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Old 10-09-2007, 08:45 AM
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Default Diesel motors 101-----common rail vs. not common rail?

Good day all,

I keep hearing the term "common rail". What does this mean?

Secondly, why do these newer diesels smoke less(technical reasons please) and get better fuel economy?

Thanks, Paul
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Old 10-09-2007, 08:50 AM
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Default RE: Diesel motors 101-----common rail vs. not common rail?

I know there are people here that will explain--- all I know is that a diesel with common rail is quite ie Durmax
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Old 10-09-2007, 09:18 AM
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Default Re: Diesel motors 101-----common rail vs. not common rail?

Seaswirl Paul - there was a good article in the magazine Soundings maybe 9 - 12 months ago about the various diesel types. I'll try and dig it up and gt yo the specific month if I still have it. They described the various pros and cons of the older mechanical diesels and the newer common rail, electronic diesels. I'd relay what I remember from the article here but I'm sure I'd screw it up!
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Old 10-09-2007, 10:05 AM
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Default Re: Diesel motors 101-----common rail vs. not common rail?

Common rail is the new technology of how fuel is distributed to the injectors. When I buy my next inboard boat, Common Rail engines will be my number one priority. Better economy and virtually no smoke.

Here's the little that I know. A computer lets the engines know when to spray fuel to the injectors. It also tells it how much. Basically, it gives the engine fuel ONLY when it needs it, and doesn't over do it. No smoke because there is no wasted fuel that needs to be burned. Better fuel economy because there is no excess fuel being used.

Anyone correct me if I'm wrong.

It's a very complicated technology. Your best bet is to google it.
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Old 10-09-2007, 10:11 AM
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Default Re: Diesel motors 101-----common rail vs. not common rail?

Common rail diesels have an injector system similar to a gasoline engines' EFI. The electronic injectors are all located on one high pressure line and are opened sequentially by an ECM rather than by cams, belts, wheels, etc. No traditional diesel timing pump. They get better fuel management due to the ability of the ECM to adjust timing and duration of the spray based on load, air temp, etc like a gas motor

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Old 10-09-2007, 10:42 AM
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Default Re: Diesel motors 101-----common rail vs. not common rail?

To throw another wrinkle into it, my new Cummins QSM-11's are electronic but NOT common rail. My non-technical explanation of electronic but not common rail is that the injector set-up is similar to what most are familiar with but the injectors themselves are more complicated and controlled by a computer to provide the correct amount of fuel that the engine needs for the conditions. Common rail is supposed to be the next step. I can tell you that the QSM-11's are incredible. After trolling all day, when getting onto plane, my previous 3208's blew a black cloud you could see from the space station. The QSM's blow no smoke.
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Old 10-09-2007, 11:15 AM
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Default Re: Diesel motors 101-----common rail vs. not common rail?

The term "Common Rail" is a bosch trademark, instead of having an injection pump that metered fuel sent to individual injectors the CR system has a main pump that supplies a fuel rail with constant fuel pressure. From there an ECU actuates the individual injectors as needed rather than having the injectors actuated by a camshaft or a rack. The QSM-11 is a fully electronic high pressure direct injection with some parts sourced from Bosch but otherwise cummins, its got what I consider a near flawless injection system. There are other engines in the cummins quantum series that are called "common rail" since they're using all bosch components. I am not impressed with these engines and prefer the cummins system. Caterpillars version of this is called ACERT.
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Old 10-09-2007, 11:57 AM
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Default Re: Diesel motors 101-----common rail vs. not common rail?

Each cylinder get fuel on the intake stroke of it's firing order. No raw fuel being pumped into the cylinders that are not firing. Same as Sequential, Multi-Port Fuel Injection that is on most automobiles now days. The Fuel Rail is Common to all Cylinders and the Injector is controled by a solonoid and not the old style injection pump........... The Solonoid is controlled by an ECM. This system is head and shoulder ahead of previous systems, I only wonder why it took so long.
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Old 10-09-2007, 12:54 PM
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Default Re: Diesel motors 101-----common rail vs. not common rail?

Quote:
steveyacht - 10/9/2007 11:57 AM
Each cylinder get fuel on the intake stroke of it's firing order. No raw fuel being pumped into the cylinders that are not firing.
Actually the fuel is injected "near" TDC of the compression stroke. The heated generated by compressing air from the intake stroke is what cause the ignition.

Quote:
This system is head and shoulder ahead of previous systems, I only wonder why it took so long.
Making an injector that could open against the extreme pressure being delivered by the pump was "a challenge" !

The Ford PowerStroke 7.3L and 6.0L (previous generations) used "unit injectors", as did the equivalent Navistar engines. Sort of a half way to common rail.

The current PowerStroke 6.4L and DuraMax use "common rail". One of the reasons that common rail systems are quieter is that all of the fuel in not injected at one time. It is done in "spurts".
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Old 10-09-2007, 12:58 PM
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Default Re: Diesel motors 101-----common rail vs. not common rail?

TheOldWizaerd........... you know, you are right. I never considered the extreme compression ratio differences between gas and diesel engines. Thank you. And yes, the TDC comment is 100% correct, I over simplified.
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Old 10-09-2007, 01:51 PM
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Default Re: Diesel motors 101-----common rail vs. not common rail?

Paul,

if i could just add a few points of interest from my reading. Common rail works off extremely high pressures which aids in fuel atomization. The improved atomization, in conjuction with electronic control of the actual injection event(s), which is further adjusted by many engine/sensor parameters, is whats largely responsible for a decrease in emmisions and an increase in economy.

One drawback to common rail is that due to the extreme pressures involved, an injector failure can dump large amounts of fuel into a cylinder in short order.
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Old 10-09-2007, 06:02 PM
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Default RE: Diesel motors 101-----common rail vs. not common rail?

jav1 said it all. Common Rail allows delivery of the fuel to the injectors at extremely high pressure causing injection of fuel to the cyliners in very tiny fuel particles enabling very thorough combustion. Fuel pressures are getting up into the 25,000 lb/sq inch range and higher
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Old 10-09-2007, 07:26 PM
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Default RE: Diesel motors 101-----common rail vs. not common rail?

I also have a question on this topic, I currently have TAMD 71 B Volvos . I know once these diesels are started they will continue to run , I can loose an alternator or batterys and I am getting home. Now if I owned Common Rail and say my alternator crapped out and then my batteries were to loose charge would this not cause issues with the electronic components thus causing breakdown status?
Curious
Thanks
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Old 10-09-2007, 07:29 PM
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Default Re: Diesel motors 101-----common rail vs. not common rail?

Yes. I am aware of a recent incident with a very well known, high quality builder powered by Cummins QSC540 common rail engines that completely shut down due to low voltage. Long story short, both alternators had gone bad(which seems strange that both went bad at the same time).
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Old 10-09-2007, 09:18 PM
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Default Re: Diesel motors 101-----common rail vs. not common rail?

I had mech injected engines and then went for the newer 'common rail' replacement version. Difference in smoke is incredible (don't see any now). Difference in fuel burn at cruising revs is noticeable, but difference at trolling revs is around 20%. Big saving if you spend a lot of time chasing billfish. Downsides are had one of the sensors start creating intermittant false alarms at around 2000hrs, replaced it and no more alarms. Had some trouble with starting one engine after a wiring loom connection got badly wet with salt water due to a pump connection failure. Had to fix the corroded connector pins, no big issue, but took quite a while to find the problem due to the complexity of these types of engines. Only other problem was when we got a load of water in the fuel and some injectors were damaged. These 'common rail' engines are less tolerant of water in the fuel than mechanically injected diesels. Now have Racor separators in front of the proprietry manufacturers seperators. No more problems. Overall, after 3000hrs I love the improvements in the 'electronic' engines and would not go back, but have learned to be more careful with water issues by better sealing the wiring loom connectors and putting the Racors in place.
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Old 10-10-2007, 06:31 AM
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Default Re: Diesel motors 101-----common rail vs. not common rail?

There's a fair amount of debate on boatdiesel with respect to this (newer) versus older technology diesels. In new boats, buyers are loosing the choice since teir 2 emmisions standards are stringent enough that very few purely mechanical engines meet the spec. That leaves electronic engines with commonrail at or near the top of the food chain. There are some other hybrid injection technologies that are even newer but not mainstream yet.

In older vessels, I'm still a proponent of a good mechanical diesel over an electronic engine for recreational use. With limited use, I feel more comfortable with a power plant I can diagnose and service at sea that isn't reliant on electrical power + computers + software + wiring + sensors plus pristine fuel to run. Commercial is also a different market and any technology that reduces emmisions and improves operational efficeincy is very desireable and worthwhile.
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