Optimizing the Aneroid Fuel Control on Cummins PT Pumps

[MikePhua]

Introduction to the PT Fuel System
The Cummins Pressure-Time (PT) fuel system, introduced in the early 1950s, marked a significant advancement in diesel engine technology. This system utilizes a positive-displacement fuel pump that delivers a precise amount of fuel to the injectors based on engine speed and load. A key component of this system is the Air-Fuel Control (AFC), which adjusts fuel delivery in response to turbocharger boost pressure. The aneroid, a diaphragm-based device within the AFC, plays a crucial role in this adjustment process.
Understanding the Aneroid's Function
The aneroid operates by sensing changes in atmospheric pressure, which correlates with turbocharger boost. As boost increases, the aneroid diaphragm moves, adjusting the AFC lever to regulate fuel delivery. This mechanism helps control the amount of fuel injected during acceleration, thereby managing engine power and reducing excessive smoke. Proper calibration of the aneroid is essential for optimal engine performance.
Challenges in Fine-Tuning Without an Air Screw
Some PT pumps are designed without an external air screw for AFC fine-tuning. This design choice can complicate the adjustment process, as operators must rely on internal components to achieve the desired fuel delivery characteristics. For instance, after rebuilding a PT pump, one operator discovered that the AFC screw was only one turn open from the full stop, leading to noticeable improvements in power upon adjustment. This experience underscores the importance of proper AFC calibration for engine performance.
Adjusting the AFC Without an Air Screw
In the absence of an external air screw, fine-tuning the AFC requires careful manipulation of internal components:

• Star Wheel Adjustment: Located within the AFC housing, the star wheel controls the rate at which the AFC lever moves in response to boost pressure. By adjusting the star wheel, operators can modify the sensitivity of the AFC, affecting fuel delivery during acceleration. Turning the star wheel toward the engine increases sensitivity, while turning it away decreases sensitivity.
  
• Aneroid Spring Tension: Adjusting the tension of the aneroid spring alters the force required to move the AFC lever. Increasing tension can reduce the amount of fuel delivered, while decreasing tension allows more fuel to be injected. This adjustment is typically made by rotating the aneroid housing.
  

Considerations for Safe and Effective Adjustment
To accurately diagnose and adjust the AFC, consider the following steps:

• Inspect Hydraulic Fluid: Check the fluid for cleanliness, proper level, and signs of contamination.
  
• Bleed the Hydraulic System: If air entrapment is suspected, bleed the system to remove trapped air.
  
• Examine Cylinder Seals: Look for signs of wear or damage on the cylinder seals.
  
• Test Control Valves: Operate the control valves and observe the lift arm response for any inconsistencies.
  
• Check for Leaks: Inspect hoses, fittings, and connections for any signs of hydraulic fluid leaks.
  

Maintenance Tips to Prevent Spongy Lift Arm Cylinders
To prevent issues with spongy lift arm cylinders, adhere to the following maintenance practices:

• Regular Fluid Changes: Change the hydraulic fluid at intervals recommended by the manufacturer, typically every 500 to 1,000 hours of operation.
  
• Use Quality Filters: Install high-quality filters and replace them as per the maintenance schedule to prevent contamination.
  
• Monitor Fluid Levels: Regularly check and maintain the hydraulic fluid at the recommended levels.
  
• Inspect Seals and Hoses: Periodically inspect seals, hoses, and fittings for signs of wear or damage.
  
• Calibrate Control Valves: Ensure that control valves are properly calibrated and functioning correctly.
  

Conclusion
Fine-tuning the AFC on a Cummins PT pump without an external air screw requires a thorough understanding of the system's components and careful adjustment of internal elements. By methodically adjusting the star wheel and aneroid spring tension, operators can achieve optimal fuel delivery characteristics, enhancing engine performance and efficiency. Always prioritize safety and consult technical resources when performing such adjustments.