SIMULATION MODEL OF PNEUMATIC

PROCEEDINGS OF THE INSTITUTE OF VEHICLES 3(107)/2016
Piotr Mróz1, Sebastian Brol2
SIMULATION MODEL OF PNEUMATIC-HYDRAULIC DRIVE
1. Introduction
The accumulation of energies is very important today. Gathering and storage of heat
from solar radiation examined since the early XIX century. Now is known that the
accumulated amount of heat depends heavily on participation of gas in Earth's
atmosphere [1]. Theorizes the impact human activity has on emissivity the so-called
greenhouse gases. It is estimated that about 20% of greenhouse gas emissions are
emissions from road transport [2].
Vehicle manufacturers through legal restrictions of emissions (f. e. Euro standards,
standards CO2), are obliged to implement technical solutions that contribute to fewer
and fewer greenhouse gases emissions by their vehicles. These solutions are mainly:
start / stop system, diesel - particulate filters, reactors for the chemical processing of
exhaust gases, alternative fuel, alternative drive, hybrid systems (plug-in systems).
Based on the literature, it was observed a trend in development of hydraulic hybrid
vehicles. These are vehicles in which the mechanical drive system is assisted by
hydraulic system (parallel hybrid - PHHV) or a mechanical system for the most part
replaced the hydraulic system (serial hybrid - SHHV).
Table 1. shows the strengths and weaknesses hydraulic hybrid vehicles (HHV) on
the background of the more popular and more studied hybrid electric vehicles (HEV).
Table 1. Advantages and disadvantages of hydraulic hybrid vehicle
ADVANTAGES
Better fuel economy than HEV (in some
cases)
Lower life cost than HHV
Lower accumulator degradation
Lower investment cost
DISADVANTAGES
Lower energy density in accumulators
than in other solutions
Lower range on hydraulic drive itself
Excessive power waist collected in
braking
Higher CO2 emission compared to PHEV
or EV
Hydraulic hybrid vehicles use energy stored in hydraulic accumulators. It was
estimated that they collect less energy than electric batteries, but have a high power
density [3]. In addition, an attempt to estimate the cost-effectiveness of reducing CO2
emissions through the introduction of a variety of hybrid technology [4]. Estimates show
that to reduce emissions can the most contribute hybrid plug-in. These solutions, where
it is possible to recharge energy for hybrid hydraulic replacement tank of compressed
gas on a fully charged or refueled the tank, as currently is the case for combustion
1
2
mgr inż. Piotr Mróz, doctoral student of Opole University of Technology,
dr hab. inż. Sebastian Brol, Opole University of Technology Professor, Department of Road and Agricultural
Vehicles, [email protected]
45
vehicles. Such solutions are now largely unexamined, but it is estimated that they can
bring the most cost-savings [5].
High impact on the profitability of the development of the hydraulic hybrid is only a
sort of hybrid. For comparison (Fig. 1.), the parallel hybrid electric and hydraulic bring
savings on a similar level. While the SHHV brings approx. 20% more savings than
SHEV.
Fig. 1. Precentage improovement in economic savings [5] where: CONV – conventional
vehicle, SHEV – series hybrid electric vehicle, SHHV – series hybrid hydraulic vehicle,
PHEV – parallel hybrid electric vehicle, PHHV – parallel hybrid hydraulic vehicle,
EV – electric vehicle, HEHV – hybrid electric hydraulic vehicle.
Known solutions of hydraulic hybrid vehicles usually use an internal combustion
engine as a generator of high pressure (series hybrid) or can work independently
(parallel hybrid), where the hydraulic system supports their work, eg. when climbing or
acceleration of the vehicle.
On the development of technology of hydraulic hybrid vehicles work many
companies (including Eaton, Bosch-Rexroth). At the present moment it was not found
any deployed passenger vehicle that use a hydraulic hybrid technology. Many vehicle
prototypes are available for testing for the media and presented at the motor shows. And
over from 2007 - 2016 was observed increase in the number of scientific papers about
the hydraulic hybrid vehicles.
Hydraulic hybrid vehicles have a relatively low range on hydraulic drive, which is
due to the low volume (20 - 40 dm3) of hydraulic accumulators. Fully discharge an
accumulator is after 300-1000 meters of driving. Distance could be increased if it were
possible to use an unlimited amount of hydraulic fluid.
2. Pneumatic-hydraulic drive
Based on the literature a solution was found that could significantly improve the
range of hydraulic hybrid vehicles. Figure 2. shows a the pneumatic-hydraulic drive unit.
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Fig. 2. Pneumatic-hydraulic power unit according to Shaw [6] where: AT - air tank,
DPV - directional pneumatic valve, AOC - air-oil energy converter, DHV - directional
hydraulic valve, HM - hydraulic motor.
This concept allows pumping hydraulic fluid by compressed gas from one
accumulator to another and to move the hydraulic motor. In such a system, the hydraulic
motor can work independently without the participation of the internal combustion
engine. In carrying out the cycles of pumping, it is possible to an unlimited number of
pumping hydraulic fluid at a limited number of compressed gas, the amount of which
can significantly increase the range of the vehicle compared to the known solutions are
hydraulic hybrid vehicles.
There is not known by authors any paper, where Shaw would develop his solution
for the vehicle, so an attempt to build a similar, modified pneumatic-hydraulic drive unit.
ATV-type vehicle equipped with a prototype of the pneumo-hydraulic drive unit, and in
table 2. below summarizes the most important parameters before and after the
modification of the drive system.
Table 2. Features of vehicle before and after modification
Features of vehicle before modification
GVW 360 kg
Engine capacity 149 cm3
Max. power 7,1 kW (7000 rpm)
Max. torque 10 Nm (6000 rpm)
Type of gear automatic
Max. velocity 65 kph
Drag forces (total)
Features of pneumatic-hydraulic drive
Tank capacity 44 000 cm3(30 MPa)
Gas Nitrogen
Accu. capacity 2000 cm3 (2 pcs.)
Max. torque 41 Nm (@15 MPa)
Gear 2 stage
Max. velocity 45 kph
50 N - 180 N
At the present moment, the most important modification of the Shaw’s basic system,
include specially paired hydraulic valves, which enables the system to maintain a
constant direction of rotation of the hydraulic motor, regardless of the direction of flow
of hydraulic fluid between hydraulic accumulators.
3. Simulation model
During an earlier work [7] simulation model in MatLAB Simulink environment
(Fig. 3.) was prepared,
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Fig. 3. Scheme of simulation model of pneumatic-hydraulic drive
where: T – air tank, A – pneumatic-hydraulic accumulator, LV - loading valve, DV –
drain valve mT0 – initial mass of gas in tank, mA0 – initial mass of gas in accumulator,
TT0 – initial temperature of gas in tank, T A0 – initial temperature of gas in accumulator,
pT0 – initial pressure of gas in tank, p A0 – initial pressure of gas in accumulator, p AP –
preset pressure of gas in accumulator, mT – mass of gas in tank, T – mass flow rate of
gas in tank, TT – temperature of gas in tank, T – temperature flow rate of gas in tank, pT
–pressure of gas in tank, VT – volume of gas in tank, mA – mass of gas in accumulator,
A – mass flow rate of gas in accumulator, T A – temperature of gas in accumulator, A –
temperature flow rate of gas in accumulator, pA –pressure of gas in accumulator, V A –
volume of gas in accumulator, TA – mass flow rate of gas between tank and
accumulator, TTA – temperature of gas between tank and accumulator.
The modeled gas process was adiabatic expansion of nitrogen from the tank to the
gas side of pneumatic-hydraulic accumulator. It was assumed that T = TA = A,
because pipe and valve connecting tank and accumulator, is not modeled. For simulation
it was also assumed a constant parameters and variables range, which shown in table 3.
Table 3. Significant constant and variable parameters of the simulation model
Parameter
Molar mass of nitrogen
Initial mass of gas in tank
Initial mass of gas in accumulator
Initial temperature of gas in tank,
Initial temperature of gas in accumulator,
Initial pressure of gas in tank,
Initial pressure of gas in accumulator,
Preset pressure of gas in accumulator
Volume of gas in tank
Volume of gas in accumulator
Sectional area of the flow
Constants of gas
Coefficient of discharge
Coefficient of thermal capacity
Density of nitrogen
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Symbol
Value
Unit
MN2
mT0
mA0
TT0
TA0
pT0
pA0
pAP
VT
VA
A
R
Cd
κ
ρ0
28
19,35
0,003
293,15
293,15
30
0,1
2÷15
0,044
0,0002÷0,002
7,85×10-5
8,31
0,85
1,4
1,25
kg/mol
kg
kg
K
K
MPa
MPa
MPa
m3
m3
m3
J/mol·K
kg/ m3
The model uses the ideal gas law, whereby calculation of mT0 and mA0 is given by
(1).
mT 0
pT 0VT
RTT 0

,
M N2
m A0
p A0V A
RTA0

,
M N2
(1)
Subcritical and supercritical mass flow rate of gas was calculated based on equations
(2)[8].
2
 1




 pA   pA   
pA
2





Cd

A

p




T
0 
  p   for p  0.528


1
p
T

 T   T  
m T  m A  
,
 1

p

 2   1
Cd  A   pT   0 
 for A  0.528

pT
  1

(2)
Subcritical mass flow rate of gas is when a ratio of the pressures in accumulator and tank
is greater than 0.528 [8], if not, it’s supercritical mass flow rate of gas.
Temperature change rate (3)[8] is different for tank and accumulator due to expansion of
gas from the tank and compressing gas in accumulator, also does not include heat
exchange with the environment, because the adiabatic character of the process.
  TT  m T  m T  TT
TT 
,
mT
  TTA  m TA  m T  TA
TA 
,
mA
(3)
In the simulation modeling of the process of pumping gas through the orifice.
Temperature of gas between tank and accumulator T TA calculated on the basis equation
(4)[9].
T
TTA  T
dt
p 
  A 
 pT 
 1

,
(4)
Using above equations and ideal gas law, can be calculated the pA and pT (5).
mT 0 
pT 
m T
dt RT
T
M N2
VT
m A0 
pA 
,
49
m A
dt RT
A
M N2
VA
,
(5)
4. Results of simulation
Simulation model allow estimate duration of the time of emptying accumulator what
means - expansion of the gas in accumulator from preset pressure to assumed
atmospheric pressure. Filling accumulator means - the gas in predetermined volume of
accumulator is compressed to preset pressure of gas in accumulator.
Simulations were carried out for parameters showed in table 3. Figure 4. compared
the emptying time of accumulator for two cross-section areas of valve.
Emptying time of
accumulator, t, s
6
Cross sectional area of valve: 20 mm2,
5
Cross sectional area of valve: 50 mm2
4
3
2
1
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Initial preset pressure of gas in accumulator, pAP, MPa
Fig. 4. Emptying time of accumulator through valves of cross-sectional areas:
20 and 50 mm2
These results show that doubling the cross-sectional area of valve allows reduces twice
the emptying time of accumulator. It is important for a control method for this type of
drive.
After emptying accumulator we should fill-in accumulator to preset pressure of gas.
But, after emptying, accumulator is assumed to be 90% full of oil. What means, we can
filling 10% volume of accumulator. Figure 5. show filling time of accumulator to preset
pressure from different pressure of gas in tank pT.
b)
0,50
0,40
0,30
0,20
30 MPa
25 MPa
20 MPa
15 MPa
10 MPa
Filling time of accumulator,
t, s
Filling time of accumulator,
t, s
a)
0,10
0,00
1
2
4
6
8 10
Initial preset pressure of gas in
accumulator, pAP, MPa
0,50
0,40
0,30
30 MPa
25 MPa
20 MPa
15 MPa
10 MPa
0,20
0,10
0,00
1
2
4
6
8
10
Initial preset pressure of gas in
accumulator, pAP, MPa
Fig. 5. Filling time of accumulator to 10% of volume through valve of cross-sectional
areas: a) 20 mm2 and b) 50 mm2
50
These results show impact of gas pressure in tank for filling time of accumulator and that
this time can be reduced by increasing cross-section area of the valve.
Based on simulation results and parameters from table 3., it was estimated that at
preset pressure of gas in accumulator 2 MPa is possible to make 151 cycles of emptying
and filling accumulator. Based on the assumed constant and parameters from table 3. to
appointed, for preset pressure of gas in accumulator p AP in range 2÷15 MPa, empirical
equation (6) allowing to estimate the possible number of cycles of emptying and filling
accumulator for any set pressure in this range.
i  4818,6  p AP
1,149
,
(6)
The equation can successfully applying this ratio to the range of the vehicle, thereby
facilitating the driver or autopilot, maintaining the driving profile set to overcome the
greatest possible distance.
5. Conclusion
 Manufacturers of components for hydraulic hybrid vehicles working to improve
the design of engines and hydraulic pumps and improve their efficiency and
reliability, and lower production costs.
 To improve control of hydraulic systems it is vital to increase the pressure at
which they operate hydraulic valves and improve their reliability.
 To improve the collection of hydraulic energy it is important to increase the
density of accumulated energy.
 The simulation model allows the modeling of the process of filling and emptying
of pneumatic-hydraulic accumulators,
 Simulation allows to estimate the duration of the filling and emptying of the
accumulator,
 This enables to estimate the number of cycles, and range the vehicle,
 These data allow to develop control strategies pneumatic-hydraulic drive unit.
References:
[1]
Wydawnictwo Naukowe PWN: Definicja efektu cieplarnianego (szklarniowego)
(pol.). [dostęp 11 kwietnia 2008],
[2]
United States Environmental Protection Agency (EPA) website,
http://www.epa.gov/climatechange/ghgemissions/sources/transportation.html
(link is external),
[3]
Baseley S., et al., Hydraulic Hybrid Systems for Commercial Vehicles, SAE
technical paper, 2007,
[4]
PSA Peugeot Citroën and Bosch developing hydraulic hybrid powertrain for
passenger cars; 30% reduction in fuel consumption in NEDC, up to 45% urban;
B-segment
application
in
2016,
Website,
http://www.greencarcongress.com/2013/01/psabosch-20130122.html,
[5]
Jia-Shiun Chen: Energy Efficiency Comparison between Hydraulic Hybrid and
Hybrid
Electric
Vehicles,
Energies 2015, 8, 4697-4723; doi:10.3390/en8064697,
[6]
Mróz P., Brol S.: Conception control system of pneumatic-hydraulic drive
system, Proceedings of the Institute of Vehicles x(xx)/2016, Warsaw, 2016,
51
[7]
[8]
[9]
Shaw D., Yu J., Chieh Ch.: Design of a Hydraulic Motor System Driven by
Compressed Air, Energies Vol. 6, pp. 3149-3166, 2013.
Federal Emergency Management Agency, U.S. Department of transportation,
U.S. Environmental Protection Agency.: Handbook of chemical hazard analysis
procedures, Washington, 1989, 1-800-367-9592 in Illanois pp. 394,
Bailyn, M. (1994). A Survey of Thermodynamics. New York, NY: American
Institute of Physics Press. p. 21. ISBN 0-88318-797-3.
Abstract
The article presents a simulation model of pneumatic-hydraulic drive. The main
point of this work is mathematical model, which is used to simulate the work of
pneumatic-hydraulic drive. This is further used for control system synthesis and to
formulate the control rules using Simulink.
The process modelled is generally adiabatic expansion of a diatomic gas. Additional
the low of gas from main reservoir to hydro-pneumatic accumulator is modelled and
simulated. The model uses the equation of state of ideal gas by Clapeyron and process
included subcritical and supercritical mass flow of gas.
The simulations show that the time to empty the accumulator is more than 3 times
longer than its filling. That determines the way of control of developed pneumatichydraulic drive.
Keywords: pneumatic-hydraulic, simulation, model, drive, vehicle, hydraulic hybrid
MODEL SYMULACYJNY NAPĘDU PNEUMATYCZNO-HYDRAULICZNEGO
Streszczenie
Artykuł przedstawia model symulacyjny pneumatyczno-hydraulicznej jednostki
napędowej. Głównym punktem artykułu jest model matematyczny, który
wykorzystywany jest w symulacji pracy pneumatyczno-hydraulicznej jednostki
napędowej. Wyniki służą do syntezy układu sterowania i sformułowania reguł
sterowania, używając środowiska MatLAB/SIMULINK.
Modelowany proces jest procesem adiabatycznego rozprężania dwuatomowego
gazu z zbiornika gazu do pneumatyczno-hydraulicznego akumulatora gazu. Model
wykorzystuje równanie stanu gazu wg Clapeyrona i uwzględnia podkrytyczny i
nadkrytyczny przepływ masy gazu.
Symulacje pokazały, że opróżnienie akumulatora trwa ponad 3-krotnie dłużej niż
jego napełnienie, co determinuje sposób sterowania rozwijanym układem
pneumatyczno-hydraulicznym.
Słowa kluczowe: pneumatyczno-hydrauliczny, symulacja, model, układ napędowy,
pojazd, hydrauliczna hybryda
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