Abstract

The research presented here is the comparison studies between different commercially available techniques for measurement of soot and particulate matter (PM) emissions from passenger car diesel engine. The compared devices are filter paper-type smoke meter, photoacoustic spectrometer, opacimeter, differential mobility spectrometer and laser-induced incandescence. The focus is to study static and dynamic transient exhaust emissions from the location position closer to the actual combustion event—downstream of the turbine, position characterised by the higher temperature and higher pressure of the emission gas—than the standard measurement position, in the tailpipe of the exhaust manifold. The main task is to compare an accuracy and sensitivity of individual devices at static and dynamic soot and PM emissions.

Keywords: soot emission, particulate matter, diesel engine, smoke meter, photoacoustic spectroscopy, opacimeter, differential mobility spectrometer, laser-induced incandescence

## 1. Introduction

The combustion engine-driven vehicles are one of the main consumers of petroleum natural resources and are mainly responsible for air pollution in the heavily traffic regions, in the cities and in large metropolitan areas [1, 2]. To minimise the pollutant emission from vehicles, an effective combustion control and monitoring plays a very important role [3]. Measurements of diesel emissions are usually performed by using various measurement techniques [4]. The limits for exhaust emission of newly produced vehicles are regulated in the European Union (EU) by European emission standards and are defined in a series of European Union directives. These regulations are usually amended every half of the decade and are published in a form of standards like first Euro 1 norm set in year 1992 to current Euro 6 valid from year 2014. In the case of passenger cars, the emission standards are defined for particulate matter (PM), total hydrocarbon, non-methane hydrocarbon (NMHC), carbon monoxide and nitrogen oxide emissions. The basic idea of these norms is to contribute to the reduction of emissions by newly produced vehicles. However, with the strict limits given by the European regulation on pollutant emission, measurement of low-level emission concentrations from the engine is very

challenging. This is mainly due to measurements of such as low concentrations and/or very short time duration peak emissions. Thus, the sensitivity of method and applied measurement techniques represent an important issue [5, 6]. Another important factor for minimising overall emissions is a high temporal resolution of the measurement device, mainly during the measurement of fast transient emission peaks. These are the emissions produced due to the rapid acceleration or deceleration phase of the vehicle. Nowadays there exist many different commercially available techniques for PM and soot concentration emission measurement, based on gravimetric analyses [7], paper filter-type smoke meter [8], measurement of continuous opacity [9, 10], differential mobility spectrometers [11–13], measurement of photoacoustic spectroscopy [14–16] and measurement of laser-induced incandescence (LII) [17–20].

The main differences between these techniques are in methodology and in measurement principle. Another issue is the sensitivity of the measurement itself, sampling time and dynamics response to fast emission detection. Additional criterion is the position—location of sensor during the measurement of emissions from engine. The optimal position of the sensor with respect to the measurement of the fast dynamic response would be as close as possible to the emission source, directly into the combustion chamber of the engine. The second alternative would be upstream of the turbine and the third possibility is downstream of the turbine in the exhaust manifold. The first position—directly into the combustion chamber—is due to the geometrical restrictions that are difficult to measure, and therefore the nearest possible location is in the exhaust manifold. However, due to high instantaneous pressure and temperatures of soot in this part, the measurements are usually performed far from exhaust manifold or directly in the tailpipe. The placement of the sensors into the tailpipe is introducing additional delay in measured emission signal and can negatively influence measured results. A next drawback of tailpipe position is the change in gas emission dynamics due to measurement in lowerpressure zone; however, high-pressure information can be very helpful in order to better characterise, control and minimise emissions during combustion process.

Up to now, the systematic studies dedicated to the differences in sensitivity and accuracy of transient PM emission by comparing available commercial devices were made by Viskup et al. in [21, 22]. The accuracy and reliability of measured emission data are very important information to eliminate toxic emissions from vehicle engines and to meet the future EU directives.

In this chapter, the comparison between different techniques for measurement of soot and particulate matter static as well as dynamic transient emissions is shown. The analysis presents differences in measured emission by individual techniques from a position closer to the actual combustion event, downstream of the turbine in the exhaust manifold. Measurement at this position is influenced by higher temperatures and higher pressures of emission gases than standard tailpipe position. However, this will influence the possibility of using a particular device as well as techniques for such a non-standard measurement location. High concern is given to comparing the device's sensitivity and dynamics at static as well as fast transient emission peaks, because these produce a main fraction of total emissions during the standardised test cycles from passenger car diesel engines [23]. The obtained results will allow better understanding of PM emissions, support dynamic emission modelling for control design and contribute to the development of virtual emission sensors [24, 25].

#### 2. Instrumentation

Because this chapter is dedicated to the comparison between different commercial available measurement devices for the soot and particulate matter emission measurements, the basic measurement principle of each method is briefly discussed.

Comparison of Different Techniques for Measurement of Soot and Particulate Matter Emissions… DOI: http://dx.doi.org/10.5772/intechopen.91186

### 2.1 Basic principle of differential mobility spectrometer

The basic principle of differential mobility spectrometer (DMS) is in charging of PM particles formed during the combustion and classifying them on the basis of its mobility. Particles of PM, which enter into differential mobility spectrometer, are charged by means of corona discharge. Each particle is charged proportionally to its surface area. Charged particles are then introduced into a strong radial electrical field inside a classifier column. Positively charged particles are further drifting through a sheath flow in the direction of the numerous segmented collection electrodes. Particles are then attached to different distances of the segmented electrode rings down the classifier column, according to their drag charge ratio. The current produced by particles on every sensitive electrometer is used to determine particle size and number of concentration. Basic schema of DMS is shown in Figure 1.

#### 2.2 Basic principle of laser-induced incandescence

The laser-induced incandescence technique uses the laser radiation with short pulse duration and high power to irradiate particles formed during the combustion event. This process induces incandescence light emitted by the particulates, which is consecutively detected by fast photodiode, photomultiplier tube (PMT) or fast avalanche photodiode detector (APD) with nanosecond (ns) response time. The excitation of the soot particles by laser-induced incandescence is typically performed with pulsed laser radiation, e.g. Nd:YAG with pulse duration of 10 ns at fundamental laser wavelength 1024 nm with laser fluence 0.4 J/cm<sup>2</sup> or frequencydoubled 532 nm at 0.2 J/cm<sup>2</sup> [26]. The basic optical arrangement of LII setup is shown in Figure 2.

The LII scheme works as follows. The pulsed laser beam with Gaussian profile is converted into a homogeneous vertical sheet by using a combination of cylindrical and spherical lenses to achieve uniform intensity over spatial profile. Beam is then expanded and directed into the burner. Laser power meter and beam profiler monitors the emerging laser beam from the burner cell. Signal from LII emission is collected perpendicularly by the collecting lens and directed through attenuator into the beam splitter. Split signal is further filtered with interference filters and imaged into fast detector. The LII signal is sampled with nanosecond intervals with fast digital oscilloscope and further processed by the computer.

### 2.3 Basic principle of opacimeter

The basic principle of the opacimeter is measurement of attenuation of emitting light from the light source, which is transmitted through soot media and

Figure 1. Schema of differential mobility spectrometer.

#### Figure 2.

Optical arrangement of laser-induced incandescence.

consequently absorbed. A light transmitting-type opacimeter measures a soot concentration based on absorbed and scattered light. The measured density value is then the superposition of the black smoke (soot) due to high temperature fuel combustion, blue smoke hydrocarbon vapour and white smoke water vapour. The concentration of the soot is calculated from light attenuation using the Beer-Lambert law of light absorption. The measured results are then expressed as opacity (%) or in absorption coefficient in (m�<sup>1</sup> ). Basic schematic arrangement of the opacimeter is shown in Figure 3.

#### 2.4 Measurement of soot concentration from the opacity

Because the opacimeter measures the opacity in %, it is necessary to recalculate the opacity values into soot concentrations. For this reason, the steady-state measurement outputs of the opacimeter and the smoke meter empirical correlation function can be used as:

$$\text{Operity} = \text{1.67.FSN}^2 + \text{1.56.FSN} + \text{0.619} \tag{1}$$

Comparison of Different Techniques for Measurement of Soot and Particulate Matter Emissions… DOI: http://dx.doi.org/10.5772/intechopen.91186

which has been derived in [27], to obtain the filter smoke number (FSN) from opacity measurements. The soot concentration can be calculated from the empirical correlation equation [28].

$$\text{Soot}\left(\text{mg}/\text{m}^3\right) = \frac{1}{0.405} 4.95. \text{ FSN. } e^{(0.38. \text{FSN})} \tag{2}$$

By this empirical correlation method, it is possible to obtain the soot concentration values in (mg/m<sup>3</sup> ).

#### 2.5 Basic principle of paper filter-type smoke meter

The basic principle of the smoke meter is in measurement of exhaust gas emissions sucked through a filter paper. The blackening of filter paper is measured with reflectometer and indicates the soot content in the exhaust gas. Smoke collected by the filter and the blackening of this filter depend primary on soot concentration and the effective filter length—exhaust gas volume related to the filter area. The measuring principle of the smoke meter is shown in Figure 4.

Rolled filter paper is fed through exhaust gas chamber where the soot particles are adsorbed at the surface. Afterwards the filter paper is further moved in the direction of the measurement section. Here the blackening of filter paper is measured with reflectometer and further directed out from this chamber. Used filter paper is rolled on the spool.

#### 2.6 Basic principle of photoacoustic spectroscopy

The basic mechanism of photoacoustic spectroscopy is in measurement of acoustic response after absorption of modulated laser radiation by the PM. The laser radiation is absorbed by the PM and leads to heating and consequently to thermal expansion of the particles accompanied by acoustic waves in surrounding gas of the photoacoustic cell. Local expansion is modulated with the frequency of the light source. Generated sound waves resulting from the modulation of the light are

Figure 4. Measuring principle of paper filter-type smoke meter.

proportional to the absorbed laser energy. Sound waves are recorded by means of sensitive microphone built in the resonator. Intensity of the acoustic waves is proportional to the photoacoustic signal. Acoustic resonator acts as a longitudinal resonator for the amplification of the photoacoustic signal. Speaker is used to determine the exact resonance frequency and for controlling of microphone. Schematic arrangement of photoacoustic spectrometer is shown in Figure 5.
