diesel question
Discussion
How does the injection system in a diesel work exactly.
I've got a vauge idea but I'm not sure. The fuel pump compresses the fuel and it goes off to the injectors (at constant pressure??). But unless you're at full throttle I know that the fuel consumption will be less that the pump throughput. So the fuel is returned somewhere. but where?
Plus what is the difference between the old system and the new common rail stuff? Is it simply that the common rail is at a higher pressure and features direct injection or is there something else.
Why the name of common rail if the old systems deliver fuel directly to the injectors via pipes as well?
Andy
I've got a vauge idea but I'm not sure. The fuel pump compresses the fuel and it goes off to the injectors (at constant pressure??). But unless you're at full throttle I know that the fuel consumption will be less that the pump throughput. So the fuel is returned somewhere. but where?
Plus what is the difference between the old system and the new common rail stuff? Is it simply that the common rail is at a higher pressure and features direct injection or is there something else.
Why the name of common rail if the old systems deliver fuel directly to the injectors via pipes as well?
Andy
The 'old' system uses a pump which is also the throttle and timing. this directly injects an amount of fuel depending on the throttle position into the combustion chamber at a given point in the 4 stroke cycle, thus firing and giving power.
The new common rail diesels run much like a conventional fuel injected car thus having a 'common fuel rail' with individual electronic injectors that fire's the fuel into the combustion chambers again depending on throttle position. Diesels do not have a conventional butterfly throttle like a petrol as the amount of fuel that gets forced into the combustion chambers determines the amount of power or revs the engine produces.
The new common rail diesels run much like a conventional fuel injected car thus having a 'common fuel rail' with individual electronic injectors that fire's the fuel into the combustion chambers again depending on throttle position. Diesels do not have a conventional butterfly throttle like a petrol as the amount of fuel that gets forced into the combustion chambers determines the amount of power or revs the engine produces.
Hi Andy
I'll try and answer a few of your Q's as simply as possible.
Fuel is drawn up from the tank and passed through a filter by a lift pump (sometimes called a transfer pump), The low pressure fuel is then passed to the high pressure side via this pumping unit. The diesel is compressed to the point where it overcomes the pressure of the main spring within the fuel injector.
As the diesel pressure begins the overcome the spring, it lifts a needle off it's seat allowing the fuel to pass through tiny holes and out of the injector into the combustion chamber in the form of a finely atomised spray.
As the compressed fuels pressure drops, the spring then closes off the fuel to the cylinder.
Fuel is returned to the tank in alothough it's route can take a number of ways depending on the systems design, it MUST leak back from the injectors to either the fuel pump where it picks up with the pump fuel return, or it can go back to the filter head before going back to the tank.
Leak off fuel is very important in the system. It's used to lubricate the pump and injectors and also used to heat the diesel to prevent waxing (when needed)
Re the differences between old and new types of injection.
Common rail is
Quiter
cleaner
provides a higher degree of fuel control
to name but a few advantages.
Instead of having a slight delay and a single fuel burn, common rail systems can offer multiple injections per firing cycle (pilot injection), so the flame front is already traveling as the main fuel load is being instantly injected.
Focus Cmax has the ability to use 6 injection pulses per firing stroke, compared to a single injection of the old gen kit.
On full revs, you can expect to see about 44.000 psi at the fuel rail pressure sensor, compared to about 16.000 on an old system, the higher the pressure, the finer the atomisation which leads to a cleaner more powerful burn.
Hope that helps with some of it.
I'll try and answer a few of your Q's as simply as possible.
Fuel is drawn up from the tank and passed through a filter by a lift pump (sometimes called a transfer pump), The low pressure fuel is then passed to the high pressure side via this pumping unit. The diesel is compressed to the point where it overcomes the pressure of the main spring within the fuel injector.
As the diesel pressure begins the overcome the spring, it lifts a needle off it's seat allowing the fuel to pass through tiny holes and out of the injector into the combustion chamber in the form of a finely atomised spray.
As the compressed fuels pressure drops, the spring then closes off the fuel to the cylinder.
Fuel is returned to the tank in alothough it's route can take a number of ways depending on the systems design, it MUST leak back from the injectors to either the fuel pump where it picks up with the pump fuel return, or it can go back to the filter head before going back to the tank.
Leak off fuel is very important in the system. It's used to lubricate the pump and injectors and also used to heat the diesel to prevent waxing (when needed)
Re the differences between old and new types of injection.
Common rail is
Quiter
cleaner
provides a higher degree of fuel control
to name but a few advantages.
Instead of having a slight delay and a single fuel burn, common rail systems can offer multiple injections per firing cycle (pilot injection), so the flame front is already traveling as the main fuel load is being instantly injected.
Focus Cmax has the ability to use 6 injection pulses per firing stroke, compared to a single injection of the old gen kit.
On full revs, you can expect to see about 44.000 psi at the fuel rail pressure sensor, compared to about 16.000 on an old system, the higher the pressure, the finer the atomisation which leads to a cleaner more powerful burn.
Hope that helps with some of it.
the amounts of fuel used are govened by the pumping and metering mechanisms within the injection pump.
simplist form of pressure generation consists of a cam ring and rollers, the fuel enters a pumping chamber, the rollers are forced together by the cam ring and the pressurised fuel is sent down a delivery port to the injector.
a mechanical govenor (centrifugal) is employed to limit rpm by reducing the fuel into the pumping elements as speed increases towards max.
The pump is also advanced by fuel pressure to deliver the fuel at the correct time in relation to engine speed.
Diesel injection is an incredibly complex area, I find it more indepth than petrol in some respects.
simplist form of pressure generation consists of a cam ring and rollers, the fuel enters a pumping chamber, the rollers are forced together by the cam ring and the pressurised fuel is sent down a delivery port to the injector.
a mechanical govenor (centrifugal) is employed to limit rpm by reducing the fuel into the pumping elements as speed increases towards max.
The pump is also advanced by fuel pressure to deliver the fuel at the correct time in relation to engine speed.
Diesel injection is an incredibly complex area, I find it more indepth than petrol in some respects.
andytk said:
Nope. I'm even more confused now. (but cheers for the responses)
How does the old style system meter the fuel? Is it to do with the leak off rate? So the more you put you're foot down the less leak off occurs and the more fuel goes into the cyclinders?
Andy
The mechanical injection pumps use small pistons driven by a cam. The stroke of the piston is varied according to the position of the throttle (and a mechanical governor).
nighthawk said:
the amounts of fuel used are govened by the pumping and metering mechanisms within the injection pump.
simplist form of pressure generation consists of a cam ring and rollers, the fuel enters a pumping chamber, the rollers are forced together by the cam ring and the pressurised fuel is sent down a delivery port to the injector.
a mechanical govenor (centrifugal) is employed to limit rpm by reducing the fuel into the pumping elements as speed increases towards max.
The pump is also advanced by fuel pressure to deliver the fuel at the correct time in relation to engine speed.
Diesel injection is an incredibly complex area, I find it more indepth than petrol in some respects.
Living in a very rural part of the world a few acquaintances were in the habit of modifying the govenor on Nuffield tractors to get a bit mph, 35mph wasn't that unusual.
Guess it had an effect on more than the engine, pretty difficult to get an exchange clutch when the clutch is in wee tiny bits collected in a tin bucket.
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