Diesel Hybrid!

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