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01/05/2008
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Do hybrid powertrains really deliver the best of two worlds, asks Andrew Noakes, or are they just environmental window dressing?
 Colin Chapman of Lotus, one of the most ingenious structure and suspension engineers in the history of the car, had little interest in engines. Sketching concepts for new designs, it is said he would draw chassis details and suspension geometry in some detail - but when it came to the powertrain he would simply draw a box, and label it 'engine'.
Approach the design of the perfect passenger car powertrain from a similarly detached perspective and it is easy to isolate the essential factors for a successful design. Performance factors like power, range and fuel consumption are obvious, as are the economic issues of production cost and feasibility, reliability and maintenance. Packaging requirements and crash performance might be key design factors, while the powertrain contribution to NVH will probably be an important influence on customer satisfaction. In the modern world, tailpipe emissions and end-of-life recyclability are also likely to be important.
The rise to prominence of the internal combustion engine, and its continued dominance over the years, has come from the exceptional breadth of its ability across these areas. Decades of development have optimised or at least improved every area, but IC powertrains still have drawbacks – and modern concerns over the use of fossil fuels and the problems of global warming are throwing those drawbacks into sharper focus. Emissions are an increasing concern, as is poor efficiency in urban driving where the gasoline engine in particular is working at the lower end of its speed range and at part-throttle, well outside the most efficient part of its operating envelope.
One of the solutions is to use a hybrid powertrain, which has two power sources working together. Where the dominant power source is an IC engine, the secondary power source can take over during low-speed running to minimise operation of the IC engine in its inefficient range, and the two sources can work together to deliver higher levels of acceleration when demanded. Although mechanical systems using flywheels to store energy have been successfully used in public transport vehicles and are likely to make an appearance under the new F1 regulations in 2009, recent developments in passenger cars have concentrated on gasoline/electric and now diesel/electric hybrid powertrains.
Electric motors deliver excellent efficiency even at low speeds with potentially low emissions, and consume no power at standstill. Adding regenerative braking systems to recover energy which would otherwise be lost is relatively simple, giving a further boost in efficiency. But despite their advantages in the urban mode, electric motors cannot yet replace IC engines because of their poor open-road performance and range, problems which even the latest improvements in battery technology and the development of fuel cells have yet to solve. Combining an electric motor and IC engine into a hybrid powertrain, however, might provide the best of both worlds.
In their early days hybrids were classified as 'series' or 'parallel' depending on the architecture of the energy flow paths, but recent designs have blurred the distinction. In a series hybrid the two energy sources - battery and IC engine - both provide power for an electric traction motor. As there is no mechanical connection between the engine and the wheels, engine and vehicle speeds are decoupled so the IC engine can largely be operated within its region of minimum specific fuel consumption, to the benefit of economy and emissions. Meanwhile, the excellent torque characteristic of the electric motor - which can deliver maximum torque from zero speed - means complex transmissions are unnecessary. On the debit side, a large and heavy electric motor is needed as it must provide all the power to drive the car, and there are significant losses in the conversion of energy from mechanical to electrical and back. This form of hybrid is most commonly seen in diesel/electric railway vehicles.
In a parallel hybrid the IC engine power and the output from a battery-powered electric motor are combined by a mechanical coupling to drive the vehicle. If the IC engine also drives a generator to recharge the batteries on the move you have a series/parallel hybrid, the best-known example being Toyota's Prius which has sold more than 900,000 units since 1997. Toyota's Lexus brand has introduced hybrid models based on similar technology in the SUV and executive sectors, though it is difficult to see how even a hybrid drivetrain can add much environmental credibility to a 2.3 tonne limo motivated by a 5.0 litre engine. Delivering 'the performance of a V12' with a mere V8 is hardly the environmental breakthrough the world has been waiting for.
All Toyota's current hybrids, in common with the majority of hybrid and electric vehicles available so far, use nickel metal hydride cell technology to store electrical energy. Although lithium ion cells have twice the energy density of nickel metal hydride cells, they have disadvantages which have yet to be overcome. Chief among these is their fragility: they are susceptible to damage by vibration or puncture, which can short circuits or lead to thermal 'runaway', leading to fire or even explosion. To minimise the risks, lithium ion cells must be protected from physical damage, kept cool, charged within strict limits and carefully monitored. Manufacturers of lithium ion mobile phone and laptop computer batteries have been forced to execute recalls in recent years because of problems with lithium ion cells. Similar problems with batteries big enough for automotive applications would be much more serious.
In addition to the safety issues, lithium ion batteries have yet to demonstrate that they can last the life of the vehicle without major deterioration in performance. Cost will also continue to be an issue until production volumes increase.
Although some analysts believe Toyota has invested heavily in Fuji Heavy Industries largely because of its involvement in lithium ion development, the next generation of Toyota hybrids will be sticking with nickel metal hydride cells. Honda's replacement for the Civic Hybrid, due in 2009, will also use nickel metal hydride cells. But the tide may be turning for lithium ion technology. Toyota has used it in some smaller domestic-market applications already and at one stage planned to introduce it in the next-generation Prius, but appears to have had a change of heart. The Tesla electric roadster, which is now entering production, and the Honda FCX Clarity fuel cell vehicle to be unveiled later this year are both based on lithium ion cells. Daimler has released details of its own Li-ion batteries for hybrids, which are plumbed into the vehicle's climate control system to keep them cool.
GM is currently funding two joint-venture projects - one with A123Systems and Cobasys, the other with Johnson Controls and French company Saft - which are developing two different lithium ion technologies for the 2010 Chevy Volt. Though GM prefers to call Volt an 'extended range electric vehicle' it is a series hybrid, with a small IC engine driving a generator which charges the Volt's batteries on the move to increase its autonomous range from the 40 miles it can achieve in battery-only mode to as much as 640 miles. Unlike conventional hybrids, the initial battery charge is provided by a connection to grid electricity which, in theory at least, is a more efficient method of energy use than generating electricity on the move using a small-capacity generator. An overnight charge from the grid, using cheaper off-peak energy, is intended to provide enough range to cope with an average commute.
'Plug-in hybrids' might be able to play a significant role in the future, but like electric-only vehicles they only make real sense if there is a shift towards more sustainable generation of grid electricity.
Volvo has also shown a plug-in hybrid concept, called ReCharge, which has four individual wheel motors, a 1.6 litre flexible fuel IC engine and a battery-only range of 100km. Volvo is one of several European OEMs to show hybrid technology demonstrators. PSA is another, but like Volvo it has yet to turn any of the hybrid concepts it has displayed into a production vehicle. VAG displayed a hybrid Golf concept at the Geneva show earlier this year and a production version is likely to follow, while a hybrid Audi Q7 is due for launch later this year. Porsche, BMW and Daimler are all developing premium-sector hybrids for production (the latter pair collaborating on hybrid driveline technology) but European OEMs have so far favoured 'mild hybrid' strategies using crankshaft starter/generators and intelligent control of auxiliaries to recover energy otherwise lost during braking. Research by transmission design consultant Romax Technology suggests that in urban conditions a simple stop/start system can cut idling fuel consumption by more than 80%, saving a typical vehicle 0.4 litres/km.
In Europe compression ignition (CI) engines are still widely favoured over full hybrids – no doubt influenced as much by the OEMs' many years of costly investment in CI engines and their popularity in the European market as by any significant advantages in service. Whether such a view will persist remains to be seen. If fuel prices continue to rise, and demand for hybrid vehicles continues to drive production volumes higher, hybrids could become more cost effective than diesels. Already hybrids are much more widespread in Japan and in the US, both markets which have yet to take CI vehicles to their hearts.
In the meantime there are promising areas for hybrid development. Because the IC engine used in a hybrid powertrain does not need to work over a wide speed range, expensive systems for changing cam phasing, valve lift and induction length are all rendered obsolete and the engine can be optimised for a much more restricted range of operation. The smaller speed range is also a good fit for emerging homogenous charge compression ignition (HCCI) combustion strategies, which cannot operate at the extremes of the engine's operating speed range and struggle to deal with transient conditions, but which have the potential to improve efficiency in mid-range steady-state operation. Because a wide operating speed range and swift response become unnecessary, it is also possible to consider alternative engine types which have so far been unacceptable for passenger vehicle use, such as gas turbines.
Lithium ion technology is also developing. Researchers in Japan have created flexible lithium ion batteries and at MIT lithium ion chemistry has been built into cellulose sheets, impregnated with carbon nanotubes to form the cell's anode. Ultimately it might be possible to mould these new forms of lithium ion battery to the shape of body or trim panels, solving problem of packaging bulky batteries within the vehicle structure.
Ultracapacitors are an attractive alternative to electrochemical cells for the storage of electrical energy. Energy density is currently not competitive with most rechargeable battery chemistries, but capacitors do have the advantage that their capacity can be fully utilised - many battery designs have acceptable service life only when very high or low charge states are avoided by careful management, reducing their usable capacity. Ultracapacitors can also deal more effectively with the high power levels produced by regenerative braking, making them ideal for use in 'microhybrid' stop/start systems.
Progress in all these areas is bound to make a positive contribution to the way passenger cars use ever more limited energy resources. What's disappointing is that so many of the latest hybrid designs are applying the technology to vehicles which are intrinsically unnecessary and wasteful, where a more effective method of improving fuel efficiency would simply be to downsize to a more appropriate vehicle. The real benefit to the consumer in these cases is not the improvement in fuel consumption or the reduction in emissions which results from the adoption of a hybrid powertrain, but the more trivial massaging of the buyer's conscience: a hybrid is seen as environmentally responsible, even in practice it returns fuel economy no better than a conventional rival vehicle. It's something which some hybrid brands have been quick to capitalise on in their advertising campaigns. Most car buyers, it seems, have little real interest in what goes on under the bonnet - something they share with at least one of our industry's engineering icons.
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Author
Andrew Noakes
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