Diesel combustion study at full load using CFD and Design of Experiments

<div class="htmlview paragraph">A multi-zone, direct-injection (DI) diesel combustion model, the so-called RK-model, has been developed and implemented in a full cycle turbocharged engine simulation code. The combustion model takes into account:</div> <div class="htmlview paragraph"> <ul class="list disc"> <li class="list-item"> <div class="htmlview paragraph">transient evolution of fuel sprays;</div> </li> <li class="list-item"> <div class="htmlview paragraph">interaction of sprays with swirl and walls;</div> </li> <li class="list-item"> <div class="htmlview paragraph">evolution of near-wall flow formed after spray-wall impingement depending on impingement angle and swirl;</div> </li> <li class="list-item"> <div class="htmlview paragraph">interaction of near-wall flows formed by adjacent sprays.</div> </li> </ul> </div> <div class="htmlview paragraph">In the model the fuel spray is divided into a number of zones with different evaporation conditions. The piston bowl is assumed to be a body of revolution with arbitrary side shape. Submodels of soot and NOx formation are included. The model has been validated by experimental data obtained for high-speed, medium-speed and low-speed engines over the whole operating range; a good agreement has been achieved without recalibration for different operating modes.</div> <div class="htmlview paragraph">Predictions of spray tip penetration, spray angle and ignition delay were validated by the published data obtained for different diesels including diesels with multiple injection system and injection timing after the TDC. Formulas for computation of these characteristics were derived.</div> <div class="htmlview paragraph">Computational research and optimization of sprayer nozzles orientation for different piston bowl shapes has been performed. Analysis of fuel sprays evolution in contact with walls as well as distribution of fuel in characteristic zones has been done for part load and full capacity. Conclusion about dependence of optimal piston bowl shape on BMEP was made.</div>

<div class="section abstract"><div class="htmlview paragraph">The concept of hot surface assisted compression ignition (HSACI) was previously shown to allow for control of combustion timing and to enable combustion beyond the limits of pure homogeneous charge compression ignition (HCCI) combustion. This work investigates the potential of HSACI to extend the operating limits of a naturally aspirated single-cylinder natural gas fueled HCCI engine.</div><div class="htmlview paragraph">A zero-dimensional (0D) thermo-kinetic modeling framework was set up and coupled with the chemical reaction mechanism AramcoMech 1.3. The results of the 0D study show that reasonable ignition timings in the range 0-12°CA after top dead center (TDC) in HCCI can be expressed by constant volume ignition delays at TDC conditions of 9-15°CA. Simulations featuring the two-stage combustion in HSACI point out the capability of the initial heat release as a means to shorten bulk-gas ignition delay.</div><div class="htmlview paragraph">Engine trials were conducted to map the operating limits in HCCI and HSACI mode for an engine speed of 1400 1/min as a function of intake air temperature (148-173°C) and relative air-fuel ratio (λ = 2.0-3.0). Results show that HSACI extends the lean limit by more than ∆λ = 0.4 and reduces the minimum required intake temperature by at least 5 K compared to HCCI. Comparative experiments of HCCI and HSACI reveal that HSACI benefits from higher engine load, lower ringing intensity and lower NO<sub>x</sub> emissions without deteriorating efficiency. Experimental data concerning the occurrence of instable combustion in HCCI and the role of initial heat release in HSACI agree with the trends predicted by the 0D models.</div></div>

<div class="htmlview paragraph">To limit pollutants from transport and to reach a low level of exhaust emission, Low Temperature Combustion (LTC) modes such as Homogeneous Charge Compression Ignition (HCCI) are being studied. HCCI process is based on the auto-ignition of a highly diluted air/fuel mixture. It allows very low level of NO<sub>x</sub> and particulate emissions. But several problems must be solved: firstly the limited combustion range, the difficulty to control the combustion process, the noise but also the high HC and CO emissions. Two ways appear as very promising to improve HCCI combustion: engine technology and fuel formulation.</div> <div class="htmlview paragraph">In this work, attention is paid on initiation and HCCI combustion control through fuel formulation. This article shows the effect of acetal addition (and in particular butylal addition) in a Diesel base fuel.</div> <div class="htmlview paragraph">It seems to be difficult to have only an “HCCI engine” because the operation zone of the engine is too small. More likely, it will be a Diesel/HCCI engine. The advantage of acetal is to reduce significantly particulate emissions of a Diesel engine. This is an advantage for a Diesel/HCCI engine, when the engine works in Diesel combustion but it is necessary to get a better understanding of the influence of acetal addition in a Diesel base fuel during the HCCI combustion.</div> <div class="htmlview paragraph">Butylal can have a good potential to improve the HCCI combustion by delaying the initiation of a Diesel base fuel but due to its versatile aspect, it is difficult to control, for all the experimental conditions, its effect on the initiation and the HCCI combustion of a Diesel base fuel.</div>

<div class="htmlview paragraph">The reduction of NOx emissions required by the future Euro 6 standards leads engine manufacturers to develop Diesel Homogeneous Charge Compression Ignition (HCCI) combustion processes. Because this concept allows reducing both NOx and particulates simultaneously, it appears as a promising way to meet the next environmental challenges.</div> <div class="htmlview paragraph">Unfortunately, HCCI combustion often increases CO and HC emissions. Conventional oxidation catalyst technologies, currently used for Euro 4 vehicles, may not be able to convert these emissions because of the saturation of active catalytic sites. As a result, such increased CO and HC emissions have to be reduced under standard levels using innovative catalysts or emergent technologies.</div> <div class="htmlview paragraph">The work reported in this paper has been conducted within the framework of the PAGODE project (PSA, IFP, Chalmers University, APTL, CRF, Johnson Matthey and Supelec) and financed by the European Commission. The objectives are to develop advanced new catalyst formulation for efficient CO/HC conversion at low temperature, possibly coupled with new emerging technologies based on non-thermal plasma concept and adapted to the specific HCCI exhaust emissions.</div> <div class="htmlview paragraph">This paper presents one of the first steps of the study: the definition of the oxidation catalyst boundary conditions under HCCI combustion. For this, we perform a detailed characterisation of the CO and HC emissions on a NADI™ dual mode engine (HCCI at low load and switch to conventional combustion at high load), based on a 2.2-liter, 4-cylinder for passenger car.</div> <div class="htmlview paragraph">The regulated pollutant emissions (CO, THC) have been measured with standard gas analyzer devices. Moreover, hydrocarbon speciation has been done with a C1-C9 gas chromatograph (GC) coupled with a FID system. In addition, aldehydes and ketones emissions have been analyzed by means of a High Pressure Liquid Chromatograph (HPLC).</div> <div class="htmlview paragraph">The results show that HCCI combustion tends to form higher methane emissions than conventional combustion. Moreover, high EGR rates lead to higher oxygenated emissions, in particular formaldehyde. If aldehydes are easily converted when the catalyst is warm, methane is not oxidized at these temperatures.</div> <div class="htmlview paragraph">The next steps of the program will be the definition, the fabrication and the tests of an HCCI-dedicated efficient oxidation catalyst to comply with CO and HC emissions standards. Non-thermal plasma reactor will be also mounted in the exhaust line to help the oxidation reaction.</div>

<div class="htmlview paragraph">A naturally aspirated in-line six-cylinder 2.9-litre Volvo engine is operated in Homogeneous Charge Compression Ignition (HCCI) mode, using camshafts with low lift and short duration generating negative valve overlap. This implementation requires only minor modifications of the standard SI engine and allows SI operation outside the operating range of HCCI. Standard port fuel injection is used and pistons and cylinder head are unchanged from the automotive application. A heat exchanger is utilized to heat or cool the intake air, not as a means of combustion control but in order to simulate realistic variations in ambient temperature. The combustion is monitored in real time using cylinder pressure sensors.</div> <div class="htmlview paragraph">HCCI through negative valve overlap is recognized as one of the possible implementation strategies of HCCI closest to production. However, for a practical application the intake temperature will vary both geographically and from time to time. It is therefore very important to gain knowledge of how this variation affects the combustion process.</div> <div class="htmlview paragraph">The operating range in HCCI mode for this specific engine stretches from 650 rpm to 4000 rpm and from 1 bar IMEP to 4 bar IMEP. The emissions of NO<sub>x</sub> are approximately two orders of magnitude lower in HCCI mode compared to SI mode. The emissions of CO are slightly lower while the emissions of HC are slightly higher for HCCI operation. When run in HCCI mode the engine's efficiency is improved throughout the operating range by as much as 10 percentage units compared to SI operation. The effect of inlet air temperature variation on HCCI combustion through negative valve overlap is insignificant when the engine runs well inside of the HCCI operating range. The effect is however prominent when the engine is operated close to the border of the operating range.</div>

<div class="section abstract"><div class="htmlview paragraph">A gasoline compression-ignition combustion system is being developed for full-time operation over the speed-load map. Low-temperature combustion was achieved using multiple late injection (MLI), intake boost, and moderate EGR for high efficiency, low NOx, and low particulate emissions. The relatively long ignition delay and high volatility of RON 91 pump gasoline combined with an advanced injection system and variable valve actuation provided controlled mixture stratification for low combustion noise.</div><div class="htmlview paragraph">Tests were conducted on a single-cylinder research engine. Design of Experiments and response surface models were used to evaluate injection strategies, injector designs, and various valve lift profiles across the speed-load operating range. At light loads, an exhaust rebreathing strategy was used to promote autoignition and maintain exhaust temperatures. At medium loads, a triple injection strategy produced the best results with high thermal efficiency. Detailed heat release analysis indicated that heat losses were significantly reduced. At higher loads, a late-intake-valve-closing strategy was used to reduce the effective compression ratio. For all tests, intake air temperature was 50 C.</div><div class="htmlview paragraph">3D CFD simulations of fuel injection, mixing, and combustion were important to understand the emissions formation processes. With multiple late injections and low-to-moderate fuel pressure, spray penetration was low, mixing was fast, and wall wetting could be avoided. Fuel sprays were characterized in a spray chamber. Injection rate was measured using a rate tube.</div><div class="htmlview paragraph">Results showed that ISFC was very low. Minimum ISFC of 181 g/kWh was measured at 2000 rpm-11 bar IMEP. For IMEP from 2 to 18 bar, engine-out NOx and PM emissions were below targets of 0.2 g/kWh and 0.1 FSN, respectively, indicating that aftertreatment for these species may be reduced or eliminated. It was found that combustion noise levels, characterized by several noise metrics, could be effectively controlled by the injection process. Measurements of exhaust particulate size distribution indicated very low particle count, especially for a preferred injector with low levels of in-cylinder swirl.</div><div class="htmlview paragraph">Collectively, these results demonstrate the potential feasibility of full-time GDCI using RON 91 gasoline at low-to-moderate injection pressures with high fuel efficiency. While more development work is needed, there is good potential for a practical GDCI powertrain system based on these concepts.</div></div>

<div class="htmlview paragraph">Homogeneous Charge Compression Ignition (HCCI) is effective for the simultaneous reduction of soot and NOx emissions from diesel engine. In general, high octane number and volatility fuels (gasoline components or gaseous fuels) are used for HCCI operation, because very lean mixture must be formed during ignition delay of the fuel. However, it is necessary to improve fuel injection systems, when these fuels are used in diesel engine.</div> <div class="htmlview paragraph">The purpose of the present study is the achievement of HCCI combustion in DI diesel engine without the large-scale improvements of engine components. Various high octane number fuels are mixed with diesel fuel as a base fuel, and the mixed fuels are directly applied to DI diesel engine. At first, the cylinder pressure and heat release rate of each mixed fuel are analyzed. The ignition delay of HCCI operation decreases with an increase in the operation load, although that of conventional diesel operation does not almost varied. In addition, the mixed fuel containing much high volatility fuel produces the higher peak of heat release rate of premixed combustion.</div> <div class="htmlview paragraph">The exhaust emissions (CO, CO<sub>2</sub>, THC, NOx, PM) in case of each mixed fuel are measured. Low NOx emission can be achieved by HCCI operation using the mixed fuels despite low volatility of base fuel. However, PM emission is relatively high in case of HCCI operation, and then PM is separated into SOF and ISOF. As a result, it can be confirmed that the high PM emission is derived from the increase in SOF, and the high SOF emission results from the base fuel in the mixed fuels.</div>

<div class="htmlview paragraph">The effects of Exhaust Gas Recirculation (EGR) on diesel engine exhaust emissions were isolated and studied in earlier investigations (<span class="xref">1</span>,<span class="xref">2</span>,<span class="xref">3</span>,<span class="xref">4</span>,<span class="xref">5</span>). This paper analyses the heat release patterns during the combustion process and co-relates the results with the exhaust emissions. The EGR effects considered include the dilution of the inlet charge with CO<sub>2</sub> or water vapour, the increase in the inlet charge temperature, and the thermal throttling arising from the use of hot EGR.</div> <div class="htmlview paragraph">The use of diluents (CO<sub>2</sub> and H<sub>2</sub>O), which are the principal constituents of EGR, caused an increase in ignition delay and a shift in the location of start of combustion. As a consequence of this shift, the whole combustion process was also shifted further towards the expansion stroke. This resulted in the products of combustion spending shorter periods at high temperatures which lowered the NO<sub>x</sub> formation rate. In addition, the longer ignition delay period provided more time for the fuel top penetrate which could have led to larger amounts of gases in the flame envelop, thereby lowering the combustion temperature and resulting in lower NO<sub>x</sub> formation.</div> <div class="htmlview paragraph">Although the increased ignition delay period is expected to cause increasing amount of fuel being burned during the pre-mixed burning period, however, the reduction in oxygen availability associated with the application of EGR diluents reduced the rate at which the fuel burnt in the pre-mixed phase. Moreover, the shift of the combustion process towards the expansion stroke resulted in earlier quenching of the combustion process, that is, shorter combustion duration, which yielded higher levels of products of incomplete combustion in the exhaust.</div> <div class="htmlview paragraph">The different effects of inlet charge thermal throttling (associated with the use of hot EGR) were simulated and the heat release patterns were analysed. The reduction in the inlet charge oxygen, resulting from throttling the inlet charge, caused an increase in the ignition delay; however, this increase was largely offset by reductions in ignition delay due to increased inlet charge temperature and decreased inlet charge mass.</div>

<div class="htmlview paragraph">A DoE analysis was carried out to investigate the effects of the compression ratio, injection timing, injector nozzle hole size and number on performance and emissions in a diesel marine engine, aiming to find out the optimal combination between all the abovementioned parameters.</div> <div class="htmlview paragraph">The study was performed on a six cylinder in line, 100 liter total displacement, diesel marine engine, by means of a 1-D engine simulation fluid-dynamic code, coupled with a multi-zone combustion model for oxide of nitrogen (NOx) and particulate (PM) prediction.</div> <div class="htmlview paragraph">A preliminary detailed validation process, based on an extensive experimental data set, was carried out on the engine model concerning, in particular, the predicted heat release rate, the in-cylinder pressure trace and NOx emissions for several operating points of a propeller load curve.</div> <div class="htmlview paragraph">Afterward a full factorial Design of Experiments (DoE) analysis was carried out in order to find out the optimal combination between compression ratio, injection timing, injector nozzle hole size and number aiming to reach the lowest brake specific fuel consumption value while complying with NOx emissions and peak firing pressures constraints.</div>

<div class="section abstract"><div class="htmlview paragraph">The carbon-neutral biodiesel is a promising renewable substitute for fossil diesel that renders the traditional oxides of nitrogen-particulate matter (NO<sub>x</sub>-PM) trade-off into a unidirectional NO<sub>x</sub> control problem. Low-temperature combustion (LTC) modes such as homogenous charge compression ignition (HCCI) are attractive for obtaining ultra-low NO<sub>x</sub> and PM emissions. Studies on utilizing biodiesel fuel for HCCI combustion mode are sparsely available. Moreover, biodiesel emulsions in the HCCI combustion mode have not been attempted so far. Based on this premise, the present work explored the potential to utilize biodiesel and its emulsions having 20% and 25% water by volume under HCCI operating conditions. Biodiesel was prepared from a non-edible Karanja oil. The biodiesel emulsions were prepared using a heated magnetic stirrer apparatus with 3% by volume of the raw Karanja oil as a surfactant. A production light-duty diesel engine is modified to run in external mixture preparation based HCCI mode. An air-preheater and a fuel vaporizer aid in vaporizing the low volatile biodiesel injected into the inner surface of the vaporizer at 300 bar pressure using a common rail direct injection (CRDI) system. The engine compression ratio was fixed at 15, the maximum operable value with biodiesel. The results show that the engine could be operated only up to 20% of the rated load with biodiesel. With biodiesel-water emulsion, 30% of the rated load could be achieved. The maximum cylinder pressure was higher, and the heat release rate was advanced with emulsions compared to neat biodiesel. The brake specific fuel consumption was reduced by 35%, and the brake thermal efficiency increased by 2.4%, along with ultra-low NO<sub>x</sub> and smoke emissions with emulsions. The present study shows the potential to utilize biodiesel-water emulsions in HCCI mode with reduced exhaust emissions. Suitable charge dilution methods must be adopted to extend the HCCI operating load range with emulsions.</div></div>

<div class="htmlview paragraph">A modified version of KIVA II code was used to perform three-dimensional calculations of combustion in a DI diesel engine. Both an ignition delay submodel and a different formulation of the fuel reaction rate were implemented and tested.</div> <div class="htmlview paragraph">The experiments were carried out on a single cylinder D.I. diesel of 0.75 I displacement equipped with sensors to detect injection characteristics and indicated pressure. A fast acting sampling valve was also installed in the combustion chamber to allow the measurement of main pollutants during the combustion cycle, by an ensemble average technique. Computational and experimental results are compared and the discrepancies are discussed.</div> <div class="htmlview paragraph">Today the demand for light duty engines that produce less emission and consume less fuel is increasing. Thus, if limits on CO<sub>2</sub> emissions are established, the direct injection diesel engine for light duty applications will become an attractive option. Also, manufacturers of heavy duty diesel engines are facing increasingly stringent emission standards. New research efforts aim at improving DI diesels emission behaviour.</div> <div class="htmlview paragraph">Given the complexity of the interaction between the different processes in diesel combustion, comprehensive models of in-cylinder phenomena prove to be useful in providing guidelines for prototype development. Multidimensional analytical models of in cylinder phenomena have been greatly improved over the last decade, given the progress of the efficiency of computer hardware and diagnostic techniques. In particular many authors [<span class="xref">1</span>, <span class="xref">2</span>, <span class="xref">3</span>, <span class="xref">4</span>, <span class="xref">5</span>, <span class="xref">6</span> and <span class="xref">7</span>] have numerically simulated the interaction between the airflow field and the spray and have compared it with experimental data. However, both three dimensional calculations of combustion in the presence of fuel sprays and the corresponding comparisons with experimental data are hardly found in literature. In most cases the comparison between numerical and experimental results is limited to the pressure history and to the fitting of some pollutant emissions late in the expansion stroke.</div> <div class="htmlview paragraph">In particular, limiting the review to diesel combustion, Zellat and others [<span class="xref">8</span>] present calculations of a swirl chamber engine at different loads, injection timings, and speeds, obtained with a modified version of KIVA code. In their results, the computed in-cylinder pressure agrees well with the experimental one. Takeneka and others [<span class="xref">9</span>] used KIVA code to perform calculations of DI Diesel combustion for both swirl chamber and quiescent chamber systems. No details about the modifications introduced in the original code are provided.</div> <div class="htmlview paragraph">Computational comparisons with in cylinder measurements, performed in a quiescent chamber system by Kamimoto [<span class="xref">10</span>], show discrepancies between computations and measurements. Gonzales and others [<span class="xref">11</span>] also used the KIVA II code with the Reitz atomization model and a single step Arrhenius mechanism to describe combustion of tetradecane in a D.I. quiescent chamber engine. The paper focuses on the spray submodel effect on the accuracy of pressure history prediction. It was demonstrated that, if adequate grid resolution is used, prediction of combustion is sensitive to the details of combustion and fuel injection models.</div> <div class="htmlview paragraph">In the present paper a detailed comparison between experiments with a fast acting sampling valve and 3D calculations is presented for a medium duty swirl supported DI engine. Calculations are performed by KI-VA II code with the adoption of the TAB jet break up sub-model and wall impingement modelling. In addition, a different formulation of combustion model is discussed. The current status of the “numerical engine program” in progress at Istituto Motori of CNR, is thus defined.</div>

<div class="htmlview paragraph">The investigation of pumping efficiencies of classic FIE lead to the development of an hydraulic unit injector (HUI). In the HUI the pump plunger is not directly driven by a camshaft but by a distributor-type fuel-injection pump (VE-pump) which simultaneously operates the control.</div> <div class="htmlview paragraph">The HUI was designed with a computer model. The differential piston increases the pressure by the multiplier 4. The VE-pump produces only 30 MPa for an injection pressure of 100 MPa. The high pressure chamber is filled through a small check valve.</div> <div class="htmlview paragraph">Cavitation in the pipe is eliminated by a specially designed delivery valve and a preloaded spring for the differential piston.</div> <div class="htmlview paragraph">A reduced needle shaft (3 mm diameter) leads to a very fast opening and closing of the nozzle, thus allowing to set the opening pressure to 40 MPa.</div> <div class="htmlview paragraph">The HUI's were driven on the pump test bench up to the engine speed of 5000 rpm and reached a peak injection pressure of 120 MPa. On the engine test bench a bmep of 0.8 MPa at a smoke number 4 was reached with a 1.7 l naturally-aspirated engine. The minimum specific fuel consumption was 225 g/kWh. The ignition delays are 2-4°CA shorter than usual due to the small 0.17 mm nozzle holes. This results in a considerably lower NO<sub>x</sub> concentration in the exhaust gas. On the other hand there is still a high smoke emission at part load which cannot be explained:Smoke number = 2 at bmep = 0.3 MPa and n = 3000 rpm.</div>

<div class="section abstract"><div class="htmlview paragraph">The Homogeneous Charge Compression Ignition (HCCI) combustion eliminates the issues of higher particulate matter and nitrogen oxides emissions that prevail in the traditional compression ignition (CI) combustion mode. The complete replacement of traditional fuels with renewable fuels for internal combustion engines is challenging because significant infrastructure changes in the production and delivery systems are required to ensure renewable fuel availability and economic feasibility. Thus, the use of renewable acetone blended with traditional gasoline has been proposed in the present study to smoothen the transition from the traditional CI to the HCCI engines. HCCI experiments were performed in a light-duty diesel engine at 1500 rpm rated speed. By varying the volumetric proportion of the acetone in the gasoline from 20% to 40%, the HCCI engine load range from 20%-60% was achieved, significantly higher than the limited diesel HCCI load range of 20%-38%. An ignition-quality enhancer, 2-Ethylhexyl nitrate, at 6 vol.%, was blended in the acetone-gasoline to prevent the higher cyclic IMEP fluctuations at the lower engine loads. The empirical correlation of the start of combustion (CA10) established on a rapid compression machine facility was validated for the HCCI engine. The combustion phasing (CA50) was predicted using CA10 and fresh air equivalence ratio. A double-Weibe function was developed to characterize specifically the high-temperature heat release rate during the HCCI combustion and determine the extent of its fast-burning core region. An in-depth energy balance study was conducted to elucidate the reason for the indicated thermal efficiency trends. The acetone-gasoline-fuelled HCCI engine generated smoke and nitrogen oxides emissions that were less than 0.0007 g/kW-h and 2.5 g/kW-h, respectively, at all the engine loads. This study concluded that lower exhaust emissions could be obtained without compromising engine performance when the acetone-gasoline blends are used as fuels for operating the HCCI engine.</div></div>

<div class="htmlview paragraph">Heat release data have been obtained from a 5 in bore direct injection diesel engine operating over a range of conditions including off-design conditions, such as running at low air to fuel ratios, using six-hole injector nozzles in place of the standard three-hole nozzles, and reducing compression ratio and thereby swirl ratio at tdc.</div> <div class="htmlview paragraph">The paper shows that when operating with adequate utilized air fuel ratios (A/Fs) the experimental heat release data could be correlated with a single zone combustion model and that changing the number of nozzle holes or compression/swirl ratio had little effect on the correlation after the initial period of heat release.</div> <div class="htmlview paragraph">At inadequate A/Fs a modification to the model to allow for partial combustion was introduced. This improved correlation and enabled reasonable heat release prediction in the cycle synthesis program.</div> <div class="htmlview paragraph">Examples are given of heat release and cycle pressure diagrams computed using these models.</div>

<div class="section abstract"><div class="htmlview paragraph">Strict emission regulations and the need of higher efficiency of future dual fuel engines require an optimized combustion process. For getting a better understanding of the in-cylinder combustion process optical investigations represent a powerful tool. For medium speed dual fuel engines, optical investigations are pretty rare respectively not available. Especially the avoiding of knock events within the combustion process is a key development topic to realize high engine load and high engine efficiency.</div><div class="htmlview paragraph">For the investigations a fully flexible dual fuel test engine was used. The engine is operated with a natural gas / air cylinder charge which is ignited by a small micro pilot diesel injection within the gas mode.</div><div class="htmlview paragraph">Beside standard development measurement techniques like cylinder pressure measurement and exhaust gas emission measurement, optical measurement techniques were used to gain information about the in-cylinder combustion process of the pilot fuel combustion as well as the combustion of the natural gas / air cylinder charge. It is desirable to realize an entire visualization of the combustion within the combustion chamber under real engine conditions at full load. This means the need for a minimal invasive measurement technique with small modifications of the combustion chamber design and surface temperatures.</div><div class="htmlview paragraph">Due to these requirements a tomographic, fiber based measurement technique was used synchronal to a high speed visualization of the in-cylinder OH* chemiluminescence. The fiber based measurement technique uses 128 optical fibers in which every single fiber measures the luminescence of a special observation volume. The raw data of the single fibers is used to calculate, based on a tomographic algorithm, the in-cylinder light emission intensity distribution to visualize the combustion in a horizontal cutting plane over the whole bore diameter. To complete the information of the combustion by the third dimension, a synchronized, intensified high speed camera was used to record the OH* chemiluminescence. Both measurement techniques in combination give a far-reaching image of the in-cylinder combustion process.</div><div class="htmlview paragraph">To increase the understanding of the in-cylinder processes and to optimize the combustion process, typical combustion relevant parameters like pilot injection nozzle, pilot injection timing, pilot injection pressure and excess air ratio were varied. In this publication the basic combustion mechanisms are described for two different injection timings at full load. In addition the isolated pilot fuel combustion in pure air for an injection timing which gives a transient point between partly premixed and strongly premixed combustion is shown.</div><div class="htmlview paragraph">The measurement results show an ignition process which is strongly influenced by the reaction kinetic mechanisms. Especially for early injection timings a long ignition delay was measured which result in a well premixed pilot fuel within the natural gas / air mixture. Moreover, the flame propagation speed was calculated and a reaction kinetically controlled post flame combustion process was observed.</div><div class="htmlview paragraph">In addition a different mechanism for in-cylinder pressure oscillation, which usually is detected as engine knock, was shown which differs from the generally known mechanism in spark plug ignited passenger car engines.</div></div>