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SAE 2011 High Efficiency IC Engines Symposium - Session 2 - Frontiers in Light-Duty Engine R&D Presenter Ulrich Spicher, Karlsruhe Inst. of Technology

In this work, heat loss was investigated in two different HCCI single cylinder engines. Thermocouples were adapted to the surfaces of the cylinder heads and the temperature oscillations were detected in a wide range of the engine operation conditions. The local heat transfer is analyzed with port fuel and direct injection, for different engine parameters and operating points. It is shown that the spatially averaged measured heat loss in HCCI operation represents the global heat loss well. The spatial variations are small in the operation map presuming stable operating points with low cyclic variations and good engine performance. Furthermore, the heat loss measured in HCCI operation is compared to the heat loss detected in homogeneous and stratified DI-SI operation in the same engine. It is shown that the local heat losses in stratified DI-SI operation show large variations, depending on the direction of the flame propagation.

In the present paper the results of a set of experimental investigations on LSPI are discussed. The ignition system of a test engine was modified to enable random spark advance in one of the four cylinders. LSPI sequences were successfully triggered and exhibited similar characteristics compared to regularly occurring pre-ignition. Optical investigations applying a high speed camera system enabling a visualization of the combustion process were performed. In a second engine the influence of the physical properties of the considered lubricant on the LSPI frequency was analyzed. In addition different piston ring assemblies have been tested. Moreover an online acquisition of the unburned hydrocarbon emissions in the exhaust gas was performed. The combination of these experimental techniques in the present study provided further insights on the development of LSPI sequences.

In this work, heat loss was investigated in two different HCCI single cylinder engines. Thermocouples were adapted to the surfaces of the cylinder heads and the temperature oscillations were detected in a wide range of the engine operation maps. The resultant heat transfer profiles were compared to the heat losses predicted by existing models. As major discrepancies were stated, a new phenomenological model was developed that is well-manageable and describes the heat loss in HCCI mode more precisely than existing models. To analyze the insulating effect of deposits, the heat transfer equation was solved analytically by an approach that allows consideration of multiple layers with different material properties and thickness. This approach was used for the first time in conjunction with engines to calculate the heat flux at the surface of deposits and the deposit thickness.

The two-stroke SI engine remains the dominant concept for handheld power tools. Its main advantages are a good power-to-weight ratio, simple mechanical design and low production costs. Because of these reasons, the two-stroke SI engine will remain the dominant engine in such applications for the foreseeable future. Increasingly stringent exhaust emission laws, in conjunction with the drive for more efficiency, have made new scavenging and combustion processes necessary. The main foci are to reduce raw emissions of unburned hydrocarbons via intelligent guidance of the fresh air-fuel mixture and to improve performance to reduce specific emissions. The flow velocity in the electrode gap of the spark plug is of great interest for the ignition of the air-fuel-mixture and the early combustion phase of all kinds of SI engines. In these investigations, the flow velocity in the spark plug gap of a two-stroke gasoline engine with stratified scavenging was measured under various conditions.

This paper presents the results of experimental and numerical investigations on pre-ignition in a series-production turbocharged DISI engine. Previous studies led to the conclusion that pre-ignition can be triggered by auto-ignition of oil droplets generated in the combustion chamber. Analysis of more recent experiments shows that a modification of the engine operation parameters that promotes spray/lubricant interaction also increases pre-ignition frequency, while modifications that enhance the speed of chemical reactions (thereby favoring auto-ignition) have little or no influence. The experimental and numerical findings can be explained if we assume the existence of a substance (originating from lubricant/fuel interaction) that displays extremely short ignition delay times.

Rapeseed oil can be a possible substitute for fossil fuel in Diesel engines. Due to different physical properties of rapeseed oil like higher viscosity and higher compressibility compared to diesel fuel, rapeseed oil cannot be easily used in conventional Diesel engines without modifications. Especially incomplete combustion leads to deposits in the combustion chamber and higher exhaust gas emissions. These unfavorable characteristics are caused primarily by insufficient mixture preparation. The adjustment of the injection system will improve the mixture preparation and the combustion of a Diesel engine, operated with rapeseed oil. The nozzle geometry is the main parameter of the whole injection system chain to realize a better combustion process and so higher efficiency and lower exhaust gas emissions.

Natural vegetable oil like rape seed oil is a potential substitute for regular fuel for diesel engines. Compared to other biogen fuels like rape seed methyl ester (RME), pure rape seed oil is neutral towards groundwater and it needs considerably less energy and additives for production. Different physical properties of rape seed oil compared to Diesel fuel are the reason why conventional Diesel engines can hardly be used satisfactorily with rape seed oil without being modified. Poor exhaust-emission behavior is caused by the incomplete combustion. Due to poor spray atomization of vegetable oil, an increased fuel entrainment in the lubricating oil, carbonization in the combustion chamber and deposits at injectors and valves are further drawbacks of injection systems designed for conventional diesel fuel. The preheating of this fuel can solve some problems.

A multi-optical fiber technique is presented, which enables one to detect the flame propagation during non-knocking and knocking conditions in real production engines. The measurement technique is appropriate to detect knock onset locations and to describe the propagation of knocking reaction fronts. With this knowledge, the combustion chamber shape can be optimized, leading to a better knock resistance and higher combustion efficiencies. Results of flame propagation under non-knocking and knocking engine operating conditions are presented. In addition, correlations between knock onset locations and areas in which knock damage occurs are shown for different engines. Presented are the effects of combustion chamber modifications on the combustion efficiency, based on the analysis of the optical fiber measurements.

The paper presents an application of a quasi-dimensional (QD) model for the combustion simulation in a two-stroke engine. In contrast to 0D-models the QD-models provide an opportunity to describe the development of the combustion process in dependence on the actual thermodynamic state in the combustion chamber. The QD-models enable to couple the flame propagation with the combustion chamber geometry and with the flow field. An extensive sensitivity analysis is performed for the QD-model by varying the parameters of the QD-model itself and of the operating points. The constructed QD-model is examined under various conditions (engine speed, the delivery ratio and the air to fuel ratio) and shows a good agreement with experimental results.

The cyclic changes of the cylinder pressure are mainly influenced by the primary inflammation phase, which in turn depends on the local air/fuel ratio and the residual-gas fraction at the spark plug. The ion-current measurement technique is based on the conductivity of the mixture during the internal combustion. It is therefore possible to use the signal for combustion diagnostics when using the spark plug as a sensor. This article demonstrates the potential of ion sensing at the spark plug and in the combustion chamber to detect sources of interference which prevent an optimal combustion process. Comparing the ion signals of consecutive combustion cycles delivers explanations of phenomena that could not yet be sufficiently characterized by cylinder-pressure indication. The results allow new fundamental approaches to the optimization of the combustion process.

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 a combination of Rayleigh-scattering, laser-induced incandescence (LII) and extinction which enables simultaneous measurements of temporally and spatially resolved soot concentration, mean particle radii and number densities. These first investigations show that 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.

Spray-guided gasoline direct injection demonstrates great potential to reduce both fuel consumption and pollutant emissions. However, conventional materials used in high-pressure pumps wear severely under fuel injection pressures above 20 MPa as the lubricity and viscosity of gasoline are very low. The use of ceramic components promises to overcome these difficulties and to exploit the full benefits of spray-guided GDI-engines. As part of the Collaborative Research Centre “High performance sliding and friction systems based on advanced ceramics” at Karlsruhe Institute of Technology, a single-piston high-pressure gasoline pump operating at up to 50 MPa has been designed. It consists of 2 fuel-lubricated sliding systems (piston/cylinder and cam/sliding shoe) that are built with ceramic parts. The pump is equipped with force, pressure and temperature sensors in order to assess the behaviour of several material pairs.

Investigations of the fuel injection processes in a spark ignition direct injection engine have been performed for two different fuels. The goal of this research was to determine the differences between isooctane, which is often used as an alternative to gasoline for optical engine investigations, and a special, non-fluorescing, full boiling range multicomponent fuel. The apparent vaporization characteristics of isooctane and the multicomponent fuel were examined in homogeneous operating mode with direct injection during the intake stroke. To this end, simultaneous Mie scattering and planar laser induced fluorescence imaging experiments were performed in a transparent research engine. Both fuels were mixed with 3-Pentanone as a fluorescence tracer. A frequency-quadrupled Nd:YAG laser was used as both the fluorescent excitation source and the light scattering source.

Diesel engines face difficult challenges with respect to engine-out emissions, efficiency and power density as the legal requirements concerning emissions and fuel consumption are constantly increasing. In general, for a diesel engine to achieve low raw emissions a well-mixed fuel-air mixture, burning at low combustion temperatures, is necessary. Highly premixed diesel combustion is a feasible way to reduce the smoke emissions to very low levels compared to conventional diesel combustion. In order to reach both, very low NOX and soot emissions, high rates of cooled EGR are necessary. With high rates of cooled EGR the NOX formation can be suppressed almost completely. This paper investigates to what extent the trade-off between emissions, fuel consumption and power of a diesel engine can be resolved by highly premixed and low temperature diesel combustion using injection nozzles with reduced injection hole diameters and high pressure fuel injection.

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 NOx catalysts may reduce this advantage. Therefore, a Bowl-Prechamber-Ignition (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.

High frequency ignition (HFI) and conventional transistor coil ignition (TCI) were investigated with an optically accessible single-cylinder research engine to gain fundamental understanding of the chemical reactions taking place prior to the onset of combustion. Instead of generating heat in the gap of a conventional spark plug, a high frequency / high voltage electric field is employed in HFI to form chemical radicals. It is generated using a resonant circuit and sharp metallic tips placed in the combustion chamber. The setup is optimized to cause a so-called corona discharge in which highly energized channels (streamers) are created while avoiding a spark discharge. At a certain energy the number of ionized hydrocarbon molecules becomes sufficient to initiate self-sustained combustion. HFI enables engine operation with highly diluted (by air or EGR) gasoline-air mixtures or at high boost levels due to the lower voltage required.

This paper presents a special optical fiber technique which allows to measure temperatures in SI engines using the emission bands or respectively emission lines of the temperature radiation of diatomic molecules. The measurement technique enables the detection of average temperature in a small volume element. These temperatures are used to determine the local NO concentrations using the extended Zeldovich-mechanism. First, theoretical background of both temperature and NO-determination and measurement technique including optical fiber sensors are described. Finally, the temperature and NO dependence versus crank angle are presented and discussed at different combustion chamber locations for different engine operating conditions.

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. Today's production ECUs have no or only very low feedback regarding the process in the combustion chamber. As long as this data is missing, the design of the maps in the ECU can only be a compromise, since production tolerances and aging processes have to be considered in advance. Disturbances in the combustion process may not be detected at all. If more knowledge about the course of combustion is provided, especially the start of combustion (SOC), various operating parameters, such as the pilot injection quantity or the beginning of current feed to the injector, could be adjusted more precisely and individually for every cylinder.

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 NOx emissions. The need for expensive lean NOx catalysts in combination with complex operation strategies may reduce the advantages in efficiency significantly. The Bowl-Prechamber-Ignition (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 NOx 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.