Alternatives to four-stroke engines promise an increase in efficiency

21st February 2013

For 100 years the four-stroke internal combustion engine has been the dominant automotive powerplant, but there is a pressing need to improve fuel efficiency, emissions and power density beyond what can be achieved through fine-tuning this concept. Paul Stevens reports on some innovative alternatives.

On a day-to-day basis it can be difficult to see how the marketing messages associated with a product are shifting. But look across a number of decades and it can be seen how much aspirations have changed and how the products have developed to reflect that. The marketing of cars in the 1980s, for example, was all about levels of equipment. A car that offered electric windows, a radio-cassette player and a sunroof was considered quite special, yet those features soon become standard equipment in most manufacturers' brochures. Through the 1990s, a key area of product differentiation was coefficient of drag (Cd). If a manufacturer could claim that its latest vehicle had the lowest Cd in its class, then that was a major selling point. By the end of the decade, though, it was difficult to find any mention of Cd even in the small print.

Automotive marketing through the early years of the twenty-first century was strongly focused on safety, with the introduction of the European NCAP ratings, and manufacturers competing vigorously to offer ever more effective crumple zones and increasing numbers of air bags. Just a few short years later, though, high levels of safety are largely taken for granted.

A focus for differentiation within the automotive industry today is fuel efficiency and pollution. Sources of fossil fuels look to be running dry, and petrol (gasoline) costs have risen steeply, so engine manufacturers are under pressure to come up with designs that do more with less. And with global warming now broadly accepted as a scientific fact, the need to reduce levels of carbon dioxide and other greenhouse gases has increased in importance. This, again, directly impacts on engine design.

European legislation to cap the levels of CO2 that vehicles produce is getting ever more stringent, and similar measures are being adopted in the USA and elsewhere. At the same time, vehicle tax paid by the consumer is evolving across many countries to reward consumers who buy vehicles with lower emissions and penalise those who do not.

Unlike the issues that drove the marketing in earlier decades, the need for fuel economy and reduced emissions are unlikely to go away. In the long term we can expect to see a far greater reliance on electric vehicles with fuel cells or on cars powered by compressed natural gas or liquefied natural gas.

Split-cycle engine

What, then, are the innovations in internal combustion engine design that will carry us through the next decade or so, meeting the need to increase efficiency and reduce emissions? For one, earlier this year the Scuderi Group unveiled a proof-of-concept prototype split-cycle engine. The new engine, which its designers claim has the ability to revolutionise the long-term viability of the internal combustion engine, was unveiled as a naturally aspirated one-litre petrol unit (Fig.1). Scuderi Group expects it to produce up to 80 per cent fewer toxins than a typical internal combustion engine and, when fully developed with turbocharged and air-hybrid components, to achieve significant gains in fuel efficiency.

Scuderi's technology divides the four strokes of a conventional combustion cycle over two paired cylinders: one intake/compression cylinder and one power/exhaust cylinder. Unlike conventional engines that require two crankshaft revolutions to complete a single combustion cycle, the Scuderi engine requires just one. Alongside the improvements in efficiency and emissions, studies show that the Scuderi engine is capable of producing more torque than conventional petrol and diesel engines.

Split-cycle engines have been around since 1914 and, over the years, many split-cycle configurations have been developed. However, none has matched the efficiency or performance of conventional engines. In particular, previous split-cycle engines have had problems relating to poor breathing (volumetric efficiency) and low thermal efficiency.

The breathing problem is caused by the high-pressure gas trapped in the compression cylinder. This trapped high-pressure gas needs to expand before another charge of air can be drawn into the compression cylinder, which effectively reduces the engine's capacity to pump air and results in poor volumetric efficiency. Scuderi's engine solves the breathing problem by reducing the clearance between the piston and the cylinder head to less than 1mm. This design requires the use of valves that open outwards, enabling the piston to move very close to the cylinder head without interference with the valves. Almost 100 per cent of the compressed air from the compression cylinder is therefore pushed into the crossover passage.

With regard to thermal efficiency, this has to date been significantly worse than in a conventional Otto cycle engine because previous split-cycle designs have all tried to fire before top-dead-centre (BTDC) - like a conventional engine. In order to fire BTDC in a split-cycle engine, the compressed air trapped in the crossover passage is allowed to expand into the power cylinder as the power piston travels upwards. But, by releasing the pressure of the compressed air, the work done on the air in the compression cylinder is lost. The power piston then has to recompress the air in order to fire BTDC. In a conventional engine, the work of compression is done only once, leading to much better thermal efficiency.

In Scuderi's design, the thermal efficiency problem has been solved by breaking from conventional design best practice and instead firing after top-dead-centre (ATDC). Firing ATDC in a split-cycle arrangement eliminates the losses resulting from recompressing the gas.

Consequently, the technology provides a simple but elegant solution to the problem of how to meet modern demands for increased engine efficiency, improved power, downsizing and lower emissions. Early projections indicate that drivers of standard vehicles could see a 50 per cent gain in fuel efficiency over conventional engine designs when the engine is implemented with all of its turbocharging and air-hybrid features, and performance should be as good as or better than a conventional hybrid electric vehicle - but with even less environmental impact. And, of course, the Scuderi engine would work in electric hybrid vehicles too.

Five-stroke engine

A second approach to improving on conventional engine designs is Ilmor's five-stroke petrol engine, developed to deliver fuel economy and emission levels comparable to those of current diesel engines, but without the problems of particulates and NOx emissions that plague diesels.

Ilmor's five-stroke concept engine utilises two high-pressure (HP) fired cylinders operating on a conventional four-stroke cycle, which alternately exhaust into a central low-pressure (LP) expansion cylinder, whereupon the burnt gasses perform further work and improve thermodynamic efficiency. By decoupling the expansion and compression processes, the LP cylinder enables the optimum expansion ratio to be selected independently of the compression ratio.

The proof-of-concept engine has shown some very promising figures, and Ilmor is now looking to produce a second-phase development engine for in-vehicle testing. Performance targets for the engine are a 10 per cent improvement on fuel consumption over current four-stroke engines in a package that is up to 20 percent lighter than existing production engines; power density should also be much higher.

But what of developments in two-stroke engine technology? The Z engine developed by Aumet Oy is a diesel powerplant that combines the best features of two-stroke and four-stroke processes (Fig.2). The Z engine cylinder produces work on every crankshaft rotation like a conventional two-stroke engine, but features an exhaust port that is more akin to that used in the four-stroke cycle. In operation, fuel is injected into the cylinder when the piston is near TDC and ignites spontaneously. The expansion stroke follows, and the exhaust valves are opened before BDC. As the piston rises, it pushes the rest of the exhaust gases from the cylinder. Near TDC, the exhaust valves are closed and the intake valves opened, with the intake air being compressed externally to high pressure. After the intake valves are closed, the final compression is done in the cylinder. The cylinder temperature rises to the self-ignition temperature during final compression.

A prototype engine was unveiled in 2003 and has since undergone further testing and development.

Switching modes

Suppose, though, you could combine not only the best features of two-stroke and four-stroke technologies but also both modes of operation. This is exactly what the 2/4Sight engine does, with the promise of fuel savings of up to 27 per cent. Developed by Ricardo and a consortium of automotive partners, the 2/4Sight petrol engine uses novel combustion, boosting, control and valve actuation technologies to enable automatic and seamless switching between two- and four-stroke operation, thereby delivering significant performance and fuel economy improvements through aggressive downsizing (Fig.3).

The ability to operate in two-stroke mode delivers increased power (and reduced fuel consumption) during acceleration, while switching to four-stroke mode gives high efficiency at cruising speeds. The idea is not a new one, having been tried before in the 1980s and 1990s, but the required technology to control the valves and switch modes of operation 'on the fly' was not sufficiently advanced. A key breakthrough came with Ricardo's development of a patented mechanical cam switching system that was capable of delivering the required switching performance for the control strategies developed on the test bed.

Air handling on the 2/4Sight concept is based on two-stage boosting and intercooling using a Rotrex supercharger and Honeywell turbocharger. One of the problems with the traditional two-stroke engine, as discussed above, is the total-loss lubrication. However, the 2/4Sight's boosting technology and the innovative valve actuation eliminates the need to use the crankcase for the intake charge. This means that no oil is burnt and emissions are minimised.

The research prototype engine is based on a single bank of a 2.1litre V6, which, in six-cylinder 2/4Sight configuration, is intended to deliver levels of performance and driveability more usually associated with a V8 petrol engine of 3 or 4litres capacity. Simulation results indicate that vehicle acceleration, including launch from rest, can be maintained with a 2.0litre V6 2/4Sight petrol engine replacing a 3.5litre baseline powerplant. This would deliver fuel savings of 27 per cent over the New European Drive Cycle (NEDC) and would reduce the vehicle CO2 emissions of the baseline from 260g/km to 190g/km.

While the engines discussed above offer significant potential, it is notable that none is yet in production. Scuderi says it is in discussion with engine manufacturers from Asia, Europe, the USA and India, and is hopeful of seeing its engine in a production vehicle by 2012.

Aumet's Z engine has demonstrated how savings in manufacturing costs can be achieved over those for conventional four-stroke engines, so the company is now seeking a partner to deploy the Z engine in a production vehicle. Certainly it is an attractive proposition: the components used in the Z engine are similar to those already used in conventional internal combustion engines and compressors, hence there would be no need to make significant changes to the component supply chain.

As for the 2/4Sight engine, having completed development of the prototype, Ricardo and its partners are currently negotiating potential sources of funding and support for a vehicle demonstration programme. This may be a little way off, but Ricardo says that, in addition to validating the 2/4Sight concept, the research project is also delivering significant benefits in terms of its many constituent technologies that are likely be applied in the more immediate term.

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