The NOVA-1 4.5cc Side Port Spark Ignition
Created: March 30, 2012
The Nova 1 was originally designed a long stroke, plain bearing, side-port diesel (compression ignition) engine. It's a quaint old thing which even today retains a strong appeal to model engine makers, world-wide, as evidenced by the many who have built one—a surprising number going to the effort of making their own pattern to sand-cast the crankcase. Chris Dunn (Australia) recently joined these ranks, but Chris decided to be a bit different, adapting the basic design to spark ignition.
Chris retained the original swept-volume (aka displacement, but not capacity) of 4.5cc. The head was made thicker to accommodate the spark plug thread, reclaiming some height as the engine no longer required space for the contra-piston (four fins, as compaired to seven on the diesel). For the technical, Chris aimed at a geometric compression ratio of 8:1. He also elected to make his cooling fins a little thicker than the ultra-thin 0.020" (0.5mm, actually) specified on the original plans—not a bad move as fins that thin will warp away from the tool during cutting unless you use a backed-off cutting tool and flood the job with coolant.
The crankcase pattern was modified slightly to make casting easier, and a set of points which use a flat on the rear of the prop driver added. The tapered crankshaft section of the original was retained, making it easy the set the timer arm to whatever position proves most convenient for operation. All the other parts, including the nicely made and beautifully soldered brass fuel tank close to the original plans.
Here's the proof of the pudding: the Nova-1 SI smoking out the workshop (ambrosia to model engineers, something less sweet to others). Chris reports that his engine was initially hard to start, but he got there, turning an APC 10x6 at 3,600 rpm on the first run. By the second run, it had loosened up to give 4,000 rpm, although was still hard to start (Chris was hand-starting the engine in the best traditions etc). After about 8 minutes accumulated running time, he found it would start after a couple of chokes and a few flicks with the spark retarded 25-30°. The fuel used was un-leaded and oil in a 9:1 ration.