MODERN OPTICAL MEASUREMENT TECHNIQUES APPLIED IN A RAPID COMPRESSION MACHINE FOR THE INVESTIGATION OF INTERNAL COMBUSTION ENGINE CONCEPTS

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1 MODERN OPTICAL MEASUREMENT TECHNIQUES APPLIED IN A RAPID COMPRESSION MACHINE FOR THE INVESTIGATION OF INTERNAL COMBUSTION ENGINE CONCEPTS P. Prechtl, F. Dorer, B. Ofner, S. Eisen, F. Mayinger Lehrstuhl A für Thermodynamik, Technische Universität München, D Garching, Germany 1. INTRODUCTION The development of new combustion concepts requires an efficient investigation and improvement of the applied systems. For a correct understanding of the processes inside the combustion chamber of an internal combustion engine, optical measurement techniques represent a modern method of investigation. Especially for larger sizes of engines, a system is necessary which allows a fast adaptation to the desired parameters and an efficient conduction of the experiments. For this purpose, a rapid compression machine (RCM) in a 1:1 scale for large-bore engines (piston displacement approximately 14 l) with a variable optical access has been developed. The background of the RCM setup was a research project concerning a large-bore compression ignition (C.I.) engine, using hydrogen as fuel. This project is carried out in cooperation of two departments of the Technische Universität München with the industrial partner MAN B&W Diesel AG Augsburg. The Lehrstuhl A für Thermodynamik (LAT, department of thermodynamics) investigates the injection and combustion process by means modern optical measurement techniques in the rapid compression machine. In addition to the gaseous fuel injection systems, new generations of Common Rail Systems for truck diesel engines have been investigated. These experiments have been carried out for another research project dealing with the reduction of toxic substances in the exhaust fumes of diesel engines. Both projects are supported by the Bayerische Forschungsstiftung (Bavarian Research Foundation). 2. EXPERIMENTAL SETUP The experimental setup used for the investigations is based on a fully optically accessible rapid compression machine. The machine simulates a single compression stroke under realistic conditions. The piston speed can be adjusted to different operation conditions. For the ship diesel engine the speed is rpm. The compression ratio, the load pressure, the intake temperature as well as the injection timing can be varied. In addition to this fact, a swirl or a turbulence can be engendered in the combustion chamber. The experimental setup allows a variety of investigations under different conditions. It is also simple to replace the injection device or the nozzle geometry. The principle operation of the RCM is based on the opposite movement of two concentric pistons, a compression piston and a mass balance piston. The cross-section of the RCM is shown in Figure 1.

2 Figure 1: Operation of the Rapid Compression Machine (RCM) Both pistons are connected with an oil cushion. The oil flows through large overflow openings from the outer chamber in front of the balance piston into the inner chamber behind the compression piston. This means that the center of gravity is kept at every position of the pistons. This fact allows a strong acceleration of the pistons without any exterior forces and therefore reduces vibrations of the entire machine to a minimum. The vibration free operation of the machine allows the use of sensitive optical measurement techniques. The optical access is achieved through a window in the bottom of the piston with a diameter of 200 mm and if necessary also through two windows in the cylinder head. The material of the windows is quartz glass and, for the use with a UV laser, synthetic quartz glass. The driving energy, pressurized air, is supplied to the balance piston through large tubes. The start procedure of the RCM works as follows: In the starting position the large overflow openings which connect the inner and outer oil chambers are closed with the compression piston. This means that the inner piston works as a large, fast switching valve. After the bypass valve has opened, the compression piston moves slowly forward until it opens the large overflow openings. At this moment the full driving force accelerates both pistons until they are decelerated from the compressed air in the combustion chamber. The RCM allows a maximum combustion pressure over 20 MPa. The injection system can be triggered according to the piston position or to the pressure level. The piston speed can be adjusted with the driving pressure and the damping system at the bottom of the RCM. The intake air is injected through tangential air ports into the cylinder head in order to engender a swirl in the combustion chamber. The cylinder head can easily be replaced for the use of different injection devices.

3 Figure 2: Schlieren/Shadow setup with the RCM This setup allows the use of optical measurement techniques [1,2], such as the laser induced (predissociation) fluorescence (LIF/LIPF) [2] for the detection of several species like OH or the fuel (Hydrogen with Aceton tracer molecules), and fast imaging techniques, such as the high-speed digital video and Schlieren techniques for the visualization of the mixing and combustion processes. A typical setup using the high-speed video camera is shown in Figure 2. A setup using the Schlieren/Shadow method for the simultaneous visualization of the fuel injection and the combustion is shown in Figure 3. It is found that for the diagnosis of the injection and combustion phenomena a time resolved sequence of images is of great interest, especially if strong cyclic variations in real engines are observed. For this reason only methods which deliver sequences of images are considered. These techniques are restricted to experimental setups which use a laser with a high repetition frequency (>15 khz) or which do not need any additional light sources. The key detection system for the image data is a digital high-speed video system. For the observation of the combustion process the emitted light (self-fluorescence) is observed by using the digital high-speed camera at a frame rate of 13.5 khz. In addition to the conventional data such as the pressure, the piston displacement, the needle lift and other required signals, the images are stored digitally. In a next step the data is analyzed, combined with the images and recorded to a digital mpegfilm. These films give a good impression of the location and the spatial distribution of the ignition and the combustion process. Figure 3: High-speed video imaging setup with the RCM

4 The first subject to be discussed, is the investigation of the ignition behavior of the hydrogen diesel engine. The experiments are carried out in the RCM by varying of the compression ratio, the load pressure and the flow condition (variation of the swirl and the turbulence). The compression ratio reaches a value up to 22, the compression end pressure is between 5 and 11 MPa. The temperature reaches values over 950 K. The hydrogen is injected with a pressure between 25 and 33 MPa and a maximum injection duration of 15 ms. In a next step the influence of the injector geometry is analyzed. For these experiments, nozzles with a hole number of 6 up to 24 and a slot nozzle with nearly equivalent crosssections are manufactured. These measurements were carried out under the same pressure conditions. The second subject is the investigation of two different types of Common-Rail injection systems of a truck engine. For this purpose, the piston and cylinder head were adapted to the appropriate diameters. The injection systems were tested under conditions where an unusual engine behavior was observed. 3. RESULTS AND DISCUSSION In the first series of experiments for the hydrogen engine, it has been shown that the compression ratio has a significant influence on the ignition delay. Higher temperatures lead to shorter ignition delays [3]. However, large variations of the ignition delays have been observed in all the experiments. The ignition occurs statistically distributed in the combustion chamber. The analysis of the high-speed films proves the assumption that an ignition of a single jet does not lead to the ignition of other hydrogen jets as it is shown in Figure 4. It has also been observed that several jets have not ignited. It can be inferred that different areas of combustible hydrogen mixtures of the injected jets in a single compression cycle can have different ignition delays. As a result, cycle to cycle variations in the pressure recordings are expected, and the possibility of miss-fire cannot be excluded. Possible reasons can be statistical effects on the ignition due to the compression end temperatures near the auto-ignition temperature of the hydrogen and the varying mixing conditions. It is likely that there is an influence of impurities in the injected hydrogen, the intake-air from the evaporated lubricating oil film of the piston. The cyclic variations of the combustion were also observed in a single cylinder test engine and could be explained with these observations. All experiments with hole numbers up to 10 show that a burning jet hardly ignites the neighbor jet. As discussed above, this behavior leads to large variations in the pressure recordings. As a result of these observations, a nozzle system with a spatially connected combustible mixture distribution was set up. Experiments conducted with nozzles with 18 holes or a slot show a much better combustion behavior than those with 6 to 10 holes. Once auto-ignition occurs, the flame propagates very fast over all injected jets. Due to the equivalent cross-section of the nozzles, the diameter of the holes decreases with the increase of the number of holes. A smaller bore diameter reduces the momentum of the injected hydrogen significantly (~d 2 ). Owing to a smaller momentum the jet is slowed down faster and a reduced penetration speed of the jets can be observed. Due to the lower speed, the auto-ignition begins in the vicinity of the nozzle. The best ignition behavior is observed with a slot nozzle. Under these conditions, an earlier and a better reproducible ignition can be achieved. On the other hand, a longer combustion duration is observed. The flow conditions have significant influence on the combustion process. Nozzle geometries with low jet velocities and large ignitable areas have good auto-ignition properties, yet induce insufficient conditions for a fast and effective overall combustion. The flow energy for the mixing process is lost in highly throttling nozzles. The nozzles with 6 holes show, in contrast to nozzles with more bore holes, better combustion properties as soon as the hydrogen is ignited. The combination of both advantages leads to the application of nozzles with a series of large and small bores. The existence of the small bores ensures an early and reliable ignition in the vicinity of the nozzle and a fast flame propagation which leads to the ignition of the surrounding jets. The large bores, however, cause a high momentum of the injected hydrogen, which entails a good mixing. Figure 5 illustrates the ignition and the combustion process of a combined 18-hole nozzle with 6 x 0.6 mm and 12 x 0.4 mm bores. A fast flame propagation around the nozzle is achieved within 0.6 milliseconds. In addition to this fact a faster penetration of the jets from the large bore holes to the wall of the cylinder is observed which results in a more efficient use of the whole combustion chamber. Nozzles of these types are, at present, the best combination of a reliable ignition and of an efficient combustion. However, the variations of the ignition processes and the pressure rise rates are higher than those observed in real diesel engines. The possibility of setting up a configuration with late internal mixing and spark ignition, such as that used in a stratified charge engine for example in the new type of GDI (gasoline direct injection) engines, is also of great interest. For this reason experiments have been set up with a small modification to the cylinder

5 head of the RCM, the insertion of a spark plug. With the same nozzle geometry and a reduced compression ratio, the auto-ignition can be avoided which enables a reliable and a fast ignition of the hydrogen. Due to the optimized arrangement of the holes in the nozzle, a fast flame propagation around the nozzle is achieved within 1 ms. Figure 6 shows a spark ignited combustion. The spark-plug timing is set 1 ms after the beginning of the hydrogen injection. With a late injection near TDC and an immediate ignition with a spark plug, higher compression ratios can be realized, as opposed to processes with an external or early internal mixing. An overview over the research activities on hydrogen engines is given in [4] and a more detailed discussion of the above discussed results in [5-7]. Figure 4: Compression ignition of hydrogen, injection near TDC, 6 hole nozzle, time for ignition of all jets is longer than 3.5 ms Figure 5: Compression ignition of hydrogen, injection near TDC, 18=(6+12) hole nozzle, time for the ignition of all jets is reduced to 0.6 ms

6 Figure 6: Stratified charge engine, spark ignition of the hydrogen, injection near TDC, 18=(6+12) hole nozzle, spark ignition 1 ms after injection begin6 ms However, the understanding of the combustion process requires, in addition to the flame propagation behavior, the knowledge of the mixture formation before the ignition. For these investigations a Schlieren/Shadow setup is used, as shown in Figure 3. By means of these measurements a detailed investigation of the injection process is possible. The jet penetration length, the jet opening angle, the jet deflection and the spatial distribution can be determined, down to a time resolution of 25 us. The injection process of the above discussed 18 hole nozzle is shown in Figure 7. The images are recorded with an image rate of 13.5 khz. It can be seen that the jets, which are injected from the large bores have a faster propagation than the jets, which are injected from the small bores. From these results the possible locations and the timing of a spark plug can be determined. In the above shown pictures of Figure 6 the spark plug is located in the ten o clock position of the nozzle, in the direction of a large bore hole. So the fast jets can reach the spark in a minimum span of time. Figure 7: Mixture formation of a hydrogen injection 18=(6x0.6mm+12x0.4mm) hole nozzle The second series of experiments were carried out to investigate the behavior of new prototypes of Common-Rail Systems for truck engines. In experiments with test engines, driven at low load conditions, a higher amount of pollutant emissions was observed. Especially these conditions were investigated in the RCM to a better understanding of the mixture formation and the ignition process, which caused these unexpected high emissions. Under these conditions a high potential is seen to reduce the overall emission for the prescribed test cycles. Two types of nozzles were investigated. The terms of the manufacturer for

7 the injectors are DLLZ-type and DSLZ-type. The typical behavior of a DLLZ nozzle for a injection with pre-injection is shown in Figure 8. Figure 8: Diesel combustion with pre-injection, DLLZ, very symmetric injection of small amounts of diesel, unintended post-injection of diesel Figure 9: Diesel combustion with pre-injection, DSLZ, asymmetric injection of small amounts of diesel, no post injection of diesel The small amount of diesel of the pre-injection, which is used to preheat the combustion chamber and to reduce the noise emission is very symmetrically distributed in the vicinity of the DLLZ-nozzle. It also ignites symmetrically. The problem of this nozzle type is the closing of the needle. After the closing of the needle a very small amount of diesel is injected with low momentum and thus with an insufficient mixture formation which might be the cause of a higher soot emission. This problem is reduced with the DSLZ-type of injectors. The typical combustion process of this type of injectors is shown in Figure 9. At the end of the injection process no diesel fuel is injected after closing the needle. However, this version of injectors have problems at the beginning of the injection process, when very small amounts of diesel are injected. A vibration of the needle during the opening phase causes a very asymmetric fuel jet, which is injected into the combustion chamber during the pre-injection. Ignition occurs on the bottom right side of the nozzle. The injection of the main diesel amount is also asymmetrical, yet on the opposite side of the nozzle. The jet impinges immediately on the walls and a asymmetric combustion occurs at the beginning of the injection. With these investigations the reasons of an unusual behavior can be found and information for a purposeful improvement of the injection devices can be achieved.

8 4. CONCLUSIONS The developed setup for the investigations of a large-bore C.I. engine with full optical access allows a detailed analysis of all relevant processes. The modern optical techniques enable the analysis of the mixing, the ignition and the combustion. The results obtained with this setup contribute to a better understanding of the very fast processes. Furthermore this setup allows an efficient testing and development of engine components. The combination of the compact RCM and the high-speed measurement techniques offer a powerful research tool. By using this system, a time resolved analysis of a fast process can be attained within a single experiment. In order to get an overview of the injection process of about 10 ms duration, the system delivers in a single shot 135 pictures. The time between two experiments is 2-5 minutes. The five minute period includes the cleaning of the piston window inside the combustion chamber which would be difficult to realize using a glass engine. The concept of the RCM has been proved successfully in the last years. The good quality of the experiments and the fast execution of the experiments have led to the construction of small RCM with a piston displacement of 0.5 l for the study of new injection systems for car diesel engines. The new system has been used successfully for nearly one year. All projects were supported by the Bayerische Forschungsstiftung and industrial partners: MAN B&W Diesel AG Augsburg, BMW, Audi, MAN Nürnberg. REFERENCES [1] Mayinger F., Optical Measurement Techniques, Springer-Verlag, Berlin, Heidelberg, New York, Tokyo, 1994 [2] Eckbreth A.C., Laser Diagnostics for Combustion Temperature and Species, Abacus Press, 1988 [3] Siebers D., Naber J., Hydrogen Combustion under Diesel Engine Conditions, XI World Hydrogen Conference, , 1996 [4] Das L. M. Hydrogen Engines, A view of the Past and a look into the future Int. J. Hydrogen Energy, Vol. 15, , 1990 [5] Dorer F., Prechtl P., Mayinger F., Investigation of Mixture Formation and Combustion Processes in a Hydrogen Fueled Diesel Engine, Hypothesis II, August 1997 [6] Zeilinger, Wiebicke, Rottengruber, Woschni, Investigation of a directly injecting hydrogen large bore C.I. engine, XII World Hydrogen Conference 1998 [7] Dorer F., Prechtl P., Mayinger F. High pressure hydrogen injection system for a large bore four stroke diesel engine: investigation of the mixture formation with optical measurement techniques and numerical simulations., XII World Hydrogen Conference 1998 Web-Sites: