Intake Manifold Design for Single TB IM's with a Plenum
B18B IM (left and closest to you in side view pic) and ITR IM (right): Notice the ITR IM has shorter runners with larger diameters compared to the longer tunnel ram runners of the B18B.
Skunk2 IM for the GSR with it's shorter than ITR stock runners.
When race engine builders talk about fuel injected engine "parts integration", one topic of the discussion is planning out where you want your powerband to be located along the rpm range .
The induction system can be "tuned" or designed to have features which can improve the way the cylinder fills and determine where PEAK TORQUE will be located along the rpm range. This is what we call intentional "powerband location" placement.
The three features of an intake manifold with a plenum that
determine peak torque location are it's:
- plenum volume
- runner length
- runner area
But before we proceed with how these 3 features affect cylinder filling, we should first understand how air flows in an intake manifold.
I. RAM AIR THEORY
Dry air is thought to behave like a compressible elastic fluid. In the "Ideas: Flow capacity, flow velocity, and flow quality" article, we discussed the differences between laminar versus turbulent fluid flow. However, instead of looking at fluid dynamics, mass air flow can also be looked at in terms of it's acoustic behaviour or behaviour as a sound wave and it's frequencies.
Sound waves travel as undulating pulsations up and down an IM runner. These pulses have a frequency or resonance and carry energy. You'll be surprised to discover that air isn't just sucked into the engine but also can be forced through the engine's intake valves even in naturally aspirated setups.
Figure 1. Air flow down an intake runner as a sound wave (acoustic
resonance).
In a naturally aspirated engine, on the intake stroke, the piston
drops creating an area of low pressure in the combustion chamber that is less than atmospheric pressure and as the intake valve opens, the air from the outside is set in motion down the IM runner.
Once air (as a sound wave) has been set into motion down an IM runner, it does NOT simply stop when the intake valve is closed shut and wait for the intake valve to re-open.
Instead, when the intake valve closes shut, this air sound wave bounces off the backface of the valve and travels at the speed of sound back up towards the IM plenum (rarefaction wave). This reflected wave has a frequency, amplitude, and negative pressure associated with it.
Once the wave reaches the plenum, the resonance wave is isolated and the plenum chamber behaves like a resonance chamber. What is a resonance chamber?
The analogy used by most mechanical engineers to explain how a resonance chamber works is that it acts like an oscillating spring (i.e. imagine the plenum acts like the spring) with a block attached on the end of the spring (imagine the air wave in the IM runner to behave like the block) . As the block compresses the spring, the spring builds or stores up energy and when the spring uncoils, the block is given a push or energy as it travels away from the spring's compressed position.
Like our block and spring, the air resonates ( or compresses the spring) at a certain frequency (spring bouncing back and forth) inside the plenum and gains energy (pressure) . The air wave is then bounced back at the speed of sound down the IM runner towards the intake valve again. But this time it has been given an extra "push" from the resonance chamber. The new sound wave going to the intake valve has a positive pressure and is travelling at a higher tone or energy (higher sound frequency).
The bouncing back and forth of sound waves from the closed intake valve to the plenum and then back down again occurs over several intake valve openings continuously. Why does this happen?
These reflected resonance waves don't reach the intake valve when it re-opens and therefore continue to reflect. This continues until several reflected air sound waves (or columns) stack up (amplified) at the closed intake valve. The energy (or pressure) of these amplified ( or stacked up ) reflected waves build up until they reach a maximum energy (and pressure).
The trick to resonance (or sound) tuning of the IM is to have these maximally amplified waves arrive at the intake valve just as it opens . The basic mechanism of intake manifold "tuning" or design is to provide high pressure at the intake valve so that the mass flow rate into the cylinder is boosted at a given engine speed or rpm. We do this "tuning" by changing the IM runner length and diameter (area).
By building up pressure from stacked resonating (or reflected) air sound waves (or columns) and releasing this "boost" at a specific rpm, you can get higher cylinder filling [ i.e. achieve a volumetric efficiency (VE) greater than the cylinder swept volume. The engine breathes at a VE > 100% ] . The reflected positive pressure waves from the plenum, when it arrives at the right time, actually pushes in more air into the cylinder beyond the effects of the piston sucking in air. Not only do you control the location of where peak torque occurs by varying runner length and diameter, you get a gain in power by using the plenum's resonance effect. This is what we call " acoustic supercharging".
Since Mopar was one of the first to use ram theory in a street car, check out:
http://www.chrysler300club.com/uniq/allaboutrams/ramtheory.htm
it has a nice calculation to show how many times an air sound wave bounces back and forth before it finally reaches an intake valve that is open at your desired rpm.
Plenum volumes will vary in size depending upon the application but the general rule is that FI setups require larger plenum volumes than N/A setups. So an STR or Venom IM with a huge plenum is too big for a N/A motor. Some experts suggest that the plenum volume for a peak torque somewhere from 5000-6000 rpm should be equal to 50-60% of the "equivalent" displacement in a 4 banger. On an N/A setup the equivalent displacement = actual displacement. On FI setups, the equivalent displacement = how much volume of air is blown into the motor.
Peak volumetric efficiency occurs at peak torque. So when we "release" these built up amplified waves just at the right time into an opening intake valve, we get peak torque at that rpm.
Therefore, by designing the IM with a certain plenum volume and runner
size, you can control at what rpm the engine will achieve peak torque and more importantly, you will have more power gain at that peak torque rpm from acoustic supercharging.
Here's a nice summary of resonance tuning using ram theory for an IM :
quote:
Originally posted by Jim McFarland
Every physical system has one or more "natural vibration" frequencies that are characteristic of that system .
An organ pipe is a common example of how a resonant condition is displayed. Based upon the physical dimensions of an organ pipe, a flow of inlet air may produce a resonant tone or pitch.
Changing the pipe's dimension, given the same amount of input air, could produce another resonant point or tone.
With regard to an engine's intake {or exhaust} system, it is possible to dimension a passage to accommodate specific cylinder displacements and engine speed so that a "resonant" condition helps produce an increase in total air flow {intake or exhaust}. In it's simplest form, this amounts to "tuning" an inlet {or exhaust} passage. Physical dimensions of the passage are constructed to provide a resonant tuning point {particularly relative to rpm and valve timing} at which a "boost" in flow is produced. This results in an increase in cylinder filling {volumetric efficiency} and potential gains in torque.
Notice that with invididual throttle bodies (ITB's) you lose this resonance effect because the reflected wave escapes out into the engine bay (or the atmosphere) and is not stored and returned by a plenum/acoustic chamber. ITB's do NOT use ram theory to get that extra kick at peak torque because they usually in race form do not have a plenum. In some street ITB's, a plenum is attached for practical reasons (sound deadening and filtering). They rely on very very large amounts of passive cylinder filling based on the piston's effects and use tuned air horn height and tapered diameter (with an S-shaped velocity stack opening) to get the N/A pressure boost effect
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Posted 9/30/2002 12:59:50 AM
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