The Complex Napier Sabre 24 Cylinder Aero Engine Explained Simply

The Napier Cyber H24 engine that powered mainly the Hawker Typhoon and Tempest in World War II is incredibly complex. I mean, just look at it. But by the end of the video, you will understand the reasons behind the major engineering decisions made, as well as solid basic knowledge of how the Napier Cyber engine works. And it will all be explained in a way that is easy to understand. Reciprocating internal combustion engines for airplanes reached its peak during World War II. Maybe not in terms of efficiency, but definitely in terms of sheer power output. By 1938 already, the Cyber produced 2050 horsepower, and the final large-scale production variant, the 5A, produced 2600 horsepower. But it didn't stop there. The Cyber 7 produced 3500 horsepower, although admittedly this was not a production variant. But let's take a few steps back to understand why and how such powerful engines came into existence. Ever since the first powered flight of 1903, the challenge was on to develop engines that delivered more power while remaining as lightweight as possible to power increasingly advanced aircraft. This development race intensified during World War I and only stopped with the advent of the turbojet engine after World War II, which made piston engines redundant for military aviation purposes, with the exception of some low-power and fuel-efficient engine requirements. Even before World War II, engines more powerful than the very promising B-12s, like the Rolls-Royce Merlin, were required. And with the technology and materials available at the time, there was only one realistic way of achieving it, and that was by increasing the amount of cylinders. But this presented some problems. Engines couldn't be made much longer while remaining practical for the aircraft of the time. Increasing the amount of cylinders without increasing engine length led to some interesting cylinder layouts, like the W engines, X engines, and the layout we're discussing in this video, the H engine. Another one was the radial engine, which I've explained in detail in a previous video. Inline engines, however, have a problem in that they are generally unbalanced, which causes vibration. Whether the vibration stems from primary or secondary order imbalance, the magnitude of the vibrations are proportional to the weight of the reciprocating components, most notably the pistons. And since excess vibration is really bad for engine longevity, these imbalances effectively limit how large these engines can be made. From two cylinders inline through to six inline, only the six has perfect balance in all aspects without using a balance shaft. Balance shafts cancel out an engine's imbalance, but balance shafts increase engine weight, complexity, manufacturing cost, maintenance cost, and parasitic load on the engine. It's no wonder that balance shafts are a rarity in aircraft engines. Engineers would rather increase complexity in areas that increase power or efficiency. I've extensively covered how the inline six achieves perfect balance in a previous video, linked in the description. That makes the inline six the least amount of inline cylinders that can achieve perfect balance. And it's not a coincidence that these big engines are almost universally made up of multiple rows of inline sixes. The V12 is made up of two rows of inline sixes, the W18 is made up of three rows of inline sixes, the X24 consists of four rows of inline sixes, and the H24 also consists of four rows of inline sixes. These engines all inherit the perfect balance from the rows of inline sixes. The H24 of course can be seen as two stacked flat 12 engines, which brings us to the first peculiarity of the H24 engine in that it has two crankshafts. This is where the engine's extreme complexity starts. Having two crankshafts drive a single propeller requires some gearing to make it work. On the Sabre, both crankshafts rotate in the same direction. A spur gear on each crankshaft drives two compound reduction gears on the front of the engine. Four helical gears fixed to the compound reduction gears in turn drive the propeller shaft gear. The total gear reduction ratio from crankshaft to propeller is 0.27421, meaning the gearing reduces the propeller's speed to almost a quarter of the engine's speed. This slow turning speed allowed a very large diameter propeller to be used without the propeller tips exceeding the speed of sound. Large diameter propellers can more efficiently absorb an engine's power and convert it to thrust. This contributed greatly to the high level speeds the Typhoon and Tempest achieved. Each crankshaft, secured by seven main bearings, has the same crankpin layout as an inline six, with crankpins being rotationally separated by 120 degrees. The pistons in each cylinder bank move in three pairs of two. This resulted in the firing interval of the engine being every 60 degrees, the same as a flat 12 engine. So if the H24 engine has 24 cylinders, twice as many as the flat 12, isn't it supposed to fire twice as often? Actually, in this case, no. Instead of firing one cylinder every 30 degrees of crankshaft rotation, the Sabre simultaneously fires two cylinders every 60 degrees of crankshaft rotation. One cylinder at the top crankshaft and one at the bottom, and thus it really is one flat 12 engine stacked on top of another. The H24 engine uses fork and blade connecting rods. Instead of being connected to the crankpin side by side like normal conrods, these consisted of the blade rod running inside the big end of the fork rod. This was a very popular conrod arrangement at the time and did have some minor but notable advantages. By using side-by-side conrods, the cylinder banks need to be slightly offset. This actually makes the engine slightly longer compared to fully aligned cylinder banks, which is made possible with fork and blade conrods. This also makes the slightly lighter by negating the need for the dead metal present on offset banks that serves no purpose other than making the two halves join flush. Fork and blade connecting rods also deliver power pulses from the pistons exactly in the center of the crankpins, compared to side-by-side conrods which create a slight twisting force on the crankpins and thus put more strain on the crankshaft. An important point to note is that the big ends of the conrods of the opposing cylinder banks share a crankpin, which is quite obvious considering the fork and blade conrods. This is not like the well-known Boxer 4 and 6 aero engines, or even Boxer car engines for that matter, where the opposed conrods each has its own crankpin separated by 180 degrees of crankshaft rotation. This property of each conrod having a separate crankpin is specifically what makes it a Boxer engine. The Sabre's two flat 12s are not Boxer engines. Moving further outwards from the conrods, we get the combustion chamber and cylinder head. Missing here are the inlet and exhaust ports, as well as poppet valves. By the mid-1930s, it seemed to many engineers that poppet valve engines had reached its peak. While this did not turn out to be the case, it did at the time drive research to different intake and exhaust mechanisms, one of which was sleeve valves. In contrast to poppet valves controlling intake and outlet from the cylinder head, sleeve valves allowed intake and exhaust through ports in the cylinder walls. A sleeve with holes was situated between the cylinders and the pistons. The sleeve moved up and down, and in the case of the Sabre, also had a rotating or swiveling movement, and the sleeve moved to align its openings with those in the cylinder walls, opening and closing the ports as appropriate for the stage of combustion in progress. This up, down and swiveling movement was achieved by a sleeve valve drive, not totally dissimilar to a normal camshaft drive. The engine had two sleeve valve driveshafts, one for the left cylinder banks, upper and lower, and one for the right cylinder banks. The sleeve valve driveshafts were driven from compound gears at the front, at crankshaft speed. Worm gear on the sleeve valve driveshaft in turn each drove a worm wheel with a crank arm connected to the sleeves, which gave it both the reciprocating and swiveling movement, now reduced to half crankshaft speed. Each worm wheel drove two sleeves, spaced at 180 degrees, one on the upper and one on the lower bank. Sleeve valves had several advantages over poppet valves, especially during the 1930s when it was designed. For starters, it reduces the height of the engine, or in the case of the H engine, its width, compared to engines with overhead camshafts. Although it was later noted that sleeve valve radial engines did not have a much, if at all, smaller cross-section than poppet valve radial engines. Not having valves overhead means spark plugs could be placed for optimal ignition without competing for space with valves, which also required optimal placement. In addition, a cylinder head has very limited space for inlet and exhaust ports. This limited volumetric efficiency, which is the efficiency of the intake of fresh air and outlet of spent air. To increase the total area of these ports, more valves are needed. Using more than four valves, while possible, becomes very complex. Sleeve valves, on the other hand, can have more and larger openings without competing for space with spark plugs and with less restriction on available space. This, in theory, increases the volumetric efficiency of sleeve valve engines by improving fuel and air intake and exhaust scavenging. Furthermore, a poppet valve engine is RPM limited to avoid valve float. This is when the piston speed is faster than the valve spring can return the valve, meaning incomplete and inefficient combustion and can even result in engine damage on interference engines when the pistons and valves collide. Stronger and stiffer valve springs can be used to combat this, returning the valve faster. But this increases the force required to compress the valve springs, which increases the parasitic load on the engine as well as increasing wear on the valve train components. Poppet valves were also subjected to intense heat, being exposed to the combustion events with no effective way to cool the valves at the time. Sleeve valves had a larger area of contact with the cylinder, allowing better heat dissipation. In the 1930s, it was also thought that the hot exhaust valve would cause pre-ignition at higher compression ratios. But some considered the best advantage that sleeve valves had over poppet valves was the ability to make intake air swirl or tumble. Intake swirl is predictable and consistent rotational motion of air as it enters the combustion chamber. This improves the combustion flame speed and reduces cycle-to-cycle variations in flow and pressure. With sleeve valves, the inlet passages could much easier be modified to improve intake swirl than poppet valves' inlet ports, which of course meant that a higher compression ratio was possible on sleeve valve engines. Research at the time showed that sleeve valve engines could be one compression ratio higher than a comparable poppet valve engine, thus increasing efficiency. The core idea of the Sabre engine design was that using more cylinders, with smaller and lighter pistons running at higher rpm, would make it possible to outpower the leading aero engines of the time. They were not wrong. The Sabre was the first aero engine to be rated for service at over 2000 horsepower. The 5A variant of the Sabre produced 2600 horsepower at 3850 rpm with 15 psi of boost from 36.7 liters of displacement. Other than using relatively small and light pistons, the cylinders were also oversquare with a bore of 127 millimeters and a stroke of 121 millimeters. With a shorter stroke, higher rpm was simpler to achieve. Just for the sake of comparison, the Rolls-Royce Griffin V12 with the same displacement had a bore of 152 millimeters and a stroke of 168 millimeters, which is undersquare. Each cylinder on the V12 Griffin had a displacement of about 3 liters. By contrast, the Sabre's cylinders each had a displacement of about 1.5 liters, half the size of the Griffin's. The Griffin 65 put out 2035 horsepower at 18 psi of boost with a specific power of 0.91 horsepower per square inch, compared to the 1.16 horsepower per square inch of the Sabre. This was partly possible due to the higher operating rpm of the Remembering that horsepower is torque times rpm. Increasing rpm increases horsepower. Based on these numbers, the Sabre is superior. But that is not the full picture, as we have yet to consider engine weight. The Griffin 65 came in at a dry weight of 898 kilograms to the Sabre 5A's 1134 kilograms. That's 236 kilograms heavier, and the additional horsepower came at additional weight the airplane had to carry at all times, which meant less weight available for things like fuel. The specific fuel consumption of the Sabre 5A is 0.5 pounds per horsepower hour. Although the Sabre had a much higher horsepower per cubic inch ratio, power to weight ratio was very similar to the Griffin. This might not be a 100% fair comparison, but it is nonetheless very interesting, since these two engines had identical displacement. Production of Sabre engines stopped shortly after the war, but not before the Sabre 7 achieved a takeoff rating of 3500 horsepower at 3850 rpm with 20 psi of boost, partially made possible by water methanol injection. The Napier Sabre had a single-stage two-speed centrifugal supercharger. Due to only being single-stage, high altitude performance was lacking. The Sabre 2A powered Hawker Typhoon 1B had a service ceiling of 31,800 feet, and the Sabre 2B powered Tempest Mach 5 could climb to 36,500 feet, which is not horrible for a single-stage supercharged fighter, considering the Bf 109's 39,000 feet service ceiling with a single-stage supercharger. Nonetheless, that wasn't enough to make it any sort of good high altitude fighter or interceptor. But at low and medium altitudes, the Tempest was the fastest piston-engined fighter of the war, reaching an impressive 435 miles per hour at 17,000 feet. The supercharger was driven by two torsion bars running through the hollow sleeve valve drive shafts, which drove a compound gear, which in turn drove the supercharger. The supercharger had a low gear, or moderate supercharging mode, and a high gear, or full supercharging mode. The Napier Sabre was more compact than it may seem, especially for a 24-cylinder engine. With that compactness, cooling had to be very effective, and this was accomplished with liquid cooling to keep the heat generated by the 2,000-plus horsepower engine under control. The liquid was a 70-30 mix of water and ethylene glycol, respectively. Most engine accessories were also neatly mounted on top of the engine to retain maximum compactness and to enable easy external access for maintenance. These were the propeller governor, vacuum pump, distributor units, magnetos, air compressor, generator, hydraulic pump, and supercharger oil pump. These were all driven by the top auxiliary drive shaft. The bottom auxiliary drive shaft drove the coolant pump, fuel pumps, and another supercharger oil pump. With the advent of the turbojet engine, the H24 Sabre, same as most other high-power piston engines, disappeared virtually overnight. Even the most powerful and reliable piston engines couldn't compete with turbojets on power or reliability, and the only application of aero piston engines that would survive were low-power engines, below about 500 horsepower, usually nine cylinders or fewer, mostly for civilian use. Another thing that disappeared with these engines are sleeve valves, which did not replace poppet valves in mainstream use. With higher octane fuel becoming available, as well as advancements in metallurgy and other improvements in poppet valve technology like sodium-filled valves, many of the early advantages of sleeve valves completely disappeared. Add to that the lower manufacturing cost of poppet valve engines, it's not much of a mystery why sleeve valves are a thing of the past, other than for some small-scale niche applications. Sleeve valve engines also tended to be heavier per cubic inch of displacement than their poppet valve counterparts, which is not too difficult to imagine looking at the components required in a sleeve valve engine. As I promised, this is the basics of the Napier Sabre H24 and should give anyone a basic understanding of the engine. If you stuck around till now, I thank you and I'll see you in the next one.