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| steeda |
Here the effect of runner length, port entry area and minimum port area on dyno performance are studied.
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Part 1: Runner length
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| Measurement data |
When the intake valve is opened, air starts flowing into the cylinders. However, the flow of air is stopped abruptly after the intake valve is closed. At this circumstance, the stacked air builds up a high pressure nearby the intake valve and then this pressure wave will make its way up the pipe away from the cylinder. Once the valve is opened again, the air will be dragged into the cylinder. An ideal runner length is that if the air will arrive back at the intake valve just as it opens for the next cycle.
If the runner length is too short, the pressure wave is disappeared before the intake valve is opened again. If the runner length is too long, the pressure wave travels a very long distance until it is reflected by the intake manifold system. Hence the pressure wave may be unable to re-enter the intake valve at a proper time.
In the red zone, the engine runs very quickly. A long runner length is NOT preferable for high-rpm performance because there is no time for the pressure wave to complete an ideal oscillation. In contrast, the use of a long runner length enhances low-rpm performance.
Engineers usually design multiple reflection of the pressure-wave in a small car. To allow the multiple reflection of air, the runner length should be reduced. Assume the intake runner pipe is cut in half, the pressure wave will travel up and down the pipe twice before the intake valve is opened again.
Before optimizing the runner length, it is advised to calculate the trial length analytically. Assume you want to boost the engine performance particularly at 6000rpm, you need to take into account the intake-valve timing into the calculation. If the intake-valve keeps opening until 260 degree, that's mean the intake-valve will be closed between 260 to 720 degree. A simple mathematics tells you that 720-260=460 where the intake-valve is closed. While 6000rpm is equivalent to 628 rad/sec, the engine takes 0.01 second to rotate once. By multiplying the angular ratio of 460/360, the intake valve is closed during 0.01 x 460 / 360 = 0.0127 second. The speed of sound is 343m/s at room temperature and therefore the pressure-wave should travels 343 x 0.0127 = 4.38m in order to meet the proper time. The "4.38m" refers to the distance traveled by the transmission and the reflection of the pressure-wave and therefore the trial runner length is roughly 4.38/2 = 2.19m (~7 feets). What? 7 feets? This is too long. Now you should use the mentioned multiple reflection of the pressure wave to decrease the runner length by at least twice. (due to the limited size of vehicle!!!).
At low-rpm region, the torque is slightly larger in a longer pipe. While the engine runs slower at low rpm, the air molecules do not need to move rapidly to catch the proper time of pressure-wave and hence a longer primary length favors low-end torque.
Part 2: Port entry area
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| Measurement data |
We observe that the dyno performance in red zone is better when the port area is increased from 1.5 to 6 square inches. A large port entry area is desired in high-speed circuits because more air can flow into the chamber. In contrast, the low-end torque & horsepower are higher if the air flows through a narrower entry port. As the intake valve opens a little at low-rpm region, a large amount of air moves toward the valve may create a turbulence-like feature. In addition, the air molecules move faster in a narrower port that can make the intake process more effectively.
Part 3: Minimum port area
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| Measurement data |
Another trick to improve engine performance is to setup a minimum port area. Here we demonstrate how the minimum port area affects the dyno results. When the maximum port entry area equals to the minimum port area, the horsepower and torque are plotted in orange and pale blue color, respectively. We treated these orange and pale blue curves as a control experiment. Now we fix the maximum port area as 3 square inches and monitor the minimum intake area nearby the valve. The dyno performance is optimized at the minimum area of 1.5 square inches. This is due to the gain in kinetic energy of air molecules in a narrower tube and presumably actuates the intake process. However, the dyno performance is getting worse at the minimum area of 1 square inches because there is insufficient amount of air flows through the pipe. At the end, it pales the efficiency of combustion engine.