Rolls-Royce motors meet Lada Fuel!
When Britain sent Hurricane fighters to Russia in 1941, their Merlin XX engines needed the special 100 octane petrol developed for the RAF. Although a supply was shipped there to startwith, the Russians themselves were unable to produce any more. Their fuel was not only lower grade but very variable in quality. Henry Broquet, a British technician, collaborated with them and came up with the tin alloy catalyst, a workable answer that enabled the Merlins to run on lower grade petrol satisfactorily. Why was the grade of fuel so critical? After all, car engines can be detuned to run on cooking petrol by retarding their ignition or fitting thicker head gaskets. You simply could not do that with a thoroughbred Rolls-Royce aero-engine, even though a special low compression 600 hp Merlin called the Meteor was produced for tanks. In any case, the Hurricane was already outclassed by newer German fighters such as the Me 109F and the FW 190 and needed every ounce of power it could get.

Extreme Demands
Aircraft ask far more from their power plants than any ordinary car or boat. During the 1939-45 war period, designers pushed aero-engines to the very limits of contemporary technology. Engine life was not the main consideration (the average life of a Lancaster bomber with its four Merlins was 20 flying hours). It was literally a matter of life or death to get the maximum possible power (bhp per litre) for take-off and combat, coupled with the best possible economy (specific fuel consumption) for range. Fuel quality was critical to the performance that designers could get out of their engines.

Power
The power output of a piston engine depends upon firstly the revolutions per minute (rpm) and secondly the pressure developed in the cylinders (brake mean effective pressure or bmep). Friction and mechanical stress limit the rpm, but fuel is one of the main factors limiting the attainable bmep. This is mainly due to the problem of detonation. If the fuel/air mixture is compressed too much, it may not burn evenly but explode suddenly. This gives a very high peak pressure and a sharp blow on the piston instead of a steady push. The energy from the fuel does no useful work but is wasted as heat. Severe or prolonged detonation can blow holes in pistons, burn out exhaust valves and even knock off cylinder heads. Car drivers can hear the characteristic pinking sound caused by the shock waves hitting the cylinder walls, but there is too much background noise for pilots to hear it in the cockpit. They have to watch their cylinder head temperature gauges and be sensitive to rough running. Easier said than done in a dog fight.

Octane Rating
Petrol's resistance to detonation is given by its Octane Rating. This is the percentage of iso-octane (full chemical name 2,2,4 trimethyl pentane)contained in a mixture of iso-octane with normal heptane that reproduces the same knock characteristics as the petrol when tested under standard conditions. One of the ways to increase octane rating is to dope the fuel with tetra-ethyl lead. What this does is to steady the rate of combustion and inhibit the sudden temperature rise ahead of the flame front that triggers the explosive ignition. A drawback is that lead deposits tend to build upon the spark plugs so ethylene dibromide is often added, to combine with the lead and take some of it out in the exhaust.

Pre-ignition and Plug Fouling
Another problem with mediocre fuel is that it may have poorly combustible components that have not been refined out, and they leave unburned deposits on the cylinder head and valves. These impair cooling and may get so hot that they act like diesel glow-plugs, igniting the mixture before the spark plugs fire, too early before top dead centre. The effect of this pre-ignition is similar to detonation and equally damaging to the engine. Characteristically a hot engine will "run-on" after the ignition is cut, as though it was a diesel. (Inefficient pre-war side-valve engines used to need decarbonising every ten thousand miles or so because they built up these unburned deposits.)

Superchargers
By the mid 1930s all big military aero-engines were supercharged. The fuel/air mixture in a supercharged engine is fed into the intake manifold under positive pressure instead of being sucked in. The pilot can read this manifold pressure on a boost gauge. The reading roughly correlates with the bmep being developed, and hence the load on the engine. Unlike a turbo-charger in a car, an aero-engine supercharger serves two purposes. It increases power by forcing a greater mixture charge into the cylinder, but it also counteracts the drop in air pressure with altitude. In many designs the throttle butterfly is not fully open at low altitude even with the throttle lever fully forward, but an automatic boost control worked by an aneroid capsule progressively opens it as the aircraft climbs. Eventually full-throttle height is reached, above which power starts to fall off. Some engines have two-speed superchargers which then change gear and run at higher rpm to maintain power to a still greater altitude.

To obtain even more boost, two-speed two-stage superchargers were introduced, with an intercooler between the stages. They grew nearly as big as the basic engine itself. Figures for the Griffin engines of the Avro Shackleton give some idea of the boost and weight of mixture fed in by such large superchargers. At take-off power, each engine used 250 bhp just to drive its own supercharger.

Overboosting
In most designs, full supercharger capacity could not be used at low altitude because it would have over-boosted the engine. However, in combat or other emergency, the pilot could often push the throttle lever through a tell-tale wire or gate on the quadrant and demand emergency boost at the risk of wrecking his engine. Never mind the red lines on the gauges! A broken wire would warn the groundcrew to check the engine afterwards. (By the way, Italian aircraft worked the other way. Their throttles were pulled back for full power and pushed forward to idle. This caused some tricky moments when captured examples were flown by Allied test pilots.)

"Infinitely Variable Gearing"
Fixed-pitch airscrews were on their way out for powerful fast aircraft by the end of the Thirties. They could be likened to having only one gear in a car, so that it was impossible to get the right setting for both take-off and high speed. A constant-speed unit (CSU) enables the pilot to select the engine rpm he wants. The CSU maintains this, within its operating range, by adjusting the airscrew blades to coarser or finer pitch as airspeed or throttle setting varies. For maximum power the settings would be fully fine pitch (that is maximum rpm), full throttle, and rich mixture. However, this is not the way to get maximum miles per gallon, as any car driver knows.

Range
To fly as far as possible on a given fuel load depends not upon maximum power, but on maximum efficiency. This is obtained by setting the lowest rpm, highest boost and weakest mixture possible, maybe 20% below the chemically correct ratio. Weak mixture gives less bmep, but more critically causes higher cylinder temperatures. Coupled with high boost, it increases the risk of detonation.. The cylinder head temperature gauge is therefore critically important when flying for range, and if it goes into the red sector, boost must be reduced and rpm increased. Fouling and burning-out of spark plugs is also a problem. British plugs were better than American ones in the early 1940s. B17 Flying Fortresses had their Wright Cyclone engines refitted with British plugs when they arrived in the UK.

The Japanese Navy were experts in range flying. Cruising at 152 mph their Zero fighters could manage 1,130 miles on internal tanks only - about 9 miles per gallon, not bad for a 1,120 hp engine! They could stretch their flight times to ten hours or more by hanging on the prop at 115 mph with the mixture so lean the engine was on the very fringe of cutting.

Britain's Super Fuel
During the 1939-45 war, the British were leaders in supercharger design. At the same time however, they continued to use conventional carburettors for their aero-engines. A carburettor needs volatile petrol because it is sucked in by the airflow and has to vaporize readily. Fortunately the UK had developed a special 100 octane fuel which became available to Fighter Command in March 1940. It enabled the boost of Merlins to be increased from 6.25 lb to 12 lb at 3,000 rpm without detonation, thus producing an extra 300hp compared with running on 87 octane. To tell them apart, the new fuel was coloured green and the lower octane blue.

Since the 1920s, British aviation fuel had included up to 20% benzole, which is an aromatic. American aircraft fuel systems did not like the solvent effect of this. Early 100 octane produced about 1937 for the US Army Air Corps was a blend of ordinary paraffinic petrol and iso-octane. (The iso-octane was obtained by the alkylation process first developed by British petrochemical engineers in 1935 at the Anglo-Iranian Oil Company, now BP. The Americans paid the company 8 million dollars in royalties for using it.) This American 100 octane had only about a 2% aromatic content, and did not give the rich mixture response that would enable 12 lb boost pressures in British aero-engines.

To obtain this, the answer was to replace the ordinary paraffinic component with a highly aromatic petrol distilled from Venezuelan crude oil. Tetra-ethyl lead was added at 3.66 cc per gallon. By 1940 seventy percent of our supplies of this improved 100 octane came from three Esso refineries, two in the USA and one in the Caribbean. Most of the rest was produced by Shell from Borneo crude. The crews of tankers carrying this volatile fuel did not have much chance if they were torpedoed on the way to Britain.

Fuel Injection
Germany and Russia did not have the luxury of ample high grade petrol. The Germans could actually have read about the new British 100 octane in a press article in 1939, although it was supposed to have been kept secret. They did not apparently realise its significance. Their reliance on 87 octane was one of the reasons for them using fuel injection for their aero-engines. (Effectively, a carburettor gives the engine what it asks for, whereas fuel injection gives it what it needs!). The Americans had begun to use continuous injection about 1935. It is only practicable with a supercharger. The fuel demanded by the throttle setting is pumped through an injection nozzle into the eye of the impeller. Here it is atomised and vaporised by both the churning action of the impeller and the heat rise as the air is compressed. A satisfactory combustible mixture can thus be obtained from less volatile petrol.

The Germans went on to develop timed injection to each cylinder, which is more efficient and can work with even lower grade fuels. However, the tiny amount of fuel each cylinder needs every firing stroke is very difficult to meter accurately. In effect the Germans used the best engineering solutions whereas the Allies used the best petrochemical solutions to meet the same problems.

Squeezing the Last Ounce of Power
The Germans eventually produced 96 octane petrol for the Luftwaffe, but evidently not the highly aromatic type. To get extra power for take-off or combat they turned instead to water-methanol injection. This increased the power of the Daimler-Benz engine of the Messerschmitt 109G, for instance, by 22%. It was fairly ruinous to the sparking plugs, apparently. What the water did was to cool the mixture, thus increasing its density, and at the same time reducing the risk of detonation at high boost. (The same effect can be noticed to a lesser degree in the way a car seems to run better in cool rainy weather.) Allied pilots on the tail of a FW 190D could see the trail of steam when the German pilot opened up to full boost. At high altitude the Germans also used nitrous oxide injection which gave a similar percentage increase in power. These injection systems meant extra servicing and supply problems, as well as more to go wrong in the aircraft, but they got that vital extra power.

Having the benefit of aromatic high octane, as described earlier, the British and Americans obtained a similar result by giving the engine a super-rich mixture at full boost. Some of this did not burn but acted as a coolant to stop the temperature in the cylinder rising to detonation point ahead of the regular flame front. This explains double octane figures such as 100/150 Octane, the latter being the super-rich rating. 100/150 fuel enabled Tempest fighters to sprint after the V1 flying bombs in 1944.

Russian Engineering
People make jokes about Lada cars, but they forget the Mig 29 Fulcrum, or that the Russia was first in space. During the war the Germans learned the hard way not to underestimate Russian engineering when the T34 tank appeared. (They even considered copying a captured example and putting it into production themselves, but eventually incorporated its lessons into a new design of their own; the Panther.) Russian aero-engines were technically advanced, but rugged and easy to maintain in order to cope with freezing temperatures and primitive servicing facilities. According to Czech and French pilots who flew with the Soviet Air Force, it was usual to drain off all the coolant if the aircraft was parked for any time. Even a glycol mixture would have frozen. The lubricating oil also had to be heated before the engine could be turned over to start. Legend has it that Russian groundcrew used to light fires in drip-pans under their engines to thaw them.

The Lavochkin LA-5 fighter is an example of Russian aero-engineering. It had a close-cowled 14 cylinder two-row radial with a two-speed supercharger and fuel injection, not unlike the BMW engine of the FW 190. This produced a maximum continuous 1650 hp, and an overboost of 1,850 hp limited to two minutes, all this on about 87 octane . The cylinder head temperature had to be watched very closely as this engine was prone to blowing cylinder heads off. Prolonged cruise at low rpm fouled up the spark plugs and the throttle had to opened briefly every quarter of an hour to burn off the deposits. A superior feature of many Russian fighter aircraft was that exhaust gases were tapped off, cooled and filtered, then fed into the fuel tanks to reduce the danger of explosion in combat.

Merlins in rough company!
The Hurricane Mk. IIs that were sent to Russia were fitted with the Rolls-Royce Merlin XX which gave 1,280 hp for take-off and 1,850 at 21,000 ft. on 100 octane fuel. Although the Hurricane was a strong and easily maintained aircraft, its finely engineered engine must have seemed like a thoroughbred race-horse amongst rugged cavalry chargers, needing a special diet instead of the rough oats eaten by its comrades in arms! (The Merlin was also more awkward to service than the average Russian and German engine. Who described the pre-war Rolls-Royce car as the triumph of workmanship over design?!)

Tin the Answer
The Russians were already aware of research on the use of a tin catalyst to improve combustion, and had good resources of tin. The Soviet regime was so pathologically secretive it was unlikely to have shared this knowledge with the Western Allies. However, having been given a large number of Hurricanes which they could not use in combat until the fuel problem was overcome, they were ready to cooperate with Henry Broquet to see if this catalyst phenomenon might be the answer. He succeeded in producing a practical application that worked, and could be used in the roughest active service conditions. There is some suggestion that the catalyst may have been put into the refuelling bowsers, rather than in devices fitted to the aircraft themselves. The catalytic action apparently breaks up the long chain molecules of the less volatile elements in petrol, improving combustion efficiency and discouraging detonation. In effect it reproduces the benefit of a high aromatic content. Another advantage in bitter Russian temperatures was the reduction of waxing in the fuel lines.

Lead is Dead, Long Live Tin!
Big European and American oil companies saw no need to take up this development. They had invested a lot in tetra-ethyl lead and had no incentive to try something completely different. But lead is to be banned from petrol by the year 2000. Millions of motorists and boat owners with engines designed for 4-Star are going to have problems. True, they can buy expensive "super unleaded" instead, which is supposed to equal 4-Star.  However, this has all sorts of otherdetonation suppressing additives instead of the lead. Some of these additives themselves are not very nice. Furthermore, they still leave the problem that some designs also rely on the lead to protect valve seats from erosion and stop valves from sticking.

Henry Broquet's well-proven tin catalyst is a better answer than tetra-ethyl lead anyway, improving the fuel before it goes into the engine so that combustion is more efficient. This reduces deposits on the cylinder head and valves, and also the amount of waste heat going into the exhaust. As a result exhaust valves and their seats are cooler and do not need a protective coating of lead. Finally, more efficient combustion means less exhaust emissions.

If anyone needs proof that it works, they need look no farther than those hard-pressed Merlins in Russia!

R H Robson, MA, RAF(Ret)

Copyright 1998 R H Robson. All rights reserved.

This article may be freely quoted in personal discussion. It may not be republished in any form for commercial gain without the express permission of the author.