A UK government challenge to produce a
medium/small vehicle with CO2 emissions of less than 100 g/km has seen
engineering company Ricardo team up with PSA Peugeot Citroën and UK research
group QinetiQ to develop an innovative diesel full-hybrid. This article first
appeared in the Ricardo Quarterly Review and is used with permission.
The diesel engine is universally acknowledged as
the most efficient internal combustion engine in widespread land transport use.
It can demonstrate its efficiency in a variety of ways – hauling 40 tonne
payloads across continents, wafting busy executives along hectic Autobahns at
200 km/h or, more recently and at the other end of the power scale, winning the
gruelling 24 hours of Le Mans by a handsome margin (see
Audi R10 Diesel Race Car.)
More recently still, JCB and Ricardo demonstrated
the most extreme application of the diesel’s inherent performance potential when
the 1500 horsepower Dieselmax streamliner took the world land speed record for
diesel-powered cars at Bonneville Salt Flats in Utah in August 2006, raising the
bar to over 350 mph (see
350.092 mph... Breaking the Diesel World Speed
Record).
Yet it is with low numbers for CO2 that the diesel
is still primarily associated: here’s where the diesel’s inbuilt efficiency can
be exploited to maximum effect to generate fuel economy figures that just a
decade ago would have been dismissed as wishful science fiction thinking. But
while the diesel is unrivalled as a near-perfect power source for everyday
running, even the most efficient of power units will be wasting energy if it is
asked to operate outside its optimum regime or if it continues running when it
is not needed to accelerate the vehicle or maintain its speed.
That’s why, when the UK government’s Department
for Transport issued its Ultra Low Carbon Car Challenge (ULCCC) to the auto
industry in 2003, Ricardo was quick to propose a diesel-fuelled full-hybrid.
Only this combination of a peak efficiency engine with a truly optimised system
of energy management would ensure that the demanding target of 100 g/km CO2
emissions could be achieved, reasoned the Ricardo team. Considering that that
target had to be met on a C-class medium sized car meeting market-competitive
performance criteria and with no loss of comfort or amenity, the challenge was
indeed a daunting one.
Teaming up with industry benchmark diesel engine
producers PSA Peugeot Citroën and energy storage specialists QinetiQ, the
Ricardo consortium – which had by this time acquired the name Efficient-C – was
awarded the £3 million program to go ahead and develop a demonstrator vehicle
for the new hundred-gram technology; Ricardo chief engineer David Greenwood was
appointed project leader, and work began in February 2004.
Efficient-C’s target: 3.75 litres per 100 km
In a way, the name says it all. With C standing
for the element carbon as well as the C segment in which the eventual production
car would compete, the design demanded maximum efficiency in every aspect of its
engineering if it was to achieve the necessary step-change improvement in fuel
consumption, and thus CO2 and greenhouse gas emissions.
Chasing a saving of almost one third over 2003-era
models meant that no system on the vehicle would be immune from scrutiny in the
quest for absolute energy efficiency. Air conditioning, steering, braking and
even the engine cooling circuits were all to be re-thought along energy saving
lines – and of course the powertrain, where the major gains stood to be made,
would come in for the biggest engineering effort of all.
By now the responsibilities of the three technical
partners had been more formally defined, with PSA Peugeot Citroën contributing
its expertise in diesel engines, stop-start systems and hybrid componentry and
QinetiQ exploiting its military experience to provide the high voltage network
and advanced battery systems. Ricardo, as project leader, took on the hybrid
control strategy as well as the key responsibility for overall vehicle
integration.
However there was one surprise when the project
went public in early 2004: the choice of the very roomy but relatively
utilitarian and unstreamlined Citroën Berlingo compact MPV as the demonstrator
vehicle.
“The choice was deliberate,” said project leader
Dave Greenwood. “The Berlingo uses the same engine packages as the mainstream
Peugeot and Citroën hatchbacks, so the technology is transferable. By going for
99 g/km CO2 in the bulkier Berlingo, we know this will translate into under 90
g/km in a more aerodynamically efficient vehicle such as the Peugeot 307 or
Citroën C4. We also want to show that the technology can be applied to a basic
family car and that it does not require advanced streamlined carbon-fibre
bodywork or any limiting of practical utility.”
Breakthrough in CO2 Emissions
Robert Peugeot, vice president of innovation and
quality at PSA, underlines the seriousness of the situation and the need for a
stepchange in the control of CO2 emissions: “We are extremely concerned over the
medium and longterm evolution of CO2 emissions and new technology is one part of
the solution,” he told an audience of stakeholders and press at the presentation
of the Efficient-C project. M. Peugeot speaks with unparalleled authority on the
matter: PSA has a 30 per cent European market share of vehicles emitting under
120 g/km CO2, while in the under 110 g/km banding its share is 60 per cent.
“The work carried out with our partners in the
Efficient-C programme shows that remarkable performance can be delivered by
means of hybridisation of a base vehicle fitted with an HDi engine,” added M.
Peugeot.
This is a view strongly supported by Greenwood: “I
think that diesel hybridisation is the only currently available technology-based
solution capable of bringing a significant breakthrough in terms of consumption
and CO2 emissions in the European market.”
Program Definition
Though the UK government’s challenge did not
specifically require it, Ricardo and its partners decided at an early stage that
the base concept should be expanded to provide the useful additional benefit of
zero emission pure electric operation for sensitive city centres.
“For this reason we selected a diesel
full-hybrid,” said Greenwood. “We chose a 1.6 litre HDi engine, almost identical
to the standard unit in the Berlingo, as our starting point, and linked it to a
288 volt, 23kW motor generator.“
Right from the start, the concept was conceived to
maximise commonality with non-hybrid variants and to minimise the likely cost of
production, another of the Challenge’s stipulations.
Where the consortium chose to depart from the
specifications, however, was when early simulations had shown that the concept
could improve significantly on the stipulated performance threshold of 0-100
km/h acceleration in a maximum of 16 seconds.
“We believed we could improve upon the Challenge’s
acceleration target and that 0-100 km/h in less than 13 seconds was achievable,
together with a top speed of at least 150 km/h,” said Greenwood. “The decision
was also made to fit a diesel particulate filter in order to reduce particulate
emissions to a barely measurable level.”
By now the Efficient-C architecture had become
finalised. A five-speed automated manual transmission was chosen as the most
efficient means of keeping the engine in its optimum operating regime, while the
electro-hydraulic clutch was positioned between the engine and the electric
motor generator so as to allow independent electric operation without the diesel
engine running.
Unusually for a hybrid, Efficient-C has a separate
12 volt starter generator for firing up the diesel engine, rather than relying
on the integrated electric machine as on gasoline hybrids. “It’s for
refinement,” says Greenwood. “Because of the diesel’s higher compression ratio,
the main electric machine could not restart the combustion engine without the
passengers noticing: with the separate starter we can restart the engine while
it is decoupled from the electric drive system, then seamlessly blend the two
power sources.”
The electrical drive architecture is completed by
a DC/DC converter and an advanced lithium-ion battery, located under the luggage
compartment floor; the 288 volt array is managed by an efficient control system
monitoring everything from state of charge to temperature to ensure maximum
performance and life expectancy.
Innovative Control Strategy
Overseeing the operation of almost every system on
board, including the all-important management of current flows and energy
recuperation, is an advanced supervisory control system based on Ricardo’s rCube
prototype controller.
“Almost half our total project effort went into
this control system,” says Greenwood. “The aim is the seamless integration of
all systems and functions. We have five CAN networks – most cars have one or two
– and there are six additional microprocessors.”
More than 70MB of bespoke control code were
written for the project, says Greenwood. This compares with less than 40MB for
most cars.
The substantial built-in electronic capacity
allowed the project engineers to implement many innovative powertrain control
strategies, including no fewer than six different operating modes. It is the
task of this control system to interpret the driver’s demands and respond to
them in the globally most effective manner.
Underlying all the thinking, however, is the
golden rule that the diesel engine must only be run when it makes sense from an
energy efficiency point of view.
“Diesel engines are most efficient at roughly
one-third speed and two-thirds of the rated torque,” explains Greenwood. “That’s
why we don’t run the engine in the non-efficient areas of its operating map.”
As a result, Ricardo’s map of a typical mid-sized
diesel’s fuel efficiency shows three distinct areas of operation: the
traditional broad speed-load area required for the standard MVEG consumption and
emissions cycle, a smaller island of peak diesel efficiency within this area,
and a distinct no-go zone at light load and low rpm where electric rather than
internal combustion power is called for.
What is innovative on the Efficient-C map,
however, is the way in which the diesel is more often used in its area of high
efficiency. Dave Greenwood explains: “What we’re doing here is increasing the
load on the diesel engine so as to move it into the most efficient portion of
its map. We do this by generating current and storing it for later use.” In this
way, Efficient-C is able to score an effective win-win, keeping the diesel
engine in its sweet spot while also generating current in the most efficient way
possible.
Six Modes of Operation
Besides the innovative efficiency boosting mode
just described, Efficient-C can also operate in five other different ways, the
controller system switching the vehicle seamlessly between modes without the
driver or passenger being conscious of the change. The only driver-selected mode
is that of electric-only drive for zero-emission city zones.
Under steady cruise conditions, the conventional
IC mode is most likely to be invoked. With the diesel engine driving the wheels
directly, the automated transmission is responsible for ensuring the powertrain
is always in the right gear.
For hard acceleration, where greater torque is
required than the diesel engine alone can supply, the electric machine draws
power from the battery to send additional torque to the wheels. In contrast, for
low-speed urban driving and for pull-away from rest, electric operation is much
more efficient: accordingly, the diesel engine remains stopped and the battery
powers the motor-generator directly.
With the vehicle slowing from speed, the diesel
engine is automatically shut down and the motor-generator captures the vehicle’s
kinetic energy and stores the regenerated current in the battery for later use.
“When the driver lifts off the throttle,” says Greenwood, “we shut down the
engine and perform light regenerative braking to replicate the engine braking
effect. When the brakes are pressed we increase the amount of regenerative
braking until the capacity of the motor is saturated, and only then do we engage
the foundation hydraulic brakes. In this way, during normal city driving almost
all braking is done electrically.”
The final mode, designed to cope with the reality
of dense contemporary traffic where the vehicle may be stationary for a
substantial period with current-consuming functions such as lights, wipers, air
conditioning and audio in continuous operation, sees the diesel engine fire up
and connect directly to the generator for a short time to keep the battery
topped up to the appropriate level. This does not happen very frequently: the
battery capacity is such that these electric loads can be supported for around
20 minutes before the top-up is needed.
Across all six of these modes the low-temperature
cooling circuits are able to protect the motor and power electronics from
overheating, while the DC/DC converter enables the 288 volt motor generator to
provide a more efficient 12 volt supply than would have been possible via a
conventional 12 volt generator. This is especially important, given the
electrical load represented by such key functions such as the 12 volt
electro-hydraulic power steering and the electric vacuum pump for the brakes.
Since it draws up to 6 kW, the air conditioning compressor is powered at 288
volts as it is far more efficient to supply this at high voltage and low
current.
Mission Accomplished; Mission Exceeded
With the modesty typical of a confident, dedicated
engineer, Dave Greenwood confirms that all the targets of the Ultra Low Carbon
Car Challenge were met or exceeded. “We achieved a 30 per cent improvement in
overall consumption and CO2 emissions compared with the already highly efficient
base HDi engine,” he affirms. “This puts even the high roofline Berlingo into
the 99 g/km CO2 bracket.”
Particularly impressive is the 45 per cent fall in
in-town consumption, confirming the effectiveness of the hybrid system in
stemming urban emissions. This figure is even computed with the battery charge
state stable for continuous in-town operation – and if the car drives into and
out of the town along a dual carriageway, the in-town performance is even
better.
“We’ve managed a small improvement in performance,
too,” he says with some satisfaction. This makes Efficient-C a strong candidate
for the status of one of the world’s most efficient internal combustion
powertrains.
Breaking down the 30 per cent efficiency
improvement, Greenwood says 6 per cent of the total savings come from reduced
rolling resistance and slightly improved aerodynamics, 4 per cent is contributed
by the engine stop-start system, 8 per cent by the optimisation of the engine’s
operating regime, and 12 per cent comes from the storage and re-use of
regenerative braking.
Compared with other advanced technology vehicles,
he says, Efficient-C has class-leading powertrain efficiency. The current
best-in-class gasoline hybrid vehicle, the Toyota Prius, notably emits slightly
more tailpipe CO2, even though its energy requirement in terms of mass, rolling
resistance and aerodynamic drag is some 25 per cent less.
[Ah, but the Prius
is a production car that’s been available for nearly 10 years! – Ed]
But perhaps the most significant endorsement comes
from the company that has been the most consistent backer of the diesel hybrid
concept. “The technical breakthrough is here,” said Robert Peugeot at the
project’s rollout: “It’s here in front of us.”
The Next Frontier
The next challenge, says M. Peugeot, is to move
from this project experience to an affordable car. “Three thousand pounds
(€4300) is clearly too much for the customer to pay,” he says, referring to his
own engineers’ estimation of the likely extra cost of a hybrid C4 or 307 over
the price of a conventional HDi diesel edition. “We have to work and do further
research to progress from this prototype to put an affordable car onto the
market.”
“Customers are very pragmatic,” observes M.
Peugeot. “They make the final decision, and we find that they are not prepared
to pay for more than a certain level of technology – though it might be possible
to make them change their mind if suitable incentives were in place.”
Those incentives should not be
technology-specific, he adds, but must be geared directly to CO2 emissions and
be linear, rather than artificially banded, in their application. “Then it’s our
job, as an automaker, to set the right products in front of the customer.”
The success of the Efficient-C project in
producing the world’s most energy efficient C-segment five-door car demonstrates
that the diesel hybrid is indeed the most convincing CO2- reducing solution yet
devised – the ‘right product’ so elegantly described by M. Peugeot. All that now
remains is for those same engineers to apply equal ingenuity to the perhaps
tougher task of bringing the on-cost of diesel hybridisation down into the £1500
target zone that PSA has identified as the extra amount the European customer is
prepared to pay.
Technical Specifications
1.6 litre HDi engine: 92 hp at 4000 rpm
Electric motor: permanent magnet synchronous, 23
kW peak power, 130 Nm torque
Transmission: five-speed automated manual,
electro-hydraulic clutch
Brakes: combination of electrical regenerative and
conventional hydraulic
Fuel tank: 60 litres
Kerb weight: 1374 kg
Key Performance Indicators
|
Berlingo Multispace |
Efficient-C |
Maximum speed |
158 km/h |
171 km/h |
0-100 km/h |
14.8 sec |
13.4 sec |
80-120 km/h |
17.9 sec |
12.3 sec |
Fuel consumption, urban |
6.7 lit/100 km |
3.7 lit/100 km |
Extra-urban |
4.7 lit/100 km |
4.0 lit/100 km |
Combined |
5.4 lit/100 km |
3.75 lit/100 km |
CO2 emissions |
143 g/km |
99 g/km |