SOME FACTORS CONTRIBUTING TOWARDS THE SUCCESSFUL LUBRICATION OF SMALL EUROPEAN TWO-STROKE PETROL ENGINES 590375
The ever increasing number of two, three, and four-wheeled vehicles on the roads of Europe, propelled by small two-stroke engines, each consuming per mile many times the lubricating oil used by the average American car has focused attention on the necessity for investigating the fundamentals governing the lubrication of these units.
This Paper deals with such an investigation, and shows that the average mileage which many of these vehicles can cover before requiring engine overhaul can be substantially increased by proper attention to the selection of base oil and additive. Design and operating factors governing the performance of these engines are also considered.
It is shown that although many engines will run on oils ranging in viscosity grade from SAE 10W to SAE 50, provided that the fuel to oil ratio is decreased with decreasing oil viscosity, the most satisfactory oil depends largely upon the design of the particular engine and upon the operating conditions.
The most common cause for complaint in most countries is that of port blocking, and the rate at which this occurs is greatest when using oils of high viscosity. Such oils, however, provide better protection against piston seizure under adverse conditions. The most irritating complaint, particularly in Great Britain, is undoubtedly connected with the sparking plug. This trouble usually takes the form of a complete engine cut caused by the so-called whiskering, but can virtually be eliminated by the use of special additives. Complaints of this type can be minimised by careful attention to both carburation and ignition systems.
Complaints from some countries of rapid corrosive wear through rusting of the ball and roller main bearings are bound up with the use of the vehicles for exceptionally short journeys at low temperatures, so that the working temperature of the crankcase is low. These troubles are made worse by the designer failing to ensure that the bearing lies in the path of the oncoming charge of fuel and oil, and often also by incomplete mixing of the fuel and oil before it enters the engine. The remedy is to maintain the crankcase at as high a temperature as possible both during standing and running, to make sure that the fuel and oil are properly mixed, and to use in the oil one of the special additives developed to counteract this problem.
The oil chosen for two-stroke lubrication must largely be a compromise between conflicting requirements, but in the author's view, suitably treated medium V.I. SAE 30 oils containing 10% to 20% of bright stock have the best all around properties.