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The Legacy of the Cummins 855 Engine
The Cummins 855 series, particularly the NTC and NTA variants, has long been a cornerstone of heavy-duty diesel power. Introduced in the 1960s and refined over decades, this 14-liter inline-six engine became a favorite in trucks, generators, and construction equipment. Its reputation for durability and torque made it a global workhorse, with hundreds of thousands of units sold across North America, Asia, and Europe.
Cummins Inc., founded in 1919 in Columbus, Indiana, built its brand on innovation in diesel technology. By the time the 855 series peaked in popularity during the 1980s and 1990s, Cummins had already established itself as a leader in fuel system design, particularly with its Pressure-Time (PT) system—a mechanical marvel that defined the engine’s performance characteristics.
Understanding the PT Fuel System
The PT system stands for Pressure-Time, a unique approach to fuel delivery that relies on low-pressure fuel supplied to injectors, which then meter the fuel based on camshaft timing. Unlike common rail systems that use high-pressure electronic control, the PT system is entirely mechanical, making it rugged and easier to service in the field.
Key components include:
Despite its durability, the PT system is not immune to wear. Over time, components degrade, leading to performance problems that can be subtle or catastrophic.
Typical symptoms include:
Injector Wear and Replacement Strategy
Injectors in the PT system are mechanical and rely on precise timing and fuel atomization. Over time, internal wear reduces their ability to meter fuel accurately, leading to misfires, rough idle, and poor emissions.
Best practices include:
The PT pump is the heart of the system. When it fails, symptoms range from sluggish throttle response to complete engine shutdown. Rebuilding the pump is often more cost-effective than replacing it, especially for legacy engines.
Rebuild checklist:
Preventing Air Intrusion and Fuel Starvation
Air leaks are a silent killer in PT systems. Because the pump operates under vacuum before pressurizing fuel, any breach in the supply line can introduce air, causing surging, stalling, or poor load response.
Prevention tips:
Shutdown Control and Safety Features
The PT system includes manual and automatic shutdown options. In older equipment, a pull cable physically cuts fuel flow. More modern setups use air cylinders or solenoids triggered by sensors or emergency switches.
Recommendations:
While PT systems are robust, they lack the precision and emissions control of modern common-rail systems. Retrofitting an 855 engine with electronic governors or hybrid fuel systems is possible but costly.
Pros of retrofit:
Conclusion
The Cummins 855 PT fuel system is a testament to mechanical engineering that still powers equipment decades after its introduction. Its simplicity, reliability, and field-serviceability make it ideal for harsh environments and legacy fleets. By understanding its operation, recognizing wear patterns, and applying preventive maintenance, operators can keep these engines running strong—proving that sometimes, old-school technology still holds its ground.
The Cummins 855 series, particularly the NTC and NTA variants, has long been a cornerstone of heavy-duty diesel power. Introduced in the 1960s and refined over decades, this 14-liter inline-six engine became a favorite in trucks, generators, and construction equipment. Its reputation for durability and torque made it a global workhorse, with hundreds of thousands of units sold across North America, Asia, and Europe.
Cummins Inc., founded in 1919 in Columbus, Indiana, built its brand on innovation in diesel technology. By the time the 855 series peaked in popularity during the 1980s and 1990s, Cummins had already established itself as a leader in fuel system design, particularly with its Pressure-Time (PT) system—a mechanical marvel that defined the engine’s performance characteristics.
Understanding the PT Fuel System
The PT system stands for Pressure-Time, a unique approach to fuel delivery that relies on low-pressure fuel supplied to injectors, which then meter the fuel based on camshaft timing. Unlike common rail systems that use high-pressure electronic control, the PT system is entirely mechanical, making it rugged and easier to service in the field.
Key components include:
- PT Pump: Driven by the engine gear train, it regulates fuel pressure based on RPM and load.
- Mechanical Governor: Controls maximum fuel pressure to prevent overspeeding.
- Injectors: Meter fuel based on how long the inlet port remains open, governed by camshaft rotation.
- Fuel Filters: Primary and secondary filters clean diesel before it reaches the pump.
- Shutdown Mechanism: Manual cable, air cylinder, or solenoid cuts fuel flow to stop the engine.
- PTG Pump: A PT pump with an integrated governor.
- Scavenge Pump: Optional component on larger systems to return excess fuel.
- Overflow Line: Returns unused fuel to the tank, helping cool and clean the system.
Despite its durability, the PT system is not immune to wear. Over time, components degrade, leading to performance problems that can be subtle or catastrophic.
Typical symptoms include:
- Hard Starting: Often caused by low fuel pressure, air leaks, or worn injectors.
- Loss of Power Under Load: Indicates poor fuel delivery, possibly due to pump wear or clogged filters.
- Excessive Smoke: Black smoke suggests overfueling; white smoke may point to incomplete combustion or injector malfunction.
- Surging or Hunting: RPM fluctuations at idle or under load can stem from governor issues or air in the fuel lines.
- Fuel Leaks: Cracked lines or loose fittings allow air ingress and fuel loss, reducing efficiency and increasing wear.
Injector Wear and Replacement Strategy
Injectors in the PT system are mechanical and rely on precise timing and fuel atomization. Over time, internal wear reduces their ability to meter fuel accurately, leading to misfires, rough idle, and poor emissions.
Best practices include:
- Replace injectors every 5,000–7,000 operating hours in high-load environments.
- Use OEM or certified remanufactured units to ensure proper flow rates.
- Inspect injector tips for carbon buildup and erosion.
- Test spray patterns during overhaul to confirm atomization quality.
The PT pump is the heart of the system. When it fails, symptoms range from sluggish throttle response to complete engine shutdown. Rebuilding the pump is often more cost-effective than replacing it, especially for legacy engines.
Rebuild checklist:
- Replace seals, bearings, and plungers.
- Clean galleries and pressure regulators.
- Calibrate governor spring tension.
- Test pressure output across RPM range.
Preventing Air Intrusion and Fuel Starvation
Air leaks are a silent killer in PT systems. Because the pump operates under vacuum before pressurizing fuel, any breach in the supply line can introduce air, causing surging, stalling, or poor load response.
Prevention tips:
- Use reinforced fuel lines with proper clamps.
- Pressure test the supply system annually.
- Replace pickup tubes and foot valves in aging tanks.
- Install clear return lines to monitor air bubbles during operation.
Shutdown Control and Safety Features
The PT system includes manual and automatic shutdown options. In older equipment, a pull cable physically cuts fuel flow. More modern setups use air cylinders or solenoids triggered by sensors or emergency switches.
Recommendations:
- Test shutdown function monthly.
- Lubricate manual cables to prevent sticking.
- Verify solenoid voltage and response time.
- Install redundant shutdowns in critical applications like mining or marine.
While PT systems are robust, they lack the precision and emissions control of modern common-rail systems. Retrofitting an 855 engine with electronic governors or hybrid fuel systems is possible but costly.
Pros of retrofit:
- Improved fuel efficiency
- Lower emissions
- Enhanced diagnostics
- High initial cost
- Complex integration
- Loss of mechanical simplicity
Conclusion
The Cummins 855 PT fuel system is a testament to mechanical engineering that still powers equipment decades after its introduction. Its simplicity, reliability, and field-serviceability make it ideal for harsh environments and legacy fleets. By understanding its operation, recognizing wear patterns, and applying preventive maintenance, operators can keep these engines running strong—proving that sometimes, old-school technology still holds its ground.
