Gas dynamics of resonant exhaust pipes. Drawing, description

PERSONAL TRANSPORT: GROUND, WATER, AIR
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Gas dynamics of resonant exhaust pipes. Personal transport

Personal transport: land, water, air

Directory / Personal transport: land, water, air

The use of resonant exhaust pipes on motor models of all classes can dramatically increase the athletic performance of the competition. However, the geometrical parameters of pipes are determined, as a rule, by trial and error, since so far there is no clear understanding and clear interpretation of the processes occurring in these gas-dynamic devices. And in the few sources of information on this subject, conflicting conclusions are given that have an arbitrary interpretation.

For a detailed study of the processes in the tuned exhaust pipes, a special installation was created. It consists of a stand for starting engines, a motor-pipe adapter with fittings for sampling static and dynamic pressure, two piezoelectric sensors, a C1-99 two-beam oscilloscope, a camera, a resonant exhaust pipe from an R-15 engine with a "telescope" and a home-made pipe with blackening surfaces and additional thermal insulation.

The pressure in the pipes in the exhaust area was determined as follows: the motor was brought to resonant speed (26000 rpm), the data from the piezoelectric sensors connected to the pressure taps were displayed on an oscilloscope, the sweep frequency of which was synchronized with the engine speed, and the oscillogram was recorded on photographic film.

After developing the film in a contrast developer, the image was transferred to tracing paper at the scale of the oscilloscope screen. The results for the pipe from the R-15 engine are shown in Figure 1 and for a home-made pipe with blackening and additional thermal insulation - in Figure 2.

Gas dynamics of resonant exhaust pipes
Rice. 1. Change in pressures in the resonant exhaust pipe R-15

Gas dynamics of resonant exhaust pipes
Rice. 2. Changing pressures in a homemade heat-insulated exhaust pipe

On the charts: P dyn - dynamic pressure, P st - static pressure, OVO - opening of the exhaust window, BDC - bottom dead center, ZVO - closing of the exhaust window.

An analysis of the curves makes it possible to reveal the pressure distribution at the inlet of the resonant tube as a function of the crankshaft rotation phase. The increase in dynamic pressure from the opening of the exhaust port with a diameter of the outlet pipe 5 mm occurs for R-15 up to approximately 80°. And its minimum is within 50 ° - 60 ° from bottom dead center at maximum purge. The increase in pressure in the reflected wave (from the minimum) at the moment of closing the exhaust window is about 20% of the maximum value Р. The delay in the action of the reflected exhaust gas wave is from 80 to 90°. Static pressure is characterized by an increase of 22° from the "plateau" on the graph up to 62° from the moment the exhaust port opens, with a minimum located at 3° from the moment of the bottom dead center. Obviously, in the case of using a similar exhaust pipe, blowdown fluctuations occur at 3° ... 20° after the bottom dead center, and not at all at 30° after the opening of the exhaust window, as previously thought.

The homemade pipe study data differs from the R-15 data. An increase in dynamic pressure to 65° from the moment the exhaust port is opened is accompanied by a minimum located 66° after the bottom dead center. In this case, the increase in the pressure of the reflected wave from the minimum is about 23%. The delay in the action of the exhaust gases is less, which is probably due to the increase in temperature in the thermally insulated system, and is about 54°. Purge fluctuations are noted at 10° after bottom dead center.

Comparing the graphs, it can be seen that the static pressure in the heat-insulated pipe at the moment of closing the exhaust window is less than in R-15. However, the dynamic pressure has a reflected wave maximum of 54° after the exhaust port is closed, and in the R-15 this maximum is shifted by as much as 90"! The differences are related to the difference in the diameters of the exhaust pipes: on the R-15, as already mentioned, the diameter is 5 mm, and on the heat-insulated one - 6,5 mm. In addition, due to the improved geometry of the R-15 pipe, it has a higher static pressure recovery factor.

Conclusions

The data presented in previously published studies do not give a reliable idea of the dependence of static and dynamic pressure on the angles of rotation of the engine crankshaft and on the characteristics of resonant tubes.

The efficiency of a resonant exhaust pipe largely depends on the geometric parameters of the pipe itself, the cross section of the engine exhaust pipe, temperature conditions and valve timing.

The use of counter-reflectors and the selection of the temperature regime of the resonant exhaust pipe will make it possible to shift the maximum pressure of the reflected exhaust gas wave by the time the exhaust window closes and thus sharply increase its efficiency.

Thermally insulated exhaust pipes of well-chosen geometry will give, with a simultaneous increase in engine temperature, an increase in dynamic pressure at the moment of closing the exhaust window, which will further increase engine power. For a more complete understanding of the meaning of such physical quantities as static and dynamic pressure in tuned exhaust systems, as well as the influence of temperature regimes, we can recommend the last two editions from the list of references.

Authors: V.Fonkich, O.Kuznetsov

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