Up Up and away

[John Dammeyer]

Yikes,

Phil,  please turn off the HTML

OK.  I knew that sooner or later you'd pop in with that comment.  8-).

I don't agree with you.  Altitude does make a difference.  I can throw a bunch
of advanced Physics and Math to prove it,  (I stayed up really late one night),
but let's use common sense and intuition to come up with the result that
Bernoulli later proved.

Air is a fluid and has mass.  To move air from one place to another requires
force.  In the case of air the force is a difference in pressure between two
locations.  The key element to understanding why a lower external pressure
(outside the throttle plate) will move less air into the cylinder for an
equivelant MAP is that the difference in air pressure results in a different air
velocity.

The internal combustion engine is not a static environment but one in which time
plays a factor.  The intake valve is open for a period of time.  During that
time the piston moves down and creates a low pressure area inside the cylinder.
This causes the higher pressure area in the plenum to force air to move into the
cylinder at a velocity dependant on the difference in pressure between the two
regions.

At the same time air from the atmosphere is moving past the partially open
throttle plate at a velocity dependant on the atmospheric pressure and the MAP.
The intake valve closes before the air pressure equalizes to the value of
atmospheric pressure.  Air continues to flow into the plenum past the throttle
plate and the pressure would equalize to atmospheric if it were not for the next
intake valve opening for the next cylinder intake stroke.

So the MAP is a reflection of the velocity of the air mass flowing into the
system and the velocity over time tells you how much air has actually entered
the cylinder while the intake valve was open.

Now because the air does have mass it doesn't stop flowing just because the
valve has closed.  It piles up around the intake valve and the pressure inside
the manifold starts to increase at a rate faster than the difference in air
pressure.   When the runners are the correct length this all works together to
create a sort of supercharger like puff into the next opening intake valve which
actually improves the breathing of the engine.

The faster the piston goes down,  the more extreme the difference in pressure
between the intake manifold and the inside of the cylinder and the faster the
air will flow.  You can simulate this with a pressure gauge and a bit of hose.
Suck at the end of the hose until the gauge reads 10"Hg. and then block the hose
with your tongue.  If there are no leaks the pressure will stay at that value.
Put a pinhole into the side of the hose and you have to continue to suck to
maintain a value like 15"Hg.  Suck faster and you can draw the pressure down to
10"Hg. but you're also moving more air.

Now I am not saying that a Pulsewidth[MAP,RPM] table will not produce a nice
clean precise fuel mixture for an engine.  It does.  But,  the MAP and the RPM
only tell you how fast the air in the intake manifold will travel into the
cylinder and not how fast the replacement air moves that is drawn into the
manifold past the throttle plate.  Yes,  on average a higher altitude will
create an average lower MAP and lean off the mixture a little;  but not enough
simply because the Dyno runs for that MAP and RPM value had a different pressure
outside the throttle plate and the replacement air moves at a higher velocity.

The velocity of the airmass and the time the intake valve is open is the
important part of the equation.  This is why Mass Air Flow Sensors like the ones
on SAABs etc have an automatic altitude compensation and work so well with Turbo
Chargers;  which in effect, change the pressure outside the throttle plate
causing a higher velocity air flow (and a resultant higher MAP) and more air
into the cylinders.

Phil,  a while ago you posted the comment that no one had made a case for a
barometer in a vehicle.  Hope this helps.

Regards,

John


>Date: Fri, 08 Oct 1999 01:18:11 +1000
>From: Phil Lamovie <phil at injec.com>
>Subject: Re: Up Up and away
>
><HTML>
>
>
><P>Hi All,
>
><P>Recent postings regarding MAP sensors are in need of a little
><BR>massaging.
>
><P>Lets start with the definition&nbsp;&nbsp;&nbsp; manifold <B><I><U>ABSOLUTE
>PRESSURE</U></I></B>
><BR>means what it says. Not relative or delta but absolute.
>
><P>Any ECU mapped by speed/density <B><I><U>AUTOMATICALLY</U></I></B> corrects
><BR>for engine load with altitude.
>
><P>Question 1
>
><P>How does the engine differentiate between part
><BR>throttle vacuum at sea level and full throttle at 10,000 feet ?
>
><P>Answer It doesn't. There is no difference.
>
><P>There is no need to check and record MAP value before startup.
><BR>As the pressure varies absolutely so should the fuel table value.
>
><P>Speed density demands only RPM and MAP (air temp correction is
><BR>not mapped but mathed. same for all other basic corrections)
>
><P>If the table has 16 speed values and 16 load points you will
><BR>have sufficient date to run the engine reasonably.
>
><P>Pilots had to correct mixtures only with badly designed fuel systems.
>
><P>The mech/injected engines had no such problem.
>
><P>Phil Lamovie
><BR>Injec Racing</HTML>
>