Diesel Engine Power Insufficiency

2. Engine power insufficiency mainly refers to when the engine operates at high or full loads at the rated speed, constantly emitting black smoke, and the speed is unstable.

2.1 Reasons for Diesel Engine Power Insufficiency

1. Low-pressure oil circuit obstruction;
2. Governor’s failure;
3. Insufficient intake and exhaust not fully discharged;
4. Reduced fuel injection quantity;
5. Poor fuel atomization;
6. Incorrect fuel injection advance angle;
7. Insufficient compression force;
8. During assembly, the fit gaps of moving parts are too small, increasing resistance.

2.2 Fault Diagnosis for Diesel Engine Power Insufficiency

2.2.1 First, observe whether the diesel engine can reach the rated power speed.

The rated highest speed is specified on the factory nameplate, and if necessary, the maximum speed screw on the fuel injection pump can be adjusted. The smoke color is normal when operating at rated speed under no or light load. However, when operating under full load, if the speed is unstable and black smoke is emitted continuously, the governor spring will likely weaken. In this case, remove the governor from the fuel pump and check the spring strength using a comparative method. A weak spring must be replaced. Additionally, insufficient fuel supply can cause the same symptoms. For example, a broken or poorly sealed spring or steel ball in the one-way check valve of the fuel injection pump can prevent sufficient fuel supply to the plunger. Other fault causes listed in section (1.3.5) above may also lead to insufficient diesel engine power.

2.2.2 When the diesel engine has insufficient power, analyze the fault based on the symptoms.

Typically, when the engine has insufficient power, abnormal exhaust smoke color and strange noises often occur, so these symptoms can be used to diagnose the causes of power insufficiency.

2.2.3 White smoke emission.

This is caused by low combustion temperatures, leading to water vapor formation. For example, water in the fuel lowers the combustion temperature; low-quality fuel or mixing with other fuels; or using unsuitable fuels. If there is water in the fuel, first drain the fuel from the fuel pump, diesel filter, and fuel tank, allow it to settle in a container, and remove the water before reuse. If the fuel is murky and has a strange odor, it indicates that the oil quality is abnormal, and it does not meet the required oil grade. Drain all the diesel fuel from the fuel lines, clean the system, and replace it with new fuel. If the fuel quality is normal and the white smoke persists and is heavy, consider the following situations:

(1)  Cylinder head gasket erosion or damage, commonly referred to as “blown head gasket.” If the break is at the water chamber and cylinder sleeve junction, coolant will leak into the cylinder due to pressure differences (mainly during the intake stroke of the diesel engine), causing white smoke. Meanwhile, fuel and gas will also leak from the damaged part into the cooling water chamber, turning the coolant black.
(2)  Sand holes or cracks in the cylinder sleeve or cylinder head exhibit similar failure symptoms to cylinder head faults. Use cylinder shutdown and oil cut-off methods to identify the specific location.
(3)  If the above two causes are ruled out, but heavy white smoke still occurs, use cylinder shutdown methods to identify the fault. If the white smoke disappears after shutting down a cylinder, it indicates the cause of the white smoke is likely poor cylinder sealing, which could result from ring sticking, severe wear, or broken rings, causing severe mechanical wear of the cylinder sleeve. Regarding the valve mechanism: excessive valve wear, carbon buildup on valve stems, causing poor valve seating and sealing, or a valve gap too small that prevents proper closing as the engine temperature rises, causing incomplete sealing.

Other component causes: Related parts such as piston pin seats, connecting rod copper bushings, connecting rod bearings, and crankshaft journals (crankshaft pins) wear significantly, causing the piston to fall below the set top dead center position. This occurs due to accumulated machining errors and wear, lowering the compression pressure.

In routine service, one cause of white smoke is rod deformation, which occurs during engine operation as the fault damages the engine. The deformation of the connecting rod happens after the cylinder head gasket blows. When the gasket fails and coolant mixes with the cylinder, after stopping, the coolant gradually seeps into the cylinder. Due to the poor sealing effect of piston rings and the compressibility of water, the coolant accumulates above the piston. The amount of water depends on the piston position and shutdown duration. If the engine is started while this coolant is inside, the connecting rod deforms as it cannot overcome the pressure. After deformation, the piston does not reach the top dead center, changing the compression ratio and causing abnormal combustion, resulting in white smoke from the exhaust.

(4) Delayed fuel injection timing causes incomplete combustion, resulting in white smoke.
(5) Low ambient temperature and insufficient engine temperature may also cause white smoke, which disappears as the engine temperature rises.

Unburned fuel particles with a diameter of about 1.3 microns result in white smoke, while particles below 0.4 microns cause blue smoke.

2.2.4 Blue smoke emission indicates poor combustion in the diesel engine, which can lead to power insufficiency.

Blue smoke, typically from burning oil, is characterized by a blue color, high opacity, a pungent odor, and increased oil consumption. The main causes of blue smoke are:

Severe wear of the cylinder sleeve, piston rings, and piston ring grooves, causing engine oil to enter the combustion chamber, resulting in blue smoke;
Damage or deformation of the valve stem seals, causing oil to leak from the valve guides into the combustion chamber or exhaust outlet, leading to blue smoke;
Excessive oil level causes oil to enter the cylinder for combustion, resulting in blue smoke;
Poor-quality or incorrectly rated engine oil causes poor lubrication, leading to piston ring sticking, reducing combustion chamber sealing, and resulting in power loss and increased oil consumption.

If the fuel injectors are malfunctioning, it can also lead to reduced power and blue smoke from the exhaust. Misalignment, wear, or low injection pressure may cause poor atomization and incomplete combustion, resulting in blue smoke and power loss.

Reduced fuel injection angle can also cause blue smoke and affect diesel engine power.

When the power of a turbocharged engine is insufficient, some faults are manifested as blue smoke. Common turbocharger faults include:

1. Clogged air filter;
2. Blocked compressor outlet piping;
3. Air leaks between the compressor and engine intake manifold;
4. Air leaks between the engine intake and cylinder head;
5. Clogged exhaust pipe;
6. Compressor impeller or diffuser blade contamination;
7. Worn turbocharger bearings leaking oil;
8. Turbocharger damage.

When turbocharger failure is confirmed, the underlying cause must be identified. Our turbocharged diesel engines use exhaust gas turbochargers, with the turbine and compressor connected to the diesel engine’s exhaust and intake systems. Therefore, the working quality of the turbocharger directly impacts the engine. Thus, when diagnosing turbocharged diesel engine faults, always first diagnose and eliminate diesel engine issues. Once the engine operates normally, proceed to diagnose turbocharger faults. Do not disassemble or dismantle the turbocharger prematurely.

2.2.5 Exhaust Black Smoke Emission.

If a diesel engine emits black smoke, it indicates incomplete combustion. Short bursts of black smoke are normal during startup, acceleration, or overload operation. However, if black or gray smoke persists during normal or low-load operation, the engine is not working properly and power is insufficient. There are multiple causes for this:

(1) Intake system blockage: This includes excessive air filter clogging, blockage by debris in the intake pipe, or deformation and blockage of connecting intake hoses.
(2) Incorrect valve clearance adjustment: If there are abnormal sounds during operation, such as a popping sound and a mixture of black and white smoke, it suggests improper valve clearance adjustment. Push rods, lifters, and camshaft cams may wear abnormally due to material and heat treatment issues. Also, loose or damaged valve adjustment screws can lead to improper operation. Inadequate fuel supply can also cause black and white smoke. Poor cylinder sealing, such as issues with piston rings, cylinder sleeves, and other sealing components, can lead to black smoke.

2.2.6 If the fit clearance of major engine components is too small, it can increase resistance during engine operation, leading to power loss. The main areas prone to failure include:

1. Piston and cylinder sleeve fit;
2. Connecting rod bearings and crankshaft crank pin;
3. Main bearings and crankshaft journals;
4. Camshaft and camshaft bushings;
5. Idler shaft and idler shaft bushings.

When the fit clearance of these parts is too small, friction increases, lubrication deteriorates, and the moving parts can “seize,” leading to power loss. In repairing this type of failure, the engine model and the required fit sizes for each part should be followed, and these fit sizes should be adjusted as per the repair manual.

Some power insufficiency faults do not emit smoke but show a gradually decreasing speed. These usually occur due to insufficient fuel supply. It is necessary to analyze the causes of fuel system issues, such as low-pressure pipe obstruction, clogged diesel filter, dirty fuel pump inlet filter screen, vacuum formation in the fuel tank, insufficient fuel supply from the pump, aging or damaged fuel lines, or loose, unsealed joints. High-pressure fuel system and governor issues include plunger wear and governor faults causing fuel volume adjustment errors. High-pressure fuel system faults should be repaired by professionals and tested on specialized fuel pump test benches.

Improper assembly can increase resistance and cause power consumption. The key aspects include:

(1) Axial clearance: The clearance between the crankshaft, camshaft, and intermediate gear shaft should be adjusted as required.

A. The thrust washers adjust the axial clearance of the crankshaft. If the clearance is too small, the thrust washers should be ground thinner to adjust. The upper and lower washers should be placed on the same plane and ground using the “8” shape method, with sandpaper placed flat on a smooth glass or marble surface.

B. The camshaft axial clearance is adjusted by replacing the thrust plate thickness or grinding.

C. The intermediate gear’s axial clearance is adjusted by replacing the intermediate shaft.

(2) Fit between holes and shafts: Parts include crankshaft and main bearings, connecting rod bearings and crank pins, camshaft and bushings, connecting rod bushings and piston pins, cylinder sleeves and pistons, intake valves and guides, timing gear mesh, etc. Main bearings and connecting rod bearings are adjusted by scraping to the correct clearance, while connecting rod copper bushings, guides, and camshaft bushings are adjusted by standard pressure reaming to achieve proper clearance (methods available in repair manuals). During engine assembly, lubricating oil must be applied to all moving contact surfaces to prevent parts from being damaged due to temporary oil shortages during startup.

2.2.7 Fuel system failures can also cause a diesel engine to have insufficient power and emit black smoke.

The quality of fuel injection significantly impacts power. After replacing or cleaning fuel injectors, fuel injection pressure tests should be conducted. During the test, the injection speed should be controlled at 60 times per minute. The injectors should not drip oil or spray unevenly. The atomization should be uniform, and the return oil amount should be checked. If the return oil amount is too high, the cause should be analyzed, such as assembly technique or cleanliness. If the sealing surface of the nozzle is contaminated, it can lead to high-pressure oil leakage, reducing injection pressure and increasing return oil.

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