Who wants to get started?
I’m sure Adam remembers the formulas, without even looking at the link :lol
Volumetric efficiency is just the percentage of air that is filling your cylinder at a specific speed/load condition vs. the volume of the cylinder.
At zero rpm if you opened the intake valve the cylinder would fill.
As engine speed increases the air needs to flow so quickly that the viscosity of the air limits the amount of flow. You don’t get all the air into the cylinder as you could have if you gave it more time.
Simple as that.
Its mostly shear stress, and throttling losses.
Solution: bigger ports, bigger valves. Don’t restrict the airflow.
You maximize volumetric efficiency my minimizing air velocity. Of course this makes a shitty engine which is only good for top end, or a really sweet diesel.
Turbocharging can really help maintain a good Ev by increasing density, and therefore reducing velocity.
^ uhm
yeah i agree, sounds good to me
Don’t be so modest kid, go blow him up!!
Is this why a Cam is beneficial? Leaves the valves/ports open for a longer period of time?
God, I haven’t looked at shit like this since I was mapping out what turbo to buy…
was this a stock Z28?
very interesting read. but who here has that Auto Tap thingy?
Any combination of longer, or more open will lead to greater volumetric efficiency.
More open causes less restriction from the valve because there is more curtain area for the air to flow through.
Longer allows more time for air to flow into the cylinder.
Cams are a bit specific to the type of fuel injection, the manifold, and the combustion chamber setup including valve arrangement.
IF you have fuel injection, and most people do, you may want to keep the intake valve closed for a small portion of the down-stroke to build a bit of vacuum in the cylinder. This is done so that when the intake valve opens, the velocity of the air increases at a high RATE. If you can ACCELERATE the air to high speed quickly you can turn the injector on sooner.
In order to have an excellent distribution of atomized fuel in the air, you don’t want to have the injector (which is digital) spraying into air that is not moving at the correct velocity. If you open the injector too soon the air is not moving fast enough and you will have a mixture that is too rich in the beginning of the intake stroke. If you open the injector too late, after the valve is already open, you will have let in just air which is too lean because it contains no fuel.
Even though the combustion chamber has high swirl characteristics, the resultant mixing is more like a marbled bread than a homogeneous mixture. The marbled bread fuel distribution is the concept behind fuel stratified injection, which is very complicated. FSI is an attempt to get better more even atomization with fuel injectors, either direct or indirect injection.
An ideal cam would wait just the right amount to open the intake valve, accelerating the air at an incredibly high speed, and open the injector is soon as the intake valve was opened.
Of course the valve has to close as well. This further reduces the quality of the mixture. If you wait too long to close the valve you velocity will drop down to a level where the injector is too aggressive for that air velocity and you will once again have a rich mixture. Air velocity as a function of stroke is somewhat of a truncated sine wave with a spike in the beginning.
The initial spike is the sudden rush from the previously mentioned vacuum, which then reduced quickly to a function of the change in volume created by the moving piston. The piston velocity is a sine-like function, with the highest velocity at mid stroke and reducing from there to BDC. The intake valve will usually close somewhere on the bottom of the down-stroke.
Clearly cam theory is far more complicated than understanding Ev, but it is obviously one of the most important factors in cylinder filling.
Overlap is essentially the best solution to this complex problem by the way. With overlap you can let the air in the cylinder without fuel, while you let the intake velocity get up to speed. This plain air sweeps the remainder of the combustion gasses out of the cylinder. Combusted gasses are hot, and take up space in the chamber. They can also lead to spontaneous combustion or knock.
Also the overlap cools the walls of the intake chamber which allows for slightly higher compression ratios without knock as well.
Formulas suck Vlad. I like to pretend they don’t exist and we as humans just randomly throw numbers and shit together to make all this up, yet still make cool things that work :lol 1320(John???) nailed it on the head on all accounts though, no need to go into depth any further than he did.
Some companies(like VAG) also released their data showing how the newer DI motors have a bit higher VE as well. It’s pretty cool shit looking at CFD comparisons of some dino four banger to a 2.0FSI
Fluid dynamics is fun :lol Ug brain hurts just mentioning that…
Don’t forget at high RPMs there is an inertia effect of the mass of air moving toward the combustion chamber in the intake port and causing it to keep going even after the piston reaches BDC…
Tuning the “pulses” with the variation of intake runner lengths and careful selection of the exhaust header can have significant effects on VE at these higher speeds.
What engineers are basically doing is taking into account the cam timing / actual lift / engine speed / flow area etc and choosing an intake runner length that results in enough inertia so that air flow does not want to change direction when intake valve closing occurs and the piston reaches BDC. This helps VE.
I always got a laugh out of the term “jerk” :lol
I thought about taking dynamics into the discussion but i thought that it would be perhaps too much of a concept to add into an already complex discussion. Which turned out to be a non-discussion unfortunately.
To be honest with you resonance of air might be beyond my capabilities to fully describe. Obviously Gas dynamics is a graduate level course everywhere.
Clearly its inertia and momentum applied to a control volume approach of fluid mechanics, but with the variations in density it is certainly not something I could compute numerically. Its out of my league.
Its best explained in this analogy: (for others)
You have 100 people running full speed shoulder to shoulder in a very dense grid. Suddenly an invisible wall drops down and the front row of the people smash into it and all the other runners plow into the back of them creating a higher density.
With the actual density things are so much more complicated.
- The geometry of the intake is fixed, but the velocity is highly variable.
- The volume ratio of the intake to cylinder is NOT constant
- The rate of change of the volume ratio is not LINEAR
- Due to valve overlap there are portions of the intake event that are not control volume, nor are they resonant.
- pressure wave propagation and reflections in HIGHLY complex geometry.
- Mixed phase fluids (air and fuel)
- Different phases before and after the injector.
I might be so bold as to say ill bet nobody can do these resonance calculations accurately on a curved intake without CFD.
You could perhaps do it on paper with a velocity stack with simple conical or cylindrical geometry with a VERY open port and a straight shot into the cylinder but this is very limited geometry.
The port bowl is complex geometry. lots of reflections and changes in momentum here. the valve stem and curtain area section would be its own challenge. Not to even mention the sinusoidal graph of velocity vs time.
This is major league calculations here.
You would have to do a Runge-Kutta 4th order approximation of momentum and velocity of the particles of the whole system to really get a handle on the locations of the nodes at any given point, and how they evolve and move around the intake as pressure and speed was varied.
I couldn’t even hazard a guess at the nodes of any intake system at all. not even the simple velocity stack and straight port arrangement.