Common Rail Diesel Engines

Submitted: Thursday, May 01, 2003 at 08:32
ThreadID: 4676 Views:22666 Replies:3 FollowUps:4
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Hi guys,

A question from a complete novice when it comes to diesel engine types.

Can somebody enlighten me: What is a common rail Diesel Engine and without going into too much technical detail, how does it differ from say a Turbo Direct Injection Intercooled Diesel.

Thanks in advance

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Reply By: GUPatrol - Thursday, May 01, 2003 at 10:46

Thursday, May 01, 2003 at 10:46
A conventional diesel, direct injection or indirect injection has an injector pump driven by the engine (often mechanical pump) but later models range from elctronic timing control to semi electronic, these kind of pump have mechanical plungers to inject diesel into the engine (via injectors) at a set pressure (the pressure depends) indirect injection uses less pressure and is less precise.
This kind of pump requires pipes to each injector and at every stroke there is a noise as the diesel is driven to high pressures to be properly atomised.
A common rail diesel (ie: Land Rover Discovery) does not have a precise pump with a plunger, instead it has a rail where all injectors connect and a very powerful electric fuel pump to keep the pressure constant (Very high pressure), each injector is operated individually electronically by the ECU (Electronic control unit), that way the injection can be quieter and more presise and each injector can be controlled individually even cutting some out and some in depending on the circumstances (you can notice this listenging to a TD5 while being driven on slow tracks).
There is another type as used by caterpillar where there is a common rail but the injector is operated by engine oil pressure (ie Jackaroo engine).

AnswerID: 18886

Reply By: StephenF - Thursday, May 01, 2003 at 11:31

Thursday, May 01, 2003 at 11:31
Normal diesel engines take their timing from the injection pump (like the distributor on a petrol engine), which also provides the fuel pressure to the injectors. The injectors are spring-loaded and open in response to the pressure pulse from the pump. In common-rail diesels the pump plays no part in the timing but simply pressurises a rail of high-pressure fuel to which all of the injectors are connected. Injection timing is provided to each injector electronically.

Direct/indirect injection relates to the combustion chamber. Direct injection means that the fuel is injected directly into the cylinder, usually into a depression in the top of the piston. With indirect injection the fuel is injected into a spherical pre-combustion chamber in the head where it ignites then spreads via an opening into the main combustion chamber.

In a normal diesel the air is sucked into the cylinders at atmospheric pressure. In a turbo-diesel the air is compressed by a turbocharger before entering the engine, thereby allowing more air (and more oxygen) into the cylinders. Because the air is heated as it's compressed it loses density, which negates some of the benefit of turbocharging, so an intercooler removes some heat from the air to regain some density.

AnswerID: 18887

Follow Up By: Member - Paul T- Thursday, May 01, 2003 at 17:10

Thursday, May 01, 2003 at 17:10
Thanks guys

Your answers helped to clarify the differences heaps.

I guess there are advantages with the Common Rail Diesel as well as disadvantages when compared to other diesel types. Probably burns fuel more efficiently and reduces emissions, but also adds something else that can fail?

FollowupID: 11841

Reply By: Damian M - Thursday, May 01, 2003 at 21:12

Thursday, May 01, 2003 at 21:12
I thought I would add a little more to this discussion. Being an Auto Electrician, I have a Little knowledge in this area...

I would like to add that the Ford, F250 and others that use the 7.3litrel Powerstroke Engine have a very interesting injector system.

The Injectors are operated ny oil pressure, This is not unusual in itself, but, They are operated at 110 volts!! Yep, that's what I said, 110volts.
So Please, If you come across one of these engines or someone has broken down in one and you are hoping to help, BE VERY CAREFUL.
Making contact with this voltage can be Lethal. It only takes about 200milliamps to electrocute someone and there is more than that available.

As to how a Oil Pressure Injector works. It involves 2 Chambers in the Injector. The Oil Chamber is 7 times Larger than the Diesel Chamber. That means that they operate at 7 to 1 Ratio. In other words, If you have 20psi oil pressure you will have at most 140psi pressure in the Diesel Chamber. This is why you have a high voltage to operate the injector. It take quite a bit of force to activate the Solenoid in one of these injectors.
The Engine Management Computer operates the Injctors according to what the Engine requires. The Injectors are Modulated (Open & Closed) many times a second, for a specified amount of time, to supply the right amount of fuel to the Engine. This is know as Pulse Width Modulation.

I could go on forever about the ins & outs of Fuel Injection, But it'll bore you to tears.
Even this is a bit Lengthy, Sorry. I couldn't help myself..

Regards to you all.
AnswerID: 18938

Follow Up By: StephenF - Friday, May 02, 2003 at 09:31

Friday, May 02, 2003 at 09:31

Don't be sorry - some of us like to know how things work.

FollowupID: 11894

Follow Up By: Savvas - Friday, May 02, 2003 at 15:42

Friday, May 02, 2003 at 15:42

That's quite interesting. Is the level of voltage also why some modern diesel engines emit a large amount of interference on HF and some UHF 2 way radios ?
FollowupID: 11921

Follow Up By: Member - Paul T- Friday, May 02, 2003 at 16:08

Friday, May 02, 2003 at 16:08

very interesting stuff: does this mean that most common rail diesels will have injectors operating at this voltage, or is it only with vehicles such as the ones you mentioned.

FollowupID: 11926

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