NEVIS is a ground-breaking internal-combustion engine, and more particularly an engine concept that comprises an innovative exhaust valve and intake system, hence its name NEVIS (New Exhaust Valve and Intake System).

In illustrating the concepts behind the NEVIS engine, this document uses published data from the field of mechanical engineering as well as test results derived from proven engines.

In terms of the fluid dynamic characteristics of the NEVIS engine, time and cost has limited the numeric data currently available.  What has been set out in this document has been derived through simplified mathematical modeling.


Only four types of engines have been commercialized on a large scale in the history of engines: the four-stroke engine, the two-stroke engine, the Wankel engine and the gas turbine. Many other engines and power sources have been studied and tested such as the electric engine, the steam engine, the Stirling engine, the fuel-cell engine, hybrids and engines operating with compressed air, but the current stage of their development does not allow these alternatives to adequately compete with the main four internal-combustion engines because of excessive production costs, inefficiency or inferior functionality.

In the 1970s, the gas turbine seemed capable of replacing the traditional four-stroke engine within the automotive sector because of its simplicity and contained dimensions. In aeronautical applications, the gas-turbine engine showed even better efficiency than reciprocating engines: it was durable and powerful, produced little vibration and operated in a relatively clean manner. On the other hand, acceptable efficiency was obtainable only if a very high temperature could be reached in the combustion chamber. Accordingly, it was necessary to build the turbine with special materials, too expensive for large-scale automotive production.

The success of the Wankel engine, that possesses similar characteristics to the gas turbine without its prohibitive costs, has been hindered by its modest fuel efficiency and durability only recently improved. Nevertheless, it continues to be used in those applications where fuel consumption is not the main concern <!--


Until a few years ago the two-stroke engine had a large number of defects, such as high fuel consumption, excessive emissions and irregular functioning at low rpm or idle speeds. However, recent advances have led to very respectable levels of efficiency and functionality along with dimensions that are almost half the size for the same power output. A good example is provided by AVL two-stroke diesel engine[1] with a controllable exhaust timing system.


The following goals were pursued in the development of the NEVIS engine:

a)     Reduction in wasted energy from the exhaust as well as from heat radiation

b)     Improved efficiency at all rpm and power loads

c)     Optimal combustion for improved consumption and performance as well as reduced emissions

d)     Application versatility (aeronautical, automotive, nautical, etc.)

e)     Fuel versatility (gasoline, diesel, hydrogen and biofuels)

f)      Engine block modularity allowing for a wide range of power options

g)     Design simplicity excluding the need for complex or expensive technology and precious materials

h)     Compactness and reduced weight allowing for easy assembly and maintenance

All these goals were taken into account in developing the NEVIS engine and have been applied through the adoption of five key concepts inherent in the engine’s design:

1)     The first concept incorporated within the NEVIS engine relates to a method conceived and experimented by Kadenacy to obtain the scavenging of a two-stroke engine by means of the inertia of the air found in the intake duct and withdrawn by the depression existing in the combustion chamber immediately after the exit of exhaust gases. This method provides considerable advantages in terms of efficiency and overcomes the need to use turbines or compressors generally implemented for scavenging needs.

2)     Importantly, the second concept included within the engine addresses the serious limitations of the Kadenacy effect that is typically only useful within a very small range of rpm. To resolve this issue, the NEVIS engine incorporates a controlled annular exhaust valve of ample size. This exhaust valve allows for variable duration and phasing of its opening by varying the amount of residual pressure remaining from combustion expansion and thus giving the opportunity to correctly phase the scavenging at all rpm’s and loads. At the conclusion of the scavenging phase, the quantity of air that must be kept in the combustion chamber can also be regulated for the varying load demands of the subsequent combustion. Consequently, a new cycle has been implemented within the NEVIS engine. This new cycle allows partial loads to have an expansion stroke greater than the compression stroke like the Miller cycle but with the advantages of a two-stroke cycle. In contrast to what happens with butterfly throttling in the intake duct, the air is free to enter copiously into the combustion chamber with a better fluxing efficiency, thus providing an optimal scavenging even at partial loads and at minimum rpm.

3)     The third concept is based on the adoption of a special shaft for the transformation of the alternate motion of the pistons into rotary motion. This shaft is a sort of sinusoidal camshaft similar to those adopted in engines with cylinders arranged coaxially around the shaft (see the DYNACAM 12- cylinder engine below). However, the new shaft differs in many aspects. First, it has reduced mass and dimensions. Furthermore, it allows for very low average piston velocities with the cam providing for constant acceleration and deceleration of the piston along with the capability of allowing a brief full stop at both top and bottom dead centers in order to provide more time for combustion and scavenging. The shaft incorporated within the NEVIS engine also provides the ability to complete three combustion cycles within a single shaft revolution. 



NEVIS BP compet 4

The DynaCam engine, an updated design of

this is now called the Axial Vector Engine


4)     A variable compression ratio is the forth concept inherent within the NEVIS engine. This is made possible at all rpm and power loads by the simple regulation by of an annular screw element within the shaft itself - unlike the complicated mechanisms necessary in traditional engines to achieve this capability.

5)     The fifth concept incorporated within the NEVIS engine is the adoption of annular pistons which enhance the engine’s thermal efficiency and allow for a light and compact structure as well as a rational integration of the engine’s other key concepts.






Tech Nevis 3 way view of NEVIS engine



1st NEVIS Prototype






1,000 cc

Power density Kw/L





80mm internal - 178 mm external




  Estimated:     250/187 »  @ 2.000 rpm

Average piston velocity


7.5 meters/second

Engine Block


Steel /aluminum






64 x 36

Power/Weight ratio (Kw/kg)



Compression ratio

7:1  to  38:1

Injectors per cylinder                                 


Sparkplugs per cylinder                              


























Tech Nevis shaft and 3 lobed disk

Triple-lobed disk attached to the engine

shaft of the cylinder block


The shaft is hollow and has anterior and external grooves along with posterior and interior ones. One groove allows the joining to the shafts of other modular engine units that may be added. A substantially cylindrical support coaxially connected to the engine shaft, via respective grooves, hastwo protrusions encompassing it with a cyclic undulated profile. Between the two protrusions operate three couples of ball bearings attached to three supports of an annular piston;  when they are pushed by the pistons on the inclined parts of the profiles the resultant forces cause the rotation of the profile <!---->support, and therefore of the engine shaft.

Conversely, when it is the support on the engine shaft to set the ball bearings in alternate motion  through its rotation, the ball bearings will follow the constant accelerations and decelerations caused by the undulations of the profiles that, by the way, are flat at their vertices to allow the pistons to briefly stop at the dead points.  One of the two profiles is used to push and the other to call back the ball bearings depending on whether it is the engine shaft to drive the pistons’ motion or visa versa.  Likewise, the ball bearings invert their task to push and to decelerate the piston with every stroke.

The piston velocities and accelerations are represented in the graphic below where, for simplicity, only one operative cycle has been considered, while in reality the cycles are three for every revolution.





            Tech Nevis three way view of piston

Side, Top and Bottom views of the NEVIS piston


Tech Nevis piston with bearing on lobed disk

Bearing at the base of the annular piston running along triple-lobed disk


The piston is structured such that its top surface can be interchanged in order to allow for the possibility of future variations of the NEVIS engine to have different types of combustion; i.e. either spark ignition or compression ignition in which a thicker piston top and cavities would be used to allow correct combustion.

An internal thread, close to the external segments,  enables a secure and easy assembling of the two possible tops; the blocking is ensured by two bolts opposing each other on the thread to prevent  unscrewing (the prototype piston crown was forced into its position after being cooled in liquid nitrogen).

The interchangeable crown of the piston can be of aluminum, but the structural elements of the piston are best suited for materials like steel which have the advantage of restricted expansion and of greater strength of the cavities for the segments that are often subjected to wear. Furthermore, the robustness required for the support of the ball bearings would be difficult to achieve if it was made of aluminum.

The function of the cylindrical surrounding wall of the piston is to be considered as structural support for the annular piston, or as an obstruction of intake ports to avoid oil leakage from the basement, but no longer as a surface able to contain the lateral pressures caused by the traditional connecting rods that here have been eliminated together with all balancing problems.

While the external segments are traditional, the two <!--

-->internal elastic rings of the piston obviously tend to contract towards the inside; their hardened face is also internal and they require accurate definition and experimentation.

The annular piston  is subject to forces that cause it to rotate on its axis when the profiles of the cam engage the ball bearings to move. To solve this problem, a further 6 small ball bearings have been appropriately inserted on the walls of the basement block to contrast the guides at the sides of the   of the piston ball bearings support, the guides force the piston only in its reciprocating movement .

A thermodynamic analysis of a the annular piston has been carried out by the University of Catania to provide the correct structure considering the heat and mechanical stress to be undergone. Nevertheless, a further optimization of the piston weight is achievable while maintaining the pistons necessary level of strength.  



Tech Nevis 3d open view cross section Small


One of the most serious limitations of the normal two-stroke engines is due to the dimensions of the of the intake and exhaust ports that have very modest space on the cylinder walls. In annular chamber of the NEVIS, the total surface of the intake can be 3-4 times larger, because intake ports can be distributed along the entire lower part of the larger cylinder wall of the chamber, being  the exhaust port located in the high part of the cylinder. The annular exhaust port is even larger than the intake.  To this must be added the fact that the piston stops at the bottom dead point holding the intake ports  completely open for a certain period.  

The combustion chamber was designed in order to have unidirectional scavenging to achieve an efficient fluxage and a uniform distribution of temperature, which, together with the cooling of the internal and external cylinders of the chamber, helps prevent undesirable deformations of the chamber near the intake ports that typically occur in classic two-stroke engines.

The head is well cooled due to the absence of a traditional valve that normally subtracts space to cooling liquid

To achieve a certain degree of symmetry of in the expansion of the flame front and for security reasons in aeronautical applications, three spark plugs have been positioned on the head at a distance such that the farthest point that has to be reached by each flame front of is 77.5 mm.  This is not a lot but neither is it so short as it is 40% more in comparison with a traditional combustion chamber having a bore of 92mm and with a spark plug positioned in the center. Nevertheless, this difference has little importance considering that, due to the standstill of the piston, the flame front has almost triple the time to cover the 77.5 mm of distance (comparing the duration of the spark advance with a four-stroke engine, it is like having 150° instead of 60° of engine shaft rotation). The spark plugs used are of small dimension and commercially available.

Near each spark plug there is a fuel injector to allow a good spray and a good ignition anywhere in the chamber. The additional cost of a spark plug and two injectors per cylinder is compensated by the fact that a single annular piston realizes in the same amount of time a number of operative cycles comparable to four normal pistons, as later explained in greater detail.


The NEVIS exhaust valve operates by closing an annular fissure a few millimeters high; in the prototype this is 4.5 mm.  Accordingly, its shape is annular and it lifts and it shuts down like a guillotine from the point of contact with the cylinder toward the engine head and vice versa.

Like the annular piston, it involves less inertia to complete a lift which is half that of a traditional valve lift. Its larger mass, due to considerable diametrical dimensions, is distributed on 6 short stems symmetrically distant in order to avoid undesired flexion of the ring that is, in any case, lighter than 6 traditional valves.


NEVIS Exhaust valve



The sealing of the annular valve has to be ensured at the top with an internal boarded edge of the valve that lies on another boarded edge created on the engine head, where a thin trapezoidal spring ensures the desired sealing due to its shape and elasticity. For the realization of this trapezoidal spring, it was necessary to choose a material also capable of retaining its elastic properties at relatively high temperatures since as it can be affected by the blow-by of gases despite being protected within the boarded edge of the head.

The shape of the trapezoidal spring must ensure that the valve has a hermetic closure variable in height, considering that it is not possible to have a perfect matching of the head with the cylinder block and that the valves are integral with the head; if the coupling is not strictly accurate leaks may occur from the superior edge of the valve, or <!--

-->if the head is too distant from the cylinder block, leaks would occur from the lower part of the valve that is not able to close the fissure completely, as it already knocked against the upper edge of the head. With the trapezoidal spring mentioned before such problems are avoided, considering that the amplitude of the spring flexion will be higher than the amplitude of the coupling tolerance of the head with the cylinder block.



Tech Nevis sealing ring of exhaust valve

Sealing Ring of Exhaust Valve


In any case, the spring requires small oscillations and has to withstand a relative amount of mechanical strain; it can therefore be realized with a thin section that, together with the ample diameter, will not require elevated pressure to obtain the desired vertical deformation. The edge where the trapezoidal spring lies is removable from the top of the cylinder head to allow the disassembling of the valve.

The lower part of the valve presents no sealing problems, as it can be considered like a valve with a very large diameter; a traditional coupling is therefore easily created with the point of contact between the valve and the higher part of the cylinder block with an inclination of the edge of contact of 30-45 degrees. Of course, the contact surface between the valve and the cylinder will be covered with material similar to that of traditional valve seals.

Traditional exhaust valves have a very serious disadvantage: they open towards the inside of the combustion chamber and struggle against the normal outflow of exhaust gases which, after having increased the valve temperature, burn the stems too. This often leads to the necessity of using more resistant materials (chromium silicon steel, austenitic steel with a high content of nickel chromium) or realizing complex parts, such as hollow valves or parts partially composed of metallic sodium or <!--

-->lithium or potassium salts that improve the heat transmission from the head to the stem of the valve.

The temperatures that can be reached are very high in comparison to those of the intake valves:   the combustion chamber is therefore destined to have considerably different temperatures in the two respective areas, with all the consequences this entails for pre-ignition.

The NEVIS exhaust system, avoids these problems in terms of the outflow of exhaust gases; in fact, for the greater part of the expulsion of gases, the “guillotine” valve is well protected in its position above the exhaust fissure. For this reason, it is not exposed to exhaust gases and it does not hinder their flow.

The small quantity of heat that may be absorbed when it is closed is easily drawn through the stems and the lower edge that is close to the cooling liquid.

Six radial deflectors gently direct the exhaust gases toward the duct that collects the exhaust in order to utilize in the best possible way the kinetic energy of the exhaust.



Tech Nevis exhaust views

Exhaust assembly




In a future version of the NEVIS engine, the closure of the valve will be operated by means of a desmodromic timing distribution not disclosed here because object of a new patent request.

The current system to recall the valve in the NEVIS engine utilizes good-sized springs with a reduced number of spires that are calibrated in order to provide sufficient pressure for the sealing in its resting position and sufficient strength to bring down the valve after the lift and avoid detachment of the roller tappets from the cams.

The roller tappets for the annular valve consist of three ball bearings; each ball bearing is pivoted to a structure whose top side holds a recalling spring; the same axle of each ball bearing has room to join two rocker arms on each side connected with the two relative stems of the valve to be lifted.


Tech Nevis Image of Bearing and Rocker Arm of Exhaust Valve System

Image of Bearing and Rocker Arm of Exhaust Valve System



If two stems are lifted at the same time by one roller tappet, then the lifting of the valve can be effectuated with three cams acting on three roller tappets. 

The adoption of rockers helps eliminate the vibrations caused by the alternate motion of the valve.

The stems are very short and being fitted, but not welded, to a rather cold valve, they shouldn't be subject to appreciable lengthening when the engine’s operating temperature rises. Anyhow, their elongation does not cause the roller tappets to come close to the cams , but to draw away from them. For this reason the adjustment of the roller tappets is not necessary and the initial positioning has to be done by putting the roller tappets in contact with the surface of the disc or flat support of the cams that lies below , obviously in the point where it's flat and not where the cams are located.





Tech Nevis Unassembled accelerator

Unassembled accelerator



To vary the loading of the engine, it is necessary to vary the duration of the exhaust opening: this is possible through the special cams illustrated above. They are free to slide along the perimeter of their support that has an ample diameter in order to allow the three cams a sufficiently extended area to develop on the plane along the perimeter.

The regulation of this slide is obtained with a “regulating cylinder” that has internal and external helicoidal grooves and is located coaxially between the cylindrical part of the support of the cams and the concentric cylindrical part of the sliding cams. They are coupled via their respective helicoidal grooves; consequently moving back and forth the “regulating cylinder” it is possible to obtain the desired sliding of the cams. Although the “regulating cylinder” rotates at the same engine rpm, it needs to be supplied with a ball bearing fixed on its edge that can be grabbed while rotating This ball bearing has three protrusions on the external ring that are inserted in inclined fissures of two other concentric cylinders, one cylinder being fixed to the cover of the head, the other one free to rotate on its axis to solicit, with the edges of its contrasting fissures, each ball bearing protrusion for a lift or a lowering of the “regulating cylinder”. The external cylinder, free to rotate, has teeth on its lower edge engaged with a smaller gear that has an axle to transmit the rotation to the external part of the block, allowing the adjustment of the accelerator through extended devices.


Like the accelerator, the timing of the lifting phase is variable.  The NEVIS engine achieves this by varying the angular position of the support of the cams to the coaxial engine shaft.

This variable timing system comprises a “regulating cylinder” with a helicoidal groove both internal and external which  is located coaxially between the cylindrical part of the support of the cams and the engine shaft. They are coupled via their respective helicoidal grooves. To cause the variation of the timing, the “regulating cylinder” can be moved back and forth of a few millimeters by a ball bearing fixed on it that can be moved up and down thanks to three small external protrusions inserted in oblique guiding fissures of two external concentric cylinders: one of the cylinders is fixed to the block, while the external one can rotate on its axis. In this way, the oblique fissures are made to rotate each in an opposite direction so that the contrasting push of the edges of two guiding fissures, internal and external, applied on each ball bearing protrusion causes a lifting or a lowering of the “regulating cylinder.” The external cylinder , free to rotate, hasteeth  on its lower edge engaged with a smaller gear that has an axle to transmit the rotation to the external part of the block, allowing the adjustment of the phase through computerized actuators.


The compression ratio can be easily varied by initiating a minimal movement of the piston cam support parallel to the axis of the engine shaft. The movement is regulated in an extremely precise way with a “regulating cylinder” that is also the support of the big double ball bearing holding the piston cam support. The “regulating cylinder” is screwed to the engine cover, consequently, if it is twisted, it can bring the piston near the cylinder head. It is also has grooves on its lower edge, inserted with the grooves of another twisting-cylinder equipped with teeth on its lower edge engaged with a smaller gear that has an axle to transmit the rotation to the external part of the block, allowing the adjustment of the compression ratio through a computerized actuator.


The principal innovation of the NEVIS engine is the system used to vary load needs. The extended opening of the exhaust valve allows the piston to expel the air that has replaced the exhaust gases through the cleaning phase. The longer the valve stays open, the smaller the amount of air available for the combustion that follows, and as the compression ratio may be varied as desired, it is possible to have very small quantities of air charge. In other words, the load is reduced but not the efficiency of the engine that totally utilizes the expansion of the combustion with an expansion stroke that is inversely longer in relation to the load entity.

If the opening time of the exhaust valve is short and the air <!--

-->is prevented from flowing out of the exhaust duct, the compression ratio can turn back to the initial proportion and significant loads can be achieved, especially if the inertia of the air in the intake duct causes some supercharging. For the sake of efficiency, it is better not to run the engine under pressures that are too high as this requires greater depressions in the combustion chamber that can only be obtained using the kinetic energy of the exhaust gases which, flowing out at high velocity, cause depressions.  The more these are intense and durable the higher the velocity and the quantity of the exhaust gases. Thus, anticipating the opening of the exhaust valve when there is still a certain pressure in the combustion chamber improves the filling process, but causes a loss of efficiency as expansion doesn’t take complete advantage of the pressure given by the combusted gases.

The use of ducts with variable geometry and lamellar valves could widen considerably the range of rpm at which the aspired versions of this engine can be utilized.

In those cases in which turbine compressors would be used, or passing from a gasoline version to the diesel version, it will be possible to reduce or increase the compression ratio as needed by reprogramming the software that assists the actuators of the timing and of the variable compression ratio system, in order to allow optimal regulation for any possible load need.

One cycle is completed in 120 degrees of engine shaft rotation. However, to establish a direct comparison with the four- or two-stroke classic cycle diagrams, it is necessary to consider NEVIS shaft rotation 4.5 times slower.

Accordingly, one degree of its shaft rotation is comparable to 4.5 degrees of traditional four- or two- stroke shaft angle rotation. In this way it is possible to represent graphically a diagram that in 540 degrees describes the entire new cycle including the standstill of piston at top and bottom dead points (the rectilinear parts of the oval diagram that follows). To be more clear, if the four-stroke cycle needs 720 degrees of shaft rotation to be completed and the two-stroke needs 360 degrees, the NEVIS cycle occurs in 120 degrees. What we are trying to do is to compare the single cycles, and if the shaft rotation angle degrees are considered as a function of time, expanding the time is like increasing the amount of degrees. Therefore, the timing of a single stroke can be represented with 180 degrees instead of 40; if rotation timing is 4.5 times slower, similarly the graphic description of an entire NEVIS cycle can be expanded on the diagram in 540 degrees instead of 120 as represented in the first diagram that follows.

Diagrams of the timing system of the NEVIS engine are represented below.

Tech Nevis cycle


Tech Nevis Bortone Cycle

There is no need for counterweights to balance the NEVIS engine as long as it is realized using two, four or six piston modular blocks. The only vibrations which need dampening are the torsional ones of the engine shaft. A connection with the engine shaft through normal commonly used elastometers can easily absorb a vast range of frequencies, and, as in traditional engines, the recourse to vibration reducers is useful to avoid shaft rupture due to stress.


A difficulty, never surmounted, linked to the direct injection of gasoline in two-stroke engines resides in the insufficient vaporization of the fuel. Diesel fuel can burn if sprayed in small drops, but gasoline must be transformed into vapor in order to burn properly. To obtain vaporization within a limited amount of time, heat is required. With respect to the necessary heat, direct injection in the combustion chamber provides a hotter ambient temperature than the intake duct; on the contrary, the amount of time necessary for vaporization is very modest (in the traditional two-stroke engine only 100 degrees of engine shaft rotation are available). Some engine designs, like the two-stoke Orbital, utilized a pre-mixture of gasoline and compressed air, or resorted to higher pressure of the injection to obtain better fuel pulverization .

In the NEVIS engine, the amount of time available for vaporization is 2.6 times longer, as can be easily deduced from the diagram of the timing system that shows an interval of 260 degrees between the closing of the exhaust and the spark ignition.


As set out previously, the NEVIS engine’s cooling is more efficient notwithstanding the reduced heat dispersion. The reason is as simple as it is intuitive: If one tries to cool down the centre of a piston it is necessary to absorb a great amount of heat from the periphery until the desired temperature is progressively obtained at the centre of the piston, but at that point the peripheral parts will necessarily be at a much lower temperature than the centre and this against the fact that the peripheral structure is much stronger than the centre of the piston. In other words, excessive heat will be absorbed by the peripheral part in order to cool the centre of the piston. This does not happen with the annular piston because the centre of the piston, just like the periphery, is cooled directly by cooling liquid; the heat absorbed in the periphery can therefore be calibrated without excesses and without the risk of having some hotter points because of interference with other structures or with other cylinders that, in normal engines, are located one next to the other, while in NEVIS they are disposed along the shaft one after the other and well distanced.


The choice of double ball bearings is also linked to the kind of lubrication selected for the engine that happens through controlled spraying, which is preferable to the conventional method as it absorbs less power and allows dispersion within one segment that should be at the short base of the piston and that, by itself, can prevent oil from entering the intake duct. This nebulizing system also does not require periodical oil changes and makes it possible to eliminate the oil pan that would otherwise negate some of this engine’s advantages, its compact size with its very low baricenter.  Otherwise, the engine’s modularity would require as many oil pans, or worse, scavenger pumps, as the number of cylinder blocks utilized.


-->The roller bearings, on the other hand, do not suffer from the problem of the absence of lubrication during the start-up phase.

A number of holes in the wall of the engine block allow the oil flow down towards the lowest part of the block where other holes suck up and recover the modest quantity of excess oil due to the depression created by two normal external gear pumps that need to be selected based on the number of modular engine units desired; the same applies to the water pump, the injector pump and the gasoline pump.


In considering exhaust hydrocarbons (HC), indubitably one of the reasons of their presence relates to the un-oxidized part of the charge located in the thin part of the first segment (crevices); in the annular combustion chamber this thin part is more extended than in a traditional cylinder having the same displacement, but, as explained later in greater detail, at least three pistons of the same total displacement would be required to have a proper comparison and the crevices in such circumstances are surely much greater.

Actually, in the traditional two-stroke engine the concentration of HC reaches  particularly high levels because the charge mix of air and gasoline (diluted during the scavenging phase) in the combustion chamber together with the residues of burned gases and part of the gasoline comes out from the cylinder without burning. This does not happen in the NEVIS engine as during the scavenging phase there is no gasoline in the chamber yet, while at the end of the washing phase only clean air is left that can be mixed with the injected gasoline with correct stoichiometric and compression ratios and nothing can escape from the chamber because the exhaust valve is closed.

In contrast, traditional four-stroke engines, even though there are no substantial gasoline exhaust losses (unless there are significant overlappings between the opening of the intake and of the exhaust), the concentration of HC reaches particularly high levels when the engine works with strong depression in the intake duct (which happens at minimum rpm and during deceleration) because the fuel is very rich in gasoline, the expulsion of the burned gases is less complete and the compression ratio is very low.

In other operating conditions, the presence of HC in the exhaust gas is due to an incomplete combustion of the fuel in the substratum attached to the walls of the combustion chamber where the temperature is lower than that required for oxidation. This is also a function of the mean temperature in the combustion chamber. As set out previously, in reference to the surface/volume ratio, the mean temperature in the NEVIS engine is uniform and more elevated, particularly around the exhaust valve that represents one of the walls of the crevices upon the first segment: the wall is not surrounded externally by cooling liquid, resulting in less heat loss from the chamber and reducing the substratum mentioned before.

With the NEVIS engine there are ultimately many possible solutions to further limit HC: thermal reactors and various catalytic devices can help, but what is truly useful is the consistent reduction of the problem at its origin, and that means a correct and complete combustion with all power loads. The design of the NEVIS engine, with possible adjustments, makes this achievable. HCCI -- Homogeneous Charge Compression Ignition is a possibility for the NEVIS engine.  This relies upon a very lean (high proportion of air to fuel) and well-mixed air-fuel mixture that is compressed until it auto-ignites. The resulting spontaneous burn produces a flameless energy release in a large zone almost simultaneously -- very different than the spark/gasoline burn or the compression/diesel burn.  The NEVIS engine adapted for HCCI would not only increase efficiency further but would also do away with NOx without the PM (particulate matter) emissions of a diesel. 



-->In all probability, these are the ways to achieve very low emissions in the future before the advent of hydrogen and NEVIS can provide an adequate answer of this potential even with hydrogen as BMW was able to demonstrate with its hydrogen-fuelled 12 cylinder engine built in the early 1980’s.



The principal reasons that prohibit normal two-stroke engines to achieve a high mean effective pressure when compared to four-stroke engines are due to the following factors:

1.     The dimensions of the intake ports are necessarily small because sufficient lateral space is needed for the exhaust ports that should not overextend and excessively limit the expansion (in four-stroke engines valves are much larger).

2.     The cleaning phase is not properly carried out at all rpm and at all loads because the throttle partializes the air entering in the duct of the intake. Accordingly, part of the exhaust residues remain in the combustion chamber and create problems for the subsequent oxidation. This occurs with less intensity in four-stroke engines.

3.     Loss of gasoline from the exhaust, unless direct injection is utilized, due to the contemporary opening of intake and exhaust ports at the end of scavenging phase. The four-stroke engine, on the contrary, has a longer time to carry out the vaporization even with direct injection.

4.     Part of the expansion cannot be exploited because of the anticipated opening of the exhaust port that is high on the cylinder wall, and in correspondence of which the piston still receives a strong pressure. In a four- stroke engine the problem is less severe due to the delay of the opening of the valves that begins at about 60° of angle of crank with regard to the bottom dead point; in a two- stroke engine the opening of the exhaust at 70 degrees before BDC causes the missing of about 15% of useful stroke.

All the drawbacks of the traditional two-stroke engine are overcome by the NEVIS engine.

1)     In NEVIS, the intake and exhaust ports are considerably bigger than the ones in the classic two- stroke and they are more than three times bigger than those of the four-stroke engine. The flow is considerably higher.

2)     The scavenging of the NEVIS engine differs from that of the traditional two-stroke and it can be compared to four-stroke efficiency;  in fact, it can be complete at all rpm and at all loads with an optimal expulsion of all combustion residues due to the new cycle, to the particular unidirectional outflow and to the longer scavenging phase.

3)     The variable lifting law of the exhaust valve and its regulating timing system, in combination with the new “loading” method, allow to avoid completely and at all operative conditions losses of any quantity of fuel at the end of the scavenging phase; in fact, the exhaust valve during this precise phase will always be closed. The NEVIS engine also provides a time for vaporization that is 2.6 times longer than in the normal two-stroke engine: this allows the use of direct injection operating with relatively modest pressure and ensures good injection even at high rpm and high loads.

4)     Considering the modest height of the intake ports and the fact that the <!--

1.     -->opening of the exhaust begins at a point similar to that of a four-stroke engine (55-60 degrees of angle of crank) with respect to the bottom dead point, there is a longer-lasting pressure at the end of the stroke.


For all these reasons, it should be clear that it is wrong to consider the single cycle of the four-stroke engine nearly twice as efficient as a single cycle of the NEVIS engine, as it generally occurs when comparing the normal two-stroke to the four-stroke engine.

In only one cycle, the NEVIS engine can produce a mean effective pressure highly superior to the single cycle of the two-stroke engine and only pessimistically equal to the four-stroke engine cycle. At partial loads and low rpm not even the four-stroke engine can sustain the comparison, due in part to the fact that the compression ratio can be modified during the operative cycle and in part because there is the advantage of a longer stroke in the expansion phase than in the compression phase, as in a variable and extreme Miller cycle.

The annular piston can at first appear a reckless and counterproductive choice. Actually, it doesn't seem likely that the segments on the external side of the piston as well as the internal ones can achieve a reduction in friction (it is well known that the perimeter of a circle is much shorter than both the perimeter of a doughnut covering the same area of the circle. Even less promising seems to be the increased mass, due to the increase of the diametric dimensions of the annular piston . But as often happens with things that are not immediately evident, a deeper analysis is necessary to ensure correct valuations.

To start, let us compare the annular piston with a traditional one, say with a bore of 86 mm and a stroke of 86 mm, for a unitary displacement of 500 cc, a so-called super squared as in the most recent applications. The data show that the surface involving the friction of segments in the classic piston amounts to 23223.44 mm² and it can be considerably reduced by increasing the diameter of the piston with respect to the stroke (obviously maintaining the same displacement).

However, increasing the diameter of traditional pistons beyond a certain limit is not advantageous because of the cooling difficulties of the centre of the piston, which, being too far from the cooling liquid, is not able to drain all the heat that it accumulates; thermal efficiency deteriorates because of a bad surface/volume ratio and the time to complete combustion increases due to the longer distance that the front of flame has to cover.

For these reasons bores bigger than 80-90 mm are rarely used in unitary displacements reaching 500 cc. With the annular piston it is possible instead to have much bigger diameters, as the central part of the piston can be easily cooled down. A surface of the top of the piston around 3-4 times bigger than the piston with a bore of 86 mm , allows to reduce the stroke of the piston to 25 mm.


-->If the diameter of the internal hole of the annular piston is 80 mm, enough to leave room for cooling water and the engine shaft , the external diameter of the piston has to be 178 mm if we want to obtain a 500 cc volume in the combustion chamber . In this way, no point of the piston is farther than 24.5 mm from the cooling liquid , as in a classic small piston with a bore of 49 mm .

At a second glance, an interesting surprise is the reduced surface that the segments of the annular piston involve with their alternate motion and that amounts to 20.253 mm² against the 23.223mm² of the traditional piston; in other words, a 15% bigger surface than the annular.

NEVIS repeats the cycle three times in one revolution of the shaft; the four-stroke engine completes an entire cycle in two revolutions of the engine shaft. Accepting the fact that both engines give similar power output per single cycle, it would be incorrect to compare two of the kind monocylindrical engines, of same displacement, because to obtain comparable power output the four stroke should have at least a number of revolutions per minute six times faster, (even an extremely short stroke would not be able to significantly improve the average velocity of the four-stroke piston beside also having to overcome a number of serious mechanical, thermal, and fluid-dynamic problems ).

Without forgetting that NEVIS needs at least two cylinders to be balanced, a more fair and reasonable comparison between NEVIS single cylinder and a four-stroke engine would be to consider for the latter a unit with four cylinders of a 25% bigger total displacement and same NEVIS stroke (doubling the prototype from 25 mm to 50 mm). In this way, there is a certain approximation of fundamental parameters, such as medium velocity of pistons, dimensions of intake and exhaust ports, volume of charge in expansion, and other aspects that are an essential condition while searching comparable power output.

The following data can clearly evidence all the potential of the NEVIS engine.

To avoid mixing too many complex phenomena to be analyzed at one time, the variable compression ratio, the Kadenacy effect and Miller cycle, that advantage NEVIS, are not considered here although they represent further consistent prerogatives of efficiency and functionality added to an already more efficient and simpler basis than the four stroke.


Engine specifications compared



4 Stroke







1,000 cc

1,333 cc



80 mm internal

178 mm external

92 mm



50 mm

50 mm


Top Piston Surface

19,848 mm                              

26,576 mm


Pistons Surface / Intake Ports Surface     




Pistons Surface  / Exhaust Ports Surface 




Surface Intake Port                                    

3,720 mm

1,250 mm


Timing Of Intake Opening                        

42° x 4.5 = 189°



Degrees Of Intake At Max Opening       

20° x 4.5 = 90°


Surface Exhaust Port                                

4,470 mm

1,160 mm


Timing Of Exhaust Opening                       

43° x 4.5 = 193.5°



Degrees Of Exhaust At Max Opening  


Surface Of Combustion Chambers       

800 cm

1,110 cm


Segments Circumference                          

559 + 251 mm                             

289 mm  


Total Segments Circumference           


1,156 mm    


RPM at Average Piston Velocity of 7.5m/S                

at 1,000 rpm               

at 4,500rpm       


Segments Crowling Per Minute                

22,065 cm                         

52,020 cm


Degrees For Vaporization                        

58° x 4.5 = 261°



Number Of Combustions                            

3,000 x 3

2,250 x 4




The useful time for vaporization appears to be the only data point modestly superior in four-stroke engines, but for all other parameters the difference in efficiency is consistent; the four-stroke has higher friction and dispersion of heat from the vast surfaces of its four cylinders which alone are factors that make a much larger efficiency impact than the 10% longer vaporization timing.

The fact that the piston has a very limited mean velocity, should not be misunderstood as a slowdown of movement: In fact, the time the piston uses to compress the charge is no longer than the time used by the four-stroke engine; and as a matter of fact, the piston moves the same quantity of charge in the same time and in the same volume. The combustion is also happens in the same amount of time of the four-stroke engine and takes place in a constant volume and in a condition in which the flame front experiences a more uniform temperature. The greater uniform temperature in the combustion chamber allows the adoption of a higher average compression ratio. In all engines, particularly during combustion, which is the hottest phase, there is, in proportion, the largest transfer of heat to the cylinder walls. In the NEVIS engine a smaller mean volume in this phase adds to the capability of further reducing the volume of the chamber at partial loads given the variable compression ratio. Reducing the volume of the chamber means a better surface/volume ratio resulting in less heat loss from the walls. The longer stopping of the piston at the top dead point does not generate a greater heat loss, as the time needed to complete the combustion does not increase nor does the velocity of the flame front decrease. The result is therefore not a longer time of heat dispersion, but simply a smaller volume of the combustion chamber for the duration of the combustion.

The versatility of the NEVIS engine offers the opportunity to attract the interest of many sectors. This is an important evolution that provides further guarantees for commercial success. Actually, there is no previous historical example of the complete modularity of the NEVIS design around a single block: one is therefore free to add units, ranges of different displacements for different horsepower needs, while maintaining unchanged all the prerogatives of efficiency, durability, low cost etc.

The adoption of a hollow shaft allows using a transmission shaft, or passing axles, in automotive applications. The shorter version of the engine with two cylinders can be designed with the cylinders at the sides of a specific gearbox and two converters, so that its axles passing through the cavity of the engine shaft can transmit motion directly to the drive wheels without any need of a clutch or a differential. If the space is limited or more cylinders are needed, it would be possible to position the engine longitudinally with respect to the vehicle, distributing in this way the engine’s torque between the anterior and posterior axles without the need of gears, reducers, or double shaft transmission. In any case it is possible to use the 2-, 4- or 6-cylinder engine in a traditional way connecting a clutch and a gear box directly to the engine shaft. In aeronautical applications this engine shows even more evident advantages considering the reduced weight and frontal section, but also the very high specific power together with low consumption.

In the 2-cylinder 2000cc versions the power and the torques that can be obtained are even greater than those of a four-stroke engine with 8 cylinders and a displacement of 2700cc. Furthermore, it must be underlined that there is no need for gear reducers for a propeller; in fact, revolutions of the shaft are 4.5 times slower.

A considerable advantage is the possibility to easily utilize two counter-rotating propellers by placing, also in this case, a transmission shaft in the cavity of the engine shaft and inverting the rotation by means of gears on the back, or it is possible to make two engines run in the two possible rotating senses independently, and transmit the motion to each propeller with a shaft passing through one of the two engines. Several possible applications exist also in the nautical sector and in hovercrafts.  Particularly in WIGs (Wing-in-Ground effect) or ekranoplans, (with significant military interest), have the need for high power during take-off, but of a far less power during normal cruising, this makes the NEVIS extremely suitable engine considering that optimal efficiency persists also at partial loads.

It is reasonable to expect a much lower specific consumption considering several factors: strokes are reduced by half; the segment length is inferior; the kinetic energy of the exhaust gases that would otherwise be wasted is used; the wider exploitation of expansion, particularly at partial loads; the optimal compression ratio at all rpm and at all loads ,which will be higher than the optimal compression ratio of traditional engines; the elimination of part of the transition or the simple gear reductions that originate a considerable absorption of the power available for the tires in automotive applications.

The NEVIS engine also has significant advantages from an industrial point of view. It has low projected production costs as well as its inherent commercial advantages, but also has the reduced investment needs for its industrialization since it is not encumbered by the necessity of a sophisticated processes, unusual machinery or expensive materials.





[1] SAE Technical Series Papers 981038, 02/1998