Laser Peening Application Review

SECTOR:
Automotive
Application:
Engine

Component(s):
Intake and Exhaust Systems – Valves, Springs, Turbo/Superchargers
Common material(s):
Cast iron
Spring Steel
Titanium alloys
Stainless steel
Nickel based alloys

Common failure(s):
Valves (Thermal fatigue, bending fatigue)
Springs (fatigue)
Turbochargers/Superchargers (Fatigue, FOD)

The costs of failure:
When looking at the cost of a component failure, there are many aspects that need to be examined in addition to just the part costs themselves.  Some of the actual costs are quantifiable, while others are inherently not.  Due to the many different applications of engine components, it is very difficult to quantify the cost of a failure for a broad application, but here are a few items to consider:

• Cost of replacement part ($)
• Cost of labor to replace part ($)
• Lost time due to replacement part ($)
• Reputation damage (UNK)
• Future sales (UNK)
• Property/Life loss risk (UNK )

Expected Improvement(s) from Laser Peening:

Laser peening provides great improvements in fatigue life of metal components, often by over 10x the baseline state.  Automotive engine parts should experience an appreciable increase in fatigue life; however, there are also many other benefits.
Laser peening imparts residual stresses that remain stable at higher temperatures for a longer time period due to the relatively low degree of cold work imparted by the process.  This means that the components will retain their material properties in hot environments where normal products may typically see reduced performance. As a result, the beneficial residual stresses of laser peening can prolong the service life of turbochargers and valves to reduce or prevent the onset of thermal fatigue.  Laser peening also increases the damage resistance to FOD and erosion of the turbine and compressor in forced air induction equipment.

How Component(s) Functions and Fails:

Engine operation is enabled through the use of valves. As with most any other valve, the function is to open and close a passage as commanded. In the case of the combustion engine, the valves serve to allow air (and sometimes fuel) into the combustion chambers, seal the chamber, and allow for combustion gases to be expelled out of the chamber to prepare for a new charge of air. The valves operate in tandem with the valve springs, camshafts, and often other components that can include push rods and lifters. The camshaft places pressure on the valve and spring to cause the valve to open.  When the camshaft pressure is relieved, the spring pressure forces the valve to close. 
As the valves are repeatedly closed, especially at rapid speeds, the valves can experience large stresses.  These stresses can initiate fatigue cracks in the valve stems which will lead to subsequent major engine damage. Thermal fatigue can also occur as the hot exhaust gases pass over the exhaust valves.
Valve springs can also fail due to the rapid acceleration rates experienced and the high number of cycles they experience.  A 4-stroke engine operating at 6000 RPM loads a valve spring 50 separate times per second! The forces necessary to open and close the valves this quickly can severely stress springs, pushing them to their limit, leading to cracking and fracture.  A failed engine spring that occurred as a result of fatigue cracking is shown in the image to the right . The fatigue cracking started on the inner diameter of the spring where the stresses are at their peak. You don’t want to see this in your engine!

Turbochargers and superchargers use compressor/turbine wheels to compress the air going to the combustion chambers. By compressing the air, more air can enter into the cylinder during an intake stroke. The increased air quantity can then support increased fuel, which leads to higher horsepower output.  In a turbocharger, a turbine wheel is powered by the exhaust stream. The compressor wheel is mounted to the opposite end of a shaft to compress the intake air for the engine. In a supercharger, a belt-pulley system powers the compressor wheel, usually through the standard accessory routing (alternator, power steering pump, etc.). The high rotational speeds of the compressors and turbines create high levels of stress in the wheels. Overspeeds can lead to failure as well as different resonance issues that develop at certain RPMs.  In many cases, cylic loading of the individual vanes on these wheels occurs and lead to fatigue cracking as shown in the image on the left.  Once the fatigue cracking starts, it is only a matter of time until the entire assembly will likely be ruined as part of a vane detaches from the wheel. This equals big costs to not only replace the turbine or compressor, but the entire assembly.

How Laser Peening Can Improve Performance:

Laser peening of metal components typically results in a corresponding boost in fatigue strength.  This increase in fatigue strength is enough to extend the service life and potentially make the part an infinite-life design (failure will never occur due to normal operation). So what tangible benefits could the addition of laser peening transition to?

Higher sustained and peak engine RPM without failure

Shown to the right is a generalized S-N plot with plots for a baseline material and the same material that has been laser peened. The plot would indicate that the laser peened material has a higher fatigue strength (endurance limit) than the baseline material and is capable of handling higher stresses for a longer period of time (higher σ level, more cycles). This translates to several potential improvements:

1. Springs capable of withstanding higher stress levels allow the valves to have longer open duration times as higher spring pressures can be achieved to return the valve to the closed position.  This allows more air to enter the combustion chamber to generate more horsepower.
2. Sustained engine operation at high RPMs can be achieved by increasing spring fatigue life.  At high RPMS, standard springs can reach a fatigue life quickly when they are severely stressed.  Laser peened springs would be able to operate at the same RPM for a longer period of time before failure occurs.  The increased RPM may be the edge needed to squeeze a few more horsepower out of an engine, especially in high performance applications.
3. Poppet valves, valve stems and valve seats take a lot of abuse in an engine.  With laser peening, the valves are capable of taking more of a beating without failure. Laser peening can help reduce wear on the seals, reduce thermal fatigue, and sustain higher inertial stresses for rapid operation.

More reliable boost from forced air intake systems

Turbochargers operate in a harsh environment, relying on exhaust gases for operation. With laser peening, the turbines and compressors have increased resistance to FOD and erosion.  Modern turbochargers also use sophisticated equipment to monitor boost pressure, open waste gates, etc.  Deficiencies in these systems can occasionally lead to overspeed events that can often initiate fatigue cracks in blades. By laser peening, these overspeed events don’t have to mean a complete turbo rebuild.

Longer life with less expensive materials

A common theme in the world of engineering is to do more with less.  This is one of the goals and main benefits of laser peening. With deep compressive residual stresses, laser peening can be applied to less expensive materials to achieve the performance of expensive materials. Similarly, while using expensive materials with laser peening it is possible to use less mass and cut down on manufacturing costs. Residual stresses are a key component to unlocking the increased performance gains available for many applications.  Engineering the residual stresses into your application is what LSP Technologies does best!
Shown below is an example of a performance valve spring.  The addition of laser peening creates a compressive residual stress (blue) on the exterior of the wire where the stresses are the highest.  Optimization of the laser peening process can also be made to improve only the areas most in need of surface enhancement (the ID) to balance performance and cost.

Conclusion

These intake and exhaust components can significantly impact the performance of an engine.  When you are seeking the absolute top performance, laser peening offers many benefits that are worth exploring.  Whether your goal is to run the engine harder, faster, or longer we are here to discuss your needs and goals and work towards engineering a solution.  Call us or contact us online at LSPT to discuss your application today!

Contact Information:
Stan Bovid, P.E.
Senior Metallurgical Engineer
LSP Technologies, Inc.
6145 Scherers Place
Dublin, Ohio 43016
(614) 718-3000 ext. 415 phone
(614) 718-3007 fax
sbovid@lspt.com