An engine capable of switching between two-stroke and four-stroke operation to reduce fuel consumption and carbon dioxide emissions could be fitted to cars by the end of the decade, according to automotive consultancy Ricardo.
The company has been leading a project, also involving Ford, to investigate the feasibility of a petrol engine that combines the advantages of two-stroke and four-stroke. This would create a smaller, lighter engine with the performance of larger power units, but with better fuel economy, reduced emissions and lower production costs than existing advanced diesels.
The results of the 2/4SIGHT project, carried out as part of the DTI-backed Foresight Vehicle programme, have exceeded expectations, said Dave Greenwood, chief engineer for advanced technology at Ricardo. The technology could contribute to efforts to reduce CO2 emissions by 2008-2012, he said. Ricardo now hopes to take the research forward and begin producing a prototype engine, providing they can secure the co-operation of an industrial partner (Ford or another OEM) and more government funding.
The 1.04-litre, three-cylinder engine concept provides the equivalent peak power of a 1.8-litre petrol engine, but with 50 per cent more torque at low speeds, making it more ‘fun to drive’.
Although the technology is ultimately targeted at the family car market, the project partners expect its first application to be in the luxury and sport-utility vehicle sectors, due to its combination of high performance and tax-friendly low CO2 emissions. Two-stroke operation produces very high levels of torque, meaning a much smaller engine can produce the same level of performance as a larger unit. The new engine would operate in four-stroke mode for the majority of its lifecycle, only switching over to two-stroke when more torque is needed than is available at the current engine speed.
Unlike the conventional two-stroke units used in mopeds and jet skis, which have no valves, the design incorporates a conventional valvetrain system. To switch from four-stroke to two-stroke operation, the speed of the valvetrain is doubled to produce twice the number of firing pulses per engine cycle. This gives the engine its power boost, and can be done either mechanically using an epicyclic gearset (a series of inter-linked spinning gears), or by use of an electro-hydraulic valve gear, in which there is no camshaft and each valve is controlled by its own actuator.
Conventional two-stroke engines also tend to produce more pollution than four-stroke units, as each time a new charge of fuel and air is introduced into the combustion chamber, part of it is pushed out through the exhaust port before it is closed. In the concept engine fuel is only injected directly into the chamber after the valves have closed, which also improves fuel efficiency. Despite this some form of exhaust after-treatment, such as a NOx trap, may still be needed to cover two-stroke operation, said Greenwood.
An air-handling system uses a combination of a fixed geometry turbocharger, an intercooler and a supercharger to ensure the engine receives the necessary airflow. Two-stroke engines require a supercharger to maintain good ‘scavenging’, in which a through-flow of air pushes the exhaust out of the exhaust port.
While the results of the project are promising, much more research will be needed before the technology will be ready to enter production, including further work on the valvetrain, air handling and control systems, said Greenwood.
Ricardo’s role was to define the engine operating strategies and carry out simulations, while Ford performed simulation modelling. Researchers at Brighton and Brunel universities analysed the combustion system, and technology specialist Ma 2T4 investigated the mechanism to switch the valvetrain between the modes.