What's the difference....

[Tumble]

between Common Rail & Direct Injection fueling in Diesels... I realised this morning that I thought they were the same thing.... i knew they were different, of course, but the way I described them to myself was the same... weird huh?


Miffed in the Isle of Man

[Big RICHARD]

Common rail is exactly what it says. The diesel is compressed in a common chamber and this chamber feeds all of the cylinders. Direct injection has an injector for each cylinder.

[J4MES]

hmm... direct injection refers to the fact that the injectors squirt fuel directly into the cylinder (because diesel ignites on compresson at 20:1, not due to a spark) and not into the inlet manifold, like with a petrol injection engine... the VW PD (pump deuse i think) sticks it in at a rediculous PSI of about 2000 IIRC, and i think it runs off a common rail too - not sure about this but i believe that the fuel lines all run off a common rail from the fuel compressor to the injectors, and are pressurised - all the injector does is opens and closes when it's supposed to...

[Zak33]

Whoaah Horgunner...not true bloke

sorry

First : Indirect Injection.....this is NOTwhere fuel is sprayed into the inlet mainfold...thats only petrol cars.

On an indirect diesel car (read ALL cars for years and years, inluding some of the best like Pug 1.9 DTurbo) the air is sucked in, the valve shuts and the fuel is sprayed into a little side chamber, cast into the head, that is connected to the main combustion chamber. It was used because it was quieter.....the fuel begins itsdetonation in this little chamber and the flame front spreads out of there and down to push the piston.

Taxis and lorries, buses etc, have all been Direct Injection for along time, BUT THEY WERE NOISY ...no good for cars.

Direct Injection is where the fuel is sprayed into the actual piston chamber, not a side chamber. It is NOW quiet because a tiny amount of diesel is sprayed in as the piston rises, it catches fire and helps to "cushion" the diesel "clatter" noise.....then near top dead centre, the rest is sprayed in.

In a Common Rail diesel engine, as explained up there/\ , the fuel is ALL in ONE FUEL INJECTOR RAIL under even higher pressure than Direct Injection.

The problem for ME is that I still dontknow WHY it is quieter.....and that is the reason for Common Rail systems....they are quieter. I believe it is because the seperate fuel injectors with one common rail are easier to control and allow a better "pre charge" to be injected.....but trust me....VW did it along time ago on the first Direct Injection 90bhp 1.9 TDi Golf etc....and Peugeot do it too.....

So I am unable to tell ya WHY it is both more powerfull AND quiter than even Direct Injection......I've got an Astra here with the new 1.7 cdTi in it and its like driving a petrol car in all but low tick over......the 3.0 V6 24v Diesel in the Signum is Ok too

pull a house down with that thing

[Zak33]

Here ya go.......

Quote from : http://www.dieselforum.org/factsheet/commonrail.html

HOW DOES A COMMON RAIL INJECTION SYSTEM WORK?

The way in which fuel is injected into the cylinders determines the torque, fuel consumption, emissions and noise of diesels. Two factors are important: the fuel pressure as it enters the cylinder, and the shape and number of the injections.

A common-rail injection system separates the two functions - generating pressure and injecting - by first storing the fuel under high pressure in a central accumulator rail, then delivering it to the individual electronically-controlled injection valves (injectors) on demand. This ensures that incredibly high injection pressures (over 25,000 pounds per square inch in some systems) are available at all times, even at low engine speeds.

[Zak33]

so in essence (makes sense nowmy brain has grabbed it) the problem up to NOW has been controlling the fact that the injectors were not so easily controllable as on a Petrol engine, cos to get the fuel to spray finely enough involves MONSTER PRESSURE......and to do this with individual injectors involves SERIOUS technology on Injectors AND a good ecu to control them.

Go back a little step to grasp this. On an older diesel, the fuel injection timing was controlled by the FUEL PUMP....as it turned it delivered the fuel at preset points (like an Ignition coil throws the spark at preset points on a petrol car)

The Direct Injection allowed the fuel to be put into the cylinder directly, and if need be, in two phases, BUT STILL NOT UNDER FIRECE PRESSURE cos it was being sent from 4 different "sources".....

Now the technology has caught up, I GUESS (ie: I am trying here, but will take advice from any of you with open arms) that the ability for a fuel Injector to hold a zillion PSI behind its nozzle until a spit second timing decision by a new sort of ECU is only just possible.

PLUS....its gotta beworth doing.....and as some companies jumped from indirect straight to Common rail, it means the compaines with Direct Injection must have thought "sod that, ours go like nutters anyway, stick with ours"

Right..open field now...what does anyone else know!

And I mean KNOW

[J4MES]

er, thats what i said...