Daniel Dahl

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.

An Evaluation of Different Combustion Strategies for SI Engines in a Multi-Mode Combustion Engine

Future pressures to reduce the fuel consumption of passenger cars may require the exploitation of alternative combustion strategies for gasoline engines to replace, or use in combination with the conventional stoichiometric spark ignition (SSI) strategy. Possible options include homogeneous lean charge spark ignition (HLCSI), stratified charge spark ignition (SCSI) and homogeneous charge compression ignition (HCCI), all of which are intended to reduce pumping and thermal losses. In the work presented here four different combustion strategies were evaluated using the same engine: SSI, HLCSI, SCSI and HCCI. HLCSI was achieved by early injection and operating the engine lean, close to its stability limits. SCSI was achieved using the sprayguided technique with a centrally placed multi-hole injector and spark-plug. HCCI was achieved using a negative valve overlap to trap hot residuals and thus generate auto-ignition temperatures at the end of the compression stroke. The experiments were performed using a 6 cylinder, 3.2 liter Volvo engine equipped with cam profile switching (CPS), variable cam timing (VCT) for both intake and exhaust valves, and a spray guided direct injection (SGDI) system. In conjunction with a fully programmable control unit these features allowed the engine to be run in all the tested modes without any hardware modifications. Five operating points in the low load/speed zone of the engine map were optimized for fuel consumption using full factorial, 2-D and 3-D experimental designs (with centre points). The same operating points were used for each combustion strategy and their effects on exhaust emissions, combustion and fuelconsumption were evaluated.

HCCI/SCCI Load Limits and Stoichiometric Operation in a Multicylinder Naturally Aspirated Spark Ignition Engine Operated on Gasoline and E85

To meet demands for improvements in the CO2 emissions and fuel economy of gasoline passenger car engines advanced combustion strategies, to replace (or combine with) conventional spark ignition, must be developed and implemented. 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 that the combustion becomes too rapid, leading to excessive pressure-rise rates and large pressure fluctuations (ringing), causing noise. A potential solution to this is to use charge stratification, but charge stratification normally gives rise to increased NO x emissions. Tests with a multicylinder engine reported here confirmed that there is significant potential to increase the upper load limit using charge stratification. In addition, the possibility of operating the engine in stoichiometric conditions, using a combination of NVO and external exhaust gas recirculation (EGR) (thus allowing the increased NO x emissions to be countered using a three-way catalyst) was investigated. Stoichiometric operation was found to be possible for both homogeneous and stratified modes, across a wide operating range, with small compromises in maximum load and fuel consumption. Nevertheless, delaying the need for a mode shift, and operating in stoichiometric conditions when entering a mode shift, should be beneficial in a drive cycle.

The Origin of Pressure Waves in High Load HCCI Combustion: A High-Speed Video Analysis

Homogeneous charge compression ignition is an alternative combustion strategy for spark-ignited gasoline engines that improves engine efficiency and thus reduces CO2 emissions, which is crucial to meet targets set by legislation. However, this combustion strategy is limited to low loads, mainly due to pressure oscillations that arise if combustion is too rapid. The aim of the work described in this article was to record and identify these pressure oscillations and correlate them with the preceding combustion. This was performed using an image-intensified high-speed video camera with a sampling rate of 111 kHz filming inside the combustion chamber of an engine with optical access through a quartz piston. A strategy is described whereby video analysis is used to extract the acoustic modes resulting from combustion. In this work it was possible to detect four different acoustic modes. It is shown that the type and magnitude of these modes can be correlated to the size and position of combustion chamber zones with rapid combustion and the light intensity development (combustion speed) in these zones. It is also shown that the highest combustion rates occur in regions where combustion starts late.

The role of charge stratification for reducing ringing in gasoline engine homogeneous charge compression ignition combustion investigated by optical imaging

Homogeneous charge compression ignition offers the possibility to reduce the fuel consumption of gasoline passenger car engines. However, the combustion strategy is limited to low loads due to pressure oscillations at higher loads. A strategy for extending the homogeneous charge compression ignition load range is charge stratification, using, for example, late direct injection to prolong the combustion duration and reduce the rate of pressure rises and pressure oscillations. In this study, local temperatures and fuel concentrations near top dead centre in a gasoline engine operating in homogeneous charge compression ignition mode were measured using two-wavelength planar laser-induced fluorescence, and the following combustion was analysed using high-speed video to investigate the effects of fuel and temperature stratification on combustion in order to explain the ringing inhibiting effect of charge stratification for fuels displaying single-stage ignition. The extent of spatial distribution of combustion timing correlated well with the extent of fuel and temperature stratification. Furthermore, the gas was leaner and hotter in early igniting regions, while it was richer and colder in late igniting regions. The dampening effects of charge stratification on the combustion speed and pressure oscillations are probably due to rich conditions in the latest burning regions (where combustion is usually most intense) slowing down combustion, which explains why the strategy only works when the global air-to-fuel ratio is not excessively lean.

Valve Profile Adaptation, Stratification, Boosting and 2-Stroke Strategies for Raising Loads of Gasoline HCCI Engines

The development of high efficiency powertrains is a key objective for car manufacturers. One approach for improving the efficiency of gasoline engines is based on homogeneous charge compression ignition, HCCI, which provides higher efficiency than conventional strategies. However, HCCI is only currently viable at relatively low loads, primarily because at high loads it involves rapid combustion that generates pressure oscillations in the cylinder (ringing), and partly because it gives rise to relatively high NOX emissions. This paper describes studies aimed at increasing the viability of HCCI combustion at higher loads by using fully flexible valve trains, direct injection with charge stratification (SCCI), and intake air boosting. These approaches were complemented by using EGR to control NOX emissions by stoichiometric operation, which enables the use of a three-way catalyst. Experiments were carried out using a single-cylinder engine of passenger car size running on gasoline and controlled with negative valve overlap. By adapting the valve profiles (lift, duration and phasing) for high loads, a fuel saving of 3% at constant load or a load increase of 6% could be achieved for lean HCCI compared to those obtained using camshafts that were not adapted for high load operation. Further, using intake pressures up to 180 kPa provided almost linear increases in load for lean HCCI, stoichiometric HCCI and stoichiometric SCCI. However, lean SCCI did not profit from boosting because the charge became too lean and stratification lost its effect as a ringing inhibitor. At intake pressures exceeding 140 kPa, stoichiometric HCCI operation becomes redundant since NOX ceases to be a limiting factor. Additionally, promising results were obtained in initial tests of two-stroke operation, which yielded higher maximum loads, lower fuel consumption and lower NOX emissions than the other strategies.

Reduction of Fuel Consumption and Engine-out NO x Emissions in a Lean Homogeneous GDI Combustion System, Utilizing Valve Timing and an Advanced Ignition System

This study investigated how the amount of dilution applied can be extended while maintaining normal engine operation in a GDI engine. Adding exhaust gases or air to a stoichiometric air/fuel mixture yields several advantages regarding fuel consumption and engine out emissions. The aim of this paper is to reduce fuel consumption by means of diluted combustion, an advanced ignition system and adjusted valve timing. Tests were performed on a Volvo four-cylinder engine equipped with a dual coil ignition system. This system made it possible to extend the ignition duration and current. Furthermore, a sweep was performed in valve timing and type of dilution, i.e., air or exhaust gases. While maintaining a CoV in IMEP < 5%, the DCI system was able to extend the maximum lambda value by 0.1 - 0.15. Minimizing valve overlap increased lambda by an additional 0.1. For dilution by exhaust gases, an increase of 9% was noted for the ignition system and a further increase of 9% was obtained by minimizing the valve overlap at 1500 rpm/5.00 bar BMEP. This exchange of internal combustion residuals for external dilution resulted in a further decrease of fuel consumption but increased engine out NO x . Using air as a diluter resulted in a 6% fuel consumption benefit over exhaust gas dilution, mainly due to the enhanced combustion efficiency arising from higher oxygen concentrations. However, the lower oxygen concentration when using exhaust gases as a diluter led to 50% lower engine out NO x levels at the dilution limit.

Comparison of Lab Versus Engine Tests In the Development of a Highly Efficient Ammonia Formation Catalyst for a Passive SCR System

Commercial three way catalysts have limited capacity towards reducing NOx in the presence of excessive oxygen. This prevents lean-burn combustion concepts from meeting legislative emission standards. A solution towards decreasing NOx emissions in the presence of excess air is the use of a passive-SCR system. Under rich conditions ammonia is formed over an ammonia formation catalyst, the ammonia is stored in the SCR and in its turn reacts with the NOx under lean engine conditions. Here up-scaled Pt/Al2O3 and Pd/Al2O3 catalysts as well as a commercially Pd-Rh based three-way catalyst (TWC) are evaluated using both engine and further lab-scale tests. The purpose of these tests is to compare the ammonia production for the various catalysts under various lambda values and temperatures by means of engine and lab scale tests. The Pd/Al2O3 showed little sensitivity to temperature both under engine and lab scale experiments. The Pt/Al2O3 was affected to a large extend by temperature for both test methods. The TWC showed stable production during the engine measurements while under lab tests an increased temperature resulted in a lower ammonia yield. Differences between the engine and lab scale tests are mainly due to catalyst temperatures, space velocity, CO poisoning and uncertainties in the composition of the engines exhaust gas. Both Pt/Al2O3 and Pd/Al2O3 form ammonia although the former generate higher amounts at high temperature but are believed to suffer from CO poisoning at low temperatures.

Homogeneous Lean Combustion in a 2lt Gasoline Direct Injected Engine with an Enhanced Turbo Charging System

In the quest for a highly efficient, low emission and affordable source of passenger car propulsion system, meeting future demands for sustainable mobility, the concept of homogeneous lean combustion (HLC) in a spark ignited (SI) multi-cylinder engine has been investigated. An attempt has been made to utilize the concept of HLC in a downsized multicylinder production engine producing up to 22 bar BMEP in load. The focus was to cover as much as possible of the real driving operational region, to improve fuel consumption and tailpipe emissions. A standard Volvo two litre four-cylinder gasoline direct injected engine operating on commercial 95 RON gasoline fuel was equipped with an advanced two stage turbo charger system, consisting of a variable nozzle turbine turbo high-pressure stage and a wastegate turbo low-pressure stage. The turbo system was specifically designed to meet the high demands on air mass flow when running lean on higher load and speeds. Also, a dual coil ignition system was used for enhanced ignition ability and a lean NOx emissions exhaust after-treatment system (EATS) dummy was fitted downstream the turbo to receive representative exhaust pressures and temperatures for further development purposes. The engine was mapped running lean in various load points in the operational area of interest. It was found that the engine could sustain a high degree of dilution in lower engine speeds and intermediate loads. Fuel consumption improvements of 12% were obtained running at 1500 rpm and 10 bar BMEP at lambda 1.8. At higher engine loads, above 10 bar BMEP, it was found that the combustion stability deteriorated. The ignition could not be optimized due to knocking combustion and at the same time, combustion duration, measured in crank angle degrees, increased with increasing en-leanment and engine speed, leading to late combustion phasing and large variation in cycle-to-cycle of NMEP. This is currently limiting the operational region of lean combustion of the engine used. The load limit in lean operation was investigated, assessing combustion variations and knock phenomena under different operating conditions.