A. The background

The 4 stroke Otto engine

Internal combustion gasoline (Otto) engine is an basic prime mover for the passenger automobiles and many other applications. The problem with this engine is its inherently low thermal efficiency in order of 24 - 26%.
The egine works at well known 4 stroke cycle:
1. Intake
2. Compression
3. Expansion (work stroke)
4. Exhaust

B. The entry:

The 5 stroke cycle Otto engine.

The reason for the low thermal efficiency of the Otto cycle is the high content of unused energy in the exhaust gases. One of the ways to recover part of this energy is the use of the turbo-charger. The proposition is to employ as the the energy recovery means additional expansion piston and cylinder - in other words to add the second expansion stroke to the basic Otto cycle!
The 5 stoke cycle will be as follows:
1. Intake
2. Compression
3. First expansion (work stroke as in basic Otto)
4. Second expansion (second work stroke)
5. Exhaust

The second expansion adds about 20% to 25% of work for the same fuel consumption.

The optimal second expansion ratio is about 2.8.

The 5 stroke engine can easily be built on base of existing 4 stroke engines making it possible to realize huge fuel economy for the United States almost immediately! Considering that the life cycle of the passanger vehicles in the U.S. is about 4 years - all such vehicles could be ~ 20% more economical.

As stated before - the optimal second expnsion ratio is about 2.8. But it is possible also to convert existing 4 cylinder 4 stroke engine to 5 stroke cycle by converting 2 middle cylinders to a combined additional expansion cylinder! Of course this "combined" second expansion cylinder will give us only expansion ratio of 2, but still - it will give fuel economy improvement of 19% to 21%.