Liner failuresTags : engine-structure
I don't know what it is about component failures but we engineers tend to find them fascinating. We may have designed and produced the most elegant piece of hardware, but it's somehow only when it fails - and the more catastrophically the better - that we sit up and take note.
Sometimes it isn't the fault of the component but perhaps the environment into which it was placed. Sometimes it may have worked well in engines over many years and then suddenly as a result of a single, slight change to the rest of the engine it goes on to fail. And other times we get the strangely bizarre - the failure we could have never predicted, but for our experience, for 100 years. Given a long list of engine failures over time you will therefore not be surprised that the cylinder liner features quite prominently.
An example of the first type of failure is in the machining and preparation of the upper flange and its location in the cylinder block. Poorly machined liner seating or a small particle of dirt trapped between the liner flange and its seat can lead to sudden failure at the assembly stage when the cylinder head is finally tightened. Bending loads introduced into the brittle iron liner by the action of increasing the clamp load can introduce excessive shear stresses, which cause the liner to crack at an angle of 45º all the way around. Characterised by a noticeable 'ping' at the final stage of assembly, the heart - and wallet - drop.
Manufacturers, especially engine manufacturers, tend to be a cautious bunch. Components that have served their masters well will often be carried over into new designs, sometimes untouched or with only slight modification. A major change in liner material has, perhaps understandingly, caught out at least two luxury high-performance vehicle manufacturers. Another, however, their liner technology having been proved and apparently unchanged over many years, is starting to alarm owners with low-mileage examples, fastidiously looked after since new.
The reasons for the failure may or may not be understood by the one-time manufacturer, but with engines well out of warranty the aftermarket is trying to understand if there is a common root to the problem. Scuffed liners as a result of piston seizure would seem to be the outcome, but is this down to marginal lubrication, piston ring sticking or marginal cooling? The cause is not very clear.
Early examples are apparently rarely affected but when increasing the stroke in later versions the failures increased. In some cases, classic D-shaped fatigue cracks also appear in the liner, propagating from the coolant side, suggesting perhaps some form of cooling problem. But is this one problem or two - cooling or lubrication, or perhaps even a combination of the two? At this point the engineers among you will be wondering, and will possibly already have their own thoughts or ideas.
Finally, one of the most bizarre failures I have heard of occurred in a large marine diesel, but it could happen just as easily in a race engine. A shock wave from the combustion of a slow-speed diesel unit traversed the cylinder liner and passed into the cooling water. High coolant temperatures and low cooling water flow rates produced a condition where the repeated pressure waves coming through from the combustion chamber created what is known as cavitation in the liquid next to the liner wall. Alternating pressure spikes and then rarefactions in the liquid caused vapour bubbles to form and then implode again, eventually eroding the external wall of the liner, until eventual failure.
Bizarre or not, a one-off occurrence or a common problem, it is difficult to say, but whenever you have high thermal throughput, low coolant flow rates and rapid changes in pressure, the destructive forces of cavitation may not be very far away.
The cylinder liner may be a familiar component but it still has the ability to surprise and confuse in equal measure.
Fig. 1 - Liner installation failures
Fig. 2 - Classic D-shaped crack in the cylinder wall
Written by John Coxon