The "Kiwi" 15cc Four-stroke Petrol Engine.
By Philip Coleman - July 2002.
The first version of this well known little engine appears in January 1935 when Edgar T Westbury described it in a series of articles in the English magazine Model Engineer & Practical Electrician. This was the first major series written by Westbury for that magazine and was the start of an association that lasted over 30 years. At that time the engine had no name. The name "Kiwi" was applied sometime later when a need arose to give it an identity when a larger "stable" of engines had been created. The engine is a petrol fueled, spark ignited engine and has a bore of 1" and a stroke of 1+1/8", which gives it a capacity of about 14.5cc. The engine is an air cooled design and uses plain bearings throughout and has vertical valves operated by push-rods, with a lift of 3/32". Cams are tangential and the tqppets are convex which is not an ideal arrangement but Westbury claimed that this design gives reasonable results at low speeds. The engine has a two bearing crankshaft which would have presented problems for some amateur machinists. An interesting feature is the banjo oiling system for the big end. Negative crankcase pressure draws oil from a small tank via the front main bearing which has a narrow groove along the top to conduct the oil without upsetting too much its lubrication. The banjo collects this oil as it leaves the bearing and the oil then flows by centrifugal force directly to the big end, a point in small engines which generally receives very poor lubrication.
The flywheel is attached to the crankshaft using a collet. The purpose of this is to prevent damage in the event of hydraulic lock which might occur if the engine in a power boat drew water into the suction piping while still running. The collet would allow the flywheel to slip and might avoid destroying the engine if the boat decided to change to submarine mode. This probably happened quite frequently. The power from the crankshaft may be taken off either end. As it was designed for boat use the majority of users would have taken the drive off the rear side of the engine (not the flywheel side). The gyroscopic forces from the heavy flywheel must have had an interesting effect on the running of boats around a small circular path!
The original published design was accompanied by performance curves taken from dynamometer tests on the prototype carried out with the assistance of Dennis Chaddock. The engine produced about 0.4HP at 6,500rpm and about 0.64HP at 8,000rpm. The curves produced were quite complete and included the frictional power absorbed by the mechanical parts of the engine and BMEP (which peaked at around 78psi at 6,000rpm).
The engine was designed principally for model racing boat use, even though it is air cooled. Model boat racing was a very popular pastime in those days and the engine was designed to meet a need for a properly engineered engine which would be reliable and give extended service. Apparently many of the engines built during this time were substandard as they were designed by boat builders with little practical knowledge of mechanical engineering. Westbury complained that many of the engines of the time had open camshafts with exposed gears receiving totally inadequate lubrication and very crude cam arrangements. But then most constructors in those days were very pleased merely to get a home-built engine to run. Westbury set out to do better than this and designed engines with both strength and lightness and which had adequate lubrication, bearing surfaces, the correct materials and good performance as well. A sense of quality pervades all his designs.
Westbury was born in 1896 and served in the Royal Navy in the First World War and saw service in the North Sea and at the Battle of Jutland. He was employed as a laboratory assistant at the RAF at Cranwell in the early 1920's and while there became acquainted with Frank Whittle, inventor of the jet engine. Westbury gets a mention in Whittle's autobiography where Whittle describes how they had a team making model aeroplanes and Westbury designed and built its engine. Thus Westbury was building engines from an early time and would have had a very good opportunity to examine aero-engines in detail and was no doubt exposed to the most up to date engineering practice of his day. One can imagine him inspecting every detail that he could and "chew the fat" with the fitters and engineers involved with their maintenance. This would have served him well in his later days designing the famous engines which have been reproduced by thousands of model engineers since. During the Second World War Westbury developed small petrol-driven generators for use by the armed services. Later he assisted in the design of the famous Vincent HRD motorcycle. He had a nearly continuous run at Model Engineer from the mid 1930's until his death and described a large number of internal combustion and steam engines. He died in May 1970.
One famous engine builder was the author Neville Shute who was a keen model engineer who had nearly finished Westbury's 4 cylinder "Sealion" just before he died. He even wrote a fictional book (Trustee from the Toolroom) which he said he wrote with Westbury in mind as the main character.
Westbury obviously had a great passion about small engine design and he was without doubt a very good engineer, designer and craftsman. His engines demonstrate excellent understanding of the engineering practice of his day and he went to great lengths to ensure that his engines would be reliable, give long service and be constructed easily by amateur modellers. He was always willing to adopt new techniques and try different approaches to solve problems in construction. His writings were always very clear and thorough which no doubt helped a lot of model engineers new to miniature engine construction. We owe him a debt of gratitude for the legacy he left us in the dozens of miniature engine designs he created.
The details of the 15cc Kiwi engine in the first articles were very sketchy and only a few parts were given as dimensioned drawings. Castings were eventually made available by Geo Kennions in England.
O Burnaby Bolton of Sydney, Australia, sold a casting kit (his "IC4") which has an uncanny resemblance to the 1935 "Kiwi" 15cc engine and this set of castings and a drawing are still available from E & J Winter of Medowie, NSW. The Bolton drawings for this engine are just as sketchy and incomplete as those in the original Westbury articles and the construction of this engine should only be undertaken by experienced model engine constructors. For some reason Bolton reduced the stroke to 1+1/16", but all other details appear to be identical. The smaller stroke reduces the capacity to 13.7cc.
In 1960 Westbury updated the design and called it the "Kiwi MkII" 15cc type "KW". (It is not known what "KW" refers to). There were some substantial changes but much of the original design remains the same. All the components were fully detailed in a long series of articles in Model Engineer (starting in August 1960) and the difficult machining operations were fully described. The principal improvements in the design are:
- Ball bearings on the crankshaft. (The banjo lubrication system was abandoned, but the crankshaft was drilled to take oil direct to the big end under the action of negative crankcase pressure).
- Harmonic (i.e. circular arc) cam design with a change to flat-faced tappets.
- A larger cam casing to give more room for the gears and cams.
- A larger cylinder head with improved cooling fins and a flat top to make holding it for machining much easier.
- A better design of the rocker support pillar.
- Optional designs for a water-cooled version.
- Contact breaker and carburetor detailed.
Builders using the OBB castings should get a copy of the Kiwi Mk II drawings for all the minor parts and adopt particularly the cam and tappet details. It is not possible to fit ball bearings to the original crankcase castings.
The Kiwi was the basis of several subsequent improved versions, mostly by revisions to the cylinder head. For example the "Kittiwake" had the same "bottom end" (crankcase, cams and so on) but had a cylinder head with inclined valves. It also had an add-on wet sump and circulating oil pump. It is interesting to note that Westbury says that the performance of the improved version was not in line with expectations and that perhaps the increased surface area inside the combustion chamber for this layout offset the advantages of improved air flow.
The Kiwi engine is not suited to model aircraft use, although Westbury claims that some did take to the air. In fact he adopted the name "Kiwi" from the flightless bird of New Zealand as a reminder that it was designed for purposes other than flying! It would be quite possible to convert it to glow plug ignition (to eliminate the weight of the coil and battery) but it is really too big and heavy for its power output to be a serious contender for aviation purposes. I like it because it is well designed, robust and has a pleasing appearance. I know of one Kiwi Mk II which is regularly run in a beautiful model boat. This engine is water-cooled and was built by Alan Fern in Sydney.
Plans and castings for both the water cooled version and the air cooled version of the "Kiwi MkII" were oncew supplied by Woking Precision, but you can now get them from Hemingway Kits. Plans and castings of the Bolton engine (IC4) from E & J Winter, PO Box 124, Medowie 2318, NSW, Australia, ph/fax (02) 4981 7999 (email: firstname.lastname@example.org). Hemingway has other IC engine kits and components for sale. Winter has other IC engines as well as steam engine castings and miniature spark plugs, piston rings, small fasteners and other model engineers supplies.