The present invention relates to a method and apparatus for the active control of noise specifically but not exclusively in exhaust systems.

Conventional exhaust systems consist of a length of pipe with silencers connected to an engine. The effect is to dampen the sound and vibration produced by the exhaust gases.

As there is no perfect exhaust system there is a requirement to improve the level of sound or vibration reduction which can be achieved.

Consequently the present invention seeks to reduce the sound and vibration in an exhaust system by a method of active noise control.

Accordingly there is provided a method for reducing the noise produced by an engine's exhaust system which comprises the introduction of extra fluid to the exhaust system in addition to that produced by the exhaust output of the engine.

The term exhaust system is applicable to any hot gas flow where noise is present.

It is thought that by the addition of a fluid such as air or water into an exhaust system that the sound and vibration which this produces is in antiphase to the originating exhaust sound and vibration. This results in a reduction or cancelling of the sound and vibration. To achieve the optimum reduction in noise it is preferable- that the introduction of

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fluid into the system is suitably timed such that the noise source (exhaust) and noise produced by the fluid introduction are of different phase and ideally opposite. The total noise output of the system will actually be increased if the noise produced by fluid injection is in phase with the original exhaust noise.

To produce sound an effluent gas must contract or expand to create a volume velocity, that is, there must be a net volume change. Thus volume changes in the exhaust lead to noise.

By adding a fluid to the exhaust there is an initial increase in volume which may be offset by cooling or other effects which result in an overall decrease in volume. These volume changes can oppose those occurring naturally in the exhaust. The effects of volume increase and contraction cooling can be experienced with various fluids but addition of liquid such as water has the further effect of vaporisation thus resulting in a volume increase. The latent heat of vaporisation for a liquid introduced to the exhaust has the effect of cooling the pressure peaks. Therefore if cold water is added to the hot gases of an exhaust system the effects of heat extraction and vaporisation compete. In this case the cold water and subsequent vaporisation cool the gas causing a contraction while water vapour produced by the vaporisation causes expansion.

Preferably the fluid introduction takes place in the proximity of the outlet manifold of the exhaust.

Preferably the fluid introduced is a liquid. The boiling point of any liquid introduced should be lower than the temperature of the exhaust

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gases to ensure vaporisation of the liquid.

Preferably the fluid introduced is water.

Whilst there are various ways of introducing the fluid it is preferably injected into the exhaust system. In the case where turbo chargers or super chargers are fitted the fluid may be pressurized into the exhaust via a bleed from the turbo charger or super charger. Alternatively, fuel injectors can be used for fluid introduction to the exhaust system.

Preferably the fluid when a liquid is introduced into the exhaust system as a spray.

The effective operation of the invention's active noise control depends to some extent on adopting the appropriate procedure for a particular noise source. For example where there are large source strengths it is necessary to have low exhaust temperatures together with a fine spray of liquid, preferably water, injected into the exhaust to achieve good active noise control.

Alternatively for noise sources of high bandwidth, high Mach numbers for exhaust velocity and high exhaust temperatures should be used together with a fine spray of liquid, preferably water, and turbulent mixing of water and exhaust gases.

Bandwidth of the active noise reduction system can be increased by ensuring that forced convection occurs for the lifetime of the droplet. This may be achieved by disturbing the flow up and downstream of the

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injector or other entry system to the exhaust.

Furthermore sound level and hence active noise control may be increased by ensuring that the final temperature is less than 100°C. It is believed that this is because the latent heat has been partly extracted without vaporisation. Final temperature as used herein refers to that temperature at which there is no net change in injected fluid volume within the exhaust which yields noise.

According to a further aspect of the present invention there is provided an apparatus for achieving active noise control in exhaust systems which comprises at least one injector positioned for the injection of fluid directly into the exhaust system.

The invention is considered to have particular utility in reducing the noise levels in exhaust systems of heavy goods vehicles.

The invention will now be described by way of example only and with reference to the accompanying Drawing which shows a graph of volume change for water against initial temperature and where f\ is the volume change factor produced by injection of X moles of water (at 15°C) per mole of exhaust gas at temperature T _al .

For sound production it is necessary to have a volume change in a gas. Exhaust gases from an engine expand as they move from a high pressure area to a low pressure area. A sound absorbing system should thus be effective to produce volume contraction in the exhaust gas flow. When water is injected into a hot exhaust system there are four states

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of the isobaric process to consider. These are the initial state upon injection; the mixing, heat exchange and expansion; vaporisation; and equilibrium where the final temperature is greater than 100°C.

State

1. n _a moles of hot gas, volume V _al , Gas H ₂ 0 liquid Temperature T _al , and V _al n _L , T _L1 n _L moles of water at T _L1 T _al , n _a

2. Allow mixing, heat exchange Gas: V _a2 , T _a2 and expansion H ₂ 0: T _L2 = 100°C

3. Vaporisation Gas: V ^a o,, Taj,

H ₂ 0 vapour:

-v2 100°C, n = n,

4. Equilibrium at T _f > 100°C Gas + H ₂ 0:

V _f , T _F , n = n _a + n _L

The heat required to vaporise n _χ moles of water under constant pressure equals the enthalpy extracted from the exhaust gas (states 1 and 3) :

^H _v = = -c _pa n _a (T _a3 -T _al ) where c _L = the latent heat of vaporisation of water c _pL = specific heat capacity of water at constant pressure c _pa = specific heat capacity of gas at constant pressure

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Rearrangement gives

where X = n _L /n _a is the ratio of water to gas in the mix.

For the change from vaporisation to equilibrium, (state 3 to state 4) assuming perfect gas behaviour (valid for small X) then if there is no heat exchange with the outside environment dH = 0 and H ₃ = H _f , that is:

^C p ⁿ a ( ^T a3- ^T 0> ⁺ C _p v ⁿ v ( ^T v ₃ -T ₀ ) = ^W

where T ₀ = a reference temperature

For mixtures of perfect gases some thermodynamic properties of the mixture are just the weighted sum of the corresponding properties of the constituents (Gibbs-Dalton Law). This is applied here:

cpn = f cp PiJ.n1.

Applied above and rearranged gives:

where R = (1/X) (c _pa /c _pv )

Finally, for a perfect gas the fractional volume change from the temperature change is:

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The water must remain as vapour in the final state for this expression to be valid. A petrol engine exhaust contains typically 75% N ₂ , 10# C0 ₂ and 1~% H ₂ 0 assuming stoichometric combustion. The specific heat capacity of the exhaust gas will be dominated by that of Nitrogen. By taking the initial water as T _L1 = 15°C and molar heat capacities c _pa = 34 Jmol ^"1K"1 , c _pv = 36 Jmol ^'1 * ^"1 , c _pL = 75-4 Jmol-" ^"1 , c _L = 40600 Jmol ^"1 (these values applying at atmospheric pressure and at the relevant temperatures), gives a set of curves of as a function of X and initial exhaust temperature T _al , as shown in figure 1. The figure shows that, for water at least, there is always a net contraction in volume.

Where the invention has been applied to a 60 kw diesel generator by the injection of water into the exhaust system using a relatively simple arrangement an acoustic source was produced of comparable strength to the engine at frequencies below 2~ Hz.

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