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Burning questions
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01/02/2007
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Pressure is mounting on engine designers to increase specific power output, deal with emissions and make their products burn a wider range of fuels. They are responding to the challenge with a range of materials, gas control techniques and fuel strategies. Jeff Daniels reports
The traditional concerns of the engine designer have always been to provide sufficient power and torque, and adequate durability, with the smallest, lightest and most economical engine. To these was added a need to comply with exhaust emissions legislation. As we move deeper into the 21st century, a further concern has been added: the need for compatibility with ‘eco-fuels’.
None of the earlier concerns have gone away. Indeed they remain an important facet of technical philosophy as applied to engine design. But the range of technologies that must now be considered by the powertrain designer is wider than ever. In particular, the fuel system and the exhaust system have become almost as important as the ‘guts’ of the engine itself.
The materials scientist still has a part to play. Weight-saving has become part of engine design, and with the majority of gasoline engine blocks now cast in aluminium, magnesium is the most obvious way forward - although so far only BMW has used the material in large-scale production. Honda, arguably the other leader in engine technology as a whole, has developed its expertise in the insertion of composite reinforcements in ‘slush moulded’ aluminium which, apart from reducing the machining requirement, allows thinner walls and thus more compact blocks.
Compactness, plus the search for optimum thermal efficiency and lower friction, has led to even more interest in the inline three-cylinder layout for 1 litre engines for the most economical cars. Volkswagen now offers one such engine in its Fox and a three-cylinder diesel for the Golf, while Opel has continued its use of a three-cylinder unit in the new Corsa, Mitsubishi offers a 1.1 litre unit in its latest Colt, and Toyota has a three-cylinder Aygo. Apart from providing a better combustion chamber surface to volume ratio and a smaller friction surface, the compact three-cylinder is easier to install transversely in a small car - although it must be said that current units all have larger capacity four-cylinder counterparts which must fit into the same space. Mechanical balance and operating refinement seem no longer to be a serious issue.
More fundamentally, today’s engine designers seek to optimise efficiency by seeking total control over gas flow and the combustion event. This has involved the ability to vary valve timing and lift, the geometry of the inlet ports, and the use of ever more subtle and complex forms of fuel injection, as well as exploiting advances in sensor technology.
Early moves to control gas flow included variable valve timing and the use of swirl control valves in the inlet ports of 16-valve engines, with the object of maintaining high gas speed at low engine rpm. Today’s most advanced engines, notably from BMW and Honda, exercise total control over valve lift as well as timing. BMW does so with its Valvetronic system, launched in 2002 on its four-cylinder engines but now used across its gasoline engine range, including the new 1.6 litre unit developed as a joint venture with PSA and fitted to the Mini.
Honda has continued to evolve its VTEC system and in September 2006 announced its latest iteration, a 2.4 litre engine -combining continuously variable valve lift and timing control with the continuously variable phase control of VTC (Variable Timing Control)-. When this system is combined with optimised intake components the result, according to Honda, is a 13% reduction in fuel consumption plus remarkably low exhaust emissions. Production is planned “within three years”.
Many of the latest high-tech gasoline engines use direct injection. After a stuttering start in the 1990s with Mitsubishi and Toyota, when observers questioned whether the principle provided sufficient return (in relation to its cost) to justify its use, other manufacturers added their own interpretations. To some extent the interest moved to Europe, where BMW, Mercedes and Volkswagen all now offer models with direct injection. The focus has moved away from lean-burn operation at part-load, and towards the function of direct injection in optimising spray formation to achieve clean, efficient combustion. As with common-rail diesels, there has been a tendency to increase injection pressures (although with gasoline, the pressures are still far lower) to increase injection rates and improve atomisation.
There has also been renewed interest in forced induction. During the 1990s, interest in boosting the induction rather waned, but the technique has once again been taken up with enthusiasm.
Some modern applications depend on the ingenious use of more than one device. BMW now fits some of its engines with a twin-turbo system, the units differing in size and switching between single, parallel and series induction according to engine speed and load, to eliminate, for all practical purposes, the phenomenon of ‘turbo lag’.
Double the benefit
Volkswagen has applied its TSI system to a carefully adapted 1.4 litre engine. With TSI, a clutched supercharger works in series with a turbocharger, filling in the bottom end of the torque curve where the turbo would normally be lacking. The TSI concept is very much about downsizing: the 1.4 litre engine (with its block of grey cast iron for much increased strength) delivers up to 125kW, enough to deliver excellent performance in the Golf GT, and indeed the VW Multivan in which it has also been demonstrated.
Meanwhile, Porsche elected to develop a variable-geometry turbocharger for its new 911 (997) Turbo. VG turbos are familiar technology in turbodiesels but until now gasoline engines with their higher exhaust gas temperatures could not use them. A possible alternative, first discussed by Visteon some years ago, is to use an electrically driven compressor to provide a modest boost effect to increase low-speed response and overall efficiency; so far there is no news of a production application.
Any powertrain engineer will explain that the best way to deal with emissions is not to create them in the first place, which is the driving force behind all the work being done on induction and combustion. Mixture preparation remains the key. Yet it remains a fact that tomorrow’s exhaust emission regulations are being pitched so low that even a theoretically perfect combustion system would not be able to comply, because of the interfering effect of the nitrogen which forms the bulk of ambient air, and of impurities in fuel, most notably sulphur. Recognising this, the authorities have mandated high levels of fuel (gasoline and diesel) purity for the near future, with sulphur content reduced below 50 parts per million.
Such levels, in conjunction with highly efficient catalytic converters and extremely capable control systems whose rapid response depends on advanced sensors and formidable computing capacity, make gasoline engines capable of complying with the most demanding emissions requirements now tabled, other than the ultimate ZEV (zero emissions) standard.
Diesels remain another matter, yet new concerns about the greenhouse effect and global warming make it essential, in the view of many, that the economical, high-technology diesel should take a larger share of the market, not least in the USA.
The challenges here, as always, are NOx and particulates. After many years of patient development, NOx reduction (or storage-reduction) catalytic converters have been made to work reliably at high conversion rates. An alternative, being strongly pushed by DaimlerChrysler and Volkswagen, is metered urea injection to reduce NOx, the principle behind the BlueTec programme. The two partners see BlueTec as the route to mass diesel penetration of the US market, when it is used in conjunction with an oxidising converter and a particulate filter. Particulates can still only be reduced to very low levels by trapping and burning, which seemed little more than a theory until PSA pioneered in use in volume in the late 1990s. Now all diesel manufacturers have their variations on the theme, and the cost has been either absorbed or passed on to the consumer.
However, the biggest of all concerns for powertrain engineers today is to achieve compatibility with the coming generation of eco-fuels. The problem is that such fuels are very much in the plural. There is no sign of emerging standardisation, rather a contest between various parties and vested interests whose view, and pet solutions, range across a wide spectrum - precisely what the powertrain engineer does not need.
In the first instance, there are fuels which reduce the amount of carbon released from fossil fuels, with natural gas (essentially methane, CH4) leading the way, and hydrogen as the long-term objective. They seem to compete with the true eco-fuels, derived from vegetable matter and therefore involving carbon only in a short loop of burning, plant fixation, harvesting and reconversion into fuel. The principal eco-fuels have been the alcohols, methanol and ethanol (and possibly also butanol) which can be blended with and ultimately replace gasoline; and the esters, derived from vegetable oils and fats, which can in the same way serve as replacement diesel fuels.
This by no means exhausts the possibilities. Volkswagen for example is pushing the idea of Synfuel, a synthetic hydrocarbon derived from natural gas, and of Sunfuel, applying similar principles to biomass (and therefore genuinely an ‘eco-fuel’).
The challenge for the powertrain designer is that this range of potential fuels involves a huge range of properties - of density, volatility, combustion temperature and octane rating, to say nothing of possible chemical effects on vulnerable parts of the fuel system, and safety considerations, when they are used in pure rather than blended form. And one can hardly conceive of tomorrow’s car owner being willing to drive onto a filling station forecourt and pick the right fuel from a dozen dispensers of different kinds.
There is no doubt that all the alternatives can be made to work. Vehicle manufacturers have been at pains to provide opportunities for technical journalists and other opinion-formers to drive vehicles running on a wide variety of fuels, proving they drive normally and the refuelling process will not tax anyone capable of handling a self-service gasoline pump without mishap.
Yet this is not really the point. At some stage the largest markets, such as the EU, will have to decide which is the eco-fuel of choice, and oblige its introduction alongside gasoline and diesel, with the ultimate aim of replacing both. For the moment, if the 2007 Detroit Show provides evidence to go by, manufacturers are concentrating on ‘flex-fuel’ vehicles for the US market – the flexibility in this instance consisting on the ability to run either on gasoline or on E85 (85% ethanol, 15% gasoline).
Flex fuel a compromise
That is surely only one step, and not necessarily in the right direction, if only because flex-fuel engines inevitably sacrifice some efficiency compared with an engine optimised to run on one fuel or the other.
Standardisation on a single fuel could be a long step towards reaching the other holy grail of powertrain engineers, the homogenous charge compression ignition (HCCI) process. This calls for the uniform yet spontaneous compression-ignition of a homogenous charge already present in the cylinder during compression - as distinct from the diesel principle in which fuel is injected towards top dead centre, and spontaneously ignites shortly afterwards. Achieving smooth, stable HCCI has proved extremely difficult, but the task is certainly eased if it is based on a synthetic biofuel of consistent quality with zero impurities.
Volkswagen (which prefers the acronym CCS - combined combustion system - to HCCI) has demonstrated a 2 litre unit which, by comparison with the TDI engine from which it was derived, is 5% more economical, with very low NOx and particulate emissions.
Cracking the challenge of HCCI is one of the tasks still confronting powertrain engineers, but it is not the only one. There is, for example, the question of optimising engines which form part of a hybrid powertrain. We have seen, for example, how the gasoline prime mover in the Toyota Prius runs a high-efficiency Atkinson cycle which would simply not be possible if it was required to provide normal driveability through a mechanical drivetrain. With JD Power forecasting (at Detroit) a 2012 market for nearly 800,000 hybrid-electric vehicles from all the major manufacturers, it seems likely to prove yet another challenge to be overcome in the never-ending quest for the ultimate engine.
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Author
Jeff Daniels
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