Air in Fuel: The Diesel Engine's Silent Killer

For any diesel engine, whether powering a vehicle on the road or a boat on the water, the fuel system is its lifeblood. It's a precisely engineered network designed to deliver clean, pressurised fuel to the combustion chambers at exactly the right moment. However, a common and often overlooked issue can quickly bring this intricate system to a standstill: air in the fuel. Understanding why this happens and how to address it is crucial for every diesel owner.

What Happens If Air Gets Into the Injector Pump?

Simply put, if any air gets into the injector pump, your engine is very likely to stop. The fundamental reason for this lies in the differing properties of air and diesel fuel. Unlike fuel, which is largely incompressible at the pressures involved, air is highly compressible. An injector pump is designed to raise fuel to extremely high injection pressures, often hundreds or even thousands of times greater than normal tyre pressure. If air is present, the pump will find it far easier to compress the air than to build the necessary pressure to inject the fuel. This means that instead of delivering a finely atomised spray of fuel into the cylinders, the pump will simply compress the air, leading to a loss of injection and, consequently, the engine ceasing to operate. This is precisely why, whenever a compression-ignition (CI) engine runs out of fuel or has a fuel system component changed, it must be 'bled' – a procedure specifically designed to remove any trapped air.

Understanding the CI Fuel System

There are three main types of CI fuel systems commonly found in light vehicles, each named after the injector pump they utilise: the INLINE, DPA ROTARY. Larger engines might also employ a UNIT INJECTOR system, often associated with Cummings designs, or a PRESSURE TIME system. While the larger engine systems have unique aspects, most common systems share many components and principles. Let's first explore these common parts before delving into the specifics of the injector pumps themselves.

Common Fuel System Components

The shared components typically include:

• FUEL TANK
• FUEL PIPES
• CHASSIS OR PRIMARY "FILTER"
• LIFT PUMP (if fitted)
• ENGINE OR MAIN FILTER
• INJECTOR PIPES
• INJECTORS

Industrial installations often employ a 'DAY TANK', a smaller, high-level tank that gravity-feeds the fuel system, kept full by an electric or manual pump from a bulk supply. This may or may not replace the engine's lift pump. Regardless of the setup, there will always be filters between the tank and the injector pump to ensure fuel cleanliness.

The Fuel Tank

The fuel tank for a diesel engine is quite similar to a petrol tank, featuring a breather and baffles. Key differences might include internal coating or material due to diesel's distinct chemical properties compared to petrol. If the tank is exposed to airflow, it might be insulated with a jacket to help keep the fuel warm, preventing it from waxing in cold conditions, which can block filters. In extreme cold, an electric heating element might be fitted. All tanks, main or day, should have a drain tap at their lowest point to allow for sampling and inspection for water contamination.

The Pipes

Injector pipes are special small-bore, thick-wall steel pipes designed to withstand the extremely high pressures of injection. Their specific shape is a design feature ensuring even fuel delivery, so they should never be bent. Pressure pulses can force the pipe out of the factory swaged nipple if the pipe is forced onto the injector or pump by the nut. Other fuel pipes are typically plated thin-wall steel or, more commonly today, hard plastic with steel fittings. Some return pipes are push-fit synthetic rubber. Maintenance primarily involves ensuring clips and unions are secure. Exercise caution not to overtighten suction unions, as this can lead to slight air leaks over time, eventually stopping the engine. Always slacken the other end of an injector pipe if it needs to be moved.

The Fuel Lift Pump

Various designs exist, but most mechanical lift pumps resemble those found on carburettor petrol engines. The diaphragm and valves are made from materials compatible with diesel fuel. A significant difference is often the inclusion of a priming lever, allowing manual operation of the pump when the engine is stationary. This is invaluable for bleeding the system after running out of fuel or replacing a component. Sometimes, a separate priming pump is fitted, either in a fuel line or as part of a filter. Gravity-feed day tank systems typically don't require a priming pump.

The Primary Filter

Often omitted from modern light vehicle fuel systems, where a filter-like object is present, it might be a filter, agglomerator, or sedimenter. Whatever the type, its primary purpose is to extend the life of the main or engine filter by removing larger dirt particles and, in some cases, water.

• Normal Filter: Typically a resin-impregnated paper element, housed in a throwaway metal housing or a separate bowl. Fuel flows from outside to centre, allowing dirt to settle. Multiple O-rings should be replaced when changing the element.
• Sedimenter: A very simple unit that removes anything heavier than fuel, such as water and dirt. It has no element, using an inverted cone to create a vortex. As fuel enters, it rotates down the cone, increasing in speed due to the widening diameter. Centrifugal force throws heavier particles to the outside, where they fall into the sediment bowl. Lighter fuel passes up the centre to the outlet.
• Agglomerator: Designed to remove water droplets, it also provides some filtering. Fuel and water are forced down through the element. Water droplets gain a static charge from friction with the element material. Upon emerging, these charged droplets agglomerate (stick together) and adhere to the sediment bowl, allowing water-free fuel to discharge up the centre. The water level can often be observed and drained as needed.

Like normal filters, elements should be changed at regular service intervals, or sooner if contaminated. Sedimenter and agglomerator bowls should be regularly inspected and drained. When cleaning, always replace rubber seals to prevent leaks.

Injector Pumps: A Specialist Domain

Injector pumps are complex mechanisms and should generally be considered 'specialist only' items. Routine attention is usually limited to inspecting for security and leaks. They typically fall into three types: INLINE (mechanically governed), DPA (mechanically or hydraulically governed), and ROTARY (mechanically governed). All can be simplified into a block diagram illustrating their core functional sections: the Governor, the Metering device, and the High-Pressure Pump.

The Governor

The governor's role is to measure engine speed and throttle position, then use this information to control the device that 'meters' (measures) the fuel supplied to the engine. If the engine is slow and the throttle wide open, the governor instructs the metering device to supply more fuel for acceleration. On generators, the governor maintains a constant engine speed regardless of load. If the engine is fast and the throttle closed, the governor reduces or cuts off fuel to slow down. Governors can be Mechanical (using spinning bob weights and springs), Hydraulic (using intermediate fuel pressure and a spring), or Electronic (using ECU-controlled solenoid valves).

The mechanical governor, widely used, operates on the principle of bob weights. These weights, mounted to spin, fly outwards due to centrifugal force, their movement controlled by springs. This movement is then transmitted to the metering device. There's a constant interplay between the throttle setting and the bob weights; the spring adjusts for the difference. For example, when the driver depresses the throttle, the spring pushes the rack to the maximum fuel position. As engine speed increases, the bob weights exert force, moving the rack back to reduce fuel, balancing the engine speed. A common issue is engine surging at idle (revving up and down), often resolved by using two springs (a strong outer for high speed, a lighter, slightly longer inner for idle) or a special adjustment to tension the idle spring.

The Inline Pump

The inline pump, though less common on modern light vehicles, is still found on some industrial-derived diesel engines. It may also appear as individual pumping elements for each cylinder. The pump's camshaft is driven by the engine's timing gears, pushing plungers to force high-pressure fuel through a delivery valve to the injector. Fuel control is achieved as the governor moves a control rod horizontally, twisting the pumping element to regulate fuel delivery. A stop control pulls the rack to a 'NO FUEL' position. For cold starting, a control allows the rack to move beyond the normal maximum fuel position, supplying more fuel; it self-regulates back to idle once the engine fires.

Fuel control in an inline pump is ingenious. The plunger head features a helical slot aligned with a fuel port. By twisting the pumping element, the governor changes when this slot uncovers the port during the plunger's upward stroke. If the slot aligns early, fuel escapes, and no pressure builds (the 'no fuel' position). By twisting the element, the solid part of the plunger covers the port earlier, allowing fuel to be pressurised and injected. The angled upper edge of the recess then uncovers the port, stopping injection. The amount of fuel injected is precisely controlled by how much the plunger lifts before the port is uncovered, allowing for an infinite number of positions between 'No Fuel' and 'Maximum Fuel'.

The Delivery Valve

The delivery valve plays a crucial, often misunderstood, role. After injection, when the injector's needle valve closes, the inertia of the fuel in the injector pipe can cause pressure surges. These surges can 'bounce' the fuel, potentially reopening the injector at the wrong time, leading to poor running, excess smoke, and poor fuel economy. The delivery valve prevents this. When fuel pressure forces the valve up, it effectively occupies space within the injector pipe connection. As the element uncovers the fuel port and pressure falls, the valve re-seats, increasing the volume in the injector pipe and causing an immediate, sharp fall in pressure. This pressure drop is sufficient to bring any pressure surge below the level needed to reopen the injector.

Adjustments & Service (Inline Pumps)

Five adjustments exist for inline pumps: two typically done in a workshop, three requiring specialist equipment.

• Specialist Adjustments:
  • Timing: Involves setting the engine to a timing position and then adjusting the pump body or coupling until fuel movement from a removed delivery valve indicates correct timing. This requires experience, data, and specialist equipment.
  • Phasing: Adjusting each pumping element to start injecting at the correct time (e.g., 90 degrees apart for a 4-cylinder engine).
  • Calibration: Ensuring each element delivers the same amount of fuel for a given control rod setting.
• Workshop Adjustments:
  • Idle: Adjusting the idle stop for correct tickover speed, sometimes including an adjustment to control surging.
  • Stop Control: Crucially, ensure the stop control mechanism (e.g., cable) is free to move and has a small amount of 'slack'. If it's tight or seized, it can limit the governor's movement, preventing the pump from delivering fuel for higher speeds and power.

Service involves visual inspection for leaks and security. At extended intervals, check that pumps not fed by engine oil still have oil in their cam chamber.

DPA Pumps

The DPA (Distributor Pump Assembly) pump was developed to overcome the high cost, bulk, and complexity of inline pumps, particularly for smaller CI engines in cars and light vans. It delivers fuel to individual injectors much like a distributor delivers spark to plugs. DPA pumps come in mechanically or hydraulically governed types. Their operating principles are similar, differing mainly in how the governor valve (known as the metering valve) is moved. The pumps are driven by a splined drive, and a 'master spline' ensures correct installation.

Metering (DPA Pumps)

Metering in DPA pumps relies on a simple principle: for a given pressure and time, more fuel passes through a larger hole. The governor controls the amount of fuel delivered by simply opening and closing an orifice. A hollow metering valve twists or slides to alter the fuel flow. This simple method requires a known supply pressure, which is provided by the transfer pump and regulating valve.

Transfer Pump & Regulating Valve (DPA Pumps)

The transfer pump is a simple vane-type pump that increases the lift pump pressure. Along with the regulating valve, it provides a higher, known pressure supply that varies slightly with pump speed. The regulating valve has two purposes: to allow the injector pump to be bled (by bypassing the transfer pump when the engine is stationary and the priming pump is used) and to control transfer pump pressure during operation. When the transfer pump develops greater pressure, the regulating valve moves against a spring, returning fuel to the transfer pump's inlet side, thus controlling the pressure.

High Pressure Pump (DPA Pumps)

The high-pressure pump consists of two or four opposed plungers with a pumping chamber between them. The outer end of each plunger connects to a roller acting as a cam follower. The cams, which force the plungers together, are formed on the inside of a heavy steel 'cam ring'. The cams have a complex shape to provide correct injection pressure characteristics. The maximum outward movement of the plungers (and thus maximum fuel delivery) is set by adjustable stops during pump assembly. As the centre shaft, rollers, plungers, and pumping chamber revolve between the cams, fuel flows from the metering valve into the pumping chamber, pushing the plungers apart. The cams then force the plungers together, pressurising the fuel and sending it back up the shaft to the distributor port and then to the injectors.

Fuel Distribution (DPA Pumps)

A section of the main drive shaft is drilled up the centre. At the injector pipe end, this drilling turns at right angles to a single hole (the 'distributor port') in the shaft's side. At another point, the shaft is drilled multiple times (one per cylinder) at an angle. When the plungers move in, these holes are closed off by the pump body. When plungers need to move out, one of the inlet ports aligns with the metering valve port, allowing fuel into the pumping chamber. High-pressure fuel is delivered when the distributor port aligns with an injector. This precise timing ensures fuel is delivered to the correct cylinder at the correct moment.

Advance/Retard (DPA Pumps)

Unlike inline pumps, DPA and more modern pumps can advance and retard their injection point, improving fuel economy and reducing exhaust emissions. This is achieved by using transfer pump pressure to turn the cam ring against a spring. Higher pressures at higher speeds advance the injection, while falling pressure at low speeds allows the spring to retard it.

DPA Summary

The DPA system operates as a single, integrated unit. The pump's interior is flooded with clean, filtered fuel that both cools and lubricates its working parts. While wear can occur over extended use, requiring adjustment or renewal, this is a specialist job. If water enters a hydraulically governed DPA pump, the governor may stick, usually closed, preventing the engine from starting.

Rotary Pumps

Developed by Bosch as a competitor to the CAV DPA pump, the rotary pump shares a similar shape and size. Its main shaft runs through the pump, with a transfer pump at one end and the high-pressure pumping mechanism at the other. Instead of a cam ring, the cams are set on the face of a rotating disc, which meets a roller-type cam follower that forces a pumping element towards the end of the pump. Transfer pump pressure subtly moves the cam follower to provide advance/retard to injection. It features a mechanical governor and an electric solenoid for stop control.

Operation (Rotary Pumps)

The transfer pump is a vane-type unit, similar to other vane pumps. On modern rotary injection pumps, it often doubles as the engine lift pump and priming pump, allowing for self-priming. The regulating valve is similar to the DPA pump's, primarily controlling transfer pressure. The governor is a typical centrifugal type, but its triangular bob weights are driven through a gear ratio, increasing the governor's spin speed. This generates more centrifugal force per engine revolution, allowing for smaller, lighter weights and reducing governor inertia, which leads to quicker response and helps reduce surging. Metering involves a control spool that alters the length of the injection stroke during which fuel can be pressurised. At a certain point, a port in the pumping element emerges from the control spool, allowing high-pressure fuel to spill into the pump body, thus ending injection. Advance/retard operates similarly to the DPA pump. The stop control is an electric solenoid: energised when the ignition is on, it lifts an armature to allow fuel flow; when the ignition is off, the armature falls, blocking the inlet and cutting off fuel. The delivery valve's purpose and operation are similar to those in the inline pump.

Adjustments & Service (Rotary Pumps)

Idle speed may be adjusted via a dedicated adjuster on the pump body. Maximum engine speed, especially on generators, should not be adjusted without specialist knowledge. Routine service involves inspecting for leaks and ensuring security.

Injectors

Most injectors on small to medium industrial engines operate on the same fundamental principle, despite varying appearances. The nozzle, fitted into the combustion chamber, comes in several designs to suit specific engine types. Their purpose is to deliver fuel into the correct part of the combustion chamber as a finely atomised spray. Due to the high compression pressures and limited injection time, extremely high pressures are used, often 100 times, and briefly even 1000 times, greater than car tyre pressure.

Operation (Injectors)

High-pressure fuel enters the injector body, acting upwards on the needle valve. When this pressure overcomes the spring pressure, the needle valve lifts infinitesimally, allowing fuel to be injected through the nozzle. The nozzle's shape forms the fuel spray into the required pattern. As injection ends and pressure falls, the spring forces the needle back onto its seat. Any minute fuel leakage past the needle is returned to the tank via the bleed-off pipe.

* WARNINGS & SAFETY *

Never allow your skin anywhere near an operating injector nozzle. The extremely high-pressure spray can easily puncture your skin and lead to severe injury, including the potential for amputation. The fit between the needle and nozzle is incredibly precise; even moisture from your skin can damage the needle barrel beyond repair. Some nozzles have a very small projection; if this is accidentally knocked off or damaged, the engine will smoke, and a new nozzle will be required.

Other Injectors

Older vehicles and many trucks use injectors located by studs in the head and have pressure set by a screw adjuster in the upper body, but operate on the same principle. Some injectors use two internal springs, known as two-stage injectors, to prevent the nozzle needle from 'bouncing' at the end of injection. Nozzle types vary by engine application:

• Single-hole nozzle: Used in direct injection engines, produces a solid cone spray.
• Multi-hole nozzle: Also for direct injection, produces multiple solid cones.
• Pintle nozzle: Used in both direct and indirect injection, produces a hollow cone spray.
• Pintaux nozzle: Exclusively for indirect injection. Normally, it produces a hollow cone. During cold starting (cranking), the slow pressure rise prevents full needle lift, allowing an auxiliary spray hole to direct fuel into the hottest part of the chamber, aiding cold starts.

Stop Controls

There are three primary types of engine stop controls:

• Mechanical Cable: An operator-pulled cable mechanically moves a lever, forcing the governor into the 'no fuel' position.
• External Electric Solenoid: An electric solenoid operates the same mechanical mechanism as a cable. Electricity is supplied only while the engine is being stopped and must be cut off once the engine stops to prevent battery drain and solenoid burnout.
• Internal Electric Solenoid Valve: Built directly into the injector pump. This solenoid requires continuous electricity to lift a fuel cut-off valve, allowing the engine to run. When the engine is stopped, current is cut, the valve falls (with spring assistance), and fuel supply is halted. If an engine with this type won't start, check the solenoid wire connection and ensure battery voltage is present when the ignition is on. You'll need to turn the ignition ON to bleed a pump equipped with this system.

Service involves lubricating mechanical parts and inspecting terminals for security, tightness, and damage.

Runaway Engines

A 'runaway engine' is a dangerous condition where a diesel engine uncontrollably revs to maximum speed, often beyond its safe limits, and cannot be stopped by normal means. While less common with modern enclosed governors, several causes exist:

• Pneumatic Governor Failure: Older systems used a throttle butterfly to create vacuum, moving a diaphragm on the injector pump rack. If the diaphragm punctured or a pipe disconnected, the rack could go to maximum fuel, causing a runaway.
• Overfilled Lubricating Oil: Industrial engines often have internal crankcase breathers that direct fumes (and potentially oil mist if overfilled) into the inlet manifold. A diesel engine can happily run on lubricating oil, leading to an uncontrolled acceleration.
• Internal Fuel Leak: If fuel leaks from an internal pipe into the sump, it raises the lubricating oil level and produces flammable fumes when hot. The breather system can then direct these fumes into the cylinders, causing the engine to run on them.
• Environmental Gas: If a sufficient concentration of flammable gas (e.g., butane, propane) enters the air intake, the engine will burn this gas, resulting in a runaway.
• Severely Worn Engine: In extremely rare cases, an engine with badly worn pistons and bores could produce enough crankcase pressure to blow lubricating oil through the breather into the cylinders.

While the causes are understood, the specific method for stopping a runaway engine is not detailed within these notes.

Do I Need to Bleed a Fuel Line?

The necessity of manually bleeding a fuel line depends on the specific scenario and pump type. Many modern systems, particularly those with rotary pumps, are designed for automatic venting. Controlled venting is provided at the injection pump through the fuel drain manifold. Small amounts of air introduced by routine tasks like changing filters or the injection pump supply line will often be vented automatically if the fuel filter is changed according to instructions. In such cases, no manual bleeding of fuel lines is required.

However, manual bleeding is essential if:

• The fuel filter is not filled prior to installation.
• The injection pump is replaced.
• High-pressure fuel line connections are loosened or lines replaced.
• It's the initial engine start-up, or start-up after an extended period of no engine operation.
• The vehicle fuel tank has run empty.

Bleeding Low Pressure Lines and Fuel Filter(s)

For low-pressure lines and filters, the procedure typically involves:

1. Using an 8 mm wrench, open the vent screw.
2. Operate the priming button on the lift pump (if fitted) until the fuel flowing from the fitting is free of air bubbles.
3. Tighten the bleed screw. The typical torque value is 9 N•m (7 ft-lb).

Bleeding High Pressure Lines

Bleeding high-pressure lines requires caution due to the significant pressures involved. You'll typically need 17 mm and 19 mm wrenches.

WARNING: The pressure of the fuel in the line is sufficient to penetrate the skin and cause serious bodily harm. Exercise extreme caution.

Venting is accomplished by loosening one or more fittings at the injectors and cranking the engine to allow entrapped air to bleed from the lines.

1. Loosen one or more fittings at the injectors.
2. Crank the engine in short bursts (no more than 30 seconds at a time, with two-minute waits between engagements) to allow air to bleed out.
3. Once air is purged and fuel begins to spray, re-tighten the line fittings. The typical torque value is 30 N•m (22 ft-lb).
4. Start the engine and vent one line at a time until the engine runs smoothly.

WARNING: Do not bleed a hot engine as this could cause fuel to spill onto a hot exhaust manifold, creating a danger of fire.

CAUTION: When using the starting motor to vent the system, do not engage it for more than 30 seconds at a time; wait two (2) minutes between engagements to prevent overheating.

WARNING: It is necessary to put the engine in the “RUN” position for the bleeding process. Because the engine may start, be sure to follow all safety precautions and use the normal engine starting procedure.

Frequently Asked Questions (FAQs)

Why does air in the fuel stop the engine?

Air is highly compressible, unlike diesel fuel. The injector pump cannot build the necessary high pressure for injection when air is present, as it will simply compress the air instead of forcing fuel into the cylinders. This leads to a loss of injection and the engine stopping.

Can I bleed my diesel engine myself?

For low-pressure lines and filter changes, manual bleeding is often a straightforward process that many competent DIY mechanics can perform. However, bleeding high-pressure lines requires significant caution due to the risk of injury from high-pressure fuel. Replacing an injector pump or dealing with a completely empty fuel tank typically necessitates manual bleeding, and if unsure, it's always best to consult a professional.

What are the signs of a fuel system problem?

Common signs include difficulty starting, rough running, engine surging (revving up and down uncontrollably), excessive smoke from the exhaust, loss of power, or the engine stopping unexpectedly. Any of these could indicate air in the system or other fuel delivery issues.

Are all injector pumps the same?

No, there are several main types, including Inline, DPA, and Rotary pumps, each with distinct internal designs and operating principles. While they all perform the core function of pressurising and distributing fuel, their construction, governing mechanisms, and adjustment procedures vary significantly. Modern pumps are generally more complex and often require specialist tools and knowledge for repair and adjustment.

Why is the delivery valve important in an injector pump?

The delivery valve is crucial for preventing secondary injections and ensuring precise fuel delivery. After injection, it rapidly drops the pressure in the injector pipe, preventing pressure waves from 'bouncing' back and forth, which could cause the injector to reopen at the wrong time, leading to poor engine performance, increased emissions, and reduced fuel economy.

Conclusion

The presence of air in a diesel engine's fuel system, particularly in the injector pump, can be a major headache, leading to engine shutdown and frustrating downtime. Understanding the intricate workings of the fuel tank, pipes, filters, and the various types of injector pumps – Inline, DPA, and Rotary – is key to maintaining a healthy diesel engine. While routine maintenance like filter changes can sometimes be managed by an informed owner, the internal workings and adjustments of injector pumps are best left to specialist technicians. Always remember the critical safety warnings, especially when dealing with high-pressure fuel lines. By ensuring your fuel system is clean, free of air, and properly maintained, you'll ensure reliable and efficient operation of your diesel engine for years to come.

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