Common Rail vs Direct Injection *Diesel*

[zanakas]

Bosch passenger common rail system 3rd gen piezo injectors 1800bar.

http://www.boschautoparts.co.uk/pcDies14_1.asp?c=2&d=1

Bosch passenger unit injector system type UIS P2 exceeds 2500 bar.

http://www.boschautoparts.co.uk/pcDies14_3.asp?c=2&d=1

So the advantages of common rail is not pressure. Bosch even describes higher pressure being achievable with unit injectors:

"The UIS does away with high-pressure lines and thus enables injection pressures in excess of 2200 bar on passenger cars."

As long as the prime mover of the injector can reliably sustain the reactions (the cam), not the most reliable equipment on the PD.

Common rail systems require numerous high pressure connections and consistent high pressure. Unit injectors presumably generate intermittent high pressure running off the camshaft and one connection per injector unit requiring the high pressure seal.

Common rail has other advantages presently, but it seems fairly likely the engineering challenges will be met with unit injector systems over time and cost of unit injector systems is likely to be lower over time due to scale economies in manufacturing.

Unit injectors have the clear advantage of redundancy improving reliability. A common rail pressure failure will bring the engine down. A unit injector pressure failure will bring one cylinder down.

Assuming you could drive with one dead cylinder... in all likelihood the injector failure would cause damage beyond the injector itself. Not to mention that the CRD system, as a whole, is much simpler, and uses far fewer moving parts. I think its tough to argue the reliability edge lies with unit injection...

Do you happen to work for Bosch?

[bluey]

Do you happen to work for Bosch?

Nope. Don't even work in an engineering/automotive field. Just went looking for information from a large manufacturer of common rail and unit injection systems to try to suss out why they might be different and how they contribute to diesel clatter/nailing, of which I am not fond. Prefer facts to fairy tales.

[bluey]

BTW, Bosch apparently developed and manufatures the Volkswagen Group's unit injectors.

This interesting short article on PD vs Common rail, which says PD generates higher pressure.

This PD page has lots of photos of engine internals.

Found some interesting posts by a diesel engineer (659FBE on Sat 30 august 200 about common rail vs unit injectors. He said the main problem for CR is that the high pressure pump is fuel lubricated vs PD oil lubricated. PD is therefore better suited to multi-fuels.

[Transporter]

BTW, Bosch apparently developed and manufatures the Volkswagen Group's unit injectors.

This interesting short article on PD vs Common rail, which says PD generates higher pressure.

This PD page has lots of photos of engine internals.

Found some interesting posts by a diesel engineer (659FBE on Sat 30 august 200 about common rail vs unit injectors. He said the main problem for CR is that the high pressure pump is fuel lubricated vs PD oil lubricated. PD is therefore better suited to multi-fuels.

Bluey,
VW is changing to CR system since they know that it would be much harder to meet a new Euro5 emissions and future emission targets with PD system.

[gldgti]

Nope. Don't even work in an engineering/automotive field. Just went looking for information from a large manufacturer of common rail and unit injection systems to try to suss out why they might be different and how they contribute to diesel clatter/nailing, of which I am not fond. Prefer facts to fairy tales.

its not the injection system as such that produces diesel clatter - its the combustion noise that you hear.

the way to change the noise that a diesel makes is to vary all of these things:

- engine rpm
- combustion chamber shape
- injection timing
- number of discrete injections during a cycle
- injection pressure

all of these things have a large effect on diesel clatter.

the PD and common rail systems share 1 most important design characteristic - they are both direct injection - this fact accounts for much of the diesel clatter you can hear, and is near impossible to get rid of.

the injection equipment itself produces nearly no noise at all (in comparison) especially compared to older pump driven diesels - even the VE injector pump operates quietly.

by using CR injection, you are able to control the pressure, discrete injections, timing, advance - all on the fly and with computer maps to optimise for emissions, power, even quietness. as with all things, the tune you wind up with is a compromise that the designer believes satisfies most requirements enough.

CR allows the greatest flexibility in tuning and so ofcourse this is where all the manufacturers are heading.

if reliability were an issue, i hardly think Mercedes-Benz would have taken up CR tech so long ago - yet they did.

i spent the better part of a year researching diesel injection for my mechanical engineering degree, and i can tell you now, there's no wizardry involved in any of this stuff - nor is it i a matter of which system is quieter/noisier, torquier, free revving etc - you can tune any diesel engine to run however you like, providing you have enough flexibility in the injection system to allow you to tune the way you want it to be, and providing you can supply it with sufficient air.

CR is simply the next step in flexibility for the manufacturer.

[CatonaPC©]

So after reading all this I get the impression manufacturers are heading down the CR path, not necessarily because it's better than PD, but because it's cheaper?

From what I read, emissions are not an issue as PD can be tuned to comply with future standards. But CR certainly appears to be a simpler approach.

[Transporter]

So after reading all this I get the impression manufacturers are heading down the CR path, not necessarily because it's better than PD, but because it's cheaper?

From what I read, emissions are not an issue as PD can be tuned to comply with future standards. But CR certainly appears to be a simpler approach.

You don't know what the future emissons standards are, so how do you know that the Unit Injector System can be tuned to meet the future emissions standards?

[CatonaPC©]

You don't know what the future emissons standards are, so how do you know that the Unit Injector System can be tuned to meet the future emissions standards?

True. What I am saying is that from all that I have read CR may prove the easier option to meet those standards. But I don't see any reason why PD can't be developed to meet future standards other than cost or complexity.

[Transporter]

True. What I am saying is that from all that I have read CR may prove the easier option to meet those standards. But I don't see any reason why PD can't be developed to meet future standards other than cost or complexity.

The reason is the limitation of the Unit Injector System.

Since the unit injector is camshaft lobe operated it cannot control fuel injection as well as CR system. In CR system the fuel rail (common rail) is pressurised by

a high-pressure pump (the radial piston pump on the cars) that pressure is generated independently of the fuel injection cycle. ECU in CRD system uses fuel

injector similar to the one in todays EFI cars and it can inject fuel onto the cylinder at any time which result in the better control of the fuel injection.

In the Unit Injector System it is the diesel fuel high fuel pressure, which lift the nozzle and fuel is injected (the same diesel fuel pressure like on old mechanical

pump system - only much higher). ECU uses the solenoid in the Unit injector for precise control of injection timing, quantity and pressure.


Somebody made the decision that Common Rail is more suitable at this time.
It is possible that The Unit Injector System can come back in the future, who knows, but the one thing I know that CRD has less soot from the exhaust when there is no DPF fitted.

[recalibrate_ecu]

Developmental progress

Hi,
Also the early versions of the common rail could not pressurise diesel as highly as a pd injector. Howver, the common rail system is cheaper - hence the 1.9 dti engine found in the holden astra/ saab 9-3 and alfa 147.

More recently common rail systems have developed to inject up to 2200 bar - yes that is almost 32000 psi which will puncture a hole in your finger should you decide to place it in the way.
More commonly (except audi r15 tdi race lmp1) injection pressure are around 1600 -1800 bar.
So common rail can now inject at the same pressure as pd and is cheaper.
Hope this helps?

Or Volkswagen Audi Grupe

[recalibrate_ecu]

Diagnostic check

Before I start the process I will complete a diagnostic engine check to ensure your vehicle doesn't have any faults.
If for any reason you require the vehicle to be returned to the 'stock' map I hope to do this for a nominal charge of approx. $50.

[Shifter]

Interesting!

A proper comman rail diesel engine, opposed piston, turbo charged 4 cylinder J type Doxford. This engine was proably built sometime in the mid to early 60's but Doxford comman rail (6000 psi) diesel engines have been around since the 30's not new technology. Interesting to note that most high speed marine diesel engiens are all mostly comman rail dervivatives.