INTRODUCTION - Toro · ENGINE FAILURE ANALYSIS: 4-CYCLE TWIN CYLINDER INTRODUCTION This...

ENGINE FAILURE ANALYSIS: 4-CYCLE TWIN CYLINDER

INTRODUCTION

This Productivity Series section is designed to review procedures used to determine the failure of a Toro Power Plus twin cylinder 4-cycle engine.

An engine failure analysis is a systematic way of analyzing an engine failure. By analyzing the failed components, the actual cause of the failure can be found. Correcting the cause prevents a repeat failure.

The cause of an engine failure is also used to determine if the failure is covered by the warranty.

Systematic Approach

A systematic engine failure analysis begins well before the engine teardown. It begins when the customer notifies you of the problem.

Information collected at the beginning of the engine failure analysis may determine where to focus your attention during the remaining steps of the analysis.

EFA Form

The Engine Failure Analysis form is used to document your findings during the inspection process.

The first two sections of the Engine Failure Analysis Form require information from the customer. Both sections should be completed when the engine is brought in for repair.

The first section covers basic customer and product information. The second section pertains to the fuels and oils used in the engine.

Page: 1ENGINE FAILURE ANALYSIS: 4-CYCLE TWIN CYLINDER


INTRODUCTION (cont'd)Customer Interview

Additional information will be helpful in your diagnosis. Have the customer describe the failure. Ask follow up questions on any unusual events to clarify their description of the failure.

EXTERNAL INSPECTION

Perform the initial inspection of the tractor while the customer is present. Look at the general maintenance of the engine. Check the condition and amount of oil in the crankcase. Inspect the air filter element and cooling fins for signs of dirt and debris.

Record the information on the Engine Failure Analysis form.

Oil Inspection

Drain the oil from the crankcase. Inspect the oil, watching for any clues for the cause of failure. Clues include color, smell, metallic flakes, dirt, water, fuel, broken parts, or sludge.

If the oil shows anything abnormal, save a sample to show the customer, or to send out for analysis.



EXTERNAL INSPECTION (cont'd)Air Filter Inspection

Remove the air filter cover. Inspect the throat of the carburetor for signs of dirt or debris before removing the filter element.

If dirt is present in the carburetor, check both sealing surfaces of the element for cracks, tears and warpage. Examine the element surface for holes or other signs of damage.

Cooling System Inspection

Inspect the cooling fins and chaff screen for debris. If blockage is present, watch for signs of overheating during the internal inspection.

Cylinder Compression

Check the compression in both cylinders. Compression should be 75 - 115 psi (517 - 793 kPa.)

Watch for a worn cylinder, piston rings, head gasket and/or valves during the internal inspection if the cylinder compression is low.

Record the findings from the external inspectionon the Engine Failure Analysis Form.



INTERNAL INSPECTION

A complete engine failure analysis does not need to be time consuming. The following rules will help minimize the time it takes to effectively diagnose an engine failure:- Inspect each component before taking it apart.

- Disassemble only what is needed to make the failure determination.

- Measure components only if you need todetermine they are within specifications.Remove the engine and begin the internalinspection.

LACK OF LUBRICATIONSymptom - Bearing Metal Transfer

Metal transfer in bearing areas is a sign of a “Lack of Lubrication” engine failure.

Oil provides lubrication for moving parts inside an engine. Increased friction occurs when a bearing area is deprived of oil. Friction causes the temperature of the bearing surfaces to rise.

Metal transfer between the bearing components occurs when temperature of the bearing surface reaches the melting point of the bearing material.

Cause - Low Oil Level

In lack of lubrication failures, bearings at the end of the oil supply are likely to show damage first.

Oil is pumped from the flywheel side of the engine through the oil transfer tube, to the PTO crankshaft bearing and then to the #2 connecting rod journal.

When the engine runs with a low oil level, metal transfer will occur between the #2 (PTO Side) connecting rod bearing and the crankshaft rod journal.



LACK OF LUBRICATION (cont'd)Cause - Loss Of Oil Pressure

The oil transfer tube supplies fresh pressurized oil to the PTO side of the engine. If this tube leaks or is damaged, oil flow to the PTO side crankshaft bearing and #2 connecting rod bearing will be lost.

A leak in the oil pickup tube or a worn oil pump would cause a similar problem.

Be sure to inspect these components for damage when rebuilding an engine.

Cause - Oil Passage Blockage

A lack of lubrication failure can be caused by a blockage in an oil passage. In this case, a bearing plate gasket was constructed from a piece of paper.

The home-made gasket covered the oil supply hole. This blockage prevented oil from flowing to the PTO side crankshaft bearing and #2 connecting rod journal causing the failure.

An oil passage blockage will cause metal transfer in the areas that the passage lubricates. The remaining bearings in the engine will be unaffected.

Note: Always use original equipmentmanufacturer parts.



DIRT RELATED FAILURESSymptom - Accelerated Wear

A common cause of engine failures is dirt ingestion. Dirt is an abrasive. As dirt works through the engine, it accelerates wear in areas such as the cylinder wall, piston rings, piston skirts, and bearing surfaces.

A proper engine failure analysis needs to determine the dirt’s point of entry. Finding the point of entry will reduce the chance of a repeat dirt ingestion failure. The point of entry will also be used in warranty determinatian for engines which are still in the warranty period.

Symptom - Gray Finish On Wear Surfaces

Because dirt accelerates wear, the areas most effected by dirt ingestion are the normal wear areas of an engine.

Dirt cuts fine scratches, which align with the parts direction of movement, into bearing and cylinder wall finishes leaving them with a dull gray appearance.

If this was a lack of lubrication failure we would see smearing of the metal, but instead we see fine scratches caused by dirt.

Symptom - Embedded Dirt

The connecting rod bearings experience an impact load when the spark plug ignites the fuel/air mixture. If dirt is present in the bearing area when this impact occurs, it can be pressed into the bearing surface.

This connecting rod bearing shows pieces of embedded dirt verifying dirt was present in the engine.



DIRT RELATED FAILURES (cont'd)Cause - Dirt Ingestion

The most common way for dirt to enter an engine is with the intake air stream. The high velocity of the intake air allows it to pick up and carry dirt.

During the external inspection, the carburetor and air filter were examined for signs of dirt. If dirt was present in the carburetor, the dirt bypassed the air filter or one of the carburetor shaft seals. if there are no signs of dirt in the carburetor, examine the engine’s gaskets and seals to find the dirt’s point of entry.

Cause - Contaminated Oil

Poor maintenance, such as adding oil using a dirty container or funnel, may be the source of dirt in the engine. Lack of maintenance, such as not changing oil at the required interval, may allow contaminants to build up in the oil.

Either of the above cases will allow dirt to be circulated through the lubrication system. Wear will be concentrated in areas of high oil flow such as the crankshaft bearing journals.

Example - Dirt Ingestion

Dirt is found on the throttle plate verifying that it’s source is the intake air. Find the entry point because dirt does not always come through or past the air filter element. Check the throttle shaft and the choke shaft for signs of wear to the shafts or the carburetor body.



DIRT RELATED FAILURES (cont'd)Example - Dirt Ingestion (cont’d)

With the choke plate removed, dirt is found in the lower end of the carburetor. This verifies that the dirt previously found on the throttle plate was not inadvertently knocked into the carburetor when the air filter was inspected.

From here, dirt will enter the combustion chamber past the intake valves.

Example - Contaminated Oil

This connecting rod bearing shows scratches which were caused by dirt. The discoloration on the bearing indicates the oil has not been changed at the recommended interval.

Example - Good Cylinder

This is an example of a good cylinder. Notice the cross hatch in this cylinder. The purpose of the cross hatch is to hold oil on the cylinder wall.



DIRT RELATED FAILURES (cont'd)Example - Worn Cylinder

In this example, dirt has worn away the cross hatch and polished the cylinder. Notice the vertical scratches caused by dirt. When the cross hatch is worn away, the oil will run off the cylinder wall. The increased space between the piston rings and cylinder wall will increase the engine’s oil consumption.

(Note: The actual engine failure will usually be caused by lack of lubrication due to failure to add oil more frequently, with the increase in oil consumption. The problem was initiated by dirt.)

HEAT RELATED FAILURESTypes Of Overheating

Heat related failures can be caused by several factors. Some failures are caused by dirt and debris build up on the outside of the engine. Overheating can also be caused by running the engine with a low oil level (oil transfers heat from the internal components to the block.) Overheating of the cylinder is caused by carbon build up in the combustion chamber. The damage caused by overheating depends on which area of the engine overheats.

Symptom - Burnt Oil

When the engine temperature exceeds the normal operating range, hot oil will tarnish the internal components leaving them with a brownish tint. In severe cases the oil will breakdown as shown here.

Overheating an engine can cause any of the following:- Increased oil consumption

- Scuffed Pistons

- Blown head gaskets

- Oil Breakdown



HEAT RELATED FAILURES (cont'd)Symptom - Exhaust Valves Edge Curling

A close examination of the exhaust valves will show the outer edges are curled.

Extreme heat flowing past the exhaust valve during an overheating condition will soften the outer edge of the valve. Opening and closing of the valve will cause the softened outer edge to curl.

Cause - Limited Air Flow

Air cooled engines rely on air flow past the block’s cooling fins to maintain a safe operating temperature. A reduction in airflow will cause overheating to occur.

The most obvious cause of limited air flow is debris plugging the cooling fins or chaff screen. Another cause is failure to operate the engine at full throttle during loaded operation. Both situations will cause hot spots in the cylinder and overheating of the engine.

Symptom - Piston Scoring

The top edge of this piston has overheated. The outer edge has actually melted due to heat build up in the combustion chamber. Notice how the edge is cut back and gouged.

Carbon deposits in the combustion chamber will ignite the fuel/air mixture causing pre-ignition.



HEAT RELATED FAILURES (cont'd)1Cause - Carbon Build Up

Carbon has excellent insulation characteristics. Carbon build up in the cylinder head prevents heat from transferring from the combustion chamber to the outer cooling fins.

Failure to decarbon the cylinder head at the recommended interval will lead to piston overheating. Factors which accelerate the build-up of carbon in the combustion chamber include poor quality fuels, improper carburetor adjustment, and failure to operate the engine at full throttle.

MISCELLANEOUS FAILURESImproper Parts, Assembly or Maintenance

Although not as common as the failure modes already discussed, several other failure modes are possible. The following section gives you an overview of some of the other failures which you may encounter.

Wrist Pin Retainer Failure

The wrist pins are held in place by wrist pin retainers. If one of the retainers fail or is not properly installed, the wrist pin is free to move. Eventually, as the wrist pin wears a groove into the cylinder wall, the engine will fail.

Notice the groove in the cylinder wall pictured here, it was caused by the wrist pin.



MISCELLANEOUS FAILURES (cont'd)Oil Seal Failure

Dirt and debris will collect on the outside of the engine. If it is not kept clean the dirt can find it’s way to the oil seal area and cause a failure like this.

The seal area on the crankshaft has been worn away by debris that built up on the crankshaft. The oil seal no longer contacts the crankshaft causing an oil leak.

Fretting Failure

Fretting: Damage caused by rubbing, chafing or wearing away.

In this example we see wear caused by the improper fit of an electric clutch to the crankshaft. When there is improper fit the clutch and crankshaft rub against each other causing gouging and wear of the mating surfaces.

Broken Crankshaft Failure

Broken crankshafts are not common. When you see a clean break like this, look for excessive side loading on the crankshaft. Over-tensioned drive belts or an improper fit of the clutch can cause this failure.



MISCELLANEOUS FAILURES (cont'd)Broken Crankshaft Failure (cont’d)

Here the wrong clutch was used for the crankshaft. Notice the crankshaft does not fit flush with the face of the clutch. This application caused excessive side loading on the crankshaft and caused a failure.

A crankshaft break at the flywheel end is most likely caused by shock loading. A shock load is cause by a rapid change in crankshaft speed such as debris lodging in the flywheel. A loose flywheel can also produce a shock load.

The break is always clean as it is here (notice there is no rust in the break.) This indicates that the break did not happen over a period of time, but rather in a quick snap.

Broken Camshaft Failure

Usually a broken camshaft is the result of another failure, such as a broken connecting rod. Pieces from the broken rod strike the camshaft and cause it to break.



FAILURE DETERMINATIONAnalysis Of Information and Conclusion

With all the information recorded on the EFA form, you will be able to determine the cause of failure. If you determine that the failure is eligible for warranty, contact your Distributor Service Manager to discuss your findings.

Resolution of Engine Failure Analysis

The reason for an engine failure analysis is to find out why the engine failed. If the failure was caused by a manufacturing defect, tell the customer what you found and file the warranty claim. If the failure was the fault of the customer, use the EFA form as well as the failed parts to explain why the engine failed and what the customer can do in the future to make sure it does not happen again.


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