Electronically Monitored Common Rail Fuel Injection 4

[MotoGP]

Happy New Year to the Engine & Fuel Engineering Gods:

I have read the Post 71-111422 dated 1/01/2005 that deals with the subject in somewhat of a similar manner, however, my plight is more related to the performance of diesel engines and not so much of a safety issue.

There is a well-known manufacturer that has been producing some of their diesel engines with either the "conventional" injection systems, or with their "new-and-improved" electronically monitored common rail fuel injection system.

One of the manufacturer's customers recently bought a number of the "new-and-improved" variety diesel engines and has nothing but a "tale of woe" to tell.

Evidently, the engines were installed correctly and there were no incidents reported during the test bed trials or during their commissioning. But, very shortly after having been put into service, the customer reports that the engines, summarily, show that "any variation in the power supply to the injection monitoring system may cause the engine to stop without warning, apart from other difficulties with injection pumps etc." (sic)

I was asked what I thought about all of this, and, I was embarrassed to tell the person making the enquiry that I know precious little about electronically monitored common rail fuel injection systems...my gut tells me that, for the reported problem, the sky could be the limit of the reasons...and, for that matter, why would this problem have to necessarily be isolated to the fuel delivery system?

Would any of you good people be able to instruct me as to what, if anything, about an electronically monitored common rail fuel injection system is unique such that it could make about one-half dozen expensive diesel engines (purchased / manufactured on or about the same date, give-or-take) behave in a manner such as what was described?

As an aside, the customer in this scenario has resolved the problem by sticking-by the tried-and-true conventional injection system, with an option for installation of the common rail injection up-grade at such a time when they are convinced that the bugs are completely worked out.

I did my level best to provide you with all the information that I had to go on...if there are specific questions that I could ask the customer to provide, and if you could share that with me, that would be of great help, as well.

Thanks for your time and help.

 

[Aquaduct]

Once again, I'm in heavy on-road diesels and have only lab experience with CR systems, so remember the old grain of salt adage.

Simplistically, an electronically controlled unit injector like we use generates fuel pressure with a plunger off the camshaft. Injection timing is accomlished with on/off solenoids in the fuel channels. The solenoids control fluid pressure in chambers on each side of the pintle. The fluid then moves the pintle and injection happens for the specified time. Traditionally, the limitations of this system are that multiple injection capability and timing capability is limited by the fluid mechanics and camshaft rotation.

Common rail systems overcome this by using an engine pump to generate constant fuel pressure and a magnetic or piezo element controlling the actual pintle movement, probably against tremendous pressure (1600-2000 bar).

Although I'm only speculating, I believe electrical system variation will impact the CR injector mechanism which probably requires a greater degree of electronic control than simply slamming a solenoid open and shut. Although fuel pump design on a CR system and rail pressure might concievably be impacted by electrical fluctuations, the pumps I've seen don't have any problems there. I do know that one of the problems that we've seen with CR systems in test cells is fuel flow variablity. Unit injectors tend to be rock solid.

Again, for us, CR is not a given. Our fuel guys believe that we will be able to eventually get similar performance from unit injectors, even though that technology is somewhat behind CR in current capabilities.

For what it's worth, that's my unqualified opinion. Your mileage may vary.

 

[andyv8]

A CR Diesel is very similar to a petrol engine, in that all of the injection events are controlled by a microprocessor (ECU) with inputs from cam/crank/temp/pressure sensors.
Any variation in power supply to the ECU outside of allowable limits will essentially stop the ECU from functioning and hence stop all signals to the injectors.
If you think of it more like a car petrol engine rather than a traditional mechanical injection diesel engine, then you won't go far wrong.

 

[Fredbert99]

Hi MotoGP,

I think I know the supplier and customer you are talking about. I have also noted the comments in the replies to your original thread.

"any variation in the power supply" could mean any number of things. Is it the voltage thats changing, the supply frequency or phase?. Are they changing with respect to time?. All of these could have an effect on the control system, depending on how that control system uses the supply. If it uses the frequency as a timing frame then any change will affect when the control system expects things to happen. The same can happen if it happens to use the "ramp up" of the input as a timing trigger and the phase changes. If the voltage changes and is used as a reference, then trigger values can be achieved too early or too late. It is not a simple case of "this happens, that is the effect". To state that the problems were caused by the supply, in a control situation, indicates to me that they actually did a bit of digging to find the cause. It's normally "line 33 needs a comma", or that type of response.

The hardware problems I would expect from a newly developed system, though it would have been better for both parties if they had been found sooner than later. Also, in a common rail system you do not have "injection pumps", they are just pumps, that feed the rail. They have no timing relative to the injector cycle as in the traditional mechanical system.

Aquaduct did a good job of describing the CR system, though I would love to know where we could get actuators that would operate against CR pressures (which are about right), Piezo crumbles under those loads, sorry. Another system is used to start and stop injection at the moment, though if we could use Aquaducts method, you can be sure we would for the control it would give us.

I've just read back over my post, and to be honest I'm not sure I've helped, other than to point out that large diesel common rail FIE is not a straight forward system. But to my mind it is the way forward.

 

[jejtopgun]

Hi MotorGP,

I will try to bring an answer to most of the questions in this thread.

What is a "variation in the power supply". If you are mentioning the ECU, this should not be a problem. You have to notice however, that common rail ECU are operating under 13,6V. Below the 11.2V, the watchdog will coming in action and shutdown the system. It is known to us that the low voltage on the ECU and really high rpm are chagin the way the injectors are reacting and can create some massive changes in the fuel delivery. However, I have not seen anything like this. Could it be that the generator is not suppling enough power, causing the voltage to drop when a high ampere load is required for the injector ?

There is mainly three types of common rail system. The first one with normal solenoids. This system has not got enough power to open the injector himself. A little trick is to have the solenoid controlling a small hydraulic valve to open the common rail. The second type is simply replacing the soleinoid by a piezo stack. This is almost the same thing, however, the hydraulic can be designed to react faster with increase of froce from the piezo. The third type is direct piezo actuated. This is for the moment under development from Bosch, Siemens and Delphi and will only be coming on the market in few years.

Back to your problem MotorGP, il will first make sure that the power supply is not under micro power cuts. Check with an oscilloscope that the power supply is not dropping quickly and back again. You will not notify it with a normal voltmeter, but the ecu will reset everytime. The second is to make sure that cam and crank signals are not "disturbed" by any noize. This will also have for effect to make the ECU goes in safe mode, shutting the engine, reducing the common rail pressure.

Hope I helped

Have fun