Injection characteristics of a liquid phase lpg injection

Proceedings of the Institution of Mechanical
Engineers, Part D: Journal of Automobile
Engineering
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A study on the injection characteristics of a liquid-phase liquefied petroleum gas injector for air-fuel
ratio control
Hansub Sim, Kangyoon Lee, Namhoon Chung and Myoungho Sunwoo
Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering 2005 219: 1037
DOI: 10.1243/095440705X34621

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1037

A study on the injection characteristics of a liquid-phase
liquefied petroleum gas injector for air–fuel ratio control
Hansub Sim1, Kangyoon Lee2*, Namhoon Chung 2, and Myoungho Sunwoo2
1Department of Automotive Engineering, Jinju National University, Jinju, Republic of Korea
2Department of Automotive Engineering, Hanyang University, Seoul, Republic of Korea

The manuscript was received on 7 September 2004 and was accepted after revision for publication on 23 February 2005.

DOI: 10.1243/095440705X34621

Abstract: Liquefied petroleum gas (LPG) is widely used as a gaseous fuel in spark ignition engines
because of its considerable advantages over gasoline. However, the LPG engine suffers a torque loss
because the vapour-phase LPG displaces a larger volume of air than do gasoline droplets. In order to
improve engine power as well as fuel consumption and air–fuel ratio control, considerable research
has been devoted to improving the LPG injection system. In the liquid-phase LPG injection systems,
the injection rate of an injector is affected by the fuel temperature, injection pressure, and driving
voltage. When injection conditions change, the air–fuel ratio should be accurately controlled in order
to reduce exhaust emissions. In this study, correction factors for the fuel injection rate are developed
on the basis of fuel temperature, injection pressure, and injector driving voltage. A compensation
method to control the amount of injected fuel is proposed for a liquid-phase LPG injection control
system. The experimental results show that the liquid-phase LPG injection system works well over the
entire range of engine speeds and load conditions, and the air–fuel ratio can be accurately controlled
by using the proposed compensation algorithm.

Keywords: liquefied petroleum gas (LPG), air–fuel ratio, fuel injection rate, port fuel injection,
correction factor, compensation algorithm

1 INTRODUCTION engine suffers a torque loss because the vapour-
phase LPG displaces a larger volume of air than
Liquefied petroleum gas (LPG) is widely used as an do gasoline droplets [2]. Therefore, it is necessary
alternative fuel for automobiles due to its efficient to develop a port fuel injection system for LPG-
combustion characteristics and low pollution. LPG fuelled vehicles in order to improve engine power as
has a high octane number, which prevents engine well as to meet the strict emission requirements.
knock, and a relatively high hydrogen-to-carbon ratio, Considerable research has been devoted to improving
which results in substantial reduction in the emission the LPG injection system [1–4]. The power perform-
of carbon dioxide [1]. The LPG-fuelled vehicle, which ance of a liquid-phase LPG injection engine is nearly
is commercially available, uses vapour-phase LPG the same as that of a gasoline engine, and the exhaust
as a fuel and has a vapour-mixing system in order emissions are lower [5]. In addition, the heavy-duty
to meter the LPG into the intake air upstream. LPG injection engine, which is a converted heavy-
The vapour-mixing system does not provide a fast duty diesel engine, has superior power performance
air–fuel ratio control in transient engine operations to a diesel engine [6]. However, several challenges
owing to the relatively long distance between the arise in the liquid-phase LPG injection system. The
fuel mixing system and the lambda sensor installed LPG is stored as a saturated mixture in the vehicle,
in the exhaust system. In addition, compared with and the pressure in the LPG tank is the vapour
gasoline-powered engines, the vapour-phase LPG pressure corresponding to the fuel temperature.
Saturated liquids have a tendency to vaporize with
* Corresponding author: Department of Automotive Engineering, any pressure drops or temperature increases. Vapour
Hanyang University, 17 Haendang-dong, Seongdong-gu, Seoul lock can occur in the fuel lines or injectors, and this
133-791, Republic of Korea. email: bikeman@ihanyang.ac.kr affects the amount of injected LPG [2]. Icing can

D17204 © IMechE 2005 Proc. IMechE Vol. 219 Part D: J. Automobile Engineering


1038 Hansub Sim, Kangyoon Lee, Namhoon Chung, and Myoungho Sunwoo

occur around the injector tip owing to the heat of Figure 2 shows the saturated vapour pressure
vaporization [1]. In order to prevent vapour lock and for various mixtures of butane and propane as
icing, high pressure needs to be maintained within a function of temperature. The saturated vapour
the fuel rail, and a thermal insulator can be attached pressure increases with increasing fuel temperature
to the injector tips [5]. The quantity of injected LPG and propane content. Considering that the propane
varies with injection pressure, fuel temperature, and composition of LPG is less than 30 per cent in the
injector driving voltage at constant injection duration. Republic of Korea, the saturated vapour pressure is
In this study, injection characteristics according to less than 0.6 MPa over the temperature range from
changes in fuel temperature, injection pressure, −20 to 40 °C.
and injector driving voltage are examined through
injection experiments. A compensation algorithm for
the variations in injection conditions is proposed in 3 CHARACTERISTICS OF FUEL INJECTION
order to control accurately the amount of injected
fuel. This compensation algorithm is verified through 3.1 Fundamentals of fuel injection
engine tests for air–fuel ratio control.
Figure 3 shows a cross-sectional view of an LPG
injector. The amount of injected fuel through an
injector is determined by the well-known orifice flow
2 PHYSICAL PROPERTIES OF LPG
equation

S
LPG is composed of butane (C H ) and propane 2r Dp
4 10 m =C A f (3)
(C H ). Butane consists of normal butane (n-butane) ˙
3 8 f D d 1−(A /A )2
and isobutane (i-butane). Therefore, the density of d u
liquid-phase LPG can be expressed as

A B
v v v −1
r = p + nb + ib (1)
LPG r r r
p nb ib
Figure 1 shows the density for various blends
of butane and propane as a function of tempera-
ture. The density of n-butane varies from 0.528 to
0.629 kg/l, and the density of propane varies from
0.430 to 0.566 kg/l over the temperature range from
−30 to 60 °C. At a fuel temperature of 20 °C, the
density of liquid-phase LPG varies from 0.501 to
0.579 kg/l. The saturated vapour pressure of LPG in
the fuel tank is expressed as [7]
p =M p +M p +M p (2)
vapour p p nb nb ib ib

Fig. 2 Saturated vapour pressure of LPG as a function
of fuel temperature

Fig. 1 Density of LPG as a function of fuel temperature Fig. 3 Cross-sectional view of an injector

Proc. IMechE Vol. 219 Part D: J. Automobile Engineering D17204 © IMechE 2005


Injection characteristics of a liquid-phase LPG injector 1039

under the assumption of incompressible flow. The decreasing fuel temperature. The rate of change in
discharge coefficient is determined from injection the propane temperature correction factor is steeper
experiments of liquid-phase LPG. Therefore, the than that of butane.
amount of injected fuel is described as a function of
the fuel density and the injection pressure difference. 3.3 Influence of the injection pressure
The injection pressure, which is the difference
3.2 Influence of the fuel temperature
between the fuel rail pressure and the intake port
The density of LPG depends on the fuel temperature, pressure, varies with the engine operating conditions.
as shown in Fig. 1. The fuel density is largely affected Therefore, the amount of injected fuel fluctuates
by the fuel temperature, which is influenced by according to the injection pressure. In order to
the ambient temperature and heat transfer from the analyse the influence of injection pressure on the
engine compartment. As a result, the injected fuel injected fuel quantity, the fuel injection rate at an
quantity varies with the fuel temperature. In order to arbitrary injection pressure is expressed as
analyse the influence of fuel temperature on the
m =m
˙ ˙ F (7)
injected fuel quantity, the fuel injection rate at an f,i f,base f,Dp
arbitrary fuel temperature is expressed as The pressure correction factor F is defined as
f,Dp
m =m F (4)
S
˙ ˙ Dp
f,i f,base f,r(T) F = f,i (8)
The basic injection rate m
˙ at a standard tempera- f,Dp Dp
f,base f,base
ture and pressure difference is defined as Figure 5 shows how the pressure correction factor
changes with injection pressure based on three
m
˙
f,base
=C A
2r
D d
f,base
S
DP
1−(A /A )2
d u
f,base (5) pressure differences: 0.49, 0.98, and 1.96 MPa. The
pressure correction factor increases as the pressure
and the temperature correction factor F is defined difference increases. The higher the pressure differ-
f,r(T)
as ence, the lower is the rate of the change in the
pressure correction factor.
F
f,r(T)
=
S
f,i
f,base
r
r
(6)
3.4 Influence of the driving voltage
Figure 4 shows the temperature correction factors The driving voltage of an injector creates a magnetic
for propane and butane at various fuel temperatures. field in the solenoid. The magnetic field intensity
The temperature correction factor increases with induced in the solenoid is proportional to the driving
voltage. Therefore, the amount of injected fuel varies
with the driving voltage because of changes in the

Fig. 4 Temperature correction factor for propane and Fig. 5 Pressure correction factor as a function of
butane pressure difference




opening delay of an injector. The injected fuel quan- dead time increases at high injection pressures and
tities under arbitrary and standard driving voltages low driving voltages [8]. In this study, it is assumed
are described as that the dynamic quantity of injected fuel is pro-
portional to the injection duration. The amount of
m =m
˙ t (9)
f,Vi f,Vi f,Vi injected fuel under arbitrary fuel temperatures and
and injection pressures can be expressed as
m =m
˙ t (10)
f,Vbase f,Vbase f,Vbase m =m t
˙ (14)
f,i f,i f,i
respectively. An equal amount of fuel must be
injected in order to maintain a constant air–fuel ratio and the basic amount of injected fuel also can be
under identical engine operating conditions. As a expressed as
result, the injection duration at an arbitrary driving
voltage is expressed as m =m
˙ t (15)
f,base f,base f,base
m
˙
t = f,Vbase t (11) In order to investigate the influence of fuel tem-
f,Vi m˙ f,base perature and injection pressure on the injected fuel
f,Vi
If the voltage correction factor F is defined as quantity, it is further assumed that the injection
f,V duration is constant, and that the amount of injected
m
˙ fuel depends substantially on the fuel temperature
F = f,Vi (12)
f,V m
˙ and the injection pressure. From equations (14) and
f,Vbase
(15), the injected fuel quantity is expressed as
then equation (11) can be rewritten as
t m
˙
t = f,base (13) m = f,i m
F
f,Vi f,i m˙ f,base
f,V f,base
Figure 6 shows the voltage correction factor as a =F F m (16)
f,r(T) f,Dp f,base
function of driving voltage. The voltage correction
factor has an increasing tendency according to the Therefore, the ratio of injected fuel quantity can be
increase in the driving voltage. determined as

S S
3.5 Compensation of the injection quantity m r Dp
f,i =F F = f,i f,i (17)
The dynamic quantity of injected fuel changes m f,r(T) f,Dp r Dp
f,base f,base f,base
irregularly at the beginning of injection because of
the injection dead time and is mainly affected by the The injected fuel quantity m is proportional to the
f,i
injection pressure and battery voltage. The injection square root of the density ratio and the square root
of the pressure difference. The air–fuel ratio must
be maintained at a constant value under identical
engine operating conditions regardless of variations
in the fuel temperature and injection pressure.
Therefore, an equal amount of fuel must be injected,
and the equation

m =m (18)
f,i f,base
is satisfied. Substituting equations (9), (14), and (15)
into equation (18), the injection duration is expressed
as a function of temperature correction factor and
pressure correction factor as

t
t = f,base (19)
f,i F F
f,r(T) f,Dp

Fig. 6 Voltage correction factor as a function of driving Considering the effects of the temperature, pressure,
voltage and driving voltage on the amount of injected




fuel, the injection duration under arbitrary fuel tem- puter through the engine management system. The
peratures, injection pressures, and injector driving quantity of injected fuel is estimated on the basis of
voltages can be described as measurements of the air mass flowrate at the inlet
of the throttle body and the air–fuel ratio in the
t
t = f,base (20) exhaust manifold. The fuel temperature and injection
f,i F F F
f,r(T) f,Dp f,V pressure are changed during engine tests in order to
investigate the influence of fuel temperature and
injection pressure on the amount of fuel injected.
4 EXPERIMENTAL DETAILS Figure 9 is the block diagram of the fuel injection
controller which regulates the amount of injected
The tested engine is a water-cooled Hyundai gasoline fuel.
engine with an electronic engine control and a port
fuel injection system. The engine specifications are
given in Table 1, and the tested engine is shown in 5 ANALYSIS OF TEST RESULTS
Fig. 7.
Figure 8 shows a schematic diagram of the experi- 5.1 Preliminary test
mental set-up for the fuel injection control system.
The injectors used in this study are identical with Various temperatures in the engine are measured
those of a gasoline engine, and the injection pressure during the test run in order to investigate the effect
is regulated by pressurized nitrogen gas. A pressure of the changes in fuel temperature. Figure 10 shows
transducer and a thermocouple are installed in the temperature variations of the coolant, fuel, and
the fuel rail to measure the fuel temperature and intake air from engine start-up to the coolant tem-
injection pressure. The amount of injected fuel and perature of 90 °C. This test is conducted at steady-
ignition timing are controlled by a personal com- state operating conditions at an engine speed of
2000 r/min, an intake manifold absolute pressure of
Table 1 Engine specifications 0.05 MPa, a fuel injection pressure of 0.59 MPa, and
an injection duration of 4.3 ms. The engine has been
Item Specifications
soaked at a temperature of 0 °C before this test run.
Type In-line four cylinder, DOHC The coolant temperature varies from 0 to 90 °C while
Bore×stroke (mm) 75.5×83.5 the fuel temperature varies from 0 to 40 °C. Hence,
Swept volume (cm3) 1495
Compression ratio 9.5 : 1 the basic amount of injected fuel is determined
Firing order 1–3–4–2 under the conditions of a coolant temperature of
Valve timing
IVO 5° BTDC
90 °C and a fuel temperature of 40 °C. In addition, the
IVC 35° ATDC pressure difference across the injector is 0.64 MPa,
EVO 43° BTDC and the injection duration is 4.3 ms.
EVC 5° ATDC
Figure 11 shows the basic injection duration as a
IVO, intake valve open; IVC, intake valve closed; function of the engine speed and the intake manifold
EVO, exhaust valve open; EVC, exhaust valve closed; pressure. The tested engine is operated under steady-
DOHC, dual overhead cam; BTDC, before top dead
centre; ATDC, after top dead centre. state conditions at a coolant temperature of 90 °C, a
fuel temperature of 40 °C, and a relative air–fuel ratio
of 1.0. The relative air–fuel ratio is defined as the
ratio of the actual air–fuel ratio to the stoichiometric
ratio [9]. The basic injection duration increases with
increasing intake manifold pressure and varies from
7.4 ms at low engine speeds and low engine loads
to 10.3 ms at high engine speeds and high engine
loads. These results show that the characteristics of
injection duration for a liquid-phase LPG injection
system are similar to those of a spark ignition engine.

5.2 Fuel temperature influence test
Figure 12 shows the estimated and the measured fuel
injection rates as functions of fuel temperature. The
Fig. 7 Photograph of tested engine measured injection rates are calculated using the




Fig. 8 Schematic diagram of experimental setup (EMS, engine management system; CAS, crank-
shaft angle sensor; TDC, top dead centre signal; MAP, manifold absolute pressure)

Fig. 9 Block diagram of fuel injection controller

measurements of the air mass flowrate upstream 0 to 40 °C. The estimated temperature correction
of the throttle body and the air–fuel ratio in the factors are determined using equation (6). The
exhaust manifold. The measured injection rates vary measured values are smaller than the estimated
from 0.761 to 0.796 g/s and increase with decreasing values. The temperature correction factors and the
fuel temperature owing to the fuel density increase. differences between the measured and the estimated
The lower the fuel temperature, the larger is the dis- values increase according to the decrease in the fuel
crepancy between the estimated and the measured temperature. This feature is a result of the decrease
injection rates. This feature results from the increase in the injection rate due to the increase in fuel
in fuel viscosity at low fuel temperatures. Figure 13 viscosity.
shows the estimated and measured temperature The relative air–fuel ratios, with and without
correction factors for fuel temperatures ranging from compensation for the effect of fuel temperature on




Fig. 10 Temperature variations during engine warm-up Fig. 12 Injection rate as a function of fuel temperature

Fig. 13 Temperature correction factor

Fig. 11 Basic injection duration (MAP, manifold
pressure)

injection duration, are plotted in Fig. 14. With tem-
perature compensation, the relative air–fuel ratio is
maintained at around 1.0 with an error of 1 per cent.
Without temperature compensation, the relative air–
fuel ratio varies from 0.9 to 1.0. Consequently, it is
appropriate to introduce the temperature correction
factor in order to compensate for the changes in fuel
temperature.

5.3 Injection pressure inﬂuence test
The injected fuel quantity varies with the injection
pressure. This test is conducted under steady-state Fig. 14 Relative air–fuel ratios with and without
operating conditions at an engine speed of 2000 r/min, compensation




an intake manifold absolute pressure of 0.05 MPa,
a coolant temperature of 90 °C, and constant fuel
temperature of 40 °C. The basic injection pressure
is 0.59 MPa, and the injection duration is 4.3 ms.
Injection pressure is controlled by nitrogen gas
through a pressure regulator.
Figure 15 shows the estimated and the measured
fuel injection rates as functions of injection pressure.
The measured injection rates vary from 0.690 to
0.973 g/s and increase with increasing injection
pressure. At injection pressures above 0.59 MPa, the
measured values are smaller than the estimated
values, and the difference becomes larger. This is a
result of the increase in the opening delay time of
the injector.
Figure 15 also shows the pressure correction factors Fig. 16 Relative air–fuel ratios with and without
calculated using the injection rate at an arbitrary compensation
injection pressure and the basic injection rate. The
estimated pressure correction factors are calculated
from equation (8). The pressure correction factor and 5.4 Evaluation test
the difference between the measured and estimated
values increase according to the increase in injection In order to validate the developed feedforward com-
pressure. The measured values are smaller than the pensation algorithm, the tested engine is operated
estimated values above the basic injection pressure. at a constant-speed mode and at a constant-torque
This feature also results from the increase in the mode respectively. The relative air–fuel ratio is
opening delay time. measured using a lambda sensor. Figure 17 shows
Figure 16 shows the relative air–fuel ratio as a the throttle movements used in the tests and the
function of injection pressure with and without corresponding response of the intake manifold
compensation for the effect of injection pressure on pressure. The injection control algorithms used for
injection duration. With pressure compensation, the the evaluation tests are the feedforward compen-
relative air–fuel ratio is maintained at around 1.0 sation algorithms proposed in this study and a
with an error of 1 per cent. Without pressure com- typical feedback air–fuel ratio control using a lambda
pensation, the relative air–fuel ratio varies from 0.76 sensor.
to 1.07. Therefore, it is advantageous to introduce the Figures 18 and 19 show the relative air–fuel ratios
pressure correction factor in order to compensate for at constant-speed mode and constant-torque mode
the changes in injection pressure.

Fig. 15 Fuel injection rate and pressure correction Fig. 17 The throttle movement and the intake manifold
factor as functions of injection pressure pressure




ratios at constant load torque and constant engine
speed are similar. Therefore, the relative air–fuel ratio
can be maintained at the target value of 1.0 with an
error of 2 per cent by using the feedforward compen-
sation algorithm for steady-state engine operations.

6 CONCLUSIONS

There are many factors that influence the injection
characteristics. In this study, three factors that
influence the amount of injected fuel in a liquid-
phase LPG engine are investigated. These factors are
the fuel temperature, injection pressure, and injector
Fig. 18 Air–fuel ratio at constant engine speed driving voltage. In order to compensate for variations
(2000 r/min) in these factors, correction factors are proposed and
verified through engine tests. The following con-
clusions can be drawn from the validation tests of
the compensation algorithms.

1. The injection conditions that influence the
injection rate are investigated and mathematically
formulated to implement an air–fuel ratio con-
troller for a liquid-phase LPG injection engine.
2. A temperature correction factor is proposed to
compensate for the effect of fuel temperature on
injected fuel quantity and verified through engine
tests. The relative air–fuel ratio using the non-
compensated algorithm is 0.9 at a fuel tempera-
ture of 0 °C while the relative air–fuel ratio using
the temperature-compensated algorithm is 1.0.
3. In order to compensate for the effect of injection
pressure on injected fuel quantity, a pressure
correction factor is proposed. This factor is veri-
fied through engine tests. The relative air–fuel ratio
Fig. 19 Air–fuel ratio at constant load torque (49 N m) is maintained at around 1.0 with pressure com-
pensation while the relative air–fuel ratio varies
from 0.76 to 1.07 without pressure compensation.
respectively. The dotted curves represent the tested 4. A voltage correction factor is proposed and
result of only the feedforward algorithm, and the applied to the fuel injection controller in order to
solid curves represent the result of both the feed- compensate for the variations in battery voltage.
forward plus feedback control. In Fig. 18, excursions This factor varies from 0.78 to 1.12 in the driving
in the relative air–fuel ratio are observed at the voltage range 9–15 V.
moment of throttle transients, and the lambda spikes 5. The feedforward control scheme to compensate
of the feedforward compensation are higher than for the changes in injection conditions is applied
those of the feedforward plus feedback control by and evaluated during the engine experiment.
3–7 per cent. Meanwhile the relative air–fuel ratio is 6. The relative air–fuel ratios for the feedforward
maintained at the target value of 1.0 with an error of compensation algorithm and feedforward plus
2 per cent for both control cases with a constant feedback control algorithm have values of 1.0
throttle input. In Fig. 19, the lambda spikes of the with an error of 2 per cent except for fast throttle
feedforward compensation are higher than those of transients. In order to reduce the lambda spikes
the feedforward plus feedback control by 1 per cent. during fast throttle movements, conventional feed-
During constant throttle input, the relative air–fuel back control techniques should be incorporated.




ACKNOWLEDGEMENTS 7 The Korean Institute of Industrial Educators LP Gas
Technology, 1979 (Semunsa, Seoul).
8 Song, C. S., Lee, Y. J., and You, S. J. A study on the
This research is supported in part by MOST (Ministry
analysis of dynamic characteristics of the solenoid
of Science and Technology) under the National valve of automotive transmission. J. Korean Soc.
Research Laboratory (NRL) grant MI-0203-00-0058- Precision Engng, 1995, 12(8), 122–130.
02-J00-00-031-00, and part of the project ‘Develop- 9 Heywood, J. B. Internal Combustion Engine Funda-
ment of Partial Zero Emission Technology for Future mentals, 1988 (McGraw-Hill, New York).
Vehicle’, and we are grateful for their financial
support.
APPENDIX

Notation
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