Thursday, 18 December 2014

Turbocharger failures at Daf Trucks

Daf Trucks in the UK appears to have suffered a spate of problems with turbochargers fitted to its 12.9-litre MX commercial vehicle diesel engines.

According to Daf Trucks, not all MX engines are prone to the turbocharger problems; those more at risk are the 510bhp and 460bhp ratings. Turbocharger failure rates on lower rated (i.e. 410bhp and 360bhp) seemingly are no worse than other diesel engines.

According to David Wilcox, writing in the journal Commercial Motor, because the two top ratings are affected, the problem manifests in XF105 tractor units, rather than the XF tractors or rigid vehicles.

Most are Euro 5 models and all have Borg-Warner waste-gate turbochargers. Euro-6 MX-13 engines carry Holset variable geometry turbochargers. Holset of Huddersfield is part of Cummins Inc.

According to Wilcox, the cause of the problem is low-cycle fatigue of the turbochargers’ aluminium compressor wheels. The weakness in the material is generated by cyclical loading and unloading of the impeller during repeated acceleration and deceleration of the vehicle.

Deformation of the impeller blades eventually exceeds the elastic limit of the nickel-coating aluminium alloy. This causes fatigue cracks and subsequent blade break-up and eventually failure of the turbocharger.

BorgWarner Turbo Systems acknowledges that modern commercial vehicle turbochargers are subject to very high loads due to the wide range of applications in which they are used. Borg-Warner Turbo Systems admits too that in many cases where there are extreme loads, a compressor impeller made of an aluminium alloy determines the service life of the turbocharger.

In particular, material fatigue can result from extreme loads, especially when the loads are cyclical loads occurring at low frequencies. This phenomenon is also known as low cycle fatigue (LCF), notes BorgWarner Turbo Systems.

BorgWarner Turbo Systems engineers note that various measures can be taken to increase the service life. For example, the circumferential speed of the compressor impeller can be reduced by changing the aerodynamic design of the impeller or by controlling or reducing the charging pressure.

The company’s engineers add that high-strength aluminium, for example, that manufactured using the HIP technique, is often employed to reduce variations in the strength of the material. The high-end solution using aluminium consists of milled compressor wheels which were also developed by BorgWarner Turbo Systems.

The increasing number of engine applications with high cyclical loads as well as the necessity to have higher charging pressures to remain within the stricter emission regulations have made necessary an additional innovation step beyond the moulded aluminium compressor wheels.

In order to provide a customer with a technology that can withstand these loads and also ensure a service life that is just as long as it would be under standard conditions, BorgWarner Turbo Systems has initiated a special development program.

These demands on the compressor impeller have made it necessary to use higher quality materials since the potential for improvement inherent in aluminium is “just not sufficient” anymore.

Engineers at BorgWarner Turbo Systems decided to use a titanium alloy that is not only very hard, but also provides an excellent strength-to-thickness ratio.

With the titanium compressor impeller, BorgWarner Turbo Systems claims it is now able to offer different compressor impeller technologies for special application requirements and special cyclical loads. In this manner the customer can select between cast or moulded aluminium compressor impellers or between cast or moulded titanium compressor wheels depending on the area of application.

According to Wilcox, for 460bhp MX engines, Daf Trucks claims it has taken a “pro-active approach” to manage the risk, operator-by-operator. It claims the overall turbocharger failure rate on 460bhp engine is seven per cent, but this is greatly influenced by duty cycle.

If Daf Trucks assesses the risk as no more than with any other engine it takes no action. But if the risk is higher Daf Trucks will replace the turbo with an upgraded titanium-bladed version at no cost. It has replaced 3,000 turbochargers on 460bhp MX engines in the UK under these circumstances.

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