The need for gearbox
Internal combustion engines today drive most of the automobiles. These internal combustion engines work either on the principle of spark ignition or diesel principle. In addition to the many advantages of the internal combustion engine, such as high power to weight ratio and relatively compact energy storage, it has two fundamental disadvantages:
• Unlike the electric motors, the internal combustion engine cannot produce torque at zero speed.
• The internal combustion engine produces maximum power at a certain engine speed .
• The efficiency of the engine, i.e. its fuel consumption, is very much dependent on the operating point in the engine's performance map.
With a maximum available engine power Pmaxand a road speed of v , the ideal traction hyperbola Fideal and the effective traction hyperbola Feffec can be calculated as follows:
(1)
Hence, if the full load engine power Pmax were available over the whole speed range, the traction hyperbolas shown in Figure 7 would result. However, the Pmax is not available for the entire speed range. The actual traction profile of the ICE (Fengine) is shown in Figure 7. From Figure 7 it is evident that the entire shaded area cannot be used.
Figure 7: Traction force vs. speed map of an internal combustion engine without gearbox |
In order to utilize the shaded area, shown in Figure 7, additional output converter is required. The output converter must convert the characteristics of the combustion engine in such a way that it approximates as closely as possible to the ideal traction hyperbola ( Figure 8).
|
Figure 8: Traction force vs. speed map of an internal combustion engine with gearbox |
The proportion of the shaded area, i.e. the proportion of impossible driving states, is significantly smaller when an output converter is used. Thus, the power potential of the engine is better utilized. The Figure 8 shows how increasing the number of gears gives a better approximation of the effective traction hyperbola.