As an example, the especially interesting innovation potential electromechanical tractor transmission will be examined in more detail.

Generally clients and the market are increasingly demanding variable transmissions for a further increase in productivity, as a component of precision farming or even for newly developed applications⁶. These transmissions are also robust, need little maintenance and have a favourable degree of efficiency.

Currently, there are four well-known solutions for this problem. The were each awarded a gold medal on the fair Agritechnica 97. A further solution is an electromechanical transmission. This solutions results from the obvious application of new technological developments from mechanical engineering and electronic technology.

The area of favourable applications of electrical drives has been greatly expanded mainly due to developments in the past ten years (figure 5). Amongst others, these are:

- The extensive expansion of the speed regulating range due to the development of high performance frequency converters with switching frequencies up to 50kHz.

- The remarkable expansion of the constant power area due to the application of field-oriented control with rotor position feedback and field damping.

- The reduction in size due to optimisation of the current flow and the minimisation of losses using simulations, calculation using the finite-element method and the improvement in materials as well as

- The development of suitable control and monitoring software.

At the same time there occurred a distinct cost reduction due to larger production numbers and the resultant falling fixed costs. In the case of several technical problems this development already led to the substitution of complex mechanical transmissions and controls by mechatronic solutions (figure 6). In the same way the hydraulic drives often used in diesel-fuelled railtrack vehicles have been nearly completed replaced. A similar process can be found in the machine tool industry. Here, the spindles are currently mainly driven directly by electric motors and no longer using complex, mechanical adjustable mechanisms. Industrial trucks such as fork-lifts and tow-trucks also increasingly use electrical instead of hydraulic transmissions. The only area which currently does not apply electrical transmissions in the passenger vehicle area, due to the low performance density, i.e. due to the size and weight, and is currently only sensible for niche products such as hybrid busses and battery-powered vehicles. However, this could change in the future when fuel cells or the electrical split-transmissions currently in development are marketable for passenger vehicles. In contrast, the substitution of mechanical or hydrostatic split transmissions in agricultural tractors must already be considered in earnest; this is the area where there is a distinct congruence between the special requirements on the product and the characteristics of electrical transmissions.

It is common knowledge that tractor transmissions are more expensive than comparable industrial and passenger vehicle transmissions due to their size, the transmitted forces and their positioning ranges (figure 7).The trend towards power shift or hydrostatic split transmissions leads to further cost increases. However, continuous cost reductions for electrical transmissions lead us to expect long-term cost advantages. System caused disadvantages such as a slightly larger installation area and higher weight must not necessarily be negative. Regarding the degree of efficiency and the areas of application electrical transmissions are at least equal to load-switched or hydrostatic split transmissions. In fact, they have several advantages (figure 8). These are displayed by:

- variable, frictional transmission from standstill and through slow speeds up to maximum speed,

- gentle, jolt- and wear-free starting, braking and reversing, even under maximum load,

- free programmability for automatic drive, driving in the optimum operating range, driving using cruise-control, transmission of the maximum pulling load, use of maximum performance etc.,

- no need for starter motor, generator, constant water pump, belt-drive, thermostat, fly-wheel, clutch, operating brake, reverse and (power) gearbox,

- ideal motor power take off-shaft operation with electronic speed control at variable driving speeds due to almost complete de-coupling of engine and gearbox speeds,

- additional availability of the maximum engine power in the form of three-phase rotary current as well as

- low operating costs due to simple construction with near maintenance-free components and a high total degree of efficiency.

The basic form of an electrical tractor transmission is shown in figure 9. In this case a combustion engine drives a generator and the power take off-shaft.

The power necessary for the drive is taken from the generator as three-phase rotary current, rectified and then transformed back into rotary current with variable voltage, strength and frequency with the help of a modern, high-performance frequency converter with IGBT-semi-conductors. This current then drives an electric motor which in turn is connected with a conventional final drive. The operation of the drive and the choice of the individual transmission strategy is carried out using a hand lever or foot pedal as well as via a user terminal. If necessary, this basic form permits the use of several variants. If one replaces the combustion engine with a turbine, then the generator must be adapted for the different input speed and a further electric motor must be installed for the power take-off shaft. When producing electrical energy using fuel cells the rectifier must be connected directly to these cells and the working principle of the generator must be reversed, i.e. it must be used for driving the power take-off shaft. It is also possible to use this basic form as a component of an electric split transmission: in addition to the electrical power on taps an additional mechanical power share from the combustion engine drive to the power take-off shaft and leads both shares together in a planetary gearbox which is connected to the final drive. A similar but not equal drive is already available on the market as a passenger vehicle transmission in form of the Toyota-Hybrid-System⁷. At the moment, however, the basic form displayed in figure 9 is the most suitable variant for fulfilling the demands on a modern tractor transmission.

This basic form was therefore logically chosen for the realisation of a prototype (figure 10). This prototype is called - Eltrac-. It is being developed within the framework of a project funded by the Technologie-Programm Wirtschaft (TPW) (Economic Technology Program) of the land North Rhine Westphalia. It is a series production New Holland tractor with a 100 kW engine, in which the reverse and power shift transmission were replaced by an electric transmission. This prototype is slightly larger than a series-production tractor. On the one hand this is due to the fact that the electrical components had to be installed into a series-production tractor, and on the other hand that the generator and frequency converters were initially air-cooled models for cost and availability reasons. However, in the case of series production and the use of exclusively water-cooled components the original measurements can be achieved. Despite this, the prototype is a fully operable tractor on which the named advantages can be tested and verified practically.

Even though currently only a few explicit quantitative measurements are available, the driving and working tests conducted so far have made a very good impression. The comparison of the realised electromechanical transmission with power shift and hydrostatic split transmissions is therefore mainly of a qualitative nature. Figure 11 compares eight criteria of a full power shift transmission, individual hydrostatic split transmissions with eight, four and one gear(s) as well as an electromechanical transmission. As the electromechanical transmission contains a full, short-term overload-capable electrical power transmission, it has advantages regarding reversing as well as the maximum starting torque. The near complete de-coupling of the speed of the combustion engine from the driving speed permits fuel savings during transport operation. The additional power provided by three-phase rotary current permit the use of immersion pumps, stall air-conditioners etc. without the use of a power take-off shaft generator. No extensive measures for maintaining the oil transmission media are necessary for constant operation. In addition, the electromechanical transmission has improved cold-running behaviour. A further advantage is it's very simple and robust mechanical construction.

This comparison closes the description of the electromechanical tractor transmission-innovation potential.