Autolube

Description

A description of the autolube system was provided in the workshop manual when it was introduced (available at Scooterhelp). The description was hard to follow as it was in badly translated English. Here we describe the system based on that information with extra information from Vespa Lab research.

The automatic lubrication device for oil/petrol mixture (autolube) has been designed by Piaggio to ensure the perfect lubrication of the two stroke engine at all speeds and for any use condition. Prior to the autolube (or for engines that have subsequently disabled autolube or are using a different carburetor), a 2% mixture in the petrol is used, this works well for full power operation but caused issues with poorer fuel economy and increased engine carbonisation.

The 2% oil ratio was used in order to ensure adequate lubrication during the most hard use conditions i.e. at high rpm and at maximum loading. However tests have shown that a lower quantity of oil is sufficient and more convenient when using the vehicle at lower rpm and with the limited throttle opening. In other words different engine operating conditions need different quantities of oil.

The autolube adjusts oil mixture in line with the variable conditions of use the vehicle is subjected to. It does so by dosing the oil quantity in relation to the rpm and throttle position. This reduces oil contamination of the spark plug, cylinder, piston and exhaust system and also improves economy.

How it Works

A Vespa is equipped with two different tanks, one for fuel (1) and one for 2 stroke oil (2). In order to ensure oil in the tank is sufficiently full a sight glass level is provided (3). This is glass is normally full of oil and becomes empty when oil is at the reserve level. When the glass is approximately half full (depending on the position of the scooter) the oil tank will accept a full 1 litres.

The petrol (gasoline) is conveyed to the carburetor by means of the normal gravity system. The oil (5) is sucked by a oil pump piston (P) whose stroke is governed by the radial position of a sliding pin on the inclined cog. The pin position itself is controlled by the throttle position by means of a lever attached to the throttle cable.

Each position of the throttle twist grip corresponds therefore to a specific position of the sliding pin and therefore the piston stroke. This means that the quantity of the oil sucked changes according to the throttle opening (pump stroke) and engine revolutions (rotations of the pump).

Oil Pump Detail

The pump is a 35mm wide cog with a sloping top. Looking at it from the side and rotating it to see maximum slope, the high side is ~6.8mm and the low side is ~4.4mm. This cog sits in an oil sump, which has an input oil feed and an output oil path.

There is a pin which changes position with throttle, travelling along the radius of the cog (from between 6mm to 14mm from the centre – judging by the marks made on a used one). The cog is sprung to push upwards against the pin so that, as it rotates, the “up down” travel changes with the slope. When the pin is at 14mm from the centre (at full throttle) the travel is ~2mm and when it is 6mm from the centre (idle) its travel is 0.8mm. Based on a full throttle "pump" is 37 microlitres of oil and a no-throttle pump is 16 microlitres. 



The oil pump, “pumps” once every 83 revs of the crank (i.e. the cog rotates once)

Let’s look at an engine that idles at 1,000 and red lines at 7,000. Note that without actual measurement of fuel flow and actual pump flow (as opposed to calculated flow), this is all theory.

Regardless of throttle position changing pump travel – the oil delivered at full revs is 7 times that delivered at idle (assuming constant pump efficiency). If fuel delivery was directly proportional to revs, then this should indicate a fairly constant ratio of fuel to oil (i.e. more fuel = more oil). Also the carb is meant to maintain a correct and contsant air/fuel ratio over all rpm ranges, so for each spark the same amount of fuel is consumed.

If we now take into account the pump stoke ratio (i.e. strokes between 2mm to 0.8mm) – at full throttle the pump is pushing ~2.5 times more oil through than at idle. So if we assume that 7,000 revs is a 2% oil mix ratio, then idle is 0.8%! 

Here is a excel spreadsheet used for some calculations: oil_pump_calculations.xls

More Information?

To completely dispell rumours of a constant 2% across all ranges, it would be good to:

• measure actual oil throughput with the pin at idle and full throttle (on a lathe maybe)
• measure actual fuel flow under different throttle positions/revs and conditions using a Fuel Flow Meter
• with oil/fuel known for different rpm's, we can calculate ratio for the whole range
• Is the pump cog case hardened? Can the slope be adjusted to pump more oil for tuned engines?

Additional Reading

On  Dan's Motorcycle Two Stroke "AutoLube" Oil Pumps page, he states for motor bikes “At an idle the pump mixes the oil with the gas from a 120 to 1 ratio on up to 20 to 1 ratio at 8000 or more RPM.”, which is roughly difference of 6 times.

In the Vespa Labs articles section there is a good article on Premix Ratios Two-Stroke_Oil_Premix_Ratios.pdf

More Stuff From the Workshop Manual (that needs further cleaning up)

Important: When dismantling, overhauling or reassembling the autolube device the piping can remain without oil. In order to permit the autolube device a progressive and sure filling of the above piping, after said operations it is advisable to fill up the fuel tank with a of mixture of gasoline (petrol) and mineral Oil SAE 40. The mixture should be at 2%. After this refueling the following ones should be carried out with (petro/) gasoline only.

The pump unit, composed by a piston with concerning sliding housing, is put into rotation by a gear transmission (crankshaft. mixer shaft ratio: 1/85J. The piston «P» has on the rod, a proper smoothing that for the effect of the mentioned rotation alternately realises the opening and closing of the oil inlet pipe (No.5) and of oil supply pipe (No.6) the latter, provided with valve and ball set spring. In such a way the pump realises alternatively the suction period (A in fig.) and delivery period to the Venturi (B in figure). where the oil itself is mixed with the gasoline (petrol) sucked by engine.

Notice: Before starting the engine take care that the pipe which conveys from tank the oil is completely full; when air bubbles are present, take off the tube from engine. Flow down some oil so that the above bubbles are eliminated and reconnect the pipe to the engine.