Introduction We have heard about two and four stroke engines. Two stroke engines got its name from the fact that the required strokes are completed in one revolution. In short there is one power stroke in one revolution. In the case of four stroke engines the four strokes are completed in two revolutions, or there is a power stroke in two revolutions. Then how about a six stroke engine. The name of the engine has nothing to do with the number of revolutions or anything of that sort.
This engine got its name due to its construction. A six stroke engine derived its name from the fact that it is a mixture of two and four stroke engine. This engine is a radical hybridization of two and four stroke engines. This engine combines the top portion of two stroke engine and the bottom rather the middle section of a four stroke engine. These types of engines have many advantages compared to OHC four stroke engines. They are as follows 1) Increased torque and power output 2) Better fuel economy ) Cleaner burning with reduced emission 4) Longer service intervals 5) Reduced tooling costs Six stoke engines were developed in the year 1998 by Malcolm Beare. This technology is under going tremendous research works for improving the six stroke or Beare technology as it is popularly known. This type of engine is not commonly available because of two main reasons 1. This technology is patented by Ducati 2. Research works are going on for improvement of this technology Construction and Working
The six stroke engine basically works just like a four stroke engine. The major difference is in the construction. The major drawbacks of conventional engines were poppet valves, its basic problems being and – inertia, inhibiting flow especially the exhaust valve hot-spot in the combustion chamber. So a six stroke engine was simplified with the objective of improving efficiency and increasing performance compared to a conventional engine by overcoming the drawbacks of poppet valves, by means of a rotary alve application to four-stroke engine. Of course, a two-stroke doesn’t suffer such problems as it had no poppet valves. So these drawbacks were resolved by he basic taking the components of a rotary disc induction two-stroke engine, and grafting them on to a four-stroke to produce the best of both worlds. Below the cylinder head gasket, everything is conventional, so one advantage is that the Beare concept can be transplanted on to existing engines without any need for redesigning or retooling the bottom end.
But the cylinder head and its poppet valves get thrown away. To replace the camshaft and valves, Beare has retained the cam drive belt and fitted an ultra short-stroke upper crankshaft complete with piston, which the belt drives at half engine speed just as it previously drove the cam. This piston drives up and down in a sleeve, past inlet exhaust ports set into the cylinder wall, very much like on a two-stroke: these are all exposed during both inlet and exhaust strokes.
This being it’s only function, the rotary valve is lightly loaded, reducing lubrication and sealing problems. During the compression and expansion strokes, the upper piston seals off both ports, leaving the pressure contained between the two pistons, with the lower one a conventional flat-top three-ring design, while the conical upper one (so shaped to aid gas flow during both inlet and exhaust cycles by guiding it towards the ports) has two rings – one compression, one oil.
In the combustion phase, twin spark plugs provide ignition via the stock Ducati CDI and a pair of Harley coils – one per cylinder – and not only does the engine run on pure petrol (no need to add oil, because all required surfaces are positively lubricated, in spite of the application of two-stroke technology), it’s also happy on low octane unleaded fuel. Obviously there are no valve seats to suffer from lack of lead, and Malcolm says the compression ratio can be increased significantly from the Ducati motor’s 10. 6:1 quite safely because of the lack of hotspots, without problems with detonation.
So now the claimed advantages of all this start to come to light – allowing a higher compression while still happy with low octane unleaded make this an efficient and cleaner engine. There are no poppet valves to float or bend. This is a much more cost-effective way of achieving this than expensively machining a set of closing rockers for all the valves in a cylinder head, quite apart from the unwanted inertia such a system still entails. But there are other, much more significant apparent spin-off benefits from the Beare design.
First of these is fuel economy: Malcolm Beare claims his engine is 35% more economical at low revs/throttle openings than an equivalent conventional engine and 13% less thirsty at high rpm/full throttle, in spite of the doubled-up carbs. That should mean fewer hydrocarbon and CO2 emissions, because you’re using less fuel to achieve the same performance. Ducati-based prototypes Beare discovered the six-stroke version produced the same torque as a four-stroke 1,000rpm lower down the scale, as well as producing exponentially more torque as revs rose.
But in a commercial application, perhaps the most attractive benefit is the reduced number of moving parts, compared to a four-stroke design, so the six should be cheaper to make. Not as few as a two-stroke, but what you appear to be getting here is improved performance and torque, coupled with the inherent advantages of a four-stroke, on the cheap. Finally, as the upper two-stroke piston is driven at half engine speed, it should have twice the life of the lower four-stroke one. Referring to the graph, the intake begins at 0 degrees on the X-axis.
The effect of the additional volume changes that the upper piston has on the volume of the engine is all positive from a thermodynamic point of view. If the engine were a normal 4 stroke the cylinder capacity would be 340cc. Of note – maximum volume at the end of the intake stroke occurs at 173 degrees instead of 180 degrees- the change in volume is 308cc which is less than a 4 stroke (340cc)- yet the total volume at the end of the intake stroke is 415cc as opposed to 375cc for a conventional stroke. This means that the extra volume is best swept by gas velocities and not mechanical movement, and therefore mechanical input energy is less.
Also, maximum volume is before bottom dead centre 173 deg. Consequently valve timing, if the same as a 4 stroke, is more radical and is of longer duration in relation to engine volume and hence volumetric efficiency is considerably improved. The change in volume during the compression stroke is slightly greater than a 4 stroke after the ports are closed. The expansion stroke is much greater than a 4 stroke; both from T. D. C. to B. D. C. and from T. D. C. till the exhaust port is open. It is possible to leave the opening of the exhaust port later than in a 4 stroke because maximum volume is not reached until after B.
D. C. -548 deg. Instead of 540 deg. Hence the 6 stroke system is better from a thermodynamic point of view because more energy is extracted from the expansion process. During the critical combustion period the rate of change in volume in the 6 stroke is less than a 4 stroke. Minimum volume is not reached until after T. D. C. , at 361 deg. This is because of the phasing of the upper piston. It is retarded in reaching its T. D. C. until 20 deg. after T. D. C. (380). This is much better from a thermodynamic view in that combustion occurs at a more constant volume; hence ignition timing is not as critical as in a 4 stroke.
There is room in the combustion chamber for up to 4 spark plugs and two direct injectors if needed. The change in volume during the exhaust stroke is less than a 4 stroke. This means that the negative pumping work is less than a 4 stroke. Extractive gas velocity is very important. Easily accomplished at T. D. C. with a fully open exhaust port The design can cope with various runner diameters and lengths because the reed valves allow any positive pressure pulses to pass through and cancel any negative ones, as well as providing good secondary atomisation.
Hence at low revs the long thin runners are in tune and at higher revs the shorter fatter ones take over with no need to shut down the long thin ones or visa versa as would be necessary with a normal 4 stroke. Swirl is in one direction at low revs and moves to tumble when the flows are in balance reverting to swirl in the other direction as the short fat ones predominate. A good spread of torque is achieved. Construction Issues The mass of the reciprocating parts in the head is about the same as a 4 stroke but the accelerations are much slower so energy absorption is less.
The piston speed of the upper piston is about a quarter of the main piston; therefore its service life should be at least twice that of the main piston. There are no service adjustments necessary. There are no valves to drop or get hit if a timing belt snaps and the effective rev limit is only what the main piston will stand. The design has similarities to the Atkins and Miller designs in that the expansion stroke is larger than the intake stroke. Per single cylinder the number of parts in the Beare design head is fifteen compared to a single overhead cam 4 stroke of approx. 40 to 50 parts.
The design also allows the production of a single piece engine (i. e. head cast with the block) further reducing machining and therefore costs. The tips of the reed valves are positioned close to the intake port windows, thus achieving a similar result to variable cam timing. At low throttle ; revs the petals only partly open and keep gas velocity high . At full throttle ; high revs they fully open to allow maximum flow. The exhaust disk does not touch anything and is only subject to sub-atmospheric pressure, not gas flow; and therefore its service life is practically infinite. The exhaust valve is a piston port.
The simplest layout for car engines is the flat 4 or V4, with internal central chain drive to the heads. This layout allows access to 3 sides of each cylinder, with exhaust discs each end of the motor and reed valve blocks both sides of each cylinder. For in-line layouts the drive chain or belt is at the end with a row of exhaust disks down one side and a row of intake disks or reed blocks down the other side. A right angle drive is taken off the drive chain with a very light internal drive chain to the disks, or a direct drive is taken off the drive chain with light right angle drive at each disk.
Advantages (1) The Six stroke engine is fundamentally superior to the 4- stroke because the head is no longer parasitic but is a net contributor to, and an integral part of the power generation within the engine. (2) The Six stroke is thermodynamically more efficient because the change in volume of the power stroke is greater than the intake, compression, ; exhaust strokes. (3) The compression ratio can be increased because of the absence of hot spots. (4) The rate of change in volume during the critical combustion period is less than in a 4 stroke. 5) The absence of valves within the combustion chamber allows design freedom. (6) A one-piece engine from crankshaft to upper shaft becomes feasible. No head gasket. (7) The valving is desmodromic (8) There are no valves to drop or bounce. (9) The rev limit is only what the bottom end can stand. (10) Gas flow on intake increase of 20%. (11) No possibility of engine damage if the timing belt slips or snaps (12) The reed valves are so close to the intake ports that their tips become the virtual port opening. This achieves variable port area ; variable engine demand valve timing.
The tips open late ; small amounts with low throttle settings ; open early ; fully at full throttle (13) Increased torque and power output (14) Better fuel economy (15) Cleaner burning and reduced emissions (16) Longer service intervals (17) Reduced tooling cost (18) Low cost of manufacturing (19) Low machining cost due to absence of valves (20) Higher compression ratio (21) Small size (22) Less number of parts compared to four stroke engine (23) High torque at low rpm (24) 35% more economic at low throttle and 13% more economic at high throttle when compared to OHC four stroke engine Conclusion
From the above given data it can be easily understood the Beare technology or six stroke engines are the technology for the future. The project is well patented and is undergoing heavy research works. Any product takes time to establish itself in the market. Six stroke engines with all the desired qualities of a two stroke and four stroke engines will be hitting the market soon. From the above given data it is clear that six stroke engines are better compared to two stroke and four stroke engines. It is sure that six stroke engines will surely be the main stay of automobiles in the near future.