Common Rail vs Direct Injection *Diesel*

[vw50]

Hello all:

Could one of you 'tech heads' please explain the diff; pros; cons; of common rail and direct injection vw diesel engines. I know only the basic stuff, but i see that some new models will have common rail.

thanks,

vw50

[brackie]

Wow....This could be a long one! In a nutshell:

Most large truck/machinery diesels are, and some early passenger diesels were, direct injection
"direct injection" means just that...the diesel is atomised by the injector and sprayed directly into the combustion space offered by the ascending piston
direct injection diesels are generally noisier (more "diesel knock") than IDI (InDirect Injection) diesels
indirect injection was adopted by engine developers who saw it as a way of offering more petrol engine-like characteristics to buyers of diesel engined passenger cars and light commercials... eg less noise, smoother running and a more compact engine
direct injection is generally more fuel efficient than indirect injection as the fuel isn't forced to "swirl" in the pre-combustion chamber, a process that saps some of its potency
glow plugs to preheat the fuel in the pre combustion chamber for easier starting aren't so important in direct injection diesels (although they are still used to make the car more petrol engine-like)
direct injection became applicable in passenger car diesel only after computerisation of the fuel system. This combined with modifications to the pump(s) and injectors solved all of the problems of earlier direct injection enginesCommon Rail

until VW brought in the PD engines of the later TDIs, all diesels had a separate injection pump to provide pressurised diesel to each injector. These were either multi-element (in-line) pumps or distributor pumps like the Bosch VE used on VWs until PD was introduced
injection pumps are expensive and complex with very fine tolerances and are only serviceable by people who know what they're doing (having said that they are extremely durable and should outlast the engine if the fuel is kept clean and water-free)
common rail injection involves a high pressure fuel pump delivering extremely high pressure fuel to a pipe (the "common rail") that delivers fuel to the injectors. Unlike other systems, the fuel is under a constantly high pressure.
Each injector is computer controlled in order to deliver the exact amount of fuel to the cylinder at the exact time it's needed. This negates all of the "guesswork" that evolves no matter how well the fuel injecting system is designed or maintained in a mechanical systemThere's a whole lot more to this than this brief account, however it may help your understanding.

I stand fully prepared to edit and amend any inaccuracies in the account above :)

I'll move this thread to the "Diesels" forum

[gldgti]

well done brackie,

vw50, you should understand that diesel technology is actually developing now, and so there are no cut and dried answers like which is better. different diesel fuel injection and combustion systems are suited to different applications, the fuel you want to use, how you want the engine to behave etc etc etc at length.

it is interesting to note that many companies started using common rail injection before vw did, and vw are very clear about this - they feel/felt that common rail is not up to the performance and reliability standards of PD (pump duse) (someone correct my spelling).

[Transporter]

:)
well done brackie,

vw50, you should understand that diesel technology is actually developing now, and so there are no cut and dried answers like which is better. different diesel fuel injection and combustion systems are suited to different applications, the fuel you want to use, how you want the engine to behave etc etc etc at length.

it is interesting to note that many companies started using common rail injection before vw did, and vw are very clear about this - they feel/felt that common rail is not up to the performance and reliability standards of PD (pump deuse) (someone correct my spelling).

Isn't it the other way arround?
PD system is not able to meet future emission requairments.
Common rail is.
VW aknowledges that and starts fitting common rail system to their engines.
Temporarily they see window of oportunity with DPF fitted and after exhaust treatment.
Unit injector system is not as flexible as common rail and I don't think it is more reliable either. Common rail system allows multiple injections and pre injection or after injection. Maximum pressure today is 2000 bars. Verry close to max pressure in PD system. There is less black smoke (soot) from the exhaust of the common rail than PD TDi-Before fitting DPF or after exhaust treatment.
I've had MB Vito van which was common rail it was reliable and exhaust fumes were not as black as from VW Transporter T5 PD TDi.
I was driving them for six years I never saw black exhaust smoke in the rear view mirror. It is not a case of VW Transporter. I can make the car behind me disapear in the clouds of black smoke if I accelerate from lights little bit faster than normall driving requaires.
But I like driving my PD TDI's.

[brackie]

Thanks for that info. We dieselers are always hungry for knowledge and up to date information.

This sort of thread should really be archived so I'm going to make it a "sticky".

What do you guys think about a FAQ page?

[phaeton]

Already added one ;) (while you were away)

EDIT Thanks for all the info guys ;)

[Seano]

Why bother? No need.

Wikipedia does it all better (up to date, no typos, articles that are planned and therefore well structured and well written, compared to a dynamic group dialogue).

Point your browser to:

http://en.wikipedia.org/wiki/Common_rail (the Wikipedia version doesn't mention Lucas, so the diesel engineering teachers and textbooks I had are obviously worthless fools!)

http://en.wikipedia.org/wiki/Pumpe-duse (hard to understand why the article doesn't mention VAG, Audi or VW; but then the manufacturer of my car, my diesel engineering teachers and texts are obviously worthless fools! Not to mention the questionability of representing u-umlaut with 'u' instead of 'ue'!)

http://en.wikipedia.org/wiki/Turbocharged_Direct_Injection (at last a mention of VAG - even if it refers to the 'Volkswagen Group" and not the *correct* Volkswagen Audi Gesellschaft.)

So Wikipedia STILL does it better then........<ha ha ha> .......forgive my cynicism.

But gee this comment hardly adds to this thread......does it? Oh well.

Lets add something then........a new diesel injection concept with delivery pressures of 160,000psi, developed in Australia and can apparently be retrofitted. Currently seeking investment http://www.greendieselcorp.com/

..and the use of LPG in conjunction with diesel to make a bigger bang and reduce fuel use http://www.dieselgasaustralia.com.au/

[phaeton]

Why bother? No need.

Wikipedia does it all better (up to date, no typos, articles that are planned and therefore well structured and well written, compared to a dynamic group dialogue).

(at last a mention of VAG - even if it refers to the 'Volkswagen Group" and not the *correct* Volkswagen Audi Gesellschaft.)

Wikipedia is more correct than most encylopedias but not always 100% accurate as its free for anyone to edit the pages ;)

VAG actually means Volkswagen Aktiengesellschaft (stock trading company ;))

[Transporter]

Wikipedia is more correct than most encylopedias but not always 100% accurate as its free for anyone to edit the pages ;)

VAG actually means Volkswagen Aktiengesellschaft (stock trading company ;))

Or Volkswagen Audi Grupe

[Tigga1]

Or Volkswagen Audi Grupe

that would be Gruppe...:)

[zanakas]

Hello, mates...

A yank here (Good old USA) , where we have a (very) small, but very loyal and passionate group of VW diesel owners.

A few comments:

VW developed PD as an "in-house" response to common rail...they didn't want to be saddled with 3rd party royalties, etc.

Actually, unit injection (PD) has been around for quite some time, and used very effectively, in most Us diesel engines, though the tide is now turning...

Ultimately, unit injection cannot reach the pressures of common rail, nor can it control the fuel delivery as precisely, so we are seeing a shift to common rail across the board (most diesel engines in the US are now common rail).

The bulk of diesels here are heavy-duty, powering large trucks or industrial equipment. Passenger cars are a very small market. Nevertheless, the transition is the same.

[bluey]

Hello, mates...
[...]Ultimately, unit injection cannot reach the pressures of common rail, nor can it control the fuel delivery as precisely, so we are seeing a shift to common rail across the board (most diesel engines in the US are now common rail).
[...]

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."

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.

Certainly I was impressed by the lack of usual diesel engine noise in the common rail Tiguan diesel.

[gregozedobe]

My understanding is that they changed because VAG Common Rail engines can be tuned to produce less pollution than their PD engines.

This is because CR allows more injection events per combustion cycle than is practical for PD. Having more injection events gives more precise control over what happens during combustion, thus producing less undesirable gases (NOx ?).

I have read reports that the VAG CR 4 cyl engines are much smoother and quieter than their PD equivalents. Of course some of this can probably be attributed to being a more modern design.

IIRC there's not much in it for fuel economy, and the PD is said to produce more torque at low revs.

Basically there's not much you can do about it if you want a PD motor except buy now before they change over, because in the future they will all be CR whether you like it or not.

[bluey]

I was suggesting that any particular performance/noise differences between those two diesel injection technologies may be due to present performance of present products and not necessarily a fundamental constraint of the technology which could be improved by further research and development.

[bluey]

[...]
VAG actually means Volkswagen Aktiengesellschaft (stock trading company ;))

A bit lateral to the thread topic (and perhaps best moved elsewhere...)

I understand Volkswagen itself uses:
* Volkswagen AG, AG being the German equivalent for Australian Pty Ltd.
http://en.wikipedia.org/wiki/Aktiengesellschaft

* Volkswagen Group, which holds Audi, Seat, Skoda, Bentley, Lamborghini, Bugatti, Scania and other miscellaneous related companies.
"You can download a full list of the holdings of Volkswagen AG and the Volkswagen Group from this page."
http://www.volkswagenag.com/vwag/vwcorp/content/en/brands_and_companies/automotive_and_financial.html

Volkswagen Audi Group (VAG) apparently dates to 1978-1992.

The website uses VWAG rather than VAG. Ticker symbol is VOW. Present information suggests VAG is a historical abbreviation no longer used by Volkswagen or the Volkswagen Group.

[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 (http://www.bhpplus.co.uk/pages.php?pageid=19) vs Common rail, which says PD generates higher pressure.

This PD page (http://www.myturbodiesel.com/1000q/pumpedusedesc.htm) has lots of photos of engine internals.

Found some interesting posts by a diesel engineer (659FBE on Sat 30 august 2008) (http://www.honestjohn.co.uk/forum/post/index.htm?t=66473) 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 (http://www.bhpplus.co.uk/pages.php?pageid=19) vs Common rail, which says PD generates higher pressure.

This PD page (http://www.myturbodiesel.com/1000q/pumpedusedesc.htm) has lots of photos of engine internals.

Found some interesting posts by a diesel engineer (659FBE on Sat 30 august 2008) (http://www.honestjohn.co.uk/forum/post/index.htm?t=66473) 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]

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]

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.

http://www.vwwatercooled.com.au/forums/images/imported/2009/08/4CYLINDERJTYPEONTEST-2.jpg