Heavy-duty diesel automotive engines, “are not overly efficient,” states Mark Firth, executive director of product line management and marketing at Cummins Turbo Technologies. “They convert only about 45% of the energy created when burning the fuel and creating usable power, the rest is lost in waste heat,” he explains.
Cummins’ current crop of heavy-duty turbochargers – now crucial to making modern engines more efficient – operate at efficiency levels in the mid 50%. While Firth points out that this is some 3% better than the benchmark level of five-to-six years ago, he highlights that a 3% increase in turbocharger efficiency equates to just a 1% increase in fuel economy. Therefore, helping customers continuously improve the efficiency of the engines they make – as well as keep up with emissions legislation – is what drives Cummins’ own efforts to improve their products.
As well as manufacturing some 300,000 units a year – roughly a seventh of the global company’s total output – Cummins’ UK base in Huddersfield is also home to the largest of its four global technical centres, the others being in the US, India and China. The West Yorkshire centre employs some 300 of the graduate-level engineers – out of a global total of around 650 – engaged in R&D and product support. Firth says the four can operate as a single ‘virtual’ operation through their use of a common CAD system – the Pro/ Engineer package from Parametric Technology Corporation – and they also share product development projects. A range of turbochargers for 2-to-6-litre capacity diesel engines, introduced two years ago, for instance, was jointly developed between the UK, Indian and Chinese technical centres, as it was primarily for the Chinese light commercial van market.
Close co-operation with clients, in the development of products as well as the adaptation of them to customerspecific applications, is a fundamental aspect of Cummins’ operations. At any one time, Firth states, the firm is likely to have as many as 30 R&D projects underway globally and three or four specific product development initiatives, as well as multiple customer programmes intended to match current turbochargers to particular engines.
Firth explains that when a customer chooses an existing product, some work will always be necessary to enable it to be fitted to a particular engine – not least because different external geometries will require different interfaces. “The interface to the engine is the exhaust manifold,” he clarifies. For commercial and technological reasons, close co-operation with one or two ‘lead clients’ during initial product development projects is also integral to the company’s strategy.
A lead client, says Firth, will provide Cummins with details of its requirements in the form of specific air flow and boost pressures for the compressor stage – “they want the air to arrive at the engine at a certain density” – and similarly the flow, pressure and temperature at which the exhaust gases will leave the engine and arrive at the turbine. Space limitations, connection features and price range will also enter the equation.
Interestingly, far from keeping details of its own product development initiatives under wraps, Cummins has a deliberate policy of letting its clients have access to advanced prototypes of intended product features, so that a demand for such features can be written into the specification they deliver to Cummins. It is a tactic which functions as both an iterative means of accelerating product development and also as a form of marketing.
One fundamental point about the company’s automotive turbochargers – there are exceptions elsewhere in its product range – is that they are all ‘radial’ rather than ‘axial’ pieces of equipment. This means that after the airflow has passed through the compressor stage, it is turned through an angle of 90° to enter the engine and after it has exited the combustion chamber it is once again turned through 90° to enter the turbine stage. “The flow enters the compressor axially and is then turned radially – and it enters the turbine radially and is then turned axially,” Firth says. Another equally essential point is that the company’s turbochargers are just that – turbochargers driven by the engine’s exhaust stream – and not superchargers driven by a mechanical link to the engine’s crankshaft. Firth is emphatic that this represents a deliberate judgement on Cummins’ part about the relative merits of the two approaches. “We believe they are a fundamentally better product,” he states. Variable geometry innovations
Firth points to a number of innovations the company has introduced over the last several years to make its superiority even more marked. These include electrically-actuated variable geometry and machined-from-solid turbine blades. The former, he explains, means that though the core machine remains the same “the turbine can be reoptimised for different flows,” which means that the maximum amount of energy can be returned to the compressor stage. The clever aspect of the Cummins approach is that it utilises a nozzle system that can insert or retract a series of fixed vanes into or out of the flow to act as a ‘moving wall’. The use of fixed rather than ‘swing’ vanes is a unique aspect of Cummins’ implementation of the variable geometry concept and one of the consequent benefits is that the whole system is highly robust. In fact, Firth adds, the system enables the flow to be stemmed entirely if required, so that it can act as an engine braking technique.
But the company’s disavowal of the supercharger concept does not mean it is dismissive of what is, in effect, the reverse concept – ‘turbocompounding,’ in which there is a link to the crankshaft not to extract energy from the engine to drive the turbocharger, but to return it to enhance fuel efficiency. In this case, the drive unit is not the turbine stage of the turbocharger itself but a supplementary ‘power turbine’ unit that is positioned beyond it in the exhaust flow. Firth says the concept is not particularly new – the company has had turbocompounding products since the late 1980s – but at the moment it is effectively confined to use on larger engines, typically in the 12 to 15-litre volume range, with power outputs of 300kW or above. However, he can foresee that situation changing as, for instance, a currently nascent trend towards greater use of hybrid power units in the commercial vehicle sector becomes more marked.
In the early part of this year, Cummins revealed that work carried out at its UK technical centre had resulted in a much enhanced test methodology for turbocompounding systems that should make their development more efficient and their applicability more widespread. The system, known as the Emulator, was developed as a continuation of the PhD studies of Dr Kai Zhang at the University of Loughborough. Now working at the Huddersfield centre, Zhang says the methodology aims to find a way of countering the problems for accurate and speedy testing posed by the inherent complexity of a turbocompounding system in which the power turbine is linked back to the engine and turbocharger combination through a geared driveshaft.
Zhang explains that previous development methodologies have involved one of two extremes; the construction of complete turbocompounding assemblies, which are both expensive and inflexible; or purely software-based systems, which, though capable of much more rapid iteration, provide only approximate representations of intended real-life systems. The Emulator, mulator, however, provides a clever compromise between these two approaches by replacing the power turbine element of the system with a simple valve that can be opened or closed to different degrees to represent the performance of different power turbine and associated gearbox configurations. Consequently, explains Zhang, the valve can produce a range of different ‘back pressures’ in the rest of the system, allowing the overall performance of different engine, turbocharger and power turbine combinations to be estimated quickly and accurately.
Zhang says this simple expedient means the potential effect of a wide range of variations in power turbine configurations can be explored – for example wheel size, housing size and gear ratios. All that is necessary to achieve these theoretical variations is for an operator to enter a few simple commands into a computer control system. According to Zhang, the approach improves both the accuracy and speed of testing compared with the alternatives. On the former count, he says that though the degree of enhancement, compared with a purely software-based approach, is difficult to estimate, on the latter the technique probably halves the time required as well as helping cut costs. At the moment, he concedes, the Emulator is restricted to simulating static configurations, but he is confident it could be further developed to become a dynamic system, capable of simulating as large number of variables in a single test sequence.
Meanwhile Cummins is also working to improve the products themselves. Ivan Kaiser, new product design leader, manufacturing, outlines one purely mechanical innovation – the replacement of conventional ‘journal’ bearings now commonly used in turbocharger assemblies with ‘rolling element’ counterparts. The difference between the two is that the former are fixed in place so that the rotating shaft slides over their surface, while the latter, as their name indicates, also rotate in sympathy with the movement of the shaft.
Kaiser says that Cummins has worked with a major manufacturer of bearing systems to develop the application of rolling element bearings in turbocharger systems and now has a number of prototypes in place with clients of its own. It is likely to be at least three more years before standard products emerge using this type of bearing, but when they do Cummins expects them to add a percentage point to the overall efficiency of the units involved. Such an advance goes against the long-held notion within the industry that turbocharger technology had arrived at a point from which only negligible further improvements might be possible.