The Most Complicated Diesel Engine Ever Had 3 Blocks, 18 Cylinders, 36 Pistons, And Was Shaped Like A Triangle

There have been a lot of weird engines throughout history. The opposed-piston engine is an oddball, as is the Wankel and the hub-mounted piston engine. But there is an undisputed king of complicated and weird. That engine is the Napier Deltic, and it’s so complicated that it’s hard to summarize in a single sentence. But I’ll try: This diesel monstrosity was an opposed-piston menace that had 18 cylinders, 36 pistons, was shaped like a triangle, and thundered around the United Kingdom in ships and locomotives. That’s only the start.

The opposed-piston engine is a relative rarity in the modern world. Don’t mistake “opposed-piston” for the engines found in Subarus and BMW motorcycles. In a horizontally-opposed engine, you get a piston in each cylinder firing outward and sideways rather than straight up and down like in a typical piston engine. These engines are often called “flat” engines, and depending on the application, these engines can be short and compact.

An opposed-piston engine can also be a compact solution, but these engines work differently than a boxer, a flat-four, or similar. In an opposed-piston engine, two pistons share a single cylinder and instead of moving out, these pistons move toward each other before firing and coming right back to each other again. They also don’t have to be horizontal.

Opposed-Pistons Are Rooted In History

As the Gas Engine Magazine writes, one of the very first opposed-piston engines was the “Atkinson Differential Engine” (shown below) invented by James Atkinson in England in 1882. Apparently Atkinson thought Nikolaus Otto’s four-stroke engine wasn’t terribly efficient and figured he could do better. In Atkinson’s engine, two pistons shared the same cylinder and fired at each other, but each fire happened with each rotation of the crankshaft. As a result, the pistons weren’t really coming at each other, but following each other.

Opposed-piston engines would then begin to show up all over early automotive history. The famed Gobron-Brillié car that hit 95 mph in 1904 did so with opposed-piston power. These engines also found homes in motorcycles, watercraft, and even in some rail experiments. In my retrospective about the Commer TS3 opposed-piston diesel, I noted other efforts:

They weren’t the only ones, the Swiss Sulzer ZG9 was an opposed-piston engine before World War II that found a life powering generators. Meanwhile, the Junkers Jumo 204 of 1929 was another opposed-piston diesel engine and it was bolted to aircraft. There were opposed-piston engines as early as 1905. Scottish car manufacturer Arrol-Johnston had a rope-start opposed-piston engine back then.

The opposed-piston fever even hit America, where Ransom Eli Olds sought to solve the problem of early diesels being so heavy. His diesel [shown below] would dramatically cut down on weight with a radical new design.

Opposed-piston engines have several advantages. By making pistons punch each other in a single cylinder, you can eliminate numerous parts that would be needed in a piston engine with vertical cylinders. Opposed-piston engines don’t need cylinder heads or valvetrains as the movement of the pistons handle the compression of the fuel-air mixture. This reduces material and engineering costs, reduces weight, reduces bulk, reduces friction loss, and in theory, should also reduce complexity.

Early piston engines had a problem with scavenging. When an engine operates, burned exhaust gases are supposed to leave the combustion chamber to be replaced with a charge of fresh air for the combustion process to start all over. This sounds simple, but in practice, early piston engines struggled to scavenge all of the exhaust gases efficiently. Opposed-piston engines were better at scavenging than other designs of their day.

How Opposed-Piston Engines Work

Here’s how a basic opposed-piston engine works. In a single cylinder, there are two pistons, each with its own connecting rod and crankshaft. When the engine is at the bottom of its stroke with both pistons farthest apart, an exhaust port on one end is open while an intake port is open on the other end. Fresh air enters the cylinder as exhaust gases are pushed out. Both pistons begin moving toward each other, closing the ports and creating a swirl of fresh air as the compression stroke continues.

As both pistons close in on eachother and the engine reaches inner dead center (this engine’s equivalent of top dead center), fuel is injected from the middle of the cylinder. The heat and pressure generated during the compression stroke ignite the fuel at near where the pistons meet, starting the power stroke.

As the pistons blast away from each other, the exhaust ports open, blowing exhaust gases out. Once the pistons reach their farthest distance apart, the intake ports open, pushing in fresh air and scavenging the exhaust gases. Then, the cylinders start coming back at each other, starting the process all over again.

Early versions of the opposed-piston engine called for the pairs of cranks to be synchronized with gears. Exhaust scavenging also wasn’t particularly efficient, as early opposed-piston engines exposed their intake and exhaust ports at the same time. Ransom Olds and Commer figured out solutions to these problems. Their designs called for all cylinders to ride on the same crankshaft and for the piston on the exhaust side to operate ever so slightly ahead of the intake side. The changes helped scavenge exhaust gases much better while also making for a simpler engine.

Each company and engineer seemed to have their own reasons for opposed-piston engines. Ransom Olds wanted to reduce the weight of the insanely heavy engines of the 1930s, while others, like Commer, needed powerful engines that fit into smaller spaces.

The Napier Deltic, too, was born out of necessity.

The Need For Speed

The year was 1943, and the world was at war for a second time. According to the Model Engineer magazine, which published a deep dive in 1953, the British Admiralty had learned a few lessons in World War I that it wanted to apply in the second war. Britain saw a need for fast and lightweight torpedo boats, something it was sorely lacking at the time.

In the 1930s, the Germans built several Schnellboote, fast boats, that the Allies called E-boats. These attack vessels were relatively speedy and their diesel engines made them a bit less susceptible to fire than the gasoline-powered attack boats from Britain. But there was another problem. The Brits couldn’t just plop a diesel engine into an attack boat because the diesel engines of those days were usually pretty weak-sauce.

Thankfully, engineers had already been trying to solve this problem. One proposal came from N. Penwarden, a draughtsman at the Admiralty Engineering Laboratory, by D. Napier & Son Limited. Before the war and in the 1930s, D. Napier & Son purchased a license to the Junkers Jumo 204 aviation engine and began developing it into the Napier Culverin.

The Junkers engine was one of the most successful early implementations of opposed-piston technology. The 28.5-liter aircraft engine featured six cylinders, 12 pistons, and punched out 700 HP. Junkers also solved the scavenging problem by timing the pistons to open the exhaust port sooner. But Junkers did not solve the problem with needing two crankshafts for the cylinders. What it did do was experiment with piling on power with engines of different configurations, including a diamond and a rhombus.

The Napier Culverin had the same basic design. This engine featured six vertical cylinders with a total of 12 pistons contained within them. Crankshafts were found at the top and the bottom of the engine and were linked by a gearset. Unfortunately, the 821 HP firepower produced by the Culverin wasn’t enough for the demand of fast attack boats, so engineers went back to the drawing board.

The Napier Deltic

A technical manual by D. Napier & Son explains that the firm’s engineers created the Deltic essentially by taking three Culverins and placing them into a huge, mostly aluminum triangle. From D. Napier & Son:

The basic structure of the engine is formed by three cylinder blooks and three crankcases held together by high-tensile steel tie-rods passing through the cylinder blocks and crankcases and torque loaded to form a basic assembly of great strength and rigidity.

Two important design aspects arise from the triangular lay-out. Firstly, a phase angle different between the inlet and exhaust pistons of one cylinder arises automatically; the exhaust piston leads the inlet piston by 20° and the bottom crankshaft rotates in the opposite direction to the two upper crankshafts. This phasing and engine timing will be discussed in detail in a later chapter. Secondly, in any one crankcase, the crank-pins carry blade and fork type connecting-rods. The pistons attached to the fork rods control the exhaust ports of one bank of cylinders, end the pistons attached to the blade rods control the inlet ports of the other bank of cylinders adjacent to that crankcase. The clearance volume between the two opposing pistons in any one cylinder forms the combustion space. Thus the load on each crank-pin and the power transmitted by each crankshaft is the same, with a large saving of weight owing to the elimination of cylinder heads,
valves and valve operating mechanisms.

At the driving end of the main triangle assembly, a casing contains phasing gears which link the three crankshafts to a single output gear. Connecting each crankshaft to the phasing gears is a quill-shaft designed in conjunction with a vibration damper to eliminate torsional vibration over the engine range of operating speed. These quill-shafts allow differences of expansion and slight mal-aligment between the crankshafts and phasing gears: by the use of differing numbers of teeth at each end of the quill-shafts the vernier combination so formed is used for phasing the crankshafts. The phasing gear case also contains auxiliary gear trains to drive some of the engine auxiliaries, and in certain installations, auxiliaries may be driven directly from the quill-shafts. The means of transmitting the drive from the output gear of the phasing gear case.

Hopefully, your brain isn’t melting looking at that. If it is, I’ll try to simplify this. The Deltic, which is named after the Greek letter Delta, creates one huge engine block out of three engine blocks formed together into a triangle. Remember how the Junkers design has a crankshaft at the top and the bottom of the engine to run the pistons? Well, in mashing these engines together into a triangle, each vertex of the triangle has a common crankshaft. That way, you don’t have a three-block engine with six crankshafts.

The three cranks of a Deltic were linked with phasing gears so that what’s essentially three engines can operate one output shaft.

Here are some more juicy details from Napier:

A ‘geared in’ type turbo blower unit consisting of a centrifugal compressor and a single stage axial-flow turbine, is mounted on a sandwich piece at the free-end of the triangle. Port timing is so arranged that the blower completely scavenges and pressure-charges the cylinders. The engine is lubricated on the dry sump principle, with engine driven pressure and scavenge oil pumps. Certain parts of the engine are supplied with oil at a reduced pressure either through a pressure reducing valve or through restrictors taking their oil supply from the main pressure system. The main oil system also supplies oil to the clutch pumy when a bi-directional gear box is fitted. The oil is cooled by passing it through a heat exchanger or radiator before returning it to the service tank.

A closed cooling system is used, circulating distilled water inhabited with ethylene glycol. The coolant is ciroulated through the exhaust manifolds, cylinder blocks, and where applicable, through the turbine casing, by an engine driven pump. A thermostatically controlled heat exchanger or radiator is included in the system, using a raw water or air supply for extracting heat from the engine coolant.
The method of starting the engine may vary with the type of installation; alternative methods used are air-starting, or by motoring the engine through a generator, where the generator is directly coupled to the engine.

For air starting, high pressure from an air reservoir is admitted to one bank of cylinders, by means of a distributor valve, to motor the engine. Mounted on the free end of the bottom crankcase is an auxiliary drive gear box containing a train of gears driven by a flexible drive shaft from the bottom crankshaft, a similar to the blower flexible drive shafts. This train of gears may drive various engine auxiliaries and in certain applications may have an auxiliary power take-off shaft.

It’s noted that one particularly difficult challenge arose from this process of sorta mashing three engines together. The challenge was how to get the pistons to move in the correct sequence. Remember that in the base engines, two cranks moved 12 pistons in six cylinders. Now, the triangle of engines were sharing common cranks.

Note: The technical manual is 15 chapters and long enough to fill a book. Click here to read more of it.

Penwarden figured out that the optimal way to get every piston to move on time was to have one of the three cranks to run counter-clockwise. This required changes in gearing to run that crank “backward” and yep, it looks as wild as you think it is.

The Deltic even kept with the times and was engineered to be efficient at scavenging. For that, engineers made the exhaust piston lead the intake piston by 20 degrees. By doing this, the exhaust port opened before the inlet port, and then the exhaust port closed, after exhausting gases, in time for a charge of fresh air to come in from the compressor. It’s also noted that in terms of firing, a Deltic had an ignition charge every 20 degrees of crankshaft rotation. Oh don’t worry, I have a graphic for the firing order and yes, it’s properly chaotic.

Put it together and you have a comically complex engine with three crankshafts, 18 cylinders, and 36 pistons. If that’s not crazy enough for you, I’ll note that the Deltic also had three compressors to help with scavenging. Curiously, the Deltic also had three camshafts driven by the gear system. Since there were no valves to actuate, the cams were there solely to operate the injectors and pumps for the engine’s banks of cylinders.

As you can imagine, a project this absurd took a while to materialize. The first prototypes didn’t see action until after World War II in 1947. Production commenced around 1950 with the Deltic D18-11B. This 18-cylinder beast was capable of producing 2,500 HP for 15 minutes or 1,875 HP continuously, but that assumed a 10,000-hour time between overhauls.

The spec sheet of an 18-cylinder Deltic was also properly wacky. These engines had a bore of 5.125 inches and a stroke into the next postal code of 7.25 inches. The engine ran at a compression ratio of 19.26:1 and had a displacement of 88.3 liters. Of course, no sheet of numbers is complete without dimensions. A Napier Deltic 18-cylinder was 6 feet, 2.5 inches wide, 7 feet, 1 inch tall, and was 10 feet, 11 inches long. Dry, a Napier Deltic 18-cylinder weighed 8,727 pounds.

The Deltic Goes Into Service

If you’re still wondering why on Earth an engine like this existed, there’s another explanation. The Brits had a captured German E-Boat on hand, a vessel that used a triplet of Mercedes-Benz diesels with power said to be about on the level of the Deltics.

The Admiralty replaced two of the three German diesels with Napier Deltics and the difference was apparently shocking. The crazy Deltics were half of the size of the big Mercedes diesels and a fifth of the weight. Yet, they also just made ridiculously great power for their size.

This was perhaps the apex of the opposed-piston experiment. These were supposed to be relatively compact engines that pumped out huge power numbers. At the time, the Napier Deltic did just that. The engines were also remarkably good at their jobs, and the Royal Navy put these engines into any craft that needed to move swiftly, including countermeasures vessels, minesweepers, and fast attack craft.

A smaller 9-cylinder version of the Deltic was also built and were used in vessels like minesweepers or used for power generation in ships that already used Deltic 18s for propulsion.

Boats, Trains, And Fire Trucks

The Navy wasn’t the only entity to have fun with these engines. From 1959 to 1962, English Electric constructed British Rail Class 23 and British Rail Class 55 locomotives that used Deltic diesels as their prime movers.

The British Rail Class 23, which was introduced in 1959, used a Napier Deltic T9-29 9-cylinder, 18-piston diesel good for 1,100 HP. These 75 mph locomotives, often called the Baby Deltic, were known for operational issues including cylinder liner cracking around the injector hole, turbo bearing failures, and seized pistons. Only 10 of these locomotives were built, and it’s said that most of them had some sort of problem most of the time.

The more famous implementation was in the British Rail Class 55 locomotives that were often called the Deltics. These majestic machines had pairs of Napier Deltic D18-25 engines, together making a combined output of 3,300 HP. When these locomotives were introduced in 1961, they were the most powerful single-unit diesel locomotives in the world.

Keep in mind that “diesel” is the operative term there. In 1961 there were more powerful electric locomotives and Union Pacific also had its insane gas turbine-electric locomotives that were also more powerful. Steamers also made more power. But for a brief period, and by brief, I mean just a single year, the Brits had the glory. I’ll let Curbside Classic explain why British railfans absolutely love these things:

So what makes the Deltic special? The looks and the colours of the prototype; the sheer bulk of the production versions. The impact of these huge engines pulling expresses at a steady 100mph for mile after mile. The rarity – only 22, compared to 200 Class 40s and over 500 Brush Class 47 diesels. But most of all, the noise. Ah, the noise. Compare the noise of any other British diesel to a Deltic and you’ll get it. It’s like comparing a bus to a racing car. The former is slow, steady and chugs along.

But the Deltic spins frantically, emitting huge clouds of black smoke as the engine starts up (a recurring problem caused by oil build up in the exhausts, and source of more than one fire) and settles in a fast, frantic idle; then, the second engine starts (this beast is twin engined, remember), and it all happens again. In the cavernous stations at King’s Cross, York, Newcastle Central and Edinburgh Waverley, an idling Deltic drowned any PA system, and a departing one would seemingly be audible for minutes.

Shutting down the engine was no less dramatic, as the complex gearing around six crankshafts rattled to a stop. Growing up just 100 yards from the ECML, two miles from Wakefield station on the London – Leeds line, we always knew when a train was Deltic hauled minutes before it passed between the houses across the road, and the noise had a direct line of sight to our house.

Amazingly, Napier Deltics even made their way to America by way of Nasty-class patrol boats.

The New York City Fire Department also used Napier Deltic engines to power their gigantic “Super Pumper System” trailers, which were designed to extinguish the biggest monster fires. How crazy was the pumper? It fired 8,800 gallons of water per minute at a blaze. But it was also great as a hub. As Curbside Classic notes, when the NYFD responded to a postal annex fire in 1967, a Super Pumper System trailer was able to provide water to a tender truck, three satellite units, two ladder trucks, and hand hoses all by itself.

A Finicky Legend

Sadly, while the Napier Deltic may have been a super engine, it was also high-strung and difficult to repair, requiring specially-trained technicians. These engines had lots of success in their roles, but operators found the engines easier to swap out and repair later rather than to immediately repair when things went wrong. The Deltics were also somewhat limited in scope. They were small and light for ship and railway engines, but not small enough to be used in highway tractors. Eventually, more conventional diesels also caught up enough in power that operators eventually just moved to those engines instead.

Despite that, the Deltic still lasted an impressively long time. The ‘Baby Deltic’ locomotives were retired after about a decade of use, but the bigger ‘Deltic’ locomotives survived into 1980. Six of the 22 British Rail Class 55s have been preserved, too, which is pretty awesome.

The Deltics fared better on the seas. The last Deltic-powered crafts were the Hunt-class mine countermeasures vessels, which kept their Napier Deltic 9-59K engines until they were replaced by Caterpillar C32 engines beginning in 2012.

Everything about the Napier Deltic diesels was simply absurd, from their crazy configurations to the fact that they were still somehow smaller than equivalent engines in their day. Unless someone protests, I think these engines are deserving to be called the weirdest engines ever made. Yet, the coolest part is that the Deltics weren’t some crazy one-off thing, but an engine that worked well enough to be used well into the modern day. Maybe as the world looks to alternative fuel sources we’ll see something bonkers like this again.

Top Photo: Phil Sangwell/ Flickr / Wikimedia Commons