MÉTODOS DE MUESTREO DE ACEPTACIÓ N
4. La autoridad responsable (consumidor) considera como deseable la inspección reducida. El consumidor es quien debe decidir si los ahorros que se
3.5 PLANES DE MUESTREO PARA UNA PRODUCCION CONTINUA
To evaluate the optimisation result, a simulation is conducted here to compare the recovered air mass between two control strategies. This aspect of the research uses the
103
simulation model built in Chapter 4. The main difference of two control strategies is the initial air tanks pressure. The new control strategy is based on the optimized result. The relevant bus powertrain parameters are summarized in Table 5-5.
Table 5-5 Two air tanks pneumatic hybrid city bus parameter data
No. of cylinders (-) 6 Cylinder bore (mm) 105 Piston stroke (mm) 132 Displacement volume (l) 7.25 Compression ratio (-) 17.5:1 1st gear ratio (-) 6.9 2nd gear ratio (-) 4.13 3rd gear ratio (-) 2.45 4th gear ratio (-) 1.49 5th gear ratio (-) 1 Final drive ratio (-) 5.125 Air tanks volume (l) 151 Additional mass of air starter (kg) 15
Additional mass of air tanks (kg) 200
In Chapter 4, the research had developed two kinds of pneumatic hybrid powertrain differentiated by the number of the air tanks. The previous simulation result showed that compared with one air tank model, the fuel consumption of two air tanks model can be further reduced. As a result, the two air tank pneumatic hybrid engine is chosen to be the research object here to evaluate the control strategy and optimisation procedure. The two air tanks pneumatic hybrid engine structure is summarized and shown in Figure 5-11. In this configuration, two air tanks can separately receive the air from the cylinder in the CM and drive the air starter motor when cranking. There is no connection between these two air tanks in order to simplify the model.
104
Figure 5-11 Two air tanks pneumatic hybrid engine structure
The previous optimisation result shows that 5.2 bar air tank pressure is the best pressure for the braking energy recovery. The amount of air mass supplied to the air tank decrease as the air tank’s pressure increases during the braking. So the control strategy should keep the pressure of the air tank as low as possible to enhance the efficiency of energy recovery. But in order to start the vehicle without using an additional electric starter, the controller should always keep at least one air tank’s pressure is above 5.2 bar to support one engine cranking. The control strategy will compare the pressure of two air tanks at regular intervals. When the city bus starts to decelerate, the controller chooses the lower pressure air tank to recover the air. When the engine must be restarted, the lower pressure air tank is chosen for the air starter to reduce the pressure in the air tank in order to enhance the energy recovery efficiency in the future deceleration. Meanwhile, the controller monitors the pressure of two air tanks to keep at least one air tank at a sufficient pressure for the next cranking. The simulation considers the extreme case, with only enough air to support one cranking event, where the initial pressures of two air tanks are 5.2 and 3 bar respectively. If the vehicle cannot recover enough energy, after the next engine stop it will not start again.
Figure 5-12 shows the air tank pressure of the two air tanks model for the whole Braunschweig driving cycle. From this figure, it can be seen that the pressure of the air tanks increase respectively from 3 to 7 bar and 5.2 to 8.2 bar after one driving cycle. That means the air tanks become almost “Full” from almost “Empty” at the beginning.
105
This also proves that the braking energy is sufficient to implement stop-start operation during the driving cycle.
Figure 5-12 Air tanks pressure of two air tanks model during Braunschweig driving cycle
Figure 5-13 Air tanks pressure of two air tanks model during MLTB driving cycle
Figure 5-13 shows the pressures in the two respective as they vary during the MLTB driving cycle. The green line indicates the limitation of the lowest pressure of air tank to implement the stop-start operation and the best energy recovery pressure. The simulation starts with the initial air tanks pressure are 5.2 and 3 bar respectively. At the end of the simulation, the two air tanks pressure are 5.5 bar and 5.8 bar which means that the two air tank model can recover enough energy to support all the stop-start
106
operations during the MLTB driving cycle. In addition, the lower air tank pressure results in higher energy capture during the CM. As a result, the energy recovery potential of the two air tanks system will be higher.
The simulation results with both the Braunschweig and MLTB driving cycles in Figure 5-12 and Figure 5-13 also demonstrates that the braking energy recovery rate relates to the speed and number of stops of the vehicle. The Braunschweig driving cycle has less Stop– Start operations which can produce a higher mass of recovered air per vehicle stop, so can keep the pressure of air tank at a higher level compared with the MLTB driving cycle. Figure 5-14 shows a comparison of air mass recovery of two air tanks over the Braunschweig driving cycle. Tank 1 recover 2841.4 g and tank 2 recover 3536.1 g respectively. Compared with the pressure at the start of the cycle, it can be found with a lower initial pressure more air can be recovered in a single driving cycle because the lower air tank pressure can enhance the net recovery of energy.
Figure 5-14 Air mass recovery during the Braunschweig driving cycle
Table 5-6 shows the comparison of air mass recovered during the Braunschweig driving cycle between the new control strategy and the previous one in Chapter 4. The previous initial air tanks pressure, 5.2 bar and 4.5 bar, is shown to recover 6377.5 g air, 171.4 g less than the newly optimized initial air tanks pressure, 5.2 bar and 3 bar. This is because the lower air tank pressure can recover more air mass during the braking which is not only proved by the optimisation result, but also confirmed in Chapter 3. As a result,
107
the simulation starts with lower air tanks pressure can recover more air mass during the braking. Although the increment is only 2.7%, it still proves that optimising the initial air tanks pressure is worth doing for the research. This is because during the Braunschweig driving cycle, the optimised control strategy still can support all the Stop-Start operations which mean the air amount used to drive the air starter is not change. The 2.7% increment all come from the energy recovery efficiency increased during the braking. This means the optimisation of the initial air tanks pressure can increase the energy recovery efficiency.
Table 5-6 Comparison of air mass recovered during Braunschweig driving cycle between the new control strategy and the previous one
Air mass in the previous
control strategy (g) Air mass in the new control Strategy with optimisation result (g) Start End Increment Start End Increment Air mass in tank 1
(g) 840 3681.4 2841.4 840 2926.1 2086.1
Air mass in tank 2
(g) 500 4036.1 3536.1 500 4962.8 4462.8
Total air mass
recovered (g) - - 6377.5 - - 6548.9
Improvement (-) - - - (2.7%) 171.4
5.7 Conclusion
This analysis of vehicle behaviour through different bus drive cycles and during braking events, including a comparison of two configurations of the hybrid braking system points to the fully controllable hybrid braking system is the focus of the investigation.
The funding includes
(i) It indicates that in the urban area the bus’s braking power can reach to 80% of the total traction power, and the braking of the city bus is very gentle with most of the deceleration rate below 0.2 g, compared with the passenger car.
(ii) The fully controllable hybrid braking system is fit for the requirement of the pneumatic hybrid regenerative system which can either work at high efficiency
108
to recover as much braking energy as possible and supply the corresponding braking force.
(iii) The optimisation result shows that the 5.2 bar air tank pressure is the best pressure for braking energy recovery.
(iv) The model with the newly optimized initial air tanks pressure can enhance 2.7% the total air mass recovered during the braking compared with the previous control strategy which means the air tanks pressure is the function of the energy recovery rate.
These findings give the fundamental fact for the future research to develop the pneumatic hybrid city bus control strategy which not only can realize the Stop-Start function, but also other functions such as Boost function which will be discussed in Chapter 6.
109