Amin Velji

Investigations of the Formation and Oxidation of Soot Inside a Direct Injection Spark Ignition Engine Using Advanced Laser-Techniques

In this work the formation and oxidation of soot inside a direct injection spark ignition engine at different injection and ignition timing was investigated. In order to get two-dimensional data during the expansion stroke, the RAYLIX-technique was applied in the combustion chamber of an optical accessible single cylinder engine. This technique is based on the quasi-simultaneous detection of Rayleigh-scattering, laser-induced incandescence (LII) and extinction which enables simultaneous measurements of temporally and spatially resolved soot concentrations, mean particle radii and number densities. These investigations show that in our test engine the most important source for soot formation during combustion are pool fires, i.e. liquid fuel burning on the top of the piston. These pool fires were observed under almost all experimental conditions.

A Basic Experimental Study of Gasoline Direct Injection at Significantly High Injection Pressures

In gasoline direct injection engines with stratified-combustion strategies only a short time is available for mixture preparation. Therefore, investigations are carried out to evaluate the influence of high injection pressure up to 50 MPa in order to optimize the mixture preparation. Two types of multi-hole injectors are analyzed in a pressure vessel under various pressure and temperature conditions. Laser light sheet visualization technique is applied in order to determine spray characteristics like shape, angle, penetration depth and spray width. To determine the velocity of the air surrounding the spray, a PIV (Particle Image Velocimetry) measurement technique is used. Droplet sizes and velocities are measured with a Phase Doppler Anemometer (PDA) in different positions in the spray center and at the spray edge. Spray visualization experiments show the influence of evaporation on spray propagation at higher temperatures. With increasing chamber pressure, a widening of the jet is observed, while its length remains nearly constant. PIV-measurements show a torus-like flow at the spray front and a distinctive air entrainment into the spray, which depends on injection pressure and chamber pressure. Analyses of droplet velocity and diameter with PDA reveal high velocities of very small droplets at higher injection pressures. However, droplet velocity does not increase in the same dimension as does the fuel pressure.

Ion Current Measurement in Diesel Engines

Contemporary diesel engines are high-tech power plants that provide high torques at very good levels of efficiency. By means of modern injecting-systems such as Common-Rail Injection, combustion noise and emissions could be influenced positively as well. Diesel engine are therefore used increasingly in top-range and sports cars.

Investigation of Cycle-to-Cycle Variations of In-Cylinder Processes in Gasoline Direct Injection Engines Operating With Variable Tumble Systems

To operate gasoline direct injection engines at part load and in stratified mode the mixture formation has to fulfil several requirements. The complexity of this process requires - regarding a suitable mixture transportation and vaporisation of the fuel - an adjusted design of the combustion chamber and the intake ports to reliably place an ignitable mixture at ignition timing near the spark plug at any speed and load. Due to the inhomogeneous mixture distribution during stratified operation, the first combustion period is very sensitive to cycle-to-cycle variations. A reproducible mixture movement with high kinetic energy is necessary for stable engine operation with low fluctuations in the combustion process. Because of the high relevance of these facts, the effects of an adjustable air guiding system in the inlet manifold on in-cylinder flow, ignition and combustion using optical measurement techniques were investigated.

Measurements of spatial flame propagation and flow velocities in a spark ignition engine

Knowledge of flame propagation and flow characteristics enables a more detailed description of in-cylinder processes as well as assessing the influences of engine operating conditions and design parameters, particularly in regard to fuel economy and emission control. Spatial flame propagation in a single-cylinder spark-ignition engine is studied by a measuring technique that uses optical fibers coupled with photomultipliers. The propagation process is monitored through a large number of optical fibers arranged in a matrix in the combustion chamber wall. The spatial flame front shape, the flame volume, and the flame front velocity as a function of time are derived from these measurements. A dual beam laser-doppler-velocimeter (LDV), operating in forward scatter mode, is used to measure flow velocities of fluid motion in a motored and fired engine. Measurements are performed at several points within the combustion chamber. Signal processing is done by a counter. By averaging measurements over several engine cycles, mean flow velocity and RMS values are obtained. It was found that increasing the air-fuel ratio decreases the flame propagation rate and the burning velocity, whereas mean flow velocities of fluid motion are hardly affected. Swirl flow, generated by a shrouded inlet valve, leads to higher mean flow velocities, higher flame front velocities, and higher burning velocities, and the flame front is deformed.

A New Flame Jet Concept to Improve the Inflammation of Lean Burn Mixtures in SI Engines

Engines with gasoline direct injection promise an increase in efficiency mainly due to the overall lean mixture and reduced pumping losses at part load. But the near stoichiometric combustion of the stratified mixture with high combustion temperature leads to high NO x emissions. The need for expensive lean NO x catalysts in combination with complex operation strategies may reduce the advantages in efficiency significantly. The Bowl-Prechamber-lgnition (BPI) concept with flame jet ignition was developed to ignite premixed lean mixtures in DISI engines. The mainly homogeneous lean mixture leads to low combustion temperatures and subsequently to low NO x emissions. By additional EGR a further reduction of the combustion temperature is achievable. The BPI concept is realized by a prechamber spark plug and a piston bowl. The main feature of the concept is its dual injection strategy. A preinjection in the inlet stroke leads to a homogeneous lean mixture with an air-fuel ratio of λ = 1.4 to λ = 1.7. During the compression stroke a second direct injection with a small amount of fuel (about 3 % of the total fuel mass) is directed towards the piston bowl. The enriched air fuel mixture in the piston bowl is transported by the piston motion towards the prechamber spark plug. Due to the pressure difference between main combustion chamber and prechamber the mixture is transported with a highly turbulent flow into the prechamber. After reliable ignition of the enriched mixture in the prechamber, flame jets penetrate into the main combustion chamber and ignite the lean mixture. Numerical and experimental investigations were carried out in a modified 3-valve single cylinder engine for part load operation. The in-cylinder flow including the mixture process in the main combustion chamber and in the prechamber was investigated by CFD simulation, so that the local mixture composition could be predicted. With extended test runs and measurements the functionality of the BPI concept has been proved. For the quantification of the mixture enrichment in the prechamber spark plug ion current measurement has been found as an appropriate measurement tool [11][12]. At part load operation in BPI mode significant reductions in fuel consumption and NOx emission have been achieved compared to stoichiometric operation. Further investigations at full load have been carried out on a single cylinder engine and a 4-cylinder production engine to analyse the influence of prechamber spark plugs on one hand and the influence of lean operation on the other hand on the engine process. Homogeneous full load operation with the prechamber spark plug has shown a reduction in knock sensitivity. Due to significantly reduced cyclic fluctuations the maximum knock amplitudes at the knock limit was reduced.

High Pressure Fuel Pump for Gasoline Direct Injection based on Ceramic Components

Modern direct injection spark ignition engines (DISI-engines) require increasing fuel-injection-pressures in order to accelerate mixture preparation. Therein the fuel-pump is an essential component. Non-conventional materials offer a high potential to realize high pressure combined with low wear and friction. An exemplary high pressure fuel pump was developed in order to evaluate the use of different combinations of ceramic materials and steel as sliding parts. Forces and friction coefficients can be retrieved as a function of the crank angle in the sliding contacts. The leakage in the gap between cylinder and piston was analyzed and an analytical model was developed. Important effects of clearance, stroke frequency and surface roughness on forces and friction coefficients are presented for different combinations of materials and fuels.

The BPI Flame Jet Concept to Improve the Inflammation of Lean Burn Mixtures in Spark Ignited Engines

Spark ignited engines with direct injection (DISI) in fuel stratified mode promise an increase in efficiency mainly due to reduced pumping losses at part load. However, the need for expensive lean NO x catalysts may reduce this advantage. Therefore, a Bowl-Prechamber-lgnition (BPI) concept with flame jet ignition was developed to ignite premixed lean mixtures in DISI engines. It is characterised by a combination of a prechamber spark plug and a piston bowl. An important feature of the concept is its dual injection strategy. A pre injection in the inlet stroke produces a homogeneous lean mixture with an air fuel ratio of λ = 1.5 to λ = 1.7. A second injection with a small quantity of fuel is directed towards the piston bowl during the compression stroke. The enriched air fuel mixture of the piston bowl is transported by the pressure difference between main combustion chamber and prechamber into the prechamber. After the ignition of the mixture strong flame jets penetrate the main combustion chamber and initiate the main combustion process. Numerical and experimental investigations were carried out in a modified 3-valve single cylinder engine. The in-cylinder gas flow including the mixture process in the main combustion chamber and in the prechamber was investigated by CFD simulation so that the local mixture composition could be predicted. With extended test runs and measurements the functionality of the BPI concept has been proved. The BPI flame jet concept operates in all areas of the engine map and even with all kind of DISI processes.

Effect of implementing large-scale charge motion, reducing hydraulic flow of the injector and increasing injection pressure on particle emissions of a GDI engine at WOT and boosted operation:

Gasoline direct injection (GDI) shows advantages compared with port fuel injection (PFI) regarding efficiency and specific power. Due to stricter regulations for fuel consumption (via the regulation of carbon dioxide), GDI engines are becoming increasingly favourable compared with PFI engines. Therefore the share of GDI engines, especially in combination with turbocharging, is increasing in most of the markets with CO2 regulations. Challenging for GDI engines is the mixture formation process due to the short time between fuel injection and the start of combustion. Thus, the injector needs to provide a fine fuel atomization in a considerably short time. The generated spray pattern thereby interacts with the in-cylinder charge motion to generate an appropriate air–fuel mixture. Because of this challenging mixture formation process, the formation of soot in local fuel-rich areas is possible. Thus GDI engines emit more particles compared with PFI engines and need special attention on the mixture formation process. To understand the reasons for the increased particle number (PN) emissions, a project concerning the cause of particle emissions was started at Institute of Internal Combustion Engines (IFKM) in Karlsruhe in 2011. During the project, different causes for PN emissions were identified. This article discusses the possible reasons for particle emissions under high engine load and low engine speed and shows some possible solutions to reduce the emission of particles. Discussed possible solutions to enhance the mixture formation process are the generation of a large-scale charge motion (tumble and swirl), the reduction of the hydraulic flow of the multi-hole, solenoid-activated injector and an increase of the rail pressure up to 50 MPa. The reduction of the hydraulic flow and the increase of the injection pressure lead to smaller average droplets and thus to a faster evaporation. An implementation of a large-scale charge motion enhances the mixture formation process and leads to a reduction of the emitted PN concentration at high engine load. This is shown for wide open throttle (WOT) as well as for boosted operation. The reduction of the hydraulic flow of the injector by reducing the bore hole diameter at constant number of holes and spray targeting of the injector leads to smaller droplets. By increasing the injection pressure, the injection duration as well as the average droplet size can be reduced and leads to a better homogenization and a further reduced PN concentration.

Large Eddy Simulation (LES) with Moving Meshes on a Rapid Compression Machine: Part 2: Numerical Investigations Using Euler–Lagrange-Technique

The flow inside a simplified one-stroke engine with squared cross section has been calculated with compressible Large Eddy Simulation (LES) using our code SPARC and compared with the measurements on the same geometry. The one-stroke engine has a turbulence generator, which can ether generate a tumble or homogenous turbulence depending on the configuration. By waiting different amount of time after the turbulence generation process a variable turbulence level can be achieved. During the up going motion of the piston the turbulent fuel mixture is compressed and ignited by a row of spark plugs. The simulation has been using more then 8 million points for the space discretization. A space conservation law was used to calculate the grid motion with Euler-Lagrange technique. The mesh was refined in the shear layers and close to the wall so that y+ < 1 results almost everywhere. A comparison between Miles (monotonically integrated large eddy simulation) approach and conventional subgrid scale modelling (dynamic Smagorinsky) showed very similar solutions. Mean and fluctuating velocities at TDC are compared with available experimental findings.