What is up Engineheads? Today, we'll be doing a detailed comparison of three different kinds of
gasoline fuel injection systems. Port Fuel Injection Direct Injection And Dual Injection. We'll see how each system works. And we'll see how they differ from each other. And of course, we're going to examine
the benefits and drawbacks of each setup. So, let's get started. So, both port and direct injection
essentially do the same thing. As the name implies, they inject fuel. So, that the fuel can mix with the air
and create a combustible air-fuel mixture. Which when combusted,
creates combustion pressures, which drive the piston downward,
which spins the crankshaft, which then ultimately turns the wheels of the vehicle. Now they may do the same thing, but port and direct injection differ in
the location of where they inject the fuel. As the name again implies, port fuel injection injects the fuel
into the intake port of the engine. Before the intake valve. Whereas direct fuel injection injects the
fuel directly into the combustion chamber. After the intake valve. This means that in the case of port injection, you're usually going to find the injectors
somewhere on the intake manifold of the engine. While in the case of direct injection,
the injectors are going to be either on the valve cover,
or if they're not there, you're going to find them underneath the intake manifold.
Protruding directly into the cylinder head. Although the location of fuel injection
may seem as something trivial. In reality, it plays a fundamental
role in the design of the engine. And noticeably influences performance,
efficiency, emissions, and maintenance concerns. Now, both systems: Port and direct injection, consist of essentially the same parts. A fuel tank or fuel reservoir, a fuel pump, some fuel lines, and fuel injectors. Now, the engine control unit, or the ECU,
measures the amount of air coming into the engine. Using various sensors. And then tells the injectors how much fuel to inject, so that the amount of fuel injected corresponds to
the amount of air entering the engine. All with the goal of creating combustion
that is as close to ideal as possible. But direct injection has a more
challenging task than port injection. Because fuel is injected directly into the combustion chamber, the direct injectors must spray the fuel with sufficient force to overcome the pressures generated by the engine's compression. And if you have ever compression tested an
engine, you know that the upward movement of the piston inside the engine can easily
generate pressures in excess of 100 psi. Now, port injectors don't have to
combat this compression pressure, because they're injecting
fuel before the intake valve. Which means that they're
injecting either into atmosphere (If they start injection before the intake valve opens) Or they're injecting into a vacuum (If they start injecting after the intake valve opens) This means that port injection needs somewhere
between 40-65 psi to operate properly. This is obviously than the compression
pressures generated by the engine itself, which means that if you try to use a port injector, to inject fuel directly into the combustion chamber,
the fuel would actually never leave the injector. This is why to ensure that it can easily
overcome the pressures generated by the engine's compression, direct fuel injection usually
operates at fuel pressures upwards of 2,000 psi That's 140 bars
Which is 140 times the pressure of earth's atmosphere And it's also 20 times the pressure
of the engine's compression. But of course to generate such high pressures,
direct injection needs additional parts. First of all, it can't work with a simple
one fuel pump setup like port injection. Instead, it needs two fuel pumps. The first one is an in-tank
low-pressure fuel pump, which sends fuel down the lines from the tank. To a camshaft driven, high-pressure fuel pump,
which ramps up the pressure to the required amount, and then sends it into a fuel rail
from where it gets distributed the injectors. The injectors themselves are also
radically different to port injectors. They're more advanced, because they need to
be capable of rapidly opening and closing against very high fuel pressures. And also because their tips are located
directly inside the combustion chamber. They must be capable of operating
properly, while being exposed to the extremely harsh conditions
created by the engine's combustion. Port injectors of course don't
have any of these concerns, which is why in general they're much
less expensive and far less complex than direct injectors. In general, a direct injection
system because of the increased complexity and increased number of parts,
increases production costs and vehicle prices. So, as we have seen the location
of the injection determines the pressure at which the system needs to operate. And it also drives up the
number and complexity of parts. But if you have paid attention, you might have
also noticed that the location of the injection also determines the timing of the injection. As we have said, port injectors
inject either into vacuum or into atmospheric pressure, if injection
starts before the intake valve opens. And when does the intake valve open? Of course during the intake stroke of the engine. And this tells us that port injection
occurs during the intake stroke. In fact, it can occur during any other
time during any other stroke of the engine. But the fuel actually gets into the
chamber during the intake stroke. When it comes to direct injection, you may recall that it has to face the
compression pressures of the engine. And when does the engine
generate compression pressure? Of course during the compression stroke. And this tells us that direct injection occurs
during the compression stroke of the engine. In fact, in most modern more recent systems, direct injection occurs during the
later stages of the compression stroke. Right before combustion occurs. And although in some older system,
a direct injection would occur at early stages of the compression stroke, or even earlier during the intake stroke,
depending on the engine speed and engine load In general, direct injection starts much later. And is of much shorter duration
than port fuel injection. So, how does the timing of the
injection affect the engine? Well, for one, it enables a higher compression
ratio in the case of direct injection. Now, don't be confused. Compression pressure, is the pressure generated by the upward movement of the
piston inside the engine. But the compression ratio, is
the ratio between the largest and the smallest cylinder volume of the engine. The higher the compression ratio, the more we compress the air-fuel
mixture inside the cylinder. The more we compress it, the closer we bring the piston
to the heart of the combustion. And the closer the piston to the combustion,
the more of the combustion's energy can be transferred onto the
piston, and turned into motion. In other words, a higher compression
ratio can lead both to better performance and higher efficiency. So, how exactly does different
injection timing enable a higher compression ratio? Well, to understand that,
we need observe port and direct injection side by side. And as you can see, in the case of port injection,
the fuel enters the cylinder much earlier, and spends more time in the cylinder,
than in the case of direct injection, where the fuel enters only at the
late stage of the compression stroke. Now, you have to remember that
the insides of an engine are hot. Temperatures inside the cylinder are always high, because combustion occurred there
just a few milliseconds ago. The intake valves also carry a lot of heat, because they're constantly exposed
to the heat of the combustion. And the air-fuel mixture
coming inside the cylinder of a port injected engine, has to
pass right along the intake valve. And it's picking up heat from
the valve, as it does so. And all this heat inside the engine means, that the more time the air-fuel mixture
spends inside the engine, the hotter it gets. And the hotter it gets, the greater the chances of
its spontaneous combustion. Or knock. Now, knock is spontaneous abnormal
combustion of the air-fuel mixture, that occurs after the spark plug is fired. And it occurs outside the evenly propagating
flame front initiated by the spark plug. Now, for the air-fuel mixture to
spontaneously self ignite, it needs heat. And it needs a lot of heat. And this heat can come from two main sources. The first source is the compression
of the air-fuel mixture itself. The more we compress a gas, the closer we
bring its molecules together, causing them to bump against each other more, increasing
their friction and thus the heat of the gas. Now, the air-fuel mixture is of course also a gas. Which means that the more we compress it, the
higher the compression ratio, the greater the heat of the air-fuel mixture, and the greater
the chance of its spontaneous self-ignition. Now, the second source of the heat
comes from the combustion itself. When the spark plug fires, it initiates a flame front which spreads
evenly outward from the spark plug. Of course, this flame front exerts heat and
pressure on the uncombusted air-fuel mixture outside of this flame front
And if the uncombusted air-fuel mixture is already hot enough from the
compression, then the added heat and pressure from the combustion can cause it
to ignite spontaneously leading to knock. And if knock persists long enough and if it's
strong enough it will destroy the engine. Now, as we said the air-fuel
mixture spends more time inside a port injection engine than
it does in a direct injection engine. Which means that it picks up more heat, and it's already at a higher
temperature when combustion starts. Now, when we add the heat of compression and the heat of combustion onto the already
higher base heat of the air-fuel mix inside a port injected engine, it means that
we can enter 'Knock territory' more easily. And result is, that we have to be
somewhat conservative with a compression ratio inside a port injected engine to prevent knock. Now, in the case of direct injection, as we said, the fuel enters the system later, at a late stage of compression, which means that the air-fuel mixture spends
less time inside the cylinder, picking up heat. Which results in a lower base heat once combustion starts. And because of this we have more
room to increase the compression ratio of the engine in a direct injection setup. And as we said, a higher compression ratio
can lead to better power and more efficiency. And this is one of the key reasons why so much research and development has been invested into direct injection. The other key benefit of direct
injection is that it can reduce fuel consumption and harmful emissions. It can do this,
because it sprays fuel directly into the combustion chamber. Which means that the fuel sprayed by the injectors, is the same amount of fuel that actually ends up
in the combustion chamber, and can be combusted. But this is not the case with port injection. Because fuel is injected
outside the combustion chamber, the amount of fuel released by the injectors
is not necessarily the same amount of fuel that ends up in the combustion chamber. Some of the fuel may end up
as droplets that accumulate on the walls of the intake manifold
or the intake of the cylinder head. Also, the intake valve might close
before all the air-fuel mix enters the cylinder. And the result is, that the difference
between fuel injected and fuel combusted, is greater in port injected engines. Resulting in less accuracy and less
control over the injection process, which can negatively impact
emissions and fuel economy. But there are two sides to every coin. And what may seem as drawbacks of port injection, can also be its benefits in different scenarios. As we said, the air-fuel mixture spends more time
inside the cylinder of a port injected engine. And although this allows it to pick up more heat,
and ultimately limits the compression ratio, it also gives the fuel more time
to vaporize and mix with the air. To achieve good combustion you want to
burn all the fuel inside the cylinder. And to do that you want the air-fuel
mixture to be as homogeneous as possible. In other words,
you want the air and fuel well mixed together And one way of doing that,
is tumbling the air-fuel mixture as much as possible. Now, there's plenty of time for tumbling the
air-fuel mix inside a port injected engine. It first occurs when the air and fuel enter
the cylinder together and swirl around. And then it occurs some more during
the entire compression stroke, when the piston rapidly pushes
the air-fuel mixture upward. But unfortunately, direct injection doesn't have nearly the same amount of time
available to vaporize the fuel. As we said, most recent setups start injecting
only in the second half of the compression stroke. Now, direct injection makes up for this, by having extremely high fuel pressures
which dramatically improve fuel atomization. And also it uses tricks like cavities,
special cavities, in pistons. Against which fuel is sprayed, and against
which the air-fuel mixture tumbles and swirls. But despite this, direct injection can suffer from poor air-fuel mixture
homogenization at low engine speeds. At low RPMs, the piston speeds are also lower. Which means that the piston pushes
against the air-fuel mixture more slowly, causing it to tumble less. Which means that at low RPMs, direct injection engines can experience little pockets of
unburned fuel within the air-fuel mixture. This of course results in
less than perfect combustion And increased emissions And reduced efficiency at low RPMs. This means that depending on the engine
and fuel system design, in some cases, a port injected engine may
have less emissions and better efficiency than a direct
injected engine at low RPMs. Another problem the direct injection faces,
is supplying enough fuel at high RPMs. Let's imagine an engine running at 6,500 RPM. At 6.5K rotations per minute, one single
engine-rotation or revolution, lasts only 9 milliseconds. Which means that one single
engine-stroke is 4.5ms. And as we said, most recent
direct injection setups, only start injecting in the second
half of the compression stroke. Which means that there's only 2.25ms of time available to inject all the fuel that's needed. To put this into perspective, the average human blink is 100-150ms. Now, even if we were to somehow start injecting
at the very beginning of the intake stroke (which never happens) This gives us only 18ms to inject the
fuel before combustion actually starts. Now, port injection doesn't
have to worry about the start of the combustion event. Because the injector sits behind the intake valve. Which is closed during the combustion stroke,
and at all other strokes except the intake stroke, which means that it's keeping the
injectors away from combustion. In other words, in the case of port injection, we can start injecting during
the compression stroke. And keep injection throughout the
combustion and the exhaust stroke, all the way until the next intake stroke,
when the intake valves open again. The fuel will simply accumulate
behind the intake valves, and enter all at once, when the valves open again. This means that if you install injectors with sufficient capacity inside a port injected engine, you will never run out of fuel. In other words, you can rev as
high as your internals can survive. The port injection setup will never
be an obstruction to your RPM limit. But that's not the case with direct injection. Because as RPMs increase,
the time frame for injection becomes so small, that no amount of injector capacity
or fuel pressure can make up for it. And this is why on average direct injection
only engines are limited to about 6,500 RPM. There are exceptions with more RPM, with higher RPM limits that are achieved
at the expense of emissions or efficiency, but the common RPM limit is
usually around 6,500 RPM or less. Now, the location of injection carries one
final benefit in favor of port injection. Now, gasoline is a great solvent. Wanna clean an old greasy gunky part? Just get some gasoline & a brush, and watch magic happen. And as we said, the backs of the intake
valves in a port injected engine are constantly exposed to the stream of
gasoline coming from the injectors. Which means that they can never get dirty. They're constantly being cleaned. Of course, this is not the case
in direct injection engines, because the injector is
inside the combustion chamber. And thus it never sprays fuel
on the back of the intake valves Which means that all sorts of carbon deposits,
gunk and other junk from the vehicle's PCV system can accumulate on the back of the intake
valves in direct injection only engines. Now, over time these deposits can pile
up so much on the backs of the valves, that they lead to a distorted and
reduced cross-section of the intake port. Which means that less air
can come into the engine. Which of course reduces performance
and/or leads to a rough running engine. Now, there are ways to try and prevent these
deposits. Using oil catch cans or fuel additives. But results are mixed. And the need to eventually mechanically clean
the backs of the valves is simply inevitable. And it usually happens when the engine has
around 100-150 thousand kilometres on it. Now, this is of course a time
consuming and expensive job. Which sometimes even requires the
complete removal of the cylinder head, to access the valves and everything. Of course, this leads to increased
maintenance cost for direct injection engines Something that many see as the
main drawback of these engines. Another potential issue with
direct injection only engines. Especially earlier models of these engines. Is that the constant introduction of
fresh, sometimes poorly vapourized fuel, would actually dilute or wash away the
protective layer of oil on the cylinder walls. This of course would lead to increased wear
and tear, and reduced engine longevity. However, newer models have overcome
this issue by changing the direction of the injection away from the cylinder walls. Which means that most newer direct injection only
engines, don't really suffer from this issue. Another problem that the deposits in the
backs of the valves can contribute to is LSPI. Or Low Speed Pre Ignition. Now, as we've said: Knock is abnormal combustion that
occurs AFTER the spark plug has fired. Pre-ignition is abnormal combustion that
occurs BEFORE the spark plug has fired. And pre-ignition is in most cases
worse and damaging the knock Because the piston is moving into the abnormal
combustion, instead of running away from it. As in the case of knock. Now, Low Speed Pre Ignition occurs during
low speed and the high engine load scenarios. As we've said, during low engine
RPM/low engine speeds, the pistons and direct injection only engines have trouble
tumbling and mixing of the air-fuel mixture. Because the piston moves slowly. Leading to pockets of unburned fuel
floating around in the air-fuel mixture. At the same time, a higher load scenario means, that the ECU will instruct the injectors to inject more fuel. So in other words, you need to floor it from
low RPM in a direct injection only engine, to create the preconditions needed for LSPI. Now the other thing that needs to happen, is that a particle from the back of the valves,
needs to fall off and enter the chamber. Or maybe an oil droplet makes it pass
the piston rings, and enters the chamber. Then this oil droplet and/or particle can mix with the unmixed fuel folding
around the air-fuel mixture. And then this newly formed combo will get
exposed to the very high compression ratio, of the direct injection only engine. And BOOM! The combination gets self-ignited
before the spark plug fires, leading to pre-ignition. The results are often catastrophic. And if pre-ignition keeps
occurring and is left untreated, the engine will likely receive catastrophic
damage, and will need a full rebuild. Of course high mileage engines are more prone to this, because they will have more deposit
on the back of the intake valves. And also they will have more wear and tear
on the cylinders and the piston rings, making it easier for an oil droplet
to make it into the chamber. So here we have an overview of
both port and direct injection. And as we can conclude; Neither setup is really ideal. Direct injection can improve performance,
reduce fuel consumption, and improve emissions. But at the cost of increased
maintenance and some potential problems. So faced with the drawbacks of both setups, and the need keep meeting
efficiency and emission standards, car manufacturers have decided to combine
direct import injection into a single setup. Thus, creating dual injection. Interestingly enough, combining
the two types of injection, stacks up the benefits but
gets rid of the drawback. The only expense is an increased
number of moving parts. And increased production cost. Because now, instead of only one injector, each cylinder has to have two injectors,
both a direct injector and a port injector. So, how does dual injection work? Well, if you watch this video,
you can probably already guess. At low engine RPM, we're going to
rely on the good fuel vaporization and air-fuel homogenization
properties of port injection. Which means good combustion and
reduced emissions at the low RPMs. It also means that we're spraying fuel
at the backs of the injection valves, which means no deposits, no
increased maintenance costs. And reduced potential for low-speed pre-ignition. As RPMs and piston speeds
increase, injection steps in. The high piston speeds means good air-fuel mixing. And the super-accurate nature of direct injection means, that we can increase the
compression ratio of the engine. Leading to improve performance and efficiency. While at the same time reducing emissions
and fuel consumption at higher engine speeds. Want to rev high and keep
making power at high RPMs? No problem. When direct injection runs
out of time to do its thing, the ECU can simply instruct the
port injectors to rejoin the game at maximum engine loads near the RPM limit. They're going to supply the extra fuel the
direct injection doesn't have the time to supply. And Voila! All the fuel you need at any RPM. And also, all the benefits
and none of the drawbacks. The reason why more and more manufacturers
are packing dual injection into their engines. And there you have it. Something that I hope is a comprehensive overview of the three different kinds
of gasoline injection technologies. I hope you enjoyed watching this video. And I hope you learned something in the process, or maybe answer to a question you
might have had about this topic. As always, thanks a lot for watching. I'll be seeing you soon, with
more fun and useful stuff. On the D4A channel.
