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If we compare a petrol engine and an electric engine car, it is sufficient for the electric engine car (e.g Tesla Roadster) just to have 1 gear (forward), whereas petrol engine can have up to 7. Why is it sufficient for the electric engine to have a single gear? Is it because it can deliver the required torque always?

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Yes, electric motors can always deliver the required torque. What really matters is the efficiency of the conversion. Over a long range of speeds, the motors may convert 90 percent of the energy to its mechanical form. You know, it's just some electromagnetic fields that may become arbitrarily stronger - depending on how much energy is sent into them - and that always do the required work.

On the other hand, petrol engines are only highly efficient over a relatively narrow interval of the number of revolutions per minute (RPM) of the engine. It takes a certain time for the fuel to get burned or explode, for the pressure to spread in the cylinders, and whatever else needs to be done in each cycle. Moreover, at too high frequencies, the motor could get overheated and damaged, among other things. That's why petrol engines want to keep the number of RPM relatively constant and a gearbox is needed to translate the fixed frequency of the motor to a variable speed of the car.

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Another important point is that the combustion engine not only has a favored RPM range, but also a favoed output range. If the vehicle is moving at a given speed, but needs little or no torque to maintain speed (say on a slight downhill), the engine is still turning, and various dissapative processes (friction of engine parts, and gas being moved in pistoms etc.), so efficiency during very low torque conditions is low. At low RPMs, heat loss via conduction through cylinder walls grows as a percentage of the heat of combustion, lowering efficiency.

All told, the relative efficiency between these two types of propulsion will be highly dependent upon the driving cycle. Are there lots of starts and stops (or up hill/down hill)? That would favor electric. If the need for is steady continuous power, the internal combustion engine can be tuned for those conditions (load and RPM), and sized for that level of power, rather than for rapid acceleration, and will look much better.

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Any kind of motor will have a characteristic relation of torque to speed of rotation. Typically at any speed, there is a maximum torque available from the motor.

With combustion engines, the shape of the torque-vs-speed curve is fixed (afaik) and the engine must be operated at significant speed in order to deliver enough power to be effective at moving the car. Since the load conditions vary widely from stopped to cruising at top speed, it's necessary to add a device, the multi-gear transmission, to mechanically transform power at the engine's preferred speed/torque ratio to power at the load's required speed/torque ratio.

Meanwhile, the torque-vs-speed curve of electric motors can be dynamically changed, by alteration of the field strength and commutation angle inside the motor. This means that with modern power electronics, the electric motor can be reconfigured at will, so as to grind out large torques at low speeds, or smaller torques at high speed, to efficiently deliver mechanical power that is already at the correct speed/torque ratio, obviating the need for a mechanical transmission.

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