158 5 Non-electric Hybrid Propulsion Systems
The combination of mainly mechanical components makes this engine sys- tem a very cost-efficient alternative to hybrid-electric propulsion systems. The additional weight can be kept very small since only a low-pressure air tank is used. Of course the fuel economy potential offered by the additional degrees of freedom can only be realized by a well-designed supervisory control system.
In Chap. 8.8 a case study is presented in which this problem is analyzed.
5.6.1 Modeling of Operation Modes
Basically, two independent groups of operation modes of the engine system have to be investigated:
• combustion engine mode, and
• several pneumatic modes.
All the modes can be best explained by plotting the thermodynamic cycle in ap–V diagram [137]. Since for any polytropic expansionp·vγ is constant, a double logarithmicp–V diagram is used in Figs. 5.25 – 5.30. In these figures, all polytropic processes are represented by a straight line. The dashed lines represent operating cycles with a higher torque than those depicted with solid lines.
Conventional ICE Mode
The cyclic operation in conventional ICE mode (throttled operation) is rep- resented in Fig. 5.25 as a succession of eight ideal transformations: (i) a poly- tropic compression from bottom dead center (BDC) to top dead center (TDC), (ii) an isochoric combustion at TDC, followed by (iii) an isobaric part of the combustion, (iv) a polytropic expansion from end of combustion to BDC, (v) an isochoric expansion at BDC from cylinder pressure to exhaust pressure, (vi) an isobaric exhaust stroke from BDC to TDC, (vii) an isochoric expan- sion at TDC from exhaust pressure to intake manifold pressure, and (viii) an isobaric intake stroke from TDC to BDC.
The manifold pressure varies from levels below ambient to higher than ambient if the turbocharger supplies enough fresh air. For a comparison with a typical efficiency map of the conventional combustion mode the reader is referred to Fig. 3.2.
Pneumatic Supercharged Mode
The pneumatic supercharged mode is used for operating points at which the turbocharger does not yet provide enough air. As schematically shown in Fig. 5.26, during compression the charge valve is opened (CVO) and closed again (CVC), at the latest when the tank pressure is reached. The tank pres- sure and the timing of this short opening determines the amount of additional
5.6 Pneumatic Hybrid Engine Systems 159
EVO
TDC BDC
logV logp
IVO EVC
IVC
Fig. 5.25.Double logarithmicp–V diagram of the conventional engine mode. IVO:
intake valve opening, IVC: intake valve closing, EVO: exhaust valve opening, EVC:
exhaust valve closing. Dashed line: cycle with higher torque.
air in the cylinder. The amount of fuel in the cylinder has to be metered to match the amount of air coming both from the intake manifold and from the tank.
EVO
TDC BDC
logV logp
IVO EVC
IVC CVO
CVC pt
Fig. 5.26.Double logarithmicp–V diagram of the pneumatic supercharged mode.
Nomenclature as in Fig. 5.25, plus CVO: charge valve opening, CVC: charge valve closing.
Pneumatic Undercharged Mode
For a demanded engine torque lower than that achieved at naturally aspirated full load conditions, the excess air can be used to fill the tank. As shown
160 5 Non-electric Hybrid Propulsion Systems
schematically in Fig. 5.27, during the compression stroke the charge-valve is opened at a crank angle where the cylinder pressure is equal to the tank pressure. The duration of the opening determines the amount of air remaining in the cylinder. For this mode, a direct fuel injection system is crucial because for safety reasons no fuel is allowed to enter the air tank. Accordingly, fuel injection can start only after CVC.
EVO
TDC BDC
logV logp
IVO EVC
IVC CVO
CVC pt
Fig. 5.27.Double logarithmicp–V diagram of the pneumatic undercharged mode.
Nomenclature as in Fig. 5.26.
Pneumatic Pump Mode
The pneumatic pump mode is used for recuperating energy during braking.
A two-stroke operation, illustrated in Fig. 5.28, yields the highest braking torque. The pressure tank is provided with enthalpy which can be used later in the pneumatic motor mode. The braking torque can be varied by changing the timing of the charge valve and of the intake valve simultaneously. For instance, the dashed-line cycle of Fig. 5.28 provides a less negative torque at the crankshaft than the solid-line cycle, thus a smaller braking torque results.
Starting with ambient pressure at BDC, a polytropic compression brings the cylinder pressure up to the pressure of the tank. Then, the charge valve opens and an enthalpy flow into the tank starts. The timing of the charge valve closing (CVC) determines the braking torque. The highest braking torque is reached if CVC is at TDC. Leaving the charge valve open for a longer period reduces the braking torque because the tank pressure acts on the piston down- stroke again. When the cylinder pressure again reaches manifold pressure, the intake valve remains open until it closes at BDC.
The thermodynamic performance of the pneumatic pump operation is de- scribed by the coefficient of performance (COP). The COP is defined as the
5.6 Pneumatic Hybrid Engine Systems 161
TDC BDC
logV logp
IVO IVC
CVO CVC
pt
Fig. 5.28.Double logarithmicp–V diagram of the pneumatic pump mode. Nomen- clature as in Fig. 5.26.
1.2 1.6
2
2.4
3
4 6
pmi [bar]
Tank pressure pT [bar]
!2 !1.5 !1 !0.5 0
0 4 8 12 16 20
Fig. 5.29.Iso-COP lines for the pneumatic pump mode.
variation of the internal energy in the air tank divided by the engine work out- put per cycle. The latter is normalized asmean indicated pressure, in analogy to the approach of Sect. 3.1.2. Thus the COP is calculated as
COP = ∆Ut
pmi·Vd
. (5.49)
The variation of the internal energy in the air tank is, by definition,
∆Ut=cv·∆(ϑt·mt), (5.50)
162 5 Non-electric Hybrid Propulsion Systems
where ϑt is the temperature of the air accumulated in the tank and mt its mass. Using the ideal gas law pt·Vt =mt·R·ϑt, the value of ∆Ut can be related to the variations of pressure
∆Ut=Vt ∆pt
γ−1 , (5.51)
where γ =cp/cv = (cv+R)/cv is the ratio of specific heats (approximately 1.4 for air). Since the increase of internal energy is not only caused by the engine work output, but also by the “free” enthalpy of fresh air inducted by the engine, the term coefficient of performance is chosen rather than efficiency.
This definition corresponds to that used for heat pumps.
Figure 5.29 shows typical values of the COP of the pneumatic pump mode as a function of pmi (i.e., indicated torque). For a given tank pressure, only a limited range of brake torques can be applied in an efficient way. Higher torques would be possible, but then the additional work would not lead to a higher enthalpy flow to the tank, but rather would be lost by throttling, back- flow, or heat transfer. Depending on the temperature of the engine, it might thus be preferable to use the conventional brakes to provide this additional braking torque. Highest levels of COP can be achieved when the tank is empty.
Pneumatic Motor Mode
The pneumatic motor mode enables a fast start of the engine and even the launch of the vehicle. High and smooth torque transients can be provided running the motor in two-stroke mode. The pneumatic motor mode enables fast stop–start, stop-and-go operations without causing any emissions and launching the vehicle without using the clutch.
The load control in this mode is effected by varying the charge valve closing (CVC) and exhaust valve closing (EVC) simultaneously, as shown in Fig. 5.30.
The efficiency-optimal timing is discussed in [242].
Figure. 5.31 shows the COP of the pneumatic motor mode as a function of tank pressure andpmi. In contrast to the pump mode operation, the range of feasible torques is much larger. Since the pneumatic motor is running in two- stroke mode, a very high torque can be applied on the crankshaft. An average charge-discharge efficiency of≈ 70% can be assumed, neglecting mechanical friction.
5.6 Pneumatic Hybrid Engine Systems 163
TDC BDC
logV logp
EVO
EVC CVO
t CVC p
Fig. 5.30.Double logarithmicp–V diagram of the pneumatic motor mode. Nomen- clature as in Fig. 5.26.
0.1 0.2
0.3
0.3
0 0.4
0.47
pmi [bar]
Tank pressure pT [bar]
0 5 10 15 20
0 2 4 6 8 10 12 14 16 18 20 22
Fig. 5.31.Iso-COP lines for the pneumatic motor mode.
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6
Fuel-Cell Propulsion Systems
This chapter contains three sections. The first section briefly describes the application of fuel-cell systems as stand-alone energy sources for powertrains or, in combination with a storage system, as fuel-cell hybrid powertrains. The second section introduces some thermodynamic and electrochemical models of fuel cells, as well as some fluid dynamic models of the complete fuel-cell sys- tem. The last section introduces the on-board production of hydrogen through fuel reforming and presents some system-level models of methanol reformers.
The main objective of this chapter is to introduce models that are useful in the context of energy management. Readers interested in the low-level control of fuel-cell systems are referred to [203].