Ingemar Denbratt

HCCI Operation of a Passenger Car Common Rail DI Diesel Engine With Early Injection of Conventional Diesel Fuel

The possibilities of operating a direct injection Diesel engine in HCCI combustion mode with early injection of conventional Diesel fuel were investigated. In order to properly phase the combustion process in the cycle and to prevent knock, the geometric compression ratio was reduced from 17.0:1 to 13.4:1 or 11.5:1. Further control of the phasing and combustion rate was achieved with high rates of cooled EGR. The engine used for the experiments was a single cylinder version of a modern passenger car type common rail engine with a displacement of 480 cc. An injector with a small included angle was used to prevent interaction of the spray and the cylinder liner. In order to create a homogeneous mixture, the fuel was injected by multiple short injections during the compression stroke. The low knock resistance of the Diesel fuel limited the operating conditions to low loads. Compared to conventional Diesel combustion, the NOx emissions were dramatically reduced. The smoke emissions also showed a significant reduction, while CO and HC emissions increased substantially. The HCCI combustion mode is characterized by a much more rapid heat release and higher fuel consumption, due to the lower compression ratio and the high HC and CO emissions.

Direct Gasoline Injection in the Negative Valve Overlap of a Homogeneous Charge Compression Ignition Engine

An engine with variable valve timing was operated in homogeneous charge compression ignition (HCCI) mode. In two sets of experiments, the fuel was introduced directly into the combustion chamber using a split injection strategy. In the first set, lambda was varied while the fuel flow was constant. The second set consisted of experiments during which the fuel flow was altered and lambda was fixed. The results were evaluated using an engine simulation code with integrated detailed-chemistry. The auto-ignition temperature of the air-fuel mixture was reached when residual mass of the previous combustion cycle was captured using a negative valve overlap and compressed together with the fresh mixture charge Inducted. When a pilot fuel amount was introduced in the combustion chamber before piston TDC, during the negative valve overlap, radicals were formed as well as intermediates and combustion took place during this overlap provided the mixture was lean. This coon was observed during both the experiments and simulations. Combustion in the gas exchange phase increased the mixture temperature in the main compression stroke, hence advancing the auto ignition timing. The degree of main combustion advancement was found to be dependent on the level of oxygen available in the negative valve overlap and the pilot fuel amount. Calculations showed that intermediates present in the mixture at the start of the compression stroke could result in a significant advancement of auto-ignition and heat release.

Laser-induced fluorescence of formaldehyde in combustion using third harmonic Nd : YAG laser excitation

Formaldehyde (CH2O) is an important intermediate species in combustion processes and it can through laser-induced fluorescence measurements be used for instantaneous flame front detection. The present study has focussed on the use of the third harmonic of a Nd:YAG laser at 355 nm as excitation wavelength for formaldehyde, and different dimethyl ether (C2H6O) flames were used as sources of formaldehyde in the experiments. The investigations included studies of the overlap between the laser profile and the absorption lines of formaldehyde, saturation effects and the potential occurrence of laser-induced photochemistry. The technique was applied for detection of formaldehyde in an internal combustion engine operated both as a spark ignition engine and as a homogenous charge compression ignition engine.

Simulation of a Two-Stroke Free Piston Engine

The free piston internal combustion engine used in conjunction with a linear alternator offers an interesting choice for use in hybrid vehicles. The linear motion of the pistons is directly converted to electricity by the alternator, and the result is a compact and efficient energy converter that has only one moving part. The movement of the pistons is not prescribed by a crank mechanism, but is the result of the equilibrium of forces acting on the pistons, and the engine will act like a mass-spring system. This feature is one of the most prominent advantages of the FPE (Free Piston Engine), as the lack of mechanical linkage gives means of varying the compression ratio in simple manners, without changing the hardware of the engine. By varying the compression ratio, it is also it possible to run on a multitude of different fuels and to use HCCI (Homogeneous Charge Compression Ignition) combustion. Furthermore, the reduction of the number of moving parts will decrease engine friction and thus increase efficiency. In this paper, BOOST and SENKIN have been used to investigate engine performance for different fuels. A dynamic model of the complete free piston engine was created that predicts the piston motion and frequency. The gas exchange was simulated with the commercial 1- D code BOOST, which solves the gas dynamic equations. The high-pressure cycle of the commercial 1- D code BOOST was replaced by detailed chemistry calculations in the SENKIN code. For combustion reduced mechanisms of Diesel (n-heptane and toluene), gasoline (iso-octane, toluene and n-heptane), natural gas (methane, ethane, propane and n-butane) and hydrogen have been used. All mechanisms consisted of about 60-100 species. The results show that a decreased cetane number requires higher compression ratios in order to position the ignition properly. The higher compression ratios give an increase in engine speed, power and efficiency.

HCCI Combustion Using Charge Stratification for Combustion Control

This work evaluates the effect of charge stratification on combustion phasing, rate of heat release and emissions for HCCI combustion. Engine experiments in both optical and traditional single cylinder engines were carried out with PRF50 as fuel. The amount of stratification as well as injection timing of the stratified charge was varied. It was found that a stratified charge can influence combustion phasing, increasing the stratification amount or late injection timing of the stratified charge leads to an advanced CA50 timing. The NOx emissions follows the CA50 advancement, advanced CA50 timing leads to higher NOx emissions. Correlation between CA50 can also be seen for HC and CO emissions when the injection timing was varied, late injection and thereby advanced CA50 timing leads to both lower HC and CO emissions. This trend can not be seen when the stratification amount was varied, increased stratification amount leads to higher CO emission and for operating condition with late CA50 timing the HC emissions also increase with increasing stratification amount. Optical studies, with high speed CCD camera, show that an increase in stratification leads to poor combustion quality near the cylinder walls, due to leaner mixtures near the cylinder walls and this results in higher HC and CO emissions. The maximum rate of heat release depends on stratification amount - a larger amount gives a lower rate of heat release but the main heat release is advanced. Varied injection timing results in different phasing of the main heat releases. The use of charge stratification for HCCI combustion can lead to a larger operating range, due to its effect on combustion phasing and rate of heat release, since the upper load range is partly restricted by too high rates of heat release leading to high pressure oscillations and the lower load to late combustion phasing leading to high cycle-to-cycle variations. Copyright

Effects of Injector Parameters on Mixture Formation for Multi-Hole Nozzles in A Spray-Guided Gasoline DI Engine

This paper focuses on ways of improving the spray formation from spray-guided multi-hole gasoline direct injection injectors. Work has been done both experimentally using laser diagnostic tools and numerically using Computational Fluid Dynamics. Laser Induced Exciplex Fluorescence (LIEF) measurements in a constant pressure spray chamber and optical engine measurements have shown that injectors with 6-hole nozzles and 50-degree umbrella angles are unsuitable for stratified combustion because they produce steep air-fuel ratio gradients and create a spray with overly-deep liquid fuel penetration as well as presence of liquid fuel around the spark plug. In order to study injector performance, numerical calculations using the AVL FIRE™ CFD code were performed. The numerical results indicate that by increasing the injector umbrella angle, the extent of piston wall wetting can be decreased. Also, changing the pattern of holes in the nozzle changes the spray pattern, enabling its optimization with respect to ignition and flame propagation. Furthermore, PDA and Direct Imaging experiments showed that increasing the l/d ratio by reducing the hole diameter resulted in a decrease in the mean droplet sizes (D32). The spray angle was found to increase with decreasing l/d ratios. It has also been shown that by choosing a suitable l/d ratio it is possible to control the local AFR and cross-flow velocity at the spark plug. Copyright

The Influence of PRF and Commercial Fuels with High Octane Number on the Auto-ignition Timing of an Engine Operated in HCCI Combustion Mode with Negative Valve Overlap

A single-cylinder engine was operated in HCCI combustion mode with different kinds of commercial fuels. The HCCI combustion was generated by creating a negative valve overlap (early exhaust valve closing combined with late intake valve opening) thus trapping a large amount of residuals (~ 55%). Fifteen different fuels with high octane numbers were tested six of which were primary reference fuels (PRFs) and nine were commercial fuels or reference fuels. The engine was operated at constant operational parameters (speed/load, valve timing and equivalence ratio, intake air temperature, compression ratio, etc.) changing only the fuel type while the engine was running. Changing the fuel affected the auto-ignition timing, represented by the 50% mass fraction burned location (CA50). However these changes were not consistent with the classical RON and MON numbers, which are measures of the knock resistance of the fuel. Indeed, no correlation was found between CA50 and the RON or MON numbers. However, when only the PRFs were considered, a correlation was found between the auto-ignition timing and the RON number. Although a substantial difference in auto-ignition timing between PRF 70 ON and PRF 98 ON was expected, the difference was only 1.5 CAD. Furthermore, the differences in auto-ignition timing between all fuels spanned only 4.5 degrees. It was found that the reason for the different behavior of the fuels during these measurements was the method of generating the HCCI combustion mode. Retaining residuals with a negative valve overlap makes the combustion mode less sensitive to different fuel qualities. This is because chemical reactions during the negative valve overlap affect the auto-ignition timing proportionally to the amount of unburned hydrocarbons remaining from the main combustio

Spray Shape and Atomization Quality of an Outward-Opening Piezo Gasoline DI Injector

The spray formation and consequent atomization of an outward opening piezo-electric gasoline DI injector have been experimentally investigated in a constant pressure spray chamber. The sizes and velocities of the droplets and the resulting spray shape were evaluated, under different boundary conditions, using Planar Mie scattering and Planar Laser-induced Fluorescence (PLIF) in combination with Phase Doppler Anemometry (PDA) analyses and high-speed video photography. The use of piezo-electric actuation for gasoline DI injectors provides an additional means to control the atomization and spray shape that is not available with solenoid-driven injectors such as swirling and multi-hole type injectors. For instance, with piezo injectors up to four injections per cycle are possible, and the fuel flow rate can be controlled by adjusting needle lift. The captured high-speed video images show that a hollow-cone spray forms as the fuel exits the outward-opening nozzle. Shortly after the start of injection, the momentum exchange with the surrounding air creates a recirculation zone at the leading edge of the spray. The images also show that the position and size of this recirculation zone depends on the chamber back pressure and that it too can be controlled by the injection timing. In addition, the PDA analysis shows that the creation of the recirculation zone, with a fuel pressure of 20 MPa, results in good atomization, slow-moving droplets, and helps guide fuel to the spark plug in stratified charge operation. The effects of flash boiling on the injectors sprays were also investigated. The results indicate that flash boiling affects the spray shape less than when multi-hole injectors are used. Furthermore, it has been shown that the use of multiple injections per cycle can decrease the spray penetration and provide an extended window of ignitable mixture at the spark plug.

In-cylinder soot imaging and emissions of stratified combustion in a spark-ignited spray-guided direct-injection gasoline engine

The combustion in a spark-ignited spray-guided gasoline direct-injection engine operating in a stratified mode has been studied by in-cylinder imaging of the fuel, OH*, and soot distributions. Information on the fuel distribution was obtained by laser-induced fluorescence imaging of the aromatic molecules in the gasoline. The OH* and soot distributions were simultaneously visualized by detection of the natural emissions at 306 nm (OH*) and around 530 nm (soot) using two intensified charge-coupled device cameras. In addition to the in-cylinder observations, engine-out soot emissions, NOx, and HC were measured. The engine was operated at a speed of 2000 r/min and an indicated mean effective pressure of 2.5 bar, with a fully open throttle, resulting in a globally lean combustion with a fuel–air equivalence ratio of about 0.25. The gasoline was injected in single or double injections by an outward-opening piezo-actuated injector. The combustion was ignited efficiently at locally fuel-rich conditions. The soot formation and oxidation were investigated for the two injection strategies, each with three injection timings and two fixed ignition timings. The results showed that soot was efficiently formed and oxidized. From the in-cylinder measurements, it could be seen that the soot luminescence intensity quickly rose and then declined, while the combustion temperature was still increasing. Furthermore, the OH* intensity was still increasing as the soot luminescence was declining. The soot incandescence peak intensity occurred at a crank angle degree close to 50 per cent mass burned, and the OH* intensity peak arose later, shortly before the maximum soot temperature around top dead centre (TDC). When the injection timing was retarded, with constant ignition timing with respect to injection, it was found that the total soot luminosity increased. In addition, less OH* chemiluminescence was observed during the decrease of the soot incandescence, implying conditions less favourable for efficient soot oxidation in the later part of the combustion for retarded injections. This was confirmed by the engine-out soot emission measurements, which showed increased soot levels as the injection was retarded. It was also found that fuel impinged on the spark plug during the injections, resulting in a persistent jet flame close to the spark plug in the centre of the cylinder, which is believed to contribute to engine-out soot emissions.

Piston Temperature Measurement by Use of Thermographic Phosphors and Thermocouples in a Heavy-Duty Diesel Engine Run Under Partly Premixed Conditions

Piston temperature experiments were conducted in a single-cylinder heavy-duty diesel research engine both by use of optical temperature sensitive phosphor and of thermocouples mounted on the piston surface. In the former case, a thin coating of a suitable thermographic phosphor was applied to the areas on the piston surface to be investigated. The optical measurements involved the use of an optical window and of an endoscope. The possibility of using optical fibres into guide light in and out of the engine was also investigated. Results of the optical and of the thermocouple measurements were compared and were also related to more global data with the aim of exploring the use of thermographic phosphors for piston-temperature measurements in diesel engine. Thermographic phosphors thermometry was found to represent an alternative to the thermocouple method since it easily can be applied to various piston geometries.