You will find the figures mentioned in this article in the German issue of ATZ 04I2007 beginning on page 280.

Eigenschaften von Allradfahrzeugen

Characteristics of All-wheel

Drive Vehicles

Authors:Hans Dieter Sommer, Markus Schmidand Johanna Rohracher

The picture of a 4WD sports car in full action shows only one extreme as-pect of the wide application range of 4WD-driven vehicles. This paper, however, focuses on the characteris-tics applied in everyday use. It is a well-known fact that 4WD systems provide better traction. For that rea-son, this Magna Steyr-article mainly describes the aspects of cornering behavior on low friction values, and the fuel consumption.

1 Introduction

In the course of the growing model variety of passenger vehicles– SUV, CUV, etc, the Eu-ropean all-wheel drive vehicle market sees an annual growth rate of approximately 130.000 vehicles, Figure 1 [1]. In order to maintain the positive image of the all-wheel drive the latter’s characteristics must be an-alyzed continuously and improved in con-sideration of customer requirements and within the bounds of technical and econom-ic possibilities. Below, some of these charac-teristics are dealt with in detail.

2 Characteristics

There have been innumerable discussions as to which characteristics of an all-wheel

drive vehicle are immediately perceived by the users and included in their rating of a vehicle. Figure 2 shows some of the charac-teristics that are effected most by the type of 4WD-system that is selected.

The majority of the quoted characteris-tics is extensively self-explanatory. Control behavior means the quality of the control systems with regard to efficiency as well as smooth, jerk-free operation. System availa-bility means that the 4WD-system or parts of the system can be switched off, e.g. ow-ing to overload. Driving safety includes the self-steering properties as well as the ESP-compatibility of the 4WD-system. Weight is not an issue, but fuel consumption certain-ly is. Low-noise function is not discussed either. It is a requirement that is made on a vehicle independently of its drive system. The rating and weighting of issues, of

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course, depends on the manufacturer. Safe-ty aspects will always have to be priori-tized.

3 Traction

Traction has been the original target in the development of a 4WD. Thus it is the best-analyzed characteristic of a 4WD and need not be discussed in detail. It is clear that a wheel-loads-dependent tractive power dis-tribution at constantly low wheel slips must be strived for, above all for driving on ascending gradients with transverse incli-nation and for cornering [2].

4 Driveability

A test report published in an opinion-mak-ing automotive magazine compared opera-tion and handling of twelve 4WD-vehicles in winter. The most frequently mentioned characteristics in this report can be as-signed to the term „driveability [3]. All eval-uations in this report were divided into five groups, Figure 3.

As expected, the essential 4WD-charac-teristics – traction and driveability – have been mentioned most frequently and for the most part positively. However, the han-dling has partially been described with terms such as „sluggish“ or „slow-respond-ing“. Four vehicles in particular have been found fault with for „understeering in tight curves“. A behavior discussed over and over again in connection with 4WD-vehicles. Apart from conventional maneuvers, un-dersteering of a 4WD-vehicle can be tested by means of the so-called „turn-into” maneuver and the hairpin-bend maneuver used by Magna Steyr.

5 Turn-in Maneuver

The turn-into maneuver simulates the turn-ing into a road that is covered with snow or ice. This maneuver is performed to evalu-ate the „turning-in“ behavior and the vehi-cles’s behavior during the acceleration phase from turning into the road to driv-ing on the straight road. Figure 4 shows how this maneuver is carried out in practice on a wet tile lane. The evaluation focuses on the maneuver “turning into the curve”. Fig-

ure 5 shows the simulation result for turn-ing into the curve (yaw angle speed versus steering angle).

The steering wheel angle speed is 150 deg/s at a constant speed of 15 km/h. It can

be clearly seen that the vehicle with locked transfer case (“rigid”) requires the greatest steering angle. A customer perceives such a vehicle rather as being sluggish and “slow-ly responding to steering input”. The front-driven vehicle required the smallest steer-ing angle.

In addition, Figure 5 shows the results of measurements made on the test lane. Measurements and calculation correlate well, both show the same tendency with regard to the “slow response to steering in-put “. Since the test speed was constant, the stability limit range was not taken into con-sideration.

6 Hairpin Bend Maneuver

Hairpin bend maneuvers are carried out on roads with low friction values, small curve radii and constant ascending gradients Fig-

ure 6. The quasi-stationary effect of the climbing resistance on the vehicle moving at a constant speed is an advantage because it allows to analyze the influence of differ-ent drive systems and their control algo-rithms on the driving behavior in tight curves by means of a simulation calcula-tion. In dependence of the drive system, the kinematic and drive-dependent slip can produce slip and wheel forces that in turn cause the „steering behavior“ to vary.

Figure 6 shows the calculation results of maneuvers with one and the same vehicle but different drive systems: front and rear drive, central differential open and partial-ly locked, as well as torque vectoring. The torque distribution of the central differen-tial was 40/60 at a static axle load distribu-tion of 55/45. The behavior was measured at the apex of the hairpin bend.

Up to a lateral acceleration of approx. 4.5 m/s² (μ = 0,5), the vehicle with front wheel drive required the smallest steering wheel angle input. This vehicle shows a very flat steering angle input history, which can be attributed to the yaw moment that in turn is produced by the tractive force. This means that within the range relevant to the customer the vehicle with front wheel drive is perceived as being the most agile system. Increase of lateral accelera-tion up to the stability limit range produc-es the well-known understeer effect. Rear-wheel driven vehicles require a greater steering angle. At approx. 4.5 m/s², the vehi-cle suddenly begins to oversteer.

As expected, the required steering angle input of the 4x4 „ideally frictionless“ lies between the two extremes front-wheel drive and rear-wheel drive. However, the

maximum lateral acceleration of the 4x4 vehicle is considerably higher. Owing to the friction, each transfer case is locked to a certain degree. The resultant wind-up causes the vehicle’s „slow response to steer-ing input“, which results in a higher steer-ing angle input requirement and is thus noticed by the driver.

To show the potential of an “ideal 4WD-vehicle”, a system with torque vectoring was simulated. The drive torque distribu-tion between front and rear axle is infinite-ly variable. The rear axle is equipped with a superposition unit, which was proposed by Magna Steyr and allows torque superposi-tions of 900 Nm Figure 7, [3].

This allows of producing a yaw moment that is used for a yaw speed control: an equivalent control system was already on-vehicle-tested by Magna Steyr [4]. This 4WD-system reduces the steering input require-ment and allows of a high maximum lat-eral acceleration.

Particularly remarkable are the compa-rably softer transitions in the stability limit range, which is the reason for the feeling of safety the 4WD-vehicles convey. A drive sys-tem mimimizing the frequency of brake intervention to maintain driving stability and traction is, of course, advantageous with regard to comfort and wear. In this context, the hybrid drive offers new possi-bilities. If, for example, the rear wheels are driven by electric motors, targeted torque distributions at the rear axle can further influence the characteristics of the vehi-cle’s dynamics.

7 Fuel Consumption

Talking about the 4WD means that soon the issue of increased fuel consumption comes up [5]. Figure 8 shows an overview of the increased fuel consumption of 27 4WD-vehicles compared to the consumption of the 2WD basis vehicles. Provided that data in accordance with EU standards were en-tered, gas- and diesel-powered passenger vehicles of all displacement/weight classes were taken into account. The consumption was measured during “mixed urban/inter-urban operation”.

A simulation model was used for con-sumption calculations. The results allow to evaluate the effects of weight, rotational masses and degree of efficiency. A vehicle with a gasoline engine and a manual trans-mission was modelled. In contrast to the 4x2 model, the 4x4 model’s torque distri-bution was lossy. It was possible to install the 4x4-rotational masses in the 4x2 vehi-

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DEVELOPMENTAll-wheel Drive Vehicles

cle in order to pinpoint the effect of the increased weight and the additional rotat-ing masses. The other vehicle features, such as motorization, transmission, tire charac-teristics, aerodynamics, etc. were identical.

The two main influential factors for in-creased consumption are the additional weight and the 4WD’s degree of efficiency. The effect of the additional rotating masses is comparatively small. A weight increase of 5% of the 1.800 kg vehicle results in an average consumption increase of approx. 7.5%. 3.1% of this increase are caused by the additional weight, 4.4% by the 4WD-system itself Figure 9.

Furthermore, in order to compare the simulation results, the increased consump-tion was determined on the basis of the catalog data of three vehicle series of one vehicle manufacturer and entered on the diagram in Figure 9. For that purpose the consumption of convertibles, sedans, sta-tion wagons and – as far as available – their 4WD versions was compared. All vehicles are equipped with the same basis version of engine, displacement and drive system. Effects caused by different engine applica-tions, measurement procedures, air drag coefficients, etc., cannot be excluded. Nev-ertheless, the results are well comparable with the simualtion data.

However, the additional weight cannot always be attributed to the weight of the specific drive components – which is cur-rently tried to be reduced – but also to the number of additional components neces-sary for a 4WD-version. Taking these factors into consideration during concept and de-sign activities for the basis vehicle could have a weight-reducing effect.

Figure 10 shows the characteristic field of a transfer case determined by Magna Steyr on the basis of simulation calculations. The improvement potentials are obvious and universally valid: friction losses at low speeds and churning losses at higher speeds. In the course of advanced develop-ment, studies on the calculability of the oil flow are carried out in order to influence and optimize the oil flow losses in the transmission Figure 11, [7]. Regarding opti-mized bearings and reduction of losses by means of low-viscosity oils, refer to the rel-evant literature [8,9].

Since 4WD-vehicles are equipped with a number of propshafts, the bending angles should be as straight as possible to mini-mize the loss. Owing to the limited produc-tion number, optimization activities for 4WD-components could entail cost-benefit-optima that differ from those of high-vol-ume production transmissions.

8 Cost/Benefit

There is no such thing as the „best 4WD. It is always a question of which system for a vehicle or vehicle series results in the best cost-benefit ratio [10]. A major aspect is the issue of which brand-specific characteris-tics should be emphasized and which pos-sibilities the vehicle concept provides. If the 4WD-characteristics of Figure 2 are evaluated according to criteria of different vehicle brands, the evaluation of the same 4WD-systems results in different cost-bene-fit ratios. As an example, Figure 12 shows the subjective evaluations of characteristics of 4WD-components on a price-proportion-al chart.

In one case, all characteristics have been taken into consideration, in the other case only the driving behavior and agility. The evaluations were made with having drive systems for high-class vehicles in mind. When focusing on driving behavior and agility, the particular characteristics of torque vectoring come to the fore: the vehi-cle’s agility on high friction values can be improved, just to name one possibility. These improvements, however, must be weighed against all other characteristics.

Depending on vehicle type and target customer, individual characteristics of off-road vehicles and high-performance sports cars will have to be emphasized, which again could shift the cost-benefit ratio. The decisive factor is not only the drive compo-nent itself but to an increasing extent the quality and integration degree of the 4WD-controller in the chassis controller and of the 4WD-system in the complete vehicle system. This is the only possible explana-tion for the fact that the majority of today’s 4WD-vehicles are in the top-quality range despite their different drive systems.

9 Outlook

The development of future 4WD-systems is driven by the following issues:– agility and driving stability– hybrid drive and fuel consumption– cost pressure.Today, vehicle-dynamic agility is one of the essential vehicle characteristics. This means that the number of 4WD-vehicles with torque vectoring will increase. The main focus will lie on systems integrated in the rear axle [4], Figure 7. The driving stability improved by means of this technology can also be used for comfortable spring-absorb-er tuning in vehicles with a higher center of gravity.

Regarding the reduced fuel consump-tion of a hybrid drive, parts of the addition-al weight can be invested in a 4WD drive. In dependence of the application, this addi-tional weight of a 4WD will be more than compensated by the hybrid system.

Above all in vehicles with longitudinal engines, the weight will be reduced by inte-gration of main and transfer case [9]. Auto-matically-controlled, ESP-compatible drive coupling will gain in importance, above all for small vehicles already under enormous price pressure. However, all systems will have in common an optimized integration and matching – above all of the electroni-cal systems – in the system “complete vehi-cle”. This is the only way to entirely utilize the potentials inherent in the various 4WD systems. Which systems the individual brands will implement depends on the spe-cific customer potentials and the cost-bene-fit considerations based on these poten-tials.

References[1] Lanzer, H.: Permanenter und zuschaltbarer Allrad-

antrieb und systembedingte Leistungsverzweigung,

Kraftfahrtechnisches Kolloquium, Sommersemester

1985,RWTH Aachen

[2] Stappen, H.-J.: Schnee-Kanone, Auto Motor und Sport

(Dezember 2004) Heft 1

[3] Kassler, H., Bichler, M.: Ausgeführte Triebstrang-

systeme zur Fahrdynamik-Beeinflussung, 4. Interna-

tionales CTI-Symposium, Berlin, Dez. 2005

[4] Schmid, M., Eitzinger, St.: Analyse zweier aktiver

mechatronischer Systeme zur Verbesserung von

Agilität und Stabilität mittels Simulation, 6. Grazer

Allradkongress 2005

[5] Weber, Th.: Traktion und Mobilität garantiert. In:

ATZ/MTZ Extra „Die neue 4Matic in der S-Klasse

von Mercedes-Benz“, September 2006

[6] Katalog der Automobil Revue, 2006

[7] Moshammer, T.: Simulationstechnische Optimierung

der Getriebeölversorgung zur Minimierung der Füll-

menge und Reduktion der Wärmeentwicklung, Sym-

posium 2005, Triebsysteme in der Fahrzeugtechnik,

FH Joanneum GmbH, Graz

[8] v. Petry, G.: Kraftstoffersparnis durch maßgeschnei-

derte Lager für Achsgetriebe von BMW. In: ATZ (2004)

Nr. 12

[9] Greiner, J.: Komfortabler Antrieb für entspanntes

Fahren. In: ATZ/MTZ Extra „Die neue S-Klasse von

Mercedes Benz“, Oktober 05. ALLRAD!

[10] Sommer, H.-D.: Schwerpunkte bei der Entwicklung

von Sportutility vehicles. In: ATZ 103 (2001) 2

[11] Pichler, P.: 4x4 Hybridmodule mit elektronischer

Vorderachse, VDI-Tagung Elektronik im Kraftfahrzeug,

6.-7. Okt. 2005

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