Naked Science Forum
Non Life Sciences => Technology => Topic started by: peppercorn on 28/09/2010 13:36:14
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An idea initially for non-supercharged engines where two sequential turbines in the exhaust stream are designed to be coupled to a permanent-magnet electric generator.
In exhaust and turbo design, there are problems with running the exhaust ports too hot for the thermal stress it puts on the valves, the turbo itself and the increase in ignition temps in-cylinder that can lead to NOx formation and detonation.
Wikipedia states: "Reheat is most often used to improve the specific power (per throughput of air) and is usually associated with a reduction in efficiency"
http://en.wikipedia.org/wiki/Brayton_cycle#Methods_to_improve_efficiency
- However, this efficiency expressed above doesn't take into account weight, which on a vehicle has its own potential efficiency impacts. Also airflow is inherently more constricted in a piston engine than a gas turbine.
The operation of the first 'turbo' would be as normal (except driving a generator), but with a highly insulated manifold (header) and a reheat return on the low-pressure side of turbo1 that runs cooler (but still fast moving) air back through a heat exchanger as close a possible to the exhaust ports, before passing the reheated gases through a second turbine (turbo2).
The water channels of the cooling system would be best reconfigured in the area surrounding the exhaust ports to expose as much reheat exchanger surface to the valve walls, etc.
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Assuming the weight is not too much of a penalty, isn't the big difference between the motor vehicle and the power station the dynamic power/torque output requirements of a vehicle?
I suppose you could eliminate that issue if you just use the engine to drive a generator and include a battery of sorts, so that the engine is either running at optimal efficiency or turned off. Not sure the consumers would be too happy about that, but if fuel is sufficiently expensive, they might have to put up with it.
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Assuming the weight is not too much of a penalty, isn't the big difference between the motor vehicle and the power station the dynamic power/torque output requirements of a vehicle?
Let's assume for the moment that the generator's power controller will sort out the varying conditions available under dynamic engine loads and revs. The ideal for this will be that the exhaust back pressure will not act as a brake on the engine airflow (we might employ variable geometry turbos as well).
I suppose you could eliminate that issue if you just use the engine to drive a generator and include a battery of sorts, so that the engine is either running at optimal efficiency or turned off. Not sure the consumers would be too happy about that, but if fuel is sufficiently expensive, they might have to put up with it.
We might expect the engine to work in some form of 'mild' hybrid mode - given a helping hand via the averaging work from a battery pack/e-motor from time to time. Weight penalty of extra equipment (2 turbos+small generator, extra batteries, flywheel motor/gen) is a negative at constant motorway speeds, but would (I would hope) 'pay for itself' on the combined cycle.
The primary question as I see it is can the advantages of reheated gas offer a more averagely efficient than a single well tuned turbo?
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You can save some weight if you dump the mechanical transmission and make the drive all electric (like the Volt).
The primary question as I see it is can the advantages of reheated gas offer a more averagely efficient than a single well tuned turbo?
I don't think the power generating folks would do it if it didn't, but I am concerned that the dynamic load problem will eat into the gains too much if you allow it to.
BTW, do both the turbos drive generators, or is one of them used for boost?
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You can save some weight if you dump the mechanical transmission and make the drive all electric (like the Volt).
For a ground-up development I agree that series-hybrid are perhaps a better route than a 'bolt-on enough gadgets to a piston engine to make it more efficient' approach [:D] Personally, I hope to see more along the Capstone turbine direction...
I think there's still room for what are perceived as 'mild' hybrids (including 'aftermarket' gas-saving mods) and it's in this direction I'm considering, now.
The primary question as I see it is can the advantages of reheated gas offer a more averagely efficient than a single well tuned turbo?
I don't think the power generating folks would do it if it didn't, but I am concerned that the dynamic load problem will eat into the gains too much if you allow it to.
BTW, do both the turbos drive generators, or is one of them used for boost?
I think one concern for power stations can be that traditionally reheating was a means to avoid extremely high temperatures in the HP turbine which could be potentially catastrophic for the turbine blades. Engineering logic would suppose that getting hold of power in one stage rather than two or more is almost always the best bet, but materials are limited in what temp they can stand.
Saying that gas laws are funny things and, like the advantages in intercooling staged compressors, the ability to turn heat into work at the turbine end is a complex beastie, yes?
W.r.t whether the LP turbo would be used for creating some boost or not, I would say 'I don't know' - my gut feeling is a little boost is often very advantageous, just to overcome the inherent 'drag' of the intake path. But if I were setting up a test-bed I would invest in a independently powered supercharger that could be varied to give as much flexibility as possible.
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A bigger issue would be fitting it all in the limited space available without the losses associated with all the losses in the bends and joints required exceeding the increased power output, and the added mass negating the increase as well. Using a small naturally aspirated diesel is probably better, look at http://jalopnik.com/5628752/from-canada-to-mexico-on-one-tank-of-diesel for an example ( thanks to Car Talk).