A volumetric efficiency or VE map is something
that you'll find inside pretty much every modern ECU or Engine Control Unit. Doesn't matter if
it's aftermarket or OEM, it's in there. And it is probably the single most important thing which
determines how well your engine operates and how much power it makes in all sorts of different
operating conditions and scenarios. But what's also really interesting is that this map is a
really good teacher. And that is because although doesn't seem that way upon first glance, it's just
a map, but within itself this map contains all the fundamentals of engine operation. How an engine
works - it is reflected in this map. And it doesn't matter if the engine is a car, motorcycle or truck
engine, the essence of internal combustion engines is almost always pretty much the same, and it is
contained within this map. So if you understand this map you understand engines. And in today's
video we'll explain what is volumetric efficiency we'll explain how this map works and why it's so
useful. And finally we will do a practical tuning session to demonstrate how you can tune this
map yourself. A VE map has two axes. Our vertical axis is engine load. In most cases engine load is
represented by MAP or Manifold Absolute Pressure It is simply the air pressure inside your intake
manifold. This data comes to the ECU from the MAP sensor, which is located somewhere on your intake
manifold and it measures the pressure inside it Our horizontal axis is engine RPM or rotations
per minute. This data can come to the ECU from something like a crankshaft position sensor, which
counts the number of engine revolutions and sends the data to the ECU. Alternatively, the data can
also come from something like an ignition coil The ECU registers the number of times the ignition
coil fires and based on this it can calculate engine RPM Now at idle and other reduced load
scenarios we have vacuum inside the engine In other words, the air pressure inside the intake
manifold is lower than the air pressure in the atmosphere outside the engine. This occurs because
the throttle plate is almost fully closed and it's only letting in tiny little amounts of air
into the intake manifold. But at the same time the engine is running and the downward motion of
the pistons is creating a void or absence of air above the piston. Air from the intake manifold
quickly rushes into the engine to fill that void and then that same air is mixed with fuel, compressed, burned and expelled out the engine as exhaust gas In other words, we are consuming the
air. The issue is that we are consuming more air than the throttle plate is letting into the intake
manifold. So what happens is that inside the intake manifold a unit of volume such as a cubic inch
or cubic centimeter of air actually contains less molecules of air than that same cubic inch
or centimeter of air outside the engine in the atmosphere Because we have less molecules of
air we have less air pressure and we call that a vacuum. However, the vacuum inside the intake
manifold isn't true vacuum. True vacuum would be a complete absence of air. Instead we don't have that
in the intake manifold. We have something that could be called negative pressure because
it is air pressure reduced in comparison to the air pressure of the atmosphere. Now, as the
throttle plate opens more; more and more outside air is allowed into the intake manifold. In other
words, we're allowing the atmosphere to enter the intake manifold and of course because the engine
cannot consume the entire atmosphere, pressure inside the intake manifold at fully open throttle
becomes equal to atmospheric pressure outside the engine The pressure equalizes because we have
removed the barrier between the atmosphere and the intake manifold; we opened the throttle body. Now,
a naturally aspirate engine is called naturally aspirated because it relies on nature - the pressure
of Earth's atmosphere to push air into the engine A naturally aspirated engine can never achieve
significantly higher manifold pressure than the air pressure in the atmosphere, which is 14.7 psi
If your at sea level and a bit lower if you're at a higher elevation. And that's why a naturally
aspirated engine is going to be limited to about this part of the VE map and this zero at this part
tells us that there's zero difference between air pressure inside the engine and outside the engine
in the atmosphere. Now, a boosted engine. An engine with a forced induction device such as a turbo or a
supercharger can achieve significantly higher manifold air pressure than would be possible by
relying on atmosphere alone, and that's because a turbo or a supercharger sucks in air, compresses it
and stuffs more air into the same volume Because we have more air in the same volume than would
be possible by relying on atmospheric pressure alone pressure increases well beyond atmospheric
pressure. But the beauty of a VE map is that we can use the same VE map to successfully control both
naturally aspirated and boosted engines, and that's because VE maps capture the essence of engine
operation. We could also display our vertical axis like this: instead of zero we put 14.7 PSI or 1
bar which is atmospheric air pressure Everything below that is now vacuum and lowest value would
be around 3 PSI and highest value is now around 50 psi but most aftermarket ECUs choose to display
atmospheric pressure as 0 and that's because atmospheric air pressure is a constant. We can't
do anything about it and we're using it as our reference to see what the engine can add on top
or remove from atmospheric air pressure Now let's address the numbers inside the table. 87 of what?
Well, this is actually volumetric efficiency itself It's volumetric efficiency of the engine at that
particular intersection of engine RPM and engine load So this is then 87 of nothing. It's not any
particular unit. It's percentage. It's 87 percent and it tells us that 87 percent of the internal
volume or the displacement of the engine which is bore times stroke or the entire volume
of the cylinder above the piston when the piston is at bottom dead center. And 87% tells us that 87% of this has been filled with air Now you would think that the engine fills its entire
internal volume or displacement with air at all times perfectly. Well, actually it doesn't.
The engine doesn't breathe equally well in all operating conditions and different scenarios. And
this is why we have a torque curve, and this is why we cannot achieve peak torque at 1000 RPM for
example. And that's because the engine isn't very efficient at breathing at such low rpm. And this is
again why a VE map captures the essence of engine operation, and that is because the VE map matches
the torque curve of the engine. The more efficient the engine is at breathing, the more efficient it
is at filling its entire internal volume the more air there's in the engine, the more fuel we
can add, the more powerful the combustion and the more torque and power we can make. So a volumetric
efficiency of 100 tells us that the entire displacement or internal volume of the engine
has been filled with air at atmospheric pressure So consequently, a volumetric efficiency above
100, for example 110, occurs under boost And it tells us that we have stuffed in more air into the
cylinder than would be possible with atmospheric pressure alone. And this is why volumetric
pressure efficiency goes above 100 under boost So volumetric efficiency numbers essentially tell
the ECU how much air is inside the engine at any particular intersection of engine RPM and engine
load. By knowing the mass of air inside the engine the ECU can then match that mass of air with an
appropriate mass of fuel to achieve a desired air/fuel ratio Now something else you might have
noticed is that VE actually falls off at high RPM It gets reduced. Why does this happen? Shouldn't
volumetric efficiency increase with RPM? Well, yes, it does, but up to a point. As you can see it
increases with RPM but then at overly high RPM it falls off and this happens because at high RPM the
engine simply does not have enough time to breathe The motion of the valves is synced to the motion
of the piston, and at very high RPM we have very high piston speeds so the valves are open for
an extremely short period of time. Which means that the engine simply doesn't have enough time
to ingest the air and fill its entire internal volume with air. And this is why at very high
RPM, VE as well as your torque starts falling off Okay, now let's see how all this works in practice.
We are in the car, I have my laptop, we're connected to the ECU and we can follow what happens on the
VE map, how the ECU follows the VE map in real time Now observe what happens if I take
this value, this 61 here at idle in our idle zone, and let's
change it to let's say 30 You can probably hear that and you can also
see it in the software. Now our RPM is hunting We have an unstable, a hunting idle. So let's
fix this because this is annoying And as you can see immediately
our idle is restored The question is: why did this happen? Well, it happened because we told the ECU that there is less air coming in the engine then there actually is air coming into
the engine. There is more air coming into the engine than 31% of the displacement. But we told
the ECU it's 31. So the ECU adjusted, it added less fuel and as you can see again in
the software, let's do it again so you can see Our air fuel ratio starts becoming very lean
as the ECU Cycles through the cell Again we change it, and now again we have a very
stable air fuel ratio. The question is, why did it start cycling? Well it started cycling because
the ECU added too little fuel. There's too much air too little fuel and that's a lean mixture. An overly lean mixture for idling so the torque output from the engine becomes weaker
and the engine cannot sustain the needed RPM due to this weaker torque output, and so the RPM
goes down and this brings us back into one of the other cells which have a correct VE input in
them. So this again restores proper torque output proper combustion, we have more more torque, and
then this sends us back to the incorrect cell where again we have too little torque so we
fall down and we have a cycle. A never-ending cycle unless we fix the value in the table. The
question that arises from all of this is how do we know which is the correct value? Because in
reality we have no idea how much air is coming into the engine at any point in time. We don't
really know how volumetrically efficient the engine is at all of these intersections of RPM and
engine load, which begs the question: How did this map come up? Did I make it up? Did I download it
from somewhere online? Where does this map come from? Well the map actually comes from the ECU
itself. And that's because modern ECUs like my AEM Infinity right here, in this forest of wires,
these ECUs are really clever. So when you first set up your engine, your ECU, and tell it what kind
of engine it's going to run. The ECU is asking you for some basic engine parameters. Displacement,
number of cylinders, what kind of ignition system what kind of firing order, very basic
stuff that you know about your engine already And when you give the ECU these parameters the
ECU is going to spit out a generic volumetric efficiency base map based on these parameters.
And in many cases, actually in the vast majority of cases this map provided by the ECU is going to be
enough to get you started and even driving around a bit In many cases this map is going to require
only minor corrections until it is perfect But there is more good news, because even these minor
corrections to the VE map, you don't have to do that yourself either. The ECU does that for you
as well by relying on this, let me just show you let me just get in there, and this right here
is your oxygen sensor, as you can see it sits in the exhaust stream and basically measures the
oxygen content in the exhaust stream. It's a bit more complicated, what essentially does is that
it provides a feedback to the ECU. It is telling the ECU about, very accurately, about the actual
content of the air to fuel inside the engine inside the combustion chamber, and in addition to the
wideband sensor the only other thing you need is this and this is perhaps the most universal map ever. It's called a Lambda Target or air fuel ratio target map. These are your target air to fuel ratios.
This is what you want to run at different engine RPM and engine load intersections and this very
map, the very same map, is applicable I think to pretty much any gasoline engine out there. So the
triangle of VE-map, air fuel ratio Target map and wideband sensor works like this: The ECU reads
the VE map and then it injects fuel according to the values in the air fuel ratio target map.
The wideband sensor then tells the ECU that it's off by a certain percentage. The ECU corrects
the amount of fuel injected until the air fuel ratio reported by the wideband sensor equals the
target air fuel ratio. The only other piece of the puzzle is your injector setup. You must tell
the ECU the flow rate and the offset or dead time of your injectors. Fortunately you don't
have to guess anything here as these numbers are usually supplied in the package with your
injectors. Now there is a limit to how much the ECU can correct the amount of fuel based on the
feedback provided by the wideband sensor and this amount is usually limited to 30 percent. In other
words, the numbers in your VE table can be off by a maximum of 30%. This means that if your
engine is something weird or extreme you might be ingesting 30% more air in certain parts of
the map and in this case you'll be outside of the target air/fuel ratio and the ECU won't be able
to compensate unless you properly manually adjust your VE map. Even if you have to make large manual
corrections to the VE map yourself, that's easy to do as well. All you have to do is observe the
lambda feedback value on your laptop as you're driving This is telling you how much the ECU is
correcting based on what it's reading from the VE map until it hits the target. Alternatively, if you
don't want to use a laptop, you can use a digital dash like this this one that lets you see the data
while you're driving. Now let's start the engine and we're going to observe the AFR trim
value right here, let me zoom in a bit And this is telling us how much the
ECU has to correct to meet the air fuel ratio Target. And as you can see in my case As you can see in my case we have only
minimal corrections, two, three, four five percent most of the time, and this is very much
within what the ECU can do. However if you were to see 30 minus or positive at certain RPM and
throttle openings in this little AFR trim cell then you would know that the ECU is trying
to add or remove at least 30 percent fuel to hit the Lambda Target. However as you're
driving you can see where it's happening and then you can fix that area of the VE map
until you are back within the scope of what the ECU can do. So that's pretty much it, all
you really need are basic engine parameters, your injector parameters, the world's most
universal map and a wideband sensor and you can build and fix a VE map by yourself
in minutes. Isn't modern technology great?
