Common Rail vs Direct Injection *Diesel*

[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/vwc...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 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.