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PostPosted: Thu Nov 16, 2006 00:25 
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This 'fresh air injection system' is interesting. If it was purely for the purposes of fudging an emissions test I'd guess you'd want it introduced into the exhaust as far back as possible, preferably behind the back box, in order to overcome problems with back pressure, noise and lambda sensing. With this in mind, I also seem to remember that the probe they insert is fairly long, perhaps going in as far as the back box, so maybe air would have to be introduced into the box itself.

The thing about water vapour in the atmosphere is that it doesn't only act as a greenhouse gas. As air at the surface is heated it will travel upwards (in a 'standard' atmosphere where air temp decreases as height increases) until the dew point, at which the now-saturated air will form clouds. Clouds are white (on top at any rate) and so reflect solar energy back into space, thus cooling the earth. I would be very sceptical that any amount of human addition to water vapour could overcome this balance. Once the air reaches 100% relative humidity (actual value obviously alters with temp) any additional vapour released will precipitate at once, ie drip from the H-car's exhaust.

Remember, 80% of the world is covered with water, which will evapourate at varying rates at pretty much any temperature above absolute zero. Could our hydrogen cars really add a climate changing factor to that?


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PostPosted: Thu Nov 16, 2006 00:38 
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Years ,and years and possibly more years ago the idea was mooted that water vapour /steam injection was another way of increasing power .
Anyone remember it and the outcome??


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PostPosted: Thu Nov 16, 2006 11:07 
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I think we're confusing two tactics here.

The first is injecting air into the exhaust - like Paul's M5. I have seen this on a few cars - especially ones that needed to meet US Federal emissions requirements a decade or so ago. This clearly has nothing to do wit hthe combustion process taking place in the engine and (I think) is mainly there to help the catalyst to get up to working temperature quickly and maybe to help complete the oxidation of carbon monoxide and unburned hydrocarbons.

The second is exhaust gas recirculation - which takes some of the exhaust gas and makes the engine burn it again - presumably also to try and complete any incomplete combustion that has taken place. This feeds it in BEFORE the combustion chamber.

Neither is (as far as I'm aware) that common on European petrol engines.

In the MOT test, the gas analyser has a sensor (not unlike the car's own lambda sensor) which measures how "rich" or "weak" the engine is running. Feeding air into the exhaust will affect this. This can happen deliberately or accidentally - the latter when there is a hole in the exhaust. This "fools" the gas analyser into thinking the engine is running (?) weaker than it really is so the gas analyser will record a higher lambda reading than is actually coming out of the engine and the car will fail its emissions test when, in fact, the engine is running just fine!

As far as the water vapour injection goes, yes it works. As far as I know it was first used in WWII to increase the engine power of heavily laden bombers on take-off. The idea is that the cool water vapour decreases the volume of the charge going into the engine and therefore improves power because you can fit more of it in each cylinder. This was often cited as a reason why cars seemed to run better on cold, damp days - although in years gone by it was often offset by the havoc it caused with the ignition systems! As far as I know it is still used on some rally cars to this day for the same purpose. It doesn't really work on road cars because of the size of the tank you'd need and / or the complete pain in the backside of topping it up. Added to that, water is pretty heavy stuff so the benefits are at least partly offset by the extra load that needs to be carried.


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PostPosted: Thu Nov 16, 2006 11:21 
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Mole wrote:
SafeSpeed wrote:
How much of the power output comes from expanding nitrogen? 10%? 5%?


I'd have thought the answer to that was "none". The nitrogen expands with the heat created when the petrol and oxygen burn but doesn't actually "provide" and energy into the system at all....

????? Does that make any sense?


I've tried some quick mental estimates, and I reckon it's so small as to be unmeasureable. But I do reckon it's real. The nitrogen must go from ambient to 800 deg C and that does cause expansion or pressure.

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PostPosted: Thu Nov 16, 2006 14:28 
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water injection can help to increase the the charge density as alreedy mentioned, it is also used on high performance cars running very high boost to stop detonation since it runs cooler and burns slower.

The large 2stroke marine diesels are experimenting with water injection just now to reduce the combustion temps and help prevent oxides of nitrogen forming


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PostPosted: Thu Nov 16, 2006 17:35 
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I have seen water injection in use, but not in piston engines - the Pegasus engine used in Harriers has a 50L water tank (one of most structurally complex parts of the airframe) which provides a limited period of boost when water is injected into burning mix which obviously vapourises the water instantly, thus increasing the volume of exhaust gas. This is used when maximum power is required - typically vertical take-off and landing, but shouldn't be used too much as it only last for about 30 seconds before the water is all gone (and subsequently the vertical landing capability).


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PostPosted: Thu Nov 16, 2006 17:38 
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I was under the belief that the water injection on the Harrier was more to do with the fact that, in the hover, there is less airflow to cool the engine and provide ram-air effect.


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PostPosted: Thu Nov 16, 2006 17:52 
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Perhaps in the eariler versions, the later ones had flaps around the outside of the inlets that opened at lower speeds to allow significantly more air in (or rather free hinged flaps that closed when the ram-air effect was sufficient to force them closed). There might be some cooling effect as the tank sits just behind the rear nozzles, thus helping to keep the rear avionics bay shielded from the engine. I am only repeating what I was told when I worked at Kingston-upon-Thames where they designed and built the things.


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PostPosted: Thu Nov 16, 2006 18:55 
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You may well be right, I'm a mere pilot, not an engineer ;)


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