Nissan Motor Company has announced a new type of gasoline engine

Here is a simple explanation gathered from this image http://o.aolcdn.com/hss/storage/midas/de3e84cf46030cd7b3ea39... :

The difference from a conventional engine is that the engine stroke is increased or decreased automatically. The stroke is the distance the piston travels up and down in order to turn the crankshaft. A shorter distance that is closer to the combustion chamber increases the compression ratio because there is less space for the fuel/air mixture. When the system is operating in low compression mode the piston travels more and does not reach as high. The combustion chamber then ends up being farther away and a lower compression ratio is achieved.

What is novel about this approach is how the combustion chamber was left in the same place. Past efforts by SAAB had the engine head moving the combustion chamber away from the pistons. This drastically changes the characteristics of the combustion process. Something that makes emissions control challenging. Any power loss due to a dynamic combustion chamber is removed by pressurising it with a turbocharger, just like Nissan did.

Possible issues with this design could be:

- Uneven wear of the cylinder bore due to the piston travel being dynamic.

- An overboost condition during a high compression cycle could damage the mechanism and even crack the engine block.

- Increased NHV due to regular wear and tear making its way into the valvetrain and transmission input shaft.

This is a good effort and I hope Nissan is able to pull it off. Make sure to wait two years before you buy a vehicle with this technology. Dont be the guinea pig. :)

I'm curious as to what, if anything, would preclude variable displacement from being used in diesels as well as petrol engines. Diesels also have performance/economy tradeoffs like this gasoline engines, so it stands to reason that tweaking compression should give similar benefit. I've got a feeling that rumors of diesel's impending demise are greatly exaggerated.

I suspect it does two things-

1) lets the engine have high compression AND a turbo. Typically a turbo is run with lower compression, because even today nobody can make an engine run 14:1 with a turbo. Speculating, when you floor it the compression might drop to 8:1 and the turbo kicks in; then when you highway cruise, it runs 14:1 for efficiency.

2) lets the little engine still hit big torque numbers at low speeds. This is most likely if the engine is variable stroke, as longer strokes improve low speed torque.

Diesel's fatal flaw is particulate emissions.

Attempts to mitigate those, without cheating (e.g., VW), have proven difficult.

I think you are incorrect. Most production cars (all that I know) have only a single fuel tank, thus nothing to mix.