INTRODUCTION
Gasoline and diesel fuel will become scarce and most costly (Catania
et al., 2004; Hollnagel et al., 1999).
Alternative fuel technology, availability and use will become more common in
the coming decades for internal combustion engines. Nowadays, the alternative
fuel has been growing due to concerns that the reserves of fossil fuel all over
the world are finite and at the early decades of this century will run out completely.
Furthermore, the current world energy crisis made the fossil fuel price increases.
In the other hand, the fossil fuel contributes large environment pollution.
Many types of alternatives fuels available in the world. Compressed Natural
Gas (CNG) as an alternative fuel is becoming increasingly important.
This study is conducted to studies the computational and experimental of cylinder pressure performance in the development of diesel engine convert to port injection dedicated Compressed Natural Gas (CNG) engine. The baseline engine is a single cylinder four stroke direct injection diesel engine. The computational simulation is using GT-Power software. The experimental investigation of the baseline diesel engine and port injection dedicated CNG engine are using eddy current dynamometer. The objective of this research is to investigate the correlation of characteristic pressure performance in-cylinder of port injection CNG engine model based on engine speeds variation. The results output of port injection CNG engine is compared with base direct injection diesel engine model in-cylinder pressure performance.
The computational model of diesel engine covert to port injection CNG engine has been developed in the research. In the first step, is investigated the CNG engine intake port steady-state and transient of gas flow temperature simulation using GT-Power. This research is the second step and focuses in-cylinder pressure performance of single cylinder four stroke port injection CNG engine converted from direct injection diesel engine. The aim is to give an insight into the CNG engine in-cylinder gas of low thermodynamics performance using GT-POWER simulation model, how the engine model developed and the components interact. To determine port injection CNG engine pressure performance in-cylinder the engine is the essence of modeling at small intervals of time. Appropriate summation of these gas conditions over an engine cycle then leads to an estimate in-cylinder engine pressure performance.
In the port injection CNG engine, fuel is injected by the gas fuel injection
system via intake port trans valve into the engine cylinder toward the end of
the compression stroke, just before the desired start of combustion. The gas
fuel, usually injected at high velocity as one or more jets through small orifices
or nozzles in injector tip. The gas fuel mixes with high temperature and high
pressure in-cylinder air. Since, the air temperature and pressure are in the
gas fuels ignition point, spark ignition of portions of the already-mixed
fuel and after air a delay period of a few crank angle degrees. The cylinder
pressure increases as combustion of the gas fuel-air mixture occurs. The major
problem in port injection CNG engine combustion chamber design is achieving
sufficiently rapid mixing between the injected gas fuel and the air in the cylinder
to complete combustion in the appropriate crank angle interval close to top-center
(Blair, 1999; Challen and Baranescu,
2003; Heywood, 1998; Kowalewicz,
1984; Richard, 1997).
Cylinder pressure changes with crank angle as a result of cylinder volume change,
combustion, heat transfer to chamber walls, flow into and out of crevice regions
and leakage. The effect of volume change on the pressure can readily be accounted
for combustion rate information from accurate pressure data provided of model.
Cylinder pressure versus crank angle data over the compression and expansion
strokes of the engine operating cycle can be used to obtain quantitative information
on the progress of combustion. Suitable methods of analysis which yield the
rate of release of the fuels chemical energy, or rate of the burning will
be described in the study. Any researcher have been studied in-cylinder pressure
and temperature in some bases of engine operation. Piedrahita
et al. (2003) has studied the engine cylinder pressure and temperature
under different operation parameters, such as air-fuel ration and spark angle
advance, a zero-dimensional model is applied. Eriksson and
Andersson (2002) has studied an analytic model for cylinder pressure in
a four-stroke SI engine, the study describe the in-cylinder pressure of a spark
ignited combustion engine operating close to stoichiometric conditions, as a
function of crank angle, manifold pressure, manifold temperature and spark timing.
Sanders et al. (2003) has studied of gas temperature
measurements during ignition in an HCCI engine. The measurements results were
made during the compression and early portion of the combustion phase of an
n-heptane-fueled HCCI engine. The measured pressure-temperature history was
compared to kinetic calculations of the ignition delay and showed the traversal
of the negative temperature coefficient regime. In-cylinder combustion pressure
characteristics of Fischer-Tropsch and conventional diesel fuels in a heavy-duty
compression ignition engine. Christopher et al. (1999)
has studied the in-cylinder combustion pressure traces obtained during the engine
testing were analyzed to obtain several pressure-based variables including ignition
delay, combustion duration, peak pressure, location of peak pressure, relative
quantities of premix and diffusive burn heat release, indicated mean effective
pressure and location of one-half of the mass fraction burned. Klein
and Eriksson (2002) has studied compression estimation from simulated and
measured cylinder pressure.
Most of cylinder pressure investigation is usually measured with piezoelectric
pressure transducers. In this study, the cylinder profile is investigated using
computational simulation software from the real compression ignition engine
data then convert to port injection CNG engine spark ignition. The software
have used in the research is GT-POWER software. The GT-POWER is the leading
engine simulation tool used by engine and vehicle makers and supplies and is
suitable for analysis of a wide range of engine issues. GT-POWER is designed
for steady-state and transient simulation and can be used for analysis of engine
and power train control (Semin et al., 2008).
It is applicable to all types of internal combustion engines and provides the
user with many components to model any advanced concept. The GT-POWER is based
on one dimensional gas dynamics, representing the flow and heat transfer in
the piping and in the other components of an engine system. In addition to the
flow and heat transfer capabilities, the code contains many other specialized
models required for system analysis. The GT-POWER has the capability to model
all of the aspects of the engine in the schematic and more. By being comprehensive,
the code is well suited for integration of all aspects arising in engine and
vehicle development. GT-POWER can be used for a wide range of activities relating
to application and prediction of engine design and development. A user friendly
interactive post processing tool, GT-Post, can be used to manipulate and view
all of the plot data generated by cases RLT. Important performance data can
be plotted against parameters from a multiple case run (Semin
et al., 2009).
The idealized P-V diagram used to determine the cylinder pressure (Heywood,
1998). The area of this curve must match the specified indicated output
at the engine operating condition. The transition point in the figure marks
the transition between the combustion and expansion segments of the power stroke.
It can set the slope of the combustion segment with the slope of P-V curve after
TDC attribute and GT-POWER will adjust the transition point so that both the
indicated output and the slope are satisfied. The slope is defined by Eq.
1 and 2.
where, P is instantaneous cylinder pressure between TDC and transition point, Pmax is maximum cylinder pressure or pressure at TDC, VTDC is cylinder volume at TDC, V is instantaneous cylinder volume between TDC and the transition point, m is slope of P-V curve after TDC, PIVC is cylinder pressure at IVC, Rc is cylinder compression ratio, γ is specific heat ratio and Pcomb is pressure rise due to combustion.
If P is instantaneous cylinder pressure (bar) and Vdisp is displacement volume (m3), cylinder indicated mean effective pressure (imepc) of internal combustion engine is formulated in Eq. 3.
If P is instantaneous cylinder pressure (bar) and Vdisp is displacement volume (m3), pumping mean effective pressure (pmepc) in-cylinder of internal combustion engine is formulated in Eq. 4.
MATERIALS AND METHODS
The conversion development of four stroke direct injection diesel engine to sequential port injection dedicated Compressed Natural Gas (CNG) engine spark ignition are conducted on 2006-2008 at Automotive Laboratory, Faculty of Mechanical Engineering, University Malaysia Pahang, Malaysia.
The new design and development in this engine research activities are direct injection diesel fuel system changed to port injection CNG system, reduce compression ratio, compression ignition converted to spark ignition, mechanical fuel control changed to electronic fuel control system. In the CNG injection fueling system, the port injection of compressed natural gas fuel injector nozzle is used and then using new injector nozzle multi holes geometries. The basic fueling system management of four stroke direct injection diesel engine is using mechanical fueling pump system and fuel injected directly to the engine cylinder. The fueling system management of the engine will be changed to electronic control fueling system management for CNG fueling system and gas fuel is injected sequentially via intake port before intake valve.
Computational method: Port injection dedicated Compressed Natural Gas
(CNG) spark ignition engine computational model is developed using GT-POWER
software based from real diesel engine data. According to Semin
et al. (2008, 2009) the specification of
engine is shown in Table 1. In the GT-POWER engine model development,
a typical engine cylinder is modeled using EngCylinder name and shown using
number 11, engine is modeled using EngineCrankTrain component objects and shown
using number 12, Valve*Conn and EngCylConn are connection objects. Engine parameters
are shown in Table 1.
In the diesel engine and port injection CNG engine model development using GT-POWER, a typical engine is modeled using EngCylinder and EngineCrankTrain component objects and Valve*Conn and EngCylConn connection objects. EngCylinder and EngineCranktrain are used to define the basic geometry and characteristics of engine. Both objects further refer to several reference objects for more detailed modeling information on such attributes as combustion and heat transfer. Cylinder must be connected to the engine with EngCylConn part made from the predefined object which available in the template library. While EngCylConn parts have no user defined attributes, the global cylinder number for cylinder is assigned by the port number where the EngCylConn connection is attached to the engine. Cylinder are connected to intake and exhaust ports with Valve*Conn connections. Many Valve*Conn connection templates are available to define different types of valve and their characteristics.
To develop of single-cylinder four-stroke direct-injection compression ignition
engine model and port injection CNG engine model using GT-POWER software is
step by step, the first step is open all of the selected diesel engine components
to measure the engine components part size. To create the GT-POWER model, select
window and then tile with template library from the menu. This will place the
GT-POWER template library on the left hand side of the screen. The template
library contains all of the available templates that can be used in GT-POWER.
Some of these templates those that will be needed in the project need to be
copied into the project before they can be used to create objects and parts.
Table 1: | Specification
the engine |
 |
For the purpose of this model, click on the icons listed and drag them from
the template library into the project library. Some of these are templates and
some are objects that have already been defined and included in the GT-POWER
template library. Then, the engine components size data input to the GT-POWER
library of the all engine components data. All of the parameters in the model
will be listed automatically in the case setup and each one must be defined
for first case of the simulation. According, Abu Bakar (2007),
the diesel engine model is shown in Fig. 1. The conversion
of diesel engine model data to spark ignition port injection CNG engine is shown
in Fig. 2.
In the port injection dedicated Compressed Natural Gas (CNG) spark ignition
engine model is added intake pipe and throttle, then fuel is injected using
injector in intake manifold. The engine computational model using GT-Power software
is shown in Fig. 2. A typical intake manifold is modeled using
9, engine cylinder is modeled using 11 and engine is modeled using 12, then
Valve*Conn and EngCylConn connection objects. Nine is used to define the basic
geometry and characteristics of intake manifold, 11 and 12 are used to define
the basic geometry and characteristics of engine cylinder and engine crank train.
These objects further refer to several reference objects for more detailed modeling
information on such attributes as gas flow temperature. Intake manifold must
be connected to the engine cylinder with Valve*Conn, Engine cylinder must be
connected to the engine with EngCylConn part made from the predefined object
which available in the template library. While Pipe, EngCylConn parts have no
user defined attributes, the global cylinder number for cylinder is assigned
by the port number where the EngCylConn connection is attached to the engine.
Cylinder are connected to intake and exhaust ports with Valve*Conn connections.
Many Valve*Conn connection templates are available to define different types
of valve, port and their characteristics. In this research, the simulation results
of GT-Power engine computational model performance are focused on engine cylinder
pressure. The simulation results are shown in plotting and report tables in
GT-Post, graphical RLT viewer and order analysis of flow data. The solver of
GT-POWER determines the main performance of an engine simulation based on engine
speed mode in the EngineCrankTrain object in this research. Speed mode is the
most commonly used mode of engine simulation, especially for steady states cases.
In the research imposes the engine speed as either constant or by a dependency
reference object. This method typically provides steady-state results very quickly
because the speed of the engine is imposed from the start of the simulation,
thus eliminating the relatively long period of time that a loaded engine requires
for the crankshaft speed to reach steady-state. The engine cylinder pressure
characteristics calculation is according to Heywood (1998).
|
Fig. 1: | Diesel
engine model using GT-POWER |
|
Fig. 2: | CNG
engine model using GT-POWER |
|
Fig. 3: | Schematic
of engine experimental set-up |
Experimental method: The experimental investigation of the diesel engine
pressure performance is using Dewetron Dewe5000 computational combustion analyzer.
This is the basic step of the research and conducted to collect data for engine
pressure performances as baseline reference before converted to CNG engine.
The experiment set-up is shown in Fig. 3.
The port injection dedicated CNG engine is converted from diesel engine. To
develop the port injection dedicated CNG engine, there is need of any new components
or new design and modification components. The components affecting will be
design and development in this research are piston, spark ignition, natural
gas fueling system and electronic control unit. The research design and development
of components affecting for CNG engine is based on CNG engine computational
modeling performance data, then the best performance data is used to design
and development the physically components affecting. The piston engine is modified
component in the engine and designed to decrease the compression ratio from
base diesel engine and develop using CNC machine. The spark ignition is new
component in the engine and designed for assist the CNG engine combustion, the
location of the spark plug is using the hole of the injector and modified in
size of geometries. The natural gas fueling system is new fueling system in
the engine to change the liquid fueling system and designed based on sequential
injection natural gas, where the system start from cylinder tank, pressure regulator,
solenoid and injector. In the sequential injection, the injector is located
in the intake port before intake valve in every engine cylinder. The natural
gas fueling system is operated using electronic system. The electronic control
system is new component to change the mechanical system in the base diesel engine.
The electronic control system is to manage the injection timing, spark timing
and engine combustion. The experimental of port injection CNG engine spark ignition
is to investigate the performance and exhaust gas emissions effect. The experimental
is using Dewetron Dewe5000 computational combustion analyzer and engine control
data acquisition. The engine control and data acquisition is used to investigate
the in-cylinder pressure and Crank Angle (CA) degree.
RESULTS
The results of cylinder pressure performance of direct injection diesel engine,
compression ratio modified direct injection diesel engine and port injection
dedicated CNG engine are shown in Fig. 4-11.
Engine cylinder pressure profile: The engine cylinder pressure profile
investigation results are shown in Fig. 4-10.
The results are shown that the cylinder pressure is increasing in compression
stroke to combustion ignition in crank angle negative180° Bottom Dead Center
(BDC) until around in crank angle 0° Top Dead Center Force (TDCF). In the
compression stroke, the air-fuel volume is compressed from BDC to TDC. The simulation
and experiment results are not similar. The simulation results are higher than
the experimental results. The deviation is in average 2% for CNG engine (CNGE)
and diesel engine (ODE). The compression ratio of direct injection diesel engine
is 20.28:1, the compression ratio of modified direct injection diesel engine
is 14.5:1 and the that the direct injection diesel engine cylinder pressure
is higher than the modified direct injection diesel engine and port injection
dedicated CNG engine.
|
Fig. 4: | Cylinder
pressure at 1000 rpm |
|
Fig. 5: | Cylinder
pressure at 1500 rpm |
The highest of cylinder pressure is around in crank angle 0 degree (TDCF).
From the cylinder pressure performance can be predicted that the product of
engine power from the air-fuel combustion of direct injection diesel engine
is higher than modified direct injection diesel engine and the port injection
dedicated CNG engine. The direct injection diesel engine cylinder pressure is
higher than modified direct injection diesel engine and port injection dedicated
CNG engine because the compression ratio of diesel engine is higher.
|
Fig. 6: | Cylinder
pressure at 2000 rpm |
|
Fig. 7: | Cylinder
pressure at 2500 rpm |
Engine cylinder maximum pressure profile: The highest of maximum cylinder
pressure in the combustion process both of diesel engines are shown in Fig.
5 and for port injection dedicated CNG engine is shown in Fig.
4. In the diesel engine, the maximum cylinder pressure is 84.0 bar declared
in 1500 rpm engine speed. In the modified diesel engine, the maximum cylinder
pressure is 61.1 bar declared in 1500 rpm engine speed. In the port injection
dedicated CNG engine, the maximum cylinder pressure is 76.23 bar and declared
in 1000 rpm engine speed.
|
Fig. 8: | Cylinder
pressure at 3000 rpm |
|
Fig. 9: | Cylinder
pressure at 3500 rpm |
In this operating condition, both of diesel engines and CNG engine combustion
process are most excellent than the other condition. In the diesel engine, the
1500 rpm engine speed condition is not higher and not lower for the combustion
of diesel fuel. Burned diesel fuel rate in 1500 rpm is most excellent to product
the higher pressure and power. In the port injection dedicated CNG engine, the
1000 rpm engine speed condition is not higher and not lower for the combustion
of CNG engine. Burned CNG fuel rate in 1000 rpm is most excellent and product
the higher pressure and torque of the engine. The trend of the maximum cylinder
pressure for direct injection diesel engine, modified direct injection diesel
engine and port injection dedicated CNG engine are decreased if the engine speed
is increased.
|
Fig. 10: | Cylinder
pressure at 4000 rpm |
Figure 10 shows the lowest of maximum cylinder pressure
of direct injection diesel engine, modified direct injection diesel engine and
port injection dedicated CNG engine. The lowest maximum cylinder pressure in
combustion process of direct injection diesel engine, modified direct injection
diesel engine and port injection dedicated CNG engine are shown in 4000 rpm
engine speed and the nominal is 72.82 bar for diesel engine, 52.29 bar for modified
diesel engine and 25.00 bar for port injection dedicated CNG engine. In this
case, the combustion of diesel engines and CNG engine are in lately so the combustion
process is not excellent and unburned fuel is highest, this phenomenon can be
decreasing the engine cylinder pressure performance. The port injection dedicated
CNG engine maximum cylinder pressure is lowest because the natural gas fuel
is lower in density, hydrocarbon and energy than the diesel fuel. So, the cylinder
pressure in the same compression ratio, the CNG engine is lower than the modified
diesel engine if the engines are operated in high speed. The lowest cylinder
pressure of diesel engine is higher than modified diesel engine because the
compression ratio of diesel engine is higher than modified diesel engine.
DISCUSSION
The maximum cylinder pressure effect of the diesel engine converted to port injection dedicated CNG engine in variation engine speed is shown in Fig. 11.
|
Fig. 11: | Maximum
cylinder pressure |
In the 1000 rpm engine speed, the conversion of diesel engine to CNG engine
is increase the maximum cylinder pressure 21.70%. In the 1500 rpm engine speed,
the conversion of diesel engine to CNG engine is increase the maximum cylinder
pressure 8.97%. In the 2000 rpm engine speed, the conversion of diesel engine
to CNG engine is decrease the maximum cylinder pressure 1.70%. In the 2500 rpm
engine speed, the conversion of diesel engine to CNG engine is decrease the
maximum cylinder pressure 13.53%. In the 3000 rpm engine speed, the conversion
of diesel engine to CNG engine is decrease the maximum cylinder pressure 39.12%.
In the 3500 rpm engine speed, the conversion of diesel engine to CNG engine
is decrease the maximum cylinder pressure 51.40%. At the 4000 rpm, the conversion
is decrease maximum cylinder pressure 58.56%.
The maximum cylinder pressure for CNG engine is lower than the diesel engine. It caused the compression ratio of CNG engine is lower than the diesel engine and the combustion energy output of diesel fuel is produces highest power than the natural gas fuel. Another that, the density of natural gas fuel is lower than the diesel fuel. So, in the same volume, the diesel fuel is has higher pressure than the gas fuel. In this engine conversion, the CNG engine better to operate at low speed. In the low speed the maximum cylinder pressure increasing is higher dramatically than at the medium and high speed. For all of engine speed, the conversion of modified diesel engine to CNG engine is increase the cylinder pressure in low speed, but in the high speed the engine conversion can be decreasing the cylinder pressure. In the high speed CNG engine, the fuel energy is reduced and the combustion is not completely, but in the low speed the combustion of CNG engine is completely because the combustion ignition is assisted by spark plug system and the ignition point of natural gas fuel is higher than the diesel fuel, it can be producing higher engine cylinder pressure.
CONCLUSION
Diesel engine cylinder pressure is higher than modified direct injection diesel engine and port injection dedicated CNG engine because the compression ratio of diesel engine is higher. The maximum cylinder pressure for direct injection diesel engine, modified direct injection diesel engine and port injection dedicated CNG engine are decrease if the engine speed is increased. The lowest cylinder pressure of diesel engine is higher than modified diesel engine because the compression ratio of diesel engine is higher than modified diesel engine. The maximum cylinder pressure for CNG engine is lower than the diesel engine. It caused the compression ratio of CNG engine is lower than the diesel engine and the combustion energy output of diesel fuel is produces highest power than the natural gas fuel.