Electric car
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Description
An electric car is a vehicle whose motive force is entirely from electric motor or motors. The electricity to the motor(s) may be stored in batteries or generated from an internal combustion engine, a fuel cell, solar panels etc. A hybrid car cannot fall into this category because part of its motive power is directly from an internal combustion engine. Based on a mixture of energy sources typical of large industrialised countries, such as the USA, it is doubtful whether any type of electrical vehicle would be less polluting on a "well-to-wheels" basis than a hybrid car of equivalent category, size, performance and autonomy and most of them would have a considerably higher capital cost, at the present state of development (2008).
Detailed description
Battery electric car
The performance and characteristics of a battery car depends on the type of battery. Assuming the battery is charged from a public power supply, the average well-to-wheels efficiency is very low, typically between 20 and 30 per cent, meaning that the carbon dioxide and pollutant emissions are higher than with more traditional cars.
The lead-acid type is the cheapest with reasonable lifetime, if well maintained. The main problem is that the battery is inordinately heavy and voluminous for a given autonomy, because of the need to dimension it to cater for heavy starting currents. This extra weight requires much energy for the transport of the battery itself. Recent research has suggested that a combination of a lighter lead-acid battery with a supercapacitor to provide the starting surge current may provide a much better compromise.
Nickel-metal hydride batteries are a possibility: they have demonstrated their long-term reliability in hybrid cars. They are expensive and require careful computer-controlled charging conditions. Their efficiency is slightly lower than lead-acid batteries and a 100+ km autonomy would be fairly voluminous.
Lithium-ion batteries are efficient, low in volume and weight and thus may be considered as a suitable candidate. However, they do not like heavy intermittent discharges and a supercapacitor may be needed to prevent the battery from overheating. Especially in warm climates, they need to be cooled, which puts a heavier charge on the air-conditioning system, reducing the overall efficiency of the car. In cold climates, they need to be warmed. For reasonable efficiency Li-ion batteries work best between 5 ° and 35 °C. Their lifetime is limited; their charge capacity diminishes, whether they are used or not and they are very expensive. Recent improvements have reduced, but not eliminated these disadvantages. As a rough guide, the cost of a battery to give a mid-sized car a 100 km autonomy will be over the whole cost of the rest of the car and it will need replacement every 2-5 years.
Fuel cell electric car
At the present state of development, it would seem unlikely if a fuel cell car could become mainstream within several years, if ever, even if the technology is available. In the first place, the cost of the fuel cell is currently prohibitive for wide acceptance. It requires a hydrogen distribution infrastructure which is not available in most places. The need to have a 5-10 minute wait period from switch-on to reasonable performance would deter many potential users, who are used to starting a car and driving off immediately. The lifetime of the fuel cell is limited because of contamination of the membranes. Above all, with current technology, the electrodes are platinised and there is simply insufficient platinum in the world to make millions of such cars; the cost of platinum has already risen many times to about $2000 per troy ounce over the past 20 years or so, mainly because of the comparatively small demand for catalytic converters. Fuel cell manufacturers are seeking a more abundant alternative.
Diesel/petrol/gasoline-electric car
Diesel-electric vehicles have been common, especially for very heavy applications, for decades. What is new is taking the concept down to the ordinary car. If a small NiMH battery is used as a buffer between the generator and the motor, with regenerative braking, then the use of an electric transmission becomes very economical. An average mid-sized car with mechanical transmission has an engine with a peak power output of, say, 120 kW at 6,000 rpm, but it rarely needs this power. The average power consumed is more likely to be about 20-40 kW. A small diesel engine, say about 900 cm3, running at a constant regime of 4000 rpm could provide 30 kW constantly. Going downhill, for example, this power would be excessive and the excess would be used to charge the battery. Going uphill, the generator would be insufficient to provide enough power and the difference would be supplied by the battery. The battery and the motor(s), would have to be dimensioned to supply 120 kW for short periods for emergency acceleration. This system would probably make for the most economical car, despite the weight of the buffer battery and possibly the supercapacitor, plus the motor(s). It is probable that a comfortable medium-sized car for 5 passengers could be made with a typical consumption of 2.5-3 l/100 km. There is a severe down-side to this concept and that is the driver would have to keep his average consumption to the wheels equal to the generator output, no matter whether, for example, the air conditioning was on or off. With clever electronics and software, this disadvantage could be eliminated, but it means that the diesel engine would have to turn at a speed consistent with supplying the real-time average power output for the conditions chosen by the driver. This means that the diesel engine would need to work over a range of speed and not a fixed speed, with a considerable drop in efficiency and increase of consumption. It is probable that the end result would be very similar to a true hybrid car, with the advantage of the electric transmission counteracted by the extra weight of the battery plus, say, 50 kW of generator and motor(s) capable of 50 kW constant plus occasional short-term forays up to 120 kW. The obvious drive system would be in-wheel motor/braking generators on 4 wheels, but the unsprung weight would make the ride very uncomfortable. It would actually be better to place them on the chassis, inboard, with half-shafts and constant-velocity universal joints. One big advantage of the concept is that the engine and generator could be placed anywhere for best roadholding, such as a flat-4 boxer engine midships under the floorboards.
