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AVL Efficiency Diesel Engine

The Efficiency Engine –
Cost-effective Alternative to Downsizing


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AVL Solution

AVL has developed the so-called efficiency engine as an alternative to the familiar option of downsizing diesel engines. Because of the engine's moderate power density, its peak pressure requirements are lower in some areas than those of turbocharged gasoline engines. Consequently, its mechanical friction and fuel consumption can be significantly reduced, as the comparison with a conventional and a downsized diesel engine demonstrates.

De-rating means to maintain with somewhere bigger engine displacement, and do not go to specific power extremes (power density limited at ~ 45 kW/L), which lessens the requirement for sophisticated engine design due to lower PFP (peak firing pressure). Of course, for such de-rating concept decisions the expected production volume, costs, image, regional market aspects, etc. have to be taken into considerations.
 
De-rating also offers the potential of commonality between Gasoline & Diesel engine family production. The increased number of common parts with gasoline engines this leads to increased production volumes and consequently lower cost.

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AVL Approach

Prototype Implementation of Efficiency Engine - Generation 1
For the efficiency engine the focus was on minimizing the frictional losses from the crank train and the parasitic losses from oil and water pumps. 
The defined thermodynamic peak firing pressure for the Efficiency Concept is only 110bar (design PFP 120 bar). The gas forces are hence 30-40% lower than a typical state of the art Euro 5 Diesel engine. A careful redesign of the complete crank train according to the lower loads was performed. Significant benefits are realized in terms of reduced mass and friction.
 
  1. Piston:
    The lower gas forces allow a reduction of piston pin diameter from 26 to 20 mm. A reduction in wall thickness below the combustion bowl and the elimination of the piston cooling gallery due to the moderate specific power together permit a reduction in piston compression height to only 48% of the cylinder bore without requiring bearing bushes in the piston.
     
  2. Piston Rings:
    The lower gas forces also allow a reduction in ring widths and tensions to a level more familiar from gasoline engines with corresponding reduction of friction.
     
  3. Connecting Rod:
    The small end is redesigned to the reduced pin diameter and the shank cross-section adapted to the lower gas load. The big end is also reduced to suit the optimized crankpin diameter. The lower piston height allows an increase in con rod length of 5.25mm which gives and additional friction benefit due to the reduction of side forces at the piston-line interface. Nevertheless the weight of the con rod assembly was reduced by nearly 25%. The application of cost-intensive sputter bearings at the big end could be avoided. 
     
  4. Crankshaft:
    Here the focus is on minimization of diameters of the main and crankpin bearings. In the first step, parameter variation was performed using FEM-Analysis of a single crankweb, based on a metric of bending deflection under gas load. The new layout was compared to a wide range of proven designs based on this metric. The resulting layout was then validated using the full 3D Multi-Body-Simulation and Finite Element capabilities of AVL-Excite, including Elasto-Hydrodynamic simulation of the bearings. It is noted that the crankshaft design is here limited by its stiffness – and hence the edge-loading of the main bearings. Including the proportional reduction of the counterweights, an overall saving in crank train mass of around 10% was achieved.
     
  5. Oil pump:
    Adaption of oil pump size to reduced flow requirements of optimized crank train.
 
Since the mechanical loads on the efficiency engine are comparable to a high performance turbocharged GDI engine, the concept fits well to a modular engine family line-up with common components and production facilities for diesel and petrol variants. 

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Customer Benefits

With the derating approach (efficiency engine) we also see benefits with respect to system costs and complexity. The efforts for boosting, fuel injection system and NOx aftertreatment are significantly reduced. This cost effective engine has the potential for highest production figures due to worldwide market chances, thus offering additional benefits in the cost structure.
 
The development is clearly shooting for more independent systems providing attractive reduction potential in an extended operation area. The biggest challenges at the moment are the system costs and further increased complexity. The efficiency / de-rating concept currently offers an attractive solution for the cost sensitive high volume market combining improved fuel economy with system simplification and potential cost reduction.

References

MTZ worldwide Edition: 2011-12

The Efficiency Engine – Cost-effective Alternative to Downsizing

Author(s): Michael Weissbäck; Mike Howlett; Norbert Ausserhofer; Stefan Krapf