I understand that when the intake valve is closed, the pressure increases outside of the valve, and a pulse is sent out towards the air filter.

Why is there a pulse back? I would think the pulse would dissipate after leaving the filter. Unlesssss the traveling of the pulse away from the valve causes a brief low pressure area???

 

I think you're referring to reversion. You have a column of air in the runner traveling toward the valve. The valve snaps closed and the air has to dissipate the energy it picked up while being moved through the intake. It hits the valve and bounces back up the runner.

In a TPI or LT1/4 style intake, I don't think you would have to worry about the air going back out the filter, I would assume that would only be a problem on a carb system with a single plane intake.

 

Right. But I was reading that in the length of the runners allowed for timing the PULSE BACK, so it arrived as the intake value was opening again, resulting in a greater charge.. explaining the lower runners on the L98 performing well in the lower rpms, and the very short runners for the LT4.

I dont see what causes the PULSE BACK TO the valve.

thx

 

Ah, now I see what you were asking. If you look at the TPI intake, all the runners on the plenum are offset from each other. I believe the column of air goes back into the plenum and bounces off the plenum wall on the otherside. That's my best guess :D I just thought of another thing, if the holes for the runner are aligned with each other slightly, it could bounce out of one runner, and back down another. I'll have to look at the TPI a little closer.


[Modified by JCAIRE2, 5:47 PM 10/10/2003]

 

The piston creates a suction wave that is sent up the intake runner. This suction wave is reflected as a compression wave at the plenum (expansion) that returns to the cylinder. This compression wave will fill the cylinder, and holds the mix in the cylinder while the piston travels up the bore.

At low RPM the effect is negligible because the passages are too big for significant wave strength. The effect is greatest in the midrange because wave strength is strong and timing is right (runner cross-sectional area is essential to proper wave strength). The effect is still good at peak HP RPM because wave strength is strongest and nothing is pushed back out of the cylinder because of the intense reflected pressure in the intake port.

The length of the intake runner can determine the RPM for peak torque and HP. Long intake runners create a strong returning wave strength when the runner cross-sectional area is right. The long runners will also create the RPM peaks you typically see because of the combined effects of the initial reflection, as well as the strong residual "tune" left in the runners. But the primary reflection dominates, especially for shorter intake runners.

Headers work in similar fashion. The blowdown (compression pulse) is reflected off the expansion at the collector and X-pipe as a suction wave. You adjust the scavenging wave timing for your intended RPM range by changing the length, and sometimes the taper angle of the header pipes and collector.

 

Consider the intake valve as the power driving the air in the runner tubes as it opens and closes. The "pulsing" of air in the runners is due to the fact that air has mass and so mementum; once air is set in motion by some force it takes another force to stop that motion. In this case that stopping force comes from pressure and, if you will, vacuum created by the very movement of the air.

When the intake valve opens, the piston is traveling downward creating lower pressure than in the plenum. This caues air to accelerate down the runner into the cylinder. When the intake valve closes the down rushing air stream builds up pressure at the back of the intake valve because air has mass and so momentum and it takes a build up of pressure behind the intake valve to stop the downward inrush of air in the runner tube.

Now there more pressure at the back of the intake valve than in the plenum so the air begins to rush back up the runner tube into the plenum where velocity drops off due to the large volume of air. The up rush of air has momentum so that eventually the pressure is lower at the back of the still closed intake valve than in the plenum. So the air again begins to rush down the intake runner tube toward the intake valve until pressure build up at the back of the intake valve stops the down rush of air.

At certain RPM this pressure build up will occur just as the intake valve is opening. Under this condition more air is forced into the cylinder and torque & power is increased.

This is the principle of the "tuned port" L98. Since the runner tubes are relatively long this tuning effect occurs at low to moderate RPM, explainig the good torque built by the L98.

Remember it is primarily the runner length that determines the RPM at which this "tuning" occurs; the runner diameter only effects the velocity of the air in the runner tube.

As mentioned above, a similar "tuning" effect is seen in exhaust design. Here too exhaust gasses can enter the chamber at certain RPM. One way to minimize this undesired reversion effect is to provide a step on the lower side of the exhaust port into the header tube. This step doesn't much effect exhaust gas flow out of the cylinder but tends to minimize reversion back into the cylinder.

 

"The up rush of air has momentum so that eventually the pressure is lower at the back of the still closed intake valve than in the plenum." This is what I suspected, but the book didn't explain the physics of the 'bounce". Only the effect.

Thanks guys.

 

:withstupid: All this increases the VE (volumetric efficiency) of the engine at the tuning RPM. I did my senior project on this stuff... quite interesting to say the least!

 

Thats a project you can sink your teeth into! :D

I was stationed at the SATCOM site in "nabisco"... You live in a beautiful part of the country.