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Cimac Congress | Shanghai 2013
Kyushu University. The RCEM is utilised as a research model for
GI engines. An electronically controlled high-pressure gas injection
system enables injection pressures of up to 50 MPa. Diesel
pilot sprays in dual-fuel mode as well as glow plugs are used for
ignition. Air conditions in the cylinder at the gas injection are
about 10 MPa and 550°C, simulating a current GI engine. In a
first series of experiments, a cylinder head with a cubic shaped
clearance volume and an observation view of 200mm in width
and 50mm in height is applied to analyse the spray combustion.
In the experiments, pure methane, the main component of natural
gas, is used. At first, the GI combustion is compared to the
diesel spray combustion. As a result, rates of heat release for GI
and diesel combustion are comparable, while the emissions decrease
by using gas. However, the direct photos taken with 20’000
fps show a different flame behaviour between the two fuels. Such
differences in the flame characteristics are examined in detail applying
the ’laser shadowgraph’ and the ’BDL (back diffused laser)’
optical techniques. Furthermore, in order to meet IMO Tier III
NOx regulations, the oxygen content of the intake air is reduced
as a good approximation for an exhaust gas recirculation (EGR)
system. As expected, the brightness of the flame decreases and an
NOx reduction of 75% in 17% O2 can be achieved. For a second
series of experiments, a cylinder head with a cylindrical clearance
volume is newly developed to allow different swirl velocities and
an observation view over the whole 240mm in diameter window;
the side injection system corresponds to a common two-stroke
engine. Injection nozzles with different numbers of injection
holes are tested, applying different injection pressures, and multi
flames are visualised. In conclusion it can be stated that experiments
with the RCEM help to determine emission influencing parameters
and optimisation potential, to visualise and to analyse
phenomena that have not been simulated yet.
Improvement of dual-fuel-engine technology for
current and future applications
Hinrich Mohr, AVL List GmbH, Austria
Torsten Baufeld, AVL List GmbH, Austria
Currently a renaissance of dual-fuel engines can be experienced in
the large-bore engine markets. For a very long time, this technology
was in use only in stationary power generation application,
where the market shares decreased clearly during the last decade
due to the strong improvements on pure gas engine performance
values and the extension of the pure gas engines to higher output
ranges with large medium-speed engines. In addition to the previous
sole land-based usage, several mobile applications are now
opening further markets for large-bore dual-fuel engines. These
applications are marine main propulsion (e.g. LNG carries, cruise
liners) and auxiliary usage (e.g. container vessels) as well as rail
traction purposes (e.g. long-haul locomotives). The main drivers
for the increased interest in DF engines are lower fuel costs in
comparison with the expensive HFO or diesel fuel and the opportunity
to reduce especially the NOx emissions enabling a fulfilment
of upcoming emission legislations. Furthermore, natural
gas could be a high-potential low-sulphur fuel as requested for a
lot of mobile engine applications very soon. The typical secondary
liquid fuel beside gas - which has been disadvantageous in
stationary DF applications very often - enables as back-up fuel
a very reliable mobile operation. In marine usage a DF engine
facilitates a fulfilment of the IMO emission regulations inside
and outside an ECA just by switching the mode between gas and
diesel operation. The challenges of the mobile applications are
often variable speed operation, fast load response requirements,
changing gas qualities and reliable engine operation even under
difficult operating conditions. Due to the typical two operational
modes (gas operation and diesel operation) of DF engines several
compromises must be made in engine design (e. g. compression
ratio, piston bowl shape, valve timing, etc.) as well. This leads
currently to disadvantages with respect to efficiency and power
density. With regard to the before mentioned topics, AVL investigated
the dual-fuel engine technology by systematic mediumspeed
single-cylinder engine testing. A wide variety of parameters
were evaluated and the operational borderlines were explored.
Based on the gained results, a short-term outlook with today’s DF
engine technology will be defined. AVLs future perspective of DF
engine technology will be created as well enabling the definition
of required mid-term developments.
Solutions for meeting low emission requirements in
large-bore natural gas engines
Emmanuella Sotiropoulou, Prometheus Applied Technologies, LLC, USA
David Lepley, Altronic, LLC, USA
Luigi Tozzi, Ph.D., Prometheus Applied Technologies, LLC, USA
Across the entire natural gas engine industry, operators and
OEMs are faced with increased expenses and the deterioration
of engine performance as they struggle in meeting the mandated
lower emission levels. In the sector of large-bore engines, greater
than 250mm, the field is populated by two combustion strategies
aiming at meeting lower emissions. A significant percentage
of large-bore, low-speed engines, use two spark plugs per cylinder
to enhance the combustion rate. The remaining population of
engines uses precombustion chambers with a dedicated fuel feed
and controls to generate a rich air-fuel mixture in the vicinity of a
conventional spark plug. Using a precombustion chamber adds
complexity, cost and reduces reliability. Although lower emissions
are achieved with fuel fed prechambers, the engine stability with
leaner mixtures still remains the limiting factor compromising the
performance of the engine. This paper describes a cost-effective solution
for each of the combustion strategies. These solutions aim
at extending the lean limit of operation, hence, meeting the lower
emission requirements with improved combustion stability. In
the case of large–bore, low-speed engines currently using conventional
spark plugs, it is possible to avoid the costs associated with
the conversions to fuel-fed precombustion chambers by simply replacing
the conventional spark plugs with specially designed passive
prechamber spark plugs. These highly effective designs are obtained
with the latest technology in computational flow dynamic
(CFD) that uses the CONVERGE detail chemistry CFD software.
Results from engine testing indicate that specially designed passive
prechamber spark plugs achieve stable engine operation at NOx
emission levels below 500 mg/Nm3 (1.0 g/bhp-hr). In the case of
engines that already have a fuel-fed precombustion chamber, lower
emissions can be achieved with the use of a passive prechamber
spark plug in place of the conventional spark plug to form a twostage
precombustion chamber operating with significantly leaner
mixtures. The flame jets emerging from the passive prechamber
spark plug compensates for the slower flame propagation rates
associated with lean prechamber combustion. The optimum design
of the two-stage precombustion chamber, the amount of fuel
required, the fuel injection timing and the spark discharge characteristics
are also determined with the latest technology in computational
flow dynamic (CFD) that uses the CONVERGE detail
chemistry CFD software. An electronic fuel control valve provides
the amount of fuel required at the correct timing. Furthermore, an
ignition system with a tunable high energy spark discharge wave
form achieves the desired combustion stability while maintaining
a long plug life. Engine test results from this system indicate stable
engine operations at NOx emissions levels below 250 mg/Nm3
(0.5 g/bhp-hr). The solutions demonstrated in this paper provide
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Schiff&Hafen | Ship&Offshore | May 2013