Name	NE15S	Designer	Nemett
Bore	25mm	Stroke	30mm
Type	SOHC four-stroke, glowplug ignition	Capacity	14.7cc
Production run	Home Construction	Country of Origin	UK
Photo by	Nemett	Year of publication	2006

 

Background

As most readers should know, the venerable English magazine, [the] Model Engineer, has been in continuous publication since January, 1898. Over that period, the subject of internal combustion has made sporadic appearance, reflecting the interest of the readership, and the availability of contributors. However, when IC subjects have appeared, they have mostly been quite notable: the many designs and wisdom of ET Westbury, Lew Blackmore's brilliant BR2 Rotary, Eric Whittle's Aero V8, etc, etc. Recently, a new name has appeared in the pages of the ME to address the growing interest in IC projects (see Model Engine News for September, 2005). Following the fine ME tradition, the latest IC proponent writes under the nom de plume "Nemett".

The first construction series designed by Nemett commenced in issue 4273. As the ME publishes fortnightly, their common practice is to serialize over alternate issues, so the NE15S appears in odd numbered issues.

Description

The photos and description that follows were provided by Nemett himself:

The NE15S is a single cylinder, single overhead cam shaft four stroke with air or water cooling options. The main parameters of the engine are:

Bore	25mm
Stroke	30mm
Cubic Capacity	14.7cc
Compression Ratio	9:1
Length (including carb)	200mm
Height to top of cam box	165mm
Width (crank case)	63mm
Width across bearers	88mm
Weight	1.4Kg
Cam shaft drive	Single 2.5mm pitch 6mm wide toothed belt.
Carburettor	Barrel type with adjustable fuel needle and slow running air bleed. Choke diameter 7mm.

The engine is designed to be machined from the solid and has a toothed belt cam shaft drive at the front of the engine, allowing a simple overhung crank. The crankshaft runs in sealed ball races with the big end and camshaft handled with needle rollers. Cylinder liner is cast iron with an aluminium alloy piston with two plain cast iron rings. Valves are stainless steel seating directly in the alloy head. Valve diameter 8mm with 2mm lift. Inlet opens 20deg before TDC and closes 60deg after BDC Exhaust opens 50deg before BDC and closes 30deg after TDC. Both crankshaft and camshaft are of built up construction. The engine may be run with glow or electronic spark ignition.

Performance so far (measured using a 14x7in airscrew):

• 5,750 rpm on glow fuel
• 7,400 rpm on spark.

 

The 'exploded' view shows a few more features not mentioned in Nemett's brief overview. No castings are required; the engine is machined from bar-stock. There are no rocker arms; the cams act directly on hollow, cylindrical tappets that sit on top of the valve stems and provide tangetial contact with the cams. The crankcase vent appears to be connected to the OHV cavity by a small bore copper pipe. Drain plugs are fitted to the backplate and OHV cover suggesting this is a form of lubrication. I thought cases were vented to get rid of corrosive nitros oxide gas produced by blow-bye, but I'm sure our designer has a theory and an answer.

Conclusion

The large size of the engine and simple, straight forward construction will be appreciated by model engineers trying IC as a diversion from the usual ME fare of live steam. The NE15S would be a bit on the large size to screw into the nose of a flying model, but I've little doubt someone will do just that. Hopefully the experience of building and running the engine will decide many to follow up with other types, or even to produce designs of their own. Nemett and the Model Engineer are to be commended for this effort.