FUEL EFFICIENCY AS CHALLENGE FOR FUTURE COMMERCIAL VEHICLE ENGINESThe fuel consumption and therefore the CO2 emissions of commercial vehicle engines will have to be further

reduced in the future. In the following report, AVL evaluates promising approaches towards optimising basic

engine components on the basis of a six-cylinder heavy-duty diesel engine.

FUEL EFFICIENCY AS CHALLENGE FOR FUTURE

38

DEVELOPMENT FUEL CONSUMPTION

TECHNOLOGY TRENDS

Further significant reductions in the fuel consumption of modern commercial vehicle engines require a holistic view of the powertrain architecture. Downspeed-ing is being pursued, particularly for engines in the long-haul sector, whereby the operating range of the engines is being pushed to lower engine speeds, . In order to ensure sufficient climbing ability and driveability in lower gears, the torque characteristic of future engines must be adapted a mating with an automatic double-clutch transmission is beneficial.

In combination with a further increase in power density, the thermal load increases, whereby further peak firing pressure increases of over 250bar are to be expected. This poses considerable challenges for the base engine design, structural stiffness and cooling, friction, weight and cost [1]. A careful assessment of economic viability quickly leads to a modular approach for possible fuel con-sumption measures [2]: : Conceptual measures have great influ-

ence on the manufacturing process and therefore must be considered early in the product definition phase. Examples are lightweight architecture, crank shaft offset, long connecting rod, low friction valvetrain, minimised main bearing diameter, switchable piston-cooling jets or so-called split cooling.

: Optimisation measures can be imple-mented with manageable changes in

manufacturing and final assembly lines and are therefore mainly suitable for introduction during a technical revision of existing engine families. This could be reduction of conrod big end bearing diameter, bearing clear-ance optimisation, oil circuit pressure and flow optimisation, bore distortion reduction, optimised honing parame-ters and so on.

: Add-on measures can be implemented with minor or no changes to the man-ufacturing and final assembly lines and are therefore suitable for introduc-tion to existing engine families. This includes low friction coatings, variable capacity coolant and oil pumps, elec-tronically controlled thermostat, switch-able air compressor and fan or electro-hydraulic servo steering pumps.

The following sections will discuss selected technology packages.

CRANKCASE LIGHTWEIGHT DESIGN

In cooperation with the Fritz Winter GmbH, the weight and cost reduction potential of the crankcase of a 12-l heavy duty engine was investigated. Special techniques in the casting process permit structures up to 3.5mm nominal wall thickness to be achieved. The thin cast-ing method can be applied through tar-geted control of the casting and cooling processes, whereby this method is also suitable for compacted graphite iron casting. In parallel, the force-transmit-ting structure was optimised and com-

AUTHORS

DR. HELFRIED SORGERis Executive Chief Engineer Design,

Simulation and Mechanical Development in the Engineering and

Technology Powertrain Systems Division of the AVL GmbH in Graz

(Austria).

DR. WOLFGANG SCHFFMANNis Head of Powertrain Design in the

Engineering and Technology Powertrain Systems Division of the

AVL GmbH in Graz (Austria).

GEORG VON FALCKis Head of Product Line Plant and

Production Engineering in the Engineering and Technology

Powertrain Systems Division of the AVL GmbH in Graz (Austria).

800 1000 1200 1400 1600 1800 2000

Engine speed [rpm]

Eng

ine

torq

ue [

Nm

]

2000

1000

110

150

200

50

Eng

ine

pow

er [

kW]

250

3000

Engine full load todayEngine full load future

Torque characteristic with downspeeding: shift to lower engine speeds

12I2013 Volume 74 39

putationally validated for a peak firing pressure of 220bar.

A weight saving potential of over 12 % relative to the finished component was achieved, whereby the largest part of 5 % naturally lies in the periphery of the crankcase, . The production costs for the crankcase are neutral; however the weight advantage for a future engine design would be approximately 31kg.

CYLINDER HEAD COOLING

Designing an engine for best fuel con-sumption with highest BMEP causes, apart from higher peak firing pressures, considerably increased thermal load of the valve bridge and thus increased requirements for the cylinder head cool-ing, particularly in the area of the valve bridge. In the so-called Top Down cool-ing concept from AVL, the coolant flow in the cylinder head is routed from the upper water jacket in the injector area and centrally focused on the fire deck, . This can achieve up to 15C reduced valve bridge temperatures compared to standard cooling besides structural opti-misations. A further advantage is the robustness of the coolant flow against production tolerances. The production costs are almost neutral to conventional cylinder heads.

FRICTION REDUCTION

The reduction of mechanical losses in the engine is gaining in importance, since engine optimisation is attractive

from an economic viability point of view compared to hybridisation and/or other vehicle measures. The AVL database of engine strip-down results enables a friction target value definition at an indi-vidual system level as early as the con-cept phase.

CRANKSHAFT LAYOUT

The crankshaft bearing contributes approximately 20 to 25 % of the total engine friction; correspondingly, the diameter optimisation of the main and crankpin bearings is a clear target. Combinations of main and connecting rod bearings were compared in a parameter variation, , whereby all

combinations reached the stiffness and strength targets the risk for edge load-ing is a determining factor. The fric-tional advantage of the best variant was approximately 8 %.

The increase in bearing clearance is a supplementary measure to reduce the bearing friction, whereby the limit is determined by NVH requirements. To achieve the largest possible nominal clearance, a minimisation of the series tolerance is a prerequisite. This could be achieved by grading the crankshaft and/or the housing and the bearing shells, or by reducing the manufacturing toler-ances for the individual components.

CRANKSHAFT OFFSET

By introducing an axial offset between the cylinder liner and the crankshaft, the lateral piston forces can be reduced in the relevant operating ranges. The opti-mal crankshaft offset lies in the range 10 to 15 % of the cylinder bore diameter. In commercial vehicle engines the cylinder liner requires a recess to ensure space for the conrod. The liner must therefore have an angular orientation. Such designs have been state-of-the-art for many years for marine engines with three stage conrods.

The reduction in friction can be pre-dicted and optimised using piston motion simulation to a high degree of accuracy, , and lies at approximately 10 %, or corresponds to 2,5 to 3,5 % reduction in engine FMEP. For a new engine design, the add-on cost for the

Weight reduction through thin casting on the crankcase of a heavy duty engine

Top Down cooling on a cylinder head of a heavy duty engine

DEVELOPMENT FUEL CONSUMPTION

40

implementation of an offset is considered to be almost neutral.

PISTONS AND PISTON RINGS

Piston ring development requires a bal-ance between friction, oil consumption, blow-by and wear. Increasing the clear-ance between the piston and the bore generally leads to a reduction in friction, limited by NVH requirements. A crank-shaft offset tends to permit a somewhat greater clearance at a constant NVH level, whereby the piston pin offset needs to be considered.

There is a clear trend towards narrow ring heights, which are less stiff. Coatings such as diamond-like carbon (DLC), when combined with optimised design offer sig-nificant friction reduction, currently how-ever, still with a noticeable cost impact. Compact steel pistons for heavy duty engines offer significant friction advan-tages the reduced compression heights

are typically advantageous for the engine height. Comparative measurements on a six-cylinder heavy duty engine show a 12 to 15 % reduction in piston friction.

THERMAL MANAGEMENT

Fuel consumption reductions of up to 2 % in a typical driving cycle under aver-age environmental conditions can be achieved via consistent optimisation of the engine and vehicle thermal manage-ment [4]. In part load, considerable sav-ings potential is reachable by implement-ing a map controlled coolant pump and increasing the coolant temperature to the maximum permitted level. Split cooling can be used to independently route the coolant flow to the cylinder liner and the cylinder head. Split cooling is ideally combined with the Top Down cylinder head cooling from AVL.

The cooling fan has a particularly important role in commercial vehicles. Due to the high power of up to approxi-mately 50kW (for EGR concepts), it should only be activated when sufficient cooling cannot be achieved through other means. Reductions in fuel con-sumption of up to 0.8 % are possible, if multi-stage or actively controllable fully variable cooling fan clutches are used.

ON-DEMAND AUXILIARY CONTROL

The full installation power of the steer-ing servo pump and the air compressor is very seldom required during long-haul operation. The auxiliaries normally oper-ate at reduced power and cause parasitic losses that cannot be ignored. With con-ventional air compressors without power limitation in idle, the cumulative parasitic losses over the complete test cycle that could be eliminated amount to 1.3 % of

1000 1250 1500 1750 2000 2250 2500

FME

P [

bar]

~5-8 % reduction ofcrankshaft friction(stroke/bore < 1.2)

Engine speed [rpm]

0.1 bar

Parameter variation for crankshaft dimensioning (heavy duty diesel engine, 240 bar PFP)

85

90

95

100

105

0 3 6 9 12 15 18 21

Pis

ton

FME

P [

%]

Crankshaft offset [mm]

Range ofbest configuration

Pin offset -0.5 mm

Pin offset -0.8 mm

-10 % piston FMEP

OffsetOffse

FMEP reduction through crankshaft offset (in-line six-cylinder heavy duty engine, connecting rod ratio 0.27, 1200 rpm, high part load)

Cost impact of CO2 reduction measures without development amortisation

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the total fuel consumption; even air com -pressors with power limitation could, if decoupled from the drive, save 0.3 % fuel.

SUMMARY OF THE MEASURES, COST IMPACT AND OUTLOOK

AVL employs a should-cost analysis method to make early predictions of the cost impact of concept alternatives and to involve them in the concept decision [5]. This analysis is based on manufac-turing processes and cost structures, or on best practice manufacturing pro-cesses. Using the example of a six-cylin-der heavy duty diesel engine, the addi-tional product cost when introducing the fuel reduction measures displayed are calculated individually, each without apportioning the development costs, .

The accumulated on costs for the introduced technology packages lead to an return on investment (ROI) of signi-ficantly less than one year, taking into account the total cost of ownership (TCO) view. The combined potential of all optimisation measures on the base engine offers, when compared to conven-tional designs, a 3 to 5 % fuel saving in typical long-haul operation. Supported by thermal management measures over-all improvements of up to 7 % are achievable. The optimised base engine represents in any case a starting point for further fuel reduction measures, like waste heat recovery (WHR) and electrifi-cation/hybridisation measures for the entire powertrain, whose cost/benefit ratio needs to be considered depending on the application.

REFERENCES[1] Schffmann, W.; Breitenberger, M. et al: Chal-lenges to the base engine structures of future MD & HD engine commercial vehicle engines. VDI confer-ence, 2012[2] Howlett, M.; Enzi, B. et al: CO2 Reduction Potential through Improved Mechanical Efficiency of the Internal Combustion Engine-Technology Survey and Cost-Benefit Analysis. SAE conference, 2013[3] Schffmann, W.; Weibck, M. et al: High per-formance and friction reduction challenge or con-tradiction? Future diesel and gasoline engine family with common architecture. 22nd International AVL conference Engine & Environment, Graz, 2010[4] Ennemoser, A. et al: Optimized operating strategy for auxilliaries in truck engines. In: MTZ 73 (2012), no. 3[5] Schffmann, W.; Sorger, H. et al: Lightweight Design, Function Integration and Friction Reduction the Base Engine in the Challenge between Cost and CO2-Optimization. 34th Vienna International Engine Symposium, 2013

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