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Crank Angle Resolved HC Detection Using LIF in the Exhausts of Small Two-StrokeEngines Running at High Engine Speed

Andersson, Öivind; Juhlin, Greger; Ekenberg, Martin; Johansson, Bengt; Aldén, Marcus

Published in:SAE Transactions, Journal of Engines

Published: 1996-01-01

Link to publication

Citation for published version (APA):Andersson, Ö., Juhlin, G., Ekenberg, M., Johansson, B., & Aldén, M. (1996). Crank Angle Resolved HCDetection Using LIF in the Exhausts of Small Two-Stroke Engines Running at High Engine Speed. SAETransactions, Journal of Engines, 105(961927).

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961927

Crank Angle Resolved HC-Detection Using LIFin the Exhausts of Small Two-Stroke Engines

Running at High Engine Speed

Öivind Andersson, Greger Juhlin, Martin Ekenberg,Bengt Johansson, and Marcus Aldén

Lund Institute of Technology

Reprinted from: Diagnostics and Modeling in SI Engines(SP-1212)

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961927

Crank Angle Resolved HC-Detection Using LIF inthe Exhausts of Small Two-Stroke Engines

Running at High Engine Speed

Öivind Andersson, Greger Juhlin, Martin Ekenberg,Bengt Johansson, and Marcus Aldén

Lund Institute of Technology

Copyright 1996 Society of Automotive Engineers, Inc.

ABSTRACT

In order to separate the HC-emissions fromtwo-stroke engines into short-circuit losses andemissions due to incomplete combustion, LaserInduced Fluorescence (LIF) measurements wereperformed on the exhaust gases just outside theexhaust ports of two engines of different designs.

The difference between the two engines was thedesign of the transfer channels. One engine had"finger" transfer channels and one had "cup handle"transfer channels. Apart from that they were similar.The engine with "finger" transfer channels was earlierknown to give more short-circuiting losses than theother engine, and that behavior was confirmed bythese measurements.

Generally, the results show that the emission ofhydrocarbons has two peaks, one just after exhaustport opening and one late in the scavenging phase.

The spectral information shows differencesbetween the two peaks and it can be concluded thatthe latter peak is due to short-circuiting and the earlierdue to incomplete combustion.

The flow outside the exhaust port was measuredwith Laser Doppler Velocimetry (LDV). Thesemeasurements confirm the occurrence of twoemission peaks.

INTRODUCTION

The two-stroke spark-ignition engine is a verysimple and light engine concept which finds itsgreatest use as a portable power source where theseadvantages are important. However, one inherentweakness is that fresh fuel-air mixture short-circuitsthe cylinder directly to the exhaust system during thescavenging process. This constitutes a significant fuelconsumption penalty as well as excessive emissionsof unburned hydrocarbons (HC). HC-emissions fromcarbureted two-stroke engines are about five times as

high as those of equivalent four-stroke engines due toscavenging losses of fresh fuel-air mixture [1].

Due to environmental and fuel-economyconsiderations it is desirable to minimize thescavenging losses of fuel. The transfer channelgeometry is of great importance as to achieve thelongest possible path for the fuel to travel in thecylinder before it reaches the exhaust port. Properflow patterns for the fresh charge are extremelyimportant for good scavenging and charging of thecylinder [1]. To detect when during the scavengingprocess HC is emitted, some form of fast detector isnecessary. The conventional method forcycle-resolved measurements of HC emissions fromspark ignition engines is using a fast-response flameionization detector (Fast-FID). This detector is fastenough to resolve the engine cycle at low enginespeed but has a response time of more than 4 ms [2].This response time corresponds to 34 crank angledegrees (CAD) at 9000 rpm. As the working speed ofhandheld two-stroke engines is very high(9000-20000 rpm) this method can hardly be used toaccurately measure what happens during differentphases of the engine cycle.

Investigations of engine behavior have beencarried out with increasing success during the lastdecade using different laser techniques. Some of theadvantages of laser measuring techniques are theirability to offer high spatial and temporal resolution, incombination with a favorable non-intrusiveness. TheLaser-Induced Fluorescence (LIF) technique e.g.offers a great potential for measurements of speciesconcentrations without disturbing the gas flow aswould a probe.LIF is based on absorption of laser radiation by aspecies followed by spontaneous emission of light.The emitted spectrum is characteristic for thefluorescing molecule. If short laser pulses are used itis possible to make instantaneous concentrationmeasurements of emissions in the exhaust gasesduring the engine working cycle [3].

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The aim of the present experiment was toinvestigate the composition of exhaust gases fromsmall, carbureted two-stroke engines. Quantitativemeasurements were not attempted. To getquantitative results a thorough calibration proceduremust be made, taking quenching and temperatureeffects into account. The scavenging losses were tobe separated from other emissions by means ofcrank angle resolved LIF measurements just outsidethe exhaust port.

Two measuring strategies were followed. Tovisualize the overall abundance of HC emissionsduring different phases of the engine cycle, imagingmeasurements were carried out outside the exhaustport at different crank angle degrees (CAD). Thefluorescence signal could thus be evaluated andresolved over the engine cycle. Spectralmeasurements were also carried out, allowing crankangle resolved analysis of the composition of theemissions.

EXPERIMENTAL APPARATUS

ENGINE - The engine was a 60 cc carburetedtwo-stroke engine run with two different cylinders.The difference between the cylinders was the designof the transfer channels. One had cup-handletransfer channels, the other one had channels offinger-type, see figure 1. The engine was run withwide open throttle at 9000 rpm. To enable opticalaccess, the silencer was removed. This has animpact on the in-cylinder gas dynamics. The enginetype studied does, however, not use a tuned exhaustsystem. According to the engine manufacturer thisreduces the effect of exhaust system on gasexchange.

LIF SETUP - The setup was built up around theengine which had a crank angle encoder attached tothe crank shaft. A pulsed laser and a CCD detectionsystem was triggered at the desired crank-angle bymeans of the encoder and a pulse counter. The laserbeam thus intersected the flow of emissions justoutside the exhaust port and the fluorescence signalhereby achieved was collected by the CCD system.

Figure 1: The design of cup-handle transferchannels (to the left) and finger transfer channels (tothe right).

Figure 2: The feature to the right is thefluorescence from 3-pentanone. The left feature is oilfluorescence. The features overlap, but can beresolved.

Figure 3a: Experimental setup for imagingmeasurements. The horizontal laser sheet wasfocused with a cylindrical lens.

Figure 3b: Experimental setup for spectralmeasurements. The vertical laser sheet wasunfocused.

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Laser - A tunable KrF excimer laser was used(Lambda Physik EMG 150 MSC) with a maximumpulse energy of 250 mJ at 248 nm and a pulseduration of 17 ns The emitted beam had arectangular cross section (5 mm × 15 mm). Duringthe imaging measurements the thickness of the lasersheet was decreased by means of a cylindricalquartz lens from 5 mm to 250 µm, as to increase thesignal intensity and to decrease the depth of theimaged plane (i.e. increasing the spatial resolution).

Detector - The fluorescence signal was detectedwith an image intensified CCD camera (ICCD576S/RB-T from Princeton Instruments). It consist ofan image intensifier and a CCD chip. By gating thephoto cathode voltage of the image intensifier,temporal resolution down to 5 ns is feasible.

FUEL - To simplify the data analysis it wasdesirable to minimize the number of speciesoccurring in the exhaust gases. Since gasolinecontains a variety of hydrocarbons, a simpler fuelcontaining fewer molecular species had to be used.Isooctane has a number of features in common withgasoline and was decided to be a suitablereplacement.

Dopant - To be able to monitor the emission ofunburned fuel a fluorescing dopant must be added tothe isooctane, which does not fluoresce in itself [4].Important features of the dopant are vaporizationcharacteristics [5] and solubility in the fuel [5, 6].Examples of dopants that have been used areacetaldehyde [7, 8], acetone [8] and biacety [9, 10].The vaporization characteristics are described by thedependence of the vapor pressure on temperature.Both acetaldehyde and acetone have excessivedifferences in vaporization characteristics comparedwith isooctane [11]. Also biacetyl differs differs fromisooctane in this respect [4]. There is, however, analmost perfect correspondence between theisooctane and 3-pentanone vaporizationcharacteristics [4]. An added amount of 3-pentanonewill thus be likely to follow the isooctane during thescavenging process. During the last years there havebeen LIF applications using 3-pentanone as a dopant[4, 12, 13]. The absorption spectrum of 3-pentanoneis ranging from 220 to 320 nm with a maximum at280 nm [14]. As can be seen in figure 2, 3-pentanone(diethyl ketone) has an emission spectrum between330 and 600 nm, with a peak at 430 nm.

Lubrication oil - Due to the simple construction ofthis engine-type, engine lubricant must be added tothe fuel. After some investigations an oil was foundthat fluoresces between 300 nm and 500 nm with apeak at 350 nm. Using this oil and 3-pentanone inprinciple allows separate detection of the emission ofoil and fuel.

IMAGING MEASUREMENTS

Imaging measurements were performed byplacing the laser sheet horizontally just outside theengine’s exhaust port, see figure 3a. Thefluorescence signal emitted as the exhaust gasespassed through the sheet was mirrored and imagedfrom above with a commercial Nikon camera lens (70- 210 mm zoom). If the imaging system had beenplaced in front of the port, the signal would have hadto be viewed through the exhaust gases. Imagingfrom above avoids this and thus minimizes scatteringand absorption of the signal.

The images were achieved by triggering the laserat a certain crank angle position and accumulatingthe LIF signal from 100 laser shots. This minimizesthe effect of shot-to-shot fluctuations in laser powerand fluctuations in the engine behavior.

Apart from HC the exhaust gases contain anumber of species of which CO2 and H2O are mostabundant. CO2 does not absorb at 248 nm and thusgives no contribution to the fluorescence [3]. Waterfluorescence can, however, be induced via atwo-photon transition at 124 nm (1/2 × 248 nm) [15].This process is weak and a possible contributionfrom water will be negligible. N2, O2, and low levelsof CO and NOx are also present, but neither of thesespecies contribute to the fluorescence signal whenusing this laser [3]. NO can potentially interfere, butits absorption lies near the edge of the tuning rangeof the laser and is easily avoided. As it turns out, onlythe HC components of the emissions will giveimportant contributions to the fluorescence signal.

DATA PROCESSING - Evaluation of the imagingmeasurements was performed by subtracting abackground image from each image in themeasuring series. The background image wasacquired without the engine running.

RESULTS - Four examples of two-dimensionalLIF images are shown in figures 4a-4d. The bottomof each image corresponds to the location of theexhaust port. The two images taken before bottomdead center (BDC) shows that the cup handlegeometry gives a higher intensity peak at blowdownand has more fuel concentrated in the center of theport. After BDC the opposite trend is shown. Here thefinger type geometry has much higher intensity and amuch more uneven distribution of signal over theimage. Images were acquired at several differentcrank angles. The average signal intensity within theimage, confined by the edges of the laser sheet andthe exhaust port, was calculated for all imagesacquired at various crank angle degrees. This areawas 12 by 25 mm. When plotting the herebyachieved average fluorescence signal as a functionof crank angle, a characteristic curve resulted.

A striking similarity between the two cylindergeometries investigated was the of two more or lesspronounced peaks in signal intensity.

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Figure 4a: Image of LIF signal intensity for the cuphandle geometry during blowdown.

Figure 4c: Image of LIF signal intensity for the cuphandle geometry in the scavenging process.

Figure 5: Mean LIF intensity in the evaluation areaoutside exhaust port.

Figure 4b: Image of LIF signal intensity for the fingergeometry during blowdown.

Figure 4d: Image of LIF signal intensity for the fingergeometry later in the scavenging process.

Figure 6: Standard deviation of intensity within theimages

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The first peak occurred during the blowdownphase directly after the opening of the exhaust port,whereas the position of the second peak variedbetween the cylinder geometries. The first peak wasbelieved to mainly contain burned gases and thesecond to consist of fresh fuel-air mixture. Thisinterpretation was later confirmed by the spectralmeasurements. In figure 5 these typical curves areshown, where the solid curve is the result ofmeasurements on the cup-handle type cylinder andthe dashed curve is the result of the simpler fingergeometry. Note that there is a delay of more than 10CAD between the instant where the emissions leavethe cylinder and the point where they reach themeasurement region. This delay is due to the finitevelocity of the exhaust gases, see figure 14.

The absence of a sharp separation between thepeaks for the finger-type engine is interpreted as anearlier beginning of the scavenging losses incomparison to the cup-handle engine, for which thesecond peak starts after the bottom dead center(BDC). This indicates that the cup-handle geometryis more efficient in retaining the fresh charge. If it isassumed that the course of combustion is the samein both cylinder geometries, the combustion productsand emission temperature should be the same. Asan effect, the first peak should be of equal height forboth engines in figure 5. As can be observed this isnot the case. The difference in height can beexplained by variations in laser power between themeasurements and slight changes in the alignmentof the experimental setup. It can, however, beconcluded that the height of the second peak notonly is higher in relation to the first peak for thefinger-type cylinder (dashed curve in figure 5); it isalso higher than the second peak for the cup-handlegeometry (solid curve). Based on our assumptions itcan be concluded that the absolute level ofemissions of short-circuiting fuel is higher in thefinger geometry than in the cup-handle geometry.This is also known from other types ofmeasurements on these engines.

Figure 6 shows the standard deviation of the LIFsignal within individual images as a function of crankangle position. The standard deviation of all pixelswithin each image gives a measure of how much thesignal level varies with respect to the mean intensity.It thus tells us how homogeneous the HC-distributionis across the imaged region1. It is observed that thatthe standard deviation is high during the blowdownphase for both engines, due to the exhaust gasesbeing emitted in the form of a more or less welldefined jet. During the scavenging phase the fingergeometry is more inhomogenous than thecup-handle geometry. This is due to the presence ofparticles (droplets) which has also been observed inLDV measurements on these engines, see figure 15.

1 The standard deviation used here should not bemistaken for the cycle to cycle variation of mean LIFintensity.

SPECTRAL MEASUREMENTS

As can be seen in figure 3b, the laser sheet wasplaced vertically outside the exhaust port during thespectral measurements. The fluorescence wasmirrored in the same manner as during the imagingmeasurements. The slit of the spectrometer wasplaced parallel to the beam and the sheet wasimaged on the slit by means of a 200 mm spherical,fused silica lens. Emission from the entire height ofthe sheet was thus imaged onto the spectrometerslit. This increased the signal level. Just as during theimaging measurements, the LIF signal from 100laser shots were accumulated.

SIGNAL DETECTED - When the laser lightpasses through a gas it can be elastically scatteredby molecules (Rayleigh scattering) or particles (Miescattering). Due to the strong nature of thesescattering processes at short wavelengths [2], a peakat the laser wavelength in some cases dominates thespectrum and limits the sensitivity of the detectionsystem if it is not spectrally filtered. Therefore a 10mm thick liquid butyl acetate filter was placed in frontof the detection system during all measurements.Butyl acetate has a steep cutoff between 250 nm and260 nm [16], thus suppressing the laser wavelength(248 nm) by two orders of magnitude.

The oil added to isooctane for lubrication wasfound to fluoresces between 300 nm and 500 nmwith a peak at 350 nm. This is shifted from thewavelength interval in which 3-pentanone fluoresces.Hence a spectral separation of oil and short-circuitingfuel can be obtained. A separation between freshfuel and partially burned fuel originating from thecombustion can be found due to the very reactivenature of 3-pentanone. Fuel/air-mixture that hastaken part in the high-pressure cycle will not containany significant amount of 3-pentanone afterwards.

During the spectral measurements spectra werecollected at various crank angles. A backgroundspectrum was acquired without the engine runningand subtracted. The resulting spectra had a widepeak around 350 nm and superposed on this a widedistribution around 430 nm. The feature at 350 nmconsists to a great extent of oil and probably also ofpartially burned hydrocarbons [1]. The distributionaround 430 nm originates from the added dopant,3-pentanone, and indicates the occurrence ofunburned fuel.

As predicted, spectra recorded at different crankangles reveal differences in the chemicalcomposition of the emissions. Unfortunately theoil- and 3-pentanone features could not be fullyresolved.

Spectra from the emission peak around 70 CADbefore BDC are shown in figure 7a and spectra fromthe peak occurring around 70 CAD after BDC areshown in figure 7b. The data comes from thecup-handle type cylinder, running on two differentfuel mixtures. Isooctane with a 5% (vol)concentration of 3-pentanone was used during onemeasuring series, a 2% content was used

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Figure 7a: Spectra recorded at 70 CAD before BDCwith cup-handle geometry

Figure 7b: Spectra recorded at 70 CAD after BDC(peak II in figure 5).

during the other. The upper (dotted) curves in figure7a and 7b correspond to the higher concentration of3-pentanone; whereas the lower (solid) curvescorrespond to the lower concentration. Diagram 7ashows almost no difference between the spectra.Diagram 7b, however, has a greater intensity around430 nm in the case of higher 3-pentanone content. Itcan clearly be seen that the second emission peakcontains a considerable amount of unburned fuel,whereas the first peak does not. Note that the peakat 248 nm is higher in figure 7a than in figure 7b. Thisis due to strong Mie-scattering of the laserwavelength from particles/droplets present in theexhaust gases. These particles can actually be seenin figure 4a. This conclusion is also confirmed by theLDV measurements, see figure 15. RESULTS - Thespectra from different crank angle positions wereseparated in to tree different regimes, called regionA, B and C. Region A is ranging from 240 to 265 nm,and contains the Mie-scattering signal. Region B isthe wavelength interval from 265-390 nm, i.e. mostlycovering the LIF signal from oil and partially burnedfuel. The last region, C, covers the signal from3-pentanone and

Figure 8a: LIF intensity over the entire wavelengthinterval measured. Two per cent 3-pentanone wasused.

Figure 8b: LIF intensity over the entire wavelengthinterval measured. Five per cent 3-pentanone wasused

ranges from 390 to 525 nm. Figure 8 to 11 shows theintensity within the different regions as well as thetotal signal intensity for the two cylinders.

It can be seen that the Mie-scattering trend,region A, is rather similar for the two cylinders,except for the first blowdown peak found for the cuphandle cylinder. The percentage of 3-pentanonedoes not change the trends significantly. Region Balso reveals rather similar trends for the twogeometries. Two distinct peaks can be observed.The first originates from the blowdown phase. Thereasons for the second peak is less clear. It can besignal from fresh charge oil in combination withpartially burned residuals.

Region C shows a significant difference betweenthe two cylinders. Here the cup handle type also hastwo distinct peaks whereas the finger type has acontinuous high level during the gas exchangeprocess. The first peak for the cup handle geometryis somewhat unexpected.

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Figure 9a: Signal level from Mie scattering. Twopercent 3-pentanone added to fuel..

Figure 10a: Signal from mostly oil and partiallyburned fuel. Two percent 3-pentanone added to fuel..

Figure 11a: Signal mainly from 3-pentanone. Twopercent 3-pentanone added to fuel

Figure 9b: Signal level from Mie scattering. Fivepercent 3-pentanone added to fuel..

Figure 10b: Signal from mostly oil and partiallyburned fuel. Five percent 3-pentanone added to fuel..

Figure 11b: Signal mainly from 3-pentanone. Fivepercent 3-pentanone added to fuel.

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It was expected that signal would occur only late atthe gas exchange where fresh gas is expected. Thefirst peak can be the result of the tail of the spectralfeature in region B, corresponding to oil and partiallyburned fuel. This tail reaches into region C, "leaking"information to this higher wavelength region. Thehigher level from the finger type indicates again thatmore short circuiting occurs in this cylinder. It can benoted that the signal level is increased with five percent 3-pentanone compared to two per cent. Thisshows that the signal detected in this interval ismostly 3-pentanone.

As can be seen in the figures, the signal levelcontinues to be high even after the closing of theexhaust port. This is due to the high working speedof the engine - the emissions do not have time toescape the measuring region before the next cyclebegins and the exhaust port opens again.

VELOCITY MEASUREMENTS

In a former study on the two cylinder types, laserdoppler velocimetry (LDV) was used to study the in-cylinder flow during gas exchange [17]. The samesystem has also been used to measure the flowoutside the exhaust port. The measurement volumehad the shape of an ellipsoid with a diameter of 0.1mm and a length of approximately 7.5 mm. Thesystem used was a DANTEC fiber-flow with BSAsignal processors. More information on the systemcan be found in [17].

The measurement volume was placed in thecenter of the exhaust port and traversed over the portheight. 196588 samples of velocity and crank angleposition were collected for four different heights. Theaverage horizontal velocity can be found in figure 12.The vertical component is lower and thus the velocityvector from both horizontal and vertical componentshas a shape similar to that of only the horizontalcomponent, see figure 13.

The mean velocity can be used to estimate thetime delay between events occurring at the exhaustport and detection 300 mm outside the port. With theengine speed set to 9000 rpm the time delay can beexpressed in crank angle degrees. Figure 14 showsthat the delay is smallest during the blowdownphase, resulting from a high mean velocity. In thelatter stages, the mean velocity is lower and asubstantial delay exists between the port and themeasurement location.

PARTICLE FLUX - The LDV system needsparticles to scatter light from the beam crossing. Inthese experiments, no addition of special particleswas necessary. A fairly high data rate could beachieved on the naturally occurring particles. It isbelieved that these particles mainly consist ofunburned or partially burned lubricating oil. The fuelused during the LDV measurements was commercialgasoline with a 2% oil content. The highest peak ofdata rate occurs during the blowdown phase for bothcylinders, see figure 15.

Figure 12: Horizontal velocity outside the exhaustport..

Figure 13: Mean velocity outside the exhaust port..

Figure 14: Delay from exhaust port to measurementlocation 30 mm from cylinder.

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Figure 15: Data rate for LDV measurements,indicating the flux of naturally occurring particles.

This is somewhat surprising as charge escaping thecylinder during the blowdown phase has taken part inthe combustion. Hence the oil should have beenburned or at least evaporated. This is not the case.The latter peak in particle flux most likely originatesfrom gas exchange short circuiting of fresh chargefrom transfer ports to exhaust port. The data ratefrom LDV is, however, only a very crude estimate ondroplet flux. LDV is very sensitive to particle size andvelocity bias [18] can play a major role. Despite theseuncertainties some information can be obtained fromthe data rate.

Figure 15 shows that the cup-handle transferchannels have a lower secondary peak compared tothe finger type. This is in agreement with the LIFmeasurements of the fuel concentration at the samelocation. The phasing between LIF and LDV data is,however, not the same. The second peak shows uplater with LDV, than it does with LIF. This differencecan be the result of several differences between theLDV and LIF measurements. The most important ofthese is the phase detected. LDV detects only thepresence of particles. LIF on the other hand, detectsa signal from both liquid and gas phase.

DISCUSSION

The results presented shows that LIF can be usedto obtain information on the gas exchange processfor high speed two-stroke engines. The imagingresults are very similar to the spectral and anagreement can be found between Miemeasurements with LIF and data rate detected withLDV.

There are, however, clear limitations. No attemptto calibrate the LIF signal levels to concentration ofHC was performed. This can be a major difficulty asthe

temperature of the exhaust gas will changesignificantly during the cycle. This means that we stillmust investigate how to relate the signal detected atthe blowdown phase to that of the later secondintensity peak.

The overall LIF signal as a function of crank angleposition is shown in both figure 5, which comes fromthe imaging measurements, and figure 8a-b, whichcomes from the spectral measurements. Thesecurves should in theory be identical. The reason whythey differ from each other is explained byinterferences from C2. During the imagingmeasurements the laser sheet was so tightly focusedthat the laser flux was sufficient to dissociatehydrocarbons and produce excited C2 radicals [3].During the spectral measurements theseinterferences were avoided by using a non focusedlaser sheet. However, a correlation between thestrength of the C2 fluorescence and the Miescattering was observed. When the focusing lenswas removed, the C2 interference disappeared andthe Mie scattering was enhanced. There is thus apossibility that C2 fluorescence can be used fordetecting the presence of fuel droplets.

CONCLUSIONS

Cycle resolved LIF measurements have beenperformed on simple, carbureted two-stroke enginesworking at 9000 rpm. There are two phases in theexhaust process of these engines. During the firstphase combustion products are emitted, whereasfresh fuel is emitted during the second phase. This isdue to fresh fuel-air mixture short circuiting thecylinder during the scavenging process. Two transferchannel configurations were investigated; one offinger type and one of cup-handle type. In thecup-handle case the two phases can be clearlyseparated, whereas for the finger-type cylinder thereis an overlap between the phases. It has been shownthat the finger geometry is less efficient in retainingthe fresh charge in the cylinder than the cup-handlegeometry.

ACKNOWLEDGEMENTS

We are indebted to the Swedish Board forTechnical Development, NUTEK, for their financialsupport of this work.

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