mistarose

I understand the Bendix Fuel Injection system pretty well. However there is one aspect of the "Servo Regulator" which I have trouble understanding.

The "air" side of the regulator which has Ram air (High Pressure) and Venturi Air (low Pressure) provides the force which opens the the Servo Regulator Valve via a rod connection.

So the faster the air flow, the lower the pressure on the venturi side, and the "stronger" pressure differential force opens the regulator valve furthur to allow more fuel to head over to the fuel distributor.

My question is, why does the RAM air pressure stay pretty much the same (as they say in books)? If we have increased airflow, wouldn't the Ram air pressure increase, just like the pitot tube shows a higher airspeed as we travel faster though the air?

Any ideas? I am going to talk to the company like I did before, and ask them also.

Thanks in advance.

rickair7777

I would imagine that the ram air (is it really ram, not static?) could compensate for density altitude and alleviate some mixture adjusting.

mistarose

Well yeah, the ram air is measured by impact tubes before the venturi. As far as I know, they measure the same type of pressure as the pitot tube.

The fuel injection systems I have dealt with simply "compensate" for altitude, they do have injection systems that "auto-lean."

Say for example if you increased the throttle, this would immediately result in increased airflow through the venturi. Increased Velocity, Decreased pressure.

Now there is a greater High to Low pressure force on the air diaphram side, which is connected via a rod to the servo regulator valve. The increased force away from the valve's opening, slowly allows more fuel to go to the engine.

Not sure about why the ram air doesn't increase pressure with a increase in airflow. If it was static, that may make sense, but I am 99.9% sure that they measure air pressure.

rickair7777

The pitot and venturi may not be evenly matched, they may be ratioed to favor one or the other to provide some adjustment to the fuel/air ratio at higher power settings...I would guess richer at higher power to ensure cooling..