Injector back feed
Discussion
But once started car runs fine.
I have read this on the interweb trying to find an answer, that an engine will run perfectly, no smoke and no noticeable drop in performance.
The only symptom being long cranking periods when the car has been left for some while.
What's the difference?
Is more fuel injected to just start the engine, which is never repeated even when running at full load?
I have read this on the interweb trying to find an answer, that an engine will run perfectly, no smoke and no noticeable drop in performance.
The only symptom being long cranking periods when the car has been left for some while.
What's the difference?
Is more fuel injected to just start the engine, which is never repeated even when running at full load?
First, you need to understand how a common rail solenoid injector actually works!
It seems obvious, you just have a electromagnetic solenoid, that lifts a valve and the fuel flows into the cylinder, simple hey.
Er, no. Fuel rail pressure on a modern common rail diesel engine can be up to 3,000 bar (3000 times atmospheric pressure!) which results in a force of around 3kg for every square milimeter of valve area. If the solenoid had to directly overcome that pressure it would need to be massive, and take a huge amount of power to drive it.
So, a cunning trick called "pilot operation" is used. This uses a tiny orifice to release pressure on one side of a valve, meaning that the fuel pressure itself opens the injectors! However, that released pressure can only be done by bleeding off fuel, and this is the fuel that is returned to the tank from the "leak off" port on the injector.
So, what does excessive "Leak off" mean. It means the pilot valve and injector nozzle assy is worn. These parts have tiny dimensional tollerances, to keep leak off low, and provide an efficient pilot system. As an injector wears, these tollerances increase, and more and more fuel must be "leaked off" to open the injector. And a worn pilot system goes hand in hand with worn primary orifices (ooh er misses!) resulting in a poor spray pattern and lack of stream atomisation.
On it's own, that isn't any particular great issue to a warm, already running the engine (but it is to stuff the driver can see like exhaust emissions etc)
But a cold start is a particularly difficult time for compression ignition engines. On those first few strokes of the engine, the combustion chamber walls and piston are cold, and the heat of compression (itself low due to slow cranking speeds) gets lost. The fuel itself is more viscous, and tends to be difficult to atomise, (rail pressure will also be low because the HP fuel pump is engine driven, and turning slowly)
Add in worn injectors, and instead of a nice, fine, easily ignitable spray of ultra fine fuel droplets (high surface area to volume ratio) you get a dribble of viscous, clumpy fuel, that fails to ignite on those first few compression stokes. Then, as liquid fuel, it condenses onto the cylinder walls, where is absorbs even more heat from the next compression stoke, meaning the cylinder is even harder to fire.
Eventually, enough compression cyls occur (or your battery goes flat and you have to call the AA.....) and the cylinder starts to burn, in a rough, poor, sooty kind of way. All the previously injected unburnt diesel also burns off the cylinder walls and exhaust ports, leading to the characteristic blue smokey, and rough misfirey start.
After probably something like 5o to 100 cycles (say 5 to 10 sec at idle speed), the combustion chamber is hot enough to start burning cleanly (or relatively cleanly) and the engine becomes smooth again. Once warm, the drive probably won't notice the slight reduction in combustion stability, and certainly won't notice the 10 to 100 fold increase is exhaust emissions from the poor injector atomisation.
It seems obvious, you just have a electromagnetic solenoid, that lifts a valve and the fuel flows into the cylinder, simple hey.
Er, no. Fuel rail pressure on a modern common rail diesel engine can be up to 3,000 bar (3000 times atmospheric pressure!) which results in a force of around 3kg for every square milimeter of valve area. If the solenoid had to directly overcome that pressure it would need to be massive, and take a huge amount of power to drive it.
So, a cunning trick called "pilot operation" is used. This uses a tiny orifice to release pressure on one side of a valve, meaning that the fuel pressure itself opens the injectors! However, that released pressure can only be done by bleeding off fuel, and this is the fuel that is returned to the tank from the "leak off" port on the injector.
So, what does excessive "Leak off" mean. It means the pilot valve and injector nozzle assy is worn. These parts have tiny dimensional tollerances, to keep leak off low, and provide an efficient pilot system. As an injector wears, these tollerances increase, and more and more fuel must be "leaked off" to open the injector. And a worn pilot system goes hand in hand with worn primary orifices (ooh er misses!) resulting in a poor spray pattern and lack of stream atomisation.
On it's own, that isn't any particular great issue to a warm, already running the engine (but it is to stuff the driver can see like exhaust emissions etc)
But a cold start is a particularly difficult time for compression ignition engines. On those first few strokes of the engine, the combustion chamber walls and piston are cold, and the heat of compression (itself low due to slow cranking speeds) gets lost. The fuel itself is more viscous, and tends to be difficult to atomise, (rail pressure will also be low because the HP fuel pump is engine driven, and turning slowly)
Add in worn injectors, and instead of a nice, fine, easily ignitable spray of ultra fine fuel droplets (high surface area to volume ratio) you get a dribble of viscous, clumpy fuel, that fails to ignite on those first few compression stokes. Then, as liquid fuel, it condenses onto the cylinder walls, where is absorbs even more heat from the next compression stoke, meaning the cylinder is even harder to fire.
Eventually, enough compression cyls occur (or your battery goes flat and you have to call the AA.....) and the cylinder starts to burn, in a rough, poor, sooty kind of way. All the previously injected unburnt diesel also burns off the cylinder walls and exhaust ports, leading to the characteristic blue smokey, and rough misfirey start.
After probably something like 5o to 100 cycles (say 5 to 10 sec at idle speed), the combustion chamber is hot enough to start burning cleanly (or relatively cleanly) and the engine becomes smooth again. Once warm, the drive probably won't notice the slight reduction in combustion stability, and certainly won't notice the 10 to 100 fold increase is exhaust emissions from the poor injector atomisation.
That's what I said to the mechanic, but he said this, and from reading the internet he is right.
Glowplugs aren't needed that much to start common rail engines apparently, they do it via the injectors, although they are still used to speed warm-up.
This makes send to me, because the glow plug light barely registers on diesel nowadays, where as older diesels (pre common rail) I remember having to wait for the light to go out.
Glowplugs aren't needed that much to start common rail engines apparently, they do it via the injectors, although they are still used to speed warm-up.
This makes send to me, because the glow plug light barely registers on diesel nowadays, where as older diesels (pre common rail) I remember having to wait for the light to go out.
Old diesels needed "more" glow plug for a few reasons:
1) mechanical injection mean't very low injection pressure at cranking speeds (poor atomisation)
2) They were indirect injection so the heat of compression was isolated from the preburn chamber to a degree
3) Glow plugs themselves were signficantly lower power and higher thermal inertia
4) The control system was crude, meaning getting the exact injection quantity correct for a cold start was extremely difficult.
Glow plugs play an important part in modern CR diesels, its just the driver doesn't see them operating, as it is handled automatically by the ecu. They will be being used even on 25degC starts to get a "1st compression fire" for lowest starting exhaust emissions. They are also used to ensure a clean relight on exit from extended overrun fuel shut offs (like on motorway down slopes etc, where the driver has been off throttle for more than around 5 secs)
ETA: When it's cold (less than say 5degC) the engine ecu will have pre-run the glow plugs when the car is unlocked or the ignition is switched on, before you even have hit the start button, to get some heat into them, meaning a quick (<0.5sec) glow plug operation can be "hidden" from the driver, and no wait is perceived when a start is commanded.
Failed glow plugs do occur, but they are more robust than they used to be, and typically they are monitored for correct operation by the engine ecu, and will set a fault code if they have failed. They are also very easy to test!
(id always recommend a quick check of all glow plugs for diesel starting issues as the first job, because it is so easy to do!)
1) mechanical injection mean't very low injection pressure at cranking speeds (poor atomisation)
2) They were indirect injection so the heat of compression was isolated from the preburn chamber to a degree
3) Glow plugs themselves were signficantly lower power and higher thermal inertia
4) The control system was crude, meaning getting the exact injection quantity correct for a cold start was extremely difficult.
Glow plugs play an important part in modern CR diesels, its just the driver doesn't see them operating, as it is handled automatically by the ecu. They will be being used even on 25degC starts to get a "1st compression fire" for lowest starting exhaust emissions. They are also used to ensure a clean relight on exit from extended overrun fuel shut offs (like on motorway down slopes etc, where the driver has been off throttle for more than around 5 secs)
ETA: When it's cold (less than say 5degC) the engine ecu will have pre-run the glow plugs when the car is unlocked or the ignition is switched on, before you even have hit the start button, to get some heat into them, meaning a quick (<0.5sec) glow plug operation can be "hidden" from the driver, and no wait is perceived when a start is commanded.
Failed glow plugs do occur, but they are more robust than they used to be, and typically they are monitored for correct operation by the engine ecu, and will set a fault code if they have failed. They are also very easy to test!
(id always recommend a quick check of all glow plugs for diesel starting issues as the first job, because it is so easy to do!)
Edited by anonymous-user on Friday 6th January 16:37
Very interesting.
Got the car back today, and he said the injectors were back feeding, but nothing to worry about and in-line with use they have had, and by all the same amount which is good.
So he went onto check the glowplugs, and none were working!
I'm sure it was quieter on the drive home.
Got the car back today, and he said the injectors were back feeding, but nothing to worry about and in-line with use they have had, and by all the same amount which is good.
So he went onto check the glowplugs, and none were working!
I'm sure it was quieter on the drive home.
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