We’ve seen engines that turn cylinders on and off but how about engines that switch between 2 and 4 stroke operation on the fly? The engineering firm, Ricardo, has developed a prototype of an engine that does exactly that, they call it the 2/4Sight. With careful design of intake and exhaust ports plus a control system that manages the direct fuel injection timing, the electro-hydraulic valve timing and intake boost, they pulled off what seems to be quite a feat of engineering.
The end result is projected performance from a 2 liter engine equal to a normal 3.5 liter engine, fuel savings of 27 percent and CO2 emissions are reduced as well. The added benefit is still being able to use an internal combustion engine which means all of the necessary fuel supply infrastructure is already in place.
Not only does there seem to be a lot of life remaining in the internal combustion engine, after seeing the JJ2S X4 a few days ago, it may also indicate 2 strokes aren’t quite dead yet, either. Now, if they can just scale this down to motorcycle size …
Diagram of 2 stroke cycle below:
Link: Ricardo via Autobloggreen
Nicolas says
This idea seems so “simple” and evident, has there never been any other attempts/developpements of such engines in the past ?
kneeslider says
I have a hunch the technology for the precise control necessary for everything, including valve operation, has not been available until recently.
JB says
When it is cost effective things happen .
KH says
Looks complicated. I think this Kiwi 2-stroke/4-stroke engine is far more interesting: http://www.sixstroke.com
This impressive video shows difference between 4-stroke engine head and “six stroke” head:
http://www.sixstroke.com/downloads/Six_Stroke_Head_Comparison.mpg
Phoebe says
Wow, I had never heard of the Beare 6-stroke head until now. It’s like adding another little piston on top of each cylinder…really cool. Has it been adopted in any applications yet?
Sean says
Is it just me, or are the exhaust gases escaping while the cylinder is being pushed down? That, to me, says that the downward motion should suck the exhaust fumes in, not out.
Also, I hate to be pedantic, but that’s an Aussie design, not a kiwi one.
taxman says
from what i read there is interest in some level of production of the Beare 6-stroke engine. but the inventor has run into a spot of trouble with some investors that have been taking advantage of his good nature.
i really hope that he can get things settled and move on with his engine. it truly looks interesting.
todd says
2-strokes are fairly efficient at scavenging exhaust gasses despite the fact that the cylinder is expanding during half the exhaust phase. In a mechanical system (with valves like a diesel or with fixed ports) the exhaust port is just opening when the piston is traveling down the stroke; at its full aperature at BDC; and closes on the return stroke. There are some power losses associated with this but most of the power is already spent on pushing the piston down. It’s like a bolt action rifle compared to an automatic. With automated valves you can change the exhaust timing to better suit the RPM, generally starting the blow-down phase later and leaving it open longer to facilitate exhaust wave signals and and their benefit to “super charging” the cylinder.
The only down side I can see to this design is the location of the exhaust valve/port. With both intake and exhaust valves at the top a large portion of exhaust gasses tend to stick to the top of the piston and don’t get blown out. A blower or turbo would help here…
-todd
kneeslider says
This engine is designed to have both a blower and a turbo:
“The air handling system of the 2/4SIGHT 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 is provided by an external compressed air supply.”
Without boost, I would think it would be pretty hard to make this work, no matter how well timed the valves are.
Nicolas says
Now, If you put a mechanical compressor + a turbo on a 4 stroke 2 liter V6, don’t you get equivalent/superior performances than a 4 stroke 3.5 liter V6 ?
Sean says
Seems to me that you would need an extra port that opens when the piston is at 100 degrees ATDC, that pushes air in to take the place of the exhaust gasses. Or am I missing something painfully obvious here?
B*A*M*F says
I’m not really understanding the desire to switch between 2 and 4 stroke operation. If you can have the power of a 2 stroke without the emissions issues and with the superior lubrication of a 4 stroke, the why use the 4 stroke mode at all?
JB says
got me ?
hoyt says
Performance, emissions, and fuel economy seem to be their goals.
How much of the 4-stroke portion of the engine is contributing to the “fuel savings of 27 percent” ?
todd says
emissions and lubrication issues with 2-strokes are a legacy of the typical design.
Most often a 2-stroke uses the crankcase (back side of the piston) as the “air pump” to facilitate the intake charge. Since this is where all the bearings are located they need to be lubricated well – fuel is a solvent except for diesel. With a separate air pump like a turbo and the necessary mechanical valves to avoid the crankcase the intake charge no longer needs to be oiled.
As for emissions, the lack of burning oil and the addition of timed fuel delivery (only when the ports are closed), it can be on par with a four stroke engine.
What a 2-stroke gets you is power on every downward stroke of the piston, no wasted strokes; potentially twice the power output of a four-stroke.
I believe Piaggio/Aprilia has been building clean burning, direct injection 2-strokes for some time now. Adding a supercharger to isolate the crankcase makes it even cleaner.
-todd
malcolm Beare says
Diagram of 2 stroke cycle below: the exhaust valve opens quite early at 100 degrees to allow for the blowdown period before the intake valve is oponed, this detracts from thermodynamic efficiency as a fourstroke exhaust valve would open about 40 degrees later allowing more expansion. Thus in the twostroke mode you would never get twice the power of a fourstroke or be as fuel efficient.
————————————-
The Beare Head
SWEPT VOLUME
Imagine a four stroke engine single cylinder capacity one litre
Bore 115mm stroke 96.3mm swept volume of 1000cc.
Imagine a Beare Head with a bore of 75mm and a stroke of 56.5mm a swept volume of 250cc.
Imagine that the combustion chamber volume is 100cc. the trapped volume when both upper and lower main piston are at their closest proximity with the top piston down as far as possible and the main piston at TDC.
INTAKE.
The main large piston is at TDC, the upper smaller piston is at the top of its bore or BDC, so the cylinder volume is 250cc plus 100cc equals 350cc.
As the main piston descends it is increasing volume. At the same time the upper piston is descending reducing volume.
At main piston BDC the main piston has swept 1000cc and the upper piston has descended half its bore as it is synchronized at half the main piston rotational speed. It has swept 125cc and reduced the swept volume by 125cc
Therefore the cylinder volume at main piston BDC is 1000cc add the combustion chamber, add the volume left in the upper piston of 125cc so the total volume is 1225cc
So the swept volume of the intake stroke is 1225cc minus the volume at the start of the intake stroke of 350cc
875cc
COMPRESSION.
The cylinder volume is 1225cc
The main piston ascends while the upper piston continues to descend, both pistons are reducing volume.
At TDC main piston has swept 1000cc while the upper piston has swept a further 125cc
Cylinder volume is now 100cc
So the swept volume is 1225 minus 100cc
1125cc.
EXPANSION
Cylinder volume is 100cc
The main piston descends while the upper piston ascends.
Both pistons are increasing volume.
At BDC the cylinder volume is main piston 1000cc and upper piston is 125cc.
Total cylinder volume is 1225cc
So the swept volume is 1225 minus 100cc
1125cc
EXHAUST
The cylinder volume is 1225
From BDC the main piston ascends reducing volume while the upper piston continues to ascend increasing volume
At TDC the main piston has swept 1000cc, the upper piston has increased volume by 125cc
The total cylinder volume is combustion chamber 100cc and upper piston volume 250cc
350cc
So the swept volume is 1225 minus 350cc.
875cc
The total swept volume over the four strokes is 4000cc
Intake 875cc add compression 1125cc add expansion 1125cc add exhaust 875cc
Therefore the nominal average capacity of the Beare cycle engine is
1000cc
Similar arguments and dissertations could apply to the miller cycle.
The waters could be muddied somewhat more by considering only the trapped volumes after all the ports have been closed. The Japanese used to apply this principle to two-strokes with corrected compression ratios.
There is a further complication if the upper piston crank is delayed or advanced in its rotational relationship with the main crank, or if it is a conventional crank and con rod or a scotch yoke drive. All have effects on the swept volume in regards to crank angle position.
But the net results are that the Beare cycle has advantages in gaining efficiency, pumping losses are reduced as less energy is expended to suck intake and pump out exhaust. And more energy is extracted during the expansion stroke. The expansion stroke being the largest change in swept volume, because of port timings, means that the Beare head, dual opposed piston cylinder head has similarities to the Atkins cycle and Miller cycle but is subtly different and perhaps deserves the name of Beare cycle.
If the upper piston is delayed in its rotational relationship by about 20 degrees the maximum volume no longer occurs at BDC but is at173 main crank degrees on intake and maximum volume occurs at 548 for expansion and minimum volume occurs at 361 and the rate of change in volume during combustion is less than the conventional four stroke maintaining a closer relationship to the theoretical ideal of constant volume combustion.. and therefore higher maximum cylinder pressures are achieved even though the compression ratio and open throttle cranking cylinder pressure may be the same as the conventional four stroke.
B*A*M*F says
Todd, I’m with you on the benefits of this engine over a conventional 2 stroke. What I can’t work out is why you would want to use it as a 4 stroke.
Without that dual mode thing the crank case could just as easily be bypassed on a normal 2 stroke with forced induction and direct injection.
malcolm Beare says
B*A*M*F,
I think that the philosophy is to use the twostroke mode for accelleration and when maximum power is required but then use the fourstroke mode when cruising at a steady speed for maximum fuel economy.
Sean says
There may be a couple factors in play here for switching between 2 stroke and 4 stroke modes.
The idea is that you get maximum power when accelerating by using the 2 stroke mode, and maximum efficiency when cruising in 4 stroke mode. The high power output of the 2 stroke mode means that you could have a car with a relatively very small displacement engine that gives performance equivalent to a higher displacement engine. Having a smaller engine means less weight, less space, all that good stuff. So why not use 2 stroke all the time? Well, I think the problem would be that in 2 stroke mode you may be pushing a significant amount of your intake charge out past the exhaust port during cylinder filling at BDC, so you’d have an emissions and fuel economy penalty in that situation. Also I think that the engines Brake Specific Fuel Consumption (BSFC) would be significantly better while running in 4 stroke mode at high loads where your MAP would be high and throtttle opening would be significant, as compared tu running in 2 stroke mode at very low MAP and small throttle openings where pumping losses would be greater.
todd says
with timed injection you don’t need to waste fuel out the port. You inject it once the ports are closed. You can also time it to coincide with exhaust pulses so that the fuel goes out the exhaust and rides a high pressure wave back in.
With so much energy in a four stroke lost to a wasted stroke I wonder how much economy there is. Remember, power comes from efficiency. If a cylinder puts out more power that means it is using the fuel and air more efficiently, not always more of it. A 2-stroke just requires it twice as often but not twice as much… You can gain fuel economy by severely leaning out the mixture in low power situations and making the bike lighter and more aerodynamic as well.
-todd
Simon says
Interesting stuff. It is more of an automotive topic, following the trend of downsizing car engines to meet future environmental legislation. For some motorcycle engines the extra low rpm performance would be nice. But can you imagine a big v-twin sounding like a 4-cylinder?
I gave this concept some thought before. The major problem of course is how to operate the valves. In 2T mode they have to open and close twice as often.
Using current technologies this will bring down maximum rpm (and thus horsepower) by a factor 2. Some googling reveals that the prototype has a conventional drive train using “epicyclic gears on the camshaft nose” (would like to get some detail) to switch between modes, though Ricardo prefer electro-hydrolic valve actuators. To my knowledge this type of valve actuation has been experimented with some years ago (eg. EU government ELVAS project), never to be heard of again. A pity, since all 4-strokes could greatly benefit from this kind of valve control. Anybody here familiar with the subject?
Source: http://www.vehicletechnology.org/VT_magazins/vehicle_technology_2005_Nov.pdf
I have read comments that pose the question why you would want to switch between 4T and 2T mode anyway. On this subject http://www.theengineer.co.uk/Articles/268057/The+2×4+engine.htm reads:
“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.”
I think a more viable option for the short term would be to skip the 2T/4T switching idea and copy some of the design of those large marine engines. So, using all 4 valves in the head for air intake, and having a circle of small exhaust ports in the lower half of the cylinder. This enables twice the amount of air to enter the cylinder from the top, pushing out twice as much exhaust gas out of the cylinder at the bottom. Result: more rpm / more power.