A couple of doubts...please help

[dhiru]

Hi, I am a guy from India and very eager to see turbine based engines getting into automobile industry. I started on a project of building a small turboshaft engine to be used in motorcycles...but stopped in the midway because of a couple of doubts eating my head. Maybe you experts can help me. Here they are:
1. Do all turbine based engines like turbojets, turboshafts and turboprops etc have poor efficiencies at low RPMs, or is it just the turbojet? I know they operate at their maximum efficiency at their design point, but why the efficiency gets poor at low RPMs?
2. In almost all the turboshaft engines I have known, there are one, or at max 2 free turbines which convert exhaust's velocity to mechanical power. Why there are not more than 2 free turbines like 3 or 4? Will it not help to recover more of gas's energy and add to power of of the engine? Is it due to mass, or there is some additional factor?

I stopped because of these because I want to make the engine economical. Maybe your solutions and suggestions may give me some ideas how to deal with them. Please help.

[racket]

Hi Dhiru

Theres no way to make an automotive gas turbine both economical and simple .

Gas turbines at low rpm have poor efficiencies because the airflow rate doesn't reduce by a large amount and temperatures need to stay above a certain minimum just to keep the thing spinning around, we end up heating a lot of air with very little work output ..

Theres generally no need for more than a couple of freepower wheels because theres not enough pressure left over from the gas producer turbines to warrant any more, theres only limited pressure available for expansion ............the addition of extra turbines would add complication and expense for probably no , or very little, power gain , about the only "benefit" would be reduced rpm/blade speed from the smaller pressure drops/gas speeds across each stage .

Just about all the large car makers tried getting turbines on the road, and all failed ...............turbines are simply not suitable , they're strength lies in producing "full throttle" power , not part throttle power ...............why use a thoroughbred race horse to pull a cart :-)

Cheers
John

[ashpowers]

Probably the best thing to do would be to use a turbine with freepower to turn a generator and have a battery bank that the turbine charges. The main propulsor would be electric drive motor(s) to make the vehicle move.

That way, the turbine will only be used when batteries need charging and it would run up to its most efficient output to charge the batteries. Then the battle of maximum efficiency ensues and development time/costs go to the moon.

Perhaps coupling the gas gen with a boiler to run a steam engine to also turn a generator would help to recover energy from the hot exhaust gases after passing through your freepower stage(s).

And then complexity starts to kick in.....

Look into magnetohydrodynamic generators. They are incredibly efficient and virtually no moving parts. Not exactly simple devices, but vastly less mechanically complicated than a steam engine. Waste heat from the gas producer could be employed to drive such a device.