Automotive Case Study
Automotive reliability: keeping the wheels turning
Automotive manufacturers go to great lengths to make sure their latest designs are suitable for the intended application, but if things go wrong they need to understand what is causing the problem.
Although hundreds of parts go into the creation of a car engine, some components are more important than others – and none is more important than the crankshaft. By turning the downward linear motion of the piston/con rod into rotary motion, a crankshaft is at the heart of every internal combustion engine. It drives the flywheel, which transmits power to the gearbox, via a clutch, and from there to the driven wheels.
The forces acting on the crankshaft journals are tremendous. Today’s high-performance petrol spark ignition engines create a combustion pressure of more than 11 bar (1450PSI) or more. The pressures generated by diesel compression-ignition engines are even higher.
The different firing intervals in multi-cylinder engines, combined with today’s high rev ceilings, creates stress and vibration which increases with the number of cylinders and length of the crankshaft.
The combined weight of the piston, rings, gudgeon pin, retainers and con rod small end also exert a force on the crankshaft journals. The design and production of automotive components is driven by the need for low cost, low weight and better efficiency.
This means that critical components are often designed to tight tolerances, and pre-production endurance testing is used to demonstrate that the designs so produced will meet their intended in-service life.
In the case of the crankshaft, designs which use higher strength materials, smaller diameters/sizes and lighter weight deliver lower frictional losses, greater efficiency, smoother running and lower operational costs.
However, such developments don’t come without their own issues. Higher strength materials tend to be more notch sensitive and more susceptible to fatigue cracking issues. In the case of crankshafts, the change of section at the edges of journals are a prime site for fatigue to initiate.
Axiom works closely with engine developers to investigate cracking in these locations and understand why the cracks are forming. Modern techniques and stress analysis, can produce designs which should be free from the known problems - in theory. But real-world tests can still throw up unexpected failures.
Once a crankshaft is made - forged, machined, etc. - just how close is it to the actual design on paper? Critical aspects, such as the size of the radius at the side of the journals, will have been designed to very precise measurements, but can the machining match the design with sufficient precision and accuracy, time after time after time, to ensure that each and every shaft matches the original specification?
Understanding that 99.9% of crankshafts produced will be within the tolerances of the design means that 1-in-1000 will not. For a car manufacturer selling millions of vehicles this is an unacceptable failure rate. What would you do if your car suffered a catastrophic engine failure, such as a snapped crankshaft, in the outside lane of the M1?
Getting the design right, and proving it by endurance testing of both pre-production and production units, helps to ensure the that customers aren’t left stranded on the roadside.
Axiom is ideally positioned to investigate the defects, categorise the cracks and determine whether they are due to overstress, fatigue, a manufacturing defect, material deficiency or something else. Axiom can also section and show actual machined radius profiles, which is often a factor in the initiation of fatigue issues.
Axiom has worked with a number of multinational automotive design and manufacturing organisations to improve and optimise their design. Our expertise has helped them understand how and why failures occur.
Whether it is a new design of crankshaft, a cylinder liner, valve performance or piston damage, Axiom can help your organisation to improve designs and production pieces, reduce failures and benefit from better reliability.
If this sounds like something which could also be beneficial to your company, please feel free to contact us for a no obligation review.