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Technical Paper

Validation of a Reduced Chemical Mechanism Coupled to CFD Model in a 2-Stroke HCCI Engine

2015-04-14
2015-01-0392
Homogeneous Charge Compression Ignition (HCCI) combustion technology has demonstrated a profound potential to decrease both emissions and fuel consumption. In this way, the significance of the 2-stroke HCCI engine has been underestimated as it can provide more power stroke in comparison to a 4-stroke engine. Moreover, the mass of trapped residual gases is much larger in a 2-stroke engine, causing higher initial charge temperatures, which leads to easier auto-ignition. For controlling 2-stroke HCCI engines, it is vital to find optimized simulation approaches of HCCI combustion with a focus on ignition timing. In this study, a Computational Fluid Dynamic (CFD) model for a 2-stroke gasoline engine was developed coupled to a semi-detailed chemical mechanism of iso-octane to investigate the simulation capability of the considered chemical mechanism and the effects of different simulation parameters such as the turbulence model, grid density and time step size.
Technical Paper

Towards Control-Oriented Modeling of Natural Gas-Diesel RCCI Combustion

2015-04-14
2015-01-1745
For natural gas (NG)-diesel RCCI, a multi-zonal, detailed chemistry modeling approach is presented. This dual fuel combustion process requires further understanding of the ignition and combustion processes to maximize thermal efficiency and minimize (partially) unburned fuel emissions. The introduction of two fuels with different physical and chemical properties makes the combustion process complicated and challenging to model. In this study, a multi-zone approach is applied to NG-diesel RCCI combustion in a heavy-duty engine. Auto-ignition chemistry is believed to be the key process in RCCI. Starting from a multi-zone model that can describe auto-ignition dominated processes, such as HCCI and PCCI, this model is adapted by including reaction mechanisms for natural gas and NOx and by improving the in-cylinder pressure prediction. The model is validated using NG-diesel RCCI measurements that are performed on a 6 cylinder heavy-duty engine.
Technical Paper

The Impact of Operating Conditions on Post-Injection Efficacy; a Study Using Design-of-Experiments

2018-04-03
2018-01-0229
Post-injection strategies prove to be a valuable option for reducing soot emission, but experimental results often differ from publication to publication. These discrepancies are likely caused by the selected operating conditions and engine hardware in separate studies. Efforts to optimize not only engine-out soot, but simultaneously fuel economy and emissions of nitrogen oxides (NOx) complicate the understanding of post-injection effects even more. Still, the large amount of published work on the topic is gradually forming a consensus. In the current work, a Design-of-Experiments (DoE) procedure and regression analysis are used to investigate the influence of various operating conditions on post-injection scheduling and efficacy. The study targets emission reductions of soot and NOx, as well as fuel economy improvements. Experiments are conducted on a heavy-duty compression ignition engine at three load-speed combinations.
Technical Paper

Performance and Emission Studies in a Heavy-Duty Diesel Engine Fueled with an N-Butanol and N-Heptane Blend

2019-04-02
2019-01-0575
N-butanol, as a biomass-based renewable fuel, has many superior fuel properties. It has a higher energy content and cetane number than its alcohol competitors, methanol and ethanol. Previous studies have proved that n-butanol has the capability to achieve lower emissions without sacrifice on thermal efficiency when blended with diesel. However, most studies on n-butanol are limited to low blending ratios, which restricts the improvement of emissions. In this paper, 80% by volume of n-butanol was blended with 20% by volume of n-heptane (namely BH80). The influences of various engine parameters (combustion phasing, EGR ratio, injection timing and intake pressure, respectively) on its combustion and emission characteristics are tested at different loads. The results showed that when BH80 uses more than 40% EGR, the emitted soot and nitrogen oxides (NOx) emissions are below the EURO VI legislation.
Technical Paper

Numerical Investigation of PPCI Combustion at Low and High Charge Stratification Levels

2017-03-28
2017-01-0739
Partially premixed compression ignition combustion is one of the low temperature combustion techniques which is being actively investigated. This approach provides a significant reduction of both soot and NOx emissions. Comparing to the homogeneous charge compression ignition mode, PPCI combustion provides better control on ignition timing and noise reduction through air-fuel mixture stratification which lowers heat release rate compared to other advanced combustion modes. In this work, CFD simulations were conducted for a low and a high air-fuel mixture stratification cases on a light-duty optical engine operating in PPCI mode. Such conditions for PRF70 as fuel were experimentally achieved by injection timing and spray targeting at similar thermodynamic conditions.
Technical Paper

Investigation of Late Stage Conventional Diesel Combustion - Effect of Additives

2018-09-10
2018-01-1787
The accepted model of conventional diesel combustion [1] assumes a rich premixed flame slightly downstream of the maximum liquid penetration. The soot generated by this rich premixed flame is burnt out by a subsequent diffusion flame at the head of the jet. Even in situations in which the centre of combustion (CA50) is phased optimally to maximize efficiency, slow late stage combustion can still have a significant detrimental impact on thermal efficiency. Data is presented on potential late-stage combustion improvers in a EURO VI compliant HD engine at a range of speed and load points. The operating conditions (e.g. injection timings, EGR levels) were based on a EURO VI calibration which targets 3 g/kWh of engine-out NOx. Rates of heat release were determined from the pressure sensor data. To investigate late stage combustion, focus was made on the position in the cycle at which 90% of the fuel had combusted (CA90). An EN590 compliant fuel was tested.
Technical Paper

Implementation of High-Speed Laser-Induced Incandescence Imaging in CI Engines

2016-04-05
2016-01-0725
Laser-induced incandescence (LII) is a well-established technique for tracking soot, potentially enabling soot volume fraction determination. To obtain crank angle resolved data from a single cycle, a multi-kHz system should be applied. Such an approach, however, imposes certain challenges in terms of application and interpretation. The present work intends to apply such a high-speed system to an optically-accessible, compression ignition engine. Possible problems with sublimation, local gas heating or other multishot effects have been studied on an atmospheric co-flow burner prior to the engine experiments. It was found that, in this flame, fluences around 0.1 J/cm2 provide the best balance between signal-tobackground ratio, and soot sublimation. This fluence is well below the plateau regime of LII, which poses additional problems with interpretation of the signal. This is especially true when a wide span of temperatures and gradients is present, as encountered in diesel combustion.
Journal Article

Experimental and Numerical Analyses of Liquid and Spray Penetration under Heavy-Duty Diesel Engine Conditions

2016-04-05
2016-01-0861
The modeling of fuel sprays under well-characterized conditions relevant for heavy-duty Diesel engine applications, allows for detailed analyses of individual phenomena aimed at improving emission formation and fuel consumption. However, the complexity of a reacting fuel spray under heavy-duty conditions currently prohibits direct simulation. Using a systematic approach, we extrapolate available spray models to the desired conditions without inclusion of chemical reactions. For validation, experimental techniques are utilized to characterize inert sprays of n-dodecane in a high-pressure, high-temperature (900 K) constant volume vessel with full optical access. The liquid fuel spray is studied using high-speed diffused back-illumination for conditions with different densities (22.8 and 40 kg/m3) and injection pressures (150, 80 and 160 MPa), using a 0.205-mm orifice diameter nozzle.
Technical Paper

Experimental Study on the Potential of Higher Octane Number Fuels for Low Load Partially Premixed Combustion

2017-03-28
2017-01-0750
The optimal fuel for partially premixed combustion (PPC) is considered to be a gasoline boiling range fuel with an octane number around 70. Higher octane number fuels are considered problematic with low load and idle conditions. In previous studies mostly the intake air temperature did not exceed 30 °C. Possibly increasing intake air temperatures could extend the load range. In this study primary reference fuels (PRFs), blends of iso-octane and n-heptane, with octane numbers of 70, 80, and 90 are tested in an adapted commercial diesel engine under partially premixed combustion mode to investigate the potential of these higher octane number fuels in low load and idle conditions. During testing combustion phasing and intake air temperature are varied to investigate the combustion and emission characteristics under low load and idle conditions.
Technical Paper

Effects of Different Injection Strategies and EGR on Partially Premixed Combustion

2018-09-10
2018-01-1798
Premixed Charge Compression Ignition concepts are promising to reduce NOx and soot simultaneously and keeping a high thermal efficiency. Partially premixed combustion is a single fuel variant of this new combustion concepts applying a fuel with a low cetane number to achieve the necessary long ignition delay. In this study, multiple injection strategies are studied in the partially premixed combustion approach to reach stable combustion and ultra-low NOx and soot emission at 15.5 bar gross indicated mean effective pressure. Three different injection strategies (single injection, pilot-main injection, main-post injection) are experimentally investigated on a heavy duty compression ignition engine. A fuel blend (70 vol% n-butanol and 30 vol% n-heptane) was tested. The effects of different pilot and post-injection timing, as well as Exhaust-gas Recirculation rate on different injection strategies investigated.
Technical Paper

Design and Operation of a High Pressure, High Temperature Cell for HD Diesel Spray Diagnostics: Guidelines and Results

2009-04-20
2009-01-0649
This paper first compares strengths and weaknesses of different options for performing optical diagnostics on HD diesel sprays. Then, practical experiences are described with the design and operation of a constant volume test cell over a period of more than five years. In this test rig, pre-combustion of a lean gas mixture is used to generate realistic gas mixture conditions prior to fuel injection. Spray growth, vaporization are studied using Schlieren and Mie scattering experiments. The Schlieren set-up is also used for registration of light emitted by the combustion process; this can also provide information on ignition delay and on soot lift-off length. The paper further describes difficulties encountered with image processing and suggests methods on how to deal with them.
Journal Article

Commercial Naphtha Blends for Partially Premixed Combustion

2013-04-08
2013-01-1681
Partially Premixed Combustion has shown the potential of low emissions of nitrogen oxides (NOx) and soot with a simultaneous improvement in fuel efficiency. Several research groups have shown that a load range from idle to full load is possible, when using low-octane-number refinery streams, in the gasoline boiling range. As such refinery streams are not expected to be commercially available on the short term, the use of naphtha blends that are commercially available could provide a practical solution. The three blends used in this investigation have been tested in a single-cylinder engine for their emission and efficiency performance. Besides a presentation of the sensitivity to injection strategies, dilution levels and fuel pressure, emission performance is compared to legislated emission levels. Conventional diesel combustion benchmarks are used for reference to show possible improvements in indicated efficiency.
Technical Paper

Characterization of Low Load PPC Operation using RON70 Fuels

2014-04-01
2014-01-1304
The concept of Partially Premixed Combustion is known for reduced hazardous emissions and improved efficiency. Since a low-reactive fuel is required to extend the ignition delay at elevated loads, controllability and stability issues occur at the low-load end. In this investigation seven fuel blends are used, all having a Research Octane Number of around 70 and a distinct composition or boiling range. Four of them could be regarded as ‘viable refinery fuels’ since they are based on current refinery feedstocks. The latter three are based on primary reference fuels, being PRF70 and blends with ethanol and toluene respectively. Previous experiments revealed significant ignition differences, which asked for further understanding with an extended set of measurements. Experiments are conducted on a heavy duty diesel engine modified for single cylinder operation. In this investigation, emphasis is put on idling (600 rpm) and low load conditions.
Journal Article

Butanol-Diesel Blends for Partially Premixed Combustion

2013-04-08
2013-01-1683
Partially Premixed Combustion has shown the potential of high efficiency, emissions of nitrogen oxides (NOx) and soot below future emissions regulations, and acceptable acoustic noise. Low-octane-number gasoline fuels were shown to be most suitable for this concept, with the reactivity determining the possible load range. Other researchers have used several refinery streams, which might be produced by a refinery if they were required to do so without additional investment. Some of refinery streams are, however, not expected to be commercially available on the short term. For the present investigation, n-butanol (BuOH) has been selected as a blend component in diesel, and is used from 50 - 100%. The blends then have a reactivity range similar to the refinery streams, so single-cylinder engine tests for their emission and efficiency performance can also be used to determine their applicable load range.
Technical Paper

Analysis of Transition from HCCI to CI via PPC with Low Octane Gasoline Fuels Using Optical Diagnostics and Soot Particle Analysis

2017-10-08
2017-01-2403
In-cylinder visualization, combustion stratification, and engine-out particulate matter (PM) emissions were investigated in an optical engine fueled with Haltermann straight-run naphtha fuel and corresponding surrogate fuel. The combustion mode was transited from homogeneous charge compression ignition (HCCI) to conventional compression ignition (CI) via partially premixed combustion (PPC). Single injection strategy with the change of start of injection (SOI) from early to late injections was employed. The high-speed color camera was used to capture the in-cylinder combustion images. The combustion stratification was analyzed based on the natural luminosity of the combustion images. The regulated emission of unburned hydrocarbon (UHC), carbon monoxide (CO) and nitrogen oxides (NOX) were measured to evaluate the combustion efficiency together with the in-cylinder rate of heat release.
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