This paper presents simulation of working process in a new IC engine concept. The
main feature of this new IC engine concept is the realization of variable movement of
the piston. With this unconventional piston movement it is easy to provide variable
compression ratio, variable displacement and combustion during constant volume.
These advantages over standard piston mechanism are achieved through synthesis of
the two pairs of non-circular gears. Presented mechanism is designed to obtain a
specific motion law which provides better fuel consumption of IC engines. For this
paper Ricardo/WAVE software was used, which provides a fully integrated treatment
of time-dependent fluid dynamics and thermodynamics by means of onedimensional
formulation. The results obtained herein include the efficiency characteristic of this
new heat engine concept. The results show that combustion during constant volume,
variable compression ratio and variable displacement have significant impact on
improvement of fuel consumption.
Key words: simulation, variable compression, variable displacement, constant
volume combustion
1. Introduction
The internal combustion (IC) engine is the favoured propulsion system for passanger and
freight traffic. A significant reduction of CO2 emission in mobilty sector is a major challenge for the
next years. Global concerns on the limitation of energy and reduction of the CO2emission force
automotive engineers to develop more energy efficient and environmentally friendly alternative
powertrain technologies. Considering the present development trends, trends for more efficient use of
fuel resources and the well known problem of global warming and other environmental factors,
development of IC engines will certainly move towards the reduction of fuel consumption. In this
paper one of the possible ways of reducing thermodynamic losses in the IC engine is shown.
Relatively low efficiency of today`s internal combustion engine is the consequence of several
factors. First, ordinary spark ignition (SI) internal combustion engines during running at low loads
have their thermal efficiency reduced due to the effect of the throttle valve that controls the engine
load and by the fact that the compression starts at low pressure [1]. Under part load conditions,
engines use some of the work to pump air across the partially closed throttle valve. One of the possible
solutions for improving efficiency at part load is to reduce the stroke volume by selectively shutting
offseveral cylinders of an engine at the part load conditions. As early as 1916, the potential of using a
variable displacement engine to increase the fuel efficiency at part load conditions was known and
tested. This means that instead of reducing the air–fuel mixture charge by the throttle valve at part load
conditions, the stroke volume of the engine is reduced by disabling some of the working cylinders [2].
Also, the compression ratio of the engine should bevaried according to the load and speed conditions
in order to improve efficiency [3-5].
Conventional IC engines are based on a relatively simple solution to achieve a thermodynamic
cycle while providing mechanical power. While the performance, emissions and reliability of IC
engines have been improved significantly, the fundamental principle of crank-rod-piston slider
mechanism still remains largely unaltered. In theory, the most efficient thermodynamic cycle for IC
engines is the Otto cycle [6], which consists of isentropic compression and expansion processes and
constant volume heat addition and rejection processes [7,8]. It isgenerally known that the most
important parts of the cycle which determine the efficiency are the constant volume heat addition at
high compression ratios [9, 10]. This fact provides the highest thermal potential of the various
possible thermodynamic cycles which are suitable for IC engines, and the subsequent expansion
process, which converts the thermal potential into work. In reality, neither conventional spark ignition
nor compression ignition or even the modern developed homogeneous charge compression ignition or
controlled auto ignition combustionprocesses, can achieve the efficiency level suggested by the ideal
thermodynamic cycles [11]. Only the Otto cycle delivers theoretical maximum thermal efficiency. The
traditional design of internal combustion engines using a simple slide-crank mechanism gives no time
for a constant volume combustion which significantly reduces the cycle efficiency [11].
Variable displacement and variable compression engines are gaining attention by scientist and
automobile manufactures because of their fuel consumption economy advantage. One of the
successfully constructed IC engine with variable compression ratio is certainly made by SAAB [1]. In
conventional IC engines the load regulation is balanced by throttling the intake mixture [6]. Variable
displacement concepts have been analyzed in many different scientific publications. Siewart [12]
reported a fuel economy approaching 20% for variable stroke engines over fixed stroke engines. Also
there is a several patents about mechanisms which provides variable stroke, one of them are patented
by Freudenstein and Maki [13]. Several authors [14-16] have proposed different complex mechanisms
to achieve variable displacement engine. In the paper of Yamin and Dado [17] was investigated the
effect of a variable stroke mechanism on the engine performance, the conclusion showed that the
engine performance was improved with this novel design. Also Pouliot et al. [18], proposed,
constructed and studied a five-cylinder, four-bar linkage engine and Filipi et al. [19] theoretically,
investigated the effect of varying the stroke length on a homogeneous charge engine’s combustion,
heat transfer and efficiency using gasoline as fuel. Wong et al. [20] presented and analyzed a four
cylinder engine with Alvar cycle that utilizessecondary pistons and auxiliary chambers.
On the basis of these references a further step made in this paper is to make analysis of a new
engine concept which is able to make variable piston motion. Variable piston motion (VPM) IC engine
[21] is not only able to provide variable compression ratio and displacement but also with this concept
it is easy to achieve dwell angle at top dead center (TDC) and bottom dead center (BDC). With piston
dwell at bottom dead point more complete expansion can also be achieved. In this paper was used
Ricardo/WAVE software to obtaining the improvement between this new cycle and the standard Otto
cycle. Also in this paper was presented basic description of the new engine that will be able to realize
thermodynamic cycle with increased efficiency.
2. Variable piston motion IC engine
In the following section will be presented basic parts and shape of a new IC engine concept.
Variable piston motion IC engine is presented on the fig. 1. Basic parts of the VPM engine are: 1-engine block, 2-engine head, 3-toroidal piston, 4-intake manifold, 5-exhaust manifold, 6-camshaft, 7-
valve, 8-valve spring, 9-housing, 10-flywheel, 11-noncircular gear, 12-noncircular gear, 13-noncircular gear, 14-noncircular gear, 15-stepper motor, 16-stepper motor, 17-crankcase. As can be
seen from the described illustration toroidal piston make a movement conditioned by the mechanism
consisting of two pairs of non-circular gears. In this article will not be presented detailed description
of this concept, since it is not the intention of the authors to propose a kinematic analysis of a new
internal combustion engine design but only thermodynamic features and advantages over ordinary
spark ignition engines.
VPM IC engine has a two pairs of non-circular gears (NCG). A NCG is a special gear design with
special characteristics and purpose. While a regular gear is optimized to transmit torque to another
engaged member with minimum noise and wear and with maximum efficiency, a non-circular gear's
main objective might be ratio variations, axle displacement oscillations and more. In fact this feature
of NCG is very important for synthesis of mechanism where is intermittent-motion required. This
intermittent-motion mechanism combines circular gears with noncircular gears in a planetary
arrangement. With such planetary differential gearit is possible to achieve very complex movement,
where toroidal piston is able to provide motion with variable displacement and variable compression,
also because of the characteristics of NCG, piston dwell at TDC and BDC is also feasible.
Dwell time or dwell angle is important fact during combustion process. In conventional engine
this dwell angle can be changed due to variationsof ratio between connecting rod and crank radius.
Piston dwell at TDC and at BDC are often mentioned, it should be noted that strictly, there is no dwell
period in ordinary mechanism. The piston comes to rest at precisely the crank angle that the crank and
rod are in line (TDC and BDC), and is moving at all other crank angles. At crank angles which are
very close to the TDC and BDC angles, the piston is moving slowly. It is this slow movement in the
vicinity of TDC and BDC that give rise to the term piston dwell. If the piston dwells longer near top
dead center and ignition is initiated properly, there will actually be a longer period of time for the
pressure created during combustion to press against the top of the piston. This process occurs within
the engine and its part of the thermodynamic cycle of the device. In all IC engine useful work is
generated from the hot, gaseous products of combustion acting directly on moving surfaces of the
engine, such as the top of a piston. This moving boundary of combustion chamber is the focus of this
paper. In generally moving of the piston is responsible for the volume changing during process of
combustion. In this paper was presented IC engine where this boundary, i.e. top of the piston, actually
not moving in a large portion of heat addition.
The four stroke spark SI engine pressure–volume diagram (p–V) contains two main parts.
They are the compression–combustion–expansion (high pressure loop) and the exhaust-intake (low
pressure or gas exchange loop) parts. The main reason for efficiency decrease at part load conditions
for these types of engines is the flow restriction atthe cross sectional area of the intake system by
partially closing the throttle valve, which leads to increased pumping losses and to increased low
pressure loop area on the p–V diagram. Meanwhile, the poorer combustion quality, i.e. lower
combustion speed and cycle to cycle variations, additionally influence these pressure loop areas,
illustrated in detail on fig. 2.
Figure 2. Schematic comparison of gross, pumping, net IMEP and their effect on indicated
efficiency in high and low load conditions in SI engines [2]
Cylinder deactivation is initialized by cutting off the fuel supply to the selected cylinders.
There are also several systems that shut off the valves of the deactivated cylinders too. In these
systems, the reduction in pumping losses is more thanthat achieved by cutting off the fuel supply only
[22]. In this study, methods for increasing efficiency at part load conditions and their potential for
practical use are also investigated, in fact in this article was examined case where classical approach of
engine throttling was replaced with variable displacement piston motion. In fig. 3 is presented piston
motion law that was used for simulation of working processes in variable piston motion IC engine.
Figure 3. Complex motion of the toroidal pistonthat allow infinitely many displacement of the
engine in the range from 678 [cm
3
] to 4000 [cm
3
]
3. Unconventional piston motion-new four stroke cycle
The ideal scenario is to initiate and complete the combustion event while the piston remains at
the TDC position. This provides the maximum thermal potential and eliminates the negative work due
to early ignition which is well into compression stroke with conventional engine strategies. In
addition, if the combustion event completes at the TDC, the effective expansion stroke can be
maximally extended to fully use the thermal energy as well as to provide sufficient time for post
combustion reactions, thereby reducing partial burned emissions. During operation of conventional IC
engines, the piston can only reciprocate continuously between TDC and BDC at a frequency
proportional to the engine speed. The chemical reaction process associated with combustion events,
however, essentially takes a fixed-time to complete, which is relatively independent of the engine
speed. In order to maximize the work obtained from the heat energy released by combustion, the
air/fuel mixture has to be ignited prior to the piston reaching TDC, and the ignition timing should be
adjusted according to the engine speed and the quality of the air/fuel mixture. Clearly, the early stage
of the heat release before the pistonreaches TDC results in negative work.
In this section, the new unconventional piston motion law will be presented. With this
movement, the piston is able to make such motion where heat addition can be done during piston
dwell. The design geometry creates a pause or dwell in the piston’s movement at the TDC and the
BDC, while the output shaft continues to rotate for up to 35 degrees. Adding these constant volume
dwell cycles improves fuel burn, maximizes pressure, and increases cylinder charge. Fuel burn can be
precisely controlled by maintaining a minimum volume (TDC piston dwell) throughout the burn
process, containment maximizes pressure and burn efficiency. Furthermore, holding the piston at
maximum volume (BDC piston dwell) provides additional time for the cylinder to fully charge before
closing the intake valves. The design creates unconventional four stroke cycle process. This
unconventional cycle consists of the following strokes and processes.
The first stroke consists of forced and free intake. During the forced intake, piston travels from
TDC to BDC, which draws fresh mixture into the cylinder. This part of the stroke is the same as the
intake stroke in the ordinary IC engines, the secondpart is the free intake. After the piston comes into
BDC, it stops there for a while, this dwell time depends on the optimization of the intake process and
it will not be explained in detail in this paper. However, it is very important that the piston dwell does
not last longer or shorter than the optimal calculated value. After the piston comes into BDC, the
column of fresh gases continues to flow into the cylinder by inertia, until the intake valve closes. In
this way the intake volumetric efficiency is increased. The second stroke consists of the compression
process and a combustion during constant volume. In the first part of this second stroke, the piston
travels from BDC to TDC. The ignition occurs at TDC without any spark advance, thus saving the
accumulated energy of the flywheel. Ignition begins when the piston is stopped at the TDC, while the
piston stop lasts for the time calculated by optimization to complete combustion and prevent any backpressure caused by the spark advance. Consequently, the use of energy obtained from the fuel is
maximized and the fuel consumption is decreased. The third stroke is an expansion stroke, during
which the piston comes from TDC to BDC like in a standard mechanism but with the exception that
piston again makes a dwell in BDC. In this new unconventional four stroke cycle, the entire expansion
stroke occurs between TDC and BDC. Compared to standard IC engine, in the new piston motion
movement there is no exhaust valve opening advance, which determines loss of possibly resulting
work. In the second part of this third stroke, the piston comes on BDC and stays in the same position
for a while. During this time high-pressure gases are spontaneously evacuated, while the piston is
stopped at the BDC. The last stroke is exhaust stroke, during which the exhaust gas is actually a low
pressure gas, so the piston will not require a big pumping effort going up towards TDC. In the last
phase of exhaust stroke, exhaust gases can freely leave compression volume. At the same time intake
valves slowly open and fresh charge comes into the cylinder, while the piston is still in the dwell mode
at TDC. Previously described unconventional four stroke cycle can be illustrated by fig. 4.
Figure 4. New unconventional motion of piston for the selected dwell angle of 20 [deg] [23]
4. Simulation
Within the automotive industry the most widely adopted technique for gas exchange studies is
to solve the one dimensional coupled set of non-linear equations using the finite volume or finite
difference method. This technique is used in several commercial softwares e.g., Ricardo/WAVE, GTPower and AVL/BOOST. In this paper, Ricardo/WAVE software was used, which provides a fully
integrated treatment of time-dependent fluid dynamics and thermodynamics by means of onedimensional formulation. Internal combustion engine simulation modeling has long been established
as an effective tool for studying engine performance and contributing to evaluation and new
developments [24, 25]. Thermodynamic models of the real engine cycle have served as effective tools
for complete analysis of engine performance and sensitivity to various operating factors [26, 27].
WAVE is the primary program and solver for all simulations of fluid dynamic systems, this software
can be used to model the complete internal combustion engine. The piping and manifolds of the
intake and exhaust systems are modeled using the basic WAVE flow elements. These networks are
then linked together through engine elements and sub-models, which have been calibrated to provide
accurate driving inputs for the intake and exhaust pressure-wave dynamics.
The details of the flow (as calculated in the flow network) are obtained as a solution of quasione dimensional compressible flow equations governing the conservation of mass, momentum and
energy-eq. (1-3). The flow network of both conventional and unconventional piston movement is
discretized into a series of small volumes and the governing equations are then written in a finite
difference form for each of these elementary volumes. A staggered mesh system is used, with
equations of mass and energy solved for each volume and the momentum equation solved for each
boundary between volumes. The equations are written in an explicitly conservative form as:
∑ =
boundaries
flux m
dt
dm
(1)
Equation (1): mass continuity equation.
2
2 1
4 () () 22 fp flux
flux
boundaries
udxA
CCuA dpA m u
dm D
dt dx dx
ρ
ρ
⎛⎞ − + ⎜⎟ ⎝⎠ =−∑
(2) Equation (2): Conservation of momentum equation.
()
.
() flux g gas wall
bound
dme dV
pmHhATT dt dt
=+ − − ∑
(3) Equation (3): Conservation of energy equation.
If the engine cylinder element has one zone, the entire cylinder is treated as one region. In the
latter, the cylinder is divided into two regions (unburned and burned), which share a common pressure.
The two-zone model is used to capture the chemical processes taking place during the combustion
period in more detail. Combustion models may be used either with a single or two-zone engine
cylinders, but for this research two zone modelswere used because of the problem with knock
combustion that was also examined. For the single zone model there is the energy equation refer to (4)
as below: () V P Q h m mu
nvalves
i
i i ∆ − − = ∆ ∑
=1
(4)
During combustion, the only term of enthalpy flow is mihi
due to propagation of the flame front to the
unburned zone. For the two-zone, refer to model (4), in the unburned zone we have:
0 ) (
0 1 0 0 1 1 = ∆ − + − + − ui ui u u u u u u u
h m Q V V P u m u m (5)
Using the equation of the state, it becomes:
0 0 1 1 1 0 0 1 1 = ∆ − + − + − ui ui u u u u u u u u u
h m Q PV T R m u m u m (6)
Similarly, for the burned zone we have:
0
0 1 1 1 0 0 1 1 = ∆ − + − + − bi bi b b b b b b b b b
h m Q PV T R m u m u m (7)
As a constraint, the volumes of the unburned and burned zones are summed up to the total cylinder
volume:
0
1 1 1 1 1 1 = − + c b b b u u u PV T R m T R m (8)
The last three equations are a complete set and are solved by using the Newton iteration method.
Since this article investigates unconventional piston motion, classical approach for solving
problems of volume changes cannot be applied. When the piston position differs from standard crank
piston motion, the imposed piston motion sub-model can be used for modeling the engine. The
formulation to calculate the instantaneous cylinder volume is identical to the one used in the standard
WAVE model, with the exception that the piston position, s, is linearly interpolated between points in
the user-entered profile. Smooth piston motion depends on the fine spacing of the crank angle array. In
this case enough large arrays were used to enable one-degree spacing. As far as the high-pressure part
of the cycle is considered, the most important process is the combustion. Without in-cylinder pressure
measurements, the combustion model had to be predicted based on typical forced induction Wiebe
function parameters. WAVE allows for three parameters in the Wiebe correlation to be input: 10-90
percent burn duration, 50 percent burn point, and the Wiebe exponent, described by eq. (9). In this
program, Ricardo Wave model of combustion can beselected between several options, ranging from
theoretical models with constant volume or constantpressure heat release, over Wiebe-function based
heat release models, to quasi-dimensional two-zonemodel of turbulent flame propagation. The SI
Wiebe function is widely used to describe the rate of fuel mass burned in thermodynamic calculations
[28].
1
1
WEXP
AWI
BDUR
Weθ
+
⎡ ⎤ ∆ ⎛⎞ − ⎢ ⎥
⎜⎟ ⎝⎠ ⎢ ⎥
⎣ ⎦
=− (9)
This relationship allows the independent input of function shape parameters and of burn
duration. The experimentally observed trends of premixed SI combustion are represented quite well.
In this paper the Wiebe one stage model of heat release has been chosen. The parameters of Wiebe
function were selected to achieve good agreement between modeled and experimentally recorded
pressure. Selected parameters have been successfully applied in the research [29-31]. Engine data that
was chosen for this research was presented in tab. 1. It can be noticed that valves open duration are
constant values, but position of maximum valve opening (EVMP and IVMP) are in certain ranges.
That is because of variability of piston motion, mechanism is constructed in that way that allow
different piston displacement and in the same time adjustment of valvetrain open phase.
Table 1. Main engine data
engine type spark ignition
engine cycle four-stroke
number of cylinders 2
number of valves per cylinder 4
bore 120 [mm]
stroke 30-177 [mm]
intake valve diameter 44 [mm]
exhaust valve diameter 40 [mm]
valves path 15 [mm]
EVDUR 235 [deg]
IVDUR 230 [deg]
EVMP 253.3-245[deg]
IVMP 479.3-471 [deg]
octane number 98
compression ratios 8-16
The valve train was modeled by setting up the appropriate number of valves per cylinder and
entering details about valve size, lift, and flow, for this purpose was chosen values which are different
from the conventional valvetrain. Reason for that can be found in the fact that piston dwell have
impact on valves open duration. So, in this concept, because of the piston dwell there is no need for
valve overlap, this can be seen from fig. 5. Valve data for each cylinder must be entered referencing a
valve model. The Lift Valve model was used in thisexample, so that the valve would follow a set
profile. The intake and exhaust valves were modeled using ducts and junctions, where geometry such
as length, orientation, and cross sections are specified. Heat transfer and friction data must also be
entered, in this model the selected valuesare similiar to the standard SI engine.
Figure 5. Valves lift without valve overlap for intake and exhaust valve respectively
Since in this paper was investigated only virtual engine model, for the purpose of model
calibration in this study was examined influence of selected input parameters for simulation of
ordinary IC engine. The calibration of simulating model was performed on ordinary spark ignition
engine on a test stand with adequate experimental equipment. It was realized through the comparison
of experimental and calculated results and tuning some model parameters and constants. Following the
procedure prescribed in the WAVE user manual the average values of all important values was
compared to test data. In order to validate the model with high degree of precision, it is important to
have as much engine test data as possible. For thisresearch model was calibrate to match experimental
data for 50 different operating conditions at full and partial load. In order to validate the parameters
calculated by Ricardo/WAVE software, engine data was recorded at a range of engine speeds between
2000 and 6000 [rpm]. The pressure histories were recorded in first engine cylinder and in two
characteristic points in inlet pipe of relating cylinder and compared with calculated curves. TDC must
be determined within 0.1 degrees in order to accurately calculate work (IMEP), so in order to avoid
serious error in the TDC determination caused by torsional vibration the test cylinder must be chosen
in multi-cylinder engine as the one immediately next to the crankshaft encoder.Piezoelectric pressure
transducer was used for the purpose of acquiring in-cylinder pressure data.For this experimental
investigation was used a special category of ECU (Engine Control Unit) which is programmable in
order to achieve different working parameters (air-fuel ratio, ignition timing, fuel injection, etc.).
main feature of this new IC engine concept is the realization of variable movement of
the piston. With this unconventional piston movement it is easy to provide variable
compression ratio, variable displacement and combustion during constant volume.
These advantages over standard piston mechanism are achieved through synthesis of
the two pairs of non-circular gears. Presented mechanism is designed to obtain a
specific motion law which provides better fuel consumption of IC engines. For this
paper Ricardo/WAVE software was used, which provides a fully integrated treatment
of time-dependent fluid dynamics and thermodynamics by means of onedimensional
formulation. The results obtained herein include the efficiency characteristic of this
new heat engine concept. The results show that combustion during constant volume,
variable compression ratio and variable displacement have significant impact on
improvement of fuel consumption.
Key words: simulation, variable compression, variable displacement, constant
volume combustion
1. Introduction
The internal combustion (IC) engine is the favoured propulsion system for passanger and
freight traffic. A significant reduction of CO2 emission in mobilty sector is a major challenge for the
next years. Global concerns on the limitation of energy and reduction of the CO2emission force
automotive engineers to develop more energy efficient and environmentally friendly alternative
powertrain technologies. Considering the present development trends, trends for more efficient use of
fuel resources and the well known problem of global warming and other environmental factors,
development of IC engines will certainly move towards the reduction of fuel consumption. In this
paper one of the possible ways of reducing thermodynamic losses in the IC engine is shown.
Relatively low efficiency of today`s internal combustion engine is the consequence of several
factors. First, ordinary spark ignition (SI) internal combustion engines during running at low loads
have their thermal efficiency reduced due to the effect of the throttle valve that controls the engine
load and by the fact that the compression starts at low pressure [1]. Under part load conditions,
engines use some of the work to pump air across the partially closed throttle valve. One of the possible
solutions for improving efficiency at part load is to reduce the stroke volume by selectively shutting
offseveral cylinders of an engine at the part load conditions. As early as 1916, the potential of using a
variable displacement engine to increase the fuel efficiency at part load conditions was known and
tested. This means that instead of reducing the air–fuel mixture charge by the throttle valve at part load
conditions, the stroke volume of the engine is reduced by disabling some of the working cylinders [2].
Also, the compression ratio of the engine should bevaried according to the load and speed conditions
in order to improve efficiency [3-5].
Conventional IC engines are based on a relatively simple solution to achieve a thermodynamic
cycle while providing mechanical power. While the performance, emissions and reliability of IC
engines have been improved significantly, the fundamental principle of crank-rod-piston slider
mechanism still remains largely unaltered. In theory, the most efficient thermodynamic cycle for IC
engines is the Otto cycle [6], which consists of isentropic compression and expansion processes and
constant volume heat addition and rejection processes [7,8]. It isgenerally known that the most
important parts of the cycle which determine the efficiency are the constant volume heat addition at
high compression ratios [9, 10]. This fact provides the highest thermal potential of the various
possible thermodynamic cycles which are suitable for IC engines, and the subsequent expansion
process, which converts the thermal potential into work. In reality, neither conventional spark ignition
nor compression ignition or even the modern developed homogeneous charge compression ignition or
controlled auto ignition combustionprocesses, can achieve the efficiency level suggested by the ideal
thermodynamic cycles [11]. Only the Otto cycle delivers theoretical maximum thermal efficiency. The
traditional design of internal combustion engines using a simple slide-crank mechanism gives no time
for a constant volume combustion which significantly reduces the cycle efficiency [11].
Variable displacement and variable compression engines are gaining attention by scientist and
automobile manufactures because of their fuel consumption economy advantage. One of the
successfully constructed IC engine with variable compression ratio is certainly made by SAAB [1]. In
conventional IC engines the load regulation is balanced by throttling the intake mixture [6]. Variable
displacement concepts have been analyzed in many different scientific publications. Siewart [12]
reported a fuel economy approaching 20% for variable stroke engines over fixed stroke engines. Also
there is a several patents about mechanisms which provides variable stroke, one of them are patented
by Freudenstein and Maki [13]. Several authors [14-16] have proposed different complex mechanisms
to achieve variable displacement engine. In the paper of Yamin and Dado [17] was investigated the
effect of a variable stroke mechanism on the engine performance, the conclusion showed that the
engine performance was improved with this novel design. Also Pouliot et al. [18], proposed,
constructed and studied a five-cylinder, four-bar linkage engine and Filipi et al. [19] theoretically,
investigated the effect of varying the stroke length on a homogeneous charge engine’s combustion,
heat transfer and efficiency using gasoline as fuel. Wong et al. [20] presented and analyzed a four
cylinder engine with Alvar cycle that utilizessecondary pistons and auxiliary chambers.
On the basis of these references a further step made in this paper is to make analysis of a new
engine concept which is able to make variable piston motion. Variable piston motion (VPM) IC engine
[21] is not only able to provide variable compression ratio and displacement but also with this concept
it is easy to achieve dwell angle at top dead center (TDC) and bottom dead center (BDC). With piston
dwell at bottom dead point more complete expansion can also be achieved. In this paper was used
Ricardo/WAVE software to obtaining the improvement between this new cycle and the standard Otto
cycle. Also in this paper was presented basic description of the new engine that will be able to realize
thermodynamic cycle with increased efficiency.
2. Variable piston motion IC engine
In the following section will be presented basic parts and shape of a new IC engine concept.
Variable piston motion IC engine is presented on the fig. 1. Basic parts of the VPM engine are: 1-engine block, 2-engine head, 3-toroidal piston, 4-intake manifold, 5-exhaust manifold, 6-camshaft, 7-
valve, 8-valve spring, 9-housing, 10-flywheel, 11-noncircular gear, 12-noncircular gear, 13-noncircular gear, 14-noncircular gear, 15-stepper motor, 16-stepper motor, 17-crankcase. As can be
seen from the described illustration toroidal piston make a movement conditioned by the mechanism
consisting of two pairs of non-circular gears. In this article will not be presented detailed description
of this concept, since it is not the intention of the authors to propose a kinematic analysis of a new
internal combustion engine design but only thermodynamic features and advantages over ordinary
spark ignition engines.
VPM IC engine has a two pairs of non-circular gears (NCG). A NCG is a special gear design with
special characteristics and purpose. While a regular gear is optimized to transmit torque to another
engaged member with minimum noise and wear and with maximum efficiency, a non-circular gear's
main objective might be ratio variations, axle displacement oscillations and more. In fact this feature
of NCG is very important for synthesis of mechanism where is intermittent-motion required. This
intermittent-motion mechanism combines circular gears with noncircular gears in a planetary
arrangement. With such planetary differential gearit is possible to achieve very complex movement,
where toroidal piston is able to provide motion with variable displacement and variable compression,
also because of the characteristics of NCG, piston dwell at TDC and BDC is also feasible.
Dwell time or dwell angle is important fact during combustion process. In conventional engine
this dwell angle can be changed due to variationsof ratio between connecting rod and crank radius.
Piston dwell at TDC and at BDC are often mentioned, it should be noted that strictly, there is no dwell
period in ordinary mechanism. The piston comes to rest at precisely the crank angle that the crank and
rod are in line (TDC and BDC), and is moving at all other crank angles. At crank angles which are
very close to the TDC and BDC angles, the piston is moving slowly. It is this slow movement in the
vicinity of TDC and BDC that give rise to the term piston dwell. If the piston dwells longer near top
dead center and ignition is initiated properly, there will actually be a longer period of time for the
pressure created during combustion to press against the top of the piston. This process occurs within
the engine and its part of the thermodynamic cycle of the device. In all IC engine useful work is
generated from the hot, gaseous products of combustion acting directly on moving surfaces of the
engine, such as the top of a piston. This moving boundary of combustion chamber is the focus of this
paper. In generally moving of the piston is responsible for the volume changing during process of
combustion. In this paper was presented IC engine where this boundary, i.e. top of the piston, actually
not moving in a large portion of heat addition.
The four stroke spark SI engine pressure–volume diagram (p–V) contains two main parts.
They are the compression–combustion–expansion (high pressure loop) and the exhaust-intake (low
pressure or gas exchange loop) parts. The main reason for efficiency decrease at part load conditions
for these types of engines is the flow restriction atthe cross sectional area of the intake system by
partially closing the throttle valve, which leads to increased pumping losses and to increased low
pressure loop area on the p–V diagram. Meanwhile, the poorer combustion quality, i.e. lower
combustion speed and cycle to cycle variations, additionally influence these pressure loop areas,
illustrated in detail on fig. 2.
Figure 2. Schematic comparison of gross, pumping, net IMEP and their effect on indicated
efficiency in high and low load conditions in SI engines [2]
Cylinder deactivation is initialized by cutting off the fuel supply to the selected cylinders.
There are also several systems that shut off the valves of the deactivated cylinders too. In these
systems, the reduction in pumping losses is more thanthat achieved by cutting off the fuel supply only
[22]. In this study, methods for increasing efficiency at part load conditions and their potential for
practical use are also investigated, in fact in this article was examined case where classical approach of
engine throttling was replaced with variable displacement piston motion. In fig. 3 is presented piston
motion law that was used for simulation of working processes in variable piston motion IC engine.
Figure 3. Complex motion of the toroidal pistonthat allow infinitely many displacement of the
engine in the range from 678 [cm
3
] to 4000 [cm
3
]
3. Unconventional piston motion-new four stroke cycle
The ideal scenario is to initiate and complete the combustion event while the piston remains at
the TDC position. This provides the maximum thermal potential and eliminates the negative work due
to early ignition which is well into compression stroke with conventional engine strategies. In
addition, if the combustion event completes at the TDC, the effective expansion stroke can be
maximally extended to fully use the thermal energy as well as to provide sufficient time for post
combustion reactions, thereby reducing partial burned emissions. During operation of conventional IC
engines, the piston can only reciprocate continuously between TDC and BDC at a frequency
proportional to the engine speed. The chemical reaction process associated with combustion events,
however, essentially takes a fixed-time to complete, which is relatively independent of the engine
speed. In order to maximize the work obtained from the heat energy released by combustion, the
air/fuel mixture has to be ignited prior to the piston reaching TDC, and the ignition timing should be
adjusted according to the engine speed and the quality of the air/fuel mixture. Clearly, the early stage
of the heat release before the pistonreaches TDC results in negative work.
In this section, the new unconventional piston motion law will be presented. With this
movement, the piston is able to make such motion where heat addition can be done during piston
dwell. The design geometry creates a pause or dwell in the piston’s movement at the TDC and the
BDC, while the output shaft continues to rotate for up to 35 degrees. Adding these constant volume
dwell cycles improves fuel burn, maximizes pressure, and increases cylinder charge. Fuel burn can be
precisely controlled by maintaining a minimum volume (TDC piston dwell) throughout the burn
process, containment maximizes pressure and burn efficiency. Furthermore, holding the piston at
maximum volume (BDC piston dwell) provides additional time for the cylinder to fully charge before
closing the intake valves. The design creates unconventional four stroke cycle process. This
unconventional cycle consists of the following strokes and processes.
The first stroke consists of forced and free intake. During the forced intake, piston travels from
TDC to BDC, which draws fresh mixture into the cylinder. This part of the stroke is the same as the
intake stroke in the ordinary IC engines, the secondpart is the free intake. After the piston comes into
BDC, it stops there for a while, this dwell time depends on the optimization of the intake process and
it will not be explained in detail in this paper. However, it is very important that the piston dwell does
not last longer or shorter than the optimal calculated value. After the piston comes into BDC, the
column of fresh gases continues to flow into the cylinder by inertia, until the intake valve closes. In
this way the intake volumetric efficiency is increased. The second stroke consists of the compression
process and a combustion during constant volume. In the first part of this second stroke, the piston
travels from BDC to TDC. The ignition occurs at TDC without any spark advance, thus saving the
accumulated energy of the flywheel. Ignition begins when the piston is stopped at the TDC, while the
piston stop lasts for the time calculated by optimization to complete combustion and prevent any backpressure caused by the spark advance. Consequently, the use of energy obtained from the fuel is
maximized and the fuel consumption is decreased. The third stroke is an expansion stroke, during
which the piston comes from TDC to BDC like in a standard mechanism but with the exception that
piston again makes a dwell in BDC. In this new unconventional four stroke cycle, the entire expansion
stroke occurs between TDC and BDC. Compared to standard IC engine, in the new piston motion
movement there is no exhaust valve opening advance, which determines loss of possibly resulting
work. In the second part of this third stroke, the piston comes on BDC and stays in the same position
for a while. During this time high-pressure gases are spontaneously evacuated, while the piston is
stopped at the BDC. The last stroke is exhaust stroke, during which the exhaust gas is actually a low
pressure gas, so the piston will not require a big pumping effort going up towards TDC. In the last
phase of exhaust stroke, exhaust gases can freely leave compression volume. At the same time intake
valves slowly open and fresh charge comes into the cylinder, while the piston is still in the dwell mode
at TDC. Previously described unconventional four stroke cycle can be illustrated by fig. 4.
Figure 4. New unconventional motion of piston for the selected dwell angle of 20 [deg] [23]
4. Simulation
Within the automotive industry the most widely adopted technique for gas exchange studies is
to solve the one dimensional coupled set of non-linear equations using the finite volume or finite
difference method. This technique is used in several commercial softwares e.g., Ricardo/WAVE, GTPower and AVL/BOOST. In this paper, Ricardo/WAVE software was used, which provides a fully
integrated treatment of time-dependent fluid dynamics and thermodynamics by means of onedimensional formulation. Internal combustion engine simulation modeling has long been established
as an effective tool for studying engine performance and contributing to evaluation and new
developments [24, 25]. Thermodynamic models of the real engine cycle have served as effective tools
for complete analysis of engine performance and sensitivity to various operating factors [26, 27].
WAVE is the primary program and solver for all simulations of fluid dynamic systems, this software
can be used to model the complete internal combustion engine. The piping and manifolds of the
intake and exhaust systems are modeled using the basic WAVE flow elements. These networks are
then linked together through engine elements and sub-models, which have been calibrated to provide
accurate driving inputs for the intake and exhaust pressure-wave dynamics.
The details of the flow (as calculated in the flow network) are obtained as a solution of quasione dimensional compressible flow equations governing the conservation of mass, momentum and
energy-eq. (1-3). The flow network of both conventional and unconventional piston movement is
discretized into a series of small volumes and the governing equations are then written in a finite
difference form for each of these elementary volumes. A staggered mesh system is used, with
equations of mass and energy solved for each volume and the momentum equation solved for each
boundary between volumes. The equations are written in an explicitly conservative form as:
∑ =
boundaries
flux m
dt
dm
(1)
Equation (1): mass continuity equation.
2
2 1
4 () () 22 fp flux
flux
boundaries
udxA
CCuA dpA m u
dm D
dt dx dx
ρ
ρ
⎛⎞ − + ⎜⎟ ⎝⎠ =−∑
(2) Equation (2): Conservation of momentum equation.
()
.
() flux g gas wall
bound
dme dV
pmHhATT dt dt
=+ − − ∑
(3) Equation (3): Conservation of energy equation.
If the engine cylinder element has one zone, the entire cylinder is treated as one region. In the
latter, the cylinder is divided into two regions (unburned and burned), which share a common pressure.
The two-zone model is used to capture the chemical processes taking place during the combustion
period in more detail. Combustion models may be used either with a single or two-zone engine
cylinders, but for this research two zone modelswere used because of the problem with knock
combustion that was also examined. For the single zone model there is the energy equation refer to (4)
as below: () V P Q h m mu
nvalves
i
i i ∆ − − = ∆ ∑
=1
(4)
During combustion, the only term of enthalpy flow is mihi
due to propagation of the flame front to the
unburned zone. For the two-zone, refer to model (4), in the unburned zone we have:
0 ) (
0 1 0 0 1 1 = ∆ − + − + − ui ui u u u u u u u
h m Q V V P u m u m (5)
Using the equation of the state, it becomes:
0 0 1 1 1 0 0 1 1 = ∆ − + − + − ui ui u u u u u u u u u
h m Q PV T R m u m u m (6)
Similarly, for the burned zone we have:
0
0 1 1 1 0 0 1 1 = ∆ − + − + − bi bi b b b b b b b b b
h m Q PV T R m u m u m (7)
As a constraint, the volumes of the unburned and burned zones are summed up to the total cylinder
volume:
0
1 1 1 1 1 1 = − + c b b b u u u PV T R m T R m (8)
The last three equations are a complete set and are solved by using the Newton iteration method.
Since this article investigates unconventional piston motion, classical approach for solving
problems of volume changes cannot be applied. When the piston position differs from standard crank
piston motion, the imposed piston motion sub-model can be used for modeling the engine. The
formulation to calculate the instantaneous cylinder volume is identical to the one used in the standard
WAVE model, with the exception that the piston position, s, is linearly interpolated between points in
the user-entered profile. Smooth piston motion depends on the fine spacing of the crank angle array. In
this case enough large arrays were used to enable one-degree spacing. As far as the high-pressure part
of the cycle is considered, the most important process is the combustion. Without in-cylinder pressure
measurements, the combustion model had to be predicted based on typical forced induction Wiebe
function parameters. WAVE allows for three parameters in the Wiebe correlation to be input: 10-90
percent burn duration, 50 percent burn point, and the Wiebe exponent, described by eq. (9). In this
program, Ricardo Wave model of combustion can beselected between several options, ranging from
theoretical models with constant volume or constantpressure heat release, over Wiebe-function based
heat release models, to quasi-dimensional two-zonemodel of turbulent flame propagation. The SI
Wiebe function is widely used to describe the rate of fuel mass burned in thermodynamic calculations
[28].
1
1
WEXP
AWI
BDUR
Weθ
+
⎡ ⎤ ∆ ⎛⎞ − ⎢ ⎥
⎜⎟ ⎝⎠ ⎢ ⎥
⎣ ⎦
=− (9)
This relationship allows the independent input of function shape parameters and of burn
duration. The experimentally observed trends of premixed SI combustion are represented quite well.
In this paper the Wiebe one stage model of heat release has been chosen. The parameters of Wiebe
function were selected to achieve good agreement between modeled and experimentally recorded
pressure. Selected parameters have been successfully applied in the research [29-31]. Engine data that
was chosen for this research was presented in tab. 1. It can be noticed that valves open duration are
constant values, but position of maximum valve opening (EVMP and IVMP) are in certain ranges.
That is because of variability of piston motion, mechanism is constructed in that way that allow
different piston displacement and in the same time adjustment of valvetrain open phase.
Table 1. Main engine data
engine type spark ignition
engine cycle four-stroke
number of cylinders 2
number of valves per cylinder 4
bore 120 [mm]
stroke 30-177 [mm]
intake valve diameter 44 [mm]
exhaust valve diameter 40 [mm]
valves path 15 [mm]
EVDUR 235 [deg]
IVDUR 230 [deg]
EVMP 253.3-245[deg]
IVMP 479.3-471 [deg]
octane number 98
compression ratios 8-16
The valve train was modeled by setting up the appropriate number of valves per cylinder and
entering details about valve size, lift, and flow, for this purpose was chosen values which are different
from the conventional valvetrain. Reason for that can be found in the fact that piston dwell have
impact on valves open duration. So, in this concept, because of the piston dwell there is no need for
valve overlap, this can be seen from fig. 5. Valve data for each cylinder must be entered referencing a
valve model. The Lift Valve model was used in thisexample, so that the valve would follow a set
profile. The intake and exhaust valves were modeled using ducts and junctions, where geometry such
as length, orientation, and cross sections are specified. Heat transfer and friction data must also be
entered, in this model the selected valuesare similiar to the standard SI engine.
Figure 5. Valves lift without valve overlap for intake and exhaust valve respectively
Since in this paper was investigated only virtual engine model, for the purpose of model
calibration in this study was examined influence of selected input parameters for simulation of
ordinary IC engine. The calibration of simulating model was performed on ordinary spark ignition
engine on a test stand with adequate experimental equipment. It was realized through the comparison
of experimental and calculated results and tuning some model parameters and constants. Following the
procedure prescribed in the WAVE user manual the average values of all important values was
compared to test data. In order to validate the model with high degree of precision, it is important to
have as much engine test data as possible. For thisresearch model was calibrate to match experimental
data for 50 different operating conditions at full and partial load. In order to validate the parameters
calculated by Ricardo/WAVE software, engine data was recorded at a range of engine speeds between
2000 and 6000 [rpm]. The pressure histories were recorded in first engine cylinder and in two
characteristic points in inlet pipe of relating cylinder and compared with calculated curves. TDC must
be determined within 0.1 degrees in order to accurately calculate work (IMEP), so in order to avoid
serious error in the TDC determination caused by torsional vibration the test cylinder must be chosen
in multi-cylinder engine as the one immediately next to the crankshaft encoder.Piezoelectric pressure
transducer was used for the purpose of acquiring in-cylinder pressure data.For this experimental
investigation was used a special category of ECU (Engine Control Unit) which is programmable in
order to achieve different working parameters (air-fuel ratio, ignition timing, fuel injection, etc.).
