[peppercorn (OP)]

http://en.wikipedia.org/wiki/Hydrogen_fuel_enhancement
Note the article implies a small amount of H2 added for major lean-burn gains, so no great tanks of water needed!

The temperature needed for water dissociation in the presence of a catalyst is around 1200degC.

ICE exhaust temperatures are maybe half this, but with the use of a heat pump these temperatures could be sustained. Obviously the heat pump incurs some power cost, but would it be that great?

http://adsabs.harvard.edu/abs/1981STIN...8211405P
supports the concept that these temps can be reached by heat pumps.


Please, think of this as a purely theoretical question if the concept of 1200C in your auto is too upsetting!  Just, could it be achievable?

[Bored chemist]

You probably could, but I suspect it would be more efficient to split water electrolytically.
The gains are probably not worth it. In particular, the water might split at 1200C, but how would you separate the two products (H2 and O2)?

[peppercorn (OP)]

You probably could, but I suspect it would be more efficient to split water electrolytically.
The gains are probably not worth it. In particular, the water might split at 1200C, but how would you separate the two products (H2 and O2)?

Even high temp electrolysis is only around 50% efficient, plus electricity isn't a 'throw-away' by-product like heat.

I believe there is a number of ways of separating the two products. One I read about uses a type of membrane that utilises the higher enthalpy of hydrogen than O2 to cause separation (although I can't locate the article now).

Heat pumps seem to offer the ability to turn large quantities of low-value heat into a smaller amount at a useful temperature for all sorts of applications.
Saying that, if it were that simple we'd use them instead of cooling towers on power stations, would we not?

[syhprum]

Although 600°C heat might well a 'throw away product' a heat pump with an output at 1200°C would be quite an engineering problem!.
By far the best way to produce a small amount of Hydrogen would be electrolytically via a KER,s system (Kinetic energy recovery system).
You are talking about perpetual motion machines here !! heat pumps cannot be used to increase the efficiency of the Carnot cycle!.
Power stations use cooling towers because evaporating water with the draft generated by the cooling tower structure is the cheapest way of lowering the temperature of the exhaust steam.

[peppercorn (OP)]

You are talking about perpetual motion machines here !! heat pumps cannot be used to increase the efficiency of the Carnot cycle!.

Is this a perpetual motion machine then?
Charge air chiller (thermally activated refrigeration cycle powered by waste heat)
The Carnot cycle is an theoretical principle, unrealisable in the real world. As a result 'bolt-on' external heat cycles can, in practice, increase system efficiencies.

By far the best way to produce a small amount of Hydrogen would be electrolytically via a KER,s system.

Why KERs? Genuinely!  Is there some great advantage to doing it this way? Maybe because it's also throw-away, yes?  Also complexed, though.

As I have said, even if straight electrolysis is more efficient (not sure it is, but I'll admit it's simpler) there isn't lots of throw-away electricity floating about in the average car engine.

[syhprum]

Reducing the temperature of the incoming air to an IC engine will certainly increase both power output and thermal efficiency and as you say this can be done with a waste heat refrigeration system.
Are there not simpler ways to utilise some of this waste heat ?, BMW have on an experimental basis used both thermo electric generators (fine for making Hydrogen) and steam engines, no doubt Stirling engines would also do the job.
All these schemes require radiators to provide a cold sink that may well have a detrimental effect on vehicle efficiency, the real beauty of an IC engine is the very high combustion temperature 2000°K+ that allows good efficiency with an exhaust temperature of 800°K   

[peppercorn (OP)]

Are there not simpler ways to utilise some of this waste heat?

Figures for BMW's steam engine seem pretty rubbish for the complexity added.
I like the thermo-couple idea in principle, but this is still a tech in its infancy. Not much power is available.
Stirling engines have a relatively low power density for the weight don't they?

IC engine [has a] very high combustion temperature 2000°K+ that allows good efficiency with an exhaust temperature of 800°K   

Your figure of 2000°K confuses me slightly.  Firstly that would have to be peak temp (or the cylinder would be molten) and I'm fairly sure the mean temp at ignition defines the work available.  Second temps above 1500degC lead to NOx formation so this is to be avoided.

[syhprum]

My figures a probably a little out of date no doubt they keep below 1800°K to reduce NO2 today, this is the temperature of the burning fuel which rapidly drops as the piston moves down, measures are taken to keep the cylinder walls and valves relatively cool.
The efficiency of any heat engine depends on the difference in temperature of the working fluid between when it starts and when it is exhausted.
This is why the expansion ratio (commonly misnamed as the compression ratio) is made as high as the fuel will stand.
Large Diesel engines on ships that have lots of lovely cool seawater for cooling and can achieve thermal efficiencies of 55%.