Independent Assessment
“It is estimated that a Libralato naturally aspirated engine, with an expansion volume of 1300cc and running at 3000 rpm with a bmep of 7.7 bar will produce 50 kW. This engine is calculated to deliver a
BSFC of 208 g/kWh. A simple thermodynamic model, applying basic principles of thermodynamics, has been used to predict the approximate power, and chamber pressures. While these predictions for the Libralato engine cannot yet be considered conclusive, this model has, in the past, been validated for a large number of conventional engines against test bed measurements and with other predictive methods such as Star-CD, GT-Power. AVL’s FIRE and Ricardo’s WAVE.
The combustion process in any internal combustion engine is a complex phenomenon and, while the analysis so far carried out is based on the 4-stroke gasoline engine in common use at this time, the indications are that the combustion and expansion phase in the above Libralato pressure diagram is credible at this stage of the development of the concept. There is no reason to suspect that the Libralato cycle will be inferior to the common 4-stroke and it is believed that the early CFD work planned for this project is likely to demonstrate strategies for achieving at least the minimum targets set out here.
Engine performance is largely indicated by the relationship between pressure and volume in the working chambers.
The following graph is an original drawing of the Libralato cycle principal chamber volumes across 360o of engine rotation, the measurements being taken from the small concept demonstration model. Consistent with the diagram showing the Libralato Cycle, the expansion chamber can be seen at the top left, the second stage compression can be seen at the top right and the intake volume can be seen at the bottom. These volumes are formed by the action of the leading and following rotors each rotation of the engine. This diagram is somewhat simplified for clarity, since it does not show the first stage compression volume, nor the post scavenge volume merging with the intake volume.
Original Drawing of Libralato Chamber Volumes
The following graph compares the chamber volumes generated from the Pro-Engineer model of the Libralato engine with the physical measurements from the concept demonstration model, shown above, and a typical reciprocating engine. It can be seen that the blue (Libralato) and green (RL Vol Sketch) curves from the measured values and the Pro-Engineer calculations, correlate well and if these are compared with the red (Reference) curve from the piston engine, the curves are close enough that, when taken with the pressure curves above, the Libralato cycle can reasonably be expected to deliver the anticipated power output.
Libralato and Reciprocating Volume Diagrams Compared
In comparing the Libralato with a conventional piston engine, it must be recognised that a naturally aspirated 4-stroke piston engine, which has to deliver its power one stroke in four, must therefore have a significantly higher maximum cylinder pressure than the Libralato engine (which fires every revolution) and this generates a higher mechanical cyclic load pattern. The Libralato engine can therefore offer a much less massive structure and particularly when the lower number of components is taken into consideration, a much lower mass and therefore manufacturing cost per kW output.
It is recognised that the evolving Libralato design will have to deal with higher specific thermal loading, typical of a once-per-revolution cycle. However, the indications are that the Libralato engine will demonstrate a net advantage over the equivalent 4-stroke in terms of thermal efficiency where both are naturally aspirated.”
The Engine Consultancy Ltd. Dec 2009
