A new method of testing exhaust system
designs for aircraft engines is under development at the CAFE Foundation. We have named
the graphs produced by this new method "the EPG" or exhaust pressure-graph. The
EPG shows the instantaneous pressures at different locations in an exhaust system. It will
be used in a comprehensive study to determine which systems produce the best combination
of increased horsepower, decreased back pressure and acceptable noise levels. EPG's
will be used to study the effects of header pipe diameter, length, shape and mutual
connections as well as the effects of collectors, megaphones, anti-reversion cones, ball
joints and jet thrust nozzles. The popular "tuned crossover" exhaust system,
several merged collector systems and a standard, certificated aircraft exhaust system will
also be studied. Insights gained from the analysis of EPG's will help design more
efficient exhaust systems for aircraft.
This is the first of three reports on the CAFE Foundation's EPG findings. Suggestions
from those interested in exhaust system design are welcome.
BACKGROUND
Dynamometer testing has shown many times that reducing exhaust system back pressure can
afford significant gains in horsepower. Racing engines, where noise muffling is not a
priority, have been the principal domain for such exhaust "tuning", and have
evolved through many popular pipe designs. This evolution has included a large measure of
empirical testing and unscientific passing fads. In aviation circles, the debate over the
relative benefits of straight exhaust pipes, crossover systems or merged collector systems
has been in need of a carefully measured answer.
As racing engines, particularly in motorcycles, evolved to very high RPM's, their
exhaust designs became more sensitive to high frequency sonic events in the pipes and less
concerned with mass flow events, especially as the number of cylinders increased and the
displacement per cylinder became smaller.
Horizontally-opposed 4 cylinder aircraft engines, by contrast, typically operate at
about 2500 RPM using large displacement cylinders. This affords time enough for the
exhaust pulses to be easily detected one at at time by pressure recording devices.
TUNING BENEFITS
To understand the potential benefits from a low back pressure exhaust system, it is
helpful to imagine the exhaust pipe (header) at its attachment to the cylinder head as if
it were a powerful vacuum cleaner. Vacuum applied to the exhaust port assists the exit of
the hot gases from the combustion chamber as soon as the exhaust valve opens. This vacuum
can actually help "pull upward" the rising piston during the exhaust stroke so
that the piston does not have to "work" at expelling the hot gases.
In addition, continuous vacuum in the exhaust pipe can more thoroughly empty the
combustion chamber near the end of the exhaust stroke and can initiate a helpful early
filling of the chamber with the next inducted charge of cool fuel and air from the intake
system. Such early filling is accomplished in the so-called "overlap stroke" of
the piston wherein the piston reaches top dead center with both the intake and exhaust
valves open. The volumetric efficiency of the engine, with help from this early filling,
can exceed 100%. This means that a 90 cubic inch swept cylinder volume can actually inhale
more than 90 cubic inches of inducted charge, which, in turn, will produce more power on
each firing.
The early filling washes the hot exhaust valve head, seat and guide with the cool
inducted charge. It provides a more thorough "washout" of the end gases or hot
exhaust residuals and thus give less coking of the combustion chamber, less contamination
of the crankcase oil, and less fouling of spark plugs. It has been observed that the
engine reliability history of two popular production aircraft from two different
manufacturers differ greatly even though they use the same engine. The difference in these
two aircraft is in their exhaust systems.
Exhaust systems with very low back pressure may afford the use of jet thrust nozzles on
the exhaust tailpipes without causing unduly high back pressure. Such nozzles can provide
significant gains in cruise speed at altitude.1
MATERIAL AND METHODS
For this study, the exhaust components are fabricated from mild steel tubing and parts
commonly available at automotive muffler shops. The owners of Johnny Franklin's Muffler
Shop in Santa Rosa, California and Loren Barnes at S&S Headers in Anaheim are very
knowledgeable and helpful in providing these materials. Robert Susnar of the Sani-Fit
Company has donated a beautifully made stainless steel jet nozzle reducer for these tests.
Factory Pipes of Ukiah have also contributed to these tests.
The CAFE EPG recording system uses Ed Vetter's custom-designed software package applied
to the Digital Acquisition Device and Sensor Amplification Module described in previous
articles on CAFE Foundation equipment.2 The electronic pressure sensors are connected to
.125" x 18" copper tubes which, in turn, connect to each exhaust pipe sampling
port at a location 1.25" downstream from the cylinder flange of the header pipe. The
sampling ports are all carefully made flush with the inner wall of the exhaust pipes. The
transducers are calibrated to a water manometer. A custom made Vetter inductive crankshaft
trigger is applied to the front of the crankcase of the Lycoming IO-360 A1B6 engine to
record the top dead center position for cylinder # 1.
The exhaust pipes in this study are of adjustable length by the use of specially made
6" long overlapping slip joints. These joints are secured by stainless steel hose
clamps and safety wire. The lengths being studied are those which enjoy popular use on
other aircraft and which can be reasonably fitted to a horizontally opposed aircraft
engine. In tests with merged collector systems, great care is taken to make all the
headers of equal length prior to the merge. Pipe diameters will include 1.5",
1.625", 1.75" and 1.875" headers, 1.75", 2", 2.25" and
2.5" collector outlets, and several different megaphone and nozzle designs. An
anti-reversion cone (see photo) will be evaluated, as will a 2" stainless steel ball
joint (Aircraft Spruce Company Part Number 33233). The inside diameter of the header is
die-ground to match the 1.78" outlet diameter of the exhaust port on the cylinder
head in every case.
Cabin sound level measurements are included in most engine runs along with notes on
RPM, manifold pressure, fuel flow, static thrust and outside air temperature. Pressure
transducers are connected to the intake port of the engine's #1 cylinder at the fuel
injector orifice and to the collector pipe at a location 10.5" downstream of the
merge of the four individual header pipes. To provide a method of assessing horsepower,
the digital tachometer RPM of the fixed pitch prop at full throttle and the digital fuel
flow are recorded for each particular exhaust system. Static thrust is measured by
attaching a cable to the main landing gear and tying it to a heavy truck. The cable pulls
on a hydraulic cylinder whose pressure gauge is used to calculate pounds of thrust.
RESULTS
In Figure 1, the heavy line indicates the continuous variation of the instantaneous
pressure in the exhaust pipe of cylinder #1. Note that the large pressure rise after the
opening of the exhaust valve is labeled as "P1". This primary pressure wave
dissipates and is followed by the smaller waves R3, R2, and R4. The R waves are the
reflected waves from the firing of cylinders 3, 2 and 4, in that order, as their P waves
enter the collector pipe and reflect back upstream into the header of cylinder #1. The
collector waves or C waves are the regularly repeating sequential waves recorded in the
collector pipe with each successive cylinder firing, and are labeled accordingly, e.g.,
C1, C3, C2, and C4. The V wave is the vacuum wave in the intake port which is produced by
the descent of the piston in cylinder #1 during its intake stroke. Because of the common
intake plenum on this engine, the intake recording shows each of the other cylinder's
intake strokes producing a regular sequence of smaller V waves (vacuum waves) after V1.
The average back pressure in cylinder #1 in inches of water is calculated from the
"area under the curve" and is found to agree with the back pressure observed by
connecting a water manometer to the pressure sampling port in that header. Figure 1 shows
an EPG recording of a system with an average back pressure of -37.5 inches of water or
-2.75 inches of mercury (Hg). The FAA allows production aircraft to have exhaust back
pressures of up to +2.0 inches Hg. The ambient pressure line on the Y axis of the EPG is
labeled as zero and represents atmospheric pressure at 120 feet above sea level.
Each exhaust system tested is described at the top of the EPG using the following
format: header diameter x length/ collector diameter x length/ megaphone length x outlet
diameter/ and any other qualifying information.
Figure 2 is a recording in which all pressure probes are ganged onto cylinder #1's
sampling port and this shows good agreement in probe dynamic calibration. Note that even
at the top of the P wave, the pressure is below atmospheric.
Figure 3 is a recording in which each probe is connected to a different cylinder and
serves to illustrate the uneven P wave amplitudes and imperfect valve timing of hydraulic
lifters. It is a test where straight pipes are used with no collector and no megaphone. By
showing the comparative size of the exhaust pulses the EPG may give indications of which
cylinders have the best compression or best volumetric filling.
FUTURE TESTS
These preliminary results indicate that the EPG will be a great tool for the
development of optimized aircraft exhaust design. Our future studies will examine the
effect of these exhaust systems on climb and cruise performance, and we will present EPG
results of the various systems obtained in flight. We will keep a "library" of
pipe models to be re-tested in the future using a torquemeter. We hope to repeat these
tests on an engine which uses a camshaft with a longer overlap period to see if the power
improvement can be further enhanced. The effect of these exhaust systems when used with
increased ignition timing advance is also planned for study.
Earlier results indicated that aircraft engine power output can be improved by up to
10% by the use of low back pressure exhaust designs.3 The EPG is expected to help define
which are the best designs and will become a routine part of the CAFE Foundation Aircraft
Performance Reports.
BIBLIOGRAPHY
- Pinkel, Benjamin, Turner L. Richard, Voss, Fred, and Humble, Leroy V. : Exhaust-Stack
Nozzle Area and Shape For Individual Cylinder Exhaust Gas Jet Propulsion System. NACA
Report No. 765, 1943.
- Seeley, Brien, The Technology of CAFE Flight Testing. Sport Aviation Vol. 43, No. 5, pg.
51, May, 1994.
- Seeley, Brien, Tuned Aircraft Exhaust Systems, Sport Aviation, Vol. 39, No. 11,
November, 1980
