Performance Analysis

Figure 3-14 shows the p-V diagrams for various air tank pressures at 1500 rpm simulated by the theoretical model. The figure indicates that different air tank pressure can cause different engine brake torque performance during the CM. It can be seen that the lower air tank pressure can create more brake torque which is shown in Figure 3-14 as the biggest area surrounded by the red line. The 7 bar air tank pressure lead the lowest brake torque which has the smallest area p-V diagram. This is because the lower air tank pressure causes an earlier check valve open time which resulting in long piston working time. Thus, the engine brake torque is a function of tank pressure.

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Figure 3-14 p-V diagram at 1500 rpm engine speed

Figure 3-15 not only validates the above conclusion that lower air tank pressure creates a high brake torque, but also indicates that not only at 1500 rpm, but from the engine speed from 500 to 2000 rpm, at intervals of 500 rpm, large engine speed and small air tank pressure are both beneficial to improve the engine braking performance.

Figure 3-15 Brake torque for various air tank pressure and engine speed

Figure 3-16 shows when the engine speed at 1500 rpm and 5 bar air tank pressure, the engine brake torque for the different CREB intake valve second opening duration and closing time. The IVSC gives out when the CREB intake valve will second close, for

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example, ‘IVSC=0’ means the CREB intake valve second closing happens at piston exactly reaches the TDC; ‘IVSC=10’ means it will close at 10° CA after the piston reaches TDC. The duration means the time that the CREB intake valve second opening will last. A larger number of duration means a long second opening time of the CREB intake valve. From the figure, it can be learned that, if the CREB intake valve closing time after TDC is fixed, the longer second open duration decreases the brake torque performance of the engine. In other words, it means earlier second open the CREB intake valve during the piston upstroke, resulting in lower brake torque. The reason is when the CREB intake valve second opens, the cylinder connects the auxiliary chamber resulting in an increased volume and decrease the compression ratio which leads the cylinder pressure cannot be compressed to high pressure, as a result, the low braking performance.

Figure 3-16 Brake torque for different intake timings

The figure also shows that the CREB intake valve closure after TDC can affect the engine brake performance. If the CREB intake valve closure is exactly at TDC, it always has the maximum engine brake torque output no matter how long it opens. This is due to if the CREB intake valve close after the piston reaches TDC, which means the piston start its downstroke, the cylinder pressure starts to decrease, the air from the auxiliary chamber may flow back to cylinder and do positive work to propel the piston resulting in low brake torque. Basically, the CREB intake valve closing more early after TDC can get more engine brake torque output when the open duration is bigger than 90°CA. When the second open duration is less the 90°CA, later close the CREB intake valve sometimes can

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create bigger brake torque output if the CREB intake vale second opening duration is too short. This is because, for this situation, the short CREB second opening cannot supply enough time for the air in the cylinder be compressed into the auxiliary chamber before the piston reaches the TDC. Which means the cylinder pressure can be compressed higher resulting in the high brake torque.

Figure 3-17 shows how the different actual compression ratios affect the engine brake torque performance at different engine speed. From the figure, it can be seen that the higher actual compression ratio creates more engine brake torque output. This is because, the higher actual compression ratio, the higher peak cylinder pressure can be achieved before the piston arrives TDC, resulting in high engine brake torque. By the observation of brake torque sensitivity to the actual compression ratio, it can be concluded that brake torque is a strong function of the actual compression ratio.

Figure 3-17 Brake torque for various engine speeds and the actual compression ratio

3.4 Conclusion

In this chapter, the principle of operation has been introduced. The potential of the pneumatic hybrid system to both generate a supply of compressed air and to manage the contribution of the engine to vehicle braking had been confirmed. The pneumatic hybrid engine CM is considered from a theoretical perspective based on an air cycle analysis and validated. The effects of the air tank pressure, engine speed, intake valve second

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opening time and actual compression ratio are evaluated. The simulation results demonstrate

(i) The lower air tank pressure can create more brake torque at the same engine speed which means small air tank pressure is beneficial to improve the engine braking performance. This is because the lower air tank pressure causes an earlier check valve open time which resulting in long piston working time.

(ii) The longer CREB intake valve second opening duration decreases the brake torque performance of the engine. The reason is that the cylinder pressure cannot be compressed to high pressure, as a result, the low braking performance. (iii) It always has the maximum engine brake torque output when the CREB intake

valve second closing time is right at TDC which means IVSC is 0°CA. This is due to if the CREB intake valve close after the piston reaches TDC, which means the piston start its downstroke, the cylinder pressure starts to decrease which may cause low brake torque.

The method and the result obtained in this chapter are of great help to fully understand the pneumatic hybrid engine compressor mode and supply the following fundamental knowledge for the future research.

(i) The valve timing, engine structure parameters such as the actual compression ratios can significantly affect engine braking performance during CM.

(ii) The simulation results prove that the pneumatic hybrid engine can realize regenerative braking during the CM which means it can be a supplement to the vehicle friction braking system.

(iii) The finding that the engine brake torque is a function of tank pressure supplies the basic understanding of how to control the pneumatic hybrid powertrain.

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CHAPTER 4

DRIVING CYCLE SIMULATION OF PNEUMATIC