Saturday, September 11, 2010

Efficiency

I'd really like to increase efficiency. More work for less fuel. And less money. More for less. But you can only do so much.
Physicists have proof that there is a maximum level of efficiency possible, due to the laws of thermodynamics and entropy. Any more and you're getting a free lunch, as prohibited by the rules. You will have less than 100% efficiency, and you will have entropy, and there's just no getting around that.
For car engines, for instance, the French physicist Carnot proved about 180 years ago that the most efficient possible engine using a combustable fuel would depend on the temperature (Thanks wikipedia):

"Tc" being the temperature, expressed in the absolute "Kelvin" scale, of the cold area around the engine, and "Th" being the temperature of the exploding gasoline in the engine, again in Kelvins. Siince most of us don't operate cars in absolute zero, or even Arctic conditions, efficiency isn't very good. We're also limited by the heat tolerence of the engine. Make it too hot and it just kind of melts. Or malfunctions in some other way. You can expect maybe 20% efficiency.
Of course, one way around this is to not use a heat cycle in your engine, using some other means to generate the force. Like electricity. Electrical engines are 90% efficient. But we don't use them because we can't store it well enough. Cars use huge amounts of energy, and electrical cars as exist now have tons and tons of batteries and still have a really tiny range before the batteries are all depleted.
This gives me an idea, I'll discuss it tomorrow.

6 comments:

TCG said...

Have you ever considered the 6 stroke engine?

http://en.wikipedia.org/wiki/Six-stroke_engine

It injects water into the cylinder for an extra cycle which turns the water to steam and thus utilises the heat in a limited manner.

Same with this one.

http://en.wikipedia.org/wiki/Crower_six_stroke

Where water is sprayed into the cylinder, it claims to give a 40% reduction in fuel consumption...

Though I suppose you'd have to carry a tank of water (which is heavy) in equal proportions to petrol though unless you could come how condense it. Since you directly water cool it you can use insane compressions, like the kinds on motorbikes which feel like they are going to explode of greater than 14:1. And even leaner burn mixtures (which again increases the temp of the cylinder which is why running lean burns holes in yer pistons)

Useful for British fuel prices @ £1.15 a litre. Thats $1.78 for you yanks.

Mad Engineering said...

It's a very clever idea, but at the end of the day, it's still a heat engine, and still subject to the limits of heat engines. It cannot be more efficient than a Carnot Cycle.

TCG said...

What about this then?

http://www.technologyreview.com/energy/24701/?a=f

Mad Engineering said...

They don't seem to explain where you'd get the heat and pressure to bring the gasoline to a supercritical state. Both would require energy.

TCG said...

Supercharger? The old kind which scavenged power from exhaust gases to spin up a blower at the front. Since it is merely a heat transfer you could stick the exhaust through a scavenging system dumping it into the fuel tank....

Kind o like how the F-22 has this ability.... though dumping heat into a fuel tank sounds kind of dangerous!

Mad Engineering said...

I suppose you could wrap the exhaust pipes around the fuel line to provide heat, and pressurize with a supercharger-like device. Though you really don't want the fuel to explode before it reaches the combustion chamber.
In practice, this would massively raise "Th", thus improving efficiency to Carnot-engine like levels. Nice.

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