Carbureted Engines: Altitude Effects Explained

[Walt Puryear]

I’ve been trying to understand why a carbureted engine runs richer with altitude. I understand there is not as much power since less dense air results less mass charge and that oxygen percentage is lower at higher altitudes.

Since a venturi’s drop in pressure is given generally by the simplified formula p1 – p2 = s/2 (v22 - v12) it would seem the less dense (s) air would result is less pressure drop and proportionally less fuel drawn.

Am I using the wrong formula? Is there a commensurate increase in velocity of the air due to it’s lighter weight resulting in a higher differential? Does the air/fuel ratio really not change that much and it’s just oxygen/fuel ratio?

Any insight would be appreciated.

Walt

 

[pat8126]

Does the air/fuel ratio really not change that much and it’s just oxygen/fuel ratio?

From my understanding, the airflow remains constant and only the ratio of oxygen to fuel changes with altitude.

More specifically, the vacuum created by the car's intake manifold will cause the fuel to flow at the same rate regardless of altitude. However, there will be a lower density of oxygen molecules going into the combustion chamber and thus, a rich burning environment is created.

To adjust for this problem of changing density relative to altitude, you have to calibrate the fuel rate relative to the airflow caused by the intake manifold's vacuum.

 

[Walt Puryear]

I agree that the airflow volume remains essentially the same, however air mass is 28% less at 9,000 feet. Since the venturi pressure drop is proportional to the working fluid density, it would seem less fuel would be drawn.

 

[pat8126]

the venturi pressure drop is proportional to the working fluid density

Doesn't the pressure drop only in proportion to the speed of the air flowing past the fuel jet?

The needle valves adjust how much fuel flows in the presence of a vacuum for idle and full throttle conditions. The mass flow rate does change with altitude as there will be more or less of air's constituent parts per unit area as it moves through the vena contracta. I believe this explains the inability of the carburetor to inherently adjust the fuel mixture when there's a change in air pressure. To correct this, you must change how much fuel is drawn at the baseline vacuum created by full throttle and idle throttle with the needle valves.

(Of course, I could be completely wrong.)

 

[Walt Puryear]

I think I have managed to answer my own question. Using Bernoulli’s equation with a few assumptions like laminar flow, which is certainly not the case, and a nice online calculator at http://hyperphysics.phy-astr.gsu.edu/hbase/pber.html I came up with the results that follow. Using my carburetor’s dimensions and 6,000 rpm of flow, I found the venturi’s differential pressures to be:

sea level 2.32 psi
9,000 ft 1.62 psi

Not being able to calculate the actual fuel flow, I looked at the ratio of 2gh^.5 for the differentials pressures. At elevation this figure is 83% of seal level so have assumed fuel flow is also 83%.

Air mass flow at 9,000 feet is 70% of sea level for the same cfm. My wide band oxygen sensor runs a pretty consistent14.0 : 1 at sea level.

So (.7)(14) to (.83)(1) equates to 9.8 to .83 equates to 11.8 : 1. This is surprisingly close to the wide band oxygen sensor readings of 11 to 11.5 at elevation.
My conclusion is that lower air density results in less differential but since the quantity of fuel drawn is in proportion to the square root of head, a rich mixture results.

But as Pat8126 said, "I could be completely wrong".

 

[pat8126]

It's great that you have solved the issue!

While I certainly don't know much about fluid dynamics, I think I learned a lot just over the last few days while researching your question so thank you for posting it.

 

[rcgldr]

A side issue for some carburetors is that the end of the fuel nozzle extends into the high velocity air flow, resulting in a vortice about the end of the fuel tube, which in turn lowers the pressure at the end of the tube further still (and also helps atomize the fuel), drawing more fuel than a flush mounted fuel nozzle. I'm not sure of the altitude versus pressure related effects for such a setup.