Afm Design Flaw

[KRISKARRERA 2 1 Cars]

Now this might seem really really stoopid but.....

 

I've just thought of a flaw in the design of the afm

Well Ok now imagine this, one's travelling along foot lightly on accelerator. So therefore there's no idle or full throttle signal voltage coming from the Throttle position switch to the ECU. I.E it's not a throttle pentiometer or however it's spelt. one puts ones foot on the accelerator slightly harder in order to increase speed but still not enough to make a voltage on the TPS.

Now the ECU decides how long to leave the injectors open based on the angle of the afm flap, right? BUT the greater the deflection of that flap only happens when there's an increase in piston speed and thats only gonna happen if more fuel was shoved into the combustion chamber but that's not gonna be happening unless the afm flap has moved and on ...... it's catch - 22! YOU CAN@T HAVE ONE WITHOUT THE OTHER...in theory.....Can you see what I'm saying here?

Obviously I'm wrong cos if I was right our engines wouldn't work.

so please can someone tell me exactly how it works......

[CJ_2_0_5 0]

You are limiting the amount of air into the engine with the throttle plate. Opening the throttle reduces the resitriction, hence more air and fuel can enter the cylinders which provides more power and increases engine speed.

 

The restriction provided by the throttle plate is known as a pumping loss and is one of the reasons why diesels (and BMW valvetronic or whatever they are called) are more efficient, as they have no throttle and hence no pumping loss.

 

CJ

 

EDIT: I know the BMWs do have a throttle as a backup in case the valve control fails, but you get my point .

Edited November 23, 2003 by CJ_2_0_5

[KRISKARRERA 2 1 Cars]

Oh right ok. It's a bit of a bloody agricultural design isn't it?!

[jackherer 543]

BUT the greater the deflection of that flap only happens when there's an increase in piston speed

no, the flap will be moved as the engine is placed under more or less load. if it was relative to engine speed then a crank position sensor would be all that was needed.

[C_W 3 1 Cars]

Yes. When you blip the throttle you send the flap flicking open. When are travelling down the road and prod the throttle a bit it flicks open then settles back to suit the engine speed/load. This maybe why they can seem quite responsive???

[Rob_the_Sparky 9]

Yes. When you blip the throttle you send the flap flicking open.

Which is why there is a second flap attached to the main one that acts as a damper.

[CJ_2_0_5 0]

The overswing of the AFM flap is critical to getting the correct transient fuelling. When a hotwire air mass meter is used you need to do lots of calculations to determine the rate of change of engine speed and apply extra fuel accordingly, this is all handled by the physical design of the AFM on the L-jet. An excellent piece of engineering IMO.

 

CJ

[Guest peanutsarehairy]

Em, Flux capacitor!