Air speed limitations in 2 valve IC engines

hey everybody uh i've been watching some videos uh from george bryce from star racing and uh he's really doing an awesome job so i thought i'd give it a shot and by the way if you're not watching his videos you're doing yourself a grave disservice george bryce from star racing it's awesome when somebody of his stature one of the race winningest teams in the world actually takes time out to sit and explain cam timing and all that stuff so go watch it today i'm going to talk a little bit about airspeed now for years for decades i've been talking about airspeed air speed air speed the three major rules of toning an induction system and i always like to say that i'm a velocity manager more than a cylinder head designer or induction system designer because the average airspeed and the localized velocities in the system as well as the velocity change over time are really what i do for a living and by doing that i'm going to show you i'm going to show you why or how not how but why i do that and i always talk about air speed air speed air speed that you need air speed how fast of an air speed do you need and what happens if you go too fast in other words what happens in the induction system if it's slightly smaller than you need well i will say this you're always always always better off being a little too small than you are being a little bit too large because there's a lot of dynamic effects that happen and you know we saw this in in normally aspirated engines that operate at high engine speeds like comp eliminator and pro stock engines and um what happens is is if you tuned that system perfectly to where right at the shift point you're almost you're right almost or are hitting the subsonic choke point then what happens and you'll see this on the dyno by the way too the bsfcs will spike and the fuel flow will spike well if you're kind of you're looking at that you're thinking well that's not really a good idea but in a lot of cases it was a good idea because if your bsc's spike and your fuel flow spikes and the system goes inefficient you're not using that fuel and if you think about it in a high speed engine you're dropping 12 1500 rpm and then the whole induction system has to wind up and re-accelerate again so when you drop down on the shift if you spike the fuel at the top of the curve you have residual fuel so when the engine drops down on its shift it recovers real quickly and doesn't take time or lag to re-accelerate again so that's one aspect so today i'm going to be talking about subsonic choke and i want to stipulate that at no time in the intake or exhaust system is the air supersonic it's always subsonic because there's one there's not enough energy to do that and you can't have a sonic wave in a pipe anyway so but there's one instance that i've found and people have sent me a couple of pictures of this in the past i've only seen it three times and you have to have a fast shutter rate to catch it and this is a sonic flow this nozzle the air coming out of this nozzle is so fast that the atmospheric pressure is pinching in on these shot cones right here this is a tailor line and these are shot cones which is indicative of supersonic flow which is something you'll never see come out of an exhaust pipe except in one rare instance and that's when the engine actually detonates this is a cylinder dying this is the death knells of an engine right here i'll blow it up a little bit you can see the telltale shockwave cones coming out that's a supersonic flow but it's a shock wave from detonation that engine is dying so what is the peak air speed in the system or how fast do we get it before we run into trouble and .55 mach or 400 miles an hour is the peak air speed in the induction system why do we want to get the air speed this fast in the first place and why is it my job to use to make the system as efficient as possible to turn this 14.7 psi of atmospheric pressure into 400 feet per second with the pressure drop of that cylinder if you think about this a minute the pressure drop of that cylinder and you have 14.7 psi atmospheric pressure so when that piston drops that pressure differential hands you a certain amount x amount of energy my job as a velocity manager is to efficiently turn that pressure drop into air speed and then later on in the cycle which we'll show you here in a second exchange that velocity the kinetic energy of that velocity into pressure in the radar and the later uh during intake closing so if you think about it the intake valve opens the piston is dropping well at bottom dead center the piston is coming back up and the intake is closing but air is still ramming into the cylinder or it should be the pressure is going way up so this is the point where we exchange the air speed for pressure and if you want to learn a little bit more look at volumetric efficiency and volumetric efficiency is density it's not volume so it's 124 in density not not just not uh cubic feet so and then look up inertia supercharging effect and even wikipedia has a really good article on what exactly inertia supercharging is as airspeed increases in the intake track a corresponding increase in ram effect so the kinetic energy is proportional to the square of the velocity in other words as the velocity goes up the ram pressure goes up too the inertia so the inertia supercharging effect increases inlet pressure and the later stages of closing and we can cram another 24 in the cylinder and think about that a second if you didn't have a cylinder head on the engine and you're just looking at the engine with this with the piston at bottom dead center that cylinder is a hundred percent volumetrically efficient relative to atmospheric pressure it's holding the same pressure as the atmospheric pressure that's a hundred percent volumetric efficiency in other words the density in the air in that cylinder is the same is what's outside the cylinder so how do we cram another 24 into the cylinder without bolting the supercharger on top well we use in nature's own dynamic effects of acceleration and turning that into boost at some point now we're going to get it as fast as we can but at some point the air velocity will become inversely proportional to an increase in ve in other words it's going to take more energy to accelerate the air than we get in return and power decreases so when that piston drops the differential pressure in that cylinder relative to nature's 14.7 psi gives me x amount of energy my job as an induction system designer is to utilize that energy as efficiently as possible to accelerate the air and not transfer that energy in accelerating decelerating or changing or turning corners this is why you see unlimited engines with induction systems the runners are straight straight straight or straight as possible if the 5.4 if the mach limit is reached let's say 6 000 rpm but you've designed the engine to peak at 7 000 meaning uh the camshaft the manifold runner links all the volumes everything is tuned so the engine will peak at seven thousand but somewhere in that system is a little bit too small and you hit the mach limit at six thousand what you'll see in the power band is the engine just won't shut off i mean it's not going to go up there and just say forget it what it will do is it will go up to 6000 and then just flatten and just hang there for a thousand rpm or longer sometimes and then once it comes off the pipe once it comes off the cam at seven then it will fall like a rocket ship so at any time if you ever see it on the dyno you will see that as soon as it hits the mock limit your bsfc numbers will spike meaning the whole system is going wholly inefficient exchanging velocity for pressure is exactly what a turbocharger does it's no different they're doing the same thing we are in the intake tract except they're taking the air in with a blade and then channeling into this really thin area right here which accelerates the air to extremely high velocities and then the area expands so they exchange that velocity for pressure that's all they're doing you can see it takes all this air and throws it into this narrow narrow channel now what those velocities are i don't know let's just say they're stupid fast and leave it at that so now we're going to talk about what happens in the cycle how we exchange that velocity for pressure now just ignore the harmonics in the wave action all we're going to concentrate here is the pressure in the cylinder intake valve opening here an intake valve closing here now this green line is ambient pressure i never want to get below this point i always want my pressure to be higher i want to open the intake valve when this is at least even with atmospheric or higher and i want to close that valve when the pressure is at atmospheric or higher i never want to close it later or open it or any earlier or under the atmospheric line because that's reversion that's what version is so here intake valve opening you can see this is the pressure versus crank angle right here the cylinder pressure so the cylinder pressure drops dramatically and this is about 76 degrees after tdc at max pressure drop but the velocity itself right here we're exchanging this pressure drop for an increase in air speed and then we're going to exchange this air speed for an increase in pressure right here this is intake valve closing we're actually closing it above ambient pressure right here the the latest we want to close it is right at atmospheric so if this intersect right here was right here that would be the lightest you would possibly want to close that intake valve if you close it down here you get reversion it goes right back up the intake tract and you get a gulp of air back up the intake track which is not what you want the whole idea is to build as much area under the curve as possible right in here to create as much pressure to counteract that rising piston and cram enough more air into the cylinder now remember i said that my job was to take that pressure drop that energy that that's dropping the piston this pressure drop hands me and convert that into air speed as efficiently as possible this is the whole reason we raise ports and aim things at the valve this is why we need a nice line of sight to that valve we want that airflow path to accent the natural airflow path into the bore center anytime you try to make air do what it doesn't want to do that takes energy out of the sys that spends energy into creating heat or whatever and that is not energy that goes into accelerating the air so you see here like a forgive my artistry here this was done in paint here's the runner slowly comes down the whole thing accelerates all the way to the seat this is the highest average velocity pressure drop in the system or it should be if it's not you have a problem and for decades this is why i've said you need a throat no bigger than 90 91 percent of the valve because i made that ratio up because i know that if you don't exceed 90 that the pressure drop here is going to be low enough to move air into the cylinder that's the concept so if you make this too big and you don't drop the pressure i you don't accelerate the air air is not going to move into the cylinder it's just that simple the engine becomes lazy the ve drops dramatically this is why this area right here from here to here is 80 percent of the gain this up here this is the easy part you also notice there's no big area changes this is a nice gradual curve right here at the short side where the air has to turn this area right here right here doesn't have to get very big in order to navigate the turn now let's look at the worst case scenario this is depicting like a 23 degree small black chevy a very low port now why did they do that in the first place because engineers have to work in a box we don't they have to fit everything under a hood and make everything fit in this nice little package we don't so now if you look at this this is what's really bad about these ports you can accelerate everything to here but then you've got to expand it and you've got to move the air and you've got to turn a corner in other words the port has bias it's offset to the valve guide center line there's not a good line of sight to the valve it's shutting off that window area so anytime you have to make a huge directional change or volume change or air slow the air down and speed it up really really quickly well that's just less air speed you can build and that makes the whole system less efficient so you're just not going to achieve the engine speeds or the volumetric efficiencies that you would with a port like this you also notice this area versus this area because we've had to navigate these turns and because we've had to come down here and move this air we have to make these areas much larger than would have otherwise been necessary to slow the air down to navigate these nasty turns so there you've just spent energy there so that's less air you get in the cylinder now this is a cutaway of an sb2 cylinder head and actually this is the sb2 cylinder head that was on earnhardt's car when he wern's first race at dallas this is the throat and you see everything accelerates to that valve to the throat at the very least these lines would be almost parallel and then shrink in right here everything accelerates to right here this is the highest average air speed in the system this here is the highest localized velocity in the system both work together this is what i print out this is what we call a balanced velocity profile meaning it balanced in the sense that we're not slowing the air down and speeding the air up and turning corners and having to enlarge and expand areas in order to do that in other words it's a very efficient velocity profile this is a cfd layout of the port of basically the sb2 gray being gray and red being the fastest air speeds and green being the slowest why do we need a short side why do we need this pressure drop here at the short turn why can't we just aim this thing right down at the valve it kind of makes sense well people forget that we have fuel in the system if we didn't have fuel in the system we could lean these runners back quite a bit more and if you look at the lt heads you can see they're they're a dry flow system and those ports are lean way way back they don't need a short side developed the way you do for a wet flow system this is what turns the fuel if you don't have a high enough pressure drop here then you're not going to turn the fuel i want you to take note of how that air the hair velocity starts here it's highest here and there because if you want to know the actual natural flow path into the cylinder you can just take a piece of string and take it hold it bore center and come out as straight as possible and it's going to be a straight line into the cylinder and my job is to develop these ports to have a nice line of sight not have to turn not have to make a lot of turns yet carry that fuel around the corner so what happens if you have a good port this is a bad port this is laid back this and you notice the velocity instead of being at the apex is past the apex it's laid back way too far see this velocity comes down this dumps this air velocity just takes a huge dump it can't maintain its air velocity the pressure drop is not here in this one the fuel droplets will come in this low pressure pulls the fuel down with the air if you don't have this air velocity just perfect what will happen is this is too slow and the fuel just rams right into the back wall here a lot of the fuel just keeps going it has inertia these are droplets of liquid so you get them moving fast enough they don't want to turn the air can turn five times faster than that droplet can so you've got to lower the pressure here to turn that now what happens if it does ram into the back of that bowl it deatomizes and all that nasty raw fuel trails into the combustion chamber and doesn't burn well it burns but it burns too late and it creates usable cylinder pressure so you don't get any power out of it all it does is go out the exhaust pipe still burning and heat your exhaust valves up this is another view cfd of the sb2 runner and what i did was is i had a friend of mine who works for spacex do these for me and i said okay what we need to do is we need to depict what looks right this is correct and then we need to screw it up and show us what happens when it goes all wrong so this you see the blue all the way through here you can see this white right here this is low pressure high velocity air these are the highest velocity points right right at that throat that's where you want it this is high velocity but it hasn't broke the boundary layer and it's not turbulent it's conforming so the flow zone is the whole port maybe right out to here now what happens if you have like a this is kind of indicative of like a big block chevy port or a small block chevy when you have what we call a hyper critical short turn and by hyper critical i mean this port has a nice line of sight and the amount of degrees that you have to turn into the combustion chamber is half of what you're turning here this has to turn all the way around the corner and these are what we call hypercritical short turns and anybody who has ever worked on a windsor ford or a small block chevy knows how critical these things are you can play with those things for a week trying to get it absolutely perfect and your window area of tuning on this is very very narrow so on a port like a port like this you can lay it back a little too much or bring it forward a little too much and you're really not going to see that much of a change but on something like this everything is hyper critical any tiny little change is going to affect it and it will go turbulent and break the boundary layer is what you're this is what you're seeing here here's the apex of the short turn the air can't conform anymore it can't make this turn so it breaks the boundary layer and this is all turbulence right here now look what's happened to the flow zone it shuts it down instead of using the whole port it's only using about that much this is called a turbulent choke now a turbulent choke can be a good thing or it can be a bad thing just like anything in engine design everything has its place and hypercritical short turn has its place if you think about ls7 or ls3 cylinder heads this is what they have they have a hyper critical short turn on purpose because in order to make a lot of power let's say 6 6500 rpm they needed a 2 200 inlet valve well the problem with that is is even with an 86 percent throat you can't get the air speed up high enough to lower the pressure to move the air into the cylinder at low rpm 1500 2000 2500 rpm and that's where these engines cruise at that's where they have to get best gas mileage at fifteen hundred two thousand rpm ninety-five percent of that engine's life span is spent at fifteen hundred to two thousand rpm so all this valve area goes completely to waste if they didn't have this apex hypercritical right here at 1500 rpm when you walked into the throttle the engine would lug lag lean out and even could detonate because there's not enough pressure low pressure to move that air so what they do is they spike the air speed here and make that really abrupt and what do we all do when we want to make more power than ls7 head we lay the short turn back a little bit we radius the top off what happens to the torque everything below 3000 is trash at that point you've taken the whole power band compacted it and moved it up so now everything under usually a lot of the cams anything under 4000 is complete trash so this is very very critical on an ls7 to get this right because you can by laying that back a little too much you can knock a lot of torque out of the system so this is where flow numbers will bite you too so if you have 650 lift and this goes critical at 650 lift then what happens is there's no air speed here to drop the pressure to move the air into the cylinder everything moves from high pressure to low pressure so at 1500 2000 rpm you slam the throttle to the floor if that wasn't there if you didn't have that high velocity nothing would happen it would lug so they have tuned this to be hypercritical this is both average peak average airspeed and peak localized velocity airspeed in an ls7 it has two jobs and this just supplies enough valve area to actually feed the motor to make 600 plus horsepower at 7000 rpm so here you can see the flow zone shut down the proper way to do it and the proper way to do it in two completely separate instances this port and an ls7 wouldn't even come on till 6 500 rpm so every situation has its place the port shape port shape is so important you can have two different ports that flow the same and have basically the same velocity profile and the one that is shaped properly to minimize the velocity gradients or delta difference between velocities and certain areas will always operate better or i should say make more power than the other one so and port shape is not intuitive a lot of this stuff is not intuitive if it looks aerodynamic you got a 90 chance it ain't going to work in a running engine because this stuff is not aerodynamically intuitive it's not natural although airspeed is king you cannot attain high air speeds and high efficiency without a proper port shape so the difference between if you take two ports that flow the same and have the same velocities throughout but one square and one's oval the oval will carry the fuel the wet flow will be better the the mixture will be more homogeneous and the thing will make a ton more power and it's just a fact of life port shape and induction design take into account the natural flow path remember i said stretch that wire out stretch that string to bore center and right out the runner those are the highest velocity areas any where that line intersects a wall those are the highest velocity areas in the system period my job is to move those walls and get that flow path to where i have when i look down that runner i've got a nice beautiful flow path right to bore center but i've also got to turn the fuel so i need a short side and i've got to navigate around push rods and i have a lot of other things that i have to take into account you can't manipulate or force air to do what it doesn't want to do without suffering consequences and those and the consequence of trying to move air or turn air around the corner is you're going to sacrifice some of that inertia that energy in doing that you're going to exchange that energy for something else you're going to heat the air up you're going to expand it too fast and it's going to heat it up and cause pressure that's going to completely screw your volumetric efficiency and that's it you

Channel: Darin Morgan

Views: 5,637

Rating: 5 out of 5

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Id: qhsTQn0uUOQ

Length: 28min 25sec (1705 seconds)

Published: Wed Feb 17 2021