting from strong, albeit narrow-band "supercharging" effects of sonic wave activity in their intake tracts. With the extractor effects of the exhaust system and the ramming effects on the intake side combining to aid cylinder charging, maximum power is obtained by upper limit exhaust port time areas (to make best use of the exhaust system) and lower-limit time-areas on the transfer side to minimize charge dilution and short-circuiting.
Increases in transfer port time-area tend to depress the power peak but add to the power curve at lower engine speeds. It should however be noted that excessive transfer port time-area, in combination with the wrong exhaust system, can lead to serious instability in running - yielding a major drop in peak power without adequate compensation in power range, and a power curve marked by humps and hollows. A correct approach will be able to establish time-area values that fall within the ranges according to the conditions for which the engine is intended.
Emphasis on Area:
Taking each port individually, there is every reason to make any port as wide as possible acquiring the necessary time-area values in this manner instead of extending the port-open duration. Reasons for moderating this approach do exist, however interaction between ports and in the effects exaggerated exhaust port widths have upon pistons and rings. A too-wide exhaust port will cause rings to snag or break, or wear very rapidly, and if the widening brings the exhaust port window's sides too near the transfer ports, there will be an increased tendency towards short-circuiting of the incoming charge. Obviously, excessive widening of transfer ports can also result in ring tapping and/or charge short-circuiting. We should also understand that widening an engine's exhaust port, increasing its time-area value without actually increasing its open duration, has much the same effect as obtaining the same increase by raising its height and thus increasing both time and area: that is to say, widening the exhaust port increases the speed at which the maximum power is realized, while reducing low-speed power. And the same pattern is to be observed in increases to transfer port time-area, though in the opposite direction. These effects should be familiar, particularly as regards to the exhaust port, for any increases in exhaust port only after the limit for width has been reached. There is good reason for taking this approach, for while increases in exhaust port time-area, gained by whatever change in the port window's shape, certainly will have the same general effect, increasing width to get more time-are has a much less narrowing effect on the power band than increases in height.
Width is even more important on the intake side of any engine with a piston-controlled intake port, as there are sharp limits to time-area increase gained by lengthening the port-open duration. Piston-port engines have the advantage of simplicity, but are somewhat handicapped by the fact that their intake timing is symmetrically disposed before and after top center. There is, therefore, a strong tendency for the mixture aspirated into the crankcase during the period between intake-opening and top center and intake-closing. This tendency accounts for the extraordinary influence of intake-tract resonance and gas-inertia on the piston-port engine's power characteristics. The combined activities of sonic waves and the inertia of the high-velocity mixture stream can simply overpower the rising pressure in the crankcase created by the descending piston.
Ideally, intake-closing should occur at the precise moment when the ramming pressure is at its peak and when that pressure is equal to the pressure inside the crankcase, as this condition will trap the greatest volume of air/fuel mixture inside the crankcase. Unhappily, this ideal can only be realized within very narrow engines speed range, as inertia effects diminish rapidly than planned speeds and the natural frequency of the intake tract is determined almost solely by its (and the crankcase's) dimensions, which means that it pulses at a fixed rate and only at one particular engine speed will it truly be working in phase with the motions of the piston. Worse, at very low engine speeds neither the sonic wave activity nor the ramming effects of gas inertia will be strong enough to prevent the piston from displacing part of the charge aspirated into the crankcase right back out. All of which means that at cranking speeds, when we are trying to start an engine, the total volume of the charge being delivered into the cylinder will be determined by that which the piston displaces between the point at which the transfer ports close and the point of intake opening (which also is intake-closing).
Considering the intake limitations imposed by the problem just discussed, it should be clear that the task of obtaining adequate crankcase filling in high-speed engines is not confined to establishing a suitable time-area value. Engine for the micro car is restricted in terms of port-open duration by the need for a very broad-range output characteristics, to an intake period of not much more that 160-degrees. More than that virtually guarantees that it will be too peaky to be drivable.
Intake tract tuning will be vitally important no matter what kind of time -area is provided at the port window, and it is all too easy to get the pulsation out of phase with the piston by altering the intake timing. All alterations in intake timing should be followed with careful check to determine if matching alteration of the intake tract length is not also required. Although this kind of work should be validated by actually running the engine with a stub exhaust attached, preliminary check may be run mathematically, using the formula for finding the resonant frequency of the necked flask formed by the crankcase and intake tract.