Turbocompounding is a technique in which a combustion engine works together with one or more
exhaust gas turbines. Thereby not only from the engine, but at least from one of the turbines power
output is taken. Instead of expelling wasted energy via the exhaust pipe, the heat is extracted from the
exhaust gases by a second exhaust turbine downstream to the turbocharger.
The second turbo (the turbocompound turbine) spins at 55,000 r/min. This motion is passed through
turbine gears and a hydraulic coupling, then through the timing gears to the crankshaft. Stepping down
the revs produces a useful boost in torque, which when reaching the flywheel adds momentum. This
extra driving force do not increase expenditure on fuel.
The economical green band on the rev counter provides a wide range of economical engine speeds.
The engine exhibits great flexibility. The rotation of the crankshaft benefits from the constant extra
drive coming from the turbocompounding process, helping to even out the rhythmic pressures induced
by combustion. So the engine runs more smoothly.
First air-craft engines with turbocompound were used already during the II World War. Then they
were used in marine engines. Scania Company has been producing trucks equipped with
turbocompound engines since 1991 and Volvo since 2001. Turbocompounding in short terms will be
state of the art for most suppliers. In the USA works are carried out on electrical systems
Caterpillar is focusing its research efforts on the development of an electric Turbocompound system
for heavy-duty on-highway truck engines consisting of a turbocharger with an electric motor/generator
integrated into the turboshaft. The generator extracts power at the turbine, and the electricity it
produces is used to run a motor mounted on the engine crankshaft. The electric turbocompound also
provides more control flexibility as the amount of power extracted can be varied. This allows the
control of engine boost and thus of the air/fuel ratio. To improve vehicle driveability, the system can
be run in turbo assist mode to accelerate the turboshaft.
CO2 Benefit: 8%
Additional Cost: MEDIUM
Technology maturity: HIGH
Advantages: the system does not require a specific heat rejection system
Drawback: only part of the available waste heat can be re-used
Thermoelectricity (TE) is one of the simplest technologies applicable to energy conversion. It has long
been discovered that a few materials can generate electric power from heat, and use electricity to
function as heat pumps providing active cooling or heating: such materials are said to be
Small pieces of thermoelectric material (TE elements) are connected as shown in figure to form a
Peltier couple. One element is of p-type, the other one of n-type; they are connected in series
electrically, but thermally in parallel. A multitude of couples forms a TE-module. When a heat source
is applied to the hot plate, and the cold plate is maintained at a cold temperature, a voltage difference
ΔV is created between the cold and hot plates, that can induce a current I in an external circuit: the
Peltier couple is an electric generator.
The efficiency of a TEG depends on the material ZT as well as on the temperature drop Th - Tc across
the device. The only TEG application on board vehicles described in the literature is the Hi-Z/Mack
Truck Generator. This system utilizes the waste heat of a class 8 Diesel Truck to produce about 1 kW
CO2 Benefit: >5% (state of the art materials)
Additional Cost: HIGH
Technology Maturity: LOW
Advantages: the system has almost no moving parts
Drawbacks: only part of the waste heat can be re-used and the materials should still be developed.
The Rankine cycle is a thermodynamic cycle which converts heat into work. The heat is supplied
externally to a closed loop, which usually uses water or an organic fluid as the working fluid.
generation from wasted heat. This RCS can be operated on the exhaust gases of the Internal
Combustion Engine. The system is composed of four/five different components: Exhaust gas heat
exchanger, expander, (+generator), pump, and the condenser.
The ideal Rankine cycle consists of the four following evolutions:
- The thermodynamic fluid enters the pump at state 1
as saturated liquid and is compressed to the
operating pressure of the boiler.
- The fluid enters the boiler as sub-cooled liquid at
state 2 and leaves as superheated vapor at state 3.
Heat is transferred to the fluid at constant pressure.
- The superheated vapor at state 3 enters the turbine
where it expands isoentropically and produces work
by rotating a shaft connected to an electric generator.
The temperature and pressure of the fluid drop
during this process at state 4, where it enters the
- The fluid is condensed at constant pressure in the
condenser, by rejecting heat to the atmosphere (for
vehicle applications) and leaves the condenser as
saturated liquid and enters the pump
CO2 Benefit: >15%
Additional Cost: MEDIUM
Technology Maturity: MEDIUM-HIGH
Advantages: almost the waste heat can be re-used
Drawbacks: increases the load of the heat rejection system
Progress beyond the state of the art
The plan for the NoWaste project foresees theoretical and numerical studies for different technologies
to improve the Rankine system efficiency and integration. Within the Rankine cycle architecture
development phase the best compromise resulting from the theoretical pre-studies shall be included in
Position of the evaporator on the exhaust line
In conservative designs of mobile Rankine cycles recovering waste heat recovery from the exhaust line
the evaporator is mostly situated behind the after treatment system of the exhaust gas. This
configuration has the advantage that the after treatment of the exhaust gas won’t be disturbed. On the
other hand the exhaust gas temperature decreases by 50° K to around 100° K in the after treatment
Another disadvantage of the conventional configuration is the possible corrosion and damage of the
Rankine evaporator due to soot and ammonia from a possible SCR system.
This study shall investigate e a possible configuration of the Rankine cycle evaporator between the
turbocharger outlet and the EATS inlet and determine the gain in energy recuperation by simulations.
The research content will mainly concentrate on the possibility of an adapted after treatment system to
lower exhaust gas temperatures. The advantage of this configuration would be founded in better heat
exchanger efficiency due to higher heat source temperatures which leads to reduced packaging needs.
The minimum exhaust gas temperature at the EATS inlet will limit the efficiency of waste heat
recovery. A good adaptation of the EATS configuration to low exhaust temperature could lead to a
higher overall efficiency of heat recuperation and a gain in after treatment efficiency since the after
treatment system inlet temperatures can be controlled.
Further the development of the right control strategy for the Rankine system will be crucial in order to
let the after treatment system work correctly at every moment the engine is working. A bypass of the
Rankine system evaporator is necessary in all cases.
Indirect heat exchange and heat storage
The main drawback of the Rankine cycle is the need of heat rejection during the condensation phase.
For full charge engine working points, especially at a high environmental temperature the cooling
capacities of the vehicle won’t be sufficient to cool the engine and the Rankine cycle which leads to a
necessary energy costly fan engagement.
The indirect heat exchange uses a caloric fluid or the working fluid of the Rankine cycle itself to
recover heat from the exhaust gas and transfers the heat to the Rankine cycle. To use a specific caloric
fluid can give advantages in heat recovery and heat storage efficiency but also increases the
complexity of the overall waste heat recovery system.
A heat storage device permits to decouple the Rankine cycle from the internal combustion engine and
to produce energy when the vehicle thermal management permits it. Another advantage of an
independent Rankine cycle is the possibility to develop an independent control strategy for the waste
heat recovery cycle.
The main drawbacks of this technology are increased system complexity, weight and a loss in direct
waste heat recovery efficiency if a caloric fluid is used.
In a first step a possible architecture with or without a caloric fluid shall be identified and the steady
state waste heat recovery potential shall be identified. This phase also includes investigations on
efficient heat storage systems.
In the second phase the fuel economy potential shall be analysed performing road cycle simulations
with a Rankine cycle model including indirect heat exchange. Only with this approach the overall
benefit towards a direct Rankine cycle can be determined.
If the numerical studies show an advantage using modified cycle architecture with heat storage this
technology can be taken into account in the final architecture definition phase.
Supercritical Rankine Cycles
Sub critical Rankine cycles are often constrained by low evaporation pressures, especially for organic
fluids or heat exchanger temperature pinch limits between the hot and cold source due to very high
latent evaporation energy needed as it is the case for the water Rankine cycle.
The supercritical cycle offers a good potential of overcoming these drawbacks and maximising the
Rankine cycle efficiency by using high pressure differences between the condensation and
In order to adapt the cycle to the vehicle heat rejection potential and packaging needs using a
supercritical cycle can permit to increase the condensation pressure and temperature of the working
fluid without losing cycle efficiency.
The drawback of a supercritical cycle is the need of a high pressure and temperature resistant heat
exchanger. Furthermore the expansion device needs to be adapted to high pressure ratios and inlet
The research shall concentrate on possibilities for supercritical heat exchange. In first steady state
simulation models the potential of a supercritical Rankine cycle shall be identified using different
architectures and working fluids. In a second phase the supercritical cycle shall be simulated in a 1D
physical Rankine cycle simulation model.
Optimal heat management and heat recovery from multiple heat sources
Beside the exhaust gas the engine environment offers various heat sources as the EGR, the engine
coolant or the charge air after turbo compression. These media need to be cooled down by the EGR
cooler, the front radiator and the charge air cooler.
The engine coolant as well as the charge air offers a good heat potential for preheating the Rankine
cycle working fluid, for organic Rankine cycles with condensation temperatures lower than 70°C. The
EGR can be used for pre-and superheating. These additional heat sources can lead to higher energy
recuperation whereas the necessary external cooling power for charge air, engine coolant and EGR
decreases and the Rankine cycle condensation heat rejection increases.
The study concentrates on the determination of optimal cycle architecture for the given boundary
conditions and the estimation of necessary heat exchanger dimensions.
The waste heat recuperation means to increase the heat load of the vehicle heat rejection system
proportionally to the recuperated heat.
So, for each 100 kW of recuperated heat, assuming a conversion efficiency of about 20% (very high),
the heat rejection load will increase of at least 80 kW. This means that the vehicle heat rejection
system should be the focus of a deep review.
The most simple solution is to add to the front module an heat exchanger devoted to the heat
recuperation system, but this approach risks to have a negative effect of the vehicle cooling drag and to
reduce the effectiveness of the existing heat exchangers.
On the other hand the vehicle heat rejection system is normally designed to comply with sever
condition (very high thermal load, low speed high external temperature), in other world in almost all
the operating conditions there is a heat rejection capacity available.
The approach that will be adopted within NoWaste is based on two main points:
- the use of the existing heat rejection capacity when available (almost all the operating
- increase of the heat rejection capacity using part of the body panels as heat exchangers (flat
heat exchangers) functionalising the aerodynamic under-body and of part of the side body
panels. The panel will be channelled and transformed in heat exchangers, but keeping their
original functions (aesthetics and aerodynamics). This solution allows to increase the overall
vehicle heat rejection capacity and the effectiveness increases increasing the vehicle average
speed so in coherence with the amount of recuperated heat
Whatever will be the Rankine cycle architecture it is crucial to develop a suitable and optimised
control strategy in order to overcome transient losses and to maximise energy recuperation while
influencing the engine environment as little as possible.
Inputs to strategies come from the engine environment, the Rankine cycle itself and the vehicle
environment. Control parameters within the waste heat recovery cycle can be described with bypasses,
engine and pump speeds or specific valve characteristics for the energy sources or the bottoming cycle
This project includes control strategy development tasks and work packages in almost all phases of the
project in order to develop a highly efficient and robust strategy.