Hello all.
Lubrication in the 2-strokes:
Quote from the US6,712,519 patent of
Bombardier-Rotax GmbH:
“In a conventional two-stroke engine, as used in motorcycles, snowmobiles, personal watercraft, etc., the crankshaft is usually supported by roller bearings with separate seals mounted outboard of the bearings to seal in both the lubrication and the intake charge present in a conventional two-stroke crankcase. The roller bearings can be lubricated by oil mixed in with the intake gas charge that enters the crankcase and/or by oil injection directly to the bearings. However, any oil entering the crankcase of a conventional two-stroke will enter the intake charge and be forced into the combustion chamber through the transfer ports, as is known.
Thus, this oil will be burned in the combustion process, thereby increasing exhaust emissions. As a result, decreasing exhaust emissions in a two-stroke engine necessarily requires efforts with respect to both the combustion process and the crankcase lubricating oil.
Certain approaches to limiting the amount of lubricating oil entering the crankcase in a two-stroke engine
have proven somewhat successful in reducing exhaust emissions. In one approach, the amount of lubricating oil supplied to the intake charge is merely reduced, thereby reducing the amount of oil which does not come into contact with any lubrication requiring surface but which is merely burned during the combustion process. The problem with this approach is that as the amount of lubricating oil is reduced, the probability of inadequate bearing lubrication increases, especially in engines having high power output, with predictable undesirable results.
Other efforts generally
concentrate on reducing the amount of oil supplied to the intake charge, but separately supplying smaller additional quantities of oil directly where it is needed to compensate for the leaner concentration of oil in the intake charge. For instance, the amount of oil required by the crankshaft bearings can be directly injected to those bearings while simultaneously reducing the amount of lubricating oil supplied to the intake gas charge. Thus, the overall amount of lubricating oil used is reduced, along with the exhaust emissions from burning the oil. On the other hand, the lower limit of the amount of lubricating oil supplied to the crankcase (and thus, the combustion chamber) will depend on the lubrication requirements of the engine components, such as the crankshaft and rod bearings, pistons, etc.
In any event, the oil supplied to the crankcase will either be burned during the combustion process or exit in the exhaust as unburned hydrocarbons. In recent years, as technical advances have resulted in increased power outputs from two-stroke engines,
existing crankshaft bearings have been less able to withstand this increased loading over time, especially as lubricating oil has been reduced to lower exhaust emissions.
One proposal to limit the free lubricant supplied to the intake charge is given in
U.S. Pat. No. 3,641,990 to Kinnersly, issued Feb. 15, 1972. Kinnersly discloses the use of a one-sided crankshaft supported by a pair of crankshaft roller bearings enclosed on either end by seals, with an annular chamber disposed between the two roller bearings filled with lubricant at the time of engine assembly. Kinnersly discloses that such an arrangement can be used with either a two-stroke or a four-stroke engine and that the roller bearings and seals can be separate or integral units. There is no disclosure as to what type of seal should be used or how such a seal would be integrated into the roller bearing. In a four-stroke engine, this is less significant because the crankcase pressures are generally not high enough to interfere with the seals. It is also clear that with the use of the one-sided crankshaft, that the Kinnersly engine is not intended for high performance and the severe loadings resulting therefrom. ”
END OF QUOTE
In the Rotax E-TEC 850 the crankshaft bearings are sealed according (?) the above patent (priority: 28 July 2000; i.e. it has three more years before expiration).
for the sake of a lower lubricant consumption and, consequently, for the sake of a cleaner exhaust and of a lower running cost.
However in practice, and despite the increased cost and friction (count the number of the “tight” seals required), this “innovating lubrication system” proved (judging from the complaints of the owners) not better than the conventional 2-stroke lubrication used in their previous model Rotax E-TEC 800.
The superior power density (kW/Kg or kW/cc) is the biggest advantage of the 2-strokes.
The emissions and the running cost are the big issues / disadvantages.
On the emissions and on the running cost of the 2-strokes is where the efforts should be focused.
Thanks
Manolis Pattakos
