Why the move to Common Rail (CR)? What was wrong with Pump Duse (PD)?

[Tech1e]

A few people have asked me so i thought i would try to explain.

Around the year 2001 VW/Audi introduced the PD engine. Not a new design in theory as this type of diesel injection had been used for many years in marine applications but it hadn't come this far in terms of control. A unit injector is fitted per cylinder. The injector includes its very own high pressure pump (internal pressures of 2050 bar on the 1.9 and 2200 on the later 2.0 piezo injectors). The injector and the pump is run off the camshaft, with a cam lobe pushing down and generating the pressure. The solenoid controls the amount of fuel that is required, timing if the injector is mechanical still.

This is the fundamental problem with the PD engine and what will eventually lead to its sad demise. With new legislation all diesel engines will need to run a DPF to meet Euro 5 regs (IIRC). To activate self regeneration, a very late cycle of injection needs to be added; the fuel is injected post combustion which increases exhaust temperature which starts the regeneration process. As the timing of this injection is controlled by a mechanical factor, i.e the camshaft, there is only so much that can be done. The PD engines fitted with the DPF have a different camshaft with an additional profile to the lobe; this gives another fuel injection post combustion for regeneration. However this all has to happen within 360 degrees of camshaft rotation. For there to be time for the injector to work again for the next cycle, this obviously has some serious limitations.

With the advent of 'us' using a CR system, fuel pressure is supplied by a very high pressure pump (circa 180bar, so don't go cracking any pipes off!) to a manifold (or fuel rail) that then supplies the injectors. The injectors are then controlled by the ECU. The injector can then be mapped to deliver whenever it is required giving a greater level of control. This is also why they have been able to drop the compression ratio and make the engine quieter in operation.

I hope you find this information interesting if not usefull. Its all off the top of my head so feel free to take it with a pinch of salt

[cjb]

Interesting thanks.

Wasn't one of the reasons for adopting PD that there was a licence fee for CR? I'm sure I read that once.

[edit]

So if one was working on CR presumably there's a procedure to depressurise the rail?

[devonutopia]

Which is better for the top end tuning I wonder? Wonder how long until some seriously tunes one up.

[TeflonTom]

tuning wise i should imagine they are fairly similar in potential,

[Royston]

That's a whole 5 minutes of my life I'll never get back!:P

[Tech1e]

So if one was working on CR presumably there's a procedure to depressurise the rail?

There is a sensor on the end of the rail which dumps fuel pressure when the ignition turns off so there is only pressure when its running.

[TeflonTom]

There is a sensor on the end of the rail which dumps fuel pressure when the ignition turns off so there is only pressure when its running.

yeah, tis the same on the renault ones too, the fuel still remains in the rail but it just releases the pressure

[fabia vrs mark]

very interesting. thanks

[VWD]

What we need is a translation channel - now if I started to chant about the conductanceof a circuit in mhos ,or the dataflow in mb/s or the phase angle change of this /that - wouldn't be surprised if peeps got lost .

Get most of the gist of what's being talked about -so can we have a translater in please - sounds interesting .:):)

[Tech1e]

What we need is a translation channel - now if I started to chant about the conductanceof a circuit in mhos ,or the dataflow in mb/s or the phase angle change of this /that - wouldn't be surprised if peeps got lost .

Get most of the gist of what's being talked about -so can we have a translater in please - sounds interesting .:):)

I get that when I wander into the tech shed....lol

I tried to keep it as basic as possible.

[VWD]

I get that when I wander into the tech shed....lol

I tried to keep it as basic as possible.

Got interested ,then got lost , perhaps a bit less basic

Was interesting I admit .

[apd007]

Quite an interesting article. I had no trouble following it at all.

apd007 BEng (hons) auto. MSc :P :P

[andy2407]

Thanks Ross - an interesting summary.

So essentially the CR is greener and quieter. Are there any other advantages. And any obvious disadvanages?

How do CR and PD compare in terms of initial cost, reliability, service costs, etc?

[ultra_joel88]

So which is the mechanically superior engine? In terms of design, output and efficiency?

[Tech1e]

How do CR and PD compare in terms of initial cost, reliability, service costs, etc?

Well your completely fukced if you put petrol in a CR...lol

Servicing its exactly the same, same oil and filters.

[snownrock]

After coming from a CR diesel back to the PD i miss the CR, probably something to do with going from 166bhp to 105bhp lol. Miss the (relative) quiteness of operation, smooth power delivery and general refinement, but the PD still suprises me and its bombproof, unlike the Fiat CR :(

Great article lummox, i have a Haynes book of modern car technology (uber thick) on the shelf that i dip in and out of every now and again but it whizzes over my head, normally have my brother explain it to me

[shark_90]

That's a whole 5 minutes of my life I'll never get back!:P

That took you 5 minutes to read?

[KenONeill]

It all made sense to me, well except the bit where fuel is wasted by injecting it after TDC, when we're supposed to be trying to reduce our carbon footprints. And that's a comment on the environ mentalists, not on Ross.

[rotodiesel]

A few points. The PD system does allow for ECU controlled timing of the injection, it's just that the cam profile places limitations as to how long pressurised fuel is available for late injection for DPF afterburn. (Hence the modified camshafts for the DPF vehicles).

The CR system allows for fuel to be injected at any point in the cycle but, significantly, it allows far greater freedom of pilot injection control. This enables a small pilot quantity of fuel to be injected ahead of the main fuelling shot in order to establish a flame front and also limit the rate of rise of cylinder pressure on main injection. This is why the CR engines are smoother and quieter than the PDs.

The pilot injection system on the PD nozzle (stepped plunger) is a bodge and allows no independent pilot control - that's why it doesn't work very well.

However, the PD system has one huge advantage over CR. The fuel pressurising cam and roller run in engine oil and not fuel. The lubrication of CR pumps is very marginal at the operating pressures used - which is why a dose of petrol kills them. Not so with the PD, but don't try it.

The PD is unrivalled for efficiency (pilot injection does not influence engine efficiency) and for durability - which is why so many commercial diesels use unit injectors (common speak for PD). Given the continuing problems with DPFs under conditions of heavy traffic and the dubious long term prospects of CR pumps (see how Ford are getting on), I'll take good care of my non DPF (AWX) PD - its output and efficiency are outstanding.

rotodiesel.

Edited 24 March, 2009 by rotodiesel

[Willsvrs]

However, the PD system has one huge advantage over CR. The fuel pressurising cam and roller run in engine oil and not fuel. The lubrication of CR pumps is very marginal at the operating pressures used - which is why a dose of petrol kills them. Not so with the PD, but don't try it.

The PD is unrivalled for efficiency (pilot injection does not influence engine efficiency) and for durability - which is why so many commercial diesels use unit injectors (common speak for PD). Given the continuing problems with DPFs under conditions of heavy traffic and the dubious long term prospects of CR pumps (see how Ford are getting on), I'll take good care of my non DPF (AWX) PD - its output and efficiency are outstanding.

rotodiesel.

That’s an interesting point you raise about the reliability of the CR pump. I would hope that the new VAG CR engines / fuel systems are more sophisticated and advanced than Ford’s though. Ford had a number of reliability issues with the early Mondeo tdci’s due to fuel pump or injector failure. The cams in the fuel pumps were poorly casted and would wear untimely thus resulting in metal filings contaminating the fuel system. This was an expensive fix for anyone who suffered this failure and many were not covered under warranty (my brother had to foot the bill for his 2003 tdci). I believe they’ve rectified this in recent years and have upgraded the components, but it doesn’t instil great confidence.

As I say, I would hope the VAG CR to be a more reliable unit though – certainly seems smoother and more refined than my PD130J.

[rotodiesel]

There were heat treatment and tolerancing problems with the Delphi CR pumps - I'm not fully convinced that these have been completely resolved.

Putting thigngs crudely, if you need to raise fuel to a pressure of 2000 bar in order to make a diesel run efficiently and with low smoke (good atomisation and precise timing needed) you have a real problem on your hands converting the rotational motion of the engine to the linear motion required for the pump plunger. The stresses involved are enormous, which is why all diesel fuel pumps have roller followers.

An "old" rotary pump operating at, say 150 bar will function for a long time lubricated in fuel (if it's made by Bosch). Fuel lubrication at 2000 bar is absolutely on the limit and leaves no margins for poor maintenance or poor fuel.

I have yet to see a CR pump which will last as long as a unit injector.

rotodiesel.

[Wardy]

Interesting stuff - I enjoyed the read

Steve

[sms]

so am i reading it right and saying that the pd gives worse economy and performance to cr?

[Tech1e]

A few points. The PD system does allow for ECU controlled timing of the injection, it's just that the cam profile places limitations as to how long pressurised fuel is available for late injection for DPF afterburn. (Hence the modified camshafts for the DPF vehicles).

Agreed, but as you say if the lobe has passed then fuel pressure isn't high enough, a second profile can't be very effective as you only have a very small ammount of time to play with before pilot injection starts again.

I was hoping you would read, thanks for the input.

[WW_VRS]

so am i reading it right and saying that the pd gives worse economy and performance to cr?

Don't think that was what it meant, PDs are probably more efficient however the environmental requirements mean it just is not going to be a great solution for the future. CRs rev more petrol like.