Common-rail diesels

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Tue 9 Sep 2003 11:14
Common-rail diesels - Andrew-T
Can we have a brief explanation of why the current generation of c-r diesels no longer need the 5-second warm-up of the previous generation? Is it just the high-pressure fuel, or is there more to it than that? (MM?)
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Tue 9 Sep 2003 11:42
Common-rail diesels - RichardW
Andrew,

I take it you are referring to diesels of say 6 / 7 years ago that needed a 5 - 20 second pre-heat before they would start? AFAIK all common rail (and unit injector a la VAG) start almost immediately. There are 2 reasons:

1. Ceramic plugs - these heat up in a significantly shorter time than the older style metal plugs.

2. Direct injection (DI). In the old style engines, the fuel was injected into an 'ante' chamber where it began combusting, before expanding out into the main cylinder where combustion was completed - indirect injection (IDI). This allowed the diesel 'knock' to be much lessened and is why IDI engines are quieter than their DI counterparts (think about the perkins DI engine in the Montego!), but also why they are less efficient. Moder DI engines of course use staged injection (electronically controlled) to make the combustion process slower, and hence reduce the knock. Now, when you crank a diesel engine you need to get the air to heat up so the diesel will auto-ignite (700°C springs to mind). In an IDI engine there is no way the air will be hot enough in the ante-chamber until you have heated the head up - hence the need for glow plugs all the time (the plug is located in the ante chamber, and the diesel sprays directly over it)unless you want to churn the engine over for 4 or 5 minutes! Now, in a DI engine you inject the fuel directly into the hottest part of the air, where it will ignite unless the ambient temperature is very low - hence no need for glowplugs on modern DI engines (things like old Landrovers still need plugs even though they are DI due to the amount of metal conducting the compression heat away). I suspect that the electronics play a part too, staging the injection to avoid over-cooling the air and causing a mis-fire (diesels unlike petrols do not need to run at stoichiometric ratio - they will run at virtually any air / diesel ratio). Another thought - perhaps the plugs ARE on, but they heat up so fast that they can be on whilst the engine is cranked so you don't need to wait. Hmm, got me thinking now!! Sean would know!

High pressure fuel has not got much to do with starting (I don't think) this just allows better atomisation of the fuel (diesel is liquid phase combustion) which leads to cleaner burning as the time for a particle to burn is finite, so the smaller it is the better chance it will be totally burnt by the end of the combustion stroke.


RichardW

Is it illogical? It must be Citroen....
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Tue 9 Sep 2003 11:53
Common-rail diesels - daveyjp
On older diesels the glow plugs were switched on with the ignition, then you had to wait while they warmed up.

On some newer diesels the glow plugs are switched on when the doors are unlocked, so by the time you get in your seat and get the key in the ignition the car is ready to be started.
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Tue 9 Sep 2003 12:09
Common-rail diesels - jc
There were some diesels in the past that did not use glowplugs with DI but I'm talking lorry not car-just used excess fuelling to start;there were others that used Thermostart-a coil in the inlet manifold that glowed red-hot and was then sprayed with fuel;checking one of these was working off the vehicle was most spectacular!
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Tue 9 Sep 2003 17:49
Common-rail diesels - AndyT
I reckon that glow plugs in the latest direct-injection diesels are more to do with cold start emmissions, than anything else.
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