University of Huddersfield Repository
Gu, Fengshou, Ball, Andrew and Rao, K K
Diesel injector dynamic modelling and estimation of injection parameters from impact response part
2: prediction of injection parameters from monitored vibration
Original Citation
Gu, Fengshou, Ball, Andrew and Rao, K K (1996) Diesel injector dynamic modelling and
estimation of injection parameters from impact response part 2: prediction of injection parameters
from monitored vibration. Proceedings of the Institution of Mechanical Engineers Part D Journal of
Automobile Engineering, 210 (44). pp. 303-312. ISSN 0954-4070
This version is available at http://eprints.hud.ac.uk/id/eprint/6788/
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303
Diesel
injector dynamic modelling and estimation
of
injection-parameters from impact response
Part
2
:
prediction
of
injection parameters from
monitored vibration
F
Gu,
MSc
and
A
D
Ball,
BEng, PhD
School
of
Engineering,
University
of
Manchester
K
K
Rao,
MSc
Department
of
Mechanical
Engineering, University
of
Manchester Institute of Science and Technology
Part
2
of this paper presents the experimental and analytical procedures used in the estimation of injection parameters from monitored
vibration. The mechanical andflow-induced sources of vibration
in
a fuel injector are detailed and the features of the resulting vibration
response of the injector body are discussed. Experimental engine test and data acquisition procedures are described, and the use of an
out-of-the-engine test facility to confrm injection dependent vibration response is outlined.
Wigner-Ville distribution (WVD) analysis of non-stationary vibration signals monitored
on
the injector body is used to locate regions
of vibration in the time-frequency plane which are responsive to injection parameters. From the data
in
these regions, estimates of
injection timing and fuel pressure are obtained.
Accurate estimation of injection parameters from externally monitored vibration is shown to pave the way for the detection and
diagnosis of injection system faults. Moreover, it is demonstrated that the technique provides an alternative method for the set-up,
checking and adjustment
of
fuel injection timing.
Key words:
fuel
injection
timing
injector, condition monitoring, injector
v
i
b
r
a
t
i
o
n
,
1
INTRODUCTION
The injection process plays one of the most influential
roles in the performance and emission control of a
diesel engine. Obtaining accurate values for injection
parameters is the key to the specification, set-up, adjust-
ment and diagnosis of injection equipment. This in turn
permits enhanced performance such as reductions in
fuel consumption and pollutant emissions, and increases
in power output.
It is conventional to characterize the injection process
in terms of injection pressure, needle lift and injection
rate (the latter two factors contain injection timing
information). Injection pressure is usually measured
with
a
strain gauge transducer positioned in the high-
pressure
fuel
line, and needle lift is measured with a dis-
placement sensor inside the injector
(1,
2).
Part
1
described why, due to their intrusive nature, it was in-
evitable that these measurement techniques would
adversely affect the motion
of
the needle within the
injector valve and would consequently change the fuel
injection characteristics.
A
non-intrusive measurement approach, which does
not influence injection characteristics, is attractive
because it may permit more accurate estimates of injec-
tion parameters.
A
further possible advantage of such
an approach is that in-service condition monitoring
with subsequent adjustment, fault detection and diag-
nosis becomes feasible.
Fundamental to the principle of non-intrusive mea-
surement
is
the remote detection and interpretation of
The
MS
was
received
on
4 April 1994 and was accepted
for
publication
on
28
October
1995.
DO179512
Q
IMechE
1996
impact vibration, time-frequency
analysis,
injection
pressure,
some transmitted quantity which contains sufficient
information to enable the source parameter value to be
inferred. Of the possible transmitted parameters, one of
the most likely to contain the necessary information is
the vibration response of the injector body to the oper-
ation of the injector valve within it.
Specifically, injector vibration response is due to a
combination of the needle impacting and the flow of
high-pressure fuel within the body. Using modern signal
processing techniques and computational capabilities, it
appears possible to extract injection parameter informa-
tion from the non-stationary vibration signals that are
detected by a transducer when positioned on the outer
surface of an injector body.
In this study, the vibration response of
a
fuel injector
when operating in a test engine is analysed using the
Wigner-Ville distribution
(WVD),
and estimates of
injection timing and fuel pressure are obtained. The
layout of the paper is as follows: Section
2
analyses
injector vibration; Section
3
describes the test methods
used in the vibration measurement; Section
4
details the
WVD
analysis of the monitored vibration signals; and
Section
5
compares vibration-based results with those
obtained from conventional measurement.
2
INJECTOR VIBRATION
There are two sources of injector body vibration associ-
ated with the injection process: impacts due to the
needle hitting its backstop and seat (that is mechanical
excitation), and the
flow
of
high-pressure fuel within the
galleries and chambers
of
the injector (that is fluid flow
Roc
Instn
Mech Engrs
Vol210
304
F
GU,
A
D
BALL AND
K
K
RAO
excitation). Aside from the nature of the excitation
sources, the monitored vibration is also influenced by
the dynamic properties of the injector body.
2.1
Mechanical excitation
In Part
1
the impact behaviour of an injector was inves-
tigated numerically by modelling the needle motion as
a
two-mass vibro-impact system. The retracting backstop
impact was shown to be of lower amplitude but to
contain more high-frequency components than the sub-
sequent advancing needle seat impact. The first collision
in the needle retracting impact series indicates the
instant when the needle first reaches its fully open posi-
tion. Similarly, the first collision in the needle advancing
impact series indicates the instant when the needle first
returns to its seat.
Part
1
developed a theoretical correlation between
the impacts and fuel injection parameters. The ampli-
tude of the first collision in the opening impact series
was shown to be related to the fuel injection pressure,
and the energy of the opening impact series was shown
to be related to the fuel injection rate.
2.2
Fluid
flow
excitation
During injection, fuel flows through the internal pass-
ages and chambers of the injector with
a
sharp high-
pressure wave front. This turbulent flow impinges upon
the injector body and causes it to vibrate. The forces
involved are proportional to the dynamic head of the
fuel flow and the frequency of the resulting vibration
depends upon the sharpness of the wavefront. This
means that injection at larger fuel rates
is
more likely to
produce higher frequency and larger amplitude body
vibrations than injection at smaller rates.
The presence of flow induced vibration response is
confirmed in Fig.
1
which overlays monitored body
vibration upon cylinder pressure, line pressure and
needle lift traces. From this figure it can
be
seen that
there is a vibration response commencing prior to the
opening of the needle (at a time corresponding to the
onset of high-pressure fuel supply) and continuing until
the needle is fully retracted, at which point it becomes
swamped by the high-amplitude impact response. In
this time span, there are no mechanical impacts, only
the flow of fuel within the injector.
2.3
Injector
body
response
Due to the short duration of the collisions which com-
prise the opening and closing impact series (the dura-
tion of a collision is of the order of
a
microsecond), the
injector vibration response to this excitation is a
sequence of three series of transients, with each tran-
sient being dominated by the lower body frequencies.
Within an injection cycle, the order of the excitation
events dictates that the first transient series is due to
flow excitation, the second transient series is due to
opening impact excitation, and the third transient series
is due to closing impact excitation.
Depending upon engine operating conditions (for
example the duration of injection), an individual tran-
sient response within the train of responses may
or
may
not have decayed completely before the next transient
occurs. As shown in Part
1,
the opening and closing
impact series have different spectral contents, meaning
that their associated responses will have different spec-
tral contents too. From a time trace of monitored vibra-
tion, it may not
be
possible to detect the instant of the
closing impact if it is buried in the continuing transient
response from the opening impact series. The different
frequency contents of the two responses suggest,
10
1
I
I
1
I
I
I
I
1
1
--.**.-*.-
Cylinder pressure
Injector vibration
8
----
Line pressure
2
0
-2
Closing impact response
Fluid excitation res
I
I
I
I
1
I I
I
0
0.5
1
1.5
2
2.5
3 3.5
4
Injector vibration versus cylinder pressure, line pressure and needle
lift
Time (ms)
Fig.
1
Part
D:
Journal
of
Automobile
Engineering
Q
IMechE
1996
DIESEL INJECTOR DYNAMIC MODELLING. PART
2
305
Fuel Return
Tank
however, that it should be possible to differentiate
between the two events if the monitored vibration
response is transformed into
a
time-frequency represen-
tation.
PC
computer
3 INJECTOR VIBRATION MEASUREMENT
Due to the complex nature of the injector body, its
mounting assembly and its interaction with the cylinder
head, it is difficult to derive a mathematical model rela-
ting the internal vibration sources to an external
surface-mounted monitoring transducer. For this
reason, an experimental investigation of the relationship
between monitored vibration and fuel injection was
adopted in this study.
Two types of experimental set-up were used:
a
working engine test rig, and a bench-top rig providing
an out-of-the-engine injection test facility. The test
engine was
a
Ford FSD-425 production unit fitted with
a
Bosch
V
injection system and coupled to
a
hydraulic
dynamometer. The bench-top test rig comprised
a
cylin-
der head from another Ford FSD-425 engine and fitted
with an identical injection system, but driven by an elec-
tric motor. The purpose of the bench-top rig was to
isolate those components of monitored vibration which
were due to the injection process from those com-
ponents associated with combustion, piston slap and
other engine sources. In addition to the recording of
injector vibration when in the test engine, fuel line pres-
sure, needle lift and fuel consumption were recorded by
conventional methods to enable the relationship
between the injection parameters and vibration to be
investigated.
3.1
Bench-top injector tests
In addition to the internal sources of diesel injector
vibration, it is likely that other factors, particularly
combustion noise, will also excite the injector body.
During combustion, there is a sharp rise in cylinder
pressure and a consequent shock which acts upon the
cylinder head, cylinder wall and the tip of the injector
nozzle. This shock to the injector tip, and the vibration
of the cylinder head, are likely to contribute to the
vibration response
of
the injector body.
Figure
1
(in Section 2) shows that there is no severe
cylinder pressure change at the time of injector needle
opening, and it is hence unlikely that the cylinder pres-
sure makes any significant contribution to injector body
vibration. At the time of the injector needle closing,
however, the cylinder pressure rises sharply with the
onset of combustion, and it is hence unlikely that the
injector body vibration is due to needle impacts alone.
Ascertaining the sources of excitation during the needle
closing phase is not straightforward, and this difficulty
provided the motivation for the development of the
bench-top injector test rig.
Figure
2
depicts the schematic layout of the bench-
top rig, which was designed to isolate injector vibration
response from all sources aside from those inherent to
the operation of the injector. The principal components
of the rig are a variable speed electric motor driving an
injection pump, which in turn supplies fuel from a tank
to a set of four fuel injectors, one of which is mounted
in a cylinder head. To ensure close correlation with the
engine tests, the injection pump, injectors and cylinder
head are identical to those used on the engine. Supports
shrouded in damping material, and vibration isolating
mounts are employed to minimize the transmission
of
vibration from the motor and the fuel pump to the test
injector.
Only one injector (the test injector) is mounted in the
bench-top cylinder head. The other three injectors, used
to maintain representative loading of the injection
pump, are mounted together in a dummy cylinder head
with associated fuel collection vessels. This separation
of the test injector from the load injectors is further to
ensure its vibration isolation. Fuel levels can be adjust-
ed via the injection pump rack setting in the usual way,
and the pump speed can be varied by the closed-loop
II
Fuel
Supply
Tank
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1996
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F
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D
BALL AND
K K
RAO
motor controller. Two-channel data acquisition from
the bench-top rig was made with a computer-controlled
12-bit device sampling simultaneously at
65
kHz. The
incoming data streams of body vibration and directly
measured needle motion readings were gated in soft-
ware into 1024-point segments.
From the bench-top tests, several aspects of injector
vibration response were confirmed. By comparing the
vibration signal in Fig. 3(al) with that in Fig. 1 (which
was obtained from an engine test) it can be seen that the
two body response waveforms are very similar. From
this similarity
it
can be concluded that combustion
shocks, piston slap and other engine operating sources
have minimal effect upon injector vibration response.
Such response
is
only a consequence of mechanical and
flow-related vibration sources within the injector. This
conclusion is reinforced by the directly measured needle
acceleration traces shown in Figs 3(a2) and 3(b2). Of
particular interest in Fig. 3 is that the upper (a1 and a2)
traces are from a high-load situation, where both
opening and closing needle impacts are apparent,
whereas the lower traces (bl and b2) are from a low-
load test in which it can be seen that there occurs no
opening impact of the needle with its backstop.
-150
I
401
I
-,
-
.
.
.,
. . .
-L"
0
0.64
1.28 1.92 2.56 3.2 3.84 4.48 5.12 5.76 6.4 7.04 7.68
Time(ms)
Fig.
3
Injector vibration'and needle
motion
from
bench-top testing
Part
D:
Journal of
Automobile
Engineering
Q
IMechE
1996