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Old 3rd July 2011, 10:26   #16
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Default Re: Common Rail Diesel Injection System

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Pressure (or similar quantities) can be limited in many ways, limiters ...
All that is fine, but I still don't see the difference in the context of what we are discussing.

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... Now two pieces of puzzle remain - can somebody tell me ...
[quote=vina;2415021]... this means a drop in pressure of 5MPa or about 50bar - not much really, even if rail pressure was as low as 500bar. .../quote]Difficult to figure out what you are getting at - whether to prove the obvious (ECU controlled common rail diesel injection works well) or to prove your hypothesis about something, or to understand the scientific principles behind diesel injection technology!

With your vast knowledge (and the appreciable quest for it), perhaps we can all benefit if you stay on subjects & issues that benefit all the readers on TBHP, instead of indulging in soliloquy where we are the losers?

Let us stay on the subject of CRDI technology, shall we?
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Old 3rd July 2011, 18:44   #17
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Note from Mod : Please use the EDIT or MULTI-QUOTE buttons instead of typing back to back posts.

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...


Let us stay on the subject of CRDI technology, shall we?

Pray do tell me when did I stray off the subject of CRDI? I thought I was talking about how exactly common rail stores diesel and also that the CR in CRDI stand for Common Rail.

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Originally Posted by dhanushs View Post
.... I bet most of us would love to know whats happening under the hood: Common Rail Diesel Injection System.

Recently, I was discussing with my friend at Bosch regarding the fuel consumed while coasting in gear. It was then he mentioned that, there are 7 types of injection in a Common rail diesel injection system.

Broadly classified into three.
  • Pre
  • Main
  • Post
Pre: Mostly, there are 3 types of pre injection. One of it is pilot injection. The pilot injection (as I was told) was to create a perfect burning environment for the main injection. And to avoid any hot spots and make the combustion even. Another type of pre injection is used to (As I was said) control the NVH of the engine (I'm unclear of this).

Main: As termed, the main injection, which causes the combustion.

Post: Again, mostly, there are 3 types of post injection. I'm not so clear on this regard, but then, the post injections are used to regulate dpf, and also said to be used to control the temperature. And also, while coasting, if there is any kind of injection, then its post injection.

...

No discussion of CRDI (widely considered the most advanced at this time) will be complete without mentioning its closest competitor - Unit Injection

This is still a direct injection technology and apparently generates higher pressures than Common-Rail at the moment.

Where it loses out at the moment is in that it has no capability of multiple injections (only common rail has it) and hence on-car ignition optimisation is possible to a far smaller degree than for common rail.


Gurus may kindly add more comments.

Last edited by GTO : 3rd July 2011 at 20:40. Reason: Please use the EDIT or MULTI-QUOTE buttons instead of typing back to back posts.
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Old 3rd July 2011, 23:26   #18
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Default Re: Common Rail Diesel Injection System

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Originally Posted by vina View Post
... No discussion of CRDI (widely considered the most advanced at this time) will be complete without mentioning its closest competitor - Unit Injection ...
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... when did I stray off the subject of CRDI? ...
Ahem! And you were saying ...? Of course it can be complete 'without mentioning its closest competitors'. Please stop making a FUBAR of others' threads.

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... This is still a direct injection technology and apparently generates higher pressures than Common-Rail at the moment. ...
Direct injection technology? So was the good old mechanical in-line pump and distributor pump technology!!! Indirect Injection was used only for passenger car diesels earlier.

And no, Unit Injector (Pumpe Duese) doesn't generate higher pressures. It can do multiple injections (that is not a limitation) - it is only because the peak pressure lasts for too short a time to be practical for multiple injection. Common Rail technology is more than 50 years older than PD tech, but it matured in reliability slightly later than PD technology. (Software) controllability of Injector was achieved in PD earlier than in CR.
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Old 6th July 2011, 12:28   #19
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Default Re: Common Rail Diesel Injection System

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Originally Posted by dhanushs View Post
... Another type of pre injection is used to (As I was said) control the NVH of the engine (I'm unclear of this).
...
Did some search on the internet - nothing came back until (on another thread) there was some discussion on pssobility of engine knocking in Figo TDCi

Engine knocking - Wikipedia, the free encyclopedia - all diesels knock - that is the reason of diesel clatter.

Then I went on the internet - no single place has the full story but here's the first clue Ford Transit Forum • View topic - DIESEL KNOCK

From several other sources what I have pieced together so far - pilot injection starts the burn, but quantity is limited to limit the knock (hence lower NVH) later injections come when the piston has already moved down and is moving down (which means flame front will have a harder time catching up) - so bigger injections can be had without the risk of large knock.

Multiple injections keep the fuel burn even (because fuel is being injected gradually) thus temperature and pressure do not peak at TDC and then go down - they can both be mantained closer to the overall average all the time (single injection near TDC will raise temperature/pressure very fast as diesel burns up and then both will go down as gas expands - piston moves down. multiple injection will inject more fuel ans temp/pressure reduces - peak temp/pressure can be less, work done will be similar). This helps with NVH and NOx both - the latter needs high temp and pressure in the first place.


Some other useful links:

http://www.vdo.fr/generator/www/fr/f...tasheet_en.pdf

http://www.ias.ac.in/sadhana/Pdf2002Jun/Pe1011.pdf

Last edited by vina : 6th July 2011 at 12:31.
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Old 31st July 2011, 15:50   #20
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Default Re: Common Rail Diesel Injection System

In CRDi engines, the pumps are pumping (just like normal diesel engine - direct injection). The rise of pressure is trapped due to elasticity of steel pipes. However, still the continuous pumping will cause the pipes to rupture. To adjust the pressure, the pumps are controlled, similar to fuel racks control of older bosch pumps. And this is linked to fuel pressure sensor on fuel rail.

Common rail has the advantage that the injection and pumping are independent of each and can be seperately controlled. Hence you can have various programs set for injection (pre-injection, pilot injector, pulse injection, multiple injection, etc).

In pump duese and bosch pumps, the fuel injector and pump is locked to each other. Hence they are very very difficult to control independently and will require a lot of profiling of pumps, a very very expensive project, if any one really wants to do it.

rgds
rk
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Old 31st July 2011, 20:22   #21
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Default Re: Common Rail Diesel Injection System

Pumpe Duese is not that difficult to work with. Yes, the lab work is pretty extensive. As long as the pump reaches peak pressure and sustains it long enough (cam profile), there is enough time to make the injector dance a few steps. Not like Hrithik Roshan, more like Jitendra!

The CRDI pump (which maintains rail pressure) is not controlled like the in-line and distributor pumps. There is a mechanical rail pressure regulator which controls the pressure to safe levels.
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