Title: KINETIC ENERGY RECOVERY SYSTEMS FOR PASSENGER CARS AND URBAN
BUSES
Scientific coordinator: Prof. Giacomo Mantriota
Collaborations: Politecnico di Torino; University of Surrey, Guildford
ABSTRACT
In recent years, the need for a decrease of fuel consumption of ground vehicles pushed the interest
of the automotive engineers community towards the energy recovery in braking (KERS: Kinetic
Energy Recovery Systems). It can be estimated that in urban cycles, more than 65% of the overall
energy demand is due to speed-up and it is wasted in brakes, of course. Hybrid architectures are
employed: a) to recover and store the energy which would be wasted in braking; b) reuse the stored
energy in acceleration.
Most of hybrid systems, be either still in production or in development, are based on electric-hybrid
architecture with different layouts and storage devices. However, the multiple conversion of energy
according to the path mechanical-electrical-chemical gives poor overall efficiency, that is estimated
about of 35%. A further defect of the hybrid-electric systems is the battery life. As a matter of fact,
a charge level higher than 90-95% is suggested to guarantee a long life to batteries, and this
condition is in contrast with the requirements of a profitable energy recovery that needs frequent
complete cycles of charge and discharge.
Because of these reasons, an interesting and promising solution is given by the mechanical hybrid
system, which stores the energy in a rotating flywheel connected into the driveline by a mechanical
speed regulator. The mechanical hybrid system (mechanical KERS) stores the energy in a highspeed rotating flywheel, avoiding any energy conversion. It results that the overall efficiency of the
system can be up to 70%, that is twice the value of electrical-hybrid. An additional advantage is the
small packaging of the system.
BACKGROUND
The basic principle of a mechanical-hybrid system is simple: as the vehicle decelerates, the
flywheel is accelerated. The energy transfer between the vehicle and the flywheel is obtained via a
variable drive of which the rate of ratio change determines the amount of braking (or accelerating)
torque into the driveline. For this reason, the variable drive is a key point for a profitable design of
mechanical KERS.
In the season 2009, Torotrak has manufactured a device made of a flywheel connected to a Toroidal
CVT with a ratio spread (the ratio between the maximum and the minimum value of the
transmission ratio) equal to 6. Such simple and compact architecture was optimal to meet the
amendments of Formula 1 rule, but it is maybe not optimal for application on passenger cars. In
fact, it is commonly believed that passenger cars and urban buses need larger ratio spread than those
given by a simple CVT transmission. Power Split CVT systems are obtained via coupling CVT,
ordinary and planetary gear trains and they allow larger ratio spreads. As an unfavourable effect,
enlarging the ratio spread, also increases the size and costs, and reduces the efficiency of the
system.
This is why a correct design and optimization of the Power Split device is of utmost importance in
the development of a high quality mechanical KERS. The most convenient energy recovery can be
achieved by a correct tuning of the rate of change of the transmission ratio, power flows, available
torques and efficiency.
The research activity of this project is aimed at the optimization of the Power Split CVT systems
for mechanical KERS, focusing on purpose of reducing costs and size. Studies about mechanical
KERS will be carried on, with application to passenger cars and urban buses. The application to
urban buses is expected to be promising because of the frequent start-stop sequence of their typical
driving cycle.
The research group has a 16 years experience in the field of continuously variable transmissions,
with both theoretical and experimental approaches. The investigations have dealt with: efficiency,
dynamics, power flows and efficiency of Power Split CVT, applications of CVT and Power Split
CVT to conventional topics (fuel consumption of passenger cars, fuel consumption of buses, and
non-conventional topics, metal belt CVTs, metal chain CVTs, Toroidal CVTs, IVTs (Infinitely
Variable Transmissions). Moreover some original architectures of Power Split CVT systems have
been suggested, one of which have been patented by the author.
RESEARCH PROPOSAL
The optimization of Kinetic Energy Recovery Systems for application to passenger cars and urban
buses is the main purpose of this project. In a mechanical KERS the continuously variable
transmission has a key role for the energy transfer from the vehicle to the flywheel. The wide ratio
spread required binds to choose Power Split CVT systems.
The investigation of the optimal solution for Power Split architecture is constrained by factors
which play an opposite role in determining the best choice. As an example, an increase of the ratio
spread (the ratio between the maximum and the minimum allowable transmission ratios) would
permit in theory to recover larger amounts of energy in braking, but it would reduce the efficiency
of the variable drive and it would also increase its costs and size, with a larger torque spread.
The research project will be split into four phases:
In the phase 1, simulations of the vehicle dynamics will be performed (passenger car, bus) for the
evaluation of the fuel consumption driving standard cycles. Assuming a perfect recovery of the
energy, kinematic (working velocities of the flywheel, spread of transmission ratio, rate of change
of the transmission ratio, ...) and dynamic (flywheel inertia, torques, ...) constraints of the KERS
will be determined. An ideal transmission with unitary efficiency will be considered in this phase.
The results obtained will lead to the right choice of transmission layout (CVT, Power Split CVT,
Compound Split CVT, IVT).
In the phase 2, a general purpose model to predict the performances of all possible layouts of Power
Split systems and Compound CVT systems will be developed. Some recent studies have shown the
possibility to develop generalized mathematical efficiency models of Power Split CVTs.
In the phase 3 the optimization will be formulated and solved.
The optimization of the device will be performed considering several aspects: energy saving, cost
and size.
The mathematical optimization method will operate on the design parameters of the transmissions
as variables: layout, flywheel inertia, transmission ratios (gears, planetary gears and CVT) working
velocities of the flywheel, number of axles where to connect the device into. The cost function will
be built to take into account the fuel consumption, size and cost.
The optimized set of parameters will suggest the optimal variable drive to be applied to the KERS.
The same analysis will be performed for both passenger cars and urban buses.
SOME RELATED PUBLICATIONS
1. Bottiglione F., Mantriota G.: MG-IVT: an infinitely variable transmission with optimal
power flows. ASME Journal of Mechanical Design, Vol. 130, No. 11, 2008.
2. Carbone G., Mangialardi L., Mantriota G.: Fuel consumption of a mid class vehicle with
Infinitely Variable Transmission. SAE Transaction 2002 Journal of Engines, Vol. 110,
Section 3, pp. 2474-2483, 2002.
3. Carbone G., Mangialardi L., Mantriota G., Soria L.: Performance of a City Bus equipped
with a Toroidal Traction Drive. IASME Transactions, Vol. 1, No. 1, pp. 16-23, 2004.
4. Carbone G., Mangialardi L., Mantriota G.: A comparison between the performance of full
and half toroidal traction drives. Mechanism and Machine Theory. Vol. 39, pp. 921-942,
2004.
5. Mangialardi L., Mantriota G.: Power flows and efficiency in infinitely variable
transmissions. Mechanism and Machine Theory. Vol. 34, No. 7, pp. 973-994, 1999.
6. Mantriota G.: Power split CVT systems with high efficiency. Proc. Instn Mech. Engrs, Part
D- Journal of Automobile Engineering. Vol. 215, No D3, pp. 357-368, 2001.
7. Mantriota G.: Theoretical and experimental study of a power split continuously variable
transmission system: Part. 1. Proc. Instn Mech. Engrs, Part D, Journal of Automobile
Engineering. Vol. 215, No D7, pp. 837-850, 2001.
8. Mantriota G.: Theoretical and experimental study of a power split Continuously Variable
Transmission system: Part 2. Proc. Instn Mech. Engrs, Part D, Journal of Automobile
Engineering. Vol. 215, No D7, pp. 851-864, 2001.
9. Mantriota G.: Performances of a series infinitely variable transmission with a type i power
flow. Mechanism and Machine Theory. Vol. 37, No. 6, pp. 579-597, 2002.
10. Mantriota G.: Performances of a parallel infinitely variable transmission with a type ii power
flow. Mechanism and Machine Theory. Vol. 37, No. 6, pp. 555-578, 2002.
11. Mantriota G.: Fuel consumption of a vehicle with power split CVT system. Int. Journal of
Vehicle Design., Vol. 37, No. 4, pp. 327-342, 2005.