Define the Induction route from Air Filter to Inlet valve

[Zak33]

OK...more complex.

The air has a filter, which we understand. Next the air must be sucked into the engine. NO TURBOS in this (Loewe )

We will stick with Fuel Injected cars, modern road cars. Once the air has gone through the air filter, where does it travel, in what kind of "duct" or "casting" ?

STOP your description at the INLET VALVE..go no further.

Go into well structured detail about monitoring of air flow, but not techy stuff, just basics.
Does it split in its route? What materials are we talking about? and what stops leaks?

Ready...steady....

G

[sdp]

I'm not too sure about injection cars but I think the air goes through the filter, down through the inlet which normally branches into 4 (on a 4-cylinder engine) and from there it goes into the inlet ports on the cylinder head.

The air is being sucked in by vacumn created by the pistons as they go down in the bores.

Then just before the air reaches the inlet valves petrol gets injected into it in a fine mist. There's some sort of flow detection gizmo which along with other condition monitors helps the system tell how much petrol is needed.

Inlets are made from an alloy which has better heat conduction properties than cast iron. All the joining faces are sealed with the aid of gaskets to prevent leaks.

*I think*

[Zak33]

not bad mate.....

tell me.... you say the pistons are pulling the air in...(which is true )........surely the ROUTE the air takes is important...and maybe the size of the pipe work.

And whats the texture like inside these "pipes"

Plus....in most MODERN cars the air filter is actually located somewhere to the side of the engine bay, in a big black plastic box.........so why is that?

And in an older car, like the great Mini how does the fuel get dragged in? Cos it aint got an injector to "spray it"

And whats the name of the inlet casting?

[Russ]

bi dont know much about engines (yet) but im guessin the pipe that feeds the air into th engine would be large at the opening, then go tight at the end, as air speeds up and looses pressure when compressed.

am i right? or is it the complete opposite?

[[GSV]Trig]

Well mine goes from the airfilter up a bit of rubbery pipe thru the AFM into the SFi box then into the throttle body then the fuel gets mixed in as it goes into the inlet manifold (cast alloy) then into the cylinders..

[sdp]

Most of my knowledge of injection systems comes from the 1973 AA book of the car btw

I know the pipes (ports?) need to be smooth and a lot of people spend time and money polishing the ports and modifying them (gas-flowing them I think it's called) to achieve maximum efficency, by which I mean minimise friction and maximise airflow.

In a car like a mini you have the (imo) engineering brilliance that is the carburettor

These sit on the end of the inlet manifold with an air filter bolted on top. There are a few different types, but as I've got a SU HS4 carb and the SU's are quite common I'll try and explain how that particular one works. In simple terms the vacumn pulls up the carb's piston on the end of which is a needle. The needle is tapered and sits in the 'needle jet' from which petrol is drawn by the vacumn. As you know the air/fuel ratio needs to change depending on engine conditions.

To take acceleration as an example basically what happens when you stomp the go pedal is the butterfly throttle valve opens, which increases the air flowing through the manny, and this increases the suction in the carb chamber which makes the piston/needle assembly rises to allow more fuel into the carb to mix with the increased volume of air and so attain the correct mixture. The shape of the needle is crucial as far as getting the correct mixture goes. The piston assembly is dampered to cope with sudden changes.

The term 'variable jet' is used to describe this action. Weber carbs are fixed jet, but please don't ask me to explain how they work

I really do think they're an engineering marvel, somehow beautifully simple but horribly complex at the same time .

[Lowe]

Boo, turbos add a twist to it, but it's all pretty much the same. It goes into the inlet manifold (which has a none perfect finish to induce turbulence, helping the air and fuel mix better) then you've covered the rest. Beaten to it.

[Zak33]

Lowe...you nailed summit important there mate....

and Idont want anyone to miss it the INLET MANIFOLD is ROUGH SURFACE....it assists in getting the air swirling and turning, so for the first time we are looking for friction and resitance.

OK, lets pull this lot together.

A piston (prob one of 4) is on its way DOWN and the inlet valve is OPEN......

so there is air being drawn into 1 out of 4 cylinders.

Now follow the route the air takes.

Its gone through the front on the car into the engine bay, into an air filter box, through the filter.

It then needs to find its way THROUGH A THROTTLE VALVE....
cos otherwise this engine is gonna rev its bolts off all the time...

WE NEED TO CONTROL things like tick over and then when we hit the LOUD pedal it needs to go faster....and luckily SDP pointed it out "when you stomp the go pedal the butterfly throttle valve opens"

SO for the un-initiated....in the route the air takes, is a perfect round aperture, with a round disk BLOCKING IT.....the disk often has a TINY hole in it to allow a tiny amount of air through it.

On the outside of the casing is a lever conected to it that allows it to open, to kind of "move aside" and its got a cable connected that goes to your THROTTLE PEDAL.

It DOES NOT open like a cat flap, or a door ie it doesnt hinge on one side

It DOES open hinged in the middle.....so when it twists and opens air goes round both sides and when its WIDE OPEN its LAYING FLAT so it has no air resistance. It is a thing of beauty

THIS IS A CUT THROUGH: please be aware this one has 2 butterflies really unusual, but I am using it cos it shows them in different positions and ignore the other stuff on the left side, its just perfect cos it shows buttlerflies....and as a note, car butterflies are almost always copper/brass colour


And effectively when this throttle body/butterfly valve is CLOSED the engine piston STILL SUCKS AIR......which creates a VACUUM.....cos this piston has gotta go down IRRESPECTIVE of an air suppy or not....and that is part of what causes ENGINE BRAKING....ie your engine slowing your car down cos its drawing on air that doesnt exist

then the air needs monitoring.....new post in a second

[Zak33]

Right....so how do we monitor air flow? Its complex (no kidding) but a basic example which will work in PC geeks heads is as follows.


Electricity through a wire causes HEAT....natch

The hotter the wire gets the higher the resistance....

What if we could MONITOR resistance of a wire? thats easy

What if we has a naked WIRE stretched across the air flow....and we whacked a current through it?
What would happen when we opened the throttle and more air rushed in? What if the wire cooled down and the resitance dropped instantly.? Could we monitor that? With a small PC in a black box?

and as a further benefit, what if the ambient air temperature effected the wiring cooling? Well it would wouldn't it? Which is IDEAL cos the colder the air is, the denser it is, and therefore the more fuel we might need.

In fact, if we got a few petrol heads together to work out how much fuel was needed as a ratio of the cold air effect on this wire...and we programmed the fuel computer to spray the right amount accordingly.....wouldnt that be ACE it could be a "fuel map"

well, BINGO.......its what happens

So we want MAXIMUM air flow at full throttle....but it must get "stirred up" to help the fuel mix well......so the air goes into a MANIFOLD via one large duct 9its always been in one duct/pipe/tube up to now) having been controlled by the Throttle Body and also monitored by our air mass meter thingy....

and finally the fuel needs adding...it might get injected into the air BEFORE geting split to the cylinders (ie SINGLE POINT FUEL INJECTION) or it might get split into 4 inlets and THEN GET FUEL SPRAYED IN, ie MULTI POINT injection. Ob the whole multipoint is faster, and single point is more economical (yes TiG thats yours )

Anyone know how an injector works? roughly ?

I'm ready......and we aint even in th engine yet

[Sprockets]

Zak, the system you just described is a hot wire setup. This isn't the only way ) The early electroni systems utilised a 'metering flap' system. Where it was basically a big trap door on the inlet, which insimple terms, was connected to a big potentiometer. This was a very bad design, becasu you obvioulsy have a big obstruction which you don't want. But they were eeasy to modify for a bodge increase in fuelling, by tightening the spring . FOllowing this, was the hot wire system which you described. After that we have the 'film sensor'. This is quite a complex little device. basically there is a 'bubble' of air inside, and as the air pressure changes, the bubble changes, which via voodoo magic alters the electrical signal from it. Then finally, we have MAP sensors (Manifol Air Pressure) This is very much the same as a bubble. But these have the added advantage of being able to measure positvie pressure (needed for all you turbo fans)

All this is assuming you are using air pressure/flow to measure engine load. you can use Alpha N ampping, which just relies on a TPS for load (Throttle Position Sensor) ..but we'll leave it there

I'll leave someone else to describe the injectors. They aren't a difficult device (or i could bore you all stupid with injector theory aswell, but i think you've had enough of me for one day )

-Gavin

(PS - Just got back from ford fair, bloody brilliant, even tho i only had 3.5hrs sleep, and was nursing a hang over!)

[CocoPops]

With reference to Zak's turbulence point.

I have a BMC Carbon Fibre Induction kit, that has a cone arrangement along with some wings that make the air spin as it leaves the filter element, this causes the tubulence.

It sure does make a difference on the dyno

[Zak33]

Excellent input guys......we covered loads there.....all those Air Metering descriptions from Sprockets are fantastic.....

Cheers man

Coco points out some other methods, highlighting that SMOOTH isnt always good and things need STIRRING up

[Big RICHARD]

Rough isn't always good either. There are other ways of getting the fluids to mix. Shaping the chamber to 'spiral' air and fuel with each other would also work. This would be expensive

Aerodynamics is much more complicated that just having a rough surface, but the shape of that surface too.

[Zak33]

Originally posted by Big RICHARD
Shaping the chamber to 'spiral' air and fuel with each other would also work.

Spiral to create "swirl" is very important....and as Cocopops points out just up there, some upgrade inlets that are "smooth" because of their composition, have other stuff to get the air mixing well......BUT its still essential not to dirupt it too much as we want max air flow too.....

in production cars, that you mod yourself, its often better to leave the inlet coarse.

[RocketmanX]

Anyone know how an injector works? roughly ?

Yes I do, Roughly

Basically a plunger runs down through the center of the injector that keeps the injector closed until fuel is needed. The plunger is controlled by an elctromagnet, the electromagnet opens the plunger and allows fuel to be "sprayed" in. The amount of fuel delivered to the engine is controlled by how long the plunger is held open and called "pulse width" I believe.
Injectors are very simple in theory compared to a carburetor but require a lot of sensors to operate well where a carb is somewhat self-metering (which is BTW the brilliance behind carbs IMHO)

[Sprockets]

yep, you got the basic idea there mate Main thing to remember is that fuel is supplied to the injectors at quite a high pressure (3-5bar). ou are also correct that electronic injectors pulse, and yes, this time is referred to as the pulse width Another aspect of the injectors job is to spray the fuel in a certain pattern. some inject in a sharp pencil like stream, others, a more conical mist. All these factors are important to consider.

And yes, carbs are self calibrating to a degree. For example, you put a differnt exhaust on a FI car, it will need remapping to take into account the altered air flow characteristics, carbs on the other hand, will just supply more fuel ) (Carbs still need very careful setting up tho, don't be fooled!!)

-Gavin