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Dynamics >>23
Retrofitting
a large bore direct-injection natural gas engine
Gi-Heon Kim
and Allan Kirkpatrick, Colorado State University, USA
This
article presents research at Colorado State University on retrofitting
the direct-injection natural gas engine. It is an example of a successful
engine development study using STAR-CD. Large bore natural gas engines
have been used for many years in stationary applications such as
gas compression and electric power generation. A common configuration
of the engines is a two-stroke cycle with direct injection of natural
gas into the cylinder. Developing successful retrofit technologies
to improve engine performance and to reduce pollutant generation
has become an important issue as more stringent air emission regulations
are enacted. Poor in-cylinder mixing due to ineffective fuel delivery
is believed to be problematic in these natural gas engines. In addition,
cyclic combustion instability due to slow early-flame-growth is
one of key contributors to NOX and CO emissions. The retrofit technologies
discussed in this article are categorized into two areas: ‘Fuel-air
mixing enhancement’, and ‘Alternative ignition systems’.
Fuel-air mixing enhancement
The
specific engine modeled in this study, the GMV Cooper Bessemer engine,
is widely used in the gas compression industry, primarily in 10
cylinder versions. It is a large engine, with a 38 cm bore and stroke.
A computational model, shown Fig. 1, incorporating a moving grid
simulation of the scavenging, compression, combustion, and expansion
processes of the engine was developed and validated through comparison
with optical experimental results. The CFD computations are compared
with the PLIF (Planar Laser Induced Fluorescence) results in Fig.
2. The PLIF images are on the right hand side, and the CFD results
are on the left hand side of the pairs of images. The scavenging
flow bends the fuel jet slightly toward the exhaust ports, so that
the fuel jet hits the piston top slightly off center, producing
non-symmetric mixing in the combustion chamber. These images indicate
that the computations using STAR-CD can model the in-cylinder flow
induced by scavenging and the actual injection and mixing processes
quite accurately.
Top Dead Center Bottom Dead Center
Fig 1. Computational mesh of natural gas fired 2 stroke engine
Fig 2. CFD validation with Planar Laser Induced Fluorescence experiments
One of the promising mixing enhancement technologies is high-pressure fuel injection. Natural gas is typically injected at low pressures, 1~3 bar above manifold pressure. Since natural gas pipelines operate at pressures of the order of 35 bar, it is of interest to explore the use of pipeline gas at high pressure as the source for injected fuel. However, the cost of three dimensional engine simulations becomes considerable if the computational model has to capture the details of the complex supersonic flow structures in and near the intricate geometry of the injection valve. So, a supersonic “virtual valve” was designed for a 3D engine CFD model to reproduce the actual downstream jet characteristics, which were of crucial importance in the macroscopic engine performance. In Fig.3, the high-pressure fuel injection and mixing during the compression stroke are compared with conventional low pressure injection. The gridded dark colored regions in the figures represents a mixture richer than the lean limit of flammability, f = 0.5. The jet sweeps toward the intake ports, around the top of the cylinder volume toward the opposite side of the cylinder, and along the outer edges of the piston top. In the high-pressure injection case the flow patterns are similar, but the fuel moves with higher momentum so that the most of the volume in the cylinder is flammable at top dead center.
Low Pressure Injection High Pressure Injection
Fig 3. Fuel Injection and Mixing comparison
Alternative ignition systems
Lean burn combustion is a common solution for emissions reduction. However, if the combustion occurs in a very lean regime or mixing is not sufficient, CO and hydrocarbon emissions become unacceptably high due to ignition misfires. PCC (Pre-combustion chamber) ignition and laser spark ignition are potential retrofit technologies for obtaining stable ignition. Since a flame jet provides the ignition in the main chamber, PCC ignition is less affected by lean regions or poor mixing around the spark plug, resulting in greater stability of combustion. With a laser-based system, the spark can be positioned at any location in the cylinder.
Knowledge of the initial flow and concentration fields is required to determine the subsequent flame propagation and pollutant generation during combustion. Because of the non-homogeneity of the mixture system in the cylinder of this type of engine, the spatial fuel distribution, mean flow field and turbulence quantity field at ignition timing are critical factors determining the characteristics of heat release during the expansion stroke.
Fig. 4 presents the equivalence ratio distribution at 10 degrees before TDC, the ignition timing of the conventional spark plug. The location of the spark plug is marked as SP in the figure. As seen in the figure, there is a lean region on the spark plug side, and richer regions in the crevice around the edge of the cylinder. Selected section plots including velocity magnitude contours and vector plots are shown in Fig. 5. The scavenging induced cylinder vortex dominates the cylinder flow pattern at ignition timing. An energetic narrow bulk flow exists on the plane of symmetry flowing from the left to right side toward the spark plug. This flow direction is opposite to the direction of the flame propagation. On the other hand, the flow direction in the crevice is in the same direction with the flame propagation.
Fig 4. Fuel distribution at 10 degrees before TDC Fig 5. Flow Field at 10 degrees before TDC
Comparison of the flame propagation in conventional spark ignition, PCC ignition, and laser spark ignition is presented in Fig. 6. With spark ignition, the flame mainly propagates along the rim in the azimuthal direction, not across the center. The flame propagation during PCC ignition is quite different. The flame propagates faster across the center than in the azimuthal direction. In this case, the flame jet overcomes the adverse flow field of the main chamber. For the laser ignition computation, the laser spark location chosen was slightly off-center to the intake side. The flame initially propagates toward the chamber center, then the flame front moves radially outward. The duration of heat release is about 25 degrees of crank angle for the PCC and laser spark ignition systems, and about 30 degrees for the conventional spark case.
Fig 6. Comparison of flame propagation
In Fig. 7, the NO formation of the conventional spark ignition case and laser spark ignition case are compared. Knowledge of the NO formation region is important for NO reduction technologies such as water injection. With the conventional spark ignition, NO is mainly formed in the crevice region where the richer mixture burns at a higher temperature. On the other hand, since the combustion in the laser spark ignition case starts at chamber center, NO is mainly formed near the cylinder center.
Fig
7. Comparison of NOX formation
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