Failure Study of a Diesel Engine Using Oil Analysis

Introduction

Oil analysis is most effective when used as part of a routine program, where trend analysis can be performed. Samples are regularly pulled from equipment not on a regular trending program, primarily to do a spot check, or one time analysis. Wheres this information is very limited in the predictive information it can provide, occasionally a dramatic failure may be detected by the spot check.

Reasons for Spot Check Sampling:

• Baseline Investigation- To determine how well a piece of equipment is performing
• Suspicion of a problem- Acting on the advice of an operator, or maintenance personnel
• Confirmation of a problem- Other predictive maintenance technologies have indicated a problem exists, so correlation by oil analysis will increase confidence that the unit needs to be inspected.

Background

The Massachusetts Bay Transportation Authority (MBTA) is the public transportation provider for the City of Boston and much of Eastern Massachusetts (78 cities and towns), serving over 1 million commuters every day. The transit system is the oldest and sixth largest in the USA.. The Bus Operations division owns, operates and maintains a fleet of 980 buses, most of which use Detroit Diesel engines. Bus Operations Support unit in Charlestown, MA suspected that a problem was occurring on a City Bus engine. An oil sample from a Detroit Diesel, two-stroke cycle bus engine was taken for analysis. Different oil tests where performed to determine the condition of the engine and the lubricant. The analysis indicated severe wear and poor lubricant condition. The oil analysis report gave a recommendation for an immediate action to take the engine off the road and perform a detailed inspection. Inspection of the engine did show severe wear damage at the point of connection of the rocker arm and the push rod of one of the cylinders. The engine was then disassembled and repaired.

Oil Tests for Determination of Failure Mode

• Spectrometric elemental Analysis
• Wear debris (Ferrographic) Analysis

Spectrometric Results

Wear Metals(ppm)	Fine	Coarse
Iron	2036*	274*
Chromium	112 *	47 *
Lead	5	8
Copper	48 *	12
Tin	242 *	123 *
Aluminum	93 *	48 *
Nickel	3	1
Silver	0	0
Molybdenum	264	32
Titanium	8	0

Additive/Contaminants (ppm)	Fine	Coarse
Silicon	487 *	96 *
Boron	43	18
Sodium	61	40
Magnesium	917
Calcium	1547
Barium	1
Phosphorus	2130
Zinc	4129
Vanadium	1

Data Interpretation

The spectrometric analysis of the oil sample shows large amounts of fine and coarse Iron. Iron is most common of the wear metals and may indicate wear debris generation from different areas of the engine, such as the cylinder lining, connecting rods, piston ring and rocker arms as well as the push rod.

The chrome readings indicated wear of the cylinder lining since the cylinder lining is chrome plated and also wear of piston rings. Gray cast iron with relatively thick coatings of plated chromium up to 0.2mm (0.008in) is most commonly used for piston rings. The presence of large amounts of tin indicates wear of babbitt lined bearings.

Connecting rod and the rocker arm bearings have babbitt lined surfaces. These bearings are usually Tri-metal sleeve type, which have a hard brass or bronze layer with steel backing where the thin layer of the Babbitt is applied. Therefore the presence of tin and copper indicate wear of bearings. Silicon is usually an indication of a contamination of the oil with air born sand/dirt (silica). Magnesium, calcium, phosphorus and zinc are part of the oil additive formulation.

Ferrographic (Wear Debris) Analysis

Ferrographic analysis shows large amounts of fine ferrous rubbing wear particles with particle diameter less than 10µm along the ferrogram and light to moderate amounts of severe sliding wear particles at the entry point. The severe sliding wear particles have a particle diameter in the range of 15 to 45µm and indicate scoring and scuffing of sliding surfaces. Light amounts of nonferrous wear particles mainly copper and white metal (Babbitt and aluminum) particles are present. Heavy deposits of soot materials and carbon flake were also present. Moderate to large amounts of crystalline (sand/dirt) contaminates were also present.

Figure 1 Large deposit of fine ferrous rubbing wear particles		Figure 2 Copper and aluminum particles along the ferrogram

Most of the dust, dirt and the build up of soot and carbonaceous material are carried by the oil on to load supporting surfaces of the cylinder walls and other areas can wear the ring and the cylinder.

To determine the source of ferrous wear particles, the ferrogram was heated to a temperature of 625°F and the temper color change analyzed. Most of the ferrous wear particles change to a straw brown temper color indicating cast iron and some of the severe sliding wear particles change to blue temper color indicating carbon steel and low alloy steels.

Inspection

The failed engine part was inspected to determine the failure mode and extent of damage. The first indication of severe damage as mentioned above was the rocker arm at the connection point of the push rod.

Figure 3 Large amounts of sludge and sticky lacquer deposit on the cylinder intake ports. Some of the ports are almost completely blocked, severely restricting the airflow.		Figure 4 Rocker arm

The rocker arm bearing was also severely damaged and wiped. The cylinder shows large deposits of sludge and lacquer, restricting the ports. Wear of the cylinder lining and piston rings was also indicated. The cylinder lining shows the wear track.

Failure Analysis

The failure in the engine is caused by a mechanical damage to the rocker arm and the push rod resulting in improper opening and closing of the exhaust valve. Misalignment or fitting error in mounting and connecting the push rod to the rocker arm may have been the root cause of the damage. This leads to insufficient air intake and removal of exhaust gases. Under this condition the fuel combustion is not be complete resulting in formation of large amounts of soot and sludge as indicated in the oil analysis and inspection of the cylinder. Furthermore the build up of sludge and varnish at the exhaust ports, which restricts gas flow, also resulted in a poor air/fuel ratio.

The resulting large concentration of soot and dirt/sand material contaminated the lubricant. These abrasive contaminants will cause wear of the cylinder lining and the piston rings, generating large amounts of severe sliding and rubbing wear particles. The wear tracks found on the cylinder lining conform the abrasive wear mode.

The depth and size of the wear scar and track present on the cylinder lining also explains the different ferrous wear particle size distribution present on the ferrogram. The fine rubbing wear particles with particle size less than 4 to 6µm are generated by the polishing effect of the soot particles present in the oil. This wear mechanism (bore polishing) that is associated with engine deposits generates large amounts of fine rubbing wear particles both from the cylinder lining and the piston rings. The fine cast iron wear particles in the ferrographic analysis and the high amount of iron in the spectrometric analysis confirm this finding.

The large severe sliding and laminar platelets which turn to blue temper color on heat treating the ferrogram, indicating low alloy steel are wear particles generated from the rocker arm and the push rod. The surface damage incurred by the rocker arm at the point of contact with the push rod shows severe material removal resulting in large wear particle sizes indicated by the ferrogram up to 45µm.

The faulty motion of the rocker arm due to the mechanical damage also induced high stress on the rocker arm bearings causing wiping and damage to the babbitt lining. Inspection of the bearings and the presence of high amounts of tin and copper in the spectrometric and the wear debris analysis confirm the finding.

Conclusion

In this case oil analysis not only aided in detecting the problem, it also helped in determining the cause and mechanism of failure. Once the primary failure cause is identified it is easer to correct the problem. The primary failure mode in this case as indicated above is the mechanical damage to the rocker arm, which resulted in improper performance of the engine. Proper mounting and installation of the rocker arm and the push rod eliminated this problem .

Acknowledgment

BTS would like to thank Mr. Tim Cunnane of MBTA Bus Operations for his technical assistance for this project.