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.

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

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.

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.