Intake air

[ed knesl]

Combustion engines do not like hot intake air, I assume pulse jets
have the same problem.

Does anybody have diferent opinion ?

Is there any rule for minimum temperature differential between intake and exhaust air ?

[Bruno Ogorelec]

This is not true, strictly speaking. Old car engines always started much more easily in winter if they could draw warm air, rather than cold. Stationary gas turbines use heat exchangers to pre-heat air. What is true is that the absolute power output drops because the fresh charge is less dense if it is hot. It is also true that there must be a minimum gradient between the charge and the effluent for the thing to work. I have no idea what that spread is.

[Ray]

Well, as I see it, cool air is more dense, giving more power to a car engine they are just air pumps after all, the more air you give them, the more powerful they are. But cool dense air works at cross purposes in an engine. It makes it harder to vaporize the fuel. Liquid fuel doesn't burn in a combustion chamber, so to vaporize the fuel you want the air hot...

The reason that the old engine wouldn't start well when cold has to do with the liquid fuel that got to the combustion chamber. Until the engine heated up and was able to vaporize the droplets in the intake and combustion chamber it needed more fuel, or it would run lean and rough. They solved the problem with the choke, it caused more fuel to go into the engine and allowed the mixture to be closer to what the engine needed to run. Once the engine warmed up, the fuel would vaporize on the hot intake tract and combustion chamber and the added fuel wasn't needed to make a good mixture.

So hot air helps an engine start, cool, dense air makes more power. I don't think that a pulse jet needs hot air very much, pby only at the beginning of a run if at all, and I believe it would benefit from cool, dense air once its up to temp.

It would be interesting to measure the thrust of a pulsejet when its very cold outside, and when it is warm. Much like a car engine they are just air pumps, you put more mass in, you should get more thrust....Right?

[hinote]

Combustion engines do not like hot intake air, I assume pulse jets
have the same problem.

Does anybody have diferent opinion ?

Please see this link (it's the only one I could find with a reasonable amount of info):

http://www.schou.dk/hvce/

I recalled Smokey Yunick's "adiabatic engine" from the '80's. It ingested very high-temp air, instead of just dumping all the excess heat generated by the cycle but not used.

I think we're talking about two different issues here: feeding dense (cold) air to an engine increases its specific output--but not its efficiency.

Bill H.
Acoustic Propulsion Concepts

".......some day soon we'll be flying airplanes powered by pulsejets."

[Ray]

I recalled Smokey Yunick's "adiabatic engine" from the '80's. It ingested very high-temp air, instead of just dumping all the excess heat generated by the cycle but not used.

That's an interesting link, after reading through it I can understand why the engine made as much power as it did, and was as efficient as is it...

Smokey started with cool dense air...he heated it to vaporize the fuel, but allowed it to build pressure using a turbo as a "check valve" to prevent the hotter expanding air from preventing the further ingestion of air. This makes sense, you are prepping the engine to consume more air, with better vaporized fuel. The vaporization of the fuel lead to the better efficiency, the additional charge of air allows for better power.

If you can control pre-ignition, apparently Smokey could, the higher you raise the temperature of the air, the more it expands. If you start with a given volume of air, and you heat it to double the temperature you end up with 4 times the volume (can't remember gas law at the moment, is this thr right number?), if you don't constrain it.

[ed knesl]

Smokey's results are quite telling - this concept is worth of following.

Besides that, valveless pulse jet is an open pipe and is therefore selfregulating for the intake charge . Won't fire until the CC is
ready. Hot air may perhaps slow its frequency, but it could produce
bigger bang, because of better fuel atomizing and quicker exapansion.

I will do some more thinking about the whole issue. There is no lack of
free heat around these engines.

[mk]

Pre-heated air makes staring of (cool) pulsejets definitely more easy, getting closer to the proper acoustic length over the whole engine. I even got the patent paper Lockwood engine started with a low-pressure hot air gun.

I discovered this by using a hot air gun instead of a compressor some time ago. Just until theheater got burned. OK, it was a low-cost tool only.

Now it's a low pressure cold air source. Works quite nice for starting the "K-PT 04" series engines, even without a spark plug. I simply nozzled down the outlet port area and improved the self-cooling high-RPM 15V DC motor; which is running on 18-20V DC now.

Once heated up, IMHO, you'd want cool air entering the engine, just as for piston engines, to improve internal cooling and the power output. Note: I'm also considering special heating limits, set by flame temperature and case melting temperature.

[Bruno Ogorelec]

Bill is right -- we have to separate the issue of intake temperature from intake density. What is important is whether the same charge density will produce better results if the initial temperature is higher or not.

My instinct tells me that more energy will be liberated at the higher temperature. That is why I keep harping on the issue of pre-heat. Let's use some of the heat we are now throwing away as waste. Let's live with lower charge density. Let's compensate for it by using bigger chamber volume and port sections. They cost nothing.

However, I do not really know anything about the limits to this. How closely can the initial charge temperature approach the combustion temperature (which is the obvious ultimate ceiling) before the combustion cycle breaks down? Does anyone have an idea?

[larry cottrill]

If you start with a given volume of air, and you heat it to double the temperature you end up with 4 times the volume (can't remember gas law at the moment, is this thr right number?), if you don't constrain it.

Ray -

I don't know whose law it is anymore, either [Boyle's? Charles's? ...] but it is simpler than that, merely a linear function if everything but the temp stays unmodified. Doubling the absolute temperature doubles the volume if the gas is allowed to expand without resistance [i.e. there is no change in pressure].

Interestingly, the temperature represents the average [mean] mv^2 where m = the molecular mass and v = molecular velocity relative to the measuring device with which the molecules collide.

L Cottrill

[mk]

I don't know whose law it is anymore, either [Boyle's? Charles's? ...] but it is simpler than that, merely a linear function if everything but the temp stays unmodified. Doubling the absolute temperature doubles the volume if the gas is allowed to expand without resistance [i.e. there is no change in pressure].

Exactly.

The gradient of the linear function depends on the gas molecules. Thus every uniform gas has its spezial linear graph, wich only has to be cut out -- going towards 0K now -- at the boiling temperature. When simply lengthening the graphs each with its gradient in a linear way, they do intersect at the praktikally not reachable 0K point, thus being the intersection point with the x-axis.

Interestingly, the temperature represents the average [mean] mv^2 where m = the molecular mass and v = molecular velocity relative to the measuring device with which the molecules collide.

E.g. the walls of the casing.

[Mike Everman]

A practical consequence of warming the intake air, good or bad, is that the intake must get longer to account for the increase in the speed of sound of the warmer air, I'm thinkin'.

[ed knesl]

Bruno,

I don't think we have to concerned much with the intake air reaching
combustion/discharge temp, since ambient air is lot, lot cooler and would
take certain time to heat it up to the max treshold, also no compession is
present at that part of the cycle. There is going to be always some drop,
we just don't know how much.

I agree that larger CC and ports can compensate of any loss of air molecules due to higher charging temperatures.

Besides that - " Bigger is better ".

I guess we need to test it.