What is a 5 stroke engine?
They tend to consist of only three cylinders rather than the more traditional 4 stroke, 4 cylinder engines that many people are using today. There are two small small high-pressure cylinders and one larger low-pressure cylinder. This video has some good information and animations.
Why aren't they using them in vehicles right now?
- A secondary cylinder provides an additional expansion process enabling extra work to be extracted, which results in better thermodynamic efficiency (which means less thermal energy is wasted, hence the engine efficiency must increase)
- 5 stroke engines tend to be quite small as the firing (smaller) cylinders can be very highly rated
- The engine uses 100% conventional technology and so requires no new manufacturing techniques as such
- Intended to increase engine efficiency
- Intended to reduce emissions and fuel consumption
A few reasons as far as I can tell:
- Increased manufacturing costs due to the fact that it is not widely adopted, also referred to as the "early adopter tax"
- Not in mass production so it doesn't benefit from economies of scale (lower manufacturing cost per unit when made in higher quantities)
- It is a requirement to have two camshafts per engine, resulting in higher overheads on a production line
- Possibly higher cost of materials (much less significant compared to the first two points)
- Irregularly sized cylinders and cylinder casings, which would cause engineering difficulties and increased costs associated with that
- More points of failure, resulting in less comprehensive warranties on new cars
Is this a new thing or has it been around for a while?
From wikipedia, according to Suzuki, Ph.D., Takashi (1997). The Romance of Engines. SAE. pp. 87–94., the engine was first introduced during the early 1900's by the Eisenhuth Horseless Vehicle Company, there was and I quote:
an unusual model called the Compound with three cylinders. Two were working cylinders, the larger middle one further expanded the exhaust gases of the outer working cylinders, this concept received later the name: '5-Stroke engine'.