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Questions about stroked A084 surplus engine

  1. Jul 25, 2011 #1
    The Govt. Surplus GenSet engine A084 is a four cylinder, four stroke, 1390cc/84cu.in. aircooled flat four with a 3 in. bore and a 3 in. stroke. It was made by the thousands by Hercules and resembles a Lycoming shrunk in the dryer. It has been converted to experimental aircraft use and faily well optimized, making 45 hp @ 3200 rpm with a 54"/27" prop. It is said to be heat limited in the head. There have been some sucessful "fat fin" VW head fixes using TIG welded additions. I would like to avoid as mouch of this as I can.
    I would like to stroke this engine to 88mm. This yields a 1600cc engine. The piston speed @ 3200 rpm goes from 9.14 m/s to 10.56 m/s. To maintain the lower conservative speed the operating rpm must drop to 2534 rpm. This will allow for a larger prop disc to absorb some additional power.


    1.) Is the increase in head temperature going to be proportional to the increase in displacement, or does "stroke" heat less, given the lower rpm?

    2.) The valves are small. Does the increase in stroke allow for the use of the same valves.

    3.) If no to the above, is more lift enough to fix the situation?
  2. jcsd
  3. Jul 26, 2011 #2

    Ranger Mike

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    Personally I would NOT touch the engine..the manufacturer spent millions of dollars perfecting the engine. One critical component is longevity...it was built to live at the spec RPM and when you start to mess with design things like bore and stroke..you can not be sure about how long the thing will live..not too critical in a race car..worst case is a DNF...but in an air craft...no power= a messed up day...like death maybe..
    don't monkey with it!
  4. Jul 26, 2011 #3
    Thanks for your concern Ranger Mike. I couldn't agree more. This is a rescearch project that will be proved on an airboat. Worstcase I have to row to the shore. There is no room in the OPEM case for any tinkering. My project uses a new crank concept and cast crankcase. I'm using the A084 parts because a head with valves is $30 and cast iron/aluminum cylinders are $14. It's an engineering tinkertoy set.
    My questions are purely academic. Heat mangement is the nut to crack in small high performance small aircraft engines. Many have gone to oil or water but the weight overhead is too high for a 50 hp engine. If the prop limits rpm to under 3000 rpm, how can I optimize the things that I can change. What is optimal the Bore/Stroke ratio? What is the best cam profile?
    I'm sure people had the same concerns when the first VW took to the air and more so when someone cut one in half. Please share your knowledge and rest assured I will stay on the ground with my project.
  5. Jul 26, 2011 #4

    jack action

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    Because you keep the same bore and the same mean piston speed, you will end up with the same volumetric air flow going into the engine. So:

    Same air flow, same quantity of fuel burned, same heat release (and same power).

    Same air flow displaced by the piston, same airflow going through the valves.

    Same piston speed, same valve lift features. Although you will have the same mean piston speed, you might not have the same piston speed profile throughout the piston motion if you do not change the connecting rod length to keep the rod length-to-stroke ratio the same. Some minor adjustment may be required in valve events to be top notch (assuming you were originally).

    More info http://hpwizard.com/volumetric-flow-rate.html" [Broken].
    Last edited by a moderator: May 5, 2017
  6. Jul 27, 2011 #5
    Thanks Jack,
    I'm confused about the piston area and the total displacement. The difference in displacement between the stroked and unstroked engines is 210cc or about 15%. I understand that the velocity through the valves will be the same, but won't there be 15% more air/fuel mixture to burn?
  7. Jul 27, 2011 #6

    jack action

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    There will be 15% more mixture each stroke, but you will have 15% less stroke per unit time due to the decrease in rpm (to keep the same mean piston speed). So the same amount of mixture will be burned each minute and the same amount of heat flow should also go through your engine components. Maybe the combustion will be larger, but the «cooling» period afterward will also be longer.

    There might be some difference in combustion chamber shape (influenced by the bore-to-stroke ratio) or piston motion (influenced by the rod length-to-stroke ratio) that will affect your combustion, but it should be minimal unless you go to extreme values.
  8. Jul 27, 2011 #7
    Last edited by a moderator: Apr 26, 2017
  9. Jul 27, 2011 #8
    Thanks Pantaz, I am a member of that group. My engine uses parts from this engine in completely new ways.

    Jack, thanks, this is a little epiphany. I have been a student of the history of aircraft engines and have never understood the evolution from long stroke slow turning engines to the oversquare flat fours today. This is a new way to measure power and compare performance. If I adjust for piston speed I can compare older engines more effectively.

    For my current project, my goal was to get to an honest 50hp by adding 15% displacement to a honest 45 hp engine. Apparently I will need to increase the piston speed to accomplish my goal.

    1.) Is there a way to calculate the addition m/s required to gain 5hp, if we assump that the original 9.14m/s yielded 45 hp?

    2.) Given the above change, is lift enough to accomodate the increased flow?

    3.) And finally, I assume there is a comenserate increase in cyclinder head temp. that must be countered with addition fin area.
  10. Jul 28, 2011 #9

    jack action

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    Quick answer: you need an engine 15% faster. But you can use http://hpwizard.com/engine-horsepower-calculator.html" [Broken]. First, put the original dimensions and power and an educated guess for the volumetric efficiency to find the BMEP of your original engine (or maybe you already its fuel consumption at max power, which is great since the only unknown will be the volumetric efficiency). Then, the volumetric efficiency and BMEP shouldn't vary much, so you will know how you must modify your dimensions or mean piston speed (or even boost pressure) to reach your new goal.

    By changing the piston speed, optimal valve lift and events will most likely be different.

    There will probably be an increase. As a starting point, I would assume that the heat flow lost through the cooling system is proportional to the power produced, so 15% more heat should go through your cylinder head.
    Last edited by a moderator: May 5, 2017
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