Doctor Diesel

Back To Basics – Power, Torque, gearing and acceleration

Take a simple 12-inch diameter wheel, fix it to an axle, and attach a one pound weight to its rim at three o’clock, and the result will be one lb ft of torque applied to the axle. That’s torque, and in this context it’s strictly a static force, and only gravity acting on the weight is supplying that force. In the context of cars, we need such a force, supplied by an engine, or motor, to deliver such torque to turn the wheels, and to overcome rolling resistance at the tyres, aerodynamic resistance, and often gravity. But torque is merely a force, and energy and power must enter the picture if that force is required to do work (which is force x distance), to overcome the resisting forces; the rate at which that work can be supplied to overcome those resisting forces, is power, be it in bhp, PS, CV, or kW.

With small internal combustion engines, there is very little torque at low engine speeds, and thus very little power, as they are thermodynamically inefficient at low speeds below 1,000rpm. At the other extreme, diesel internal combustion engines run out of breath, generally at around 5,000 to 6,000rpm. So the only way to get our cars moving and accelerating continuously, and climbing steep hills when maximum power may be required, is by means of gearing, which raises the torque at the wheels, along with the engine speed, at which it can then supply more power. Known advantages of the diesel engine, specifically the turbodiesel, is that the torque is higher at low engine speeds compared with the petrol engine, and the peak torque speed band may extend from as little as 1,400rpm up to maybe 2,500rpm or more. That high torque band is where the engine is at its most efficient, converting more of the energy in the fuel into mechanical energy and losing less of it as waste heat. If you can drive your car within this high efficiency engine speed band, then you are getting more performance and more mpg for your money, in the form of fuel costs.

Not everyone can bother to make this effort, and this is where the new generations of automatic transmissions are entering fresh territory, and where electric cars are totally changing the game. With as many as eight or nine ratios in these automatic gearboxes, and also CVT transmissions, where the ratios can be infinitely varied within limits, even relatively peaky petrol engines can be programmed to stay in the optimum gear ratio in any circumstances, thereby improving engine efficiency and reducing fuel consumption.

Whilst such multi-ratio or continuously variable transmissions are merely developments of existing car technology, the characteristics of electric car motors are almost like entering a new universe. There are no thermodynamics of combustion involved, no engines with temperamental torque delivery characteristics; The switching on of an electric motor is like turning on a light bulb, it doesn’t need time to build up power and peak power of an EV is available at zero rpm, which on the Tesla for example, is 288bhp and 295 lb ft of torque; the transition from zero power to significant power is managed by electronic control systems to give smooth, controlled acceleration, with no gear changes, torque peaks, or flat spots, and with a motor with a working range of typically 0-10-12,000rpm. The torque delivery of an electric motor is a function of a managed electric current supply, not of rotational speed, and electric vehicles have a high torque potential over a larger range of speeds compared with internal combustion engines. So an EV electric motor rated at 80bhp can supply power over a much wider speed band than any diesel engine, and therefore it delivers superior performance to a diesel engine with a peak power of 80bhp. We’ll go into more detail on electric car power storage, energy recovery, and power supply in a future issue.

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