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Old 02-19-2011, 11:13 PM   #1
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Default Direct Injection Question

I was just wondering if hypothetically gasoline could be directly injected with lower fuel pressures of 5-10 bar using a regular electric pump, rather than using typical pressures of 50-100 bar. The main reason GDi systems need the high pressure is to atomize the fuel, a process that takes place in the runners in multi-port injection. When looking at the static flow of direct injectors at 5 bar, it seems as if the fuel would be sufficiently atomized for combustion. Obviously it won't work since there are no GDi systems that use lower pressures, I'm just wondering why.

Also I know that typical combustion pressures under load are around 50 bar, perhaps only 10 bar plus spring pressure on the pin would cause backpressure?
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Old 02-20-2011, 08:15 PM   #2
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Yes, but would 5bar atomize and flow quick enough to complete the injection event fast enough?

In other words, you don't want to be still injecting while the piston is coming up for compression stroke. You want it well mixed beforehand to avoid localized lean spots in the charge. Those local lean areas may cause pre-ignition for the whole volume. If not, the corresponding local rich areas may cause sooty/smoking combustion which is also bad for emissions.
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Old 02-20-2011, 08:48 PM   #3
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For starters, think about running a compression test. We typically like to see 180-200 PSI. That's roughly 13 bar. You'd need that much just to prevent air from the chamber being blown backwards through the injector. If you want actual fuel flow, double it. If you want enough fuel flow to supprt 200 hp going in within less than 180 degrees on the crank (less than 25 pct duty cycle) then quadruple it.
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Old 02-21-2011, 05:18 AM   #4
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The insanely high pressures are also to help speed up fuel delivery into the chamber.

With direct injection you have far far less time to inject fuel, e.g. with non direct injection you can inject whilst the valve is closed and the chamber is in it's exhaust cycle, you can't do that with direct injection.
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Old 02-21-2011, 03:49 PM   #5
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What about starting the engine? Does the pressurized fuel remain high in the common rail until the next startup? What if the pressure leaks down after awhile, and the first crank revolution isn't enough to bring the pressure from the cam driven fuel pump above compression pressure?

Also could our rotary vane power steering pump be used as an adequate alternative to supply high fuel pressure, assuming the internal pressure regulator is adjusted accordingly? My manual says it outputs ~80 bar at 1000 engine rpm.
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Old 02-21-2011, 05:46 PM   #6
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The pump will fire and prime the pressure I imagine, just like your non direct injection Miata does.

What are you hoping to achieve?
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Old 02-21-2011, 06:06 PM   #7
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Agreed- I can't imagine that the fuel pump would be unable to prime and generate pressure prior to the starter getting the flywheel up to speed and generating compression.

I have absolutely zero hands-on experience with direct-injection systems, be they gasoline or diesel. It may be that the injectors have reverse-flow checkvalves.

The P/S pump idea sounds interesting, though it obviously fails insofar as being able to prime the system before cranking. You'd also need to find out what kind of static pressure it can develop at 200 engine RPM.

I know that diesel engines typically use a crankshaft-driven fuel pump, but what about GDI engines? Is there some compact electrical unit that might be available?
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Old 02-21-2011, 10:28 PM   #8
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apparently, one of the manufacturers has booster fuel pumps, (attached to, and part of each injector) - if you could source those injectors, you might be able to make this work with the rest of the fuel system at stock pressures.
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Old 02-21-2011, 10:37 PM   #9
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Besides the fuel pressure…the DI gizmos have to deal with the heat and combustion pressure…indefinitely.

BTW there are folks that built one into a spark plug (or vice versa) for use in non-DI heads. Ideally things like the shape of the piston top details need to consider DI factors.
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Old 03-07-2011, 10:32 PM   #10
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Is GDi really "all that"? Say, if you were to have the same fuel pressures for DI and Multi-port Injection (assume both inject only during the intake cycle) for equal atomization and charge cooling, from a performance standpoint I see no advantage of DI. From an efficiency standpoint DI does have the advantage of only spraying fuel at a specific point by the spark plug right before ignition for A/F ratios of > 60:1 under cruise. Other than that I see no reason to add much more complexity and cost to a system for a performance enthusiast where the only benefit would be better emissions and a 5-10% increase in miles per gallon. It's great technology for 99% of automobile users, but for the high power/torque seekers DI doesn't seem like it would offer significant gains over port injection. Please correct me if I am wrong.


Here is a good article I've been reading on what is needed to operate the injectors:
http://www.drivven.com/visitor_downl...er_UM_RevE.pdf

Is it me or does it seem like carbon deposits would easily clog the holes? I suppose high pressure helps with that.
http://delphi.com/manufacturers/auto...ys/homogenous/
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Old 03-07-2011, 11:00 PM   #11
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hitting the piston direclty with fuel has a combustion chamber cooling effect. thats alittle over simplified but then again, tonights martini night..
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Old 03-07-2011, 11:27 PM   #12
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The proof's in the pudding, GDI motors can run higher compression ratios.
That's good for power production.
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Old 03-08-2011, 12:07 AM   #13
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I would assume higher compression ratios are allowed due to the added cooling DI provides on the internal components. Couldn't I just add sequential water injection and higher compression for say, $3000 dollars less for a similar effect? Albeit there would be the added complexity and *cost* of having to fill a separate tank with distilled water every so often that the average joe driver wouldn't tolerate.

I suppose it's all about getting the most bang for your buck these days, and that's what auto manufacturers use to compete.
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Old 03-08-2011, 05:36 PM   #14
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This is my rather uneducated postulation of the ideas.

You can up compression to 14:1 and use race gas

or

You can up compression to 14:1 and use direct injection

or

You can up compression to 14:1 and use water injection



Now - with that being said,

The functional relationship between race gas and GDI is equivalent to the relationship between nitrous and a turbocharger. One costs a bit more up front, the other costs a LOT more over time and can be a bitch to find.

With water injection, you're going to need to squirt water anytime you've got the car under a decent load, and if you run out of water, you're done for.

With GDI, you put fuel in the fuel tank.
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