INJECTOR SELECTION AND ATOMIZATION
Recently I have seen a lot of chatter about fuel injectors and who’s are better, up to and including a tuner that refuses to tune vehicles unless the customer buys specific injectors from the tuner. After stumbling on a post about this, I found it interesting that the tuner was making claims that, while having some technically correct parts, made it very clear that he didn’t understand what he was saying, claiming that unless people buy his injectors, the fuel will lose it’s atomization and pool in the port and on the valve, and be bad for performance. News flash: that’s exactly how the engine in question was designed to run. The biggest downside here is that for the most part, the people reading along don’t know any better. Someone who should know more than them is handing them info, so they take it as fact. One of the key things we have to keep in mind for the sake of this conversation is that we are talking about an engine that intentionally sprays on the back of a closed valve under normal operation. This gives the fuel the maximum amount of time possible to vaporize in the port. The only exception to this is on a cold start (under around 50*F), where the targeting and atomization play a role in how easily the car starts. It’s important to note though that that had absolutely nothing to do with the conversation mentioned above. Some cliff notes for those that may not be familiar with engine operation:
- There are 720 crankshaft degrees in an engine cycle.
- 0* references top dead center of the compression stroke
- The intake stroke (based on piston movement) begins 360*BTDC and ends 180*BTDC
- The targeted end of injection on the engine in question at warm idle is about 700*BTDC (340* before the intake stroke begins)
- The latest end of injection ever allowed (8192 rpm/wot) is 348.8* BTDC
- The latest end of injection allowed where these engines are typically done making power (7168rpm) is 376.5*BTDC. This is still 16.5* before the piston begins drawing air into the cylinder on the intake stroke
The bottom line is that this engine almost exclusively sprays on a closed valve, by design. I pondered on this for a few days, trying to think of a way to explain it in very simple terms. This is something that I often struggle with, so I was pretty excited when I remembered that I had heard Paul Yaw (Injector Dynamics) cover this very topic. I immediately dug through my podcasts to give it another listen, and he laid it out better than I could have. Below is a transcript of the section of the podcast pertaining to atomization that I sat down and wrote out. Some small edits were made to take out the chatter and “ums”. For anyone that would like to listen to the podcast in it’s entirety (which I HIGHLY recommend) it can be found here: https://www.youtube.com/watch?v=DjadTHBl5IQ&feature=youtu.be
Now, if someone wants to read through this and claim they know more about injectors than Paul… You might want to start getting your information elsewhere.
POWER AND SPEED PODCAST // EPISODE 94
Paul Yaw of Injector Dynamics.
Let’s consider atomization first by defining it as a measure of the average droplet size. So if we have better atomization, we have better droplets…. I have sent out injectors for doplet size testing. You can quantify that, there’s a number of ways you can look at it, and kinda scratch your head and get a feel for it, in terms of one droplet being bigger than another. But what you’ll find is that when you try to come up with a correlation inside of a running engine, it’s nearly impossible and there are a lot of reasons for that.
Initially we say a smaller droplet size has to be better, why would it NOT be better? And I’m going to agree 100% that if we spray fuel into a large open container, that the droplet size is gonna have a great deal to do with how homogenous the entire mixture is, it’s going to effect flame travel speed, because the smaller the droplets are the smaller surface area they have, and the easier if is for the flame front to vaporize the layers of fuel in front of it. Everything about that is going to be better. Unfortunately, the situation we have inside of a port, on an engine, is entirely different.
One of the things that will really stand out, I hope, is that the intake stroke on a 4 stroke engine is roughly equivalent to about 25% of a complete engine cycle…… what you have to consider ultimately is that the actual act of drawing fuel and air into the cylinder occupies 25% of the cycle. So we have this “concept” of breaking this fuel into little tiny droplets and CLEANLY shoving it past that valve into the combustion chamber where we say “wow this is great we got all these little droplets it’s gonna burn great everything is gonna be wonderful.” But the fact is, even that “concept” alone is IMPOSSIBLE if your injector duty cycle is greater than 25%. And even if we look at a system with… a large valve-open window… the amount of time that the airflow through that valve is actually HIGH, is still fairly closely approximates that 25%. THE REST OF THE TIME THAT FUEL IS QUITE LITERALLY BEING SPRAYED ON TO THE BACK OF THE VALVE.
So, if you take a bottle of windex, and hold it up into the air and spray it, I don’t know about you but I’ve always been amazed at how small the droplets are…. Now go spray it on to a window and look at what happens.. all those droplets coalesce onto the glass. And so.. that’s a better indication of what’s happening inside of an engine. Because first of all, the valve is closed for the majority of the cycle, and most of the fuel you spray in there is going to coalesce on the port surfaces. Now, to take that a step further, that’s just the FIRST step to consider in a very complex system. The next thing is that when you spray the fuel into the runner, the back of the intake valve once the motor is warmed up is very hot. What happens then is that the fuel starts to vaporize. At this point it’s very clear to consider the distinction between vaporization and atomization. Atomization, as we agreed, relates to the average droplet size, whereas vaporization describes a state change, where the fuel goes form a liquid state to a gaseous state. So that’s a good thing that’s happening there on the back of the valve, and that point if let’s say you vaporize 100% of the fuel that you put on the back of the valve, IT WOULD HARDLY MATTER IF YOU POURED THAT FUEL ON THE BACK OF THE VALVE WITH A TEASPOON, OR YOU SPRAYED IT FROM A WINDEX BOTTLE….. you lose a great deal of your atomized fuel by coalescing on the back side of the valve, then it boils off, so you’ve got a lot of gas there as opposed to a liquid, which is a good thing. When the valve OPENS, the air and the liquid fuel and the gaseous fuel are being drawn across the edge of the valve and the
valve seat. There’s a lot of shear action that helps to break droplets up, mix them up with the incoming air, particularly dependent on swirl and tumble in the head. You know, charge motion. Ant then once you get it into the cylinder you have temperature there helping to vaporize the fuel. But ultimately the point I’m trying to make is that there is a very large percentage of the fuel that no matter how hard you try to break it up into a droplet, it is going to coalesce on the surfaces inside of the port…. What you HAD in terms of atomization that you could measure, is no longer the case. When you start to think getting drawn through in a running engine, you’ll start to get a different picture of how atomization may or may not effect that.
The one area where atomization is a pretty big deal, and this is where the majority of the work gets done at the OE level, meaning fuel injector manufacturers, and why spray targeting is critical, is that during cold starts you’ll notice that all modern vehicles will essentially center the injection event over the valve event. Meaning they’re going to spray when the valve is as far open as it can possibly be, and they’re going to try and spray it in such a manner that in a perfect world it would not touch anything but air on the way in….. at that point you don’t have any heat to help vaporize that fuel. The only that vaporizes fuel at that point is the fact that it tends to be very volatile. You know when you’re filing your car with gas you see the vapors coming off, that’s what actually lights off first on a cold engine. In THAT case, the spray targeting and the atomization is pretty critical. If you could sit down and have a conversation with the people at the OE level that are designing fuel injectors, they’ll all agree that once you get that thing warmed up and going and get your foot in it, that that’s simply no longer a concern.