Manual transmission when accelerating

Drive topologies

Drive topologies

Dominik Demetz

Abstract —Electric vehicles can, in comparison to vehicles with internal combustion engines, do without differential gears and manual transmissions, have machines built into the wheel and drive the wheels selectively. This document describes these drive concepts that are intended to change the automotive world. The electrification of the drive train is explained step by step: At the beginning, the components of the drive train of a conventional internal combustion vehicle are described. Building on this, the central drive with and without gearbox, the FRID, the wheel-near drive and the wheel hub drive are explained. The efficiency, safety aspects and general advantages and disadvantages are dealt with for each topology. The whole thing is concluded with an overview that compares the drive topologies with one another.

Index Terms —Drive topologies, electric vehicle, central drive, FRID, wheel hub drive, drive close to the wheel.

I. INTRODUCTION

When designing the drive train of a purely electric vehicle, there are many new possibilities compared to the combustion engine. Since electric machines consistently deliver approximately the same torque over the entire speed range up to their rated output, gears are no longer necessary [1]. In addition, electric machines weigh less and take up much less volume than combustion engines [8]. In addition, most connections and couplings are electrical components. Compared to mechanical components, these can be placed flexibly. In a conventional vehicle, the gearbox must be directly on the axle (coupling: axle), but the converter in an electric vehicle must not (coupling: cable).

Because of this flexibility in the positioning of the components, the precise control and the affordability of electrical machines [11], con fi gurations of several machines are possible compared to "combustion vehicles" [3]. For these reasons, electric vehicles are referred to as drive types. These differ mainly in the mechanical components used and in the number and arrangement of the machines in the drive train.

II. CENTRAL DRIVE

The central drive is the most common drive in motor vehicles [2]. In a conventional internal combustion engine, it consists of an engine, a clutch, a manual gearbox and an axle gear with differential [1].

engine: The motor drives the shaft in the middle. A starter helps the internal combustion engine start because it does not work at low speeds.

coupling: The clutch couples or decouples the engine from the rest of the drive train

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Fig. 1. Drive train of a conventional combustion vehicle [19]

Manual transmission: A gasoline or diesel engine does not run over the entire speed and torque range with the same efficiency. This is illustrated on the consumption map as shown in Fig. 2. The contour lines describe the consumption at a certain speed and a certain power. The red curve describes the maximum power. A gearbox is used to operate the engine as close as possible to the most efficient operating state, both at low and high drive speeds, through suitable implementation [1]. Cogwheels convert the engine speed into

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Fig. 2. Consumption map of the Smart Forfour three-cylinder diesel engine [20]

Drive speed [12]. The power remains constant with the transmission, torque and speed change:

P. = M * n (1)

The torque is reduced by increasing the speed. By changing the gears and thus the gear ratio, a lower speed and more torque (e.g. when accelerating) or a higher speed and less torque (e.g. constant driving on the motorway) can be set as required. Instead of a manual gearbox there is an automatic gearbox or a continuously variable gearbox in some vehicles, but these basically fulfill the same function.

Differential gear: A differential gear is constructed as shown in Figure 3. When cornering, the outer wheel has to cover a greater distance than the inner one. The differential gear compensates for this mechanically by making the outer wheel run faster and the inner wheel slower. On the other hand, the torque is distributed 50 to 50 on both wheels with the same grip [10].

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Fig. 3. Internal structure of differential gear [21]

In the case of central drives, a distinction is made between rear-wheel drive, front-wheel drive and all-wheel drive. With rear-wheel drive, the engine is located in the rear of the vehicle and drives the rear axle. With front-wheel drive, the engine is in the front and drives the front axle. The same as for front-wheel drive applies to all-wheel drive, but the rear axle is also driven so that the drive force is distributed to all wheels.

Most electric vehicles today have a central drive [2]. A distinction is made between 2 forms: central motor with manual gearbox and central motor without manual gearbox.

A. Central drive with manual transmission

The gearbox enables the electric machine to be operated in the usual speed ranges (0-12000 rpm); that is, standard electrical machines can be used. The manual transmission also allows the speed and torque to be adapted to the current driving situation.

The main disadvantage of the gearbox is its efficiency. Depending on the load point, this fluctuates between 80% and 95%. Other disadvantages are: when accelerating, the inertia of the transmission parts of unused gears has to be overcome and the transmission increases the total weight of the vehicle [2]. Since electric machines are much quieter than combustion engines, the manual transmission dominates the background noise [3].

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Fig. 4. Central drive with gearbox [2]

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Fig. 5. Central drive without gearbox [2]

B. Central drive without gearbox

In the case of a drive train without a differential gear, the electric machine drives the differential with a fixed reduction. Depending on the type of machine, higher losses and negative acoustic in fl uences can occur at high speeds [8]. The efficiency of the reduction gear is 93-98%, so it is better compared to the manual gearbox (80-95%).

An example of a central motor without a manual gearbox is the Nissan Leaf [24].

III. FRONT AND REAR WHEEL INDEPENDENT DRIVE (FRID)

The FRID consists of 2 machines: one at the front and one at the rear. The differential gear remains intact on both axles. The advantage compared to the central drive and the individual wheel drives is the safety offered by this system. In the event of a fault in the electronics and consequent failure or negative torque of the machine, the vehicle can still move on without sudden stops (Fig. 6). There is also no risk of the vehicle suddenly changing direction of travel, as is the case with individual wheel drives (Fig. 7).

In addition, the FRID offers more ef fi cient acceleration and deceleration by adapting the torque to the axles. Better driving behavior can also be achieved on icy roads by simultaneously checking the slip ratio of the two axles [6].

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Fig. 6. Faults in a drive with a machine [6]

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Fig. 7. Error in a drive with several individual wheel drives [6]

The disadvantage with 2 machines is that the drive train is mostly used twice; That means: more costs, more space and a higher total weight.

Famous example from everyday life: the Tesla Model S P100D has a FRID drive with asynchronous machines and reduction on both axes [13]. The Audi E-Tron and the upcoming Mercedes EQC are also based on the FRID with asynchronous machines [14] [15]. The Jaguar I-pace, on the other hand, has a FRID with permanent magnet synchronous machines [16].

IV. NEAR WHEEL DRIVE

Compared to the central drive and FRID, the differential gear is not required for the drive close to the wheel. Instead, 2 individual motors are used for the driving axes (Fig. 8). This individual wheel drive enables fl exible, electronically controlled torque distribution on all wheels (torque vectoring) and thus improved and more ef fi cient driving behavior [3]. This is mainly relevant in the curves, where the angle can be reduced and more grip can be achieved. Torque vectoring also offers that

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Fig. 8. Drive close to the wheel [2]

Possibility of personalized driving modes: By specifically distributing the torque, the vehicle can be given how much it is under or oversteering. This function is built into the Rimac Concept One [9]. By eliminating the differential with efficiencies between 92% and 98%, the overall efficiency increases. In addition, the following applies: Nowadays, driving dynamics controls consist of targeted braking interventions on the individual wheels. This kinetic energy could be recovered by electrical control or not used at all. For this reason, components that take care of driving dynamics control in the central motor could be simplified or even replaced in the individual wheel drive [8].

However, since there is still a reduction gear, the torque and speed are comparable to those of the central motor, but the power is distributed to all motors [2]. The division of the motor creates greater flexibility in the arrangement of the components. Disadvantages of this topology are: more space, more weight and higher costs. Two machines take up more volume and require more power electronics than one machine on the same axis [2].

Example: Mercedes-Benz SLS AMG E-CELL [17].

V. HUB DRIVE

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Fig. 9. Wheel hub drive [2]

Since an electric machine with the same output is much more compact than an internal combustion engine, it is possible to install motors directly into the rims. With these wheel hub motors, the driving performance is therefore directly on the wheel (Fig. 9). It will be between Wheel hub motor with reduction gear and Direct drive differentiated. Compared to motors close to the wheel, both topologies have the advantage that there are no shafts to the wheel and thus less inertia has to be accelerated. Less inertia means reduced reaction times and higher dynamics [4].

A special form is that Wheel Hub Motor, in which, in addition to the engine, an attempt is made to build the entire drive train into the wheel. This means that a vehicle-independent platform can be created or more interior space can be gained, which opens the door to new vehicle concepts such as the Local Motors ”Olli” [18]. Mainly city vehicles, where interior space and ef fi ciency are more important than driving dynamics, could bene fi t a lot from this system. An example of such a wheel hub motor is the Protean Pd18 (Fig. 10).

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