VW Aircraft Engine Displacement
I intend to configure my VW aero engine displacement as large as is practical.
playing this game for many, many decades to be able to offer a TBO (time between overhaul) of some 1300-2400 hrs. (depending on compression ratio, turbo or non-turbo, etc.). Apparently, nothing beats displacement, at least so for direct drive engines...
VW engines come in a variety of bores and strokes, each resulting in a specific engine displacement. However, the smaller bores and strokes don't provide all that much power during take-off, whereas the extremes at the upper end produce more power than would be good for the engine case and also produce more heat than the heads could possibly dissipate.
Mechanical overload and a much more difficult cooling situation (especially so on a pusher aircraft like my IBIS) steers me clear from using the largest possible bores and strokes.
In the context of VW aero engines, 82mm or 84 mm stroke and 90.5mm, 92mm or 94mm bore have become rather common.
The following table shows the various displacements that these variables create:
|Engine Displacement (four cylinder)||82mm stroke||84mm stroke|
|90.5mm bore||2109 cc||2160 cc|
|92mm bore||2180 cc||2233 cc|
|94mm bore||2276 cc||2331 cc|
As you can see, the displacement range to play with is 2109 - 2331 cc.
Personally I wouldn't want to use the 94mm bore, as I feel that the proximity between the bore and the cylinder head bolt holes leaves precious little metal. Also, there simply remains too little landing area between the cylinder and the deck for a long lasting seal. For this reason, I'm likely going to end up with a 90.5mm bore. Why did I leave out 92mm? For one reason or another, the 92mm piston wall thickness is markedly thinner than either the 90.5mm or the 94mm. So if you have a good reason to skip 90.5mm, it might be a good idea to leave out the 92mm option from your deliberations as well.
As to stroke: 84 mm or even 86 mm would be possible (and somewhat desirable as well, as the longer stroke would increase torque in the lower rpm range), but at a cost, as this would mean that I'd have to use non-standard connecting rods and journals (borrowing from the Chevvy parts bin). Also, I'd need to remove more material from the engine case for parts clearance.
Much more practical and economical is the largest VW Type-1 stroker connecting rod that doesn't need bigger journals. This happens to be the 82mm job.
So there you have it: when push comes to shove, I'm likely to end up with a 90.5mm bore and 82mm stroke. This would provide my engine with a displacement of 2109cc.
It might seem that I'm giving up a lot of power this way. However, after having flown an IBIS with a 1835cc engine (F-PGLP) I know first hand that an IBIS doesn't need all that much power. At maximum take-off weight on a hot (appr. 32°C/90°F) & humid day at a field elevation of 1100 feet and with a slight cross-wind, we were off and away using less than a quarter of the 1460 meter long runway.
I gladly trade some power for engine longevity! Also, what I might give up on engine performance can partly be made up by a propeller design tailored exactly to my project.
Recently I learned that 94mm slip-in cylinders exist, that fit into a case that was machined for 90.5mm cylinders. Those cylinders are machined down a bit on the lower side, thereby even increasing the landing area between cylinder and case. Now this is interesting! This option would allow both a larger displacement AND an enlarged cylinder/deck mating surface. So far I don't know anyone who has experience with these, so if you are knowledgeable about these, please fill me in, as I'm still researching this topic.
Also check out: VW-derived Aero Engine >> starting