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Technical Paper

System Efficiency Issues for Natural Gas Fueled HCCI Engines in Heavy-Duty Stationary Applications

2002-03-04
2002-01-0417
Homogeneous Charge Compression Ignition (HCCI) has been proposed for natural gas engines in heavy duty stationary power generation applications. A number of researchers have demonstrated, through simulation and experiment, the feasibility of obtaining high gross indicated thermal efficiencies and very low NOx emissions at reasonable load levels. With a goal of eventual commercialization of these engines, this paper sets forth some of the primary challenges in obtaining high brake thermal efficiency from production feasible engines. Experimental results, in conjunction with simulation and analysis, are used to compare HCCI operation with traditional lean burn spark ignition performance. Current HCCI technology is characterized by low power density, very dilute mixtures, and low combustion efficiency. The quantitative adverse effect of each of these traits is demonstrated with respect to the brake thermal efficiency that can be expected in real world applications.
Technical Paper

Hydrogen Addition For Improved Lean Burn Capability of Slow and Fast Burning Natural Gas Combustion Chambers

2002-10-21
2002-01-2686
One way to extend the lean burn limit of a natural gas engine is by addition of hydrogen to the primary fuel. This paper presents measurements made on a one cylinder 1.6 liter natural gas engine. Two combustion chambers, one slow and one fast burning, were tested with various amounts of hydrogen (0, 5, 10 and 15 %-vol) added to natural gas. Three operating points were investigated for each combustion chamber and each hydrogen content level; idle, part load (5 bar IMEP) and 13 bar IMEP (simulated turbocharging). Air/fuel ratio was varied between stoichiometric and the lean limit. For each operating point, a range of ignition timings were tested to find maximum brake torque (MBT) and/or knock. Heat-release rate calculations were made in order to assess the influence of hydrogen addition on burn rate. Addition of hydrogen showed an increase in burn rate for both combustion chambers, resulting in more stable combustion close to the lean limit.
Technical Paper

Supercharged Homogeneous Charge Compression Ignition (HCCI) with Exhaust Gas Recirculation and Pilot Fuel

2000-06-19
2000-01-1835
In an attempt to extend the upper load limit for Homogeneous Charge Compression Ignition (HCCI), supercharging in combination with Exhaust Gas Recirculation (EGR) have been applied. Two different boost pressures were used, 1.1 bar and 1.5 bar. High EGR rates were used in order to reduce the combustion rate. The highest obtained IMEP was 16 bar. This was achieved with the higher boost pressure, at close to stoichiometric conditions and with approximately 50 % EGR. Natural gas was used as the main fuel. In the case with the higher boost pressure, iso-octane was used as pilot fuel, to improve the ignition properties of the mixture. This made it possible to use a lower compression ratio and thereby reducing the maximum cylinder pressure. The tests were performed on a single cylinder engine operated at low speed (1000 rpm). The test engine was equipped with a modified cylinder head, having a Variable Compression Ratio (VCR) mechanism.
Technical Paper

Employing an Ionization Sensor for Combustion Diagnostics in a Lean Burn Natural Gas Engine

2001-03-05
2001-01-0992
An ionization sensor has been used to study the combustion process in a six-cylinder lean burn, truck-sized engine fueled with natural gas and optimized for low emissions of nitric oxides. The final goal of the investigations is to study the prospects of using the ionization sensor for finding the optimal operating position with respect to low NOx emission and stable engine operation. The results indicate that unstable combustion can be detected by analyzing the coefficient of variation (CoV) of the detector current amplitude. Close relationships between this measure and the CoV of the indicated mean effective pressure have been found during an air-fuel ratio scan with fixed ignition advance.
Technical Paper

Compression Ratio Influence on Maximum Load of a Natural Gas Fueled HCCI Engine

2002-03-04
2002-01-0111
This paper discusses the compression ratio influence on maximum load of a Natural Gas HCCI engine. A modified Volvo TD100 truck engine is controlled in a closed-loop fashion by enriching the Natural Gas mixture with Hydrogen. The first section of the paper illustrates and discusses the potential of using hydrogen enrichment of natural gas to control combustion timing. Cylinder pressure is used as the feedback and the 50 percent burn angle is the controlled parameter. Full-cycle simulation is compared to some of the experimental data and then used to enhance some of the experimental observations dealing with ignition timing, thermal boundary conditions, emissions and how they affect engine stability and performance. High load issues common to HCCI are discussed in light of the inherent performance and emissions tradeoff and the disappearance of feasible operating space at high engine loads.
Technical Paper

Using Pilot Diesel Injection in a Natural Gas Fueled HCCI Engine

2002-10-21
2002-01-2866
Previous research has shown that the homogeneous charge compression ignition (HCCI) combustion concept holds promise for reducing pollutants (i.e. NOx, soot) while maintaining high thermal efficiency. However, it can be difficult to control the operation of the HCCI engines even under steady state running conditions. Power density may also be limited if high inlet air temperatures are used for achieving ignition. A methodology using a small pilot quantity of diesel fuel injected during the compression stroke to improve the power density and operation control is considered in this paper. Multidimensional computations were carried out for an HCCI engine based on a CAT3401 engine. The computations show that the required initial temperature for ignition is reduced by about 70 K for the cases of the diesel pilot charge and a 25∼35% percent increase in power density was found for those cases without adversely impacting the NOx emissions.
Technical Paper

Cylinder to Cylinder and Cycle to Cycle Variations in a Six Cylinder Lean Burn Natural Gas Engine

2000-06-19
2000-01-1941
The cylinder to cylinder and cycle to cycle variations were measured in a production type Volvo natural gas engine. Cylinder pressure was measured in all six cylinders. Emission measurements were performed individually after all cylinders, and commonly after the turbocharger. Measurements (ECE R49 13-mode) were performed with different spark gap and two different locations for fuel injection, one before the throttle and one before the turbocharger. Heat-release and lambda calculations show substantial cylinder to cylinder variations, due to lambda variations between the cylinders. The slow burn combustion chamber, with low turbulence, results in high cycle to cycle variations (> 100% COV imep) for some of the load cases.
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

Wavelet Analysis of In-Cylinder LDV Velocity Measurements

1996-10-01
961921
The object of this paper is to present a new way of analyzing in-cylinder velocity measurements. The technique is called Discrete Wavelet Transform (DWT) and it is similar to Fast Fourier Transform (FFT) with one important difference it is possible to obtain both time localized and frequency resolved information. This paper demonstrates the use of DWT calculations on in-cylinder LDV flow measurements for different combustion geometries in a natural gas converted truck engine. It will furthermore provide some information about how DWT can be used with in-cylinder measurements in the future.
Technical Paper

Cycle to Cycle Variation Study in a Dual Fuel Operated Engine

2017-03-28
2017-01-0772
The standard capability of engine experimental studies is that ensemble averaged quantities like in-cylinder pressure from multiple cycles and emissions are reported and the cycle to cycle variation (CCV) of indicated mean effective pressure (IMEP) is captured from many consecutive combustion cycles for each test condition. However, obtaining 3D spatial distribution of all the relevant quantities such as fuel-air mixing, temperature, turbulence levels and emissions from such experiments is a challenging task. Computational Fluid Dynamics (CFD) simulations of engine flow and combustion can be used effectively to visualize such 3D spatial distributions. A dual fuel engine is considered in the current study, with manifold injected natural gas (NG) and direct injected diesel pilot for ignition.
Journal Article

Heavy-Duty RCCI Operation Using Natural Gas and Diesel

2012-04-16
2012-01-0379
Many recent studies have shown that the Reactivity Controlled Compression Ignition (RCCI) combustion strategy can achieve high efficiency with low emissions. However, it has also been revealed that RCCI combustion is difficult at high loads due to its premixed nature. To operate at moderate to high loads with gasoline/diesel dual fuel, high amounts of EGR or an ultra low compression ratio have shown to be required. Considering that both of these approaches inherently lower thermodynamic efficiency, in this study natural gas was utilized as a replacement for gasoline as the low-reactivity fuel. Due to the lower reactivity (i.e., higher octane number) of natural gas compared to gasoline, it was hypothesized to be a better fuel for RCCI combustion, in which a large reactivity gradient between the two fuels is beneficial in controlling the maximum pressure rise rate.
Technical Paper

Application of High Performance Computing for Simulating Cycle-to-Cycle Variation in Dual-Fuel Combustion Engines

2016-04-05
2016-01-0798
Interest in operational cost reduction is driving engine manufacturers to consider low-cost fuel substitution in heavy-duty diesel engines. These dual-fuel (DF) engines could be operated either in diesel-only mode or operated with premixed natural gas (NG) ignited by a pilot flame of compression-ignited direct-injected diesel fuel. Under certain conditions, dual-fuel operation can result in increased cycle-to-cycle variability (CCV) during combustion. CFD can greatly help in understanding and identifying critical parameters influencing CCV. Innovative modelling techniques and large computing resources are needed to investigate the factors affecting CCV in dual-fuel engines. This paper discusses the use of the High Performance Computing resource Titan, at Oak Ridge National Laboratory, to investigate CCV of a dual-fuel engine.
Technical Paper

Combustion Chambers for Natural Gas SI Engines Part 2: Combustion and Emissions

1995-02-01
950517
The objective of this paper is to investigate how the combustion chamber design will influence combustion parameters and emissions in a natural gas SI engine. Ten different geometries were tried on a converted Volvo TD102 engine. For the different combustion chambers emissions and the pressure in the cylinder have been measured. The pressure in the cylinder was then used in a one-zone heat-release model to get different combustion parameters. The engine was operated unthrottled at 1200 rpm with different values of air/fuel ratio and EGR. The air/fuel ratio was varied from stoichiometric to lean limit. EGR values from 0 to 30% at stoichiometric air/fuel ratio were used. The results show a remarkably large difference in the rate of combustion between the chambers. The cycle-to-cycle variations are fairly independent of combustion chamber design as long as there is some squish area and the air and the natural gas are well mixed.
Technical Paper

The Potential of Using the Ion-Current Signal for Optimizing Engine Stability - Comparisons of Lean and EGR (Stoichiometric) Operation

2003-03-03
2003-01-0717
Ion current measurements can give information useful for controlling the combustion stability in a multi-cylinder engine. Operation near the dilution limit (air or EGR) can be achieved and it can be optimized individually for the cylinders, resulting in a system with better engine stability for highly diluted mixtures. This method will also compensate for engine wear, e.g. changes in volumetric efficiency and fuel injector characteristics. Especially in a port injected engine, changes in fuel injector characteristics can lead to increased emissions and deteriorated engine performance when operating with a closed-loop lambda control system. One problem using the ion-current signal to control engine stability near the lean limit is the weak signal resulting in low signal to noise ratio. Measurements presented in this paper were made on a turbocharged 9.6 liter six cylinder natural gas engine with port injection.
Technical Paper

The Importance of High-Frequency, Small-Eddy Turbulence in Spark Ignited, Premixed Engine Combustion

1995-10-01
952409
The different roles played by small and large eddies in engine combustion were studied. Experiments compared natural gas combustion in a converted, single cylinder Volvo TD 102 engine and in a 125 mm cubical cell. Turbulence is used to enhance flame growth, ideally giving better efficiency and reduced cyclic variation. Both engine and test cell results showed that flame growth rate correlated best with the level of high frequency, small eddy turbulence. The more effective, small eddy turbulence also tended to lower cyclic variations. Large scales and bulk flows convected the flame relative to cool surfaces and were most important to the initial flame kernel.
Technical Paper

Residual Gas Visualization with Laser Induced Fluorescence

1995-10-01
952463
The influence of residual gases on the cycle-to-cycle variations in engine combustion was investigated. Two-photon planar laser-induced fluorescence was used for 2D-visualization of residual gas water. In order to avoid influence from fuel fluorescence and inhomogeneities premixed natural gas was used as fuel. Measurements were conducted at different load conditions with varying inlet manifold pressure. To find out how the residual gas distribution influences the combustion process the pressure development during combustion was monitored. From the pressure information a measure of the combustion rate at different phases of the flame development was calculated. The correlation between residual gas distribution and combustion rate was evaluated on a cycle to cycle basis. The results show that with an inlet manifold pressure of 0.3 bar the correlation between residual gas fraction and rate of combustion were 0.5-0.6. At full load though, lower correlation was found.
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

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

Combustion Chambers for Natural Gas SI Engines Part I: Fluid Flow and Combustion

1995-02-01
950469
The most economical way to convert truck and bus DI-diesel engines to natural gas operation is to replace the injector with a spark plug and modify the combustion chamber in the piston crown for spark ignition operation. The modification of the piston crown should give a geometry well suited for spark ignition operation with the original swirling inlet port. Ten different geometries were tried on a converted VOLVO TD102 engine and a remarkably large difference in the rate of combustion was noted between the chambers. To find an explanation for this difference a cycle resolved measurement of the in-cylinder mean velocity and turbulence was performed with Laser Doppler Velocimetry (LDV). The results show a high correlation between in cylinder turbulence and rate of heat release in the main part of combustion.
Technical Paper

Lean Burn Natural Gas Operation vs. Stoichiometric Operation with EGR and a Three Way Catalyst

2005-04-11
2005-01-0250
Exhaust Emissions from lean burn natural gas engines may not always be as low as the potential permits, especially engines with open loop lambda control. These engines can produce much higher emissions than a comparable diesel engine without exhaust gas after treatment. Even if the engine has closed loop lambda control, emissions are often unacceptably high for future emission regulations. A three way catalyst is, today, the best way to reduce hazardous emissions. The drawback is that the engine has to operate with a stoichiometric mixture and this leads to; higher heat losses, higher pumping work at low to medium loads, higher thermal stress on the engine and higher knock tendency (requiring lower compression ratio, and thus lower brake efficiency). One way to reduce these drawbacks is to dilute the stoichiometric mixture with EGR. This paper compares lean burn operation with operation at stoichiometric conditions diluted with EGR, and using a three way catalyst.
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