Hi Flyboy,
>wotan, can you explain what is GM-1 system\fuel\thingi and how did it work?
GM-1 was the German WW2 code-name for N2O (nitrous oxide, or laughing gas).
N2O can be added to the fuel-air mixture of an internal combustion engine. In the compression cycle, it's broken up into its components, releasing oxygen that can be used for combustion.
As the challenge of high-altitude flight mainly consists of getting enough oxygen into the cylinder to maintain adequate power - which due to the decreasing air density becomes more and more difficult at altitude - N2O equates to extra power.
In fact, N2O was injected into the supercharger in liquid form through small jets, and each jet gave a constant power boost when employed. Depending on the type, you might have a 120 HP jet and a 240 HP jet, which of course added 360 HP when used in combination.
That's the resason N2O was available in steps - you couldn't add 360 HP to an engine that was already running at close to full power, or the resulting forces would destroy vital parts of the engine.
This is the main reason N2O was preferred over pure oxygen, which could be (and experimentall, was) used for the same purpose - it just gave too much power.
The nitrogen share of the nitrous oxide has a benefit, too, since it absorbs some energy on being broken up in the cylinder, controlling the detonation and allowing higher pressure.
Since N2O is injected into the supercharger as a liquid, it also gives a charge cooling effect on evaporation (cooler air means more oxygen in the same volume).
Initially, N2O was stored under high pressure to keep it liquid (laughing gas, after all, is a gas under standard conditions), but that meant the N2O vessels blew up like a bomb on being hit, so from 1941/42 on it was stored at very low temperature in an insulated tank that kept the content at less than -90 °C for as long as the sortie lasted.
(It was used by bombers like the Ju 88 and by reconnaissance planes like the Ju 86 as well, so that could be quite a long time. For fighters at readiness on the ground in the hot summer sun, though, the insulation would not have sufficed and the N2O would have begun to boil out through the safety valve after a while.)
For comparison: 0.1 kg/s of N2O injection gave extra 300 - 400 HP, virtually out of nothing.
The only drawbacks were the weight of the system (which also included compressed air bottles to force the N2O out of the insulated tank), and - more importantly - the high rate of consumption. If 0.1 kg/s gave 350 HP, that made for a specific fuel consumption of 1000 g/HPh, which compares very unfavourably to the DB601A's normal 220 g/HPh at high power :-)
So, N2O was bad for range, but great for high-altitude power.
Oh, by the way, someone mentioned that N2O was to be used for short bursts only. According to what I have read, it could be used as long as it was available and in fact short bursts were to be avoided as filling and emptying the N2O lines took some time and created some engine management difficulties.
Regards,
Henning (HoHun)
