Andreas Berntsson

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

Optical study of HCCI Combustion using NVO and an SI Stratified Charge

The effects of using an SI stratified charge in combination with HCCI combustion on combustion phasing, rate of heat release and emissions were investigated in engine experiments to identify ways to extend the operational range of HCCI combustion to lower loads. In the experiments an optical single-cylinder engine equipped with a piezo electric outward-opening injector and operated with negative valve overlap (NVO) and low lift, short duration, camshaft profiles, was used to initiate HCCI combustion by increasing the exhaust gas recirculation (EGR) and thus retaining sufficient thermal energy to reach auto-ignition temperatures. Two series of experiments with full factorial designs were performed, to investigate how the tested parameters (amounts of fuel injected in pilot injections and main injections, stratification injection timing and spark-assistance) influenced the combustion. In the optical study fourth harmonic light (266 nm) from a Nd:YAG laser was used to induce fluorescence (LIF), from 3-pentanone added as a fuel tracer, to analyze the concentration and distribution of fuel vapor within the cylinder. In addition third harmonic light (355 nm) was used to study the concentration and distribution of formaldehyde in the cylinder, and chemiluminescence signals from OH radicals were used to locate the flame front. It was found that the injection and ignition timing of the SI stratified charge were the main parameters influencing the HCCI combustion phasing. The NOX emissions were found to be significantly affected by the use of a SI stratified charge, and its injection timing. The results show that use of an SI stratified charge can extend the operational range of HCCI combustion to lower loads by advancing combustion phasing. In addition, use of NVO in combination with an SI stratified charge provides a useful, flexible means to control HCCI combustion.

A LIF-study of OH in the Negative Valve Overlap of a Spark-assisted HCCI Combustion Engine

Future requirements for emission reduction from combustion engines in ground vehicles might be met by using the HCCI combustion concept. In this study, negative valve overlap (NVO) and low lift, short duration, camshaft profiles, were used to initiate HCCI combustion by increasing the internal exhaust gas recirculation (EGR) and thus retaining sufficient thermal energy for chemical reactions to occur when a pilot injection was introduced prior to TDC, during the NVO. One of the crucial parameters to control in HCCI combustion is the combustion phasing and one way of doing this is to vary the relative ratio of fuel injected in pilot and main injections. The combustion phasing is also influenced by the total amount of fuel supplied to the engine, the combustion phasing is thus affected when the load is changed. This study focuses on the reactions that occur in the highly diluted environment during the NVO when load and pilot to main ratio are changed. To monitor these reactions, planar laser-induced fluorescence (PLIF) from OH radicals was analyzed in a series of experiments with an optical single-cylinder engine, since these radicals are known to be associated with high temperature reactions. A series of experiments was also performed using a multi-cylinder engine with varied NVO timings, which showed that the combustion phasing was influenced by both the ratio between the pilot and main injection amounts and the total amount of fuel. Data acquired from corresponding optical analysis showed the occurrence of OH radicals (and thus high temperature reactions) during the NVO in all tested operating conditions. The results also indicate that the extent of the high temperature reactions was influenced by both varied parameters, since decreasing the relative amount of the pilot injection and/or increasing the total amount of fuel led to larger amounts of OH radicals.

Reducing Pressure Fluctuations at High Loads by Means of Charge Stratification in HCCI Combustion with Negative Valve Overlap

Future demands for improvements in the fuel economy of gasoline passenger car engines will require the development and implementation of advanced combustion strategies, to replace, or combine with the conventional spark ignition strategy. One possible strategy is homogeneous charge compression ignition (HCCI) achieved using negative valve overlap (NVO). However, several issues need to be addressed before this combustion strategy can be fully implemented in a production vehicle, one being to increase the upper load limit. One constraint at high loads is the combustion becoming too rapid, leading to excessive pressure -rise rates and large pressure fluctuations (ringing), causing noise. In this work, efforts were made to reduce these pressure fluctuations by using a late injection during the later part of the compression. A more appropriate acronym than HCCI for such combustion is SCCI (Stratified Charge Compression Ignition). The approach was evaluated in tests with a single-cylinder metal research engine and a single-cylinder optical engine. The latter was used to characterize the combustion in laser-based analyses including laser-induced florescence (LIF) determinations of fuel tracer, OH and CH 2 O (formaldehyde) distributions. A high speed camera was also used for direct imaging of chemiluminescence. The effects of two main parameters were studied: the proportion of fuel injected late to create a stratified charge and the timing of the late injection. In addition, two fuels were used: a certification gasoline fuel and a blend of n-heptane, iso-octane and 3-pentanone. Both fuels were used in the metal engine for comparison. Use of a stratified charge allowed the maximum pressure -rise rates and ringing intensity to be reduced at the expense of increases in NO x and CO emissions, regardless of fuel type. Optical results indicated that both the fuel distribution and combustion were not homogenous.

Spark Assisted HCCI Combustion Using a Stratified Hydrogen Charge

Future requirements for emission reduction from combustion engines in ground vehicles might be met by using the HCCI combustion concept. In this concept a more or less homogenous air fuel mixture is compressed to auto ignition. This gives good fuel consumption compared to a normal SI engine and its ability to burn lean mixtures at low temperatures has a positive impact on exhaust emissions. However, there are challenges associated with this concept, for instance its limited operating range and combustion control. The objective of this work is to investigate a hybrid concept, based on a combination of HCCI combustion of n-heptane and SI combustion of hydrogen. The basic idea is to initiate HCCI combustion with a spark ignited stratified lean hydrogen mixture. To verify that the combustion sequence consists of flame front combustion followed by HCCI combustion, photographs of OH chemiluminescence from the combustion were taken. This was made in a single cylinder engine with optical access through a quartz window in the piston. The performance of the hybrid combustion was compared to that of pure HCCI combustion. Chemiluminescence images show an expanding flame front initiated by the spark plug. It is shown that the flame front propagation through the hydrogen charge can be used to expand the operating range of HCCI combustion, especially towards lower loads. The hybrid combustion concept gives greater scope for controlling the combustion than the pure HCCI concept. By varying the amount of hydrogen the crank angle when 50% of the energy is burned, CA50, can be phased further away from TDC.