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So I imagine a direct injection injector as a fuel injector squirting fuel directly into the combustion chamber. The amount of pressure in the combustion chamber would seem to be enough to push the fuel back into the injector.

Obviously that is not the case.

My Questions.

Is a direct injection injector any different than a port injection injector in terms of how it works?

How do they keep the pressure of combustion from escaping into the injector itself?

Do you have a diagram?

Tell me like I'm 5. Thanks.

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I will try to answer this as best as possible. There are a few factors that come into play here (Mainly the manufacturer of said engine).

Direct injection petrol engines work much like a diesel. You have a low pressure fuel pump that resides in the fuel tank itself, and a super high pressure pump that sits near the fuel rail that delivers fuel to the injectors. The high pressure pump is the first way fuel "Blow back" is avoided. It sits at a constant high pressure, which is MUCH MUCH greater than that of the combustion chamber. We know that if pressure of one atmosphere meets another and it is greater; Any fluid substance will not be able to pressurize. SO, being that the pump is pumping faster, and greater pressure that fuel should not be able to back wash into the injector theoretically.

The second step in this "Sealing" process is the injector itself. The injector has massive solenoids that allow it to open and close with great force. There should be several high strength rubber seals and a giant magnet. Unlike a normal solenoid these types of injectors can actuate both ways. Typically a standard fuel injector can only receive power to OPEN it. In a direct injection engine, they can be both opened and forced closed.

It's kind of hard to answer the question VERY thoroughly because it's not really that complicated, but if you want to ask this on an engineering forum that would probably be good too.

What it all boils down to, is that all of the seals and systems were engineered to handle it. There are MANY different types of these injectors as well, so it might be specific to that car. I hope I gave you a little insight at the very least. Check this video out too.

https://www.youtube.com/watch?v=LjJSbHxIvnM

  • I think this is the nugget I was looking for, "In a direct injection engine, they can be both opened and forced closed." – DucatiKiller Apr 22 '16 at 0:15
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    @DucatiKiller Yeah that is honestly one of the most important parts of the whole theory operation with them. – cloudnyn3 Apr 22 '16 at 0:17
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In cylinder pressure is in the 200 psi range. GDI fuel pressure is varied from 500 up to around 3000 psi. This alone is enough to keep fuel blow-back to a minimum. Carefully engineered injector pintle shape also helps. This is no different than port injectors.

What is unusual is how the pintle is moved in a GDI injector. In the most successful designs the motion of the pintle is performed by a long stack of many piezoelectric crystals. As each crystal is energized it vibrates, effectively making it larger. Add several hundred of these in a stack and you can get enough motion to move a pintle.

Article with picture of piezoelectric actuated injector

  • That's quality data. Thanks for the contribution As ever +1 ;-) – DucatiKiller Apr 22 '16 at 6:23
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Surely there is a far simpler answer to this. Fuel in injected on the "suck" stroke, when the piston crown is traveling down the bore, thus effectively creating negative pressure in the combustion chamber. There is therefore no pressure to be pushed back into the injector.

Yes the fuel is at a serious pressure, yes the injector is effectively a one-way valve but the main reason it doesn't blow back is the same reason that fuel isn't pushed back into a carburetor on an older engine. When the valve is open, the piston is traveling down the bore, it is sucking air and fuel into the engine.

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    I think he was more so asking what makes the difference between a normal injector and one designed for direct injection. Yes, it is sucked in. However, the atmospheric pressure of the cylinder can be both positive and negative. It never rests at a constant lp – cloudnyn3 Apr 22 '16 at 12:41
  • The injector is only open on the down stroke during which time the combustion chamber is at a negative pressure. Otherwise it wouldn't be able to suck any air in from the inlet manifold. – Steve Matthews Apr 22 '16 at 12:42
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    I understand that, I think he was looking for the answer as to why it can do what it does. I provided a link to a video that demonstrates the physics of it. I'm not detesting what you're saying though because you're right, I just wanted to make sure I answered it fully. – cloudnyn3 Apr 22 '16 at 12:46
  • @SteveMatthews Sure the fuel can be injected on the intake stroke but that negates the real engineering advantages gained. Might as well go back to port injectors. – Fred Wilson Apr 22 '16 at 17:51
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    @SteveMatthews - I have to disagree. GDI, and multiport, sequential, semi-seq, and continuous systems: At a 7200rpm, you'd have almost no time to do the injection. Intake valve duration is a longer event than the intake stroke, so say 270 degrees of cam. Older systems sprayed on a cold closed valve, and defaulted to batch at high RPM. The beauty of GDI is that it CAN overcome BMEP at any time, and inject fuel at any time. Even after ignition... ways to ignite the kernal, burn lean, but chase the piston down with more combustion (=more torque). Really cool things developing here. – SteveRacer May 30 '16 at 22:33

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