Stroke of Genius

Are engines combining both 2- and 4-stroke functions the way of the future?

by Anthony Smith

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This article was first published in Ricardo Quarterly Review and is used with permission.

Against the backdrop of spiralling gasoline pump prices, a revolutionary new engine concept developed by a consortium led by Ricardo offers an attractive alternative to conventional spark ignited combustion. By combining the benefits of two- and four-stroke combustion, the 2/4SIGHT engine presents the prospect of 27 per cent fuel savings over current engine technology.

Is Downsizing the Answer?

Engine downsizing has long been acknowledged as an important route to the improvement of fuel economy. All else being equal, a smaller engine has less internal friction so that less energy is wasted merely in turning its internal components. It also has less thermal inertia, which means that it warms up more quickly and is thus more thermally efficient in a typical mixed-duty, real-world operation.

Moreover, as most car engines operate at well below their point of peak efficiency in day-to-day use, by substituting a smaller capacity unit operating at higher specific load, the combustion and gas exchange process can be more efficient.

To deliver the same performance as a larger engine, however, a downsized unit must employ charge air boosting, either in the form of turbocharging or supercharging. While these measures give rise to some product cost implications, the resulting higher cycle efficiency and lower friction can offer much sought-after fuel economy and CO2 emissions improvements.

Nevertheless, there are also some very practical limits to downsizing a conventional four-stroke engine. The main obstacle to downsizing is the achievement of good low-speed torque and launch feel.

The boost system applied to a downsized engine will produce more torque, but this is limited by the onset of abnormal combustion as higher pressures and temperatures are reached; this is a problem particularly pronounced at lower engine speeds.

High cylinder pressures require larger connecting rod and crank bearings to accommodate the increase in load. This in turn can increase friction such that the benefits of downsizing can be significantly reduced.

To operate successfully, highly boosted four-stroke engines must therefore use a lower static compression ratio, which then reduces efficiency and negates the benefits of any further downsizing. Launch feel can also be a challenge for turbocharged engines due to the time required to accelerate the turbocharger from idle to generate boost pressures. Mechanically driven superchargers can help to resolve this issue but these devices also increase losses and reduce efficiency.

Taking downsizing one step further

Hybridisation, in effect using electric power to augment low-end torque, is a well-proven route to enabling further levels of downsizing. But while this approach works successfully in many products, it brings significant additional cost and complexity in the shape of the hybrid powertrain, power electronics and energy management systems.

An ideal solution to the twin obstacles to major downsizing – low speed torque performance and high specific power combustion stability – would-be a solution that is internal to the engine - and hence not requiring additional systems.

This was a challenge which had occupied the engineers of Ricardo’s technology group for some years. However, in 2004 Ricardo decided that a combustion system initially developed by the company in the late 1980s might offer a solution.

Flagship technology

Over fifteen years ago Ricardo demonstrated a poppet-valve two stroke engine concept known as the ‘Flagship’ engine; the concept was intended for premium vehicle applications where a higher performing premium vehicle two-stroke engine could be used within the same basic architecture as a more basic four-stroke used for lower performance vehicle derivatives.

The concept was demonstrated successfully as both single and multicylinder demonstrator engines but research was curtailed when it became clear that its commercial application would be limited. Yet there were aspects of the Flagship combustion technology which have since made their way into successful conventional engines. The engine had achieved its scavenging performance through the use of a top entry intake port geometry which, combined with port shielding, created a reverse tumbling motion on induction.

This same Ricardo-patented system has since found application in a number of direct injection gasoline engines where it is a well proven enabler of stratified combustion.

Two into four does go

Engineers in the Ricardo technology group realised that the intake and combustion chamber geometry originally developed for Flagship could be used as the basis of an engine capable of operating in either two- or four-stroke modes.

The new concept, named 2/4SIGHT, offered some immediate and attractive benefits for the challenge of gasoline engine downsizing.

“When we first conceived the 2/4SIGHT engine, we realised that we could potentially overcome the compression ratio barrier to further downsizing,” explains Ricardo group technology director, Professor Neville Jackson.

“By running the engine in two-stroke mode under low-speed/high-load conditions, the torque produced by each cylinder every power stroke is about half of that which would need to be produced by a four-stroke engine under the same duty. This means that the engine can retain a high compression ratio and would not require an increase in crank and rod bearing sizes. This configuration can deliver higher efficiencies than a boosted four-stroke engine with similar torque and power output.

“Two-stroke operation effectively offers a means of boosting low-end performance, in the process further increasing the opportunity for downsizing in spark ignition engines. The main benefit to the driver would be reduced fuel consumption and, possibly, better launch feel.”

The 2/4SIGHT engine would start in exactly the same manner as a typical four-stroke direct injection gasoline car engine. Its control system would monitor driver demand and when more torque is required than would be possible in four-stroke mode, the fuelling, air handling and valvetrain would be adjusted to enable switching within a single cycle and on an individual cylinder basis so that torque delivery remains smooth and uninterrupted as the engine switches between modes. Only the engine note would change due to the different firing frequency, much as in the sound made during a transmission downshift.

How it Works

In two stroke mode the 2/4SIGHT concept works by using boost air to scavenge the cylinder during a prolonged period of valve overlap. The geometry of the vertical intake port and the valve shrouding in the combustion chamber promotes a reverse tumbling motion on induction. This flow structure is particularly effective in promoting efficient scavenging.

In four stroke mode the engine operates in exactly the same manner as a direct injection boosted gasoline engine.

Building the first prototype

In early 2005, building on the very promising results of the concept study, a further programme was initiated aimed at delivering the world’s first switchable two- four-stroke engine in prototype form. With design and prototype manufacturing carried out at Ricardo, a multi-cylinder research prototype engine was tested at the Sir Harry Ricardo laboratories of the University of Brighton.

This engine was based on a single bank of a 2.1litre V6, which in six-cylinder 2/4SIGHTconfiguration would be intended to deliver levels of performance and driveability more usually associated with a three- to four-litre V8 gasoline engine. In order to develop the combustion and control systems, a single cylinder Hydra research engine was also used at Brunel University.

As this was a fundamentally new engine concept, it was essential that the prototype would enable a wide range of control strategies to be evaluated for switching between two- and four-stroke modes. To allow this, an electro-hydraulic valve (EHV) actuation system was used for the prototype development rig. While this configuration was ideal for research purposes, it was never intended as a practical solution for in-vehicle use. Instead, a more simple mechanical switching system would be implemented once the desired strategies had been evaluated.

In parallel with the prototype engine development effort in the UK, Ricardo engineers at the company’s Detroit Technology Campus undertook a study which led to the creation of a patented mechanical cam switching system which would be capable of delivering the required valve switching performance for the control strategies developed on the prototype engine with its EHV system. This not only opens the way for packaging and integration of the 2/4SIGHT engine into a production vehicle but also represents a very cost-effective means of implementation of this highly efficient combustion concept.

The air handling system of the2/4SIGHT concept is based on two stage boosting and intercooling using a Rotrex supercharger and Honeywell turbocharger. For simplicity in the initial test bed prototype configuration, however, boosting was provided by an external compressed air supply. The engine control system of the prototype was a DENSO rapid prototyping system working with DENSO gasoline direct injection and ignition components.

First ‘switchable’ fire

In late 2007 testing commenced on the prototype engine. As senior project engineer Richard Osborne explains, the achievement of first switchable firing was a major achievement for the project team.

“In normal circumstances the first firing of any research prototype engine is a major milestone in itself. For 2/4SIGHT we had three such hurdles to achieve: operation separately in two and four-stroke modes and switching under firing conditions between modes.

“We had already proven the mechanical and air handling aspects of the rig and had full confidence in the simulation work on which the design was based. Nonetheless, the team had an immense sense of achievement when we successfully achieved the world’s first firing of such a switchable engine.”

Automatic switching algorithm development

For the 2/4SIGHT engine concept to be successful, it is essential that switching is entirely demand-driven and is fully automatic. For all normal driving conditions and moderate acceleration, the engine should operate as a conventional four-stroke. Typically, two-stroke operation would be solely used when low-speed/high torque is required. This requires the control system to initiate switching based on the required duty, and to implement it in such a manner that the rate of change of torque is not interrupted in any way.

The prototype engine performed faultlessly in the subsequent development programme, validating the switching strategies through demonstration of a wide range of constant torque and load transients, and also the performance of the downsized engine in both two- and four-stroke operation.

Vehicle simulation based on engine tests

Having validated the basic engine concept, the research team went on to assess the fuel economy improvement potential of the 2/4SIGHT engine through vehicle drive cycle and acceleration performance simulation.

This work was based on the measured steady-state fuel consumption and full-load performance of the prototype2/4SIGHT engine and was carried out using a Ricardo software package that allows detailed modelling of engines, transmissions, drivelines, tyres and aerodynamics.

The baseline vehicle for the study was an 1800kg passenger car sold in the European market with a 3.5 litre naturally aspirated V6 gasoline engine and five-speed conventional automatic transmission with torque converter. To verify the validity of the models and input data, the baseline vehicle fuel consumption results were compared with published data, which were reproduced by the model to an acceptable accuracy of 1 per cent.

The simulation results indicate that vehicle acceleration performance, including launch from rest, can be maintained with a 2.1 litre V6 2/4SIGHTgasoline engine replacing the 3.5 litre 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 260 grams per kilometre to just under 190 g/km.

Published Data

Simulation Data

Baseline Vehicle

Baseline Vehicle

2/4SIGHT

Improvement

ECE Fuel (l/100km)

15.5

15.8

10.1

36%

EUDC Fuel (l/100km)

8.3

8.2

6.7

18%

NEDC Fuel (l /100km)

11.0

11.0

7.9

28%

NEDC CO2 (grams/km)

260

257

186

28%

Dr Tim Lake, 2/4Sight chief engineer and veteran of the original Flagship studies, emphasises the full extent of the achievement that these results represent:

“To put this into perspective, in two-stroke mode the test bed engine has achieved over 230 Nm per litre. This enables a two-litre switching engine to achieve over 450 Nm, which is similar to a 4.5 litre naturally aspirated four-stroke engine.

“The simulation shows that torque performance is significantly improved over the baseline vehicle –and yet it still delivers a 27 per cent fuel economy improvement.”

2/4SIGHT Vehicle programme announced

Building on the work of the 2/4SIGHTengine concept, the 2/4CAR project aims to deliver a global premium vehicle demonstrator which will realise the promise of the engine programme.

“We aim to draw on the lessons of the 2/4SIGHT research prototype engine programme and use this to create a design which can be packaged in a contemporary luxury vehicle,” explains Jackson. “This will incorporate a development of the patented mechanical switching technology developed in parallel with the engine prototype project in order to realise the switching strategies previously demonstrated on the test bed.”

Ricardo is to lead the 2/4CARproject and will work together with a consortium of partners also including the University of Brighton, DENSO Sales UK Ltd and Jaguar Cars Ltd.

If the results of the vehicle demonstrator programme live up to the great promise revealed by the research prototype engine development programme just completed, the switchable 2/4SIGHTengine concept could be extremely attractive to both automakers and the driving public in the new paradigm of high fuel prices and stretching CO2 and fuel economy targets.

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