Background:

Many diesel engine applications involve loading/unloading, think an on-highway truck going up and down hilly terrain (see country road), but specific applications are highly cyclic, passenger rail car and transit/city bus are two specific examples. Any highly cyclic application introduces particular stresses on the engine and its components that should be considered and discussed with the customer.

Low Cycle Fatigue (LCF) Failures on Turbo chargers

This discussion will focus on the Environment spine.

Due to the spooling up and slowing down for each load cycle, the turbo compressor wheel in particular can be susceptible to low cycle fatigue. This is where continual changes from low to high speed add up over time to cause fatigue failure even though the speeds and temperatures been experienced by the turbo are not in excess of the manufacturer limits (see attached article for a deeper explanation).

LCF can result in premature failure of the compressor wheel, normally a wheel burst. At best this results in a loss of power, at worst, oil can enter the combustion air system and cause an engine overspeed.

Mitigations:

Turbo Wheel Material: – The most durable compressor wheel is titanium, if this is an option then it is the best material when dealing with cyclic loading.

Engine Size and Rating: – using an engine rating that is close to its limit usually makes since since the cyclic loading application end up with quite a low load factor, however because each load cycle takes the engine close to its maximum rating, the turbo reaches high speeds. Although not always possible, a larger engine with a lower rating will result in the turbo charger not reaching high speeds and result in a longer life.

Turbo Change Out interval: It may be necessary to decrease the maintenance interval on turbo change out. This can be done when the number of cycles is known and can be done in conjunction with the turbo manufacture. This is probably a better approach than a larger engine, which could suffer from low loading and the associated problems.

Air intake temperature: Keep temperature rise between ambient air and the turbo charger to a minimum. The air temperature at the turbo compressor wheel will have an impact on the stress levels in the wheel, any increase in temperature between ambient air and the compressor whell intake will increase the stress level, the intercooler is after the turbo so even if charge air temperatures are within specification make sure the air inlet temperature rise is also within specification.

Avoiding Significnat Speed Excursions: Boost hoses that blow off under load or air intake elbows that collapse can cause significant turbo overspeed events. Each event reduces the life of the turbo. These events should be avoided at all costs. Air intake elbows should be maintained, if softened due to the presence of oil, they can more easily collapse – use silicon elbows if engine oil carry over from the crankcase breathing system is an issue in the air intake system. For boost hoses, make use of the correct hose clamps and ensure proper beading on the charge air pipes.

Loading/Unloading

When comissioning the initial installation, careful consideration should be given to loading and unloading the engine. The engine control system can generally control the rate at which load is accepted, however unloading is controlled by the connected equipment – hydraulics/alternator. In a cyclic application, rapid unloading should be avoided to avoid thermally stressing the engine. RRS (formerly MTU) best practice recommendation is to decrease load by approxiamtely 10% of rated power per second for the first 2 seconds.  After that a ramp of 40% of rated power per second is acceptable giving 4 seconds to zero power.