Why does direct injection work?

Started by MrH, June 22, 2005, 10:31:46 AM

MrH

Ok, maybe someone can help me with this.  I was just thinking about the system in general.  I think we can all agree a homogenous mixture of fuel and gas burn better, and more completely than a heterogenous mixture.  So why exactly does direct injection work as well as it does?

Does the direct injection allow for a high compression ratio, because the fuel isn't being added until closer to ignition than port injection?  So effectively, you're compressing the air more, and then adding the fuel, creating even more pressure?  Couldn't this be overcome by just upping the compressiong ratio on a port injection engine, and then it would burn more efficiently too?

Just thinking out loud.  Hopefully someone has the answers.
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#1
Well, I think what it comes down to is that it's more precise.
The fuel can be injected at the exact moment directly into the combustion chamber, and it's a very fine mist since direct injection systems run at a much higher psi. So better mixture at a better moment.
Injectors placed higher up in the stream don't mix uniformly, and can get stuck on the walls of the intake, pool and create an uneven amount of fuel delivery.

I don't quite follow your compression ratio idea, but adding the fuel closer to ignition doesn't raise your compression....
Or are you saying raise the compression and since the fuel is added later is resists detonation better? If that's the case, I don't that that is correct.

MrH

#2
QuoteWell, I think what it comes down to is that it's more precise.
The fuel can be injected at the exact moment directly into the combustion chamber, and it's a very fine mist since direct injection systems run at a much higher psi. So better mixture at a better moment.
Injectors placed higher up in the stream don't mix uniformly, and can get stuck on the walls of the intake, pool and create an uneven amount of fuel delivery.

I don't quite follow your compression ratio idea, but adding the fuel closer to ignition doesn't raise your compression....
Or are you saying raise the compression and since the fuel is added later is resists detonation better? If that's the case, I don't that that is correct.
You said a better mixture at a better moment.  The thing is, it's not a better mixture.  It's considered a heterogenous mixture, while port injection is considered a homogenous mixture.  And from what I've read, it burns more efficiently in homogenous form.

As for the compression idea.  I said it can run a higher compression ratio, due to the fact the piston is only compressing the air at first, instead of a mixture of both.  Then the fuel is added through a high pressure injector, which would increase the amount of air/fuel in that given space, and the pressure would then jump up.  Basically, you can run a higher compression ratio, due to the fact you are only compressing the air most of the time, and then jamming fuel in there too right before ignition.  Also, you can bring in more moles of air on the intake, because you are only taking air in, as opposed to both air and fuel.  So effectively, you get more air in the cylinder, and less strain on the piston, and can therefore run higher compression ratios.

Does that make sense?  I think the small loss due to the air and fuel not being uniformly mixed is overcome by the ability to run a higher compression ratio.

EDIT:  I'd think you'd be able to control the air/fuel ratio better too.  So I'm guessing direct injection engines run a tad leaner than port injection.
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BartsSVO

The small amount of fuel that is injected into the cylinder doesn't effect the static pressure inside the cylinder enough to matter. Remember that during normal cruise there are 14.7 parts air to every part of fuel.

A major advantage of direct fuel injection is it eliminates fuel pooling and gives a more uniform distribution of fuel in the cylinder. I imagine it also helps combat detonation by slightly cooling the air charge. In the end you get a more efficient and cleaner running engine.  
--Bart

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