Showing papers by "Bengt Johansson published in 1998"

Supercharged Homogeneous Charge Compression Ignition

Abstract: 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 autoignition 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 NOdx formation. Earlier research on HCCI showed high efficiency and very low amounts of NOdx, 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. Two different compression ratios were used, 17:1 and 19:1. The inlet pressure conditions were set to give 0, 1, or 2 bar of boost pressure. The highest attainable IMEP was 14 bar using natural gas as fuel at the lower compression ratio. The limit in achieving even higher IMEP was set by the high rate of combustion and a high peak pressure. Numerical calculations of the HCCI process have been performed for natural gas as fuel. The calculated ignition timings agreed well with the experimental findings. The numerical solution is, however, very sensitive to the composition of the natural gas. (Less)

Influence of Mixture Quality on Homogeneous Charge Compression Ignition

Abstract: 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 relatively 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 rations 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 in naturally aspirated mode for all tests. Stable operation could be achieved with ethanol at about 5 bar of IMEP with a gross indicated efficiency close to 50%. Stable operation of the engine was achieved with over 60% EGR. (Less)

Liquid ionization chambers for absorbed dose measurements in water at low dose rates and intermediate photon energies.

Abstract: Two new liquid ionization chamber (LIC) designs, consisting of cylindrical and plane-parallel configurations, are presented. They are designed to be suitable for high-precision measurements of absorbed dose-to-water at dose rates and photon energies typical for LDR intermediate photon energy brachytherapy sources. The chambers have a sensitive liquid layer thickness of 1 mm and sensitive volumes of 7 mm 3 (plane-parallel) and 20 mm 3 (cylindrical). The liquids used as sensitive media in the chambers are either isooctane ( C 8 H 18 ), tetramethylsilane (Si (CH 3 ) 4 ) or mixtures of these two liquids in the approximate proportions 2 to 1. A chamber filled with such a liquid mixture and with a polarizing voltage of 300 V, provides a volume sensitivity of about 10 −9 C Gy −1 mm −3 for absorbed dosemeasurements in water in an x-rayradiation field with an effective photon energy of 120 keV. In the interval 30 to 140 keV, the relative change in sensitivity is less than ± 2.5%. The leakage current of the chambers is low and stable, which implies that absorbed dosemeasurements can be done with good reproducibility at dose-rates as low as 50 μ Gy min −1 (σ 3%). The long-term calibration stability was tested for a set of five chambers over a period of more than 1 year. No systematic change in their sensitivity could be observed. The general recombination at a polarizing voltage of 300 V is less than 2% for dose-rates up to about 100 mGy min −1 . The temperature dependence at room temperature is 0.5% per ° C. The response is almost independent of the direction of the radiation for the plane-parallel LIC.

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

Abstract: 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. (Less)

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

Abstract: 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 51 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 eatrelease can be seen for different scales of turbulence The breakdown of the tumble (low frequency turbulence) into high frequency turbulence can be seen clearly This transition is not as clear with a swirling flow, where all frequencies are magnified equally at top dead center The choice of wavelet does not have a major impact on the results (Less)

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

Abstract: 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.