Originally posted by: Captain Howdy
Really even a gas turbine engine is quite a bit more efficient than typical reciprocating piston engines. Rotary engines get a bit closer, but still can't get close to the max 90% efficiency of a gas turbine used with cogeneration.
Obviously because of the complexities and the fact that gas turbines really don't like to constantly adjust output power, they will not see use in a normal passenger car.
Even if they could make them smaller there would be the problem with the inability to change output power, so you would need one heck of a hydrostatic system for that, even thinking about using a normal disc clutch with that would be hilarious.
I thought rotary engines were even less efficient, they just had a great power-to-weight ratio (like 2 strokes).
To realistically put a gas turbine into a car, it wouldn't be direct drive, because of the problems you mention. It would be a turbine hybrid, with a gas turbine to generate electricity, and a bank of batteries and an electric motor to provide the varying output that vehicles require.
Originally posted by: Mark R
No way. Gas turbines are terribly inefficient, unless operated in some kind of combined cycle system where the waste heat is used for steam/vapour production.
While combined cycle turbines are used for power generation, simple gas turbines are only used for emergency or extreme peak power, or for cogeneration. Gas turbines only advantage are low cost.
Natural gas tankers were originally built with gas turbine engines, which could run off the vaporizing gas - but this has been abandoned because the terrible efficiency of the turbines made the ships too expensive to operate. More modern, or refitted, tankers use reciprocating piston diesel engines, which have nearly double the thermal efficiency.
Same is true for small scale power production from landfill or biogas - reciprocating piston engines are preferred, unless for cogeneration with an excess heat requirement.
What are you talking about? That's completely backwards. First of all, gas turbines are much MORE expensive than piston engines, due to the engineering materials and tolerances required to make a whole bunch of high-temperature, high-speed, turbine fins.
Second, they're MORE efficient, because they can operate at higher temperatures, have fewer shocks and vibrational losses, and since they operate steady-state (not cyclical), you don't have to overbuild them as much, and cooling is easier and more predictable.
There's a reason why power plants use steam (or hydroelectric) turbines, not piston engines. They're better, but more expensive. And do you think that the turboprop would have ever been invented if it didn't provide any benefits over the tried-and-true piston engines of the previous decades?
Originally posted by: highwire
That is wrong. What is most directly to to point of this is his law that states the maximum fraction of work that can be derived from heat energy depends on the temp at which heat is introduced and how much that heat can fall while producing work before being rejected.
it is: (Tsource-Tsink) /Tsource
Turbos on diesels, high pressure ratios on fanjet engines help that fraction, but there is no way around it.
An example of Carnot applied: use the heat from the engines cooling jacket to get more power for free: (380°K-320°K)/380°K = 60/380= less than 16% of cooling jacket heat can be converted to power, as a realistic target, maybe 8%. So, with the additional apparatus required for this, half the size of the car itself, a couple more horsepower could be produced. Have I got some investors out there?
The point is this, low temp heat is junk heat .
^--I would have posted it if you didn't, thanks.
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