Model of Supercapacitor-Starter Assembly Used for Internal Combustion Engines Starting Aurel Cornel STANCA*, Paul Nicolae BORZA**, Mihai ROMANCA**, Roxana PAUN**, Sorin ZAMFIR** *Technical College "Remus Radulet" Brasov, Romania, **"Transilvania" University of Braşov, Romania E-mail:
[email protected],
[email protected],
[email protected],
[email protected],
[email protected]
Abstract - The advantages of supercapacitors at the starting of internal combustion engines (ICE), as a complementary source to classical batteries, are nowadays certitudes: the significant extension of battery life span, the reduced volume and weight, meeting the environmental demands and lowering starting time and energy.
This paper presents a design flow and sizing solution in case of design of the car’s start-stop systems based on an empirical identification of ICE behavior during their starting process based on batteries.
The paper intends to develop a model of the electrical behavior of the assembly supercapacitor-starter driven by the ICE crankshaft, model allowing reasonable estimations of supercapacitors characteristics used for ICE starting. The development of the model was based on real signal analysis on a conventional battery-starter starting system.
II. STARTER MODEL FOR LINEAR VARIATIONS OF THE
I.
INTRODUCTION
The starting of internal combustion engines (ICE) on vehicles became in our days an important research topic due to new environmental constraints imposed to vehicle manufacturers. That is why the green cars solutions start from mild hybrid cars and go to electrical vehicles. In case of mild hybrid cars as a new implementation there is the start-stop system that allows switching off the ICE during short time stops and rapid starting the ICE at driver demand. Such car’s system forces significantly the batteries on vehicles [1]. Thus, the supercapacitors combined with batteries can overpass the technical difficulties related to the reliability and availability of the system and can offer a reliable starting of the ICE even in harsh conditions that means first of all in case of low temperature (below 0°C). The supercapacitor technology offers now several solutions for fast release storage devices able to provide in short time a large amount of energy. In Tab.1. there are presented supercapacitor technologies and their technical and market parameters [2]. TABLE 1. SOA - COMPARATIVE TABLE FOR AQUEOUS AND ORGANIC ELECTROLYTE
ABSORBED CURRENT
The functional characteristics of the starter are presented in Fig.1 [3], [4] as it follows: A is Ust(Iabs) characteristic – starter voltage versus absorbed current from battery, Iabs; it is a straight line passing through (Isc,Usc) and (Inl,Unl) points, where Isc and Usc are starter current respectively voltage in short-circuit operation mode, Inl and Unl are starter current respectively voltage in no load operation mode; B is n(Iabs) characteristic – speed versus absorbed current from battery ; C is Pu(Iabs) characteristic – effective power versus absorbed current from battery; D is torque characteristic M(Iabs). The starter operation modes are the following: a) Short-circuit - in this mode the speed is zero, the absorbed current is maximum (Isc), the torque is maximum and effective power is zero; b) Full–load conditions - when engine starts to rotate, the resistant torque decreases and power is produced reaching maximum value when Iabs=Isc/2; c) No-load - when the engine started, the starter torque and power are zero, the absorbed current Inl being consumed only for loss compensation.
Unl
C
A
D
SUPERCAPACITORS
Electrolyte / Tech nology Nonaqeous woundrolled Aqueous based stacked
Voltage (V)
Energy content (Wh)
Energy density (Wh/kg)
Power density (kW/kg)
CostE (€/Wh)
CostN (€/kW)
360 – 400
300350
2.9
1.5 – 2.0
35 - 40
30-40
360 – 400
300350
target 4-6
target 2-3
target 30
target 9
Usc
B Inl
Isc/2
Iabs Isc
Fig.1. The functional characteristics of the starter.
U b − Rb ⋅ I L IL
( 1)
where Ub is off-load battery voltage and Rb battery internal resistance. Fig.2. shows time evolution of starter resistance for three different periods of the ICE starting and linear variation of absorbed current, from Isc to Inl. The equation of these plots is U b ⋅ t st ( 2) Rst = − Rb I sc ⋅ t st − (I sc − I nl ) ⋅ t
where tst is start time of ICE and t is time. III. STARTER MODEL FOR NON-LINEAR VARIATIONS OF THE
The experiments performed on two conventional ICE starting systems – passenger cars, Dacia 1310 Li and VW Golf 3 - in similar temperature conditions (-10°C ambient temperature), and having lead acid batteries of 44Ah, respectively, 45Ah capacities, with eight years of operation [7] conducted to the following results described in Tab.2. Both cars were equipped with petrol engines with engine control units (ECU). These results were obtained as statistical data processing of a lot of records (30 tests for each car) made on above mentioned cars. The specific profile of each starting system is presented in Fig.3. Load mode
ABSORBED CURRENT
Short-circuit mode 14 12
8
300 250 200
I( )
6 4
No load mode
0
0 0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
Load mode
300
U (
Voltage (V )
200
I( )
6
Fig.2. Time evolution of starter resistance at linear variation of absorbed current, for different starting periods.
150
No load mode 100
4
Starter engaging
50
Starter disengaging 0
t
250
8
0
t3
350
12
2
t2
1 1.1 1.2
Time (sec)
Short-circuit mode 14
b)
t1
50
Starter disengaging
10
Rst,nl
150 100
Starter engaging
2
a)
Rst
Rst,sc
350
U (
10 V oltage (V )
For the study of Internal Combustion Engines (ICE) starting sequence, it was used a data acquisition (DAQ) and driving platform that included a National Instruments (NI) USB6009 (for analog and digital DAQ) with the following characteristics [5]: 8 analog inputs, 14 bits/sample, 48kS/s; 2 analog outputs, 12 bits digital-to-analog conversion; 12 TTL/CMOS digital inputs/ outputs; 1 timer - 32 bits counter, with 5MHz clock; impedance of analog inputs = 144kΩ; PC interface via USB port. and an original conditioning and driving module, using as static switches the „PROFET” BTS555 transistor (Smart High side High Current Power Switch Reverse) [6] – with the following features:
Current (A )
R st ( I L ) =
Operating voltage: 5-34V; Overvoltage protection (including load dump); Overload protection; Over temperature protection; Clamp of negative voltage at output; Fast de-energizing of inductive loads; Electrostatic Discharge (ESD) protection; Current limitation (165A); Short circuit protection (520A); Bidirectional low resistance operation (2.5mΩ); Diagnostic feedback with load current sense; Open load detection via current sense; Loss of input voltage protection.
0.1
0.2
0.3
Current (A )
Quantitatively, the values of identified parameters (Isc, Inl, Usc, Unl, characteristic profiles) differ depending on: (vehicle) engine power, starter power, maintenance and wear conditions of mechanical and electrical system, temperature, battery charging state. So, the starter electric behavior is defined by characteristic A from Fig.1. and starting time. The starter equivalent resistance Rst during starting can be related to the absorbed current IL(t) and has the value given by the formula:
0.4
0.5
0.6
0.7
0 0.8
0.9
Time (sec)
Fig.3. Vehicle starting from battery supplied starter: a) VW Golf 3, b) Dacia 1310 Li.
I=
TABLE 2. EXPERIMENTAL RESULTS DURING VEHICLE STARTING. Parameter Efficient energy Wu, (kJ) Peak of released power Pmax, (W) Starter engagement time, (ms) Starting time tp, (s) Period of phase 1 of short-circuit Tsc1, (ms) Period of phase 2 of short-circuit Tsc2, (ms) Peak load period Ts, (ms) No load period Tnl,(ms) Short-circuit current amplitude Isc, (A) Current amplitude at the beginning of load mode Is1, (A) Current amplitude at the end of load mode Is2, (A) Current amplitude in no load mode Inl, (A) Oscillation amplitude in phase 1 of short-circuit mode Ivsc1, (A) Oscillation amplitude in phase 2 of short-circuit Ivsc2, (A) Oscillation amplitude in load mode Ivs, (A) Number of cycles in load mode, n Decreasing rate of oscillations amplitude in operation mode ΔA, (%) Decreasing rate of oscillations period in operation mode ΔT, (%)
( 3)
Dacia 1300 Li 0.94 – 2 2363 - 2600 20 - 30 0.8 – 1.45
VW Golf 3
during period Tsc2:
1.5 – 1.65 2877 – 2905 30 - 31 1.2 – 1.35
I=
22 – 31
29 – 43
134 – 193
145 – 180
145 -183 35 – 69
125-225 200 – 220
I (i) =
238 – 275
296 – 300
where I VS (i ) = (1 − i ⋅ ΔA) ⋅ I VS and TVS (i ) = (1 − i ⋅ ΔT ) ⋅ TVS
150 – 184
165 – 175
115 – 141
80 – 90
38 – 54
60 – 64
19 – 38
6 – 10
16 – 51
5 – 10
44 – 63
22 – 25
2-5
4–5
4-7
22 - 25
8 - 10
10 - 11
The parameters from Tab.2. are defined in Fig.4. The values of the parameters for each system (with its specific engine type, starter, wear and maintenance conditions, engine oil type and viscosity, ignition quality, battery type and wear) [4] are spread in a relatively broad range, depending on: starting type (“cold starting ” or “hot starting”), number of successive starts, initial piston position and temperature. In order to obtain the characteristic starting record of engine (see Fig. 4) a selection was made between records performed and was chosen the set of records that have presented a high coherency given by the inter-correlation function calculated between every two different records (acquired signals from starting circuit of the car in similar conditions). The higher values of short-circuit periods Tsc1 and Tsc2, peak load periods Ts, as well as oscillation amplitudes in shortcircuit and load Ivsc1, Ivsc2 and Ivs, correspond to the first starting, the “cold” starting, the most severe for the battery. There is a difference between the starter model of linear variation of absorbed current and real behavior emphasized by the profile of real diagram I(t), Fig.3. and 4. Using the identified parameters in Fig.4. and their values from Tab.2., the prior model given by equation (2) can be enhanced considering the non-linear variation as it follows: during period Tsc1:
⎞ ⎛ 4π I S1 − I SC ⋅ t + I SC + I VSC1 ⋅ sin ⎜⎜ ⋅ t ⎟⎟ 3 TSC1 + TSC 2 T ⎝ SC1 ⎠
⎛ 2π ⎞ I S1 − I SC ⋅ t + I SC − I VSC 2 ⋅ sin⎜⎜ ⋅ (t − TSC1 ) ⎟⎟ TSC1 + TSC 2 T ⎝ SC 2 ⎠
( 4)
during period (n+1/4)Ts: ⎛ 2π ⎞ IS 2 − IS1 ⋅ (t − TSC1 − TSC2 ) + IS1 − IVS(i) ⋅ sin⎜ ⋅ (t − TSC1 − TSC2 ) ⎟ ⎜ ⎟ (n + 0.25) ⋅ TS ⎝ TVS(i) ⎠
( 5)
and during interval Tnl I=
I nl − I S 2 ⋅ (t − TSC1 − TSC 2 − ( n + 0.25) ⋅ TS ) + I S 2 − I VS Tnl
( 6)
The enhanced Labview model of the battery-starter system was implemented and Fig.5. describes the signals generated by the model for the following mean values of the parameters: Tsc1=24ms, Tsc2=148ms, Ts=149ms, Tnl=55ms, Isc=260A, Is1=157A, Is2=127A, Inl=46A, Ivsc1=29A, Ivsc2=29A, Ivs=50A. For the battery, it was used the mathematical model elaborated at the Massachusetts University [8], [9], that consists of three parts: The model of the capacity; The model of the voltage; The model of the life-cycle. The present model incorporated the first two parts: (i). The model of the capacity is represented by the equation: Qmax(I)=qmax,0⋅k⋅c⋅T/(1-e-kT+c(k⋅T-1+e-kT))
[Ah]
(7)
where the constants qmax,0, k, c represent: qmax,0 –maximum capacity, for an infinitesimal current [Ah]; k – charging/discharging rate [h-1]; c – [dimensionless] ratio of available capacity and total capacity. Processing the experimental data from a battery with a nominal voltage of 12V and the capacity of 50Ah, in a discharging regime of 20h, the following values were obtained for these constants: qmax,0 = 86.1031 Ah k = 0.5874 h-1 c = 0.3747 (ii). The model of the voltage is represented by the equation: E=E0+A⋅X+C⋅X/(D-X)
(8)
X= (qmax(I)-q)/qmax(I).
where X=q/qmax(I).
(9)
At discharging, X is defined (in terms of consumed charge) by the equation
(10)
Processing the experimental data, the following values of the constants were obtained: At charging, E0=12.3097V; A=0.8616V; C=0.1798V; D=1.0260 and at discharging, E0=12.5978V; A= -0.7077V; C= -0.0823V; D=1.1311.
Starter current
TSC1
IVSC1 TS IVSC2 TSC2
IVs
ISC IS1 IS2 Inl
Tnl Time
Fig.4. Definition of parameters Tsc1, Tsc2, Ts, Tnl, Isc, Is1, Is2, Inl, Ivsc1, Ivsc2, Ivs of starting current.
U(t)/12.6V
P(t)/ 2,5kW
I(t)/300A R(t)/0.3Ω
Fig.5. Normalized signals -voltage, current, power and equivalent series resistance simulated on enhanced model.
it was implemented in LabVIEW a simulation program for ICE starting. In Fig.6. there are presented wave forms of normalized current and voltage on starter supplied by supercapacitor, simulated for two cases : a) using a supercapacitor of 50F capacity and 20mΩ internal resistance, three-cycle in-load operation; b) using a supercapacitor of 100F and 2mΩ internal resistance , four-cycle in-load operation .
U(t)/12.6V
V. SIZING METHODOLOGY OF RAPID RELEASE STORAGE
I(t)/300A
DEVICES USED FOR STARTING SYSTEM OF PETROL ENGINE VEHICLES a)
U(t)/12.6V
I(t)/300A
b) Fig.6. Wave forms of normalized current and voltage on starter when supplied by supercapacitor with a) C=50F, RSC=20mΩ and b) C=100F, RSC=2mΩ, simulated.
The battery model of the voltage reflects the fact that this parameter depends on: The status of the battery (in charging or discharging regime); The battery’s State of Charge (SoC); The series resistance of battery RB-ESR; The amplitude of battery charging or discharging characterized by the Deep of Discharge parameter (DoD) [10]. IV. SIMULATION OF VEHICLE STARTING USING THE SUPERCAPACITOR
Being given the functional model of the starter driven by the ICE crankshaft and having the supercapacitor model [11], [12]. U SC = U SC 0 + C
dU SC − RSC I , dt
(11)
in which USC is supercapacitor voltage, USC0 initial supercapacitor voltage, RSC – supercapacitor internal resistance, C – supercapacitor capacity, I – discharge current,
As a result of the analysis of the starting process of the petrol engines above mentioned, it was developed a method for dimensioning the storage of electrical energy devices consisting of the following steps: 1. Identification the ICE characteristics by performing minim 10 successive records of the voltage and current variation during starting process (V(t) & I(t)). 2. Processing of the 10 record sets for several relevant temperatures, mainly below 0°C. 3. Identification of pairs of values (starter current-time) defined by the model described in Tab.2. and Fig.4 (phases 1 of short-circuit, 2 of short-circuit , peak load and no load). 4. Calculation of required energy for the starting process, according to results of steps 1-3. 5. Verification of the sizing adopted by simulation on LabView model of ICE starting process. An example that reflects what is proposed for sizing the supercapacitor included into the starting system is presented below. It was simulated the supercapacitor starting with capacities of 50F, 75F, 100F and 125F, with internal resistance ranging 2-20mΩ [13]. The battery internal resistance was Rb=15mΩ and voltage 12.6V (the same as initial supercapacitor voltage). There were determined the efficient energy characteristics in function of supercapacitor internal resistance, Wu(RSC), Fig.7. In order to determine the range of values of supercapacitor capacity and internal resistance it was considered a „heavy” battery starting, with five-cycle, in-load operation, lasting 1.09 seconds and providing the energy of 1.69 kJ. The parameters used in starter model are: Tsc1=25ms, Tsc2=135ms, Ts=175ms, Tg=55ms, Isc=260A, Is1=160A, Is2=135A, Ig=45A, Ivsc1=35A, Ivsc2=25A, Ivs=55A. The characteristics reveal that 50F capacity supercapacitor, with any value of internal resistance cannot release enough energy required for starting and 75F capacity supercapacitor can release required energy but only for very small internal resistances. The other two supercapacitors of
1900
VI.
1800 1700
C
Wu (J)
1600
50F
1500
75F
1400 1300
100F 125F
1200
Battery
1100 1000 2
4
6
8
10
12
14
16
18
20
Rsc (mohmi)
Fig.7. Supercapacitor characteristics Wu(RSC) at ICE starting.
100F and 125F capacity, may deliver energy for internal resistance under 6mΩ, respectively, under 8mΩ. It must be noticed that the variation of internal resistance has a greater influence than capacity variation to the delivery of required energy. The behavior of 100F supercapacitor being reasonable, even at higher internal resistances, it must be foreseen how many n cycles from in load starting (implicitly starting times or starting severity) may deliver the required efficient energy. Fig.8. represents the dependencies Wu(n) for C=100F supercapacitor, for several internal resistance values. It must be remarked once again the importance of supercapacitor internal resistance reflected on efficient energy. The 100F, 20mΩ supercapacitor cannot deliver useful energy neither for 3 cycles; the 100F, 10mΩ supercapacitor may deliver useful energy at starting with maximum 3 cycles; the same rule for RSC=6mΩ and n=5 cycles. Additional simulations lead to maximum number of cycles for 4mΩ and 2mΩ: n=7, respectively, n=8 cycles. It may be concluded that a supercapacitor with 100F capacitance and an ESR below 4mΩ is adequate for harsh condition starting processes of Dacia 1300Li car. 2100
The starting model using supercapacitor-starter system described in the paper may be adapted to every starting system giving particular values for time and current amplitude parameters in short-circuit, load and no load modes. According to the simulated behavior the supercapacitor used for ICE starting can be dimensioned in such a manner to satisfy required conditions, timing and consumed energy. Further real time experiments revealed that consumed energy in supercapacitor starting is significantly reduced (approximate -35%) synchronously with the reduction of starting time for ICE (above -45%). As consequence of the new method proposed for sizing the rapid release storage device of ICE’s starting system the weight of car is decreasing and also the volume occupied by battery and supercapacitor. VII. ACKNOWLEDGEMENTS The research work presented was partially supported by Romanian Ministry of Research and Education, contract PNII-21018/2007. VIII. [1] [2]
[3] [4] [5] [6]
[7]
1900
Rsc 2 mohm
1700 Wu (J)
4 mohm 6 mohm
1500
[8]
10 mohm 20 mohm
1300
Battery
[9] [10]
1100
[11]
900 3
4
5
6
Number of cycles in-load operation
[12] Fig.8. Wu(n) characteristics for different 100F supercapacitor internal resistance.
CONCLUSIONS
[13]
REFERENCES
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