Schaeffler Kolloquium 2014 03 En

N O D H I O E A S M I O u E N l O A N G A D F J G I O J E R u I N K O P J E W l S P N Z A D F T O I E O H O I O O A N G A D F J G I O J E R u I N K O P O A N G A D F J G I O J E RO I E u G I A F E D O N G I u A M u H I O G D N O I E R N G M D S A u K Z Q I N K J S l O G D W O I A D u I G I R Z H I O G D N O I E R N G M D S A u K N M H I O G D N O I E R N GM P B D B H M G R I B D P B D l R B E F B A F V N K F N K R E W S P l O C Y Q D M F E F B S A T A T B G P D R D D l R A E F B A F V N K F N K R E W S P D l R N E F B A F V N K F NH M G R V B F P B D l D B E u B A F V N K F N K R E W S P l O C Y Q D M F E F B S A T B B S A T A T B G P D B D D l R B E Z B A F V R K F N K R E W S P Z l R B E O B A F V N K F ND O N G a u V R N H I F G D N l S E R N G M J B N D S A u K Z Q I N K J S l W O I E P N O I E P E P N N B A u A H I O G D N P I E R N G M D S A u K Z Q H I O G D N W I E R N G M DD u C y l i n d e r N d e a C t i V a t i o n S A u K Z Q I N K J S l W O Q T V I E P N V I E P E P N Z R 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C x W Z Y K F E D I O P P M N E S W l N C x W Z Y Kl S J A D S Y K J H G F D S A Y V N P I Z R W Q S C G Z N J I M N S T R E C l P Q A C E Z R W D x A Y H A S u S V N P I Z R W Q S C G Z N J I M N S T R V N P I Z R W Q S C G Z N JE K J I C K O I J G R D C K I O P M N E S W l N C x W Z Y K F E D I O P N G S A Y B G D S W l Z u K O G I K C K P M N E S W l N C x W Z Y K F E D I O P P M N E S W l N C x W Z Y KM O T M Q O G N T Z D S Q O M G D N V u S G R V l G R V K G E C E Z E M S A C I T P M O S G R u C Z G Z M O x O D N V u S G R V l G R V K G E C E Z E M D N V u S G R V l G R V K GT N u G I N R l u J G D I N G R E x O M N Y A Z T E W N F x J l R N I F Z K M N D A B O B N x Z P E W N Q M I N E x O M N Y A Z T E W N F x J l R N I F E x O M N Y A Z T E W N F xD C O S V C E S O P M N V C S E Y l J N E W C l V V F H N V R D J K u V x E S Y M N R E I W C l O M E P S C V C Y l J N E W C l V V F H N V R D J K u V Y l J N E W C l V V F H N VM O T M Q O G N T Z D S Q O M G D N V u S G R V l G R V K G E C E Z E M S A C I T P M O S G R u C Z G Z M A x O D N V u S G R V l G R V K G E C E Z E M D N V u S G R V l G R V K GA A O R u A N D O N G I u A R N H I O G D N O I E R N G M D S A u K Z Q I N K J S l W O Z W u I E P N N R A u A H I O G D N O I E R N G M D S A u K Z Q H I O G D N O I E R N G M D

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4342

Pure EfficiencyDeveloping combustion engines from the perspective of a supplier

Dr. Martin ScheidtMatthias Lang

44 453Developing Combustion Engines

Efficiency as the primary development objective

At the end of 2013, the European Union agreed new CO2 limits. As of 2020, these specify fleet emission values of 95 grams of CO2. This figure corresponds to a con-sumption of approx. 3.6 l/100 km for die-sel vehicles and 4.1 l/100 km for gasoline-operated vehicles. These limits will be the most stringent in place anywhere in the world. It is expected that it will only be possible for premium vehicle manufactur-ers (with a virtually identical mix of vehi-cles) to achieve this limit value by partially electrifying large and heavy vehicles. The plug-in hybrid drive is set to play a signifi-cant role in electrification, as it is favored by applicable legislation.

Despite increasing electrification, engi-neers across the entire automotive industry will focus on optimizing the combustion en-gine for many years to come and for a num-ber of reasons. The most important reason is the tremendous growth trajectory that the global automotive industry can expect over the coming years. Increasing prosperity means that the number of newly registered passenger cars and light commercial vehicles will grow to around 105 million units by 2020, which corresponds to a growth of 40 % compared to 2012 [1]. Emerging econ-omies, as well as newly industrialized countries such as

Brazil, Russia, China and India, will see the majority of this growth. However, there are many first-time car buyers in these coun-tries who cannot afford the costs associat-ed with drive electrification. Therefore in these kinds of markets, automotive manu-facturers that use efficient combustion en-gines to shift electrical drive components into heavy vehicles as far as is possible will be especially successful.

The second key reason, this time for the developed markets such as in Europe and the United States, is the expectation of car buyers for standard consumption figures to be approximately achieved in real life. For plug-in hybrids, this is partic-ularly the case if the distances travelled far exceed the electric range and the ve-hicle must bear the additional weight of the electrical drive components and the battery. For instance, Volkswagen has an-nounced that the plug-in version of the Golf to be introduced in 2014 will be 250 kg heavier than its comparable gasoline en-gine version. An efficient combustion en-gine with a high weight-to-power ratio can help to fulfill the expectations of end cus-tomers in this regard.

Finally, it should be noted that the Europe-an CO2 limits are particularly strict, but international legislation aiming for similar values has experienced some setbacks (Figure 2 in [2]). From the point of view of a European supplier, there is the possibility of bringing consumption-reducing tech-nologies onto the domestic market at an early stage and thus gaining a competitive advantage on a global scale.

Both EU limit values of 130 g CO2/km for 2015 or 95 g CO2/km relate to a vehicle weight of 1372 kilograms; limit values for vehicles of different weights are calculat-ed using the straight-line method and a weighting factor. As Figure 1 shows, no manufacturer in the EU currently meets the limit value for 2015; the limit values are currently only met by segments of some manufacturers fleets. In addition, the weighting factor is reduced for the 2020 target, which represents a huge disadvan-tage for manufacturers of heavier vehi-cles. On the whole, it is apparent that all manufacturers will need to put great effort into boosting the efficiency of their vehicle fleets.

Approach to improving efficiency

The efficiency of combustion engines can only be increased if the actual engine comes as close as technically possible to the at-tainable thermodynamic optimum. There-fore, the engineers first priority must be to focus on losses that occur in actual en-gines. Losses for a typical cycle, in which the vehicles aerodynamics are not taken into account, can be seen in Figure 2.

In modern powertrain concepts, the en-gines special operating states also play a vital role when it comes to standard and ac-tual fuel consumption. This applies to tran-sient states, such as increased accelera-tion. During acceleration, engines with high weight-to-power ratios and low displace-ment exhibit relatively high deviations from their optimum operating point. One of the reasons for this is full-load enrichment, which often needs to be applied at relatively low speeds to avoid knocking combustion and protect exhaust components from ex-cessive temperatures. Other operating states that should be considered in particu-lar include engine warm-up after a cold

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EU Limit Value Curve2015 target

EU Limit Value Curve2020 target

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Figure 1 CO2 fleet consumption of vehicles sold in the EU

Source: Nizzola 34.8 % exhaust gas heat

29.4 % heat loss

4.1 % charge cycle

8.4 % engine friction

3.0 % accesories

1.0 % clutch + axle drive

0.9 % transmission

18.4 % tyres

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Figure 2 Typical power losses on the efficiency chain from tank to wheel


start, and more and more frequently situa-tions in which an engine is partially shut down (cylinder deactivation) or even com-pletely stopped (start-stop/coasting).

Furthermore, the efficiency of modern engines must not be detrimental to the en-gine-out emissions of exhaust pollutants nor should it result in reduced comfort for end customers.

Potential for efficiency enhancement

There are two ways of minimizing the losses that occur within combustion engines, and they must be initiated simultaneously: first, increase the actual combustion efficiency and, second, minimize losses, especially friction and pumping losses.

Reducing pumping losses

Pumping losses depend heavily on how much the engine must be throttled at a spe-cific operating point. Or put another way: how often can operating points with low throttling, i.e. high load at low speed, actu-ally be achieved through the transmission curves.

The combination of direct injection and exhaust-gas turbo charging to en-able this kind of operation has become established on the market. It results in high specific output that can be used for reducing eingine displacement (downsiz-ing). Engines of this type tend to be oper-ated more frequently at dethrottled map points. Cylinder deactivation has a simi-lar effect, also resulting in a higher indi-cated mean effective pressure in the cyl-inders still running and thus resulting in dethrottling.

Extensive dethrottling can be achieved by closing the intake valves early (EIVC) or

late (LIVC). Both methods reduce the effec-tive compression ratio and are also known as Miller or Atkinson cycles (Figure 3). With the valve opening times thus modified, four-stroke engines experience lower pumping losses but suffer the challenge of reduced combustibility. This effect can be counter-acted by increasing the charge motion in the combustion chamber, thereby enabling improved mixture formation and more effi-cient combustion.

Ideally, to achieve complete dethrottling, it would be possible to freely select the opening and closing times as well as the valve lift for all operating states.

Today camshaft phasers, which only allow partial dethrottling, have already be-come established on the market. Ele-ments in the valve train for deactivating cylinders continue to be used. The first applications of mechanical and electro-hydraulic fully variable valve trains are now available (Figure 4).

Camshaft phase control

Camshaft phasers are manufactured in large quantities. Hydraulic systems have taken hold, and electromechanical sys-tems are being developed at the same time. The latter provide optimum adjusting speed and variability (Figure 5). However, electromechanical systems are also more costly. With this in mind, Schaeffler is not only working towards the start of produc-tion for electromechanical cam phasing systems, planned for 2015; we are also continuously optimizing the performance of hydraulic systems.

The adjusting speed of hydraulic cam-shaft phasing units is largely determined by the performance of the oil circuit. En-gine oil pressure has been consistently lowered over the past few years to reduce the power consumption of engine oil pumps. Low oil pressure is a challenging constraint when it comes to designing new and developing existing camshaft phasers. This is because the lower the oil pressure, the less energy is available to adjust the camshaft.

Miller Atkinson

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Figure 3 Dethrottling by changing intake valve timing

Valve Timing Valve Lift

Variable Valve Train

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Figure 4 Different types of variability in the valve train


Schaeffler is therefore showcasing a phaser with a secondary oil reservoir for the first time: the additional oil reservoir is located in additional bores in the camshaft phaser ro-tor in other words, right next to the oil chambers that trigger phasing when they are filled. This tank is not pressurized, it im-proves adjustment speed by providing vol-ume that does not have to be supplied by the oil pump [3].

Switching elements

Another way to increasing valve train vari-ability is provided by switching elements that vary the lift of individual valves. These

combine cylinder deactivation with switch-ing between two discrete strokes, or allow switching between three strokes. Schaef-fler is developing a mechanical solution for 3 step switching, designed to be robust enough to meet all standard requirements regarding valve train service life.

Using a switchable finger follower, a second valve stroke can be performed outside of the specified first stroke con-tour. This enables internal exhaust gas recirculation to be performed by either pushing the exhaust gas back into the intake manifold or by re-breathing ex-haust gas by opening the exhaust valve a second time during the intake phase. Schaeffler has adapted a system of this kind for a Japanese diesel engine.

Fully variable valve train

Electromechanical or electrohydraulic fully variable valve train systems offer a high de-gree of variability, the latter are already in

volume production. The electrohydraulic systems are still driven by the camshaft. Electromagnetic systems without a cam-shaft have been the subject of research for some time, but they have yet to be intro-duced which is not only attributable to the demanding electrical power requirements. The camshaft also acts as a safety element, preventing faulty actuations and thus the valve and piston coming into contact.

In 2009, Schaeffler started volume pro-duction of the UniAir electrohydraulic valve train system. This Schaeffler system in-cludes: The electrohydraulic actuator module The software required to control valve

timing; this software is integrated in the customers engine control unit

A calibration data set for the relevant applicationSince 2009, this system has been

adapted for various production engines with capacities between 0.9 and 2.4 l, and deliv-ered in high volumes to customers in Eu-rope and North and South America.

kinds of systems are aimed, in particular, at cylinder deactivation, partial dethrottling and internal exhaust gas recirculation, and are available in a range of designs (Fig-ure 6): The simplest example merely involves

shutting down individual valves and therefore also the cylinders via a switched pivot element; these types of elements have been used successfully on the market for several years.

Switchable finger followers or bucket tap-pets are also used for two-stage lift switch-ing and therefore for partial dethrottling. By using a sliding cam system, it is even pos-sible to vary the valve stroke in three stages. 3 step cam shifting systems either

Internal EGR(Retain)

Internal EGR(Re-breath)

Crossing of Valve Events

2-Step

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Switchabletappet

Switchable roller finger follower

Cam shifting system

Electro-Hydraulic Actuated

Electro-Mechanical Actuated(Enlarged Temperature Range)

Valve Deactivation(1 Valve per Cyl.)

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Switchablepivot element

Figure 6 Switching systems for varying valve lift

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Electric VCP

Engine: V6 Gasoline

Figure 5 Adjusting speed of hydraulic and electrical camshaft phasers


UniAir not only enables continuously vari-able setting of the valve lift; it also enables largely free configuration of the valve lift event within the maximum contour speci-fied by the camshaft envelope. In this way, dethrottling is possible within broad en-gine map ranges (Figure 7). It results in a fuel consumption reduction of up to 15 % in the New European Driving Cycle (NEDC).

Future generations of the UniAir system will feature new functions which will be pre-

sented in more detail at the Schaeffler Sym-posium 2014 [4]. One noteworthy function is individual control of two intake valves. This kind of activation enables a specific charge motion to be generated (especially at low loads), thereby significantly increasing com-bustion efficiency. Figure 8 shows asym-metric valve lift curves, as enabled by indi-vidual control.

From Schaefflers standpoint, the free-dom in combustion process design af-forded by the UniAir system can be ap-plied to all vehicle segments. Low-cost engines with a small number of cylinders can benefit from increased torque, while simultaneously lowering specific fuel con-sumption. In this vehicle segment the cost/benefit ratio is far superior to other measures, such as adding exhaust gas turbo charging and direct injection. Large engines benefit especially from dethrot-tling in the part load range. New functions can also support future combustion pro-cesses that can exploit the benefits of the systems extremely fast actuating mecha-nism.

Reducing friction losses

Reducing friction lossesr has always been a crucial development objective in engine de-sign. In the past, focus was placed on inter-nal friction in the cylinder, particularly friction between the piston/piston ring cylinder pair-ing. On account of increasingly stringent CO2 legislation, all other sources of loss are now also being studied. This applies in par-ticular to Crankshaft Valve train Balancer shafts Camshaft and auxiliary equipment

drives Losses caused by operating the cool-

ant and the oil pumpIn total, these friction values account for about 50 % of the friction losses of an aver-age combustion engine (Figure 9). In addi-tion, the engine heat-up process becomes more important due to the relationship be-tween friction and oil temperature. This power loss has a direct impact on standard

consumption due to the cold start section in the New European Driving Cycle (NEDC).

The valve train is responsible for a par-ticularly high proportion of friction losses that occurs at low engine speeds. Over the past 20 years, great progress has been made in this area by means of tribological

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Figure 9 Typical power loss values of individual causes of friction over engine speed for a petrol engine

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Figure 7 Valve lift curves in different engine map ranges


optimization of bucket tappets; the friction mean effective pressure has been reduced by around 50 % (Figure 10). At the same time, roller finger followers for valve control have become established they link hy-draulic valve clearance compensators with inherently low friction.

It is increasingly common for modern engines both gasoline and diesel with high specific power ratings and few cylin-ders to be fitted with balance shafts. The friction on the shaft bearing is particularly relevant due to its high speed (double crankshaft speed in a four-cylinder engine). Switching to a roller bearing arrangement while simultaneously designing lighter com-ponents (Figure 5 in [2]) can decrease a ve-hicles CO2 emissions by up to 2 %. In a four-cylinder engine, this kind of solution can reduce the weight by approximately 0.5 kg per shaft/1 kg per system.

Significantly lower friction losses can also be achieved by supporting the cam-shaft on roller bearings (Figure 11). However, if this approach is taken, it is essential to consider the assembly concept for the cyl-inder head.

The key goal for auxiliary equipment drives is ensuring seamless functionality over the service life. Transferring ever-increasing

torques and power ratings results in higher preloads in the belt drive, resulting in in-creased power loss. At the same time, dy-namic amplitudes in the belt drive are increas-ing as engines have fewer cylinders, and higher mean effective pressures; this results in high rotational irregularities. Innovative belt tensioners and crankshaft decouplers devel-oped by Schaeffler are able to transfer the in-creased torque reliably while simultaneously minimizing any power loss [5].

More dynamics, fewer losses special operating states

Optimizing steady state engine map points alone is not an effective way of improving the overall combustion engine. On the one hand, future consumption test cycles will have higher dynamic content; on the other hand, hybrid systems, in which there is no clear correlation between driving conditions and engine operating points, are being used more commonly.

Acceleration

The dynamic response characteristic of en-gines with a high degree of supercharging can be specifically enhanced by setting a positive scavenging gradient. Extremely rapid actuation of the camshaft phaser is desirable to quickly start adjusting the valve timing as required .

Electromechanical phasers allow ex-tremely high adjusting speeds of more than 250 KW/s [6]. They also provide greater ri-gidity when torque is applied between the drive wheel and camshaft, thereby achiev-ing optimum adjusting accuracy.

In addition, electric cam phasing is the only option that allows valve timing to be select-ed as required when starting the engine. By selecting the valve timing, the engine can be started with minimal compression, which results in a low-vibration start and requires considerably less starter power. Electrome-chanical phasers are largely unaffected by temperature, while hydraulically actuated systems only provide useful adjusting speeds at ambient temperatures of +7 C to +20 C, depending on the design.

However, this high performance level goes hand in hand with increased cost. Schaeffler will put this kind of system into volume production for the first time in 2015. The crank angle adjustment range will be up to 95 in this new system. It is designed to fit to the series engine cylinder head with only small changes.

Furthermore, it is of course important to bring the turbo charger up to maximum speed as quickly as possible when acceler-ating under a full load. Two-stage turbo charger systems are increasingly being used for this purpose. In these systems, the first supercharger is relatively small and has correspondingly low inertia. The use of roll-ing bearings for turbo chargers results in significantly lower frictional losses [7] and thus shorter response times. The reduction is so great that the charger could be made larger and yet retain the same response characteristics. For certain engine power ratings, a second turbocharger is therefore no longer required and considerable cost can be avoided.

Engine warm up

The high thermodynamic efficiency of mod-ern engines also has its disadvantages: sig-nificantly less waste heat is produced, which is however needed to heat the engine, transmission and, depending on weather conditions, the vehicle interior. At the same

time, the test cycles for determining CO2 and exhaust emissions demand a cold start. To distribute the initial heat produced in an optimum manner, regarding passen-ger comfort and emissions, Schaeffler has introduced a thermomanagement module (Figure 12).

In the engine warm up phase, the mod-ule can completely shut off the coolant en-tering the engine or set a minimum volume flow. When the engine is at operating tem-perature, the coolant temperature can be regulated quickly to various temperature levels, depending on load requirements and external conditions. The component has two coupled rotary slide valves that use a single drive. The first volume engine equipped with a Schaeffler multifunctional cooling water controller is the 1.8-l TFSI engine manufactured by Audi (four-cylinder in-line engine, third generation). The module heats up the coolant at a rate that is up to 30 % faster than the predecessor engine which has a wax-type thermostat. In fact, the time required to achieve target oil tem-perature is reduced by 50 % [8].

Figure 11 Drive torques for camshafts with plain bearings and roller bearings

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Figure 12 Structure of the Schaeffler thermomanagement module with integrated water pump


Compact designs for smaller engines and vehicles and further development of func-tional integration are the focus for future ap-plications [8]. Development includes a multi-functional module with separate circuits for the engine block and cylinder head (split cooling). Schaeffler estimates it is possible to save up to 4 g of CO2 per kilometer with skilful application of a thermomanagement module. A controllable water pump is a par-ticularly good solution for commercial vehi-cles whose cooling systems are designed for hill climbs, and thus allow power reduc-tion when driving on level ground.

Engine switch off

Naturally, an engine has the lowest fuel con-sumption when it is not in operation, which is why modern vehicles increasingly switch off the engine not only during test cycles but also in real traffic situations. The expecta-tion for 2016 is that two thirds of all new ve-hicles sold in Europe will feature start/stop systems; from 2019, they will be standard for conventionally powered vehicles in most segments [9]. NEDC consumption can be reduced by up to 4.5 %. In the future cycle Worldwide Harmonized Light Vehicles Test Procedure (WLTP), the percentage of en-gine downtime decreases from 23 % to ap-prox. 13 %. This means that using start/stop does not achieve the same level of re-duction. However, the WLTP is more dy-namic overall, so that vehicle coasting func-tions gain in importance.

At its simplest, coasting or better the restart of the engine at the end of the coast-ing phase -can be achieved using a belt-driven starter generator. The development target is to be able to switch from one oper-ating state to the other with the change be-ing barely perceptible or even imperceptible for the driver. However, compared to con-ventional belt drives, high torque spikes oc-cur that make new belt tensioner concepts

necessary. The wide variety of possible concepts range from using twin mechanical tensioners to a hydraulically actuated ten-sioner. Schaefflers preferred option is a de-coupling tensioner installed on the genera-tor (Figure 13).

The function of this new tensioner is ex-plained in detail in [5].

Outlook

Using the technologies outlined in this arti-cle, the efficiency of todays already very economical combustion engines can be significantly improved. Schaeffler estimates the entire remaining potential for increasing efficiency of current volume engines to be 20 % for petrol engines and 10 % for diesel engines. However, parts of this potential have already been implemented in engines now appearing on the market.

Furthermore, consistent development of the combustion engine will yield additional potential, even if existing ideas cannot yet

be covered by technology which is ready for volume production: Complete omission of full-load enrich-

ment even on gas-operated engines with a specific output of 100 kW/l or more. In addition to fuel savings, this would also reduce engine-out particle emissions

Replacing plain bearings with roller bearings in the crankshaft drive. The fundamental technical feasibility of this application has already been proven, even if an acoustically satis-factory solution has yet to be found. Studies conducted by Schaeffler have identified potential CO2 savings of around 3 %

Cylinder deactivation that shuts down each cylinder in turn instead of always shutting down the same cylinder [10]; this prevents individual cylinders from cooling down

Schaefflers viewpoint is that engine and transmission development must be even more closely coordinated in future to fully exploit this potential. After all, efficient drives will only be a success on the mar-ket if they meet customer expectations for acoustics and vibration. In turn, the de-gree by which overall CO2 emissions from road transport can be reduced depends solely on how quickly efficient drives be-come the norm. The developers of com-bustion engines and transmissions must overcome this major challenge by working together.

Literature

[1] Gutzmer, P.; Hosenfeldt, T.: Marktgerechte Reibungs-optimierung. ATZ 115, 2013, no. 11, pp. 876ff.

[2] Gutzmer, P.: Individuality and versatility: future mo-bility paradigms. 10th Schaeffler Symposium, 2014

[3] Dietz, J.; Busse, M.; Raecklebe, S.: Smart Phas-ing: Needs-based concepts for camshaft phas-ing systems. 10th Schaeffler Symposium, 2014

[4] Haas, M.; Piecyk, T.: Get Ready for the Com-bustion Strategies of Tomorrow. 10th Schaeffler Symposium, 2014

[5] Stuffer, A., et al.: Introduction of 48V Belt Drive Systems: New tensioner and decoupler solu-tions for belt-driven mild hybrid systems. 10th Schaeffler Symposium, 2014

[6] Strau, A., et. al: Quo vadis, hydraulic camshaft phasing unit? 9th Schaeffler Symposium, 2010

[7] Kropp, M., et al.: Optimising the transient be-haviour of turbo chargers, using the example of a PC diesel engine. 13th International Stuttgart Symposium, 2013

[8] Weiss, M.: Hot and Cold: Schaeffler thermo-management for up to 4 % CO2 reduction. 10th Schaeffler Symposium, 2014

[9] Kirchner, E.; Eckl, T.: The Long Path from Dis-comfort to Customer Acceptance Start-stop systems yesterday, today and tomorrow. 10th Schaeffler Symposium, 2014

[10] Faust, H.: Powertrain Systems of the Future: Engine, transmission and damper systems for downspeeding, downsizing, and cylinder deac-tivation. 10th Schaeffler Symposium, 2014

Figure 13 Schaeffler decoupling tensioner

Documents

Schaeffler Kolloquium 2014 03 En