When Ray Everitt flies overhead no-one looks up for the very good reason that, at 1,000 ft, his jodel is virtually inaudible from the ground. the prize of silence can be measured in time, money for stainless steel tubing and excess weight amounting to 18 lbs. a pretty sharp option when you consider that Ray's 65 HP engine has just gained an extra 5 HP through added efficiency.

Tuning exhaust pipes tp help maximize engine efficiency is a trick of the racing car and motor-bike fraternity. It works equally well with aircraft, although it is rarely done. Once you begin thinking in commercial terms, looking at re-certification and the cost of re-design, it is easy to see why manufacturers have stayed with the usual, primitive exhaust stubs to be found on many aircraft flying today. Happily, owners of PFA aircraft can apply for permission to carry out such a modification, always provided that it is feasible and reasonable on their particular aircraft. This is exactly what Ray Everitt has done, working hand-in-hand with Francis Donaldson, the PFA Chief Engineer.

Ray Everitt's first endeavor at hushing engines came about with a Thruster microlight. Ray, who is CFI of Red Dragon microlight school in Chirk, near Oswestry, had replaced the standard Rotax with a BMW motorcycle engine. This worked well but was far to noisy in operation and, to satisfy the CAA noise certificate for microlights, he tuned the exhaust and added a silencer. This resulted in an engine with a noise emission one fifth of that allowed by the regulations.

Ray was so pleased with the 'silent-treatment' given to the Thruster that, when he walked round his newly acquired Jodel, the primitive exhaust system shouted out for a modification. With Francis Donaldson's go-ahead, Ray used similar calculations to come up with a reasonable exhaust system that has tested well for efficiency and which has massively decreased noise levels.

On a static run-up of the engine on full power, the RPM was measured in still air with a hand-held tachometer. The result of putting the new exhaust on the engine increased the static RPM by 60, representing an increase of 5 HP on the standard 65 HP engine. Unfortunately, Ray had no equipment to measure the decrease in noise level, but it is obvious that the engine has been 'hushed' right down. Just how he maximizes power and decreases noise is down to two operations; adding a silencer and tuning the exhaust pipes so that the engine will use the least power to drive out the gases in the standard cruise. When an exhaust pulse leaves the end of an exhaust pipe a shock wave travels back up the exhaust system. If the pulse arrives at the engine just as the exhaust valve starts to open, it expels the gas without using up excess energy. You can ensure that the pulse will meet the valve at exactly the right moment by calculating the exact length and diameter of the pipe required for the pulse to arrive at the valve at this optimum moment.

Ray advises anyone interested in tuning an exhaust to begin by measuring the engine's RPM using an optical hand-held tachometer. When this is held behind a spinning propeller you will obtain a digital readout of the RPM. If you are wondering about cost and availability, most inspectors have one for calibrating the rev. counter for Permit renewals.

Naturally, you will also need to determine the capacity of your engine, by using the number of cylinders and the capacity of each. Finally, to discover the best length and diameter of pipe for your engine, you will need to know the exhaust valve timing; that is the number of degrees before bottom dead center that the valve starts to open. with this knowledge you can then set about making a tuned exhaust system by working out the following formula and applying it to your own engine:

    850 (180 + N)
L = ------------- - P
       RPM

where L = length of pipe in inches
      N = degree before BDC exhaust valve opens
      P = distance from exhaust valve to manifold
and   RPM = desired RPM

The diameter of the pipe needs to be calculated so that the volume of the exhaust pipe attached to each cylinder is twice the volume of each cylinder. The exact diameter of the pipe, incidentally, is not critical and should only be used as a guide in determining which standard sized pipe diameter should be used.

As a guide, these are Ray's calculations for the A65 engine. He began with the knowledge that the exhaust valve opens 50 degrees before BDC. The engine capacity was 700 cc per cylinder and the RPM ran at 2,200. His calculations were as follows:

    850 (180 + 50)
L = -------------- - 3 in = 223 cm = 88 inches.
       2,200

pi r^2 L = 2 * 700cc
pi r^2 * 223 cm = 1400cc
r = 1.4 cm
Diameter = 2.8 cm or 1.1 in

The nearest standard stainless steel tube available was 1.5 in.

Ray points out that, as aircraft engines turn relatively slowly, the length of pipe will be quite excessive, probably over 7 ft. To reduce the weight and complexity of the finished system, the exhaust pipe can be of a two-into-one type, with the joint at half length. the two pipes can then be joined together into a single collector and the exhaust silencer is then fitted onto the end of this single pipe.

Looking at diagram (A), you will see that, in reality, this is a four-into-two-into-one system as, of course, each of the four cylinders has its own full exhaust pipe. The reason for this is simply that the length of pipe from all four cylinders must be equal in order for the timing to be synchronized.

If you look at this overall diagram (A) of the exhaust system, you will see that two of the pipes need to curve in and out to match the length of the pipes from the two cylinders which are towards the front of the engine. Ray solved the problem of bending the 1.5in diameter stainless steel pipe by buying pre-bent pipe and TIG welding and polishing the required lengths. To measure the exact length of a curved pipe, Ray suggests tying a nut or other weight onto a length of string and then dropping it through the pipe. the string can then be marked and measured.

Looking at diagram (A), it's obvious that the extra length of pipe is going to incur a weight penalty. in the case of Ray's Jodel, it meant an extra 17.5 lbs - just about the weight of two gallons of fuel. As the weight is distributed in front and behind the center of gravity, it has made no difference to the C of G.

The junction where the pipes are siamesed together needs to follow the pattern in diagram (B). you will notice at once that both pipes are cut off squarely and do not merge smoothly together. This definite cut-off point maintains a strong pulse. Ray points out that if the pipes were to be blended together, the pulse would gradually reduce and all your tuning efforts would be in vain.

Part two of the exercise is to fit a silencer. Ray reckons that more silencers aren't fitted because there is a belief that this always results in a loss of power and engine efficiency. this may be the case with a baffle silencer, but Ray advocates using an absorption silencer. Imagine a section of exhaust pipe perforated by hundreds of tiny holes. This, in turn, is encased by a larger pipe and the space between the two is filled with either stainless steel wool or fibreglass. You will realize at once that a lot of the noise travelling down the pipe is going to be absorbed. Best of all, the engine is 'fooled' into thinking that this is just a normal exhaust pipe as it does not interfere with the flow of gas in the way that a baffle does.

Ray points out that the length of the silencer isn't crucial. the Jodel's silencer is a couple of feet in length but, basically, any length you can get hold of will be just fine. the combination of a tuned exhaust and silencer increased the efficiency of the engine at the same time as decreasing the noise level and, in this day and age, a quieter engine has got to be a better idea. Ray has to admit, though, that there is a small snag which has resulted in one of the strangest additions to scatter the flock of sheep grazing on the strip, he has to sound a car horn because they just don't hear him coming!

Addendum, by Daniel J.J. Adam

Object: Correction of your assumption concerning the echo of the exhaust: "...Just how he maximizes power and decreases noise is down to two operations; adding a silencer and tuning the exhaust pipes so that the engine will use the least power to drive out the gases in the standard cruise. When an exhaust pulse leaves the end of an exhaust pipe a shock wave travels back up the exhaust system. If the pulse arrives at the engine just as the exhaust valve starts to open, it expels the gas without using up excess energy. You can ensure that the pulse will meet the valve at exactly the right moment by calculating the exact length ..."

When the compression wave (blowdown shock wave) reach the opened end of the pipe, it's return an expansion wave to the exhaust port.

If this expansion wave reach the opened exhaust valve just before closing (ex: crank angle > plus 10 deg), but after the intake valve opend (ex: crank angle > minus 5 deg), and the effective cylinder volme is small (near TDC), the expansion wave will flow through the intake port upto the intake atmosphere increasing the aspiration.

In the graphed case, the exhaust pipe lenght, in degree, is [ (40+180+10) / 2 ] 115 degrees (divided by two because back & forth travel) at the maximum targeted RPM and Exhaust Gaz Temperature, Tk, (in kelvin).

Always in the graphed case, the intake pipe lenght, in degree, is [ (5+180+45) / 2 ] 115 degrees (divided by two because back & forth travel) at the maximum targeted RPM and Intake Gaz Temperature, Tk, (in kelvin).

Consider the wave speed, Ws, formula as [ Ws = sqrt( Gg * Ry * Tk ) ], where Ge is the "specific heat ratio" for exhaust or intake gaz, and Ry is the "ideal gaz constant". By use, the maximum targeted RPM is the RPM where you actually reach the maximum HP.

At a lower RPM than the maximum targeted RPM, the expansion wave will reach the exhaust port before the exhaust valve close, and may be before the intake valve open, therefore, causing suction into the cylinder chamber (that is very good).

Then, to resume the correction of your assumption concerning the echo of the exhaust, the echo should return before the exhaust valve closed instead of the next opening. This correction reduce the overall length (and weight) of the pipe.

Important remark: The assertion "The expansion wave, from the opened end pipe, should return at the exhaust valve closure" is applicable only and only if it's a 4-stroke engine. For a 2-stroke engine, a more complex explanation is necessary.

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