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Technical Paper

Cycle to Cycle Variations in S.I. Engines - The Effects of Fluid Flow and Gas Composition in the Vicinity of the Spark Plug on Early Combustion

1996-10-01
962084
Simultaneous measurements of early flame speed and local measurements of the major parameters controlling the process are presented. The early flame growth rate was captured with heat release analysis of the cylinder pressure. The local concentration of fuel or residual gas were measured with laser induced fluorescence (LIF) on isooctane/3-pentanone or water. Local velocity measurements were performed with laser doppler velocimetry (LDV). The results show a significant cycle to cycle correlation between early flame growth rate and several parameters. The experiments were arranged to suppress all but one important factor at a time. When the engine was run without fuel or residual gas fluctuations, the cycle to cycle variations of turbulence were able to explain 50 % of the flame growth rate fluctuations. With a significantly increased fluctuation of F/A, obtained with port fuelling, 65% of the growth rate fluctuation could be explained with local F/A measurements.
Technical Paper

Influence of Mixture Quality on Homogeneous Charge Compression Ignition

1998-10-19
982454
The major advantages with Homogeneous Charge Compression Ignition, HCCI, is high efficiency in combination with low NOx-emissions. The major drawback with HCCI is the problem to control the ignition timing over a wide load and speed range. Other drawbacks are the limitation in attainable IMEP and relativly high emissions of unburned hydrocarbons. But the use of Exhaust Gas Recycling (EGR) instead of only air, slows down the rate of combustion and makes it possible to use lower air/fuel ratio, which increases the attainable upper load limit. The influence of mixture quality was therefore experimentally investigated. The effects of different EGR rates, air/fuel ratios and inlet mixture temperatures were studied. The compression ratio was set to 18:1. The fuels used were iso-octane, ethanol and commercially available natural gas. The engine was operated naturally aspirated mode for all tests.
Technical Paper

Demonstrating the Performance and Emission Characteristics of a Variable Compression Ratio, Alvar- Cycle Engine

1998-10-19
982682
This paper is a direct continuation of a previous study that addressed the performance and design of a variable compression engine, the Alvar-Cycle Engine [1]. The earlier study was presented at the SAE International Conference and Exposition in Detroit during February 23-26, 1998 as SAE paper 981027. In the present paper test results from a single cylinder prototype are reviewed and compared with a similar conventional engine. Efficiency and emissions are shown as function of speed, load, and compression ratio. The influence of residual gas on knock characteristics is shown. The potential for high power density through heavy supercharging is analyzed.
Technical Paper

Fluid Flow, Combustion and Efficiency with Early or Late Inlet Valve Closing

1997-10-01
972937
This paper is a study of the effects of valve timing and how it influences the in-cylinder fluid flow, the combustion, and the efficiency of the engine. An engine load of 4.0 bar imepnet was achieved by setting the inlet valve closing time early or late to enable unthrottled operation. Inlet valve deactivation was also used and asymmetrical valve timing, i.e. valve timing with the two inlet valves opening and closing at different times. The valve timing was altered by switching cam lobes between the experiments. The results indicate a longer flame development period but a faster combustion with early inlet valve closing compared to the throttled case. For late inlet valve closing, a variation in the combustion duration results. As expected, the pumping mean effective pressure (PMEP) was greatly reduced with early and late inlet valve closing compared to the throttled case.
Technical Paper

Homogeneous Charge Compression Ignition (HCCI) Using Isooctane, Ethanol and Natural Gas - A Comparison with Spark Ignition Operation

1997-10-01
972874
The Homogeneous Charge Compression Ignition (HCCI) is the third alternative for combustion in the Internal Combustion (IC) engines. Here, a homogeneous charge is used as in a spark ignited engine but the charge is compressed to auto-ignition as in a diesel. The characteristics of HCCI were compared to SI using a 1.6 liter single cylinder engine with compression ratio 21:1 in HCCI mode and 12:1 in SI mode. Three different fuels were used; isooctane, ethanol and natural gas. Some remarkable results were noted in the experiments: The indicated efficiency of HCCI was much better than for SI operation. Very little NOx was generated with HCCI, eliminating the need for a LeanNOx catalyst. However, HCCI generated more HC and CO than SI operation. Stable and efficient operation with HCCI could be obtained with λ=3 to λ=9 using isooctane or ethanol. Natural gas, with a higher octane number, required a richer mixture to run in HCCI mode.
Technical Paper

In-Cylinder Flow in High Speed Two-Stroke Engines with Different Transfer Channels

1997-02-24
970357
2-D LDV measurements were performed in the cylinder of a two-stroke engine. The transfer channels of the cylinders were of two different designs: Open transfer channels and “cup handle” transfer channels. The engine was run at its rated speed, 9000 rpm. Optical access to the cylinder was achieved by replacing the standard cylinder head with a quartz window. No addition of seeding was made, since the fuel droplets were not entirely vaporized as they entered the cylinder and thus served as seeding. Results show that the flow out from the cup handle transfer channels is more directed away from the exhaust port, which promotes loop scavenging. The RMS-value, “turbulence”, was low close to the transfer ports in both cylinders, but increased rapidly towards the middle of the cylinder.
Technical Paper

Laser Sheet Droplet Concentration Measurements in a High Speed Two-Stroke Engine

1997-10-27
978494
Laser sheet droplet illumination was used to visualize the concentration of fuel droplets over the piston top area. Four different cylinder designs were examined: Open transfer channels and three types of cup handle transfer channels. Optical access to the scavenging area of the engine was achieved by removing the silencer and use a window in the top of the engine. The engines were run at their rated speeds: 9000 rpm for three of the engines and 5800 rpm for one of them. Images of the concentration patterns were captured at various crank positions, −20, −10, 0, 10, 20, 30 Crank Angle Degrees (CAD) from Bottom Dead Center (BDC). Results show that the concentration of fuel droplets is higher close to the back wall of the cylinder with cup handle transfer channels. Late in the scavenging phase, the concentration pattern is more spread over the entire cylinder area, for all types of transfer channels.
Technical Paper

Wavelet Analysis of In-Cylinder LDV Measurements and Correlation Against Heat-Release

1998-02-23
980483
Wavelet analysis was used to calculate turbulence and mean velocity levels for LDV measurements made in a four valve spark ignition engine. Five different camshafts were tested, and they produce significantly different flow behaviour. The standard cam gives tumble and with valve deactivation, swirl is produced. One camshaft with early inlet valve closing and two camshafts with late inlet valve closing were also tested. The wavelet toolbox for Matlab version 5.1 has been used for the wavelet calculations. The wavelet technique produces both time resolved and frequency resolved velocity information. The results indicate some influence of the turbulence frequency content on the rate of heat release. Correlation against heat-release can be seen for different scales of turbulence. The breakdown of the tumble (low frequency turbulence) into high frequency turbulence can be seen clearly.
Technical Paper

Measurements of Turbulent Flame Speed and Integral Length Scales in a Lean Stationary Premixed Flame

1998-02-23
981050
Turbulent premixed natural gas - air flame velocities have been measured in a stationary axi-symmetric burner using LDA. The flame was stabilized by letting the flow retard toward a stagnation plate downstream of the burner exit. Turbulence was generated by letting the flow pass through a plate with drilled holes. Three different hole diameters were used, 3, 6 and 10 mm, in order to achieve different turbulent length scales. Turbulent integral length scales were measured using two-point LDA and the stretching in terms of the Karlovitz number could be estimated from these measurements. The results support previous studies indicating that stretching reduces the flame speed.
Technical Paper

Supercharged Homogeneous Charge Compression Ignition

1998-02-23
980787
The Homogeneous Charge Compression Ignition (HCCI) is the third alternative for combustion in the reciprocating engine. Here, a homogeneous charge is used as in a spark ignited engine, but the charge is compressed to auto-ignition as in a diesel. The main difference compared with the Spark Ignition (SI) engine is the lack of flame propagation and hence the independence from turbulence. Compared with the diesel engine, HCCI has a homogeneous charge and hence no problems associated with soot and NOX formation. Earlier research on HCCI showed high efficiency and very low amounts of NOX, but HC and CO were higher than in SI mode. It was not possible to achieve high IMEP values with HCCI, the limit being 5 bar. Supercharging is one way to dramatically increase IMEP. The influence of supercharging on HCCI was therefore experimentally investigated. Three different fuels were used during the experiments: iso-octane, ethanol and natural gas.
Technical Paper

Combustion Chambers for Supercharged Natural Gas Engines

1997-02-24
970221
This work is a continuation of earlier research conducted on the effects of different combustion chambers on turbulence, combustion, emissions and efficiency for natural gas converted diesel bus engines. In this second measurement series the engine (Volvo TD102) was supercharged to enable bmep up to 18 bar at λ = 1.6-1.9. Six different combustion chambers were used. The results show that different geometrical combustion chambers, with the same compression ratio (12:1), have very different combustion performance. A high rate of heat release is favorable for lean operation, and the design of the combustion chamber is very important for the knock and misfire limits.
Technical Paper

Flame Reconstruction in Spark Ignition Engines

1997-10-01
972825
The present paper aims at discussing the flow/flame interaction in a lean burn spark ignition engine. The mean velocity and cycle resolved turbulence are measured with laser Doppler velocimetry. The cylinder pressure is recorded and a one-zone heat release calculation performed. The very early part of flame propagation is measured using two orthogonal Schlieren systems, each capturing one image of the progressing flame at a given time after spark onset. The two resulting 2D images are then, after preprocessing, used to reconstruct the three-dimensional flame. The volume of the true flame is estimated by simulating, using Markov Chain Monte Carlo techniques, a number of possible flames that are consistent with the projections on the images. The uncertainty of the estimated flame volume is given by the variation of the volume estimates. In the calculations, the volume of that part of the spark plug that is inside the flame is subtracted.
Journal Article

SI Gas Engine: Evaluation of Engine Performance, Efficiency and Emissions Comparing Producer Gas and Natural Gas

2011-04-12
2011-01-0916
The Technical University of Denmark, DTU, has designed, built and tested a gasifier [1, 8] that is fuelled with wood chips and achieves a 93% conversion efficiency from wood to producer gas. By combining the gasifier with an ICE and an electric generator a co-generative system can be realized that produces electricity and heat. The gasifier uses the waste heat from the engine for drying and pyrolysis of the wood chips while the gas produced is used to fuel the engine. To achieve high efficiency in converting biomass to electricity an engine is needed that is adapted to high efficiency operation using the specific producer gas from the DTU gasifier. So far the majority of gas engines have been designed and optimized for operation on natural gas. The presented work uses a modern and highly efficient truck sized natural gas engine to investigate efficiency, emissions and general performance while operating on producer gas compared to natural gas operation.
Technical Paper

Extending the Operating Region of Multi-Cylinder Partially Premixed Combustion using High Octane Number Fuel

2011-04-12
2011-01-1394
Partially Premixed Combustion (PPC) is a combustion concept by which it is possible to get low smoke and NOx emissions simultaneously. PPC requires high EGR levels to extend the ignition delay so that air and fuel mix prior to combustion to a larger extent than with conventional diesel combustion. This paper investigates the operating region of single injection PPC for three different fuels; Diesel, low octane gasoline with similar characteristics as diesel and higher octane standard gasoline. Limits in emissions are defined and the highest load that fulfills these requirements is determined. The investigation shows the benefits of using high octane number fuel for Multi-Cylinder PPC. With high octane fuel the ignition delay is made longer and the operating region of single injection PPC can be extended significantly. Experiments are carried out on a multi-cylinder heavy-duty engine at low, medium and high speed.
Journal Article

Effects of Post-Injection Strategies on Near-Injector Over-Lean Mixtures and Unburned Hydrocarbon Emission in a Heavy-Duty Optical Diesel Engine

2011-04-12
2011-01-1383
Post-injection strategies aimed at reducing engine-out emissions of unburned hydrocarbons (UHC) were investigated in an optical heavy-duty diesel engine operating at a low-load, low-temperature combustion (LTC) condition with high dilution (12.7% intake oxygen) where UHC emissions are problematic. Exhaust gas measurements showed that a carefully selected post injection reduced engine-out load-specific UHC emissions by 20% compared to operation with a single injection in the same load range. High-speed in-cylinder chemiluminescence imaging revealed that without a post injection, most of the chemiluminescence emission occurs close to the bowl wall, with no significant chemiluminescence signal within 27 mm of the injector. Previous studies have shown that over-leaning in this near-injector region after the end of injection causes the local equivalence ratio to fall below the ignitability limit.
Technical Paper

Flow Field Measurements inside a Piston Bowl of a Heavy-Duty Diesel Engine

2011-08-30
2011-01-1835
Combination of flow field measurements, shown in this paper, give new information on the effect of engine run parameters to formation of different flow fields inside piston bowl. The measurements were carried out with particle image velocimetry (PIV) technique in optical engine. Good set of results was achieved even though the feasibility of this technique in diesel engines is sometimes questioned. Main challenge in diesel engines is background radiation from soot particles which is strong enough to conceal the PIV signal. Window staining in diesel engine is also a problem, since very high particle image quality is needed for velocity analysis. All measurements were made in an optical heavy-duty diesel engine. Optical design of engine was Bowditch type [1]. The engine was charged and equipped with exhaust gas recirculation (EGR). The exhaust gas level was monitored by oxygen concentration and the level was matched to former soot concentration measurements.
Technical Paper

Investigation and Comparison of Multi Cylinder Partially Premixed Combustion Characteristics for Diesel and Gasoline Fuels

2011-08-30
2011-01-1811
Partially Premixed Combustion is a concept able to combine low smoke and NOx emissions with high combustion controllability and efficiency. It is of interest to be able to utilize PPC in a large operating region in order to meet the Euro VI emission legislation without relying on NOx aftertreatment. This paper investigates the differences in PPC characteristics for three fuels; Diesel Swedish Mk 1, Low Octane Gasoline (70 Octane) and US Standard Gasoline (87 Octane). Engine operating conditions, combustion characteristics, emissions and efficiency are in focus. The experiments were carried out at a range of operating points on a Volvo MD13 which is a six-cylinder heavy-duty engine. At each operating point three combinations of EGR level and λ-value were evaluated. 1. High EGR/High λ, 2. High EGR/Reduced λ, and 3. Reduced EGR/High λ.
Technical Paper

Investigation and Comparison of Residual Gas Enhanced HCCI using Trapping (NVO HCCI) or Rebreathing of Residual Gases

2011-08-30
2011-01-1772
A comparison between throttled and unthrottled spark ignition combustion with residual enhanced HCCI combustion is made. Early intake valve closing and late intake valve closing valve strategies for unthrottled spark ignition combustion are evaluated and compared. Approximately 3-6 percent relative improvement in net indicated efficiency is seen when comparing unthrottled spark ignition combustion with throttled spark ignition combustion depending on valve strategy and engine speed. The relative improvement in efficiency from spark ignition combustion to HCCI combustion is approximately 20 percent for the conditions presented in this study. The rebreathing strategies have the highest efficiency of the cases in this study.
Technical Paper

Investigating Mode Switch from SI to HCCI using Early Intake Valve Closing and Negative Valve Overlap

2011-08-30
2011-01-1775
This study investigates mode switching from spark ignited operation with early intake valve closing to residual gas enhanced HCCI using negative valve overlap on a port-fuel injected light-duty diesel engine. A mode switch is demonstrated at 3.5 bar IMEPnet and 1500 rpm. Valve timings and fuel amount have to be selected carefully prior to the mode switch. During mode transition, IMEPnet deviates by up to 0.5 bar from the set point. The time required to return to the set point as well as the transient behavior of the engine load varies depending on which control structure that is used. Both a model-based controller and a PI control approach were implemented and evaluated in experiments. The controllers were active in HCCI mode. The model-based controller achieved a smoother transition and while using it, the transition could be accomplished within three engine cycles.
Journal Article

Laser-Induced Phosphorescence and the Impact of Phosphor Coating Thickness on Crank-Angle Resolved Cylinder Wall Temperatures

2011-04-12
2011-01-1292
In order to further improve the energy conversion efficiency in reciprocating engines, detailed knowledge about the involved processes is required. One major loss source in internal combustion engines is heat loss through the cylinder walls. In order to increase the understanding of heat transfer processes and to validate and generate new heat transfer correlation models it is desirable, or even necessary, to have crank-angle resolved data on in-cylinder wall temperature. Laser-Induced Phosphorescence has proved to be a useful tool for surface thermometry also in such harsh environments as running engines. However, the ceramic structure of most phosphor coatings might introduce an error, due to its thermal insulation properties, when being exposed to rapidly changing temperatures. In this article the measurement technique is evaluated concerning the impact from the thickness of the phosphorescent layer on the measured temperature.
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