924Board.org

[TheBib]

I do not understand how the basic mechanical bosch fuel injectors work or for that matter why the engine does not flood.
If the CIS system is providing a continuous supply of fuel to the injectors and the injectors are continously spraying out gas, then why is the engine not flooded. Out of 4 strokes of the piston only one requires gas. So is the injector opening and closing and if so how does it do that with a continuous stream of fuel back pressure. Saying it another way during the other strokes of the piston when the intake valve is closed where is the fuel from that injector going if it does in fact continuously spray out? or is the injector opening and closing because of the drop in pressure and if so how can that be since the injectors on my 82 porsche 924 just go into the intake manifold and there shouldn't be that big a difference within the intake?
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TheBib

[tyfighter123]

The injectors have a valve in them so when the system is off they dont spray, only when the system is turn on the pump builds the pressure and then they spray, also when you step on the gas the incoming air will raise the air plate and the plunger moves down will more pressure be applied to the injectors and they will spray more.
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Porsche 924 1977 N/A
Mustang GT/CS 2007
Porsche 924S 1987 (parts car)(cut up and recycled)
Porsche 911S 1976
Porsche 931 1980
Porsche 931 1980 (parts car)

[TheBib]

so if i were looking inside my intake manifold at lets say injector for cylinder 1, with the engine running. Even though cylinder 1 does not have its intake valve open (lets say it is on its firing stroke or compresson stroke) the injector is still spraying gas? totally does not make sense.

i understand the air and the plate providing more gas, but an injector for a specific cylinder spraying in gas regardless of whether that cylinder is taking on air seems to not make sense to me.

what should happen is the injector should only spray when the intake valve for that cylinder is open. so the injectors would only spray in the same order as the intake valves open which is the same as the firing order.
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TheBib

[ptheskil]

Seems a bit counter intuitive but the injectors do in fact spray continuously at a rate determined by the position of the air measuring plate in the fuel distributor. When the intake valve is closed, the spray is directed at the back of the valve. The valve is hot so the fuel, which is a fine mist already, vapourises and the fuel vapour hangs around the back of the valve until it opens and sucks it (and lots of air) into the cylinder.

In practice there is always a small amount of condensed liquid fuel on the walls of the intake manifold/inlet port. Some of this evaporates every engine cycle but is replaced by new fuel from the injector so the "puddle" is a fixed volume for a given engine condition. This only becomes a problem to your AFR when you change throttle angle and the manifold pressure changes suddenly. Eg when you open the throttle, manifold pressure increases and the amount of fuel evaporating from the puddle is reduced so you get a lean AFR spike - unless you do something else to compensate for this.

The biggest drawback with the continuous injection system is that you do get a small amount of liquid fuel ingested each time the valve opens (it is spraying while the valve is open of course) and this can wash the oil film off the cylinder wall and lead to upper cylinder wear.
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1981 931 series2 Euro spec

[TheBib]

hey thanks for the answer, very complete. I am a chemical engineer who also works on cars a bunch and so it may be a nit picky question, but you gave a great answer. I was hoping the answer was that somehow the injector spring was set so that any small deflection in pressure drop near it would cause the injector to open, but that obviously would not be practical since it operates at all temperatures and subjected to extremes and needs to be reliable. I guess it works, not the most efficient set-up, but guess its better then carburators or throttle body injection like my other cars.
thanks

db
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TheBib

[Rasta Monsta]

Mechanical "always spray" injection was developed by the Porsche race program. . .due to high pressures, much more efficient fuel atomization than a carb.

CIS is running on literally millions of cars from nearly every euro manufacturer.
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Toofah King Bad

• WeiBe (1987 924S 2.5t) - 931 S3

[fiat22turbo]

BTW, most early EFI systems worked in a similar fashion. They were either firing all of the injectors all the time, or in pairs (called batch fire) and as long as you can properly control the amount of fuel injected (which is the point with CIS and EFI) the "pooled" fuel becomes less of an issue.

It is counter intuitive, but if you think about how quickly the valves open and close (even at idle) you'll see that it doesn't make that big a difference. Sequential injection does make the idle slightly more smooth and can improve power and economy a little by altering fueling per cylinder.
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Stefan
1979 924 Carrera GTS (clone-ish)
1988 944 Turbo S (Silver Rose)

[ideola]

I'm getting ~30 MPG in the 937 and I drive it hard. Pretty darn good for 30+ year old technology. How many current cars are getting 30MPG with all their computer control EFI crap???

Flow-balanced injectors, functioning WUR, and functioning fuel dizzy are the biggest keys to good running, including optimal power and economy.

FWIW, S.D. Faircloth of Jaguar Fuel Injector service is one of the few suppliers still willing to service our CIS injectors, and if asked will flow match them. It took me 5 sets of used injectors to get two flow-matched sets (within 10% tolerance). One set is currently running in the 941, the other in the about-to-be-fired-up Club Sport.

The longer I live with a properly functioning CIS system, the more I'm amazed at how well it works.
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erstwhile owner of just about every 924 variant ever made

[fiat22turbo]

My old 87 Daytona Turbo got 30mpg at 90mph on the way to Vegas. Boost gauge read 0 the entire time. The fact is that since the 90's manufacturers have been focusing on emissions and not economy. The tuning changes for emissions cause the engines to run slightly more rich to reduce NOx emissions.

Also the cars have consistently gotten heavier and heavier as people have specified that they want all of the bells and whistles. Contrary to popular opinion, the added weight isn't just due to safety equipment it also due to the extra bells and whistles.

Look at the old Geo Metro compared to the resident darling, the Prius.
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Stefan
1979 924 Carrera GTS (clone-ish)
1988 944 Turbo S (Silver Rose)

[ptheskil]

Just to add a bit more to the discussion, you can think of the difference between the CIS system and electronically controlled injectors like this:

The CIS system works at a pretty much constant fuel pressure, injects continuously and controls fuel delivery by _varying the flow rate_ of the injectors (according to the delivered fuel mass flow which is regulated by the distributor).

An EFI system works at a constant fuel supply pressure (but by the way the differential pressure across the injector is not a constant because the manifold pressure changes so this has to be compensated for) and the injectors _flow at a constant rate_. Fuel delivery is then controlled by varying the _time open_ of the solenoid valve during each engine cycle. Duty cycle if you like.

And to echo other comments, the K-Jetronic system is an excellent fuel injection system: accurate (for its day), reliable (if maintained) and easy to service. Anybody who moans about it or claims that it only works properly if you add 5th injectors, fuel pump relay jumpers or so-called cold start switches and so on, is simply experiencing a poorly maintained system. Just my view of course but I'm happy to defend it
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1981 931 series2 Euro spec

[924RACR]