How did piston engines get so powerful

[Redfax]

If you compare the BMW IV engine of the 1920s to the DB 6xx series, you see that power almost tripled, for more or less the same displacement.

What were the main technical improvements that made this possible? Was it just/mainly the supercharger or another main improvemnt which drove the advances, and everything else (beefier crankshaft etc) kind of tagged along for the ride? Or was it really a lot of smaller things none of which stand out?
What's a good book that goes into the technical nitty gritty?

 

[swampyankee]

If you compare the BMW IV engine of the 1920s to the DB 6xx series, you see that power almost tripled, for more or less the same displacement.

What were the main technical improvements that made this possible? Was it just/mainly the supercharger or another main improvemnt which drove the advances, and everything else (beefier crankshaft etc) kind of tagged along for the ride? Or was it really a lot of smaller things none of which stand out?
What's a good book that goes into the technical nitty gritty?

Try Charles Fayette Taylor Aircraft Propulsion: a review of the evolution of aircraft piston engines. https://repository.si.edu/handle/10088/18674

 

[Redfax]

Coole survey! Do you know if there's something covering basically the same ground, but in more detail?

 

[mad_max]

Higher octane fuels provided the means to produce more horsepower and because of this motors had to be strengthened for the higher pressures.

 

[Shortround6]

During the 20s and early 30s the main way of getting more air through the engine was by increasing RPM (the fuel wouldn't allow much higher compression or boost from a supercharger). The forces involved, or acting on things like pistons, rods and bearings go up with the square of the speed. So a 1900 rpm engine has 41% more stress on the reciprocating and rotating parts than a 1600rpm engine and a 2400rpm has 99-100% more stress than a 1700rpm engine.
This assumes that the engines are other wise identical (same bore and stroke, etc).
Increased power through rpm (or boost) also needed better cooling. More heat generated per minute means more heat to be gotten rid of per minute (only a bit over 30% of the heat value of the fuel wound up going to the propeller).
Higher rpm also meant better valves and valve springs were needed.
In the early/mid 30s 87 octane fuel came along and would support a higher level of boost than the older 70-77 octane fuel and a significant increase in power could be had by supercharging (above the 1-3lbs used up until then which was more intended to get good mixture distribution that actually increase power much, the engines power was limited by the leanist mixture in the hottest cylinder).
Better fuels allowed for much higher boost to be used. And it was easier. Increasing the pressure in the cylinders increased the loads/stress in a linear fashion. A 15% increase in cylinder pressure (not necessarily boost but close) increased loads/stress by 15% compared to the squared increase of loads/stress due to RPM.
For instance a DB 60? engine running at 2800rpm has 36% more stress than the same engine running at 2400rpm (assuming equal pressure in the cylinders).
An engine relying of better fuel/increased boost could go from 48in of MAP to about 65in of Map for the same increase in stress. Very rough quick calculation.

Please note that for air cooled radials, some of them more than tripled the sq in of fins on each cylinder/and head in order to handle the higher power so you needed new casting/forging/fabrication techniques to manufacture the parts (the same goes for most every other part in the engine, new alloys, new heat treatment etc).

 

[greybeard]

What's a good book that goes into the technical nitty gritty?

A good book, easy but containing all what needed to understand engine evolution is Dall'Elica al Getto, but it is in Italian.

About main reasons of specific power improvement, I would say fuel and materials.

Fuel because the progressive improvement in octane number (later performance number, when it went above the 100) allowed increasing of both compression ratio (improving efficiency and power) and charge density (by the supercharger).

Materials because they allowed to reach higher RPM, producing more power. Indeed, early engines at least up to WWI were mainly limited by RPM permitted by material resistance: if you give a glance to power curves of engines of that era, curiously you'll notice how they're straight lines, without an "hook" at their apex like modern engines. This means that they could have developed more power, since still "breathing well", if only the materials they were made of could have sustained the stress deriving from RPM's higher of the typical 1600 of that era.

 

[YGBSM]

As has been said earlier in this thread, a lot of advancements in aviation fuel were made in the decade prior to WW2. There were a lot of parallel developments, but from an American perspective, Jimmy Doolittle did a lot of good work on this front. It's no coincidence that he became an executive of Shell Oil after the war.

It's also a big part of the reason why the US was able to rapidly develop "next generation" aircraft after entering the war. Any aircraft is built upon its powerplant, and the foundation for significant horsepower increases was "primed" by the advances made in the 1930s.

As a small example comparison of radial engines in the Pacific, the primary Japanese naval fighter, the A6M 'Zero', had its horsepower pushed from the mid 900s to over 1,100 (a ~16% increase) in roughly the same time frame as the 1,200hp F4F Wildcat was replaced by the 2,000 F6F Hellcat (a ~66% increase). The Double Wasp engine enabled the creation of the Hellcat, Corsair, and Thunderbolt - all potent fighters. When the Hellcat first entered combat, several Japanese pilots made the fatal error of mistaking it for the relatively under-powered Wildcat and had rude awakenings as a result.

 

[PWR4360-59B]

Like others say the Fuel, boosting, ADI, adding more and larger cylinders, better construction techniques, and materials, better understanding of port flow and such. All light weight engines are a trade off in design and more so with aircraft engines. Aircraft engines are lacking in many areas, to correct those areas on the old WW2 engines would mean adding lots of weight, and that is not synonymous with the word aircraft.