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http://hdl.handle.net/10251/145993
Redtenbacher, C.; Kiesling, C.; Malin, M.; Wimmer, A.; Pastor, JV.; Pinotti, M. (03-2).
Potential and Limitations of Dual Fuel Operation of High Speed Large Engines. Journal of
Energy Resources Technology. 140(3):1-10. https://doi.org/10.1115/1.4038464
https://doi.org/10.1115/1.4038464
ASME International
Potential and Limitations of Dual
Fuel Operation of High Speed Large Engines
Christoph Redtenbacher
LEC GmbH
Graz, Austria
Constantin Kiesling
LEC GmbH
Graz, Austria
Maximilian Malin
LEC GmbH
Graz, Austria
Andreas Wimmer
LEC GmbH /
Graz University of Technology
Graz, Austria
Jose V. Pastor
CMT-Motores Térmicos,
Universitat Politècnica de
València
València, Spain
Mattia Pinotti
CMT-Motores Térmicos,
Universitat Politècnica de
València
València, Spain
Abstract
The aim of this paper is to identify and investigate the potential and limitations of diesel–gas combustion concepts for high
speed large engines operated in gas mode with very small amounts of pilot fuel (<5% diesel fraction). Experimental tests
were carried out on a flexible single cylinder research engine (displacement 6.24dm
3
) equipped with a common rail system.
Various engine configurations and operating parameters were varied and the effects on the combustion process were
analyzed. The results presented in this paper include a comparison of the performance of the investigated dual fuel
concept to those of a state-of-the-art monofuel gas engine and a state-of-the-art monofuel diesel engine. Evaluation
reveals that certain limiting factors exist that prevent the dual fuel engine from performing as well as the superior gas
engine. At the same NO
x
level of 1.3g/kWh, the efficiency of the dual fuel engine is 3.5% pts. lower than that of the gas
engine. This is caused by the weaker ignition performance of the injected pilot fuel compared to that of the gas scavenged
prechamber of the gas engine. On the other hand, the dual fuel concept has the potential to compete with the diesel
engine. The dual fuel engine can be operated at the efficiency level of the diesel engine yet with significantly lower NO
x
emissions (3.5g/kWh and 6.3g/kWh, respectively). Since the injection of pilot fuel is of major importance for flame
initialization, and thus for the main combustion event of the dual fuel engine, optical investigations in a spray box,
measurements of injection rates, and three-dimensional (3D) computational fluid dynamics (CFD) simulation were
conducted to obtain even more detailed insight into these processes. A study on the influence of the diesel fraction shows
that diminishing the diesel fraction from 3% to lower values has a significant impact on engine performance because of
the effects of such a reduction on injection, ignition delay, and initial flame formation. The presented results illustrate
which operating strategy is beneficial for engine performance in terms of low NO
x
emissions and high efficiency. Moreover,
potential measures can be derived which allow for further optimization of the diesel–gas combustion process.
Keywords: high speed large engine, dual fuel, diesel–gas, diesel injection, spray box, 3D CFD
Introduction
Interest is growing in using fully flexible diesel–gas dual fuel engines able to run in pure diesel mode as well
as in dual fuel mode for power generation and propulsion on land and at sea. Dual fuel operation with these
engines is characterized by the feeding of a premixed gas–air mixture into the engine and compression ignition
of the cylinder charge via direct diesel injection into the combustion chamber, cf., Ref. [1]. Benefits such as the
flexibility to adapt the type of fuel to the market, fail-safe operation, and lower nitrogen oxides (NO
x
) emissions
than diesel engines are convincing arguments for engine operators, cf., Refs. [2–8]. However, diesel–gas engine
concepts still suffer from lower efficiency and worse combustion stability than state-of-the-art monofuel diesel
engines and spark ignited gas engines when operated in the corresponding fuel mode, cf., Ref. [9]. To meet
stringent NO emission legislation, high diesel substitution rates are necessary. Investigations of a large diesel–
gas dual fuel engine concept [10] indicate that diesel fractions
1
in the range of <1% are required to reduce the
NO
x
emission level below 2g/kWh when the engine is operated at its nominal indicated mean effective pressure
(IMEP) of 24bar and without exhaust gas recirculation (EGR). Achieving adequate combustion stability within
this operating range is an issue.
First, this paper evaluates the performance of a state-of-the-art diesel–gas dual fuel concept, comparing it to
those of a monofuel gas engine and a monofuel diesel engine by providing a detailed analysis of their combustion
processes based on experimental investigations on a large high speed single cylinder engine (SCE) with a
displacement of 6.24dm
3
. The conceptual differences between these engine types are discussed and the reasons
for the disadvantages in efficiency of current dual fuel technology are explained. The aim is to show the potential
and the limitations of the dual fuel engine when it is operated at its nominal IMEP of 24bar without EGR and
with very small amounts of pilot fuel (<5% diesel fraction) so that low engine-out NO
x
emissions can be obtained.
The investigations have been limited to an operating range which provides combustion stability within a window
that allows the combustion concept to be applied to a multicylinder engine.
Note that it has already been shown that very low NO
x
emissions can also be achieved with larger diesel
fractions, cf., Refs. [3,5,11–15]. However, it must be taken into account that these investigations used boundary
conditions that differ significantly from those of the investigations in this paper. Most of these combustion
concepts were investigated at considerably lower engine loads ranging from approximately 3 to 12bar IMEP.
Furthermore, comparatively large cyclic variations of the IMEP and thus an unfavorable combustion stability
were tolerated under certain operating conditions. The investigations of Eichmeier et al. in Ref. [11] and Nieman
et al. in Ref. [12] also include higher loads of 19bar and 23bar IMEP, respectively, but comparatively high EGR
rates of 50% were employed for the investigated operating points. Moreover, the engine size in this paper differs
significantly from the displacements in the previously mentioned references which range from 0.6 to 2.4dm
3
per
cylinder.
Second, this paper analyzes combustion effects in dual fuel operation with small diesel fractions as the basis
for further improvements of the combustion concept. Reducing the diesel fraction below 5% has a considerable
influence on engine performance because of the effects of such a reduction on injection, ignition delay, and initial
flame formation. The results of the SCE investigations show that it becomes more and more challenging to ensure
stable ignition and fast combustion of the air-fuel mixture. In addition, it can be seen at very small diesel fractions
in particular that the well-known relationship between the injection timing and combustion phasing of
conventional engine concepts is no longer valid.
Effects of variations in diesel fraction and injection timing have already been investigated in Refs. [3,5,11,13–
16]. For example, Eichmeier et al. [11] studied the influence of the injection timing on combustion phasing,
efficiency, and emissions at a comparatively high IMEP of 19bar while employing an EGR rate of 55%. Krishnan
et al. [3] investigated a wide range of pilot fuel injection timings on a heavy duty SCE at a brake mean effective
pressure of 12.2bar and without EGR. A similar study of the injection timing at an IMEP of 5.1bar has been
presented by Raihan et al. [5]. Common to these three studies is a discussion of the effect that under certain
conditions, advancing the injection timing results in a retarding of the combustion phasing. Krishnan et al. [16]
examined the effects of varying the diesel fraction on a heavy duty SCE at an IMEP of 6.1bar. The results show
that larger diesel fractions yield shorter ignition delays and greater magnitudes of the initial premixed combustion
peak of the diesel fuel.
1
The diesel fraction describes the energetic amount of diesel fuel related to the total fuel energy fed into the engine.
The aim of this publication is to study the effects of varying injection timing and diesel fraction in detail by
employing additional methods apart from the SCE investigations. These methods focus on the pilot fuel injection
event since it is of major importance for flame initialization and thus for the main combustion process.
Consequently, optical investigations in a spray box, measurements of injection rates, and three-dimensional (3D)
computational fluid dynamics (CFD) simulation were conducted to obtain highly detailed information about the
fundamental effects. In contrast to the previously mentioned references, this paper focuses on studying the effects
at high load engine operation (IMEP 24 bar) without EGR.
The combination of knowledge acquired of the specific deficits of state-of-the-art dual fuel engines and detailed
information of fuel injection effects at very small diesel fractions provides the basis for further optimization of
the dual fuel combustion process.
Table 1 Single cylinder engine technical specifications
Rated speed
1500rpm
Bore
190mm
Stroke
220mm
Displacement
6.24dm
3
Conrod length
425mm
Compression ratio
Adjustable by modifying the piston geometry
Valve timing
Adjustable by modifying the camshaft geometry
Number of inlet/ exhaust valves
2/2
Swirl/tumble
0/0
Charge air
Provided by external compressors with up to 10bar boost pressure
Gas fuel supply
External mixture formation via a venturi mixer
Diesel fuel supply
Common rail system with up to 2200bar pressure
Balance of inertia forces
Four balancing shafts to compensate for first and second-order inertia forces
Experimental Setup
Single Cylinder Engine Testing. Engine testing was carried out on a high speed four-stroke single cylinder
research engine at LEC GmbH in Graz. The highly flexible SCE can be rebuilt quite easily to allow the
investigation of different engine configurations such as monofuel diesel engine concepts and monofuel gas engine
concepts as well as dual fuel engine concepts. The test bed is equipped with the latest crank angle (CA) and time-
based measurement technology for all relevant parameters. Furthermore, the engine is supplied with conditioned
charge air, burning gas, cooling water, and lubricating oil to ensure reproducible testing conditions. The back
pressure in the exhaust pipe is controlled by a flap to simulate the presence of a turbocharger. All measurements
were taken without making use of EGR. The main engine data of the investigated concepts are presented in Table
1. Table 2 summarizes the measurement technology applied on the SCE.
Table 2 Single cylinder engine measurement technology
Quantity
Instrument
Accuracy
Gas mass flow
Emerson Micro Motion CMFS015
60.25% of metered value
Air mass flow
Emerson Micro Motion CMF100
60.35% of metered value
Monofuel Gas Engine. An optimized spark ignited gas engine concept was taken as a reference for the
evaluation of the dual fuel combustion process. The cylinder head was equipped with a centrally mounted gas
scavenged prechamber including a spark plug. This setup enables highly effective ignition of a lean mixture in
the main combustion chamber, cf., Ref. [20]. The compression ratio was 12.5:1, and therefore, 0.5 points higher
than that of the dual fuel engine. Furthermore, the intake valve closing (IVC) was 20CA earlier than that of the
dual fuel engine. With this monofuel gas engine setup, a previously existing representative operating point with
an IMEP of 24bar was chosen for evaluation of the dual fuel operation. The boundary conditions of this operating
point differ slightly from those of the dual fuel operating points. The intake charge temperature was 50C and the
intake air humidity was 8g/ kg. In addition, the scavenging gradient of the monofuel gas engine was based on a
two-stage turbocharging concept.
Monofuel Diesel Engine. A state-of-the-art diesel engine concept consisting of a standard diesel injector (same
nominal nozzle flow rate as the dual fuel wide range injector) and a well-balanced nozzle/piston bowl geometry
was used as the benchmark for diesel operation. The compression ratio of 17:1 was significantly higher than
those of the dual fuel engine setup and the gas engine setup. With the gas fueled engines, such a high compression
ratio is not feasible as knocking combustion would occur. The diesel engine configuration has a 5CA earlier IVC
than the dual fuel setup. Similar to the monofuel gas engine, a previously existing and representative operating
point with an IMEP of 24bar was used for the comparison with dual fuel operation. While the intake temperature
was the same, the intake air humidity was 3.9g/kg and thus lower than that of the dual fuel engine. The scavenging
gradient of the diesel engine operating point was based on a single-stage turbocharging concept.
Spray Box and Rate of Injection Measurements. These investigations were carried out at the Universitat
Politècnica de València, CMT-Motores Térmicos. A high temperature and high pressure spray box in which the
thermodynamic conditions in an engine at the time of injection can be simulated up to a maximum temperature
of 1000 K and a maximum pressure of 150bar was used to measure the liquid spray phase (Mie scattering
technique) and the vapor spray phase (Schlieren technique) under inert conditions in a nitrogen atmosphere. A
detailed description of the measurement techniques and the corresponding optical setups is given in Ref. [10].
Rate of injection (ROI) measurements were carried out with a commercial injection analyzer from the company
IAV using the Bosch method [21], where the injector introduces diesel into a measuring tube filled with fuel.
The discharge of fuel produces a pressure increase inside the tube proportional to the increase in fuel mass. The
rate of this pressure increase corresponds to the injection rate.
Diesel mass flow
AVL Fuel Exact, MF 150KG SF
0.1% of metered valueþ0.002kg/h
Charge temperature
Resistance temperature sensor PT100 (3)
According to class A
Charge pressure
Piezoresistive pressure sensor, 0–16bar
0.2% full scale output (FSO)
Charge humidity
Vaisala HUMICAP humidity transducer
2.5g H
2
O/kg air at 45C
Exhaust gas temperature
Thermocouple type K (3)
According to class 1
Exhaust gas pressure
Piezoresistive pressure sensor 0–16bar
0.2% full scale output (FSO)
Exhaust gas emissions
AVL AMA i60 emission bench
Dependent on each exhaust gas component
NO
x
AVL CLD i60 HHD
1% FSO
Cylinder pressure
AVL piezoelectric transducer QC34C with
AVL Micro IFEM 4P3G amplifier
Cyclic temperature drift: 60.3bar; Thermo
shock error: DIMEP 61%
Crank angle
AVL 365 CC crank angle encoder
Resolution: 0.1CA
Crank angle resolved
data acquisition
AVL IndiSet 642 Advanced Plus Gigabit
Resolution: 14bit
