C172 SP fuel injection system

[baobab72]

Dear All
here i am with another doubt that puzzles me.
While in the process of familiarizing myself with the C172 SP systems for an upcoming BFR, i have come across the fuel system and a couple of questions stemmed from it:
how does the fuel/air control unit work? The book says that the fuel air control unit meters the fuel fed into the cylinders but it does nt specify the mechanic behind that; it just states that the flow is continuous and at the rate controlled by the throttle.
Why is the air from the induction system passed through the fuel air control unit prior to being fed into the manifolds?
Does the throttle control a butterfly valve within the fuel air control unit and how does its position affectes the fuel flow?
Is the fuel mixed with the fuel ouside of the intake valve or within the cylinder?
Are you provided with any kind of fuel pressure indication other than the fuel flow indicator? and where is the fuel flow pickup located?

Many thanks

Baobab72

[FlyingStone]

I'm not familiar with the fuel system of C172SP, but the principles of fuel injection engines are common.

The way I understand it, the fuel-air control unit controls fuel flow and airflow into the engine. The amount of air which enters the engine is controlled by the same principle as in carburetted engines: throttle lever controls the butterfly valve, which then controls the amount of air entering the engine. The fuel flow is then controlled with both throttle lever and mixture lever. Obviously by opening throttle wide open, you need to substantially increase fuel flow in order to maintain the same air-to-fuel ratio. But of course, you also need to control the mixture, which is virtually control of fuel flow for a given throttle (and throttle butterfly) position. To make it simple, you can imagine the fuel-air control unit as usual throttle butterfly and two valves in series - one of them is controlled by mixture lever and the other by throttle (which also controls the throttle butterfly).

Most fuel-injected aircraft engines today use continuous flow fuel injection system, which basically means that fuel is flowing out of the injector for the entire time the engine is running (i.e. without any flow interruption). Since it wouldn't be very practical to have fuel flowing into the cylinder during every cycle, the engineers came up with an idea to put fuel injectors in the intake port of each cylinder, so that the fuel can only enter the cylinder when the intake valve opens (which of course involves air entering the cylinder, too). Reason for this is very simple: simplicity. :) It is much easier to design, manufacture and control indirect continous flow injection than direct injection, since you don't have to worry about things such as very high temperatures in the combustion chamber, peak pressures, injection timings, etc. All these are factors, which could degrade engine reliability - and you certainly don't want that in your aircraft engine.

Fuel pressure, which is many times displayed to the pilot as both fuel pressure and fuel flow (you have to be VERY aware of the possibility that increased fuel pressure may actually mean less fuel flow) is usually measured at the fuel manifold (the little thing on the top of the engine, which then distributes fuel to individual injectors).

Again, I haven't flown the C172SP, but this is how fuel injection usually lookes like.

[baobab72]

Many thanks for your explanation.

If i have understood it right, the air from the air induction system is fed into the fuel air control unit which in turn meters the fuel to be added to the air so as to maintain the stoichiometric ratio. The fuel air mixture is then delivered into the cylinder via the manifolds and enters the combustion chamber when the intake valve opens. The amount of fuel air mixture from the air fuel control unit depends upon the position of a butterfly valve integral to the unit and controlled by the throttle.
The the fuel from the fuel air control unit can also be adjusted by the mixture control.
How does the fuel air control unit determine the amount of fuel to be added to the air so as to maintain the stoichiometric ratio?
If the fuel is mixed to the air in the fuel air control unit, why is fuel sprayed into the intake port? or the fuel is not mixed with air at the fuel control unit but the air from the induction system is only used to determine the amount of fuel to be added for the ratio? if so, how and where is the fuel mixed with the air? cos it was my understanding that the air from the intake port of the induction system was routed through an air box and the air fuel control unit into the manifolds which then channeled the air into the cylinder.
Sorry for the silliness of the questions.

Many thanks

Baobab72

[FlyingStone]

Not quite... See, in ordinary carburetted aircraft engine, the fuel is mixed with air in the carburettor, which is usually at the very beginning of the induction system. The mixture of air and fuel is then delivered to cylinders via induction pipes and intake ports. But this system usually ends up delivering different mixtures of fuel and air to cylinders (e.g. one cylinder may be running very lean and it may not even fire if you lean while the other may be running too rich), which isn't the optimum, if you want to run your engine at peak EGT or even LOP (lean of peak).

So, the fuel injected engines were developed, where fuel is mixed with air at each cylinder intake port. So if you have 4 cylinder engine, which I believe is your case, the air is routed through the fuel-air control unit via induction pipes to intake ports. The fuel on the other hand is routed through the fuel-air control unit, which meters the required fuel flow and sends fuel to the fuel manifold, from where you have 4 (or as many cylinders/injectors you have) fuel pipes going to each injector. You can recognise the manifold as a round "box" on the top of the engine which has 4 (could be 6 or even more) small outgoing pipes (metal, not rubber/plastic) going to the injectors. The injectors are placed in the intake port, quite close to the intake valve. So let's take a look of that again: we have air coming into the intake port via induction pipes and fuel coming into the intake port via injector. So again, basically: in fuel-injected system, fuel is mixed with air in the intake port of each cylinder (contrary to carb engines, where air is mixed only once, regardless of number of cylinders - exceptions are multi-carb engines, but I'm not very familiar with those).

Take a good look of this (http://www.airpowerinc.com/productcart/pc/catalog/IO240.jpg) and this (http://upload.wikimedia.org/wikipedia/commons/4/49/Teledyne_Continental_Motors_IO-240-B_Diamond_DA20-C1_C-GEQA.jpg) pictures, which represent Continental IO-240 engine (I couldn't find equally good picture of Lycoming IO-360). On the first picture, look at the right bottom: there is a fuel injection pump mounted to the engine itself. From there, you have a red hose, which brings fuel to the fuel-air control unit, which is located at the front top of the engine (you can even see the butterfly valve). From then, the air splits into four induction pipes toward each cylinder. The fuel on the other hand goes below the pipes to the fuel distribution manifold, which you can find on the top rear of the engine. On the second picture you can very easy see the connection from the fuel manifold to the fuel pressure sensor and then forward to the fuel pressure gague. From the fuel manifold you have 4 metal pipes, each leading to fuel injector, which is actually located perpendicular to the piston's travel and can be recognised by the fact that it is smaller than spark plug (which you probably know what to look for).