Every major manufacturer in the world is now
developing hard or soft hybrid prototypes. Malaysia’s Proton is no exception –
and they have a technological advantage. They own Lotus Engineering... In this
story we look at how Lotus designed and developed a hybrid system that could be
retrospectively engineered into a Proton Gen 2. The result is a car that can be
run as a micro-hybrid, full hybrid or pure electric car.
The Need
Increasing legislation around the world is driving
advances in fuel efficiency and emissions to address the impact of the use of
fossil fuels on the environment. Vehicle manufacturers face the challenge of
producing vehicles with improved fuel economy that offer equivalent or even
enhanced performance at a comparable cost to current models. Advanced
conventional powertrain technologies continue to offer environmental
improvements in both gasoline and diesel vehicles.
However, these alone will not achieve desired
levels of CO2 reduction that governments are targeting.
Significant reduction in vehicle weight will not
be achieved for 10 to 20 years due to current customer and legislative demands
for safer cars, which is adding weight. Low carbon fuels (NGV/CNG/bio–fuels) are
costly for global volume use and require changes in infrastructure and vehicle
technology, although they will become viable in local markets where the relevant
raw materials are available. Although recent advances in battery technology are
bringing full electric vehicles a step closer, they too will require some
infrastructure changes to provide quick charge facilities that compare with
today’s fuel filling stations. The promise of the hydrogen economy is some time
away, due to the required changes in technology and infrastructure.
In short, technology is required which complements
the evolution of conventional powertrains, while enabling further efficiency
gains until a global sustainable energy infrastructure is available. Hybrid
systems provide that technology ‘stepping stone’, building customer confidence
in electric drive trains and providing a further benefit when linked to the use
of alternative fuels. Today’s challenge for this hybrid technology is to provide
economy benefits at a low production cost.
With these issues in mind, our parent company
Proton approached Lotus Engineering to develop a parallel hybrid system that
could be applied to a Proton Gen.2 compact midsize car with a 1.6 litre gasoline
engine, with minimal impact to the base vehicle, and utilising where possible
off the shelf technologies. This ‘retro–integration’ approach is a key advantage
of the EVE (Efficient, Viable, Environmental) Hybrid which was unveiled at the
Geneva Motor Show earlier this year.
It demonstrates a realistic way for many
manufacturers to develop hybrid versions of existing vehicles at much lower
costs than developing whole new hybrid vehicle platforms.
Concept
The start of the program began with the task of
carefully selecting the technologies and approach for the EVE Hybrid.
A variety of hybrid systems were considered that
vary in the way that the drive is combined with the conventional powertrain
layout. These electric hybrids fall into distinct groups; parallel, series,
multi–mode and power–split systems.
The simpler forms of hybrid transmissions – series
and parallel – generally offer less functionality and so reduced fuel
efficiency, but are mechanically easier and cheaper to realise. Series hybrids
are efficient at low vehicle speeds but less efficient on sustained high–speed
cycles, and parallel systems are less suited to low speed work and more
efficient at medium and high–speed driving.
The multi mode and power split transmissions offer
the ability to operate in series or parallel modes depending on operating
conditions. Therefore they offer the greatest overall efficiency, but are
generally more complex to control and mechanically realise, and so are more
costly. Each system was modelled using Lotus Vehicle Simulation (LVS) to
understand the potential benefits in fuel economy and to assist in setting
targets for the next stage.
The study included basic packaging investigations
and a major goal was no modifications to the base vehicle’s chassis, body or
structure. A further important factor of the study was the ability to implement
any solutions into production in a relatively short term. This drove the
designers to use, wherever possible, existing technologies and proven
systems.
The conclusion of the concept study was to develop
a parallel hybrid combined with a continuously variable transmission (CVT) in a
twin clutch arrangement as seen in this diagram. This would allow Lotus to
demonstrate both micro and full hybrid capabilities combined with a continually
Variable Transmission with no compromise to the driving experience of the
vehicle and offering a near–term solution.
Targets
The overall targets for the programme defined
during the concept study, were:
-
Improved fuel economy of 52 mpg (5.4 litres/100
km)
-
Emissions at Euro 4
-
Acceleration 0–60 mph in 9.0 seconds (0–100 km/h
in 9.5 seconds)
-
Vehicle maximum speed 180 km/h
-
Electric vehicle maximum speed 50 km/h
-
Electric vehicle range 5km at 30
km/h
Once targets and specification’s were set,
detailed computer-based models were developed and correlated to measured
acceleration performance, maximum speed and drive–cycle fuel consumption data.
The models were used to perform an initial sizing study for the hybrid vehicle
motor and battery.
The vehicle modelling work was performed using
LVS, which enables the user to build a virtual vehicle driveline and test it
over a drive-cycle (the new European drive–cycle, NEDC was used). Once the
vehicle configuration had been determined, the vehicle model was used as the
basis for developing a control strategy.
This control strategy determines the instantaneous
power split between the motor and engine and is computed based on the efficiency
maps for the motor/generator and engine. The energy-management strategy control
algorithm was developed in Simulink (LVS is able to link directly to
MATLAB/Simulink and can be directly implemented into the vehicle controller
using autocoding). It was optimised, using the vehicle model linked to Simulink,
to reduce the fuel consumption of the vehicle.
Another key objective of the energy-management
strategy was to ensure that the battery state-of-charge was maintained within
acceptable limits.
With this complex control strategy in place, the
EVE Hybrid will consume over 25% less fuel than the standard vehicle over the
NEDC. Additionally, the computed 0-100km/h acceleration time for the HEV is over
2.5 seconds lower than that for the standard Gen2 vehicle.
Retro-Integration
Starting with the engine, various changes were
made to the Proton gasoline 1597cc CamPro unit. The main modification was the
redesign of the front–end accessory drive (FEAD) to accommodate a
starter/alternator for the stop/start functionality. This unit switches the
engine off when the vehicle stops, restarting the engine automatically when the
brake pedal is released.
As a result, noise, emissions and fuel consumption
are reduced.
This change also required a new double-acting belt
tensioner to cope with the new belt drive loading created by engine start
condition. The heating, ventilation and air conditioning (HVAC) was also
exchanged for a hybrid unit that operates as a conventional belt drive
compressor and has an additional electrically-driven capability which enables
continued air conditioning operation when the engine is stopped.
During the FEAD redesign, the opportunity was also
taken to replace the belt driven power steering and water pumps with electric
units to enable the investigation into the economy benefits of such units.
To accommodate the additional engine hybrid
functionality, the engine management system (EMS) was updated to a torque-based
unit and re-calibrated.
Due to tight packaging constraints and to minimise
any loss in overall vehicle performance, custom electric motor and power
electronics were developed in conjunction with specialist suppliers. The 30kW,
144V electric motor is positioned between the engine and transmission. It
delivers electric drive or regenerative braking via an additional clutch linking
the motor to the drivetrain. In this way it can provide the same start–stop
functionality as the starter/alternator with the additional benefit of
electrical drive or drive assist, either boosting the drivetrain performance or
providing economy and emissions benefits by operating as an electric
vehicle.
A motor/generator of this size and its power
electronics generate a great deal of heat and so are water-cooled with their own
cooling system; this required the design and packaging of an additional pump and
radiator.
An additional single–plate clutch with concentric
slave cylinder is packaged inside the motor and connects the engine to the
electric motor. This enables the engine to be switched off for electric
drive-only use. This clutch is controlled by an additional hydraulic control
pack and is supplied with hydraulic power by the electro-hydraulic power
steering pump. The clutch is controlled via servo valve and accumulator by the
hybrid control unit.
A CVT has replaced the conventional transmission,
of which the bell housing was modified to accommodate the electric traction
motor. The integration of a CVT gives benefits in fuel consumption and emissions
control. In addition, its compact package assisted in the application of a
hybrid electric drive and it provides smooth acceleration and low transmission
noise.
To store electrical energy, the EVE Hybrid uses a
30kW, 144V, Cobasys Series 1000, Nickel metal hydride (NiMH) battery pack with a
capacity of 8.5Ah. It has been installed within the boot and incorporates a
built-in battery management system and an integrated liquid cooling system,
which is cooled via air conditioned cabin air taken in through the speaker grill
and expelled through a vent at the rear of the vehicle.
Making all these systems work together required a
sophisticated custom Lotus Engineering control unit and software. These have
been developed to monitor and control all sub-control systems including the
engine management, motor/generator, transmission, HVAC, additional clutch,
electro hydraulic PAS pump, starter/generator and battery pack.
This Hybrid Control Unit (HCU) is designed around
the real–time, floating point Data Signal Processing (DSP) operating at 200MHz.
The main processor supports Mathworks RTW Autocode which has been used to
develop the control algorithms. All parameters and variables can be examined by
a firewire interface connected to an external PC in addition to an ETAS INCA
interface support via CAN.
Vehicle Systems
Various vehicle systems were impacted by the
introduction of the hybrid systems.
To maintain the integrity of the braking circuit
when the petrol engine is off, an electric vacuum pump was added to maintain the
vacuum assist for the brakes. An electro-hydraulic power-assisted steering pump
has been fitted, which allowed the retention of the existing steering column and
hydraulic steering gear. This pump also provides hydraulic pressure for the
additional clutch that engages the hybrid motor.
The front suspension was tuned to accommodate the
redistribution of vehicle mass. The 12V battery was updated to a more robust
gel–filled fibre mat unit due to the changes seen in usage and relocated to the
boot to provide more space for the auxiliary power supply, which provides 12V
from the high voltage system.
Changes to the conventional engine cooling system
required analysis and development work to implement an electrical cooling pump
and accommodate the impact of additional radiators, an oil cooler for the
transmission and a secondary radiator for the traction motor and its power
electronics. Inside the cabin the only obvious changes are that the instrument
panel incorporates an additional LCD touch screen display to show battery
charge, power split between engine and motor, fuel economy and charge/discharge
rate, and incorporates associated warning indicators where appropriate. Also
fitted to the dash are new switches to select the car’s operating mode –
micro-hybrid, full hybrid or electric vehicle. The gear lever was also changed
to match the requirements of the new transmission.
Results
When compared to the original vehicle, there are
both performance and efficiency benefits that validate the approach. Operating
in its full hybrid mode, there is an impressive 22% reduction in CO2 from
172g/km to 134g/km equivalent to a 28% improvement in fuel economy.
In micro-hybrid mode alone, there is a 5%
improvement in CO2 emissions and fuel economy.
In themselves, these figures prove the success of
the car and the project – significantly greener performance through an
affordable implementation – but actually in full hybrid mode the EVE Hybrid has
0–60mph acceleration improved from 12.6 seconds to 9.0 seconds.
