article was first published in Ricardo Quarterly Review and is used with
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
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
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
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.
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
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
“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
“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
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.
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
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
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
Automatic switching algorithm
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
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
Vehicle simulation based on engine
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
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.
ECE Fuel (l/100km)
EUDC Fuel (l/100km)
NEDC Fuel (l /100km)
NEDC CO2 (grams/km)
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