GB2404979A - Testing diesel engines - Google Patents

Abstract

The present invention is a device for testing exhaust gases from a diesel engine, comprising a monitor 18, arranged to accept a sample of the exhaust gases, to monitor at least one property of the sample (e.g. level of smoke) and to output an electrical signal indicative of that property. An acoustic sensor 50 (e.g. a microphone) is arranged to detect an acoustic signal and to output an electrical signal which has at least one data component associated with the speed of the engine being tested. A processor 26 receives the electrical signals from the acoustic sensor and the monitor and compares the data component with a threshold value.

Description

TESTING DIESEL ENGINES

This invention relates to a device and method for testing diesel engines. In particular, but not exclusively, the invention relates to a device used to initiate a diesel smoke emission test during free acceleration of an engine being tested.

Background to the invention

Diesel engines require periodic testing to determine, for instance, the amount of smoke output from the engine in the exhaust gases. Such testing may be necessary to meet statutory environmental requirements.

A diesel smoke meter (DSM) comprises an opacity meter designed for statutory smoke testing. Such DSMS generally have to be approved or calibrated to a standard for the country in which they are to be used.

In the United Kingdom, this standard is currently the Vehicle Inspectorate specification MOT/05/01/01, which covers smoke meters to be used for testing heavy good vehicles (HGV's), light goods vehicles, domestic motor cars and other vehicles, and includes DSMs to be used for reduced pollution certification, road side enforcement and by Vehicle Inspectorate testing . . . alvlslons.

The Vehicle Inspectorate specification states

that the DSM should measure accurately the free acceleration smoke (FAS) output over the vehicle's whole speed range of an unloaded engine from idle to cut-off speed (by cut-off speed we mean the speed at which the engine is limited and which it can not exceed). Under such test, the engine is accelerated against its own inertia, within a specified range of operational and measuring requirements.

The DSM can be arranged to measure the opacity of the exhaust smoke in one of three ways. The exhaust can be measured by an in line apparatus (that is an apparatus placed between the engine and the exhaust - 2 - pipe outlet), as a free air plume at the exit of the exhaust pipe (so called end of line apparatus), or within a specially designed chamber which can be arranged to take full or partial flow of the exhaust gas.

Some of the basic requirements of a DSM include the sample chamber being designed so that the pressure of the exhaust gas in the chamber should not differ from atmospheric pressure by more than a defined limit. The design should maintain correct sampling and purge air pressures at all times, which would otherwise unduly affect the opacity measurement of smoke emissions. The measurement of opacity can be made by passing the smoke sample through a chamber of fixed optical length with a light source placed at one end of this optical length and a receiver at the other.

The measurement of smoke particles in the exhaust gases is determined using the principles of light absorption. Smoke particles within the engines exhaust scatter or absorb light emitted from the light source, thus reducing the light radiation received by the receiver. The light source can be an incandescent lamp with a colour temperature in the range of 2800k to 3250k. Alternatively a green LED with a spectral peak of 560nm (+/- lOnm) can be used. The receiver should have a photopic spectral response (that is similar to the response of the human eye) with a maximum response at roughly 560nm and a response of less than 4% of the maximum below 430nm and above I 680nm. The optical measuring system should take continuous samples of light received across the test chamber throughout the FAS test period. Data from the receiver can then be processed to produce smoke I emissions readings during the test period. These smoke emission readings should be accurate and match the readings from a standard reference meter.

The DSM should be arranged to produce accurate results without the need for precise or specific alignment of the sample chamber or an exhaust gas - 3 - probe within the exhaust tail pipe. The DSM should maintain a fixed optical path length irrespective of tail pipe size or shape. To achieve this, it is favourable to use the partial flow inline method of sampling the exhaust gases and smoke into a chamber with a fixed optical path between an optical transmitter and receiver.

The DSM should be checked for correlation of the results over a range of vehicle types against a reference (or calibration) meter. To achieve the necessary degree of correlation the receiver signal can be passed through a physical response corrective filter followed by a electrical response filter and finally a peak detector. The peak signal is recorded and presented as a linear percentage absorption figure for a light absorption coefficient unit, measured in units of ml.

The DSM must be able to recognise that an FAS test is in progress so that it can automatically start data processing from the smoke meter stored data. At present, the recognition of an FAS test in progress is achieved by monitoring the smoke level output from the engine as soon as the smoke level exceeds a predetermined threshold level the DSM processor validates the stored data and proceeds with processing of said data to produce an opacity result. Such a method assumes that a discernable change in light absorption takes place when the FAS test commences.

However, as vehicle smoke emissions improve to comply with more stringent environment regulations, modern vehicles produce increasingly less smoke, particularly under acceleration of the engine. As a result, it has become increasingly difficult for smoke meters to reliably detect when an FAS test has been initiated.

Summary of the invention

Accordingly, the present invention provides a device for testing exhaust from a diesel engine, comprising: an acoustic sensor arranged to detect an acoustic signal and to output an electrical signal having at least one component associated with the speed of the engine being tested, and a processor arranged to receive the electrical signal from the acoustic sensor, and compare said signal with a threshold value.

A monitor might also be included in the device and any electrical signals from the monitor can be received by the processor for processing. The monitor is arranged to detect the smoke levels in the exhaust gas, for instance, or other properties of the sampled exhaust gas.

The acoustic signal can be associated with the changing flow of sampled exhaust gas which provides recognition of acceleration of the engine under test.

The monitor can be an optical device as described above which is used to measure the absorption or opacity of exhaust fume samples. However, other types of monitors can also used, such as devices which measure a change in refractive index or other properties of the sample. Non-optical monitors could also be used to measure a change in the sample, such as radioactive source detector arrangement, similar to those found in domestic smoke detectors. The acoustic sensor can be an electret microphone, but other types of transducer could be used, such as carbon, capacitor, crystal, glow discharge, magnetostriction, moving-coil or ribbon microphones, for instance. The at least one property monitored by the monitor can be the frequency of noise produced by the sample as it passes through the monitor or through a test chamber.

Other signals associated with the engine's speed or acceleration could also be used, such as diagnostic signals from the engine itself, or the volume of noise produced by the sample passing through the monitor,

for example.

Advantageously, the device stores the data from the acoustic sensor. The data can be used to ascertain a property of the exhaust gases when the acoustic sensor detects that the speed of the engine has exceeded a predetermined threshold value. Such a property includes the level of smoke emitted from a diesel engine. Detecting the engine speed is generally a more reliant arrangement for initiating data processesing from a smoke opacity meter. Thus, more reliable and consistent results can be obtained using the machine according to the present invention.

Advantageously, the sensor can be arranged to determine remotely whether the engine is accelerating.

This allows for the operator to use the machine without having to gain access to the engine or other peripheral engine equipment (such as the alternator or the fuel pump) which might be used to determine engine acceleration. Disposing the sensor in test chamber or monitor unit provides a compact and portable system which requires little time to set up prior to testing an engine.

Advantageously, the sensor can be a microphone arranged to detect noise signals primarily associated with the flow of exhaust gases through the monitor or test chamber. It has been found that the noise emitted from the flow of exhaust gases in the test chamber or monitor correlates to the acceleration of the engine. Preferably, the acceleration or speed acceleration of the engine is determined by the frequency of the noise. Thus, a change of frequency indicates that the engine is in a state of acceleration.

The present invention also provides a method for testing one or more properties of exhaust gases emitted from a diesel engine, comprising the step of: detecting acoustically a parameter of a sampled portion of exhaust gases to output an electrical signal associated with the speed of the engine being tested.

Advantageously, the parameter detected acoustically is associated with the engine speed, or engine acceleration state, which allows the system to more reliably detect automatically the start of an FAS test, even if the smoke content of the exhaust gases is relatively low.

Furthermore, the acoustic signal detected can be the noise associated with the sampled portion as it passes through a test chamber or monitoring system.

This has the advantage of providing a generally more reliable means for detecting the start of an FAS test.

The signal can be compared with a threshold value. An indication that a test is underway is provided when a comparison of signal and threshold indicates that the signal exceeds the threshold.

Advantageously, the threshold value can be determined prior to the step of comparing the acoustic data with the threshold. This allows the system to set a threshold value depending on the engine being testing. In other words, because the acoustic data is likely to vary between engine types, it is an advantage to set a threshold which is associated with the engine under test.

The device and method could also be used on other engines, such as petrol engines. At present, testing of a property of a petrol engine's exhaust as the engine accelerates from idle to full speed is not a statutory requirement for emission control (other than for diesel engines, of course). However, it may become desirable to conduct such tests, in which case, the present invention may also apply to testing of engines running on different fuel types. The claimed device or method could be used for such purposes, or to initiate any FAS test, or the like, on any type of engine.

Description of the figures and embodiments of the

invention An embodiment of the present invention is now described by way of example and with reference to the accompanying figures in which; Figure l is a schematic diagram of a machine embodying the present invention; Figure 2 is a schematic diagram of a test chamber embodying the present invention; Figure 3 is a schematic diagram of a data acquisition and processing circuit used in embodiments of the present invention; s Figure 4 is a flow diagram showing the process embodying the present invention.

Other methods of automatically detecting the instance when a free acceleration smoke test is being, or is about to be, conducted on a diesel engine (other than detecting a change in smoke level) have been postulated. These include detecting a change in engine speed, exhaust gas temperature, or exhaust gas pressure.

Measuring the engine speed of a diesel engine requires an analytical interface to be linked to a mechanism of the engine. These might include Piezoelectric sensors on the fuel lines to detect the high pressure pulse of fuel from the pump to an engine injector or an electrical analyser which can be connected to the alternator. Also, engine noise or engine block vibration can be analysed to provide information of an engine's speed.

These analysers are generally specialist in design and thus expensive. Also, they generally require access to the engine compartment before and during the test. This requires the person conducting the test to have a knowledge of the type of engine being tested so that the analyser is connected appropriately. The time spent insuring that the analyser is properly connected to the engine adds to the burden of the person testing that engine. Of course, some countries require compulsory engine speed measurement, in which case such analysers are necessary, although they may not be 100% reliable.

Monitoring exhaust gas temperature can be done either on entry or exit of a test chamber, or within the test chamber. This requires a sensor with a relatively quick response so that the data from the DSM is stored at the correct time (i.e. during the free acceleration smoke test). However, in most - 8 - cases, the test chamber should be heated to avoid lowering exhaust gas temperatures (thus avoiding condensation within the chamber which would result in erroneous smoke levels being detected). Heating the chamber reduces the temperature differential between the ambient gas temperature in the test chamber and exhaust gas temperature from the engine. As a result, it is difficult to obtain discernable, repeatable and consistent results from such an arrangement.

We have found that the changes during the acceleration period in exhaust gas temperature are highly erratic, showing little or no discernable change of level which we believe is necessary for satisfactory detection of an FAS test. Also, we found that different exhaust and engine sizes have an extreme effect on the exhaust gas temperature parameter. As a result, we think this method could not consistently detect the difference between idle and engine full speed required to initiate data logging of an FAS test. Thus, recognition of the start of an FAS test could not in our opinion consistently be achieved using this method.

Quick response sensors are also necessary to measure changes in the exhaust gas pressure. We found that the change in exhaust gas pressure during a free acceleration test to be small and highly erratic, proving this to be an unreliable method for detecting the start of a FAS test cycle. As with the exhaust gas temperature test, we found that different exhaust and engine sizes make a great difference to the exhaust gas pressure. Also, the Vehicle Inspectorate specification for a diesel engine smoke meter requires that the test chamber maintains correct sampling and purge air pressures, which suggests that there is little or no back pressure to the free flow of the exhaust sample. This adds to the burden of measuring the exhaust gas pressure. As a result, we believe that this method is not reliable for detecting when an FAS test has started.

The present invention makes use of an alternative to the methods and devices discussed above for detecting or recognising the start of an FAS test. The present invention utilises the sampling of acoustic noise or associated vibrations within the test chamber during the FAS test.

Referring to Figure 1, a conventional diesel engine emissions tester 10 is shown in schematic form.

The emissions tester comprises a probe 12 which is shown inserted into an exhaust tailpipe 14 of an engine or vehicle (not shown) being tested. The probe rests within the tail pipe with the entrance of the probe inserted a predetermined length into the tailpipe. The fixing clamp 16 on the probe 12 helps determine the length by which the probe should be inserted and acts to prevent the probe from being detached from the tailpipe 14 during test.

The probe is connected to a sample chamber 18 by a hose 20. The chamber comprises a sample input 22 and a sample output 24. The sample chamber 18 comprises a diesel smoke meter (not shown in Figure 1) and means for connecting the smoke meter to an appropriate data processing unit 26. The data processing unit comprises a display screen and keypad which facilitates user input to the device. A printer can be incorporated into the processor so that the test results can be printed off to provide a paper copy of the test results. Preferably, the sample chamber 18 and data processor 26 are linked by a suitable wire connection 28.

Referring to Figure 2 a schematic diagram of the sample chamber 18 is shown. The chamber comprises an inner detection region 32, a light source 34 at one end of the sample chamber and a light detector 36 at the other end of the sample chamber. The light source and detector act to meter the smoke content of the exhaust from the engine in a conventional manner. The light source and detector have a line of sight indicated by broken line 38, which passes through the centre of the inner sample chamber 32. The exhaust gases from the engine enter the sample chamber 18 at - 10 sample input 22. The sample input is linked directly to the inner sample chamber 32. Thus, the gases flow directly from the input to the inner sample chamber.

The inner sample chamber is generally T shaped, with the sample input being the leg of the T and the inner sample chamber being the bar of the T. The gases flow into the inner sample chamber and are diverted into two directions to flow along the inner sample chamber away from the sample input. This arrangement ensures that the light emitted by the light source 34 passes through a defined fixed optical path length of exhaust sample. It is appreciated that the amount of absorption and scatter of the radiation from the light source is associated with the interaction optical path length along which the light passes through the exhaust gas sample in the inner sample chamber.

The sample chamber 18 is designed to allow a free flow of exhaust gas through the chamber without causing any undue back pressure at the input 22. To achieve this, the sample exiting the inner sample chamber 32 flows around into the sample chamber volume and eventually the sample leaves the sample chamber 18 via the exit port 24. The chamber and exit port should be designed to minimise restrictions of gas flow from the sample input to the exit port.

A microphone 50 is disposed in the sample chamber to detect the acoustic properties of gas passing through the chamber. Our studies have shown that there is a change in sound pressure (signal amplitude) and acoustic property (signal frequency content) during the engine's acceleration period. Peak frequencies are dominant at various stages of engine acceleration. There is an increase of these peak frequencies as the engine gains speed. We have found that the peak frequencies are centred at 30Hz during engine idle speeds (typically 800rpm). As the engine accelerates to full speed, the peak frequencies range from 150Hz to 600Hz Typical full engine speed can range from 2500rpm to 5000rpm, depending on the rev limiter on the engines fuel pump, and the engine under - 11 test.

There are variations in the frequencies of the noise produced by the exhaust gases with the entrained smoke particles passing through the sample chamber.

These variations can be associated with different engine capacities and the number of cylinders of the engine being tested. Surprisingly, other acoustic anomalies, such as amplitude peaks appearing at particular frequencies, also occur at distinct points during the acceleration. These anomalies are different from vehicle to vehicle. As a result, care should be taken in the choice of a band of frequencies to analyse.

We have conducted lengthy studies of the noise detected by the microphone in the test chamber and it has been found that for this particular application a band pass filter with a range of 150Hz to 600Hz produces a signal of characteristics which best determine, or describe, an engine accelerating from idle to full speed.

It should be noted that the microphone primarily detects the noise of the exhaust gases passing through the test chamber. It appears that noise signals within the chamber are not dominated by the noise of the engine being tested. Of course, a small amount of engine noise is picked up by the microphone, but this is relatively low compared to the noise of the exhaust gases passing through the chamber. The placement of the microphone in the sample chamber reduces the influence of unwanted signals (such as the noise of the engine as it is idling or being accelerated up for the test).

Placement of the microphone in the chamber requires the microphone to be suitable protected from the hostile environment of the chamber. Thus, the microphone should be robust and suited to the environment that it is placed in. The microphone can be disposed in a plastic protective housing. At one end of the housing an 8mm hole can be provided to allow sound to enter the housing unattenuated. Inside - 12 the hole a lOmm cube of low density foam can be provided which acts to prevent dust or exhaust particulates entering the housing. The microphone can be fixed in the housing by a rubber grommet which s helps to isolate the microphone from vibrations in the plastic housing. The microphone housing is heated indirectly by a heater arranged on the inner chamber.

The heater maintains the inner chamber at a temperature near to 70 C. Indirect heating of the microphone housing has been found to keep the microphone in an environment which during normal use exceeds 50 C. This temperature prevents condensation of moisture in the exhaust gases on the microphone component, or in the microphone housing. The housing and foam block may effect the propagation of sound from the chamber to the microphone. Thus, the material used to protect the microphone should be a relatively thin membrane of foam or film which does not unduly effect the sound waves with frequencies below lKHz by an excessive amount.

The types of microphone suitable for this application includes Piezoelectric crystals or electret capacitor microphones. The electret capsule offers a compact or low-cost solution and generally 2s has low output impedance. Thus, the electret microphone is well suited for this application and it's use avoids the need for sensitive high impedance electronic circuits to sample the signal from an electret microphone. Furthermore, the electret microphone comprises an inexpensive capacitance unit which is permanently electrified with opposite charges on its extremities. The unit can be biased with a small voltage, typically 1.5 Volts DC.

The signal from an electret microphone is 3s relatively large compared to other types of transducers. In this application, signals in the region of lOmV to lOOmV can be produced. A low impedance electric circuit can be used to process the signal from the microphone and to produce a suitably damped signal response, due to the low output impedance of the microphone.

Acoustic analysis of the sample chamber shows that it can produce high transient signals having a highly underdamped response. This characteristic has been detected in our studies and is similar to inconsistent peak frequency levels detected during acceleration which vary from vehicle to vehicle.

Thus, it is preferable to damp the microphone signal by passing it through a low-pass filter.

Referring to Figure 3, the signal for the microphone 50 passes through a low-pass filter 54.

The filter is arranged to pass signals having a frequency of 1.5 KHz, or less. The signal is amplified by a factor of ten by an amplifier 56. This level of gain has been found to be sufficient for this application, and takes account of the highest and lowest signal amplitude produced by various vehicles under test. The amplifier also incorporates an active filter having a band-pass response of approximately 150Hz to 600Hz. Such a band pass filter 55 is shown schematically in figure 3. A single pole roll-off band pass filter is suitable in this embodiment because the characteristics of the circuit and microphone roll- off contribute to the overall response to the circuit. The characteristics of the microphone contribute to roll off at low frequencies, where as the electronic filter and sound attenuation through the microphone's protective membrane contribute to high frequency roll off.

The signal is then converted from an AC signal by an RMS-to-dc converter 58. The converter has a particular averaging response suitable to track the pattern of change of the frequencies detected by the microphone during engine acceleration. An averaging capacitor C is provided which has a time constant suitable for damping the signal. This damping has the effect of reducing underdamped transient signals which can be associated with the noise of an accelerating engine's exhaust gases passing through the test chamber. - 14

The converted signal 59 is then passed to a processor unit 60 which comprises an 8-bit ac to dc converter disposed on an embedded processors. A separate A-to-D converter can be provided between the processor and the RMS to do converter if the processor does not have an embedded converter.

Of course, all the above processing, filtering, damping and conversion can be applied to the signal in software. However, in this embodiment, the processor typically has a limited processing power and so processing the signal using an electronic circuit is preferable. During a FAS test, the data from the microphone along with opacity and RPM signal, is stored in a data storage unit 62. From here, opacity data can be processed for determining the level of smoke present in the engine exhaust gases, once the FAS test has been detected. . The diesel smoke meter consists of a measuring head, which gathers data from the optical receiver and an embedded processor which provides the operational and measuring requirements of the smoke meter.

Data passed to the smoke meter's embedded processor can include the effective light absorption figures, signals from external equipment calculating the engine rpm and the acoustic level of gases passing through the test chamber for detecting acceleration of an engine. Of course, the engine speed data from external equipment may not be necessary in countries where this data is not required for certification of an engines exhaust emissions. This data is processed by software algorithms to provide confirmation that an FAS test is in progress and to process the smoke level result.

The dc signal 59 is sampled and continuously averaged by the processor unit 60 using an algorithm shown below RA(n)= R(n- 1)+ p5 - 15 where RA is the rolling average and PS is the present sample data recording from the microphone received by the processor. Each RA value is assigned an integer identifier n.

Referring to figure 4, a diagram showing a process 100 embodying the present invention is shown.

The test is started 110, and unprocessed data from all sensors ( acoustic, light absorption, RPM signal if available) is stored 111. A base reference level of the sound detected by the microphone in the test chamber is obtained 112 during a pre-acceleration time period of approximately 320ms, whilst the engine under test is idling, just prior to a cue given to the operator to proceed with the FAS test.

The base reference value is then multiplied 114 by a factor of three and compared 116 to a default minimum value 118. The higher of these two figures (i.e. the default minimum figure and the averaged result) 120 or 122 is used as the comparison value to trigger the FAS test data logging from the DSM.

The default minimum value 118 should be set to a level which is high enough to preclude the processor from initiating data processing during engine idle, but which is low enough to allow the processor to properly recognise that the engine under test is being accelerated for the FAS test. However, the DSM apparatus is likely to be used on different engine types (that is, domestic automobile, truck or HGV eta) which have different engine acoustic signatures. Thus, to avoid having to set adifferent default value for different types of engines and different engine configurations, it is preferable to compare the default value to the noise detected by the microphone prior to FAS testing of the engine (i.e. when the engine is running at idle speed). This can be achieved by multiplying the engine idle rolling average signal by a factor of three.

As discussed above, these two values (default value and engine idle rolling average value) are compared with one another and the highest of the - 16 - values is used to determine a threshold value for use in initiating the FAS test data logging sequence. As a result, the system is able to compensate for noisy environments and/or varying noise levels from different engines being tested.

After the cue has been given 124 to the operator to commence testing the engine, the rolling averages is continued to be calculated 126. The average is then continually compared 128 to the threshold value. When the rolling average exceeds the threshold, the processor determines that the FAS test is under-way and a cue 130 is given to record a trigger point (time and value) 132. The trigger point is used to determine the point in time when the engine was accelerated. Thus, data stored (from any or all of the sensors) after this trigger point is deemed to be associated with the FAS test. The process steps from 112 through to 128 is also carried out on the light absorption data and the RPM signal data (if available), with any one of the three resulting trigger points providing validation of the start of the FAS test from the logged or stored data 111. Put another way, the trigger point is used to determine a start point of the FAS test and so data from the smoke absorption meter is processed by the apparatus shown in figure 3 from that trigger point onwards.

The process 128 takes the rolling average and constantly compares it to the threshold value until the threshold is exceeded, at which point the trigger point is stored 132. . The processor calculates the opacity result for the FAS test by processing the logged light absorption data, once it has been determined by the processor that the FAS test time period is concluded, and that the stored trigger point can validate that the FAS test took place. Calculation of the test result is achieved using known methods and the "pass/fail" result can be printed onto paper.

Our tests have shown that, in almost every case, placing the sample probe outside the exhaust tailpipe - 17 - but attached to the tailpipe such that a small amount of vibration is transmitted to the test chamber, results in the microphone/processor unit not reaching or exceeding the default threshold value. By using a default value 118, false detection of an FAS test and exhaust gas sample can be largely avoided In this instance, because a relatively small amount of vibration passes through the chamber there is little noise generated in the chamber, and hence the detected noise is also relatively small. As a result, the rolling average has been found to hardly ever exceed the default threshold value.

It is in the best interests of equipment embodying the invention to be able to avoid deliberate false readings of the engine smoke emissions by placing the probe outside of the exhaust tailpipe.

In such a case, a greatly reduced or non-existent exhaust sample is passed into the DSM and the vehicle under test could be given a successful test result, when in fact it could otherwise fail had the probe been properly inserted into the exhaust tailpipe. Embodiments of the invention described above can overcome this by requiring that test validation and subsequent data processing is only initiated when the noise of gases passing through the chamber exceed a certain value. Ultimately, this requires a minimum amount of the exhaust gases to pass through the chamber.

The embodiments of the invention are described above with reference to the use of only the noise of gases passing the chamber being utilised to determine the start of a FAS test. However, it might be desirable to use other means in conjunction with those described above to initiate the data logging. In such a case, whichever of the test means exceeds an associated threshold first can be used to validate the stored light absorption data, and process it.

Alternatively, it might be useful to have all or a majority of means exceed their associated threshold before an FAS test is deemed to have commenced. Thus, a more reliable detection of FAS test start might be achieved. The alternative means might include detecting an increase in the level of smoke in the test chamber or direct detection of the engine speed using known systems.

Other embodiments of the present invention will be considered by the skilled which are within the scope of the claims. For example, the multiplication factor applied to the base reference value might be chosen to be a different value, depending on the type of engine being tested. 19

Claims (31)

1. A device for testing exhaust from a diesel engine, comprising: a monitor, arranged to accept a sample of said exhaust, to monitor at least one property of the sample, and to output an electrical signal indicative of said property, an acoustic sensor arranged to detect an acoustic signal and to output an electrical signal having at least one data component associated with the speed of the engine being tested, and a processor arranged to receive the electrical signals from the acoustic sensor and the monitor, and compare said data component with a threshold value.

2. A device according to claim 1, wherein the processor is arranged to store data from the monitor and process the data when the processor determines that data component from the acoustic sensor indicates the speed or acceleration of the engine exceeds said threshold value.

3. A device according to claim 1, wherein the sensor is arranged to detect remotely the acoustic signal associated with the speed of the engine, or the acceleration or the engine

4. A device according to claim 1 or 3, wherein the sensor is disposed in the monitor.

5. A device according to claim 1 or 3, wherein the monitor and acoustic sensor are disposed in a test chamber.

6. A device according to any of claims 1 to 5, wherein the acoustic sensor is arranged to detect noise signals primarily associated with the flow of the sampled portion through the monitor or test chamber. -

7. A device according to any preceding claim, wherein the acoustic sensor is an electret microphone.

8. A device according to claim 6, wherein the speed and/or acceleration of the engine is determinable by the frequency of the said noise.

9. A device according to claim 8, wherein the data processor comprises a frequency filter for filtering the signal received from the microphone.

10. A device according to claim 9, wherein the filter is arranged to only pass a signal below a predefined frequency.

11. A device according to claim 10, wherein the predetermined frequency is 1.5 kHz

12. A device according to claim 1, wherein the processor is arranged to determine the threshold value by comparing a predetermined default value with a factor of a value obtained whilst the engine runs at low speed.

13. A device according to claim 1, wherein the acoustic sensor comprises, a shield arranged to protect the sensor from environmental hazards and/or condensation of liquids from said sampled portion.

14. A device according to claim 13, wherein the shield is arranged to substantially transmit noise having a frequency below 1.5 kHz.

15. A method for testing one or more properties of exhaust gases emitted from a diesel engine, comprising the step of: detecting acoustically an acoustic characteristic of a sampled portion of exhaust gases to output an electrical signal associated with the speed of the - 21 engine being tested.

16. A method according to claim 15, further comprising: comparing said electrical signal with a threshold value and, determining a trigger point when the acoustic characteristic exceeds the threshold value.

17. A method according to claim 16, wherein the threshold value is determined prior to the step of comparing the acoustic characteristic with the threshold value.

IS 18. A method according to claim 16 or 17, wherein the threshold value is determined using the following steps: detecting a reference value associated with the speed of the engine running at idle speeds, comparing said reference value with a predetermined default value, and storing as the threshold value whichever of the said reference value or default value is greater.

19. A method according to claim 18, wherein said reference value is multiplied by a factor before the comparison step.

20. A method according to claim 17 or 18, further comprising the step of: providing a cue to begin an engine exhaust test after the threshold value is determined.

21. A method according to claim 16, further comprising the step of: averaging electrical signal to provide a rolling average, and comparing the rolling average data with the threshold value. - 22

22. A method according to any of claims 15 to 21, wherein the acoustic characteristic associated with engine speed is primarily associated with the flow of the sampled portion through a monitor or test chamber.

23. A method according to claim 22, wherein the acoustic characteristic is the frequency of noise associated with the sampled portion passing through said monitor or test chamber.

24. A method according to claim 23, wherein when acoustic characteristic is the frequency of noise detected, the method further comprises the step of: filtering said acoustic data to exclude frequencies above 1.5 kHz.

25. A method according to claim 23, wherein a processor determines that the engine is undergoing an acceleration from idle speed when the acoustic characteristic exceeds a predetermined value.

26. A method according to claim 25, wherein the predetermined value is between 150Hz and 600Hz.

27. A method according to any of claims 16 to 26, wherein the threshold value is used to determine a time at which a free acceleration test commenced on the engine.

28. Use of a microphone for determining an acoustic characteristic of an sampled exhaust gas flow associated with the engine's speed or acceleration, said microphone comprising, a housing and a transducer disposed therein, said housing having an opening arranged to receive acoustic signals therethrough, and said transducer being mounted in the housing via an acoustic isolating means, said microphone being arranged for use according to any of method claims 15 to 27.

- 23 -

29. A use according to claim 29, wherein said microphone further comprises a screen device arranged to substantially prevent exhaust particulates from entering the housing via the opening.

30. A device substantially as described herein, with reference to figures 1, 2, 3 or 4.

31. A method substantially as described herein, with reference to figures 1, 2, 3 or 4.