By Maris Dislers
As a collector of 2.5cc diesels, this engine caught my eye on Ebay recently. It looked about the right size and, being a non-runner, came my way for not much cost. The Polish seller could offer no information regarding its provenance, having bought it from someone who hadn’t a clue what it was. Another interesting “wotzit” begging for identification.
This engine is clearly an amateur home-build. The workmanship is generally very good, especially the complex cylinder and rear rotor housing, machined from magnesium alloy. The very over-square design features a 15.9 mm (.626 in) bore and an 11.36 mm (.447 in) stroke for a swept volume of 2.25cc (.137 cu in). Weight is a reasonable 155 gm (5.5 oz). The shrunk-in steel cylinder liner has two side-facing exhausts timed at 160 degrees and a single inclined bypass port up front. The engine uses a steel piston, an aluminium contra-piston and an aluminium conrod with bronze bushes.
The aluminium crankcase is machined from the solid. The main bearing housing has front and back brass inserts, with a sizeable gap between them, to serve as the plain bearings for the long crankshaft journal. The crankshaft has a rotary intake port and gas passage which have no connection with the outside world. Could be a means of lubricating the bearing, but there’s a matching hole in the rear bearing piece, as if front rotary induction might have been contemplated. There’s even a centre pop mark on the crankcase where the intake venturi might go.
However, the constructor had made the crankpin long enough to engage with the rear rotary induction disc. Go figure. The steel disc has an integral pin riding in the bronze bushed housing and relying on crankcase pressure for sealing. Intake timing is a mild 130 degrees, (opening 50 degrees ABDC and closing at TDC) set for clockwise running, which was usual in Poland during the 1950’s.
The piston fit was way too tight and the pressed-in venturi was protruding proud of the disc sealing face. Clearly, this engine had never run, but deserved its chance despite having a perilously skimpy 4 mm (5/32 in.) diameter crankpin, drilled through its centre, and tiny M2.2x.45 screws holding the cylinder. An afternoon spent lapping the piston and facing the disc sealing surface had it ready for a trial run.
Starting was reasonably easy and sustained running at around 7,000 RPM was achieved with a regular 9x5 propeller. I’d gone too loose with the piston fit for really good power output, but the engine ran quite happily without the usual cooling air blast from the propeller and with commendably low vibration. This experiment vindicated the design in my opinion, although that’s the end of its career. Quit while it’s still in one piece!!
