Object StatementAero engine, 'Harkness Hornet' No.101, prototype, 4 cylinder, designed by Donald (Don) Harkness, built by Harkness & Hillier Ltd, Five Dock, New South Wales, Australia, 1929
Physical DescriptionAero engine, 'Harkness Hornet' No.101, prototype, 4 cylinder, designed by Donald (Don) Harkness, built by Harkness & Hillier Ltd, Five Dock, New South Wales, Australia, 1929
The "Harkness Hornet" is a 4-cylinder, in-line, single overhead cam, water-cooled aero engine designed for use in small aircraft. The four cylinders are bolted to the upper portion of the crankcase suggesting that they remain separate and not incorporated in a cylinder block. The upper portion of the cylinders and cylinder head are enclosed in a sheet metal water jacket. At the front and rear, lower, of the water jacket are two blanked off openings which may have been to permit drainage and cleaning of the jacket. The upper half of the crankcase is provide with brackets for attachment to the aircraft structure. The lower half of the crankcase is of sheet metal material and serves as a lubricating oil container. Two magnetos, in red, provided the electrical input to the spark plugs. Each magneto would probably be connected to each of the four spark plugs on their side of the engine. This duplication increased the reliability of ignition, and provided improved scope to ensure that the fuel combustion process was as complete as possible. The upstanding green tube houses the drive shaft to the camshaft. The drive shaft engaged with the crankshaft through bevel gears and may have run at half engine speed (if the engine operated as a four-stroke). At the top another pair of bevel gears driove the camshaft, running the length of the engine, which actuated the individual inlet and exhaust valves through rocker arms. The silver-coloured assembly contains the air supply, fuel dispensing means, inlet manifold and cooling water inlet pipe. There are four exhaust pipes. Engine operation would have been noisy without any connection to an exhaust muffler system. The pilot would also have been subjected to the smell of exhaust gases. Fresh air was drawn into the open-ended intake tube, supplied with fuel and thence through the adjustable throttle valve (to control engine speed), before passing to the cylinders via the inlet manifold.
Bore: 120 mm
Stroke: 130 mm
Normal B.H.P.: 104 at 1800 R.P.M.
Maximum B.H.P.: 115 at 2000 R.P.M.
Compression ratio: 5 to 1
Oil pressure: 60 p.s.i.
Weight: 300 lbs
There are some terms on the engine's plaque which may need definition, as follows.
"Right Hand Tractor": The propeller acts to pull the aircraft and the propeller rotates in a clockwise direction when viewed from the pilot seat.
"Firing order": The sequence of ignition initiation in each cylinder. No 1 is the front cylinder.
"Magneto advance": The spark initiated by the magneto occurs before top-dead-centre to assist the complete combustion of the fuel. (TDC occurs when the piston is at the top of its stroke, near the cylinder head)
"Valves, inlet and exhaust": In a four-stroke engine, on the inlet stroke the inlet valve opens 5 degrees (rotation of the crankshaft) before the piston reaches bottom-dead-centre (BDC) and closes 35 degrees after TDC. This is to achieve optimum air/fuel intake to the cylinder. On the exhaust stroke, the exhaust valve similarly opens 65 degrees before BDC and closes 5 degrees after TDC to achieve the maximum discharge of exhaust gases.
"Valve clearance": The clearance between tappet and valve stem, when the valves are closed is 2½ mm
Description provided by Noel Svensson.
MarksManufacturers plate on lower right hand side of engine, printed text stamped on plate, 'HARK (HORNET) NESS / AERO No 01 ENGINE / DIRECTION Right Hand Tractor / BHP Normal at 104 at 1800 R.P.M / BHP ....../ HARKNESS HILLIER LIMITED / SYDNEY / AUSTRALIA'.