Analysis of the cause of the failure of high-pressure common rail diesel engine to start

Diesel engine injection technology has gone through two development stages: traditional mechanical high-pressure injection pumps, injectors, and today's electronic fuel control systems. With the continuous improvement of the demand for diesel engine economy and power, high-pressure common rail electronic control diesel engines are becoming more and more widely used.

For electronic control system engines, under normal conditions, the electronic control unit (ECU) receives the user's start request and controls the starter to drive the engine to run; the ECU receives signals from various sensors to spray the diesel with the appropriate injection pressure at the correct injection time to ensure the best air-fuel ratio, atomization and ignition time of the diesel engine. At the same time, the diesel engine drives the rotation of the camshaft, and the intake and exhaust valves open and close at the appropriate valve timing to ensure the smooth start of the engine.

That is, the three major elements of diesel engine start-up are fuel supply, air supply, and starting speed.

In addition, diesel engine start-up is also affected by factors such as ambient temperature, altitude, and load.

This article classifies the failure of high-pressure common rail diesel engines that cannot start according to diesel engine mechanical failure, fuel system failure, and electrical system failure.

1. Overview of fault analysis methods

The diesel engine is a compression ignition internal combustion engine. For the diesel engine to start normally, it is not only necessary to spray the fuel into the cylinder at the right time and atomize it normally, but also to require the cylinder pressure and problem of the engine to reach the physical conditions for fuel ignition after the compression stroke.

Therefore, problems with the starting system, electronic fuel system, intake and exhaust system or diesel engine matching clearance will cause the diesel engine to fail to start.There are many fault points for the diesel engine to fail to start. Sometimes the fault phenomena reflected by multiple fault points are similar, but the causes of the faults are different.In order to troubleshoot the fault more accurately, shortcuts are sought in the troubleshooting process. Usually, the cause parts and the victim parts are judged, and the fault phenomena are combined with the principle structure and performance parameters of the diesel engine, and analyzed layer by layer from the outside to the inside and from simple to complex.

Commonly used methods for diagnosing diesel engine faults include "looking, smelling, asking, and cutting" and other methods:

1) "Looking": judging the fault condition by observing the color and smoke density of diesel engine exhaust;

2) "Smelling": based on the sound of the diesel engine, the sound quality and position of the abnormal sound are separated from the normal running sound, and the fault location, nature and degree are judged through experience;

3) "Asking": judging the fault point by analyzing the power and economy of the engine, and understanding the time before and after the fault and the background of the event;

4) "Cut": judging the vibration of the diesel engine and the temperature rise during the startup process through the touch and temperature perception of the hand.

2. Mechanical fault of diesel engine

1. Low cylinder pressure

Insufficient compression pressure of the diesel engine will lead to problems such as insufficient power and difficulty in starting.

Insufficient compression pressure can be found from the poor sealing of the diesel engine combustion chamber seal.

Use a special cylinder pressure gauge to check whether the cylinder compression pressure meets the technical conditions, and check whether the diesel engine has cylinder pull, piston ring wear, breakage, or piston ring matching and the sealing of the cylinder gasket.

Remove the cylinder head and inject diesel from the intake and exhaust passages to observe whether the intake and exhaust valves are not sealed tightly. If there is diesel leakage, the valves should be repaired or replaced as required.

2. Intake and exhaust system failure

Check whether the air filter and the connecting rubber pipes are blocked or deflated; for diesel engines using exhaust brakes, pay attention to check whether the exhaust brake valve is in a fully open state. If the exhaust brake butterfly valve is stuck in the closed position, it will cause poor exhaust; check whether the exhaust pipe or aftertreatment is blocked.

3. Valve mechanism failure

(1) Valve timing failure

The moment when the valve is opened and closed is marked by the crankshaft angle, which is called the valve timing.

Improper valve clearance adjustment, camshaft deformation, unreasonable camshaft design, unqualified processing, and timing gear misalignment will all cause changes in the valve timing, resulting in insufficient intake and incomplete exhaust of the diesel engine. At the least, it will affect the performance and emission indicators of the diesel engine, and at the worst, it will cause the diesel engine to fail to start.

(2) Valve leakage

Valve leakage will directly lead to low compression and work pressure in the cylinder. The valve clearance is too small, and the valve expands due to heat, resulting in the valve not closing tightly; the valve and valve seat are worn out, causing the valve to sink too much, and the valve heating will also cause the valve not closing tightly; the valve seat and the valve itself have poor sealing due to quality or assembly problems.

The valve sealing condition can be verified through air tightness test.

(3) Valve mechanism failure

Check whether the valve spring is broken, check whether the valve or valve seat ring is broken, whether the valve is burned and adhered, and whether the push rod is bent.

The failure of the valve mechanism that causes the diesel to fail to start is usually a major failure, accompanied by obvious black or white smoke, and abnormal noise of the diesel engine. When this happens, it is necessary to stop the engine immediately and conduct further inspection, otherwise the failure phenomenon will be aggravated.