54 3 IC-Engine-Based Propulsion Systems
3.3 Fuel Consumption of IC Engine Powertrains 55 MVEG–95 cycle is found to be approximately 210 N. This yields an average traction power at the wheels of approximately
P¯trac =
F¯trac·v¯
trac = 210 N·9.5 m/s
0.6 ≈3.3 kW. (3.23)
The parametertracdenotes the time fraction in which the vehicle is in traction mode (see Fig. 2.6) and the mean power ¯Ptrac is the relevant information needed to compute the engine load.8
The powertrain is assumed to include a conventional cog-wheel gear box and friction clutch. Using (3.13) the power at the input of the gear box can be estimated to be
P1= 1
egb · P¯trac+P0,gb
= 1
0.97·(3.3 kW + 0.3 kW)≈3.7 kW. (3.24) The auxiliaries, including the electric power generator, and the friction clutch consume some of the power produced by the engine. In this approach the losses caused by the auxiliaries are taken into account by an additional average mechanical power ¯Paux of 0.25 kW. This value is rather low, i.e., the vehicle is assumed to have no power steering and no air conditioning.
According to (3.16), each start causes an energy loss of Ec= 1
2·Θv·ωw,02 = 1
2 ·mv·v02=1
2 ·850 kg·(3 m/s)2≈3.8 kJ. (3.25) In the MVEG–95 on average one start from rest occurs every kilometer. Since in this cycle the average velocity is 9.5 m/s, such an event takes place ev- ery 105 s. Accordingly, the average power consumed by the starts is around 3.8 kJ/105 s≈35 W.
In summary, during the traction phases the engine has to produce the average power ¯Pe= (3.7 + 0.25 + 0.035) kW≈4 kW. Therefore, assuming a mean piston speed of ¯cm = 6 m/s,9 the engine is operated with an average mean effective pressure ¯pme of approximately 2.5 bar. This value is obtained by inserting the engine parameters into (3.5).
The efficiency of the engine at that operating point is found using (3.6), whereas the numerical values fore(cm) andpme0(cm) are taken from Fig. 3.2
ηe= pme
pmf = e(cm)·pme
pme+pme0(cm) ≈ 0.4·2.5 bar
2.5 bar + 1.6 bar≈0.24. (3.26) Therefore, the average fuel power consumed by the engine in the MVEG–
95 cycle is approximately
P¯f =trac·P¯e/ηe= 0.6·4 kW/0.24≈10 kW. (3.27)
8 To obtain the correct average fuel consumption, the factor 1/tracwill be com- pensated later in (3.27).
9 For the engine specified this corresponds to approximately 2700 rpm.
56 3 IC-Engine-Based Propulsion Systems This corresponds to a fuel flow of
∗
Vf = ¯Pf/(Hl·ρf) . (3.28) Assuming standard RON–95 gasoline and inserting the corresponding numer- ical values of Hl = 43.5· 106J/kg for the fuel’s lower heating value and ρf = 0.75 kg/l for its density yields a fuel consumption of approximately 3.1·10−4l/s or, with the value of 9.5 m/s for the average speed in the MVEG–
95 cycle, of approximately 3.3 l/100 km.
So far it has been assumed that in all brakingand idling phases the engine is shut down. While it is easy to cut off fuel in the braking phases, automatic starters that avoid idling losses are more expensive and, thus, most engines have non-zero idling losses.
The idling fuel mean pressure can be estimated from (3.6) by setting pme = 0
pmf,0=pme0(ωe,idle)/e(ωe,idle) (3.29) from which the fuel flow follows to be
∗
Vf,idle=pmf,0· Vd Hl·ρf
·cm,idle
N·S . (3.30)
Choosingcm,idle= 2.5m/sas the idling mean piston speed10and assuming a four-stroke engine yields the following numerical values
∗
Vf,idle= 4·105Pa· 710·10−6m3
43.5·106J/kg·0.75 kg/l · 2.5 m/s
4·0.067 m ≈8.3·10−5l/s. (3.31) In the MVEG–95 cycle the engine is idling for approximately 300 s. Accord- ingly, in that cycle the fuel spent for idling is approximately
300 s·8.3·10−5l/s·100/11.4 km≈0.2 l/100 km. (3.32) This figure has to be added to the 3.3 l/100 km used for vehicle propulsion.
The sum of 3.5 l/100 km is the estimated total fuel consumption for the chosen example of a lightweight vehicle and downsized engine system. Cold-start losses and other detrimental effects not considered so far are likely to increase that figure somewhat.
3.3.3 Quasistatic Method
In this section a quasistatic approach is used to predict the fuel consump- tion of the vehicle and powertrain described in the previous section. The QSS toolbox serves as the computational platform for the powertrain modeling and simulation. Figure 3.11 shows the top layer of the resulting model de- scription. Readers familiar with Matlab/Simulink will immediately recognize the characteristic elements of that software tool.
10For the engine chosen in this example this corresponds to approximately 1100 rpm.
This is a reasonable value for such a small engine.
3.3 Fuel Consumption of IC Engine Powertrains 57
0 l/100km
0 w_gb
dw_gb
T_gb J*
IC engine
J*
v J
fuel consumption
t v
a
i
driving profile
w_w
dw_w
T_rad
i w_gb
dw_gb
T_gb
gear box v
a w_w
dw_w
T_w vehicle
Fig. 3.11.Top layer of the QSS model of the powertrain analyzed in this section.
detect violation of maximum torque limits and then stop simulation
inertia engine
1 J*
w_e
T_e w_e
T_e
theta_ICE
w_e
T_e status
detect engine idling w_e_idle
J*_fco J*_idle
T_fco 3
T_gb 2 dw_gb 1 w_gb
Nm rad/s2004006008000204060 1
15 l/h 2357
10
J*_e
0
0 0
fuel consumption
fuel consumption while idling
fuel cut-off torque (depends on engine speed)
fuel consumption in fuel cut-off (usually 0) engine idling speed
Fig. 3.12.Structure of the block “IC engine” of Fig. 3.11.
As an example, the contents of the block “IC engine” are shown in Fig. 3.12. The complete model, including all necessary system parameters, is part of the QSS toolbox package that may be downloaded at the URL http://www.imrt.ethz.ch/research/qss/. The interested reader is referred to that source for a detailed description of all elements of that module.
Simulating the behavior of this powertrain yields a total fuel consumption of 3.6 l/100 km in the MVEG–95 cycle. This value correlates well with the value of 3.5 l/100 km obtained in the last section.
In fact, as Fig. 3.13 shows, the engine is operated with many different load/speed combinations. Such a variability offers many opportunities for en- ergy optimization, particularly if more than one mechanical power source
58 3 IC-Engine-Based Propulsion Systems
m/s pme
10
4 bar
0.33 0.30
3 kW 10 kW
20 kW
cm
4 8 12 16
0
0.25
Fig. 3.13.Engine operating points of the example of this section for the MVEG–95 cycle. Also shown is the average operating point used in the previous section.
and energy storage device are available. Accordingly, for hybrid vehicles the average-point method may not be applied. In these cases, reliable fuel con- sumption estimations must be based on quasistatic simulations that include the supervisory control loops. Figure 3.13 must be analyzed with some care.
In fact, the distribution of the load/speed points must be complemented by information on the frequency of these points. A representation similar to the one shown in Fig. 3.14 helps to understand which engine operation points are relevant.
ω T
n
100
300
500 20
40
rad/s Nm
Fig. 3.14. Distribution and frequency of the engine operating points (square root of the number of counts for each non-zero engine load/speed point).
4
Electric and Hybrid-Electric Propulsion Systems
While in conventional ICE-based vehicles the energy carrier is a fossil fuel, electric and hybrid-electric propulsion systems are characterized by the pres- ence of an electrochemical or electrostatic energy storage system. Moreover, at least one electric motor is responsible — totally or partially — for the vehicle propulsion.
In this chapter, first purely electric vehicles will be briefly discussed. Then various types of hybrid-electric vehicles will be introduced. The subsequent sec- tions describe the quasi-stationary and the dynamic models of typical electric components of such vehicles, including electric motors/generators, electro- chemical batteries, and supercapacitors. The modeling representations of an electric power bus, a torque coupler, and a planetary gear set are added as separate sections due to the importance of the mentioned components in most hybrid-electric powertrains. A mean-value analysis of the energy consumption of various powertrain configurations concludes the chapter.