22 automotive design
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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 per cent 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.
Fig. 3. Capable of switching automatically between two- and four-stroke modes, the 2/4Sight engine is shown here on test in the Sir Harry Ricardo Laboratories at the University of Brighton.
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
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