Diesel Injection: More Pressure! (Long Text)

Source: Automotive Review, May 29, 2003.


Diesel injection with more pressure, please!

The common rail injection system has room for improvement. The pressure of 1350 or 1600 bar is often no longer present at the nozzle. However, a high pressure is needed at the nozzle to reduce emissions. Injectors with piezo control offer advancements. The new system was presented at the Vienna Engine Symposium.
When Daimler Chrysler and Fiat introduced the first common rail injection systems from Bosch for diesel engines in 1997, the plunger pumps were already delivering higher pressures. In 1999, Siemens followed with the common rail and, instead of solenoid valves, offered the first piezoelectric actuators, which were already in their second development stage in 2000. Now, the third generation is here, which is initially intended to achieve maximum pressures of up to 1800 bar, and even more than 2000 bar.
The first VP 37 common rail pumps from Bosch, which Audi and VW used to usher in the era of direct injection for small diesel cars, delivered just over 1000 bar of maximum pressure upstream of the injector. The radial piston distributor pump VP 44, first used by Opel in 1996 and later also by BMW and Audi, started with a pressure of 1550 bar.

PRINTING COMPETITIONS
At that time, engines with pump injectors were already being used in Wolfsburg, delivering 2050 bar of pressure before the injector. Today, the VP 44 in the Audi V6 TDI even offers 2000 bar, while the common rail system from 1997 had a maximum pressure of 1350 bar. To this day, we do not know the pressure drop until the injector outlet. This also applies to the current development stage of Bosch and Siemens, which operates with a pressure of up to 1600 bar (only in the common rail?). The effective pressure at the injector is also unknown for Delphi and Denso.
Bosch and Siemens are now introducing systems into production that are designed for pressures up to 1800 bar, and are expected to reach 2000 bar, and which utilize piezoelectric actuators. For Bosch, who presented their new development several months ago, this marks the entry into the piezoelectric era, while Siemens already has considerable experience in this field. In both injectors, the piezoelectric elements are housed within the nozzle body. Compared to the first versions from Siemens, they have been reduced in diameter. To achieve longer valve travel, a significantly larger number of ceramic plates are stacked together. This has no effect on the opening and closing times, which are extremely short in both cases with piezoelectric systems.

SIEMENS COMMON RAIL
At this year's Vienna Engine Symposium, Siemens engineer Hellmut Freudenberg unveiled the latest developments from his company. These are primarily characterized by new, high-strength materials for all components exposed to pressure, which do not fatigue over their lifespan, while also allowing for reduced component sizes. A similar trend was observed in piston pumps. For example, Bosch manufactures the housings of the latest pump nozzle elements from the ball bearing steel 100Cr6. Siemens is following suit with the same steel.
All seals had to be significantly improved to remain leak-proof throughout their lifespan. The next step was to minimize the leakage of the previous systems, which had been preventing an increase in pressure. While the leakage in the previous M 1600 system was 100%, it has been reduced to only 40% in the third development stage, and it can now withstand pressures up to 1800 bar.
A 100% leakage rate means that the pressure hardly increases, even when the pump is driven at twice the speed. However, the drastic reduction in leakage within the pump and injectors also significantly reduces the power requirement, which was previously always higher than that of the pump-nozzle system. Siemens states that, with the same power requirement as before, the new pump can now supply a six-cylinder engine instead of a four-cylinder engine. Other advantages include the possibility of eliminating the need for a fuel cooler, as well as reduced system costs.
As before, the high-pressure pump operates with three pistons. It includes a pre-charging pump with a low-pressure control valve. It also features a flow control valve to ensure that it only pumps the amount needed. When operating at full capacity, the pump achieves an efficiency of up to 90% across the entire pressure range.

SLIM INJECTOR
Externally, there have been several changes compared to previous injectors. The cylindrical housing now has a diameter of only 19 mm and a central pressure connection at the top. Approximately half the length of the housing is occupied by the piezoelectric element, which acts on a hydraulic converter. This converter handles functions such as transmitting movement to the servo valve, reversing movement, temperature and wear compensation, and tolerance adjustment. Following this is a control module, and at the tip is the actual nozzle.
As usual, the injector needle is lifted from its seat by the fuel pressure against the spring force, thereby opening the injection holes. The spring pushes it back onto its seat when the servo valve closes and the fuel pressure drops. Compared to previous designs, the opening speed of the injector needle has been significantly increased, so that even with very short injection times, such as for pre-injection, the largest possible proportion of the fuel pressure in the rail is available for mixture preparation at the injection hole. For multiple injections, the rapid opening of the injector needle is essential; otherwise, only a small amount of fuel with far too low pressure enters the cylinder, resulting in poor combustion with a high proportion of pollutants in the exhaust.
The common rail technology is only now beginning to demonstrate a portion of the potential that marketing departments have been touting for the past six years. The only advantage of common rail systems so far has been the delayed pre-injection, which reduced the engine noise at idle compared to pump-injector systems.

VIBRATIONS
Bosch utilized pressure pulsations in the distributor injection pumps in such a way that the pressure before the injector was sometimes significantly higher than the pressure generated by the pump itself. However, common rail systems also experience pressure surges, but engineers actively try to minimize them. Every time an injector opens during engine operation, the pressure in the connecting pipe to the rail drops, and a pressure wave follows. When the injector valve closes, the pressure surge impacts the closing mechanism, and the wave returns to the rail. When multiple injectors are supplied by a single rail – in a four-cylinder engine, there are four – there is a complex interplay of vibrations, pressure build-up, and pressure drops within the rail.
"This naturally has a devastating effect on the fuel pressure at the nozzle, because the pressure in the system fluctuates constantly. Piston pumps have a much easier time here, especially the pump nozzle system with its extremely short fuel paths. This is a crucial reason why VW remains loyal to the pump nozzle system and, together with Siemens, is now even building a factory for pump nozzle systems in Stollberg, Saxony."
"Until now, manufacturers have been very reluctant to make statements about pressure fluctuations in common rail systems. Now, Hellmut Freudenberg has reported for the first time that vibrations within previous systems have caused pressure and, consequently, volume deviations between the injectors in an engine."
Such undesirable deviations lead to a loss of performance and an increase in pollutants in the exhaust. This is one of the reasons why Audi and VW, using swirl injection systems, were able to meet the Euro 4 emission limits, while those using common rail systems have so far lagged far behind.
There are solid reasons why Siemens uses spring force instead of hydraulics to close the injector needles in the latest system. This allows for the absorption of some of the troublesome vibration effects that previously caused multiple injections to become ineffective. However, the new engine control system also helps to increase the hydraulic stability of the entire system.
In engine dynamometer tests comparing a previous system with dual injection to a PCR.3 (third-generation piezo common rail) with five injections, the proportion of soot (particulate matter) in the exhaust gas decreased by approximately one-third, while fuel consumption decreased by about 10%.

GROWING UP.
Gradually, the common rail system is maturing. While vane-type pumps still have a significant advantage, the common rail system is slowly catching up. Some manufacturers have previously stuck with vane-type pumps, but this was not for ideological reasons, but for purely practical ones.
If Common Rail technology can provide a maximum pressure of 2000 bar by the year 2005 (introduction of Euro4 emission standards) – as Siemens suggested, indicating that this is feasible – and ensure an even distribution of fuel to the cylinders, it will gradually become acceptable for Wolfsburg as well.
Perhaps Peugeot will then be able to prevent the particles from being captured by an expensive filter, but instead prevent them from being generated in the engine in the first place, and also reduce all other pollutants.

Hi,

Sure, here is the translation of the text from German to English:

"Here are some comments from me about the text:"

"I found this text as it is and reproduced it 100%. I am neither related to nor acquainted with the author."

What was surprising to me was the fact that the pressures in the VP44 system are already as high as those in the PD system.
Even with the CR system, things aren't always "peace, joy, and happiness." It seems that the CR system, contrary to the promises of various marketing people, is neither fully mature nor problem-free. In some ways, this text even suggests that there may be more problems with the CR system than with the PD system.

hi,
Strictly speaking, PD elements are the only ones that matter.
Bring the pump and nozzle as close together as possible.
While it's certainly annoying when, for example, 'only' one nozzle is broken and you have to replace the entire component, at least you can replace individual 'pumps and nozzles.'
It's like with processors.
The drastic performance increase has only been possible due to the drastic reduction in the size of the components.
greetings

@Bertil:

You've provided a great summary of the recent developments in diesel injection systems.

Quote:

A 100% leakage means that the pressure hardly increases, even if the pump is driven at twice the speed.

Unfortunately, I can't make any sense of that statement.
"Where does this definition come from? I work in the hydraulics industry. In that field, there's a concept called the volumetric efficiency for pumps, which represents the ratio of the actual flow rate to the theoretical (geometric) flow rate of the pump."
Furthermore, the volumetric efficiency also takes into account the compressibility of the fluid, as the flow rate is always measured under pressure. This means that even a pump that is 100% dense will never achieve a volumetric efficiency of 100%.
In this case, a 100% leakage would mean that the pump has a volumetric efficiency of 0%, meaning that the external flow rate drops to 0 and the internal leakage is equal to the theoretical flow rate.
Is it meant that, at the pressure limit you mentioned, the CR pump can no longer deliver any flow because the entire flow rate is lost due to internal leakage?

Quote:

During dynamometer tests comparing a previous system with dual injection to a PCR.3 (third-generation Piezo Common Rail) with five injections, the proportion of soot (particles) in the exhaust gas decreased by approximately one-third, while fuel consumption decreased by about 10%.

Well, that brings us back to one of my favorite topics: engine efficiency. As far as I know, Fiat uses a CR (Common Rail) system (I believe from Delphi) that operates with triple injection. However, this is only used in the partial load range. At full load, only a single main injection is performed to achieve high efficiency.
A reduction in particulate emissions is primarily achieved through good fuel atomization, which requires high pressure. However, post-injection as a method for "soot afterburning" is also a common technology, but it unfortunately reduces efficiency.
The reduction in particulate emissions and the simultaneous decrease in fuel consumption are primarily due to the increase in rail pressure, and not to the improved temporal resolution of the injection process, which now uses 5 intervals. I believe this is being done to better control the temperature profile during combustion, in order to reduce NOx emissions.

Finally, a small thought to consider regarding the energy balance of injection systems. If we imagine that the common rail pump is constantly exposed to the rail pressure, then there will always be leakage at the pump.
In all other systems, the high system pressure is only built up locally and temporarily (for the duration of the injection process), resulting in a significantly smaller leakage oil flow. The power converted into heat through throttling is not insignificant and ultimately must be compensated for by the engine as mechanical output power, taking into account the (low) engine efficiency.
The heating of the fuel should not be underestimated, and it often necessitates secondary measures such as fuel cooling.

Regards,
Alex.

Deleted upon request.

Hi,

donalexo wrote:

@Bertil: You've provided a great summary of the recent developments in diesel injection systems.

I found the article very interesting.

Quote:

... In this case, a 100% leakage would mean that the pump has a volumetric efficiency of 0%, meaning that the external flow rate drops to 0 and the internal leakage is equal to the theoretical flow rate. Is it meant that, at the pressure limit you mentioned, the CR pump can no longer deliver any flow because the entire flow rate is lost due to internal leakage?

That's probably what it's meant to be.

Quote:

Well, that brings us back to one of my favorite topics: engine efficiency. As far as I know, Fiat uses a CR system (I believe from Delphi) that operates with triple injection. ...

The system from Alfa/Fiat comes from my supplier, Siemens VDO, and can even work with a five-fold injection system. SVA is the only company that can currently implement multiple injections before and after the main injection. The reason is the use of fast piezo elements. However, Bosch has already announced its entry into piezo technology, and that will likely not take long.

Hello,
especially @Rainer

Even though the initial post doesn't cover all the details, I find it quite informative and would suggest posting it in the new "Expert Articles" section – at least until the developments described become outdated.

Are there any copyright or other concerns regarding this?

It was surprising to me that the pressures in the VP44 are already as high as those in the PD system.



Didn't you know? The same thing is happening there as with the VP37.

950 bar element pressure -> 1250 bar nozzle pressure.

VP44

1400 - 1600 bar element pressure -> 1800 - 2000 bar nozzle pressure.


CU Gremlin.

Hi Ulf,

ulf wrote:

...Do any copyright or other concerns apply?

... I really can't tell you, but the source is mentioned. You can always ask them if you need to.

@Gremlin

I haven't delved deeply into the VP44 yet, so I'm not very familiar with its specifications.