A company called Ricardo with backing from Ford have developed an engine that can alternate between 2 stroke functioning and 4 stroke.
The engine (which is 1.4 litre capacity but has the output of a conventional 1.8) operates mainly in 4 stroke mode, but when greater power or torque is needed, it switches to 2 stroke operation.
The emission problem from 2 stroke mode has been overcome by doubling the valve train speed.
www.e4engineering.com/story.aspx?uid=eb657de5-a62c...9
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You're a late for an April 1st one Dude.
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April Fool or not, could it actually have potential?
I remember friends who had scooters saying that in certain circumstances the 2-stroke engine could start 4-stroking - ie firing on alternate revolutions, with consequential loss of power. So maybe the principle could be applied the other way round.
They would need to be dry-sump surely, with facility for taking the petrol/air mixture (with oil) through the crankcase?
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>>They would need to be dry-sump surely, with facility for taking the petrol/air mixture (with oil) through the crankcase?>>
Cliff - The way I envisage this engine, and I stand to be corrected, is that it would not feature a traditional 2 stroke dry sump lubrication system, but be the same as a normal 4 stroke engine. The difference would be the adaptation of the fuel/air metering to switch between modes under the control of the ECU, at the same time as valve train speed is altered. I believe this engine is potentially viable and most certainly was not designed on 1st April.
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I thought a two stroke worked because the fuel-air mixture was compressed in the crankcase when the piston was at the bottom of the stroke and pushed into the cylinder via a transfer port while the piston was still low in the bore.
I still say it's an April the 1st article.
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OK, it may be wishfull thinking rather than April the 1st. Maybe using mechanical means to fill cylinders. I always though 2strokes were rev-hungry beasts though (other than huge marine diesel engines).
www.foresightvehicle.org.uk/showdoc.asp?doc_id=105...1
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I think the essence of a 2-stroke is that two things have to happen at the same time, in order to avoid the need for separate compression and exhaust strokes. I believe it relies on the shape of the piston deflecting the incoming gas so that it doesn't mix with the outgoing - something like that anyway? The efficiency isn't quite the theoretical X2 because of some losses during this process. All the same, 2-strokes are a lot more powerful than the same sized 4-stroke. Take those old Saabs so sucessful in rallying: and even the awful Trabant was actually quite nippy.
I think the swept crankcase is a convenient "pump" for pushing in the next charge of mixture, but presumably it could actually be fed in via a turbo or supercharger and a special valve sequence.
Is this a joke, or could it actually work?
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A firm in California was developing a 2-stoke engine with a conventional crankcase and a supercharger for filling the cylinders.This overcomes all the problems that come from main/big end lubrication.
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Cliff
Yes, you're right when you describe how a 2-stroke operates, but there a few these days that actually use 'deflector' pistons. These made decent compression ratios hard to achieve. Port design makes it unecessary The scavenging efficiency is crucial to efficiency, and it's a difficult balance between effective scavenging and loss of incoming charge down the exhaust. That and the pumping power of typical crankcase compression 2-strokes are a couple of reasons you don't get double the power of a 4-stroke.
There is, as you say, no reson why a 2-stroke has to use crankcase compression, except convenience. Large 2-stroke diesels use exhaust turbochargers for this purpose, and there's the uniflow scavenged diesel which uses a turbo to blow in air via inlet ports, and exhaust valves to release exhaust gasses. The benefit of two stoke diesels is that you can have effective scavenging by air alone, because the fuel is injected on the compression stroke. You don't lose fuel air mixture down the exhaust pipe.
Regards
John S
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The hybrid 2 & 4 (& 6) stroke engine concept has in fact been around for a long time, though no-one has yet overcome the many problems it brings. Ricardo seem to be putting extra effort into the idea and they may pull it off, in fact I very much hope they do.
Just one brief example of two-stroke problems is that an engine in two-stroke mode puts a constant load on the small end and this makes it difficult to get oil into the bearing. A four-stroke small end sees a reversing load stroke-by-stroke and this helps pump oil through to lubricate and cool the bearing. A high output two-stroke would need a high-tech small end, perhaps the Melchior patented design that uses a spherical end to the connecting rod, however this would be quite expensive to make and fit.
Regarding the use of unit valve operation to facilitate the change between 2 & 4 stroke, I have mentioned the problems with this in earlier postings and won't repeat them here. Suffice to say that they have lots of weaknesses to be sorted, whether the valves be electrically or hydraulically operated.
I agree that a two-stroke is not twice as powerful as the same size four-stroke; 1.6 would be nearer the mark I think. I would guess that the cost increase of a two-stroke against the same capacity four-stroke would also be about 1.6 because today's two-stroke needs most of what the same-size four-stroke has, and more in some areas (like the small end).
Taking another example, crankcase compression is now confined to history, as are smokey exhausts. Two-strokes now need valves, not ports, to meet modern emissions regulations. When piston rings traverse a port in the cylinder wall this sets up minute ring flutter(!) that upsets the ring/bore sealing.
The problem of keeping the inlet and exhaust gases separated applies even with a diesel because all the fresh air that you send down the exhaust pipe has to be pumped and this takes energy and creates heat.
Also, if you are producing more power from the same capacity, there will be more heat to be got rid of, therefore the cooling system needs to roughly match the engine's output, not its size.
If I had to put money on it, I would say that we will not see two-stroke engines in passenger cars again. There are better ways of achieving their high-output benefits in cars, like variable-geometry turbocharged four-strokes.
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