Engine cooling systems

One of the innovative features of the new Conqueror-powered P-6 was in its cooling system. The water coolant of the earlier P-1 series was to be replaced by Prestone, a trade name for an ethylene glycol (HOCH2CH2OH) mixture.

It is not clear if the cooling system was pressurised…..I’m guessing it probably wasn’t.

http://home.att.net/~jbaugher1/p6.html

The P-6 Hawk series resulted from the installation of the new 600 hp Curtiss V-1570 Conqueror liquid-cooled engine in what were essentially P-1C airframes. The variant which is best remembered today is the P-6E, which IMHO was one of the best-looking biplane fighters ever manufactured.

The Curtiss V-1570 Conqueror engine was a evolutionary descendent of the Curtiss D-12 which powered the P-1 Hawk. The direct ancestor of the Conqueror was the unsuccessful Curtiss V-1400 engine which powered the P-2.

The first aircraft to carry the P-6 designation was the fourth P-2(Ser No 25-423), modified to race for the Army in the National Air Races of 1927. It was the first Hawk to be fitted with the new Curtiss V-1570 engine which later became known as Conqueror. Because of the use of the new Conqueror engine, the Army gave the airplane a new designation–XP-6. Stripped of military equipment, it placed second in the unlimited event of the 1927 National Air Races.

For its principal entry in the 1927 National Air Races, the Army ordered that extensive modifications be made to a stock P-1A (Ser No 26-295). It was fitted with a V-1570-1 Conqueror engine and was equipped with a set of PW-8A-type un-tapered wings complete with the skin-mounted radiators. It bore the company designation of Model 34Q. The Army redesignated this aircraft XP-6A No 1 because of its use of the Conqueror engine. It took first place in the 1927 race at a speed of 201 mph. However, the XP-6A crashed during preparations for the 1928 National Air Races.

The success of the Curtiss Conqueror engine in these two racing aircraft led to an Army contract for a service test quantity of 18 P- 6s placed on October 3, 1928. These aircraft were assigned the serial numbers 29-260/273 and 29-363/366. These aircraft were given the company designation Model 34P. The Y-for-service-test designation had just been adopted at this time, but it does not appear to have actually been applied to these planes, although they are sometimes recorded as YP-6s.

One of the innovative features of the new Conqueror-powered P-6 was in its cooling system. The water coolant of the earlier P-1 series was to be replaced by Prestone, a trade name for an ethylene glycol (HOCH2CH2OH) mixture. Prestone was a product of the Union Carbide corporation, and had an advantage of having a very high boiling point and a very low freezing point. By using Prestone instead of water, Curtiss was able to reduce the surface area of its radiators by one third. In addition, since less coolant was needed, the use of Prestone rather than water resulted in the savings of about 50 pounds of weight.

However, since the new Prestone-cooled Conqueror engines were not yet ready, aircraft 39-269/273 and 39-363/366 were delivered in October 1929 with water-cooled V-1570-17 engines as P-6s, so that they could be gotten into service as rapidly as possible. The rest of the aircraft in the order were completed later as P-6As once the Prestone-cooled V-1570-23 Conqueror engines were finally ready.

http://www.sil.si.edu/smithsoniancontributions/AnnalsofFlight/text/SAOF-0007.txt

pg 78:

Finally, at the end of the decade, a belated attempt was undertaken to challenge the air-cooled engine’s position by a new approach toward a still smaller frontal area, coupled to a much-reduced weight of both coolant and radiator by the introduction of high-temperature cooling.
Research on this “hot” cooling can be traced back to McCook Field as early as 1923. The coolant adopted was ethylene-glycol, in America known under the trade name Prestone*. Several Curtiss D-12 and Conqueror engines were converted to Prestone cooling during 1928 and 1929 and submitted to extensive tests, both at the Wright Field laboratory and, later, in the air.

One Curtiss P-1B Hawk was used with a radiator, the size of which had been reduced by 70 percent, and a P-1C Hawk was fitted with wing radiators. During the Cleveland aeronautical races of 1929, Lieutenant Doolittle put one of these fighters through its paces with an unforeseen result. The reduction in radiator size increased the diving speed to such an extent that the plane shed its wings when accelerating during an outside loop and Doolittle had to take to his parachute.

The engineering division at McCook Field had specified a coolant temperature in the radiator of 300° F. As a consequence, the cylinder-head temperature of the D-12 rose from 378° to 508° F and that of the barrel from 187° to 209° F. The aluminum block-cum-steel-barrel construction could not adapt to these temperature rises and the glycol began to seep into the crankcase at the lower joints between the barrel and the jacket.
This difficulty and other troubles that developed were impossible to remedy on the existing design. The result was a difference in opinion between Curtiss and the Army’s engineering division. The Curtiss engineers maintained that the tests showed clearly that the specification for 300° F was exaggerated, as the oil cooler was becoming bigger than the coolant radiator, which was true. To this, the engineering division retorted that the real trouble lay with the seven-year-old block design. This was true also, but the Army went further and expressed the conviction that all monoblock forms of construction were obsolete, and in this the Army would be proved wrong.

* Shortly after Prestone was introduced as an automobile antifreeze a rust inhibitor and a leak sealer were added to its ethylene-glycol base. It then became entirely unsuitable for use in aircraft engine cooling systems because the added chemicals would form a jelly-like substance if the solution became too hot—that is, at temperatures above 250° F. Some D-12 engines were ruined when an overzealous supply officer replaced glycol with Prestone in an emergency.
78

FIGURE 41 .-Curtiss Chieftain, 600 hp, 1927. (Smithsonian photo A-4666.)
Although the application of high-temperature cooling to the D-12 was not conclusive and did nothing to keep that engine in the forefront, the data gathered were to be of great consequence for the future of high-performance fighter engines. It was with glycol also that the term “liquid cooling” came into use; hitherto, all service engines had been cooled by plain water.
The Army would remain adamant for several years in its attitude toward a 300° coolant temperature, though later experience and research would show that much better results could be achieved with the ultimately used temperatures of 250-265° F.

Glycol was used by the American Army but it had to tendency to leak through gaskets and joints. Rolls-Royce experimented with it in the Schneider Trophy S.5 but abandoned it because of the same leakage problems. On 27th November 1931 the Air Ministry authorised the investigation of glycol cooling. Before that the official policy was to concentrate on evaporative cooling. Difficulties with their evaporative cooled Goshawk led RR to experiment with a composite system using both a radiator and condensers. This modified steam evaporative design based on the Goshawk was used on the PV12, later to become the Merlin, The system proved troublesome and after a few hours flight time it was decided to use pure ethylene glycol as the coolant. Just before the outbreak of WWII a pressurised cooling system was introduced to the Merlin. This used a70% water 30% glycol mixture which had proved superior to 100% glycol at atmospheric pressure and was not so prone to leakage.

Glycol was expensive and servicing manuals issued by the AM to the RAF contained instructions to “save at all costs the glycol coolant”.

Unpressurised Ethylene Glycol boils at about 130°C, unpressurised water at 100°C (obviously, but only at sea-level) so was a great advantage before pressurised cooling systems became common. Its anti-freeze and corrosion-inhibiting properties were more of a by-product.

Your engineer friend has a point, water has almost twice the specific heat capacity of Ethylene Glycol (and is more than twice as good at conducting heat); as you add it as anti-freeze (to water) you lower both of these.

However, near-boiling coolant in conditions of changing heat and pressure (and vibration) can cause cavitation that can rapidly destroy cylinder liners.

The Spitfire site referenced says that Ethylene Glycol is more efficient than water but an engineer friend who was looking into cooling a car racing engine told me that for basic cooling water is the best you can get. Something to do with specific heat capacity, but I’m not an engineer. Is he wrong? Is ethylene glycol used purely because of efficiency or is it that it stands altitude and temeperature change more effectively?

Sorry if this is off thread, but I often wondered why an expensive poisonous substance replaced a cheap safe one.

According to Bill Gunston ‘So in 1935 Rolls-Royce followed the US lead and went for ethylene glycol cooling.’

😀 I did say, “After a quick search.”

😮

I think it was the Allison V-1710 of 1931, 2 years before the Merlin. Reference: Bill Gunston’s “The Development of Piston Aero Engines” page 135. The American marketing name for ethylene glycol was “Prestone”.
It seems that it could have been used earlier, just that no-one thought of it!

After a quick search, it appears that the first aero engine manufacturedto use ethylene glycol cooling, was the RR Merlin B engine:

http://www.spitfiresite.com/history/articles/2007/08/rolls-royce-merlin-engine-development.htm

Ethylene glycol was first prepared in 1859.

According to Bill Gunston ‘So in 1935 Rolls-Royce followed the US lead and went for ethylene glycol cooling.’

After a quick search, it appears that the first aero engine manufacturedto use ethylene glycol cooling, was the RR Merlin B engine:

Thus the Merlin B became the first to use ethylene glycol cooling system;

http://www.spitfiresite.com/history/articles/2007/08/rolls-royce-merlin-engine-development.htm

Ethylene glycol was first prepared in 1859.