This post will explain the essential Tips on How to Maintain the Fuel Injection System of a Marine Engine. After reading the post, students will understand the complete knowledge of Fuel injection equipment and the maintenance and operation procedures.

The fuel system is the heart of Diesel engines and plays a vital role in the performance of Diesel engines.

This guide will provide practical knowledge to practicing marine Engineers on board. Tips have been written in a simplified way, not involving many theoretical design aspects.

Basics Philosophy

For the complete combustion, the elements needed are

Fuel

Heat

Air

Fuel: — fuel for the combustion process is supplied by the Fuel Injection pumps at the correct required pressure to the nozzles. Nozzles further break the power into tiny droplets to form the homogeneous mixture in the combustion chamber.

Heat: — Sufficient Heat is generated for the fuel to ignite in the combustion chamber during the compression stroke of the working cycles of the Engine.

Air: — Air is supplied in the suction stroke by induction process in natural aspirated and at pressure by Turbocharger in case of turbocharged Engine to complete the burning of the air-fuel mixture.

All three elements are vital for performing the complete combustion. Problems with any of these elements will reduce the efficiency of any internal or external combustion engine.

This post will explain the Fuel Injection equipment — essential equipment in the fuel injection system listed as under.

1. Fuel Injection Pump

2. Fuel Injector of Fuel Valve.

Fuel Injection Pump

The fuel injection pump is a device which helps to supply and inject the metered fuel to cylinders as per the requirement of the load and at the correct time with respect to the firing order.

Fuel Injection pump provides the fuel at pressure to injectors more than the injection pressure with a metered quantity.

Fuel Injection pumps are of the following types.

1. Jerk type Pump

2. The common rail injection system

Operating Principles

A jerk-type pump is used to inject the fuel to combustion through in fuel valve. It contains a plunger 209 closely fitted in to a barrel, as shown in the above image. The pump plunger moves upward by the cam through the roller 204. A spring 211 is mounted above the plunger to assist in its downward movement. This coil spring ensures continuous contact between the plunger and follower to the cam profile.

The plunger has a rectangular vertical groove which extends from the top to another helical groove. This helical groove controls the amount of fuel to be injected as per the requirement controlled by the governing system through the fuel control rod.

In the downward movement of the plunger, both the suction and spill ports are open, filling the barrel with fuel in the upward direction; pressure starts to build up as soon as the plunger covers the ports. The delivery valve is lifted from its seat due to the pressure of the fuel against the spring and allows the fuel to pass to fuel valve through High pressure fuel pipe.. Fuel injection continues till the plunger uncovers the spill cut-off port in its downward movement. The amount of fuel is controlled by the radial direction of the rotating plunger. The radial movement of the plunger is with respect to the barrel is performed with the rack and pinion arrangement. The delivery valve in the pump seats immediately after the fuel spill cut-off.

The distance between the pump plunger discharges to end of discharge is known as Effective delivery stroke.

The injection timing can be altered by adding and removing the shims. An increase in shim will advance the fuel timing, and a decrease in shim will retard the fuel injection timing.

By advancing the fuel timing will cause the following advantageous changes

1. Increase in peak pressure

2. Increase in thermal efficiency

3. Increase in overall fuel efficiency.

A negative effect of advanced timing

1. Excessive vibrations

2. Shock engine loading.

Retardation in fuel timing will cause the negative effects only as under.

1. Corrosion

2. High Exhaust temperature

3. After burning

4. Low thermal efficiency

Inspection of Parts

Barrel

Replace the barrel if the symptoms of heavy abrasion or damage are seen on the internal sliding bore.

Tappet Assembly

Replace the guide piston if

• Indentation on the plunger and plate on the tappet assembly seating surface is more than 0.25mm in total.
• Symptoms of heavy abrasion or damage are observed on bearing on sliding outside diameter.

Delivery Valve and others

• Replace the delivery valve if symptoms of heavy abrasions are damage is seen.

Deflector assembly

• Replace the deflector if the depth of the erosion is more than 0.5mm

Plunger assembly

Replace the plunger assembly if

• The clearance between the plunger and the barrel is more than 15 micron
• The erosion of the plunger is marked less than 0.8mm from a notch
• The depth of the erosion observed on the port of the barrel is more than 1.0 mm
• Symptoms of heavy abrasion or damage

The plunger and barrel are matched and can’t be replaced individually.

Control sleeve and control rack

Replace the sleeve and rack if

• The control rack’s movement is more than 0.2 mm because the control sleeve is firmed.
• The control rack’s movement is more than 0.5 mm on the condition that the plunger is firmed.

Plunger spring

Replace the plunger spring if

• Cracked or nicked.
• Corroded.
• Surface coating is damaged.

Delivery valve spring

Replace the delivery valve spring if

• Cracked or nicked
• Corroded.

O-rings

Replace all the o-rings in every disassembly in addition to the specified overhauls

Adjustment of the Maximum Combustion Pressure

If the Fuel Injector, Inlet and exhaust valve, Piston, Turbocharger, and charge air cooler work in perfect condition. Compression pressure Pcomp is also well within the normal limits than the Maximum combustion pressure will indicate the injection timing as per the following.

• Low Pmax will indicate the delayed timing
• High Pmax will indicate advanced timing

Injection timing is changed by adding or removing the shims from the bottom of the thrust piece above the roller guide. Measurement of “X” will be changed as per the above diagram.

Common Rail Injection System

The common rail injection system is the solid injection system. A constant pressure of 300 bars to 500 bars is maintained by a bypass valve on the main fuel header. Excess fuel is returned to the main fuel supply tank.

Operation of Common rail Injection

High pressure is supplied to a fuel header by a pimp. From the header, high-pressure fuel is forced to each injector fitted in cylinder heads. Fuel enters the cylinders at an appropriate time controlled by a mechanically operated mechanism.

The design of the header pressure must meet the requirement of the engine designed for the injector. The designed pressure must be able to penetrate droplets deep into the combustion chamber against the compression pressure. The amount of fuel entering the cylinder is regulated by the Solenoid valve operated by ECU. ECU transmits a signal to the injector to start injecting the fuel after receiving the signal from the crank angle sensor, scavenge air temperature, engine speed, Jacket water temperature, and the load.

Standard rail system helps in reducing emission, lower fuel consumption, and lower running speed. It helps in increasing combustion efficiency.

Fuel Valve (Fuel Injector)

A fuel valve is installed in the cylinder head. It is connected to a High Fuel Pressure pump in the Bosch type system or the accumulator of a common rail system by High-pressure fuel pipes. A nozzle with a multi-hole is installed in the valve, which operates against the pre-adjusted spring tension to achieve the correct injection pressure. The needle valve fitted in the nozzle closes the holes due to the spring pressure. Needle matches the proper seating angle at the valve seat, and a good bearing surface ensures the appropriate closing of wholes.

Pressure in the injector chamber develops due to fuel at high pressure delivered to the injector In the delivery stroke of the Fuel injection pump. The increase in higher pressure more than the spring pressure of the injector needle valve is lifted, and the fuel is injected into the combustion chamber. After the fuel injection, the pressure in the fuel chamber gets reduced, and the spring shuts the needle back on its set. The spill causes the sudden drop in the high fuel pressure cut-off mechanism of the High-pressure fuel pump.

Fuel valve tests

Criteria for the excellent performance of the fuel valves are listed below.

• Good atomization
• The correct spray pattern as the design of the combustion chamber ( by piston crown)
• No dribbling
• No leakage at the tip.
• Correct opening pressure.

Atomization

Fuel Atomization is when fuel is changed into droplet form by high-pressure injection and mix with air to ensure complete combustion. In atomization, we are increasing the surface area of the fuel droplet so that it can mix a short amount of time with air.

Servicing of Fuel Valve

Dismounting and cleaning

1. Dismount the fuel injection valve from the cylinder head using the unique tool

2. Use the clamping bracket for mounting the injector, and loosen the locknut (B). Release the stress of the nozzle spring by turning the screw D

3. Before dismantling the nozzle nut (H), Clean the lower part of the nozzle (J) of carbon deposits.

4. The nozzle spring (F) and the spring spindle (G) are to be taken out by unscrewing the adjusting screw (D) completely.

5. Clean all parts with kerosene or gas oil and a stiff brush (not a steel brush).

6. Clean the nozzle holes of charred coke using the supplied special drill with holder.

7. Clean the cooling space in the injector body and nozzle guide. Blow them with air after cleaning.

8. Insert the nozzle needle with gas oil in the needle guide. The nozzle needle should slide down by its weight.

9. Check the nozzle holes for the wearing out in an oval pattern. This inspection is done using a magnifying glass.

WHEN the nozzle holes

THEN

Are worn ova

Scrap the nozzles

Are not worn oval

Re-use the nozzles

All the defective parts found during the inspection must be replaced. If heavy abrasion symptoms, respectively damages are observed at the visual inspection of the parts, then replace the defective parts.

Reassembling:

Assemble the fuel valve after the careful cleaning, inspection, and overhaul, and found in good order, then assemble the fuel injection valve. When building the injection valve, proceed in the opposite order compared to the disassembly.

Pay attention to the following:

1. Lubricate the threads on the adjusting screw (D) with lubricating oil, and the threads of the nozzle holder for the nozzle nut

2. Lubricate the shoulder of the nozzle, which is in contact with the nozzle nut with an anti-seize product.

3. Wipe the plane sealing surface of the body and nozzle dry with paper.

4. Renew the sealing rings © and (E).

5. Tighten the nozzle nut as per the prescribed torque.

Pressure testing of the fuel injection valve

The effective checking of the fuel valve is obtained through pressure testing of the valve.

The pressure testing is carried out in the following way using the pressure testing apparatus supplied

1. Mount the fuel injection valve in the bracket by keeping the nozzle in a downward position.

2. Mount the test pipe on to the stand for the test of the cooling oil system

Increase the pressure by the lever.

1. Apply pressure to the cooling oil system after venting

2. Check the sealing tightness of the O-ring

3. Mount the test pipe for the test of the injection pressure and atomizing

4. Adjust the opening pressure to 320 bar using the adjusting screw (D), see fig 2, then tighten the locknut (B)

5. Recheck opening pressure.

Do not expect to chatter, but ensure that the nozzle spray from all holes is at the same angle.

1. Raise the pressure to 300 bar and hold the pressure moving the lever handle slowly downward.

There should not be more than one drip from the nozzle tip for a period of 3 to 5 secs at 300 bar pressure

Following tests are conducted on the fuel valves for the best performance

Spray pattern test

This test will indicate the uniformly and properly atomized spray of fuel through all the holes. The impression of atomization can be taken at a blotting paper kept at the specified distance as per the height at which the injection takes place in the combustion chamber and the crank angle for the injection timing.

Spray Pressure

The spray should occur from 320 Barto350 Bar; if the pressure is down than the specified pressure limit, then the nozzle should be re-adjusted/changed. Pressure is adjusted by decreasing or increasing the tension of the nozzle spring.

Dribbling

The nozzle tip of a suitable nozzle should remain dry. There should not be any leakage after and before the injection. Checking the dribbling is by having injections manually done several times quickly and inspect the nozzle tip for any leakage. Raising the pressure within 10 to 15 bar and holding it for about 10 seconds will also give a clear idea.

The reason for the nozzle dribbling is

a. Improper pressure setting

b. Dirt stuck up between needle and valve seat.

c. Inappropriate contact between needle and needle seat.

d. Needle sticking in the body.

Nozzle leak-off rate

A significantly less amount of fuel for the lubrication passes inside between the needle and the valve body. Excess clearance between the needle and the valve body will cause leak off and will reduce the amount of fuel supplied for the combustion.

Tips on How to Maintain the Fuel Injection System of a Marine Engine