Engine acoustics

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Low reflection engine test bench for investigations into engine acoustics with microphone positioning system and artificial head ( FKFS , Stuttgart)

The engine acoustics deals with all of internal combustion engines and their attachments radiated noise ( engine noise ).

Composition of the engine noise

The noise emitted by the internal combustion engine includes the sound emitted by the engine surface, the intake and exhaust system and the auxiliary units. In today's motor vehicles , combustion engines are used almost exclusively (in addition to a small number of electric drives) as drive units. It is in used in cars and trucks reciprocating engines mostly four-stroke - gasoline - or diesel engines . In motorcycles , particularly scooters and mopeds , also found two-stroke engines use.

The noise from internal combustion engines is made up of several components that can be traced back to various excitation mechanisms. A weighting of the individual mechanisms is crucial depending on the method of combustion (petrol or diesel engine), the mechanical construction (4- or 6-cylinder engine, rows - or V-engine ...) and of the operating conditions. In diesel engines, for example, combustion noises are usually the dominant source of noise, while in Otto engines, mechanical noises usually shape the engine's sound field .

In general machine acoustics, a distinction is made between "directly" and "indirectly" generated airborne noise. In the internal combustion engine, direct airborne sound excitation occurs, for example, through the intake or exhaust process. Gas exchange processes generate flow pulsations in the intake or exhaust system, which are mainly emitted from the mouths without any further energy conversion. Further causes of directly generated airborne noise in internal combustion engines can be the formation of eddies on cooling fans or excitations from toothed or V-belts .

In the case of indirect airborne noise, alternating and impact forces within the machine cause vibrations in the entire engine structure, which are referred to as structure-borne noise . This reaches the engine surface via various transmission paths and is transmitted from there either as structure-borne noise via attachment points to other components such as B. transmission , exhaust system and body are transmitted or emitted directly in the form of airborne sound, which is also referred to as "structure-borne sound-induced airborne sound radiation".

The indirect airborne noise is divided into the combustion noise and the mechanical noise. The combustion noise is caused by the effect of the gas pressure and is caused by the pressure increase in the combustion chamber during combustion. The engine parts delimiting the combustion chamber ( cylinder head and cylinder tube) and the piston-connecting rod-crankshaft bearing system are stimulated to vibrations by the periodic pressure fluctuations in the combustion chamber, which are transmitted in the engine structure to the surface and emitted as airborne sound.

Mechanical noise includes all those noise components that are excited by impact processes within the engine structure. Unevenly moving masses, such as in the crank mechanism and valve train, or rotating masses that are not completely balanced, cause alternating forces due to their acceleration, which are introduced into the engine structure.

Sources of noise in the combustion engine

combustion

The diesel engine is a leader in the commercial vehicle sector and has gained so much importance in passenger car use thanks to construction that is optimized for consumption and improved comfort that it is now predominant in new registrations there too. In the diesel engine, a distinction is made between the chamber ( prechamber injection , swirl chamber injection ) and the direct injection process (DI). In the chamber processes, which are also referred to as IDI, the diesel fuel is injected into an adjoining space (prechamber or swirl chamber), which is housed in the cylinder head. The combustion in these processes is softer and, compared to DI, also quieter, which is why chamber processes have so far been the preferred choice for passenger car diesel engines.

With direct injection, the fuel is injected directly into the combustion chamber under high pressure (up to 1200 bar or more). The injected fuel ignites in the hot air and burns. The specific fuel consumption is even lower than that of the chamber engines. For this, however, a hard engine run and higher noise emissions have to be accepted.

If one compares the noise development of engines with different combustion processes, there are significant increases in level from gasoline engines to chamber engines and from the latter to direct injection diesel engines.

piston

The piston noise, which is caused by structure-borne noise excitation by the piston , can make up a significant proportion of the overall noise in both diesel and gasoline engines. In addition to mass and gas forces in the cylinder axis direction, forces in the transverse direction caused by the connecting rod inclination also act on the piston. The necessary running clearance between the piston and cylinder enables the piston, due to these transverse forces, to perform small movements across the direction of travel in addition to its stroke movement. This component in the transverse direction of the engine is known as the transverse piston movement or the secondary piston movement. When the piston hits the cylinder wall, structure-borne noise excites the engine structure and the piston itself. Depending on the operating point, there is a different number of piston contact changes per working cycle.

crankshaft

The crankshaft and its bearing in the engine block are another major source of engine noise. Studies have shown that on the one hand structure-borne noise from the piston (stimulated by combustion) is transmitted via the crankshaft and its main bearings via the connecting rod into the engine block but also a noise excitation by impacts in the connecting rod and especially in the main bearings (in the motor housing). For operational reasons, the crankshaft is supported with play in the engine block. Under the influence of gas and inertia forces, this game is run through and when the system is changed, noise-causing impact processes are caused.

One way of reducing the structure-borne noise excitation is to make the crankshaft flexurally and torsionally rigid. The use of a torsion damper , which is mounted on the front crankshaft stub, also leads to a reduction in the crankshaft vibration in a certain frequency range and thus to a lower level of vibration excitation and noise emission.

Valve control

In four-stroke gasoline engines, a large part of the vibrations is caused by the valve control . With diesel engines, these noises hardly play a role due to the significantly larger combustion noises.

The valves are operated by camshafts. The camshafts are in turn driven by the crankshaft via the camshaft drive. Both the valve train and the camshaft drive cause vibrations that cause a characteristic sound.

The valve train includes the valves and all of the components that operate them. Depending on the design, these are usually mechanical, rarely pneumatic valve springs, the aforementioned camshafts and components such as tappets, bumpers, rocker arms, rocker arms or bucket tappets that transfer the forces from the cams to the valves. Finally, the drive of the camshafts also stimulates vibrations.

Camshafts can be driven in different ways (with decreasing frequency):

This drive runs comparatively quietly, as all of the timing chain runs that are not under tension are usually guided over rails. In the case of inverted tooth chains, this is usually not necessary if they are used to drive the camshafts below and are quite short.
This drive runs very quietly, as the plastics used have a high level of self-damping and a very high natural frequency due to their low mass.
The spur gear drive is significantly louder because of the backlash of the teeth and, in the case of straight teeth, also because of the noise caused by the meshing of the teeth. With overhead camshafts, a cascade of several gears is used. In order to reduce the noise, the large or an intermediate wheel in car engines was often made of fiber-reinforced plastic (“ Novotex ”).
The vertical shaft drive runs comparatively loudly with straight-toothed bevel gears, relatively quietly with helical gears or hypoid gears on the two deflections. The vertical shaft drive is structurally complex, but speed-stable. It was previously used in racing and aircraft engines, but in the 1930s and 40s it was also used in engines for motorcycles (Nimbus) and cars (Morris, Fiat, Crosley). Most recently, the motorcycle manufacturer Ducati installed it in its force-controlled motorcycle engines.

The noise of drive chains is mainly caused by the non-constant speed of the chain, the polygon effect. The polygon effect is more pronounced the fewer teeth the sprockets have. Due to the changing speed, the force in the chain drive is superimposed by an alternating component which is determined by the frequency of engagement of the chain links in the teeth. It can cause the structure in which the chain wheels are mounted to vibrate. The use of toothed belts has advantages for acoustic reasons. But they too can generate a tonal noise when there is resonance with the tooth mesh frequency. The acoustically least favorable are spur gear drives.

Furthermore, shocks and inertia forces when the valves are operated are responsible for the excitation of noise in the valve train. After passing through the valve clearance when opening the camshaft bearings and the bearings of the will tilt - or drag lever shock-like load, the cylinder head structure is excited to vibrations. During the phases of high acceleration, high inertia forces act on the camshafts and on the rocker arm or rocker arm bearings. When closing, the valve also springs in its seat. The use of hydraulic valve clearance compensation is acoustically very effective here.

Charge exchange

Because of the intermittent operation of reciprocating engines, muzzle noises occur in intake and exhaust systems. The sound of the intake and exhaust systems is referred to as gas exchange noise. Without the exhaust silencer, the muzzle noise would be the loudest partial sound source of the engine. With the silencers customary today, equipped with sufficient volume and technically complex, the structure-borne noise emitted from the entire surface dominates.

The exhaust system also offers the best opportunities to influence the so-called "sound" of an engine. The engine acousticians can e.g. B. assign a quiet hum to a luxury vehicle and a powerful roar to a sports car.

Cooling fan

The noise of the fan for engine cooling plays a significant role , especially in engines with high power and large displacement , which require correspondingly large cooling powers , and can cause levels in the order of magnitude of the engine noise. In the passenger car sector, thermostatically controlled electric fans are increasingly being used, which can continue to run when the engine is hot even when the vehicle is switched off and can cause noises.

The noise from fans consists of a broadband noise component and superimposed tones. The broadband noise, also known as eddy or rotating noise, is caused by vortex shedding and turbulence in the inflow of the fan. The rotating sound - characterized by single tones - is caused by periodic alternating forces that arise from the interaction of fixed and rotating structural parts of the fan and the associated periodically fluctuating flow to these parts.

The noise of the cooler flowing through depends on the flow resistance, i. H. depends on the air flow in the cooler and the flow conditions in front of the cooler. It is mainly characterized by vortex shedding, which acoustically has an impact as noise.

alternator

Since the electrics and electronics in motor vehicles are constantly expanding, there is a demand for more and more powerful alternators . With the increasing electrical power, the noise development also increased. The noise of today's generators can be of the same order of magnitude as the rest of the noise of a gasoline engine and it is often noticeable because of its tonal character (rotary sound, siren sound). Here, too, the cooling fan is an important source of noise.

Another cause of noise from alternators is the electromagnetic alternating forces that generate structure-borne noise in the rotor and stator. The structure-borne noise is emitted from the generator itself or from the components connected to it as airborne noise.

Diesel injection pump

Diesel engines require an injection pump to operate in order to inject fuel under high pressure into the combustion chamber or the antechamber or swirl chamber. After success in the targeted noise reduction in various functional groups of the engine, the injection pump can also represent an important source of noise in the diesel engine. This applies to both distributor injection pumps and in-line injection pumps.

Pressure pulses stimulate the pump housing to vibrate through structure-borne noise. The most important natural vibrations for noise emission are in the frequency range between approx. 300 Hz and 5 kHz. There is an interaction between the injection pump and the motor, to which the housing is attached to the structure-borne sound. Structure-borne sound emitted by the motor is transferred to the pump housing and structure-borne sound generated by the pump is transferred to the motor surface, where it is emitted as air-borne sound.

literature

  • Helmut Tschöke: Engine and aggregate acoustics, proceedings of the conference No. W-H030-06-172-5 of the House of Technology, Essen from June 15 to 16, 2005 in Magdeburg.
  • Martin Helfer: For the excitation and propagation of the noise excited by the piston , Stuttgart: Institute for Internal Combustion Engines and Motor Vehicles, 1994, ISBN 3-924860-21-1 .

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