Introduction to forensic engineering
Introduction to forensic engineering

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Introduction to forensic engineering

1.3 Causal analysis

The examples given in the previous SAQ illustrate a central feature of failure analysis: an implicit assumption that there are rational causes for failure, and that there are reasonably objective ways of determining those causes.

It is assumed you have some inkling of the way common machines operate. This is not going to be true of all machines, even in the home. Microwave ovens, personal computers and CD players come to mind.

With a fault on a domestic machine I expect you would look for the operating manual, which gives basic guidance on common failure modes, and how to recognise their symptoms, often in the form of a fault-finding guide. Typical fault-finding guides from car maintenance manuals are tabulated and suggest possible causes for one specific symptom.

For the cooling system part of the fault-finding guide shown in Table 1, five general symptoms are listed, and possible causes listed under each heading. It is then for the mechanic to investigate each possible fault mode systematically in turn to find the faulty component. By eliminating parts that are functioning correctly, the faulty part can be located. As internal-combustion engines work on the same general principles, the tabulated data is generic to different makes of car. Such checklists are certainly a useful start to troubleshooting, but must be used with common sense, so that unsuspected failure modes are not discounted.

1.3.1 Fault-tree analysis

One simple way of showing the inter-relationship of many different possible causes producing a single symptom is known as fault-tree analysis (FTA) (see Figures 1 and 2). An overheating car engine can be caused by insufficient coolant (Table 1, line 1). This, in turn, could be caused by internal or external leaking of the coolant. An internal leak could be caused by a broken cylinder-head gasket or a cracked cylinder head, for example. Such internal leaks usually lead to water entering the oil sump, which turns the oil milky, and is instantly recognisable. Alternatively, the coolant could leak to the exterior because of a broken hose, damaged seals or a faulty pressure cap. You will notice there are several dotted lines on the fault tree, which indicate other possible but unknown causes that might be peculiar to a model of car.

Table 1: Fault-finding checklist for an overheating or overcooling car engine

Cooling system faults
overheatingexternal coolant leakage
□ insufficient coolant in system or thermostat faulty□ deteriorated or damaged hoses or hose clips
□ radiator core blocked or grille restricted□ radiator core or heater matrix leaking
□ electric cooling fan or thermoswitch faulty□ pressure cap faulty
□ valve clearances incorrect□ water pump seal leaking
□ pressure cap faulty□ boiling due to overheating
□ ignition timing incorrect/ignition system fault – petrol models□ core plug leaking
□ inaccurate temperature gauge sender unitinternal coolant leakage
□ airlock in cooling system□ leaking cylinder-head gasket, cracked cylinder head or cylinder bore
overcoolingcorrosion
□ thermostat faulty□ infrequent draining and flushing
□ inaccurate temperature gauge sender unit□ incorrect coolant mixture or inappropriate coolant type
Figure 1: Car engine cooling system showing the major components and the coolant flow
Figure 2: Fault-tree analysis diagram showing casual connections between components for an overheating car engine

Fault-tree analysis diagrams are useful because they place analysis on a systematic footing, and give a mechanic a logical path to follow during examination. Each failure will exhibit well-defined features. If inspection of the engine near to a cooling hose shows one point where there are dried traces of impurities from the cooling system, the adjacent hose will need closer examination.

With machines where the working principle is unknown, it is often possible to evaluate failure using common sense. As you might expect, the investigation becomes a logical analysis of the way failure occurred.

If the original failure is buried under consequential damage, the task is more difficult than for simple failures. The failed component may lie deep within the machine, and the failure only inferred from the lack of a particular function. In addition, further clues to the failure may be exhibited by contact traces of wear or abrasion for example, where none would be expected. Analysis of such traces is known as trace analysis or contact analysis, and can be extremely useful in locating the source of a particular problem, as you will see later in the case studies. Forensic investigation often starts after a mechanic finds a component that should not have failed or shows defective design or manufacture.

SAQ 2

Table 2 shows the checklist of faults for the fuel and exhaust system of a car. Construct a fault tree to show causal relations between excessive fuel consumption and various car components. Feel free to add any other possible causes to your diagram not shown on the checklist. Include possible safety-critical defects. How would excessive fuel consumption be detected by the driver – rather than by the mechanic?

Table 2: Checklist for car fuel and exhaust systems

Fuel and exhaust systems faults
excessive fuel consumptionfuel leakage and/or fuel odour
□ air filter element dirty or clogged□ damaged or corroded fuel tank, pipes or connections
□ fuel injection system fault – petrol modelsexcessive noise or fumes from exhaust system
□ faulty injector(s) – diesel models□ leaking exhaust system or manifold joints
□ ignition timing incorrect/ignition systems fault – petrol models□ leaking, corroded or damaged silencers or pipe
□ valve clearance incorrect□ broken mountings causing body or suspension contact
□ tyres under-inflated

Answer

Figure 3 is my attempt to show causal relations between excess fuel consumption and faulty components. However, excessive fuel consumption could also be caused by driving conditions, because excessive starting and stopping uses up more fuel than motorway cruising, for example. The checklist shown with the question also indicates fuel leakage as another cause. As this is potentially a safety-critical defect, it should be included in the fault-tree diagram.

Detection would occur by careful monitoring of fuel consumption and mileage driven. However, some new cars have automatic monitoring systems, so that the driver is warned of any consumption problems. The driver would also normally check tyre inflation pressures, but the other possible causes shown in the checklist would normally be performed by the mechanic during servicing.

Figure 3: Fault-tree analysis for excess fuel consumption

SAQ 3

Table 3 shows the fault-finding checklist for the manual transmission system of a car. Construct a fault-tree analysis diagram for low oil levels in the sump as detected by an oil warning system, gauge, or dipstick. Add any further possible causes from your own knowledge of internal combustion engines. Briefly indicate the role of engine oil in a working engine. What are the consequences of low oil levels to engine performance?

Table 3: Checklist for car transmission system

Manual transmission faults
noisy in neutral with engine runningjumps out of gear
□ primary shaft bearings worn (noise apparent with clutch pedal released, but not when depressed)□ worn or damaged gearchange linkage/cable
□ clutch release bearing worn (noise with clutch pedal depressed, possibly less when released)□ incorrectly-adjusted gearchange linkage/cable
noisy in one particular gear□ worn synchronizer units or worn selector forks
□ worn, damaged or chipped gear teethvibration
difficulty engaging gears□ lack of oil
□ clutch fault□ worn bearings
□ worn or damaged gearchange linkage/cablelubricant leaks
□ incorrectly-adjusted gearchange linkage/cable□ leaking differential output oil seal
□ worn synchronizer units□ leaking housing joint
□ leaking primary shaft oil seal

Answer

Figure 4: Fault-tree analysis for low oil level

Figure 4 is based on Table 3, especially the final set of checks for lubricant leaks. Another possible cause that has been added includes lack of adequate maintenance (oil replacement at regular intervals, topping up when needed). In addition, the engine oil itself may be of the wrong grade or contain volatile compounds that volatalize easily, or burn in the engine easily because of internal leaks.

The engine oil is an essential lubricant to the various moving parts of the engine. It is normally pumped onto and around such components, so any loss of oil level can deprive moving parts of protection. If too low, metal will move on metal, so creating high friction and wear, with the possibility of seizure.

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