In a seminal paper, Hiroshi Uchida of the Toyota Central Research and Development Labs has given a dramatic technical insight into where turbocharging is headed. The paper, Trends in Turbocharging Technologies, shows how turbos will be used in the next decade of high fuel efficiency, low CO2 emissions engines. Linking compound twin turbocharging with electric motor assist and variable geometry turbos, Uchida explains the form in which turbos will dominate production car engineering.
So what is driving the move towards turbos? This diagram shows the trend in specific power (ie power per litre) of diesel engines being sold in Europe. As we have described in Common Rail Diesel Engine Management, Part 1, common rail diesel fuel injection and turbo intercooling have revolutionised diesel power.
High specific power will be achieved not only with more extensive use of variable nozzle turbos and variable vane turbos, but also by means of turbos boasting electric motor assist. The latter technology has been mooted for years (see Turbo Revolution - The Coming Technologies Part 2) but is now getting very close to public release. And it’s not just in diesels. The same forces are driving petrol engines towards smaller capacities and higher efficiencies – and that spells forced aspiration.
However, in many respects, turbos have been slow to change their technology. Despite over 50 years of development, conventional turbocharged engines still suffer from a lack of low-speed torque. This is because the turbo must be large enough that it doesn’t restrict exhaust flow at high loads (ie cause excessive backpressure) but must be small enough that it can spin on the low flow of exhaust gas being produced by the engine working at the bottom of its rev range.
This diagram shows the torque production versus engine speed of a variety of engineering approaches. The upper black line indicates the torque development of a large naturally aspirated engine. The lowest grey line shows the torque output of a smaller (“downsized”) engine that can produce lower absolute emission outputs and gain better fuel economy. A conventional turbo, shown by the blue line, takes about one-third of maximum rpm before it reaches full boost. A variable geometry turbo (green line) is substantially quicker to boost, but it still cannot reach full boost pressure at revs just off idle. However, a variable geometry turbo with electric motor assist (red line) can match the larger naturally aspirated engine.
And not only can the electric assist variable geometry turbo give a torque output similar to a much larger engine, it can also “recover thermal exhaust energy by acting as a dynamo at high engine speeds,” writes Uchida. “As a result of this, we can expect a significant improvement in fuel consumption.”
From all that you could assume the problems are as good as solved: just fit an electric motor to a variable geometry turbo. However, there are aerodynamic, thermal and electrical problems to be overcome.
Firstly, there is the problem of compressor surge. Compressor surge occurs when the turbo is developing a lot of boost (a high “pressure ratio”) but at a low flow rate. An electric assist turbo readily falls into this category – high boost is needed at low engine revs, where there is relatively little airflow being consumed. When surge occurs, the airflow is instable and loud noises can be created. (Interestingly, Uchida doesn’t say anything about surge causing turbo longevity problems.)
There are two ways of eliminating the problems of compressor surge. The first is to use twin differently sized turbos, with the smaller turbo used at low engine speeds. The downsides of this approach are cost, complexity and packaging. The other way of overcoming the surge problem is by bypassing compressor discharge air back to the inlet side of the compressor. However, the increase in required compressor power causes an increase in turbine drive power, in turn causing an increase in engine exhaust backpressure. As a result, fuel consumption suffers.
The surge limit can also be improved by variable geometry compressors and internal compressor recirculation using “a casing treatment on the shroud wall”.
1. Single Variable Nozzle Turbo with Electric Assist
Toyota researchers examined a variety of turbo systems that could produce high boost across the full rev range of the engine. The target boost pressure was 300kPa (43 psi) and the engine displacement was 2 litres. [Note: absolute boost figures are being used here – subtract 100 kPa (14.5 psi) to gain gauge boost figures.]
The first system used a single variable nozzle turbo with and without electric motor assist. This diagram shows the boost pressure gained with the two approaches. Without motor assist, the turbo developed about 270kPa boost by 2800 rpm. But with electric assist, over 300kPa boost was available at just over idle speed! However, the upper part of the diagram shows the required electric power to achieve the desired electric assist. Power demand peaked at no less than 5.6kW (on a 13.8V system, over 400 amps!). As Uchida somewhat dryly states: “It is generally believed that the development of a turbocharger with 5.6kW of motor assist is technically very difficult.”
This diagram shows the corrected airflow rate versus pressure ratio for the variable nozzle turbo, with and without electric assist. The turbo, both with and without electric assist, is in surge over nearly the whole of the graph.
Therefore, to achieve a single motor-assist variable geometry turbo that can develop the required 300 kPa boost pressure across the full engine operating range, “it is necessary to significantly improve the surge limit of the compressor at the same time as developing a high-power MAT [motor assist turbo] so as to achieve the target boost pressure.”
And there’s another problem. To achieve the airflow and boost pressures desired, the compressor airflow exit speed reaches 600 metres/second (2160 km/h), a speed “which is incompatible with current aluminium alloy impellers because this would over-stress the impeller. It would be necessary, therefore, to use a high-strength material such as titanium to achieve the required strength.”
“The stress in the impeller can also be lessened by reducing the backward angle of the impeller. [However, it would then be] necessary to further improve the surge limit because the surge limit deteriorates when the backward angle is reduced. Moreover, the heat-resistance of the compressor exit piping has to be improved because the temperature of the compressor discharge air reaches 523K [250 degrees C] .”
So while a single variable geometry turbo with electric assist can provide the desired boost pressures across the range of engine revs, there are problems with compressor surge, compressor construction, outlet pipe construction and the required electric power.
2. Compound Twin Variable Nozzle Turbos with Electric Assist
The next system examined used two differently sized variable nozzle turbos, the smaller of the two equipped with electric assist. As this diagram shows, only one wastegate was fitted and intercooling occurred between the turbos as well as between the smaller turbo and the engine. Also note the compounding of the turbos - one blows through the other.
This diagram shows the performance of the smaller turbo. The turbo stays inside the compressor surge limit and, with motor assist, develops high boost at low flows.
With both the smaller electric assist turbo and the larger turbo working, high boost is available across a wide range of airflows without much exceeding the compressors’ surge limits.
Importantly, “the motor assist power of 3kW is less than that of the single-stage turbocharging system”.
“Furthermore, the target boost pressure at low engine speeds can be obtained by slightly improving the surge limit of the conventional compressor. In addition, the problem of the strength of the aluminium alloy impeller is avoided because the pressure ratio of each individual compressor is lower than that of the single-stage system. However, the turbine inlet pressure in the high-pressure stage produces a high value at high engine speeds because the flow capacity of the small compressor is too low. It is thought that the turbine inlet pressure can be reduced by bypassing the small compressor at high engine speeds.“
3. Transient Performance
As you’d expect, transient performance (ie boost response with sudden throttle changes) was also greatly enhanced by the use of an electric motor assist turbo system. Toyota’s testing showed that when there was 1.5kW of electric assist, the time taken to achieve 200 kPa boost was cut by about 30 per cent, and when 3kW of electric assist was used, by about 40 per cent. However, with the higher rate of electric assist, boost overshoot was a problem.
The testing carried out by the Toyota Central Research and Development Labs indicates that the use of a compound twin turbo system with variable nozzle turbos, dual intercoolers and electric assist on the smaller turbo is viable technology.
Writes Uchida: “It seems clear, therefore, that in the future the individual development of engines and turbochargers will be supplanted by the development of turbocharged engine systems.
“Turbocharged engines incorporating new technologies are expected to make a major contribution to satisfying the demands for reduced CO and NOx emissions.“
And when you see your first twin turbo, twin intercooler, electric assist turbo Toyota – you read it here first!