Stroke of genius

Combination two/four-stroke engine concept claimed to dramatically cut fuel costs by switching between modes nears production stage. Siobhan Wagner reports.

A combined two/four stroke car engine that could deliver major fuel economy and emissions benefits is nearing production readiness after three years of testing, according to its UK-led development team.

The 2/4SIGHT concept engine is designed to automatically switch between two and four-stroke operation by using advanced combustion, boosting and control systems that can deliver high performance from a smaller unit.

Prototype testing showed that a 2ltr V6 2/4SIGHT engine could match the driving characteristics of a conventional 3.5ltr V6 claimed Ricardo, the UK automotive technology group leading the project.

This would translate into 27 per cent fuel savings and a cut in emissions of 70g/km, said the company.

The 2/4SIGHT project — which was first revealed in The Engineer in 2004 — could now enter commercial production in as little as a year if the automotive industry is sufficiently impressed by the concept.

Neville Jackson, Ricardo’s technology director, said the research group’s main challenge was to achieve a smaller engine with high low-speed torque.

Conventional four-stroke engines can be boosted with turbochargers or superchargers to produce more torque, but this is limited by abnormal combustion due to high pressures and temperatures involved at low engine speed.

‘To operate successfully, highly boosted four-stroke engines must therefore use a lower compression ratio which reduces efficiency and negates any benefits in further downsizing.’ said Jackson.

By running the engine in two-stroke mode under low speed and high load conditions, the torque produced by each cylinder in every power stroke is about half of that produced by a four-stroke engine.

‘This means the engine can retain a high compression ratio and deliver higher efficiencies than a similar boosted four-stroke engine,’ said Jackson. ‘Two-stroke operation effectively offers a means of boosting low-end performance and hence further increases the opportunity for downsizing.’

The engine starts in exactly the same manner as a typical four-stroke direct injection petrol engine. The control system monitors driver demand and when more torque is required than is possible in four-stroke mode, the fuelling, air handling and valve train adjust to enable switching within a single cycle and on an individual cylinder basis. This means that torque delivery remains smooth and uninterrupted as the engine switches between modes.

The engine uses novel direct injection technology from project partner DENSO. ‘Existing spark ignition engines use relatively low pressure fuel injection systems that spray fuel into the inlet ports of the engine,’ explained Jackson. ‘A direct injection system operates at much higher pressures and sprays fuel directly into the combustion chamber.’

He said many new engine designs have been introduced with this type of injection system because it offers both power and efficiency benefits. But what makes this system different is that the design enables better airflow for both two and four-stroke modes.

This applies particularly to the engine’s two-stroke operation. The system more effectively pushes exhausted gas out of the cylinder and draws in a fresh draught of air ready for the next cycle. The process is essential for smoothly running an internal combustion engine. Combustion is facilitated with valves that control the air and fuel flow into and out of the engine’s cylinders.

The prototype engine does not use a conventional camshaft to open and close the valves in the combustion chamber. Instead it was designed as a ‘camless’ engine, which uses a highly flexible hydraulic valve actuation system.

The system was designed only for research purposes so the Ricardo team could evaluate a wide range of possible valve control options.

In parallel with the prototype engine development effort in the UK, Ricardo engineers at the company’s Detroit facility have designed a patented mechanical cam switching system that it said offers a practical way to deliver the switching strategies developed on the prototype.

This switching system opens the way for packaging and integration of the 2/4SIGHT engine into a production vehicle, said Ricardo.

Jackson claimed the technology would be cheaper to produce than other advanced petrol and diesel systems.