From engine to wheels
Goodbye exhaust gases! Turbo-charging changes its skin, using electric boosters to increase power and reduce consumption
Turbo-charging has come a long way since its introduction in the early 80s. Its widespread use on diesel engines – of any size – is generally accepted, while up to a few years ago, the use of power-boosters on petrol-powered engines (not to mention LGP/CNG) was limited to a few luxury models. The need to increase the efficiency of petrol engines led many manufacturers to widely adopt a downsizing philosophy (see previous Pneurama articles), reducing the engine size to obtain lighter and more compact power units. Furthermore, reducing the number of cylinders - say from 8 to 6 or 4 to 3 – reduces the internal friction of the engine thus enhancing the efficiency of the same, but at the same time less cylinders also mean less power, power that can be recovered through the use of, you guessed it, a turbocharger. Turbo-charging, as we know, consists in sending compressed air to an engine, rather than have it suck in atmospheric pressure: the combustion chamber will thus be richer in oxygen molecules ready to combine with the fuel, which results in a more powerful combustion and therefore greater power. This mechanical unit is the union of two elements: a turbine and a centrifugal compressor, where the turbine collects the residual energy contained in the exhaust gas converting it into mechanical rotational energy, which is then transferred to the compressor through a shaft; the output of the compressor is then pressurized into the engine.
The turbocharger is both small and light and has the advantage of exploiting energy otherwise dispersed in the form of exhaust gases. There are some negative sides too, like the creation of a certain "resistance", which hinders the outflow of the exhaust gas, not to mention the notorious Turbo Lag, between the demand for power and the actual supply of the same. This delay can be attributed to the “boost threshold”. The boost threshold of a turbocharger system is the lower bound of the region within which the compressor operates. Below a certain rate of flow, a compressor produces insignificant boost. This limits boost at a particular RPM, regardless of exhaust gas pressure: in short, maximum flow rate at 100,000 rpm, half the flow rate at 80,000 rpm.
It is therefore important that the turbine, and consequently the compressor connected to it, always turns at a very high speed because at low revs it would inevitably slow down and lose its effectiveness: for quick acceleration, therefore, it is essential to lower the rotational inertia of the turbo-charger. This is achieved primarily by decreasing the diameter of the turbines. But since this would limit the maximum airflow rate, engines with 2 or more turbo-chargers of different sizes were introduced: the smallest "start" at low engine speeds moving on to the larger units, able to provide the needed airflow when the engine operates at high revs.
At this point technical complications and costs rise dramatically, and this is where hybrid turbo-chargers come into play. Hailed as the ultimate solution, this system promises to eliminate the turbo lag (Valeo speaks of 250 milliseconds to reach operational speed) as well as exhaust backpressure. Moreover, ECU controlled boost levels will enable tighter predictive control of in-cylinder combustion, thus enhancing the energy saving qualities of downsized engines.
Noteworthy too is the simpler installation procedure: instead of hot bulky manifolds leading the exhaust gases towards the turbine, all we find is just electric cables. Obviously not everything is perfect, these are expensive components requiring a lot of energy and while the first problem will progressively diminish with large scale production, the second is a bit more difficult to solve since such a device absorbs 7/8 kW of power (about 10 bhp) involving a considerable supply of energy. Basically, 48V electrical system will be necessary, since something like 600 peak amps will be running through the connection cables, a prohibitive value for a 12 V system.
Valeo, for example, speaks of this device in connection with its Energy Recovery System that works in the same way as an alternator/starter and an energy storage system (supercapacitors pack). The generator is driven by the engine, while driving, and supplies energy for all on-board devices, but when braking or releasing it will charge the 48 V batteries making them ready to power the electric compressor; the power of this engine/generator can further “help” the car during acceleration.
According to Valeo the entire "package", which consists of a micro-hybrid system, would have competitive costs and would provide a 15-20% fuel savings. Valeo’s commitment towards developing this technology is evident in its partnership with Audi, aimed at equipping the Q7 next year with these compressors. According to Valeo, the compressor alone can still reduce fuel consumption by 10% and improve acceleration by 27% without increasing consumption; the company, though, is still considering the option of applying this technology to traditional turbo-chargers.
Honeywell, another important component manufacturer, does not seem to be so interested in developing this technology for normal road use, but has announced a collaborative program with Ferrari for an e-turbo to be used in Formula 1. In addition, another partnership with a 'global producer' is aimed at using these innovative compressors to supply the necessary chemical reaction for ‘fuel cells’.
Even a brand closely linked to turbo-charging such as BorgWarner is carefully considering electric compressors but only as e-boosters, integrating them with traditional turbochargers. This system uses an electric compressor together with a conventional turbo, with the first that can be placed before or after the conventional turbo. By refining the combination of the two units, this system can be optimized to "expand" the engine’s power band and torque increasing efficiency and reducing consumption.
Set in the heart of the UK’s automotive industry, Aeristech, together with Volkswagen, is developing the Full Electric Turbocharger Technology which aims to eliminate mechanical links between turbines and compressors. In essence, exhaust gases feeds a turbine, which turns a generator, the energy thus developed feeds an electric compressor. This system recovers the energy of exhaust gases and the turbine and compressor are free to operate at different speeds, enabling the compressor speed to be optimized for surge while the turbine speed is optimized for efficiency.
Among the manufacturers interested in this technology we find Audi, Ford, Mercedes as well as Honda, while BMW, although considering e-boosters with some interest, continues to believe in the current turbochargers: in the evolution of its 3 liter 6 cylinder turbo-diesel engine a staggering 4 of them have been installed in pairs and set to operate at high and low pressure.