Rich Combustion Research Articles

An assessment of gasoline motorcycle emissions performance and understanding their contribution to Tehran air pollution

Motorcycles are the third most common means of transportation in the megacity of Tehran. Hence, measurements of emission factors are essential for Tehran motorcycle fleets. In this study, 60 carburetor motorcycles of various mileages and engine displacement volumes were tested in a chassis dynamometer laboratory according to cold start Euro-3 emissions certification test procedures. For almost all of the tested samples, the average carbon monoxide (CO) emission factors were about seven times higher than the limits for Euro-3 certification. No motorcycle fell within the Euro-3 certification limit on CO emissions. 125cc engine displacement volume motorcycles, which are dominant in Tehran, have the most total unburned hydrocarbons and CO emission rates, and they have less nitrous oxides (NOX) emission rates and fuel consumption compared to those of larger engine volume motorcycles. Calculation of fuel-based emission factors and moles of combustion products shows that about 40% of fuel consumed by 125cc engine volume motorcycles burns to incomplete combustion products. This proportion is lower for larger engine volume motorcycles. Approximation of relative air–fuel ratio results shows very rich combustion in selected motorcycles. Using a carburetor fuel supply system, low engine compression ratio, aging, and no catalyst could be reasons for high emission rates. These reasons could possibly result in high ultrafine particles emission rates from motorcycles. Comparison of total motorcycle pollutant emissions to that of passenger cars from previous studies in Tehran shows that motorcycles contribute to pollutant much higher than their contribution to the total fleet or total travels.

Effects of gas concentration and venting pressure on overpressure transients during vented explosion of methane–air mixtures

29 batches of vented explosion tests were conducted in a 12m3 concrete chamber filled with methane–air mixtures to investigate the effects of methane concentration and venting pressure on the development of overpressure inside the chamber. The deflagrations were vented from a square side window with a venting area of 0.64m2 upon rupture of the vent cover. The venting pressures were varied by using six different types of vent covers, and determined by performing a numerical simulation. Methane concentrations in the mixed gas varied between 6.5 and 13.5vol.%, covering both lean and rich combustion regimes. The generation conditions of the four types of overpressure–time profiles with different overpressure transients were summarized. Among the overpressure transients, the ΔP1 caused by failure of the vent cover and the ΔP4 resulted from the coupling between acoustic mode and flame were basically dominant. The rate of ΔP1 rise as well as peak value of ΔP1 and ΔP4 showed a same trend of first increasing and then decreasing with the methane concentration from lean to rich. They reached their maximum value at the methane concentration of about 9.5%, whereas the rate of ΔP4 rise was found to be insensitive to the methane concentration. The peak value of ΔP1 increased with the venting pressure, while that of ΔP4 first increased and then decreased as the venting pressure increased. In addition, the rates of ΔP1 and ΔP4 rise were both insensitive to the venting pressure. The occurrence of ΔP4 was depended on the methane concentration and the venting pressure. The differences in the behavior of these overpressure transients suggest their different generation mechanisms.

Experimental study of a laminar premixed LFG/air flame in a slot burner using Mach-Zehnder interferometry

An experimental study was conducted to investigate the influence of Reynolds number and equivalence ratio on flame temperature field and thermal flame height of laminar premixed LFG fuel. Mach-Zehnder interferometry technique is used to obtain an insight to the overall temperature field. The slot burner with large aspect ratio (L/W), length of L=60 mm and width of W=6 mm was used to eliminate the three- dimensional effect of temperature field. Two kinds of mixed fuels, LFG70 (70%CH4- 30%CO2 on volume basis) and LFG50 (50%CH4- 50%CO2) were used to investigate flame characteristics under the test conditions of 100 ? Re ? 600 and 0.7 ? ? ? 1.3. The present measurement reveals that the variation of maximum flame temperature with increment of Reynolds number is mainly due to heat transfer effects and is negligible. On the other hand, the equivalence ratio and fuel composition have a noticeable effect on flame temperature. In addition, the results show that the LFG flames compared to the CH4 ones have a lower flame temperature. With increment of CO2 volume fraction at lean combustion, thermal flame height is augmented while at stoichiometric and rich combustion, its value reduced. Thermal flame height augments linearly by Reynolds number increase, while its increment at rich mixture is higher and the effect of Reynolds number at lean mixtures is insignificant. For validation of experimental results from Mach-Zehnder Interferometry, K-type thermocouples are used at peripherally low and moderate isotherm lines.

An Experimental Study on Industrial Boiler Burners Applied Low NOx Combustion Technologies

 Abstract—Based on flue gas inner recirculation technology, a low NOx gas burner was developed and tested in a real scale industrial boiler. Some parts of combustion gases were recirculated in the combustion chamber and mixed with combustion air in the burner by its structural characteristics. And a noble low NOx combustion technology, based on partial oxidation combustion concept in a fuel rich combustion zone, was successfully applied in liquid burner. The burner was designed such that a portion of fuel is heated and pre-vaporized in the furnace then injected into a fuel rich combustion zone so that a partial oxidation reaction occurs. The effects of equivalence ratio, thermal load, and fuel distribution ratio on the emissions of NOx and CO were experimentally investigated. This newly developed combustion technologies showed very low NOx emission level, about 12 ppm when light oil was used as a fuel and about 18ppm when LNG was used as a fuel.

승용디젤엔진의 과도구간 입자상물질 저감 및 운전성능 향상을 위한 연료분사량 및 커먼레일압력 제어전략

This study proposes a control strategy of the common rail pressure with a fuel injection limitation algorithm to reduce particulate matter (PM) emissions under transient states. The proposed control strategy consists of two parts: injection quantity limitation and rail pressure adaptation. The injection limitation algorithm determines the maximum allowable fuel injection quantity to avoid rich combustion under transient states. The fuel injection quantity is limited by predicting the burned gas rate after combustion; however, the reduced injection quantity leads to deterioration of engine torque. The common rail pressure adaptation strategy is designed to compensate for the reduced engine torque. An increase of the rail pressure under transient states contributes to enhancement of the engine torque as well as reduction of PM emissions by promoting atomization of the injected fuel. The proposed control strategy is validated through engine experiments. The rail pressure adaptation reduced the PM emission by 5-10% and enhanced the engine torque up to 2.5%.

Effects of Equivalence Ratio on Asymmetric Vortex Combustion in a Low NOx Burner

This study presents a combustor with high combustion efficiency and low emissions. The flow field in a combustion chamber is significant in achieving good fuel–air mixing. The experimental and computational results of the temperature and emission of a vortex combustor are presented. Air enters into the combustor in a tangential direction relative to the combustor axis to produce vortex flow, which facilitates the recirculation of hot gas near the fuel nozzle for enhanced mixing. Propane is injected in the direction of the combustor axis at variable mass flow rates to produce variations in the equivalence ratio. The temperature measured along the center line of the combustor. The lowest temperature is measured under rich conditions first and then under lean and stoichiometric conditions. The trend of the combustor temperature at varying equivalence ratios is reproduced well in the simulation despite the increasing temperature near the inlet. An expected trend of NOx is measured at the outlet where the lowest NOx is produced by the rich combustion first, followed by the lean and stoichiometric combustion. The relative NOx emission at different equivalence ratios is reproduced well in the simulation, although the magnitude of the NOx emission is reduced. As indicated by the calculated number of swirls, the combustion under rich conditions produces about 15% more swirls near the nozzle compared with that under stoichiometric conditions as a result of the low combustor temperature. Moreover, such rich conditions produce a more uniform temperature inside the combustor than lean and stoichiometric conditions.

Lean HCCI/Rich SACI Gasoline Combustion Cycling and Three-Way Catalyst for Fuel Efficiency and NOx Reduction

Multimode combustion (MMC) concepts using homogeneous charge compression ignition (HCCI) gasoline combustion at low loads and spark assisted compression ignition (SACI) gasoline combustion at medium loads have the potential for improved fuel efficiency relative to spark ignition (SI) gasoline combustion. Two MMC concepts are compared in this paper with respect to fuel efficiency and tailpipe NOx emissions. The first concept uses stoichiometric HCCI and SACI to allow standard three-way catalyst (TWC) operation. The second concept also uses HCCI and SACI, but cycles between lean and rich combustion and uses a TWC with increased oxygen storage capacity (OSC) for potentially even greater fuel efficiency improvement. This paper performs a preliminary comparison of the two MMC concepts by analyzing two scenarios: (1) cycling between stoichiometric HCCI at 2 bar BMEP (brake mean effective pressure) and stoichiometric SACI at 3 bar BMEP, and (2) cycling between lean HCCI at 2 bar BMEP and rich SACI at 3 bar BMEP. The effects of excess oxygen ratio during HCCI operation and the frequency of oxygen depletion events on TWC performance and fuel efficiency are investigated. Results show that MMC lean/rich cycling can achieve better fuel efficiency than stoichiometric HCCI/SACI cycling. NOx emissions are moderately higher, but may still be low enough to meet current and future emission regulations.

In cylinder visualization of stratified combustion of E85 and main sources of soot formation

The combustion process and soot formation in spark ignited spray guided stratified combustion of E85 was investigated in a single cylinder optical engine with direct injection of fuel using an outward opening piezo actuated injector. The effect of engine rotation frequency, fuel quantity, injection sequence and ignition timing was studied. Combustion, soot formation and soot oxidation was analyzed using cylinder pressure measurements, images recorded using high speed video cameras, the flame emission spectrum and OH∗ chemiluminescence and soot incandescence imaging. A maximum injection duration was found to exist for direct ignition of the fuel spray. Engine rotation frequency had little effect on the initial and maximum rate of combustion. The maximum rate of combustion decreased with increasing cycle fuel mass when a single injection was used. The rate of combustion and indicated mean effective pressure increased and the combustion variability decreased when the single injection was split into multiple injections in close succession to deliver the same total fuel mass and the last fuel spray was ignited. Ignition of the first fuel spray resulted in a more pronounced change. The absence of soot incandescence during the initial flame propagation suggested flame propagation in a partially mixed fuel and air mixture with stoichiometric to fuel lean regions. A single fuel injection resulted in piston pool fires due to fuel spray impingement on the piston and was the primary source of soot formation. The pool fires persisted until after conditions favorable to oxidation of the soot had ended. Soot formation in the gas phase occurred while favorable soot oxidation conditions existed and was efficiently oxidized. The magnitude of the piston pool fires was reduced using multiple injections. The reduction is attributed to a reduction of the fuel spray penetration length and a smaller effective injection orifice area, resulting in a shorter total duration of fuel spray impingement on the piston crown. Soot formation occurred primarily in the gas phase when the first of two fuel sprays was ignited and persisted due to the second fuel spray entering an existing flame leading to fuel rich combustion.

Numerical study of combustion characteristics of ammonia as a renewable fuel and establishment of reduced reaction mechanisms

With its high hydrogen density and already existing infrastructure, ammonia (NH3) is believed to be an excellent green fuel that can be used in energy generation and transportation systems. Combustion of ammonia has certain challenges (associated with its low flame speed and fuel bond NOx emissions) that need to be addressed before its widespread use in practical systems. The primary objective of this study is to develop a reduced reaction mechanism for the combustion of ammonia which can be used to expedite the design of effective ammonia combustors through numerical simulations of realistic combustor geometries with accurate kinetics models. First we have investigated the combustion characteristics of NH3/H2/air mixtures at elevated pressure and lean conditions which are encountered in practical systems such as gas turbine combustors. Laminar premixed freely propagating flame model is used to calculate the combustion properties. The results of sensitivity study of total NOx formation with respect to the equivalence ratio indicates the possibility of localized rich combustion as an effective way to reduce the NOx concentration down to levels that are the same order as the modern gas turbine engines. In the second part of the study, by considering a wide range of conditions in terms of pressure, fuel mixture, and equivalence ratio we have developed two reduced mechanisms based on the Konnov mechanism. The reduced mechanisms are capable of predicting the total NOx emission level and the laminar flame speed at an acceptable accuracy over a wide range of conditions. Evaluating the performance of the reduced mechanisms with respect to the full mechanism and experimental data shows that the mechanisms are able to predict the combustion properties almost at the same accuracy level as the Konnov mechanism, but at a nearly five times less CPU time expense.

Effect of injector configurations on combustion and emissions in a gasoline direct-injection compression ignition engine under low-load conditions

The effect of injector configurations on combustion and emissions is investigated at low-load conditions in a gasoline direct-injection compression ignition engine. A total of five injectors with higher number of nozzle holes (10, 14) and narrow injection angles (70°, 100°) were tested while comparing with the baseline injector with eight holes and injection angle of 146°. Diesel combustion was also performed at similar operating conditions for the comparison with gasoline combustion. First, at the idle operation (engine speed = 800 r/min, engine load = 10%), combustion stability was very low with baseline injector due to the excessive mixing process. The narrow injection angle was helpful to enhance combustion stability, attributed to suppressing the formation of over-lean mixture by the fuel stratification within narrow area. However, higher number of nozzle holes were unfavorable with frequent misfires by excessive fuel/air mixing. Second, at the low-load operation (engine speed = 1200 r/min, engine load = 25%), combustion and emissions were investigated according to the injection timing. For all injectors, partially premixed compression ignition combustion was realized with early injection which exhibited low nitrogen oxide emissions resulted from lean premixed combustion. However, high hydrocarbon and carbon monoxide emissions were produced with wide injection angle due to the over-mixing and trapped fuel in squish and crevice volume. Hydrocarbon and carbon monoxide emissions were significantly reduced with narrow injection angle, attributed to the fuel stratification inside piston bowl. On the other hand, hydrocarbon and smoke emissions were aggravated in diesel combustion with narrow injection angle, attributed to fuel-rich mixture resulted from the significant fuel impingement on piston bowl. Fuel impingement occurred for both gasoline and diesel; however, evaporation process was much slower for diesel fuel due to the lower volatility. Significant pool fire and rich combustion were observed for diesel combustion, while blue chemiluminescence was dominated in gasoline combustion which meant well-premixed combustion.

Biomass explosion testing: Accounting for the post-test residue and implications on the results

This work uses the ISO 1 m3 dust explosion equipment to study the explosion properties and combustion characteristics of pulverized biomass dust clouds. An unreported feature of this apparatus is that in rich concentrations only about half the dust injected is burned in the explosion, while the overpressures remain high. This work was undertaken to try to understand the mechanisms of these phenomena, through the accounting of the debris at the end of the explosion, some of which was found in the form of impacted “cake” against the vessel wall. One possible explanation is that the residue material was biomass dust blown ahead of the flame by the explosion induced wind, impacted on the walls where then the flame side underwent flame impingement pyrolysis and the metal (wall) side material was compacted but largely chemically unchanged. The results also show that the heat transfer insulation provided by the powder wall layer contributes to the higher observed pressures. The risk of explosion with significant overpressures remains at 100% in very rich environments (equivalence ratios of up to 6) although these environments are leaner than thought due to material sequestration within the “cake”. There was little indication that a rich combustion limit was approached, this was determined in standard testing equipment that has been modified and calibrated to handle larger quantities of powder than normal.

Analysis of Completely Prevaporized Spray Flames with Water/Octane Core/Shell Structured Droplets

A series of studies on laminar flame propagation in off-stoichiometric dilute sprays of monodispersed inert or fuel drops had been investigated by large activation energy asymptotics. The present study extends previous theoretical model to consider water/octane core/shell structured drops instead of single-phase drops. The core/shell structured drops are composed of core fluid (water) encased by a layer of shell fluid (n-octane). In this study, we only deal with the case that core/shell structured drops are vaporized completely just at flame location. Namely, the discussions of this paper are restricted to the completely prevaporized mode. By varying parameters of core water radius, core-to-shell ratio, the amount of liquid loading, and the stoichiometric ratio (lean or rich burning), we examine the gasification of core water and shell fuel of core/shell structured drops upstream of the bulk flame and its relation to the internal heat transfer. The effects of drop radius, core-to-shell ratio, liquid loading, and overall heat loss or gain on flame propagation flux are reported and discussed.

Fuel Economy of a Multimode Combustion Engine With Three-Way Catalytic Converter

Highly diluted, low temperature homogeneous charge compression ignition (HCCI) combustion leads to ultralow levels of engine-out NOx emissions. A standard drive cycle, however, would require switches between HCCI and spark-ignited (SI) combustion modes. In this paper we quantify the efficiency benefits of such a multimode combustion engine, when emission constraints are to be met with a three-way catalytic converter (TWC). The TWC needs unoccupied oxygen storage sites in order to achieve acceptable performance. The lean exhaust gas during HCCI operation, however, fills the oxygen storage and leads to a drop in NOx conversion efficiency. If levels of tailpipe NOx become unacceptable, a mode switch to a fuel rich combustion mode is necessary in order to deplete the oxygen storage and restore TWC efficiency. The resulting lean-rich cycling leads to a penalty in fuel economy. Another form of penalty originates from the lower combustion efficiency during a combustion mode switch itself. In order to evaluate the impact on fuel economy of those penalties, a finite state model for combustion mode switches is combined with a longitudinal vehicle model and a phenomenological TWC model, focused on oxygen storage. The aftertreatment model is calibrated using combustion mode switch experiments from lean HCCI to rich spark-assisted HCCI (SA-HCCI) and back. Fuel and emission maps acquired in steady-state experiments are used. Different depletion strategies are compared in terms of their influence on drive cycle fuel economy and NOx emissions. It is shown that even an aggressive lean-rich cycling strategy will marginally satisfy the cumulated tailpipe NOx emission standards under warmed-up conditions. More notably, the cycling leads to substantial fuel penalties that negate most of HCCI's efficiency benefits.

Simulation of Soot Size Distribution in a Counterflow Flame

Abstract Soot formed during the rich combustion of fossil fuels is an undesirable pollutant and health hazard. A newly developed Monte Carlo method is used to simulate the soot formation in a counterflow diffusion flame of ethylene. The simulation uses a new reaction mechanism available in literature, which focuses on modeling the formation of large polycyclic aromatic hydrocarbons (PAHs) up to coronene (C24H12). Nascent soot particles are assumed to form from the collision of eight different PAH molecules. Soot surface growth includes the hydrogen-abstraction-C2H2-addition mechanism and the condensation of the PAHs. Soot coagulation is in the free-molecular regime because particles are small (not more than a hundred nanometer). The coupling between vapor consumption and soot formation is handled by an interpolative moment method. Soot particle diffusion is found negligible throughout the counterflow flame, except for a very narrow region right around the stagnation plane. The soot particle size distribution (PSD) generally exhibits a bimodal shape. The first peak corresponds to a large number of nascent particles, while the second peak results from the competition between nucleation and coagulation. Surface growth affects the PSD quantitatively, but does not change the modality. A comparison with experimental data is also provided.

Studies on the Ultra-Low NOx Burner Technology Using Partial Oxidation Reaction

A new concept of low NOx burner, based on a partial oxidation combustion concept, is successfully applied in this research. The burner is designed such that a portion of liquid fuel is heated and pre-vaporized in the furnace then injected into a fuel rich combustion zone so that a partial oxidation reaction occurs. The effects of equivalence ratio, thermal load, and fuel distribution ratio on the emissions of NOx and CO are experimentally investigated. This newly developed burner showed very low NOx emission level, about 12 ppm, when light oil is used as a fuel.

Experimental investigation of the performance of a liquid fuel-fired porous burner operating on kerosene-vegetable cooking oil (VCO) blends for micro-cogeneration of thermoelectric power

Studies related to porous burner for thermoelectric (TE) power generation have mainly focused toward achieving a specific range of power output for various applications. However, detailed analyses on the performance and emission aspects of the porous burner are lacking. In addition, physical integration between the burner and TE modules has added further complexity in this research area. Thus, this work aims to comprehend the effects of fuel–air equivalence ratio on the performance and emission characteristics of a liquid fuel-fired porous burner for micro-cogeneration of TE power. A catalytically inert Al2O3 porous medium was incorporated into a liquid fuel-fired porous burner operating on four mixtures of kerosene-vegetable cooking oil (VCO) blends: 100 kerosene, 90/10 KVCO, 75/25 KVCO, and 50/50 KVCO. Ten bismuth-telluride TE cells were arranged in a ten-sided polygon that, together with finned dissipators, formed a TE module electrically connected in series but thermally connected in parallel. The performance aspects at various fuel–air equivalence ratios were thoroughly evaluated with the corresponding temperature profiles, voltage, current, power output, and electrical efficiency. Results indicated that the surface temperature of the porous media was generally higher than the developed and exit flame temperature of the burner. Varying the fuel-air equivalence ratio significantly affected the electrical efficiency, with a maximum and minimum value of 1.94% and 1.10%, respectively. The power output steadily increased in the lean region, but stabilized as the fuel–air equivalence ratio slowly increased beyond the stoichiometric ratio. The CO emission was relatively lower at the lean region; however, significant amount was recorded in the rich combustion region. Moreover, NOx fluctuated between 1 ppm and 4 ppm over the entire range of fuel–air equivalence ratio.

English

The study presents the engine performance, fuel economy and exhaust emissions at variations of air intake pressure. In carburetor system, the air intake pressure is influenced by the degree of opening throttle plate and venturi effect which draw the fuel to the combustion chamber. The experimental work is carried out at variations of engine speed and load using a single cylinder four stroke gasoline engine attached with a dynamometer. The measured exhaust emissions compositions are used to determine the mode of combustion. The results show that the standard air intake system resulted in rich combustion which then led to incomplete combustion which is caused by less availability of air for the combustion process. Eliminating the air filter reduces the air flow restriction in the air intake system resulted in better combustion and less unburned components due to higher air availability. Higher air intake pressure is better in increasing the efficiency of combustion within a limited time to improve fuel economy, power output and exhaust emissions. Better combustion also leads to the reduced unburned components such as carbon (C), hydrogen (H2), carbon monoxide (CO) and hydroxide (OH) that resulted in cleaner emissions.

Effects of fire-fighting on a fully developed compartment fire: Temperatures and emissions

This study evaluates the effects and consequences of fire-fighting operations on the main characteristics of a fully-developed compartment fire. It also presents data and evaluation of the conditions to which fire-fighters are exposed. A typical room enclosure was used with ventilation through a corridor to the front access door. The fire load was wooden pallets. Flashover was reached and the fire became fully developed before the involvement of the fire-fighting team. The progression of the fire-fighters through the corridor and the main-room suppression attack – in particular the effect of short, medium and long water pulses on either the hot gas layer or the fire seat – was charted against the compartment temperatures, heat release rates, oxygen levels and toxic species concentrations. The fire fighting team was exposed to extreme conditions, heat fluxes in excess of 35kW/m2 and temperatures of the order of 250°C even at crouching level. The fire equivalence ratio showed rich burning with high toxic emissions in particular of CO and unburnt hydrocarbons very early in the fire history and a stabilisation of the equivalence ratio at about 1.8. The fire fighting operations made the combustion temporarily richer and the emissions even higher.

Numerical analysis of the Cambridge stratified flame series using artificial thickened flame LES with tabulated premixed flame chemistry

Detailed comparisons of LES results against measurement data are presented for the turbulent lean and rich stratified Cambridge flame series. The co-annular methane/air burner with a central bluff body for flame stabilization has been investigated experimentally by Sweeney et al. [1,2]. Three cases with varying levels of stratification in the lean and rich combustion regime are taken into account. Turbulent combustion is modeled by using the artificial thickened flame (ATF) approach in combination with flamelet generated manifolds (FGM) lookup tables. The model is adapted for stratified combustion and an alternative formulation for the flame sensor is presented. Three different grids are used to investigate the influence of the filter width and the sub-filter modeling on the overall results. Velocities, temperatures, equivalence ratios, and major species mass fractions predictions are compared with measurements for three different stratification rates and an overall good overall agreement was found between simulation and experiment. Some deviations occur near the bluff body, which are analyzed further by evaluation of atomic and species mass fractions. The stratified combustion process was further investigated and characterized by probability density functions extracted from the simulation results.

The role of dimethyl ether as substituent to ethylene on particulate formation in premixed and counter-flow diffusion flames

The role of dimethyl ether (DME) as substituent to ethylene on particulate formation has been evaluated in premixed and counter-flow diffusion flames. In the premixed flame, the equivalence ratio has been changed from 1.95 to 2.61 and dimethyl ether has been added from 2% to 30% of the total carbon fed. In the counter-flow diffusion flame, the addition of DME has been from 0% to 60% of total carbon fed. Laser induced fluorescence and incandescence have been used to follow the soot formation process: UV and visible fluorescence signals have been attributed to aromatic macromolecules and incipient nanoparticles, respectively, whereas incandescence has been attributed to soot particles and aggregates.In premixed flames results evidence that the formation of soot precursors is not so sensitive to DME addition. In very rich combustion environments, DME addition cannot completely avoid the formation of small precursors, although it can slow down the formation process. This behavior has been observed for all the equivalence ratios investigated.In the pyrolysis region of counter-flow diffusion flames, the formation of aromatic small precursors and soot particles is increased for small DME percentages, up to 20%. Then, the precursors are suppressed for larger amounts, going below the detection limit when 60% of DME is used. This suggests that DME can enhance the production of radicals and small reactive molecules in the pyrolytic side when it is added in small concentrations. For larger amounts of DME, oxidative pathways prevail and the conversion of carbon to precursors and then to soot is inhibited. In the oxidative region DME starts to effectively decrease the particle reduction also for amounts as small as 10–15%.