VW-derived aero engine | Selecting an Engine Case
The standard VW Type-1 engine case is a magnesium casting.
VW Type-1 engine case
During my visit to the 2007 RSA Fly-in (Vichy, Fance) I talked to Stéphane Malandain, who since its first flight in 1995 had put over 680 hours on his IBIS (F-PBSM), which incidentally is still equipped with it's original 1870cc VW engine.
Stéphane told me that he had done a minor overhaul at around 635 hours. He also mentioned that his magnesium engine case didn't need any attention. Although I made a note to myself about this fact, at the time it didn't change my mind about wanting an aluminium case.
Re-reading some of my communications with Steve Bennet and with the friendly and patient help of Brian Smith of ACRO Engines and Airframes Ltd., I since learned that magnesium cases that develop cracks (most notably behind cylinder #3) do so for a number of reasons, each of which can be handled if you know them:
- Overload by going for too high a compression ratio and/or using a mixture that is too far on the lean side. Having a compression ratio of well below 1 : 8.0 (and making sure the engine is not leaned too much) will solve this problem. Personally I plan on a maximum compression ratio of around 1:7.5
- un-balanced thermo-dynamic loads by having an uneven fuel metering across the individual cylinders. This will cause some cylinders to run on a leaner than average mixture whilst other cylinders might have a richer than average mixture. Going for fuel injection would solve this problem once and for all, but luckily various methods have been found to even out fuel metering on a carburettor equipped VW aero engine. For this you might want to check out an article written by Chad Stenson (this information even made it to Steve Bennet's site). Obviously, more can be done in this area, as was documented by this Sonerai forum entry by 'sonerai92a'.
- Another cause of thermo-dynamic unbalance is having engine baffling that doesn't distribute even amounts of cooling air to each and every cylinder. During initial flight evaluation, cooling air pressure differentials can be measured and if any discrepancy is found, problems can be ironed out.
- Mechanical overloading caused by a poor assembly. Using the wrong moments and not using the exact same moments where it is called for would cause such a thing.
- Other causes for mechanical overloading are vibrations that are caused by having an unbalanced engine: a non-balanced crankshaft, unbalanced connecting rods or weight differences between connecting rods and/or pistons, cylinder heads with uneven volume above the pistons - all of these factors may cause an engine to run a bit rougher than it needs to. Since each of these factors constitute a 'know enemy', it's one we can defeat.
It's all about symmetry, really. It's about ensuring that equal loads are put on equal parts, fore and aft, left and right.
Equal thermodynamic loads are a result of precise fuel/air metering per cylinder and by equal cooling per cylinder.
Symmetric mechanical loads are result of all of the above, PLUS using equal/accurate torqueing during assembly and by making sure that identical parts have identical weights and/or are balanced.
Of course it is very helpful if your propeller is not adding to any residual vibration mode, so I suggest you keep a keen eye on that part as well... :)
Now, whilst we're battling a known enemy here, it might also be of interest to note that most new magnesium cases are shipped with a reinforcement patch welded in behind piston #3 anyway, which addresses most concerns as well...
All of which leads up to my decision to not use an aluminium case after all, but use the lighter magnesium engine case instead.