14 the Engine

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Schae ffl er SYMPOSIUM 2010

14 Combus t on engine

204 Schae ffl er SYMPOSIUM 2010 205

14Combus t on engine

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205Schae ffl er SYMPOSIUM 2010Schae ffl er SYMPOSIUM 2010204

The engineUnderstanding it in its en t rety

Kurt Kirsten


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Introduc t onIn the past, the development of automobiles andtheir drive systems was influenced and pushedforward by the application of the latest technol-ogies in many cases. This meant it was possiblefor market participants to strengthen their posi-tion or the position of their product brandsby direct differentiation in a generally growingmarket.

The current market scenario is in uenced by:

Product life cycles which are con t nuously beingshortened

Increased segmenta t on

High cost pressure

Demanding customers expecta t ons

High social expecta t ons

The situa t on is also characterized by:

Increased legal requirements

A growing percep t on about the limita t ons of resources

Signi cant overcapacity of produc t on

Diff erent regional and market-segment-speci crequirements

The sum of all these requirements and challeng-es requires a holistic approach so that addedvalue can be achieved with new technologiescompared with current volume production prod-ucts. The development of added value can affectcustomers, society, legislators or producers.Mastering or controlling this complexity is theactual challenge of the present time.

Core issues forcurrent drivesystem developmentThe core issues for passenger car dri ve system de-velopment are the con ict ng aims of reducingfuel consump t on (CO 2 emissions) on the onehand and limi t ng pollutant emissions on the oth-er hand (Figure 1).

Boundary condi t ons such as costs, brand image,driving pleasure, comfort, noise and reliabilitymust also be taken into account.

Gasoline and diesel engines are posi t oned com-pletely di ff erent with regard to the con ict ngaims of fuel consump t on and emissions.

The gasoline engine is positioned very much inthe low-emission cat-egory due to its veryefficient aftertreat-ment of exhaust gas-es. The diesel engine,on the other hand, ispositioned very muchin the low consump-tion category due toits favorable, thermo-dynamic efficiencyand advantageous lowend torque character-istics. The proceed-ings of this article re-fer mainly to thegasoline engine.

Figure 2 shows a lossdistribu t on analysis re-la t ng to the NEDC us-ing a gasoline engine as

reference. The star t ng points for making improve-ments are in the area of variability in the camsha f drive. These are:

Camsha f phasing units

Par t ally-variable valve train systems

Fully-variable valve train systems

Due to the variability in the valve train, the ther-modynamic process is in uenced in the area of thelow-pressure process so that there is a posi t ve ef-fect on the pumping losses and the combus t onprocess is op t mized in the area of the high-pres-sure process. These interven t ons also have a di-rect in uence on the forma t on of emissions in the

internal combust

on engine and are, therefore,parts with direct relevance for engine out emis-sions.

The mechanical improvements refer to the mini-mization of friction losses and the reduction of parasitic losses of the accessory drives. The or-der of the mechanical losses is around 10 % to12 % of the fuel energy used. I.e. detailed opti-mization with an improvement of 10 % to 20 %with regard to mechanical losses generates a to-tal contribution of around 1 % to 2 % in the driv-ing cycle.

ThermodynamicimprovementsThe signi cantly greater poten t al for improve-ment is due to the reduc t on in the thermodynam-ic losses. The thermodynamic improvements aretargeted at enabling thro le-free load opera t on.Instead of controlling the intake normally by meansof a thro le valve, control of trapped fresh air istransferred to the valves. The required pumpinglosses are reduced due to the increase in the intakemanifold pressure. The required control of trappedfresh air is primarily undertaken by varying theopening t me of the intake valve or by the design of the valve li f curve in the form of the cam contour.

At the same t me, the charge mo t on can be direct-ly in uenced by the use of variable valve controlsystems in the combus t on chamber. The forced in- ow mo t on resul t ng from the mo t on of the pis-ton is converted into a swirl or tumble mo t on dueto the design of the intake ports. If the valves openat di ff erent t mes, the in-cylinder ow can also bestrongly in uenced. In conjunc t on with the posi-t on of the spark plug and the general layout of thecombus t on chamber, this opens up opportuni t esfor a wide range of op t miza t on measures.

Targetrange

Emissions target

CO2 target

Emissions(HC, NOx, par culates)

u e c o n s u m p

o n

Gasoline

DieselHybridiza on

Transmissiontechnology

Figure 1 Fuel consump t on/emissions

100 % energyused

78 % enginelosses

22 % effec vepower

35.3 % exhaust gas heat

30.0 % heat loss

8.5 % engine fric on

4.2 % pumping losses

4.1 % accessories +mech. losses

7.0 % acc. resistance

6.9 % air resistance

4.0 % rolling resistance

NEDC related

Efficiency chain of modern gasoline engines

Mechanicalimprovements

Fric on Damping NVH

Reference: Prof. Leohold, U. Kassel

Thermodynamicimprovements

Camsha phasingunit

Valve trains- Par ally-variable- Fully-variable

Figure 2 Gasoline engine e ffi ciency chain in the NEDC


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The percentage of residual exhaust gases which re-main in the cylinder can be directly in uenced bythe design of the valve overlap or the phasing of the valve opening. The temperature of the chargemass at the start of the compression stroke can bein uenced by adding hot residual exhaust gas di-rectly to the fresh mixture. This also enables thetemperature at the end of the compression stroketo be in uenced indirectly. This variability opensup a new alterna t ve method of carrying out op t -miza t on for modern autoigni t on combus t on sys-tems.

The possibili t es for in uencing the thermodynam-ic process can be described as follows:

Low-thro le opera t on under part load (chargecycle)

Control of trapped fresh air (load control)

Incylinder charge mo t on

Percentage of residual exhaust gases in thecombus t on chamber

Temperature at the start of the compression stroke

There are di ff erent requirements for op t mizing en-gine characteris t cs (Figure 3) depending on theload and speed:

A Longer intake and exhaust valve li f events withgreater valve overlap are desirable in the fullload range at nominal speed, in order to en-sure that the engine can breath freely (laterclosure of the intake valve).

B The intake and exhaust valve opening t messhould be shortened to op t mize the volumet-ric effi ciency in the low-speed and high-loadrange. The valve overlap must also be reduced

compared to the overlap at the nominal speed.C The pumping losses should be reduced in the

center area of the engine map by early closingof the intake valve.

D Exer t ng an addi t onal in uence on the in-cylin-der ow and maintaining the temperature of the charge composi t on are required in thelow-load range in order to posi t vely a ff ect thecombus t on process. Cylinder deac t va t on canunlock further poten t al.

E For star t ng the engine, separate measures forensuring the highest and most e ff ec t ve com-pression ra t o are also required for improvedstar t ng behavior. A limited charge throughputby means of reduced valve li f or closed valveshas proved reliable for enabling easier restart-ing with stop-start func t onali t es.

Division of variability in thevalve trainThe following are used to characterize variability inthe valve train (Figure 4):

Phasing of the valve event

Dura t on of the valve event

Maximum li f of the valve event

In Figure 5, variability is divided with regard tophase and valve li f and according to the character-ist cs of discrete or con t nuous adjustment. Thelevel of variability increases from le f to right in thediagram accordingly.

Figure 6 shows a sta t onary data map with four op-era t ng points as an example to show the in uence

of dethro ling on the engine opera t ng behavior. Itcan be seen that the in uence of dethro ling bymeans of di ff erent measures

Intake phase adjustment only

Intake and exhaust phase adjustment

Double phase adjustment & variable valve li f

is par t cularly pronounced in the lower area of thedata map and can amount to a fuel consump t onsaving of up to 12 % at a sta t onary opera t ng point.

The poten t al for improving fuel consump t on inthe driving cycle is between 4 % and 6 % comparedwith an engine with a standard valve train, de-pending on the variability selected.

The control concept and the dynamic responsebehavior are particularly important for the

transient operatingbehavior of the sys-tems. Reliable recog-nition of the currentoperation mode in in-dividual cylinders isof particular impor-tance for controlling

the air, fuel and igni-tion paths. Therefore,dynamic valve trainsystems also unlockgreater potential forfuel consumption sav-ings in the driving cy-cle.

DCombus on op miza on(Charge mo on)

C

Op miza on of pumpinglosses

Op miza on of pumpinglosses

B

Max. torque Maximum volumetric efficiency Early closure (short valve event)

A

E

T o r q u e

Engine speed

Combus on op miza on

Max. power Full li Late closure

(long valve event) Greater overlap

Figure 3 Diagram showing requirements for engine characteris t cs

LiDura on of valve eventPhasing

Figure 4 Division of variability in the valve train

Discrete

Two-step

Switchable tappet Pivot element Finger follower Shi ing cam lobe Roller tappet

Three-step Finger follower Shi ing cam lobe

Valve train

Valve liPhase adjustment

Con nuous

Electric

Mechanical Valvetronic

Electrohydraulic UniAir

Con nuous

Hydraulic

Electromechanical

Figure 5 Level of variability in the valve train


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0

1

2

3

4

5

6

Engine speed in 1/min0 1000 2000 3000 4000

Engine map

Thro led, no valve train variabilityVariable intake cam mingVariable intake and exhaust cam mingThro leless, combined camsha ming and variable valve train

M e a n p r e s s u r e i n

b a r

85

90

95100

105

B S F C i n %

85

90

95100

105

B S F C i n %

85

90

95100

105

B S F C i n %

85

90

95

100

105

B S F C i n %

85

90

95

100

105

B S F C i n %

Figure 6 Improvement in fuel consump t on by means of dethro ling

Design examples andthe in uence of variablevalve trains on the engineopera t ng behaviorFigure 7 shows an overview of current, variablevalve train systems and their market launches.

The following are used to evaluate the di ff erentsystems:

Fuel consump t on inthe driving cycle

Control concept

Dynamic responsebehavior

Timing characteris t c

An evalua t on of the in- uence of valve trainvariability on the com-bus t on process is con-ducted on the basis of

the following

ve criteria:Pumping losses

Percentage of residualexhaust gases

Temperature at thestart of the compress-ion stroke

Charge mo t on

Trapped fresh air

Figure 8 shows evaluated design examples using aspider diagram. The lling capacity of the spiderdiagram also increases with an increasing level of

variability.

In summary, the following can be noted in view of the thermodynamic improvements:

Variability in the valve train is not only relevantfor reducing pumping losses, but also assurespoten t al for reducing fuel consump t on andemissions during combus t on (especially withdirect-injec t on gasoline and diesel engines).

Downsizing and downspeeding increase therequirements for the basic layout of the drivetrain.

Stop-start func t onali t es change the load pro lefor t ming drives.

Fast and cycle by cycle variabili t es make use of the full poten t al in the transient opera t ngmode, increase the comfort of stop-startfunc t onali t es and improve the star t ng pointsfor hybridiza t on.

A good t ming drive is more than the sum of good individual components, but a comprehensiveoverall design.

Mechanical

improvementsAs already noted in Figure 2, the mechanicalimprovements refer to the friction and parasiticlosses of the accessories. Overall, the followingoptimization criteria must be taken into ac-count:

Frict on

Damping

NVH behavior

(Ford, BMW,VW, Audi, GM,Fiat, Opel,Porsche, Ferrari,SAIC, Chrysler,Volvo, )

per cylinder bank per cylinder bank per cylinder per cylinder bank

20092006

two-step(poss. three-step)

(Porsche, Honda,)

two-step

1997

con nuous

slow

approx. 4 %

(FIAT)(BMW, PSA)(Audi, )

medium

1989, 1999, ...

slow

In volumeproduc onsince:

Characteris c:

Dynamicresponse:

Control:

Fuel savings*:

* NEDC related, rela ve to standard valve train

approx. 7 % approx. 8 % approx. 8 %

con nuous

slow

2001

con nuous

fast

valve by valvecylinder by cylinder

approx. 8 % - 15 %

Camshaphasing unit

Camsha phasingunit + electro-hydraulic tappet

Camsha phasingunit + shi ing camlobe + electronicactuator

Camsha phasingunit + electro-mechanical sha

Electrohydraulicsystem(intake only)

Figure 7 Comparison of di ff erent, variable valve train systems

Pumping losses

Percentage of residualexhaust gases

Temperature at the start of thecompression strokeCharge mo on

Trapped fresh air

Camsha phasingunit

Camsha phasingunit + electro-hydraulic tappet

Camsha phasingunit + shi ing camlobe + electronicactuator

Camsha phasingunit + electro-mechanical sha

Electrohydraulicsystem(intake only)

Figure 8 Evalua t on of di ff erent variable valve train systems


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Along with the chal-lenge of transmi ngthe drive power, thereare further require-ments for the basic de-sign of the componentsin the t ming and ac-cessory drive:

Preloads should be

kept as low aspossible (low bearingloads, low fric t on)

Noise genera t on inthe t ming driveshould be minimized

Dynamic peak loadsshould be avoided

Figure 9 shows design examples.

The poten t al improvement can be derived fromFigure 2. The percentage losses due to mechanicalfric t on in the engine and the drive power for ac-cessories in the NEDC are approximately 12 % to

13 % of the primary energy.

Figure 10 shows the components which the Schaef- er Group supplies as typical engine componentsand modules. It can be seen that most parts arecomponents which are subjected to a sliding or ro-tary mo t on.

The minimiza t on of losses is par t cularly impor-

tant. In addit

on, these components are elements of vibratory systems, which can only be opt

mizedto a limited extent as individual components andmust, therefore, be op t mized in a total system ap-proach.

Figure 11 shows this rela t onship using the exam-ple of a chain-driven camsha f of a four-cylinderengine. It is assumed in the complete dynamicssimula t on that excita t on is ini t ated in the crank-sha f plane, and considera t on is given both to thebehavior of the

chain

chain blades

hydraulic tensioner

and also the valve actua t on components on thecamsha f side

nger follower

valve spring

valve

and, therefore, represents a complete systemsimulation. The challenge to come up with anoptimized system solution is to find the righttrade-off between reduced friction and ensurethe necessary level of damping. This requires thedevelopment of calibrated and validated simula-tion models which model the overall relation-ship.

SummaryFigure 12 shows an overview of the poten t al formaking individual improvements to current inter-nal combus t on engines. It can be seen that in thecase of diesel engines there is only minimal poten-t al for improvement by means of thermodynamicmeasures. Improvements of 10 % to 12 % can s t llbe achieved for gasoline engines.

In addition, an overall potential of 3 % to 5 % canbe unlocked by initiating mechanical measuresin the engine. The potential due to stop-startfunctionalities, downsizing and thermo man-agement round out the overall potential for im-

provement.With the modular range of components and engi-neering services on o ff er, Schae ffl er Engine Sys-tems is well equipped to make a contribu t on to-wards unlocking poten t al for improving fuelconsump t on.

Bank model complete with chain:EVT comparison (5000 1/min)

Valveaccelera on

Camshaming angle

Valveaccelera on

Camshaming angle

Figure 11 Complete system simula t on

Timing drive Camsha Balancer sha Injec on pumps

Drive / power transfer Accessories (FEAD)

Figure 9 Accessory and t ming drives of internal combus t on engines

4 6 %Thro ling lossesGasoline

2 3 %Fric on reduc on

5 8 %Downsizing

1 2 %

Thermal management

Diesel < 3 % Gasoline < 7 %Combus on systemop miza on

3 5 %

Stop-start Func on

Further improvements

ThermodynamicimprovementMechanical improvement

1 2 %Demand-controlledaccessories

Figure 12 Overview of the poten t al for improvements

Variable camsha phasing systemTappet

Finger follower+ pivot element

UniAir

Timing drive

Accessory drive

Balancer sha module

Figure 10 Typical components for engine applica t ons

14 the Engine

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