By William Pearce

Toward the end of World War I, a number of companies were pursuing the concept of a very large engine powering a very large aircraft. Just about every country that had extensive experience in the field of aeronautics expended resources to create the large engine and aircraft combination. As history unfolded, all of these projects came to naught, although the experience gained did pave the way for future projects.

Side view of the 800 hp (597 kW) Sunbeam Sikh I V-12 engine. Carburetors can be seen attached to the first and last cylinders. Note the two water pumps under the engine and the exposed valves.

The Sunbeam Motor Car Company based in Wolverhampton, England had added aircraft engine design and manufacture to its existing automotive business in 1913. Sunbeam’s aircraft engines were designed by Louis Coatalen, their chief engineer, and were sometimes referred to as Sunbeam-Coatalen Aircraft Engines. As with so many other companies, Sunbeam designed a large aircraft engine during the closing days of World War I. This large engine was named Sikh (or Sikh I), and it was intended for use in either large aircraft or airships.

The Sikh was a 60 degree V-12 engine. Its individual cylinders were a departure from the standard Coatalen-designed engines. The cylinders were machined from steel forgings and had welded sheet metal water jackets. Each cylinder had four spark plugs positioned under its six exposed valves. The three intake valves were positioned on the Vee side of the cylinder, and the three exhaust valves were positioned on the outside of the cylinder. The intake and exhaust valves were operated by separate rocker groups positioned above the valves. This configuration allowed all intake (or exhaust) valves to be opened or closed simultaneously. Each rocker group was actuated by a pushrod that was driven by a camshaft mounted in the Vee of the engine and geared to the crankshaft. Four magnetos at the rear of the engine fired the spark plugs.

A Sunbeam Sikh ad from 1920 touts the engine as the most powerful in the world but prophetically adds, “at the moment.” The Duesenberg H developed at the same time as the Sikh I had the same output, and the 1,000 hp (746 kW) Napier Cub would eclipse both engines later in 1920.

Two water pumps were positioned under the engine and driven by vertical shafts from an accessory gear. Each pump supplied cooling water to one cylinder bank. The Sikh had four carburetors—one attached to the first and last cylinders of each row. For each cylinder row, the air/fuel mixture flowed through an intake manifold attached to the cylinders inside the Vee of the engine. The engine used aluminum pistons mounted to H section, forked connecting rods attached to the crankshaft. The hollow crankshaft was made of nickel-chromium steel. Via spur reduction gears, the propeller shaft turned at 0.657 engine speed. The crankcase of the Sikh was an aluminum casting.

The Sunbeam Sikh had a 7.09 in (180 mm) bore and 8.27 in (210 mm) stroke. The engine’s total displacement was 3,913 cu in (64.1 L), and it produced 800 hp (597 kW) at 1,400 rpm. The Sikh had a dry weight of 1,952 lb (885 kg).

The engine was first run on 11 May 1919 and was displayed at a number of aviation shows. Although the Sikh passed British Air Ministry tests to prove its airworthiness, Sunbeam did not receive any orders for the engine. Large engines and large aircraft were simply not practical in the early 1920s, and there was little interest in airships in the immediate post-war era.

In addition to the Sikh, Sunbeam co-developed a smaller engine known as the Sikh II (or Semi-Sikh). The inline-six Sikh II was essentially half a Sikh. The cylinders were the same but they were mounted on a new crankcase. The Sikh II was direct drive without any gear reduction, and the camshaft was mounted on the left side of the engine. With the same bore and stroke as the Sikh, the Sikh II had a total displacement of 1,956 cu in (32.1 L) and produced 425 hp (317 kW) at 1,400 rpm. The engine had a dry weight of 1,120 lb (508 kg). Unfortunately for Sunbeam, the Sikh II, like the Sikh, found no applications.

The Sunbeam Sikh I as displayed at the Olympia Air Show in 1920. Note the two spark plugs positioned under the valves on both sides of the cylinder, the pushrods in the Vee of the engine, and the four magnetos. In the left corner of the picture is the Short Silver Streak. (Stilltime Collection Image)

By 1927, British airship development had been renewed, and the R100 and R101 programs were underway. Sunbeam saw a new opportunity for the Sikh engine and developed the Sikh III strictly for airship use. The Sikh III was again a 60 degree V-12 engine, and most sources say it possessed the same bore, stroke, and displacement as the original Sikh. However, some original sources (Jane’s and Flight) say the bore was increased to 7.28 in (185 mm), which would give a total displacement of 4,134 cu in (67.7 L).

The individual cylinders of the Sikh III were redesigned and refined using a carbon steel barrel and a cast steel head. In addition, the valve train was completely redesigned. Each cylinder still had three exhaust valves, but the number of intake valves was reduced to two. The valves for each cylinder were enclosed in a common rocker cover. The rockers extended though the cover and were actuated by pushrods that ran between the cylinders. On the left cylinder bank, the exhaust rocker arm protruded out the rear of the cover, and the intake rocker arm protruded out the front. This configuration was reversed for the right cylinder bank. The crankshaft was forged from nickel-chromium steel and had six throws. Each cylinder had two spark plugs that were enclosed by the rocker cover. The spark plugs were fired by two magnetos driven at the rear of the engine.

The inline-six Sunbeam Sikh II was essentially half a Sikh I. Note the camshaft and pushrod arrangement in the rear view on the left. The front view image on the right illustrates the engine’s carburetors, valves, and lack of a propeller gear reduction.

The engine used two carburetors, which, along with the intake manifolds, were positioned in the Vee of the engine. Each carburetor supplied the air/fuel mixture to three cylinders of each bank. The propeller shaft of the Sikh III was geared to the crankshaft at a 0.567 reduction. The Sikh III produced 1,000 hp (476 kW) at 1,650 rpm and had a dry weight of 2,760 lb (1,252 kg). The engine was 7 ft 2 in (2.2 m) long, 3 ft 4 in (1.0 m) wide, and 6 ft 2 in (1.9 m) tall.

The Sikh III was first run in 1928 and was displayed at shows in 1929 and 1930. However, engines for the R100 and R101 airships had already been selected. The disastrous crash of the R101 airship in 1930 caused Britain to cease all further airship development, leaving the Sikh III without any possible applications.

Only small numbers of Sikh I, Sikh II, and Sikh III engines were built. Like many large aircraft engines built over the years, the Sunbeam Sikh engines were never installed in any aircraft or airships.

The Sunbeam Sikh III was intended for airship use but never found an application. Note the new cylinder heads. The exhaust valve pushrod can been seen on the rear left cylinder.

Sources:
– Sunbeam Aero-Engines by Alec Brew (1998)
– Aerosphere 1939 by Glenn D. Angle (1940)
– Jane’s All the World’s Aircraft 1927 by C. G. Grey
– Jane’s All the World’s Aircraft 1929 by C. G. Grey
– “The Sunbeam Motor Car Co., Ltd.” The Aeroplane (31 December 1919)
– “Aero Engines at Olympia” The Aeroplane (21 July 1920)
– “The Sunbeam Motor Car Co., Ltd.” Flight (18 July 1929)