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Page last modified 14:24, 26 Jun 2009 by internetscooter
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Vespa Labs > Vespa 101 > Engine > Oil Pump

Oil Pump

From $1

    ./300px-Vespa px oil pump cog.jpg ./200px-Vespa px oil pump base.jpg ./200px-Vespa px oil pump variable pin.jpg
     

    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. 

    ./300px-Vespa px oil pump cog dimensions.jpg ./300px-Vespa px oil pump cog notes.jpg

    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

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    #1
    internetscooter says:
    I noticed on Stellaspeed someone pointed to this page as said the calculations were wrong (rathering than commenting here). I have rechecked the calcualtions and they were wrong. I have updated the spreadsheet and info.

    Previously I calculated 0.4mm travel of the pump at idle but it should have been 0.8mm. This changes the varition to 2%-0.8% from what was previously stated as 2%-0.4%.
    Posted 09:58, 26 Jun 2009
    #2
    Dedalo says:
    On the use of the oil pump with the tuned engines, some people say (probably heretically! and I'm sure the fastidious here would object) that you can run the oil pump by basically disengaging the metering arm so that it swings wide (i.e. always pumps at 2 - 2.2%). The superstitious would always want to add a few more drops to the petrol tank (which some might think sort of defeats the purpose of having an oil reservoir + pump!), but I know there are Vespa tuners out there who do this. I believe this is the method that is advocated for the Vortex carb which is regarded highly in certain circles.
    Posted 07:54, 19 Mar 2010
    #3
    internetscooter says:
    Check out the Two-Stroke_Oil_Premix_Ratios.pdf in the http://www.vespalabs.org/Vespa_101/Reference_Material/Articles section. The oil ratio isn't that precise, the engine will handle a wide range. My feeling is that you'll foul the plug with too much oil around town. On the open road, a few more drops may help, too many drops will change the viscosity and the engine will run lean! edited 08:54, 19 Mar 2010
    Posted 08:50, 19 Mar 2010
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