1 of 6 articles on Deltic loco design engineering
by Ian Strange


The origins of the Deltic engine design can be traced back to 1880s Germany. This was a 2 stroke gas engine developed by Oechelhäuser, featuring two opposed pistons in one cylinder. In the 20th century, Hugo Junkers produced compression ignition (i.e. diesel) versions which ultimately were developed into aero-engines.

What Napier produced after WW2, was a triangular engine based upon the Junkers principle, but quite different in detail. Some features of the design, such as the pistons, were unique (patented by Napier). Much of its engineering followed the current trends of the day for high-performance piston engines.


Opposed piston engines such as the Junkers, were shaped roughly like a flat slab. It needed two crankshafts, top and bottom, which had to be synchronised by a long train of gears. It still worked out light and powerful, but the triangular form for the Deltic took it a big step further. Triangulated structures (take a look at a girder bridge!) are inherently rigid, and so weight can be kept down.

The ratio of crankshafts to cylinders is also more favourable. Instead of two crankshafts for 6 cylinders, it has three crankshafts to serve 18 cylinders. The whole assembly was bolted together following the latest trend, i.e, long tie-bolts passing right through the cylinder-block from one crankcase to the next (see Appendix 1 for a technical explanation). The three crankshafts are synchronised by the phasing gears.


So, why have two opposed pistons in each cylinder? A two-stroke engine has little time to expel ("scavenge") exhaust gases and fill up again with fresh air. In this type of engine, one piston opens/closes the inlet ports, and the other piston controls the exhaust ports. The incoming charge of air has to travel the full length of the cylinder before meeting the exhaust ports. For good measure, the inlet ports are angled to give the air a rotational swirl. The end result, is a cylinder virtually full of un-contaminated air, unlike lesser two-stroke engines! This arrangement also eliminates cylinder-heads and their associated gaskets.

It is better still on a Deltic. The phasing of the cylinders means that the exhaust pistons are 20 degrees ahead of their corresponding inlet piston. Sooooo... the exhaust ports open first, letting out the bulk of the spent gases. By the time the inlets open, the cylinder pressure will have dropped ready for that air to make its entrance. The exhaust ports then close first, allowing air pressure to pile up. More trapped air equals more power.


This is one of the clever parts of the design! In most two-stroke engines, the pistons have a particularly hard time. The Junkers engines were plagued with piston failures. A lot of heat passes into the pistons, with little time to cool off between firing strokes. Heat from a piston crown normally passes to the rings and thence to the cylinders and cooling system. In a two stroke, too much heat on the rings may cause them to stick as the oil over-heats. The exhaust pistons also have to cope with hot gases passing over the piston-rings. The most common solution is to try and reduce the heat reaching the piston-rings by fitting a steel piston crown.

Napier were somewhat more clever than that! Their patented piston design employed a carefully controlled oil cooling system. Each piston was made of two main parts: Outer body, and insert (or "gudgeon-pin housing"). Oil passages between the two parts allowed heat to be carried away, and also maintaining lubrication for the rings. The absence of steel for the crown, meant that its surface temperature could be kept relatively low, and so not heat up the air entering the cylinders excessively. Some of the heat picked up by the oil, was transferred to the piston skirt from where it could pass through to the cylinders and cooling system. This effectively by-passed the piston rings (cutting out the middle man!) to prevent them from sticking.

A later piston design (see photo) was fitted with a separate crown, but this was made from Hidurel 5, an alloy of copper. Copper conducts heat extremely well, and so keeps that top surface temperature lower still. This particular alloy has high strength and other favourable properties.


Each crankpin on the crankshafts, carries two connecting-rod big ends. This is achieved using fork and blade type big ends as shown in the illustrations. In keeping with high-performance engine designs of the day, main and big-end bearings were of the thinwall type, running on a crankshaft hardened by nitriding (see Appendix 2 for more details). Like on any good racing engine, the con-rods were given a polished finish to remove any surface blemishes.


While aluminium alloy is used for cylinder blocks, separate cylinder liners of high grade steel alloy are employed. These are of the "wet" type, with the outer surface in direct contact with the coolant, and rubber seals to (hopefully!) keep the coolant in its place. The bores of the liners are chromium-plated to give a hard-wearing surface.


Four stroke engines usually rely upon atmospheric pressure to fill their cylinders with air. Two stroke engines, conversely, require some mechanical means to achieve this end, and also to scavenge the cylinders of spent exhaust gases. A Deltic engine is fitted with a scavenge-blower for this purpose which pumps air into the cylinders. The blower pressure is a relatively low 4.2 psi at full power. In fact, the engine is not actually super-charged, despite the similarities to a super-charged engine.

This blower is of the centrifugal variety. That is, it contains an impeller (or "radial fan"), shown in the photo painted silver. Air is drawn in near the centre of the impeller, and 'thrown' out at its edges, into the three air passages leading to the three banks of cylinders.

The quantity of air delivered has to include some excess air that passes straight into the exhaust system. This is normal two stroke practice to ensure that scavenging is complete.


For two stroke engines with piston controlled exhaust ports, it is normal for oil consumption to be higher than for four stroke engines. The quantity of oil that is passed, varies with engine revs, as shown in the upper of the two graphs.

As can be seen, even at idling speed, oil consumption is still quite high at over 3 pints per hour. At idling speed, the velocity of the exhaust gases is low enough to deposit droplets of oil - at least in the large chambers of the exhaust collector drum and the silencer where velocity is lowest. This oil build up then leads to the familiar emissions when the engine is next revved up.


The Deltic (as installed in the production locos) has one injector per cylinder, served by its own injector-pump (i.e. 18 injector-pumps per engine). A camshaft on each bank of cylinders (i.e. 3 camshafts per engine) drives these pumps. The pumps themselves are of the type developed many years earlier by Bosch.

The quantity of fuel delivered is varied by rotating the plunger. Bosch's original design did this using a rack and pinion system, but the Deltics use a control-rod that rotates the plunger via a pair of skew-gears. However, the term "fuel-rack" has remained in use.

Supply of fuel from the four tanks is by electric fuel pumps, one per engine.


Deltics employ "dry sump" lubrication, which is normal practice for marine engines. That is, oil is contained in a tank and drawn into the engine by a pressure pump. After draining down into the sump, it is picked up by a scavenge-pump and returned to the tank via the oil radiators. The oil radiators (roof mounted) transfer heat to the surrounding air, assisted by the cooling fans.


Liquid-cooled (although ultimately giving up heat to the air) with a water/anti-freeze mixture. The anti-freeze solution helps to reduce corrosion of the aluminium-alloy castings including cylinder blocks. These are especially susceptible to corrosion wherever they are in contact with other metals and also the rubber cylinder-liner seals.

The exhaust manifolds (and elbow pipes) are also cooled, which helps to keep down temperatures in the engine room. Coolant flowing through the cylinder blocks, follows a complex series of distribution passages that help to give equal cooling to all cylinders. The cut-away photo shows cooling passages painted light blue (including the outside of cylinder-liners). Coolant even flows through small passages drilled through the exhaust port bars (painted black on the cylinder illustrated).

After passing through the engine and doing its job, coolant is sent to the radiators mounted in the roof. Like the oil radiators, these also received air-flow courtesy of the roof mounted fans (two per engine). Between each pair of fans is the coolant header tank which keeps the system topped up and under some pressure. The higher the pressure, the higher the boiling point, allowing the coolant to run at about 85 deg. C. A higher coolant temperature (relative to ambient air temperature) allows a greater quantity of heat to be shifted. It also reduces the effect of high ambient air temperature on those hot summer afternoons.


Automatic control of each engine is looked after by the governor. This responds to the driver's power handle, and then endeavours to maintain the selected engine rpm. It also controls the power output across the rev range. It does this partly by controlling the engine's fuel supply (via the fuel-racks) and also by controlling the load on the engine. The latter is achieved with the help of the load-regulator which can vary the amount of torque that the main generator places on the engine. The governor monitors engine rpm in the time-honoured method: Centrifugal fly-weights. These spring-loaded weights move outwards under the effect of centrifugal force, as they revolve - driven by the engine.

Deltic engines have a short stroke and so rev up much more quickly than more conventional railway diesel engines. For this reason, governing is more difficult and has to be very precise. The Deltic's governor features a speed-reset system to help achieve this, partly by anticipating changes in engine revs - rather than waiting until it's too late!


Manufacturer D. Napier & Son
Built at Napier's Liverpool Works
Type D18-25B (D prefix later dropped)
No. of cylinders 18
No. of pistons 36
Swept volume 5384 cu.in. (88.223 litres)
Stroke 7¼ inches (184.2mm)
Bore 5.125 inches (130.2mm)
Continuous power 1650 bhp (1231kW) at 1500 rpm
Short term power 1800 bhp (1343kW) at 1500 rpm
Length 125 inches (317cm)
Width 69 inches (176cm)
Height 94 inches (239cm)
Blower pressure 4.2 psi (29kPa)
Weight 5 tons 9cwt 82lbs (5574kg)
Fuel Diesel fuel B.S. 2869 class A
Oil SAE 40 (e.g. Shell Rotella T40)
Coolant 70/30 mix of distilled water/glycol anti-freeze


For your added interest, the ultimate production marine Deltic was Paxman's version of the CT18-42K, illustrated above (from a brochure).

This engine developed a maximum 4140 bhp, or 4000 shp (shaft horse-power at the gearbox output). It featured a turbo-charger, but with a mechanical drive (along the centre of the triangle). The compressed air-charge passed through inter-coolers, making the previously heated air contract, and so filling the cylinders with a greater air mass. Turbo-charger pressure was 19 psi at full power.

APPENDIX 1 - Long tie-bolts

The Deltic cylinder-block/crankcase assembly is bolted together with very long, thin tie-bolts (visible in one of the above photos). The longer and thinner the bolts, the more they stretch for any given change in load. But because this stretch relieves more of the compressive load from what the bolts are clamping (i.e. cylinder block and gaskets), the actual load on the bolts varies less than for short and stiff bolts. Also, the blocks don't have to worry about absorbing the firing loads of the engine.

APPENDIX 2 - Bearings

Bearings for crankshafts and con-rod big-ends usually follow the principle of soft material supporting a relatively hard shaft. Soft material allows metal dust to become embedded. For higher performance engines, a less soft material is required, otherwise it will become crushed. This increases the risk of partially embedded metal dust abrading the crankshaft. So, as mentioned above, the crankshaft is surface hardened by the nitriding process. Also, thinwall bearings help, because they have only a thin layer of soft bearing material. The thinner the layer, the greater the pressure it can withstand. The bearing materials generally used come under the heading of "white-metal". These cover a wide range of alloys of lead and tin that have a low melting point.

Oil filtering also helps to permit the use of these higher performance bearings. Only a few years before the Deltics, the concept of oil filters on engines was still a novel one!


Colour photos by the author (thanks to the National Railway Museum).
All other illustrations originate from D. Napier & Son Limited and Paxman Diesels. These are reproduced by kind permission of MAN B&W Diesel Ltd.

Ian Strange

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