What is the fuel/air ratio in a 1.2?

[C.slicker]

Anyone know the average ratio? of course it will be proportional to the position of the accelerator but on average?

Its just I was thinking about James Mays comment when discussing the Veyron. He reckons the veyron took in as much as air in 10 minutes as he did in 2weeks. That seems alot of air?

 

[stan*]

proportional to load and rpm, but stoich is 14.7:1 by mass.

 

[C.slicker]

what does stoich mean?

Presumabely the rate of increase in fuel outstrips the increase in air, when the accelerator is pressed?

Does the engine require new air each time the piston moves up and down (i.e. each revolution)? If so is this to say that for a one litre car on average about 920-930ml of air will be needed for each revolution? I am bloody confused!

Any help appreciated!

 

[stan*]

Stoichiometric...the theoretical AFR which gives complete combustion.

Its depends on engines speed, not just load (throttle)...in any case, the ammount of air is fixed by volumetric efficiency at that event...so the mass of fuel required is dependent on the AFR you are trying to achieve for the same event.

Its a four stroke engine, so fresh charge is brought into the cylinder on the intake stroke which will be on every other revolution.

 

[C.slicker]

Ok, just given that some though and...

Assumptions:

1 litre engine
Complete combustion occurs
Plentiful supply of air and fuel
Ratio of 10-1 (14.8 is hard to work with)

If this is the case 100ml or fuel will be used every revolution and 900ml of air.

When the car is running at say 4k rpm the engine will be taking in 400 litres of fuel a minute... and 3600 litre of air per minute!

Where is the flaw in my logic?:S :S :S

 

[GordonD]

Where is the flaw in my logic?:S :S :S

Stoichiometric is 14.7:1 by mass. Repeat, by mass. Not by volume. Air, a gas, is a lot less dense than petrol, a liquid.

Air has a density of about 1.2 kgs per cubic metre. A litre of air only weighs 1.2 grams. So at an AFR of 14.7:1 that only requires 82 milligrams of fuel, which is only 111 microlitres. So divide your result by something in the order of 1000 and you've got the right answer.

(I haven't checked my arithmetic. My point is not that my numbers are right, but that you're a long way out in your arithmetic because you calculated on the basis of volume not mass. OK?)

 

[stan*]

proportional to load and rpm, but stoich is 14.7:1 by mass.

ratio by mass

 

[stan*]

doh lol

 

[C.slicker]

Oh right yeah ok.

Forgot about that minor detail lol!

But that makes the air taking over 3800 litres per minute at that engine speed!

That seems one hellllll of a lot of air! Mental.

I guess it is therefore little wonder a Veryron with a W16 engine (forget what litre now) travelling in excess of 250mph consumes more air per ten minutes than Captain slow consumes in a fortnight!

 

[GordonD]

But that makes the air taking over 3800 litres per minute at that engine speed!

That seems one hellllll of a lot of air! Mental.

Its an 8 litre 4-stroke engine. If it was normally aspirated it'd use about four litres of air per rev. So at 4,000 rpm it'd actually use 16,000 litres of air per minute. But its turbocharged. So it actually uses more than that.

I hope you don't have a job where my safety depends on your mathematical abilities.

 

[stan*]

a 1.0 litre 4 cyl engine has four 250cc chambers. It takes 2 revolutions for each cylinder to have a power stroke, therefore it takes 2 revolutions for 1000cc of air to be "taken in".

So at 4000 rpm, 2000 litres is taken in one minute.

This is assuming 100% volumetric efficiency.

 

[C.slicker]

Gordon, when I was talking about the engine taking over 3800litre per minute that was with regards to the 1litre lump, not the 8 litre veyron engine!!!

As for Stans comment:

a 1.0 litre 4 cyl engine has four 250cc chambers. It takes 2 revolutions for each cylinder to have a power stroke, therefore it takes 2 revolutions for 1000cc of air to be "taken in".


Forgot it takes two revs (as measured on dial) for all four pistons to complete their cycles.

Cheers for your help guys, all MUCH clearer now!

 

[stan*]

no probs

 

[The Fatty]

Just out of interest, why did you want to know all of that? lol. Seems very random...

 

[C.slicker]

The comment on top gear about the amount of air the veryon consumes got me thinking.

And then it just got me thinking.

I may also do some calcs as to how much difference cold air makes in relation to warm air.

Sam

 

[stan*]

A drop of 10deg.c will yield a 1.8% increase in power output. This is the common rule of thumb derived from air density based calcs.

 

[C.slicker]

Right ok, I worked it out to be a 10*C loss leads to a 1.6% increase. Must have used a different air density figure.

How hot do you reckon a air taken in by an induction kit which sits right next to the engine (as in where the standard air box was, with no cold air feed (for arguements sake lets say the car is travelling at 50mph).

Sam