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The Effect of Cavitation on Diesel in Diesel Fuel Injection Equipment

Authors:
  • Exa-flo Ltd

Abstract

Modern high-pressure diesel fuel injection equipment (FIE) meter the diesel fuel supply to the injectors and engine from one or more of the high-pressure pump, the pressure accumulator (common rail), and the injectors. The fuel metering process requires the separated fuel to be returned back to the fuel tank via control valves. The relaxation flow of the high-pressure metered fuel across the metering valves is likely to produce cavitation. There is a concern in the fuel industry and in the FIE manufacturers that metered fuel may be compositionally degraded by cavitation. This paper reports the results of two experiments conducted at City, University of London, on (1) cavitation inception in model return valves, and (2) alteration of the composition of diesel fuel as a result of cavitation. Experiment (1) involves the manufacture of optically accessible acrylic model pressure control valves characteristic of those found in diesel injectors. The models have been subjected to varying upstream to downstream fuel pressures in order to characterise the conditions required for cavitation inception. Experiment (2) has been conducted in two parts, identified by 2A and 2B. In experiment 2A, 11 commercial diesel samples have been subjected to cavitation flow tests in a purpose-built high-pressure recirculating flow rig operating at 550 bar upstream pressure. The diesel samples have been subjected to in-situ optical extinction measurements at 405 nm. The composition of these diesels has been examined before and after the cavitation tests using two-column gas chromatography. A decrease in the volume fraction of aromatics has been observed. In Experiment 2B, unadditized middle distillate diesel has been subjected to cavitation flow tests in a purpose-built high-pressure recirculating flow rig operated at 1,650 bar upstream pressure. The diesel samples have been subjected to in-situ optical extinction measurements at 405 nm. The composition of these diesels has been examined before and after the cavitation tests using uv-vis absorption spectrophotometry, and two-column gas chromatography. The uv-visible absorption spectra reveal a destruction of aromatics in the diesel sample.
The Effect of Cavitation on Diesel
in Diesel Fuel Injection Equipment
Russel Lockett, Mahesh Jeshani, Zeeshan Fatmi, Alberto Bonifacio,
Olawole Kuti, Richard Price (Shell), Nicholas Rounthwaite (Shell)
Identification of the Problem
1. Black diesel (diesel containing large concentrations of soot-like nano-
and micro-particles) in the fuel tank.
2. Soot-like microparticles and sticky brown gums found in diesel fuel filter
(often saturating the filter).
3. Carbonaceous deposits and sticky gums found in the neighbourhood of valve
seats in the pump and small clearance surfaces in injectors.
4. Hard polymeric lacquer coating internal pump, pipe, common rail, and injector
surfaces in Diesel Fuel Injection Equipment (FIE).
Where and how have these deposits originated ?
Research Programme
1. Characterise the return valve nozzle flow in Diesel Fuel Injection Equipment
(FIE).
2. Develop a high pressure recirculation flow rig for the study of the effect
of cavitation on diesel.
3. Identify suitable diagnostics for the experimental investigation into the
effects of cavitating flows on diesel fuel samples.
4. Develop a detailed chemical kinetic model for oxidation and/or pyrolysis
for surrogate diesel reflecting the composition of real diesel.
5. Gain a qualitative understanding of chemical kinetic reaction paths leading
to deposit formation.
INJECTOR STAGES
1. Resting state
2. Start of Injection
3. Injector Opened
4. End of Injection
1. Identify and characterise Diesel FIE return valve
nozzle flow.
Acrylic model
6
Experimental Setup
Experimental Setup
Optical Equipment arrangement along with Test Rig apparatus
Acrylic model setup during experiment
8
Acrylic block results
Dimensions:
Inlet Passage Ø210μm
Outlet Passage Ø225μm
Block 1 Block 2
Dimensions:
Inlet Passage Ø210μm
Outlet Passage Ø240μm
0
2
4
6
8
10
12
010 20 30 40 50 60 70
Downstream Pressure (bar)
Upstream Pressure (bar)
150micron
Above_block 2
Below_block 2
Above_block 1
Below_block 1
0
2
4
6
8
10
12
010 20 30 40 50 60 70
Downstream Pressure (bar)
Upstream Pressure (bar)
200micron
Above_block 2
Below_block 2
Above_block 1
Below_block 1
04/05/2017 9
0
2
4
6
8
10
12
010 20 30 40 50
Downstream Pressure (bar)
Upstream Pressure (bar)
Above measurements
50micron
100micron
150micron
200micron
250micron
0
2
4
6
8
10
12
010 20 30 40 50
Downstream Pressure (bar)
Upstream Pressure (bar)
Below Measurements
50 micron
100micron
150micron
200micron
250micron
0
2
4
6
8
10
12
020 40 60 80
Downstream Pressure (bar)
Upstream Pressure (bar)
Above measurements
50micron
100micron
150micron
200micron
250micron
0
2
4
6
8
10
12
020 40 60 80
Downstream Pressure (bar)
Upstream Pressure (bar)
Below Measurements
50micron
100micron
150micron
200micron
250micron
Block 1 Block 2
Observations
1. Greater pressure ratio required to produce cavitation inception for 50μm
needle lift, relative to other larger needle lift heights.
2. Greater pressure ratio is required to produced cavitation inception in
Block 2 relative to Block 1. This is due to the larger nozzle diameter
ratio between holes 2 and 1 in Block 2 relative to Block 1. This
produces a rising pressure ratio between upstream fuel pressure and
intermediate volume.
3. Cavitation is never observed in the first passage, and intermediate
control volume.
4. This design displaces the location of cavitation to the expansion region
of the second nozzle, and facilitates stable needle lift.
2. The Effect of Long-Term Cavitation on Diesel
(550 bar)
Objectives
1. Design and build a recirculating, cavitating flow rig to
support long term high pressure cavitation flow through a
diesel nozzle.
2. Develop a simple diagnostic for identifying change in
diesel composition.
3. Identify changes in composition with cavitation time.
Experimental Setup 1: Recirculating Flow Rig
Fuel
tank
Low
pressure
pump
Fuel
filter
Pressure
gauge
Injector
Viewing
device 1
Cooler
Water
out
Water
in
Fuel to viewing
device 2
Fuel from
viewing device 2
High pressure
pump
High pressure
pump motor
Front View Isometric View
Experimental Setup 2: Optical Absorption Setup
Detector 1 (Reference
detector)
Detector 2 (Transmission
detector)
Viewing device 2
Fuel out
Fuel in
Beam
splitter
405nm Laser
Mirror
10 Commercial Diesel Fuels:
Initial Fuel Composition
Results 1: 10 Commercial Diesel Samples,
Gas-to-Liquid Diesel (GTL), GTL + 20% RME
Results 2: Two Column Gas Chromatography
Results 3: GC x GC Shell Flitwick
Results 4: GC x GC Initial Fuel Composition
Results 5: Changes to Fuel Composition due to Cavitation
Observations & Conclusions
1. The experimental variation in spectral extinction coefficient is repeatable
with an uncertainty of approximately 2 %.
2. The spectral extinction coefficient increases over 40 hours cavitation
duration for all commercial fuels, indicating nano/micro-particle formation.
3. The spectral extinction coefficient for GTL diesel appears to be
unaffected during the 40 hours cavitation, indicating low variation in
composition and production of nano-/micro-particles.
4. GC x GC analysis of commercial diesel shows a trend of reduced mono-
aromatic fraction comprising the cavitated diesel relative to fresh diesel.
3. The Effect of Long-Term Cavitation on Diesel
(1,650 bar)
Objectives
1. Design and build 1,650 bar cavitating, recirculating flow
rig.
2. Extension of sample analysis to include UV-Visible
absorption spectroscopy and laser particle counting.
3. Analysis of pressure dependence and filter type.
4. Analysis of fuels and fuel properties.
Experimental Setup: High Pressure Cavitation
Flow Rig & Injectors
Results: Long Duration Tests using Old Research
Nozzles (ϕ= 213µm, v= 1.15 l/min)
Benzenoid
Band
0
0.2
0.4
0.6
0.8
1
1.2
1.4
245 265 285 305
Self-normalised absorbance at 245nm
Wavelength (nm)
0hour
30hours
60hours
130hours
0
0.5
1
1.5
2
2.5
3
3.5
190 240 290 340 390
Absorbance
Wavelength nm
0hour
30hours
60hours
130hours
Observations & Conclusions
1. Prolonged cavitation conditions the nozzle surfaces, resulting in increased fuel
mass flow rate (reduced nozzle cavitation).
2. Upstream pressure appears to determine the rate of cavitation induced pyrolysis
(indicated by variation in extinction coefficient), only in its determination of fuel
mass flow rate through the system. This suggests cavitation-induced fuel pyrolysis
is a downstream process associated with cavitation bubble collapse occurring
during downstream pressure recovery.
3. Time-dependent UV-Vis absorbance (405 nm) consistent with in-Situ
measurement of spectral extinction coefficient (405 nm).
4. UV-Vis Benzenoid band (245 nm 285 nm) shows decrease in mono-and di-
aromatic concentration with cavitation duration.
5. Broadband monotonic increase in UV-Vis Absorbance suggests nano-/micro-
particle formation, leading to a consistent increase in solid matter particle volume
concentration (Rayleigh scattering/small particle absorption regime).
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