Fine-Tuning the Aneroid Fuel Control on Cummins PT Pumps

[MikePhua]

Introduction to the PT Fuel System
Cummins' Pressure-Time (PT) fuel system, introduced in the early 1950s, revolutionized diesel engine performance by integrating fuel delivery with engine speed and load. This system utilizes a gear-driven, positive-displacement fuel pump, which maintains a constant fuel pressure to injectors, ensuring precise fuel metering. At the heart of this system is the Air-Fuel Control (AFC) mechanism, which adjusts fuel delivery based on turbocharger boost pressure. The aneroid, a key component of the AFC, plays a crucial role in this adjustment process.
Understanding the Aneroid and AFC
The aneroid is a diaphragm-based device that responds to changes in atmospheric pressure, which correlates with turbocharger boost. As boost increases, the aneroid adjusts the AFC lever, altering the fuel delivery to the injectors. This mechanism helps control the amount of fuel injected during acceleration, thereby managing engine power and reducing excessive smoke.
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, one operator noted that after rebuilding the pump, the AFC screw was found to be only one turn open from the full stop, leading to noticeable improvements in power upon adjustment.
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

• Incremental Adjustments: Make small adjustments and test the engine's response to ensure that changes lead to desired outcomes without introducing issues like excessive smoke or reduced performance.
  
• Monitor Engine Behavior: Pay close attention to engine performance, including acceleration, smoke levels, and power output, after each adjustment to assess the impact.
  
• Consult Technical Manuals: Refer to Cummins' official service manuals for detailed procedures and specifications related to AFC adjustments.
  

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.