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
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
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”.
geometry turbos have been used on diesel engines for years. They feature
adjustable guide vanes to change the behaviour of the turbine housing, allowing
the turbo to spin fast at low gas flows.
until recently, they have not been able to be used on petrol engine cars. The
reason that this has been the case is the much higher exhaust gas temperatures
produced by petrol engines. These can rise to as high as 1000 degrees C, versus
the 700 degrees encountered in diesel engine exhausts. However, the latest 911
turbo Porsche uses a specially developed variable geometry turbo featuring
exotic materials able to withstand the high temperatures.
the Porsche turbo can be seen with its guide vanes closed....
with them open.
the turbo looks like this. (1) Turbine casing, (2) Movable guide vanes, (3)
Turbine wheel, (4) Electric motor for guide vane adjustment, (5) Guide vane
adjuster, (6) Compressor casing, (7) Compressor wheel, (8) Excess pressure
valve, (9) Oil inlet, (10) Coolant inlet.
electric assist turbo adds an electric motor to an otherwise fairly conventional
turbo. A permanent magnet is mounted on the turbo shaft and the stator is
located in the bearing housing. Keeping the assembly cool is both important and
difficult. Another problem is that the outside diameter of the magnet is limited
because of the speeds at which a turbo spins – if the magnet diameter gets too
great, the magnet is liable to fly off the shaft. Using a longer magnet avoids
this problem but changes the dynamic behaviour of the shaft, especially in its
resistance to vibration.
major turbo manufacturers have been working feverishly on electric assist turbos
for at least 5 years, none has yet reached the light of day in a production
model. But, as the Toyota Central Research and Development Labs technical paper
suggests, they’re now very close.
1. Single Variable Nozzle Turbo with Electric
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
[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
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
[motor assist turbo]
so as to achieve the target boost
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
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
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
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!