Method for distributing gas flow in exhaust system of aircraft multi-cylinder four-stroke piston engine with reduced noise levels

Abstract

FIELD: aeroacoustics of piston engines.SUBSTANCE: invention can find application in reducing noise from light propeller-driven aircraft and unmanned aerial vehicles on the ground. In an aircraft multi-cylinder four-stroke piston engine, the gas flow is distributed in the engine exhaust system, taking into account the dimensional restrictions for installing the exhaust system on the aircraft. The exhaust gases from the cylinders are distributed in the exhaust system taking into account the need to ensure a minimum frequency of the fundamental tone of the engine. The lower frequency of the fundamental tone of a multi-cylinder engine provides a greater value of artificial lowering of the noise level on the ground in the dBA metric, which ensures a lower noise level for both the engine and the aircraft in which it is part of the power plant.EFFECT: reduction in the noise level of an aircraft multi-cylinder engine without increasing the weight and dimensions, and reduction in the noise level on the ground of the aircraft, which includes this engine in its power plant.1 cl, 4 dwg

Description

[0001] The invention relates to the field of aeroacoustics of piston engines and can find application in reducing the noise of light propeller-driven aircraft and unmanned aerial vehicles on the ground.

[0002] A "Method for Silencing Exhaust Noise from a Four-Stroke Internal Combustion Engine and a Device for Implementing the Same" is known [1], which consists of dividing a cyclic portion of exhaust gases into two parts, one of which is fed into a separate chamber and released into the environment, and the other into another chamber. After the exhaust process is complete, the gases are directed into the first chamber and released into the environment. The device implementing the claimed method comprises a housing divided by a partition into a main and a damping cavity, an outlet pipe located in the main cavity, and an intermediate pipe located in the housing, made in the form of a spiral integral with the inlet pipe and having a window at the initial section opening into the main cavity. The open end of the pipe exits into the damping cavity.The reduced pressure behind the exhaust organ is caused by two rarefaction waves reflected from the window and the open end of the intermediate pipe, and by the fact that the exhaust gases enter the main cavity in two parts, with the second part arriving with a time delay due to the long length of the intermediate pipe.

[0003] The disadvantage of this method is the need to install an additional housing with two chambers, one of which is used to direct engine exhaust gases. The proposed design is quite bulky. This method and device cannot be used in aviation due to existing size and weight restrictions.

[0004] A known "Method for Reducing Exhaust Noise from an Internal Combustion Engine" [2] involves converting each initial mass wave of exhaust gas flow into several smaller mass waves. This is accomplished in such a way that the rise and fall fronts of adjacent smaller mass waves intersect, and the time interval between their peaks is chosen such that the integral flow value is close to steady state. This implementation ensures that fluctuations in exhaust gas flow are converted into steady state flow.

[0005] The disadvantage of this method is the need to design a device for its implementation - an exhaust noise muffler for an internal combustion engine (ICE), which, in order to operate effectively, must have a significant volume and, as a result, dimensions, which makes it impossible to use it on an aircraft ICE.

[0006] A "System for Actively Suppressing Intake and Exhaust Noise of an Internal Combustion Engine" is known [3]. The internal combustion engine comprises an air intake system, an exhaust system, and a compensating signal generation system. The compensating signal generation system comprises a sound source enclosed in a sealed chamber, connected to the intake manifold and to the exhaust pipe via acoustic waveguides, and the vibrations of the loudspeaker membrane are controlled by a control unit containing amplitude and phase information of the compensation mode, wherein the membrane emits sound waves in opposite directions in antiphase. The invention makes it possible to achieve simultaneous reduction of both intake and exhaust noise of the engine using a single acoustic source.

[0007] The disadvantage of this method is its inability to reduce the noise of an aircraft internal combustion engine. The presence of an additional compensating signal generation system ("anti-noise") degrades the reliability and failure safety of the power plant as a whole, which is unacceptable in aviation.

[0008] The closest invention to the claimed invention in terms of its features and taken as a prototype is the "Method for Distributing Gas Flow in a Noise Muffler System" [4], in which the gas flow enters through an inlet, splits into two streams, which are distributed into individual chambers. The individual chambers move in parallel and are counter-connected in a mixing cavity. The technical result consists of reducing noise levels with minimal energy loss and reducing the aerodynamic drag of the muffler.

[0009] A disadvantage of the prototype method [4] is that the gas flow from all cylinders enters the muffler through a single inlet. Implementing this solution on an aircraft engine requires using multiple pipes, combining them into a single exhaust manifold, from which the exhaust gases are then fed to the exhaust muffler. This method significantly complicates the design and increases the weight and dimensions of the power plant, which is a critical factor in aviation. It is also worth noting that this method of distributing gas flow within the muffler system can significantly increase backpressure in the exhaust system, thereby reducing the available power of the power plant.

[0010] The invention proposed for patenting, “A method for distributing gas flow in the exhaust system of an aircraft multi-cylinder four-stroke piston engine with a reduced noise level,” makes it possible to eliminate the shortcomings of the prototype.

[0011] The main task that the claimed invention is aimed at solving is the development of a method for distributing gas flow in the exhaust system of an aircraft multi-cylinder four-stroke piston engine with a reduced noise level.

[0012] The technical result of implementing the claimed invention is a reduction in the noise level of an aircraft multi-cylinder engine without increasing the weight and dimensions, a reduction in the noise level on the terrain of an aircraft, the power plant of which includes this engine.

[0013] The technical result is achieved in that, according to the method of distributing the gas flow in the exhaust system of an aircraft multi-cylinder four-stroke piston engine with a reduced noise level, which includes the following steps:

[0014] - an analysis of the design of an aircraft is carried out from the point of view of the possibility of ensuring the distribution of gas flow in the exhaust system of an aircraft multi-cylinder four-stroke piston engine in such a way that the exhaust gases from each cylinder are directed to exhaust manifolds that combine as few cylinders as possible, while taking into account the dimensional restrictions on the installation of the exhaust system on the aircraft;

[0015] - based on the empirical method for assessing the noise of aircraft internal combustion engines, the spectra of sound power levels are determined for a multi-cylinder engine with different methods of distributing the gas flow in the exhaust system of the internal combustion engine, while the prototype engine in the calculations is the option where the exhaust gases from each cylinder enter a common exhaust manifold;

[0016] - based on the calculated sound power spectra for each of the possible configurations, the total sound power level, weighted according to the A scale of a standard sound level meter, is determined in the frequency range standardized for noise in the area;

[0017] - determine the effects of reducing the total sound power level of the internal combustion engine in the dBA metric for different variants of gas flow distribution in the exhaust system relative to the prototype engine;

[0018] - choose a less noisy option for distributing gas flow in the exhaust system of an aircraft multi-cylinder four-stroke piston engine.

[0019] In accordance with the ICAO Standard [5], the unit for assessing the ground noise of light propeller-driven aircraft whose power plants include multi-cylinder piston engines is the maximum total noise level in the frequency range of one-third octave bands of 50-10000 Hz, adjusted according to the A-scale of a standard sound level meter (with a slow response). The amplitude-frequency response of the standard A-filter according to GOST 17187-2010 (IEC 61672-1:2002) [6] is shown in Fig. 1.

[0020] The tonal frequencies in the ICE noise spectrum are usually divided into cylinder and engine frequencies. The cylinder harmonic frequency (ƒ) ц ) in the noise spectrum of a four-stroke internal combustion engine can be determined in accordance with the expression:

[0021]

[0022] where k is the harmonic number, n кв - engine crankshaft rotation speed (rpm).

[0023] It is known that the fundamental frequency of a multi-cylinder engine depends on how the engine exhaust system is organized on the aircraft.

[0024] In particular, on the MAI-223M aircraft [7], the exhaust system of the four-cylinder, four-stroke ROTAX-912 engine is designed in such a way that the exhaust gases from all cylinders are directed to the exhaust noise muffler, while the fundamental tone of the engine is the fourth cylinder harmonic. On the AN-2 aircraft [8], the exhaust system of the 9-cylinder, four-stroke ASH-62IR engine is designed in such a way that the fundamental tone of the engine is the 7th cylinder harmonic.

[0025] Thus, the frequencies of motor harmonics in the noise spectrum of a multi-cylinder aircraft engine are determined in accordance with the expression:

[0026] = kƒ ц i,

[0027] where k is the harmonic number, i is the number of cylinders in the engine connected by a common exhaust manifold. The first engine harmonic is the fundamental tone of the engine.

[0028] Thus, the lower the fundamental frequency of a multi-cylinder engine, the lower its noise level, and the lower the ground noise level in dBA of the aircraft in which it is part of the power plant. Clearly, a lower fundamental frequency can be achieved through rational distribution of the gas flow in the exhaust system of a multi-cylinder four-stroke piston engine.

[0029] The effects of noise reduction of an aircraft internal combustion engine depending on the method of gas flow distribution in the exhaust system of an aircraft multi-cylinder four-stroke piston engine on the ground can be assessed using the empirical method [9], which allows one to determine the spectra of the sound power levels of an internal combustion engine in specified one-third octave frequency bands according to the expression:

[0030]

[0031] where n n - nominal crankshaft speed (rpm); ƒ - central frequency of the one-third octave band (Hz); N e - rated effective power (kW); n кв - crankshaft rotation frequency at a given mode (rpm), - the central frequency of the one-third octave band, corresponding to the frequency of the fundamental tone of the engine (the first engine harmonic) (Hz); K1 and K2 are empirical coefficients that depend on the type of engine; V H- engine displacement (l). The nominal mode should be considered the takeoff mode of the power plant.

[0032] In this calculation, the prototype engine should be one with an exhaust system designed so that exhaust gases from each cylinder enter a common exhaust manifold. In other words, the prototype engine is the one with the highest fundamental frequency.

[0033] When calculating the spectra of sound power levels of internal combustion engines, the coefficients K1 and K2 for the prototype engine and modified engine versions are the same, which makes it possible to perform calculations in relative normalized values.

[0034] Based on the calculated spectra of sound power levels, the total sound power level, weighted according to the A-scale of a standard sound level meter, is determined, which will characterize the influence of the distribution of the gas flow in the exhaust system of an aircraft multi-cylinder four-stroke piston engine on its noise level on the terrain in the dBA metric.

[0035] Total A-weighted sound power level of a standard sound level meter L WA∑ is determined on the basis of the energy summation of sound power levels in 24 one-third octave frequency bands in the range of 50-10000 Hz according to the expression:

[0036] j

[0037] where L Wj - sound power level in the j-th one-third octave band, Aj - correction of the standard A filter according to GOST 17187-2010 (IEC 61672-1:2002) [6].

[0038] The invention is illustrated by figures, where:

[0039] Fig. 1 shows the amplitude-frequency correction of the standard A-filter (SPL, dB) in one-third octave frequency bands (50-10000 Hz).

[0040] Fig. 2 shows the exhaust system of the ROTAX-912 engine in its standard configuration, when the exhaust gases from all cylinders are directed into one exhaust noise muffler.

[0041] Fig. 3 shows a modified exhaust system of the ROTAX-912 engine, where the exhaust is organized for the cylinders in pairs.

[0042] Fig. 4 shows the calculated one-third octave spectra of sound power levels (L W , dB) of the ROTAX-912 engine for cases of organizing the exhaust from four cylinders into one common muffler and organizing the exhaust for the cylinders in pairs into two mufflers.

[0043] Implementation of the invention

[0044] To implement the claimed invention, the following sequence of actions is proposed.

[0045] An analysis of the aircraft design is performed to determine the feasibility of distributing the gas flow in the exhaust system of a multi-cylinder, four-stroke piston aircraft engine so that exhaust gases from each cylinder are directed into exhaust manifolds that combine as few cylinders as possible. This approach achieves the lowest possible fundamental frequency for a multi-cylinder engine.

[0046] Based on an empirical method for assessing aircraft engine noise, sound power level spectra are determined for a multi-cylinder engine with different gas flow distribution methods within the engine's exhaust system. The prototype engine used in the calculations is a configuration in which the exhaust gases from each cylinder enter a common exhaust manifold. This configuration is the noisiest in an ICE exhaust system.

[0047] Based on the calculated spectra of sound power levels, the total sound power level, weighted according to the A scale of a standard sound level meter, is determined in the frequency range (50-10000 Hz) standardized for noise in the area.

[0048] Then the effects of reducing the total sound power level of the internal combustion engine in the dBA metric are determined for different variants of gas flow distribution in the exhaust system relative to the prototype engine.

[0049] They select a less noisy option for distributing gas flow in the exhaust system of an aircraft multi-cylinder four-stroke piston engine.

[0050] Example

[0051] As an example, let's consider a four-stroke, four-cylinder ROTAX-912 aircraft engine with a displacement of 1.35 liters. The available power of the power plant at takeoff mode at a crankshaft speed of 5800 rpm is 73.5 kW.

[0052] In the standard configuration, the gas flow distribution in the exhaust system of the ROTAX-912 engine is organized in such a way that the exhaust gases from all cylinders are directed to one exhaust noise muffler as shown in Fig. 2.

[0053] In the modified configuration, the gas flow distribution in the ROTAX-912 engine's exhaust system is organized so that exhaust gases from cylinders connected in pairs by exhaust pipes are directed to exhaust mufflers. This means that each two cylinders operate on a separate exhaust system.

[0054] The calculated one-third octave spectra of the sound power levels of the ROTAX-912 engine according to the empirical method for the cases of organizing the exhaust from four cylinders into one common muffler (standard configuration) and organizing the exhaust for the cylinders in pairs into two mufflers (modified configuration) are shown in Fig. 4. The frequency of the fundamental tone of the engine in the standard configuration of the exhaust system is 200 Hz in the one-third octave frequency bands, and the frequency of the fundamental tone of the engine in the modified configuration is 100 Hz.

[0055] The total A-weighted sound power level of a standard sound level meter in the noise-rated frequency range (50-10,000 Hz) is 2.6 dBA lower for the engine with the modified exhaust system configuration. This is a significant reduction in engine sound power, indicating a nearly 50% reduction in engine noise.

[0056] Thus, the proposed method for distributing the gas flow in the exhaust system of an aircraft multi-cylinder four-stroke piston engine with a reduced noise level makes it possible, through the rational organization of the exhaust system, to reduce the noise level in the area of ​​the multi-cylinder internal combustion engine and, as a consequence, the aircraft on which it is installed, without increasing the weight and dimensions.

[0057] List of references

[0058] 1. Rudoy B.P., Vakhit Yu.R. Method for suppressing exhaust noise of a four-stroke internal combustion engine and device for its implementation / / Patent for invention RU 2013575 C1, 30.05.1994. Application No. 5038818 / 06 dated 05.11.1991.

[0059] 2. Bazhenov S.A. Method for reducing exhaust noise of an internal combustion engine / / Patent for invention RU 2386040 C2, 10.04.2010. Application No. 2008110654 / 06 dated 19.03.2008.

[0060] 3. Vasiliev A.V., Mokrinsky A.V. Active noise suppression system for intake and exhaust of an internal combustion engine / / Patent for invention RU 2240427 C2, 20.11.2004. Application No. 2002107739 / 06 dated 26.03.2002.

[0061] 4. Antipin V.M., Antipina E.S., Zhilin S.N., Zhilina I.S. Method of distributing gas flow in a noise muffler system / / Patent for invention RU 2386822 C1, 20.04.2010. Application No. 2008140997 / 06 dated 15.10.2008.

[0062] 5. Environmental Protection: Annex 16 to the Convention on International Civil Aviation. ICAO, Montreal. Vol. 1. Aircraft Noise. 2017. 228 p.

[0063] 6. GOST 17187-2010 (IEC 61672-1:2002) Sound level meters. Part 1. Technical requirements. - M.: Standartinform. 2012. - 32 p.

[0064] 7. Moshkov P. Study of an aviation opposed piston engine noise / / Aerospace Systems. 2022. Vol.5. No. 4. P. 607-613.

[0065] 8. Moshkov P. Study of aircraft radial piston engines noise / / Journal of Vibroengineering. 2023. Vol. 25.No. 3. P. 606-614.

[0066] 9. Moshkov P.A. Empirical method for predicting the noise of aircraft piston engines / / Bulletin of the Samara State Aerospace University named after Academician S.P. Korolev (National Research University). 2016. Vol. 15. No. 2. P. 152-161.

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