OIL & GAS – TURBO-COMPOUNDING
Society
Historical Engine Aircraft the via McCutcheon D.Kimble Commons/Wikimedia Energy cycle
in heavy land vehicles. Later systems
avoided the need for continuously variable
transmissions. However, they still required
two-stage-reduction gearboxes, combined
with fluid couplings, adding considerable
mechanical complexity to the engines.
A further issue with mechanical turbocompounding
is that at low engine loads,
T urbo-compounding has
electric turbo-compounding (ETC) has
historically been applied to
the potential to significantly improve the
reciprocating aero engines,
efficiency of gas and diesel-powered
with the power-recovery
gensets, as well as large vehicle engines.
turbine connected to the
output shaft. It was quickly realised that
MECHANICAL TURBO-COMPOUNDING
the turbine section of the engine could
The original form of turbo-compounding
completely replace the piston engine. It was
transmitted power from the turbine
more efficient at recovering energy from
mechanically. Some aero-engines used it
the hot gas, as well as being more reliable.
to drive a separate propeller, while most
Turbo-compounding piston engines quickly
transferred the energy into the main
gave way to turboprop and turbojet engines.
output shaft using a continuously variable
Since the 1950’s, most piston engines
transmission. For example, the Nomad
that recover energy from exhaust
engine (main image; see also diagrams,
gases have used that energy to drive a
which are rotated 90° relative to image) used
turbocharger. However, there is renewed
a Beier variator.
interest in turbo-compounding using
High-efficiency aero-engines quickly
electrical generation rather than turbines
evolved from turbo-compounding into
mechanically coupled to the output shaft.
turboprop engines. These proved to be
Since 2014, Formula 1 cars have used
mechanically simpler and more reliable,
turbo-compounding engines with a
albeit more expensive to manufacture.
turbocharger, connected to an electric
There did, however, continue to be
motor/generator. Outside motorsport,
occasional use of turbo-compounding
28 www.operationsengineer.org.uk May 2019 insufficient pressure is produced to drive the
additional turbine without a negative impact
on engine performance. The resulting
backpressure can reduce fuel efficiency and
increase emissions. Diversion valves may be
needed to mitigate for these effects, adding
more complexity.
HIGH-EFFICIENCY POWER GENERATION
Using turbo-compounding to generate
electrical power removes the need for
complex mechanical power transmission
and allows the load to be reduced when
required to maintain an acceptable
backpressure. In the past, it was difficult for
vehicles to use the significant amount of
electrical power produced.
However, with widespread hybridisation
of vehicles, finding a use for this power is not
an issue. ETC is now ideally suited to large
diesel engine vehicles, both land-based and
marine, and stationary power generation.
The highest efficiencies for heat engines
are typically obtained from combined cycle
power plants, which use a gas turbine for
the first cycle and a steam turbine for the
second cycle. This type of power plant can
achieve real-world efficiencies of over 60%.
By Jody Muelaner
Turbo-compounding uses a turbine to recover energy from the exhaust gases
of a piston engine, and supplies this additional energy to the engine’s output
shaft. It is, in effect, a form of combined cycle power, with a piston engine
operating as the first cycle and a gas turbine operating as the second
/www.operationsengineer.org.uk