At all engine loads, the purpose of the diesel engine fuel system is to deliver the correct amount of fuel, at the correct time, to the correct cylinder in the required condition and as a result, the system must be capable of controlling the quantity, timing, period, and sequence of delivery. There must also be control functions to ensure that the fuel is in good condition and that the delivery is appropriate for the actual load.
Systems range from the most basic mechanical hydraulic types to those that use load dependent, electronic controls to adjust quantity and timing and distribute to individual cylinders, but the basic requirements must be met regardless of how simple or sophisticated the system is.
Diesel engine basic fuel system
Fuel is drawn from the service tank by gravity or by an electrically or engine-powered supply pump and delivered to the high pressure fuel pump inlet rail, but to ensure proper injection viscosity, residual fuels must be heated after leaving the service tank.
Example of an electrically driven fuel supply pump
The high pressure pump in a basic direct fuel injection system usually delivers fuel directly to the fuel injection valves. The high pressure fuel pump can be used to control the amount of fuel entering the engine cylinder, as well as the timing of the fuel delivery, in conjunction with the fuel injection valve setting.
Sectional view of an high pressure fuel pump
The hydraulic pressure within the fuel injection valve causes the valve to open, allowing fuel to enter the cylinder combustion chamber directly.
Example of fuel injector sectional view
Example of fuel injector
The high pressure fuel pump and the fuel injection valve are typically regarded as the primary components of any diesel engine fuel system and the pump driving mechanism, as well as the features of these two components, can provide timing and quantity control.
A cam mounted on the engine camshaft is frequently used as the driving mechanism, but new modern engines are designed without a camshaft and rely on electronic fuel timing and quantity control (about common rail system you can read in here).
When using residual fuels, the fuel system necessitates a heating system with a viscosity control function and to avoid gassing, heated fuel is typically delivered under pressure to the high pressure fuel pump suction rail.
The load signal for any changes in fuel timing and quantity during engine operation is typically provided by an input from the engine governor (information about engine governor can be found in here).
The vast majority of high pressure fuel pumps used in diesel engines are jerk type, where jerk refers to the sudden, sharp movement of the pump plunger during fuel delivery. This results in a very rapid, almost instantaneous rise in fuel pressure in the supply to the fuel injection valve and in order to achieve this type of motion, the pumps must be driven by cams rather than cranks, as the fuel cam profile can be shaped to produce the desired delivery rate from the pump.
High pressure pumps on larger diesel engines are typically single units located near the cylinder cover for the unit they supply and the main camshaft controls the pumps. This means that the lengths of high pressure fuel pipes can be kept to a minimum.
Example of Main Engine with individual fuel pumps
Inline multi-cylinder pump blocks are commonly used for smaller engines. All of the high pressure pump cylinders for the entire engine are housed in these blocks. All of the fuel pump plungers are controlled by a single control rack.
Example of a fuel pump’s block
On some small engines, within the pump housing, a small sub-camshaft is used.
To ensure that piping losses are comparable, some engines position the pump block near the center of the engine, with equal length high pressure pipes to each fuel injector valve.
A high pressure fuel pump can be controlled in a variety of ways, including the quantity and timing of delivery, but the most common method is to use a helical profile machined into the pump plunger element to control the injection start and spill point by covering and uncovering the suction and spill ports in the pump barrel.
Example of different helix cut on plunger trailing edge
The start or end of injection, or both, can be varied depending on how the helix is positioned and variations include:
- constant start and variable end
- variable start and variable end
- variable start and end. This one is a type of variable injection timing.
Instead of having helix cut into the plunger, another common method is to use suction and spill valves to control the start and end of delivery. The term spill refers to the point at which delivery is terminated and fuel is spilled back to the pump suction or return line.
Both of these methods of control achieve fuel distribution to the various cylinders by having a separate pump element for each cylinder unit of the engine. In addition these two main methods described, there are other ways to gain control. For example, electronically controlled fuel injection systems have been introduced in some recent engine designs.
Common rail electronically controlled injection
Constant delivery pumps are used in this type of system to deliver high pressure fuel to the manifold, which is also known as a common rail. This fuels each cylinder through electronically controlled timing valves that control the admission of high pressure fuel to the injection valves (about common rail system you can read in here).
Some engines make use of distributor-style fuel pumps.
Unlike the previous control methods, this type of fuel pump controls not only the timing and quantity of fuel delivered, but also the cylinder unit to which the fuel is delivered. Smaller engines are more likely to use it.
It has been demonstrated that the thermal efficiency of a diesel engine is proportional to the ratio of maximum cylinder pressure, Pmax, to compression pressure, Pcomp (Pmax/Pcomp). The higher the ratio, the higher the efficiency and the lower the specific fuel consumption (SFOC).
The value of Pmax varies with load as a linear or straight line characteristic when the fuel injection timing is fixed.
The maximum value of Pmax is constrained by the strength of the combustion chamber components, whereas the minimum value of Pcomp is constrained by the need to achieve sufficient compression temperature to ignite the fuel.
The value of Pmax can be changed by adjusting the timing of the fuel injection and is normally increased by advancing the fuel injection timing and decreased by retarding it.
If we advance the fuel timing as the load decreases, the value of Pmax can be kept at its maximum, though this is only practical over a portion of the load range. This increases the ration Pmax/Pcomp, and thus the thermal efficiency, over a portion of the load range while decreasing SFOC. This is why variable injection timing (VIT) was developed and implemented (you can read about VIT in here).
From what we’ve already said about helix control, you should be able to deduce that changing the vertical position of the spill port relative to the leading edge of the pump plunger changes the point at which injection begins. Raising the spill port in relation to the plunger will cause the injection timing to be delayed, while lowering it will cause the injection timing to be advanced. This shift in relative position can be accomplished by raising or lowering the pump barrel or plunger.
As previously stated, the spill valve is used to vary the end of injection on a valve control type high pressure fuel pump, with later opening increasing the delivered fuel quantity. We also need to vary the point at which the suction valve closes, in the VIT control of this type of pump. Later closing advances the injection timing, while earlier closing retards it. When the governor output lever moves in response to a change in engine load, the linkage moves to rotate the suction and spill valve eccentrics on the pump with valve control.
Example of VIT for valve control type
Other than changing the valve timing or repositioning the barrel, there are several other ways to achieve VIT. This usually necessitates a change in the position of the cam follower or the camshaft. If the cam follower is attached to a pivoted horizontal lever on an eccentric shaft, rotating the shaft causes the follower to move horizontally relative to the vertical center line of the fuel pump and cam.
Internal and external leakage caused by erosion of pump elements and valve seats, as well as damage caused by fuel contaminants, are the most common fuel pump faults. Severe internal or external leakage can have an impact on the timing and amount of fuel delivered to the cylinder and can be caused by general wear or cavitation erosion. Cavitation happens when there is a sudden drop in pressure, causing vapour bubbles to form, which then collapse, resulting in excessive impact forces as the liquid fills the vapour space. Moreover, the high flow speed across control edges and valve seats can cause erosion as well.
Fuel contaminants can cause abrasive and corrosive damage.
Overheating of the pump elements caused by either high temperature or low fuel lubricity can cause plungers to stuck inside barrels or, in extreme cases, to seizure.
Seized plunger inside barrel due overheating
Some engines have fuel pump lubrication facilities to prevent this from happening.
Due to the extremely fine clearances of the operating parts, fuel pump maintenance is usually limited to the replacement of seals, valves, and the pump element. Typically, adjustments are limited to pump timing and the initial setting of the VIT mechanism.
Pump timing testing and adjustment methods differ depending on the type of pump.
The following are some of the more common methods for checking pump timing:
- measurements of plunger height relative to barrel;
- measurements of suction and delivery valve lift at a given crank position;
- matching a reference mark on the pump element or tappet to one on the casing; checking the point of spill cut-off
The majority of fuel injection fuel pumps are cam driven, positive displacement types. The rate at which fuel is delivered to the engine cylinders is determined by engine speed and the cam’s operating profile. The fuel cam shape is divided into several sections.
These are the base circle, the peak (also known as the dwell), the rising flank, and the falling flank. Most reversible fuel cams have identical rising and falling flanks.
When the cam follower is on the base circle and the peak of the cam, the fuel pump plunger is stationary; when it is on the flanks, it is moving. The rising flank of the cam can be thought of as the operating profile, with three distinct sections: acceleration, controlled speed, and deceleration and the rate of fuel delivery is governed by the cam profile’s controlled speed section.
Fuel pump cams can be separate components or integrated into the camshaft and only those that are separate components can be individually adjusted.
Split cams are used by some manufacturers to make replacing damaged cams easier. Although some engines still use bolt on or spline mounted cams, the majority use hydraulic tapered sleeve couplings, also known as muff couplings, to mount the cams on the camshaft.
Example of tapered sleeve coupling camshaft
The shaft is fitted with a tapered sleeve with a threaded end, and the cam, which has an internal tapered bore to match the sleeve, is mounted on it. The assembly is held together by a locknut. The cam has an oil hole that runs from the outer surface to the inner bore. This is usually blocked with a screwed plug.
The fuel injection valve’s function is to allow fuel into the cylinder while also acting as a non-return valve to prevent air and combustion gas from returning to the fuel system. Although there are numerous fuel injector designs, the majority are similar and operate on the same principle.
Example of fuel injector valve
The valve is made up of two parts: a body and a fuel nozzle that houses the needle valve. The injection spring provides positive seating for the needle valve via a thrust piece. The spring compression screw can be adjusted to change the needle valve lift pressure. As the fuel pressure acting on the needle valve rises, the generated hydraulic force overcomes the spring force, and the needle valve lifts off the seat, allowing fuel to flow to the injector nozzle and into the cylinder through the nozzle holes. The pressure drop across the nozzle hole accelerates the fuel, and as it passes through the high pressure dense air, the high velocity fuel stream breaks up into fine droplets. When the pressure in the fuel injector falls, the needle valve reseats due to spring force, and the injection stops.
When a standard type of fuel injection valve is open, fuel flows through the valve. When using hot heavy fuel oil, there is a risk that the fuel will cool down sufficiently to block the injector due to its high viscosity. To reduce this risk, it was common practice to switch to diesel fuel before shutting down the engine. Nowadays, on new modern engines, many fuel injectors have the fuel recirculating continuously, when the injector is closed.
On new engines, recirculation is accomplished by incorporating a slide valve within the main needle valve.
The main needle valve and the slide valve are seated when the fuel pressure in the valve is at normal supply pressure. Fuel is flowing through the valve body because the recirculation port is open. As the high pressure fuel pump begins to deliver fuel, the fuel pressure rises enough to lift the slide valve, closing the recirculation port. The fuel pressure is now rapidly rising, and the needle valve opens to allow fuel into the cylinder. As the failing pressure causes the needle valve to seat, ending injection, and then the slide valve to seat, allowing recirculation to resume. With these configurations, the engine can be stopped on heavy fuel oil for indefinite periods of time as long as the supply pump is operational and heating is available.
Using recirculating type fuel injection valves eliminates the need for fuel valve cooling systems because the continuous through flow of fuel removes enough residual heat to prevent the fuel injector from overheating.
The most common faults found with modern fuel injection valves are:
- Damaged or fouled needle valve seating surfaces leading to dribble or leakage;
- Scored or burnt needle valve and housing leading to seizure or internal leakage;
- Damaged or blocked nozzle holes;
- Breakage or weakening of injector spring;
- Leakage at mating surfaces;
- Carbon deposits due to overheating
The majority of modern fuel injectors are maintained through periodic testing in a test rig. Any operational flaw is normally corrected by general cleaning, replacement of the needle valve and nozzle assemblies, replacement of the injector spring, and replacement of the entire injection valve. Some injection valves may have renewable seals, while others may require mating surface reconditioning. Some recirculation types have service exchange cartridges that include the needle valve, slide valve, and thrust assembly. These types typically have separate nozzle tips that can be replaced if they become damaged.
One of the most serious risks when operating a diesel engine is the risk of a high-pressure fuel leak spraying fuel mist onto hot engine surfaces, potentially resulting in a serious fire.
Example of generator fire due fuel leak
SOLAS requires double-skinned pipes to be installed to protect against high-pressure sprays.
Example of double skinned fuel pipe
The inner pipe is typically made of solid drawn steel pipe, while the outer skin is typically made of a flexible metal sheath with a drain provision, which is usually connected to a collecting pot and equipped with a leak detection device. Some systems include a cut out for the affected unit’s high pressure fuel pump.
Some smaller engines, as an alternative to this arrangement, have all of the high pressure pipes contained within a steel enclosure that drains to an alarmed collection pot.
Furthermore, precautions must be taken to reduce the possibility of any fuel leaks reaching the heated surfaces. Any surfaces that may be impacted by fuel system leaks and are above 220 degrees Celsius, the self ignition temperature of a typical fuel, must be insulated.
Example of identified hot spot in auxiliary engine using a thermal camera
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