Naked Science Forum
Non Life Sciences => Technology => Topic started by: JimBob on 04/09/2009 19:46:38
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OK, I realize that the way to do it is to get the gas turbine to run an electrical generator and then an auto. But in the end is this method more energy efficient than an internal combustion engine. I am NOT asking about initial cost - if it is more efficient, cost cutting manufacturing methods will be forthcoming.
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I believe the problem with GTs is the efficiency drops right off in part load conditions. Also small GTs are much less efficient due to increased relative blade tip turbulence.
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I suppose if it was a hybrid vehicle you could only run the GT at an efficient speed, but you have to put a lot of energy into a GT to spin it up, so turning off and on a lot may not be such a good idea either. Don't think there's much could be done about the turbulence problem. Do tolerances also become a problem at smaller scales?
GTs seem to scale up really well. The big ones are quite efficient, but they don't seem to want to scale down. Pity.
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Isn't a gas turbine an internal combustion engine anyway?
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Yes it is. But JimBob is a bit old and decrepit, so I cut him some slack.
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BTW, some turbine enthusiasts add combustors to automobile turbochargers to convert them into gas turbines. Some of the results are quite impressive and bound to impress your neighbors (not to mention being potentially lethal!)
Automobile turbo(super)chargers may provide a clue about efficiency too. They certainly do allow smaller engines to produce a lot more power, so the power to weight ratio will be better than a larger normally aspirated engine of equal output, but I'm not aware of any car companies claiming that turbocharging improves fuel economy.
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I think a lot depends on the size, large gas turbines such as are used on aircraft and power stations and large diesel engines such as those on tankers can exceed over 55% thermal efficiency but small turbines used on cars are much lower.
Acknowlegments to Geezer I agree with all he said on the matter
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I just wanted to make the point how high the efficiency is in large sizes, IC engines are usually quoted as having an efficiency of about 25%.
I recall the liner Queen Elizabeth had its original turbines replaced by German Diesels to reduce fuel consumption so they don't always win even in large sizes.
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Was that the QE2? I thought it was initially a steam turbine. I remember they did have problems with them early on. Something to do with condensed water particles wrecking the turbine blades I think.
But I agree with you. GTs have their limitations, more's the pity.
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Automobile turbo(super)chargers may provide a clue about efficiency too. They certainly do allow smaller engines to produce a lot more power, so the power to weight ratio will be better than a larger normally aspirated engine of equal output, but I'm not aware of any car companies claiming that turbocharging improves fuel economy.
Until today! Ford claims they are able to use higher compression from the turbo. This does improve fuel economy. They are able to do this because they have a sophisticated direct fuel injection system that prevents the pre-ignition problem normally associated with higher compression.
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Ford claims they are able to use higher compression from the turbo. This does improve fuel economy.
Is this a new development?
I was under the impression that turbos always had the capability to make IC engines more efficient. They definitely have been increasing Diesel efficiency for some time. I think the efficiency issue is sometimes clouded in vehicle propulsion due to the difference between engine efficiency and vehicle efficiency - as dragging a bigger more efficient engine around is not always as efficient as a lighter overall package with a lower efficiency engine.
Geezer, can you post a link to the Ford article?
Thanks.
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Hi Peppercorn:
Yes. It's new. I read it in (don't tell anyone - Popular Science) Ford calls it EcoBoost. Should be available on their 2013 models.
In the past, turbos did not improve (thermal) efficiency. They allow a smaller engine to produce a lot more power. They are a means of increasingthe apparent displacement of an engine, but they did nothing for thermal efficiency.
To improve thermal efficiency you have to run at at higher compression ratios, and a turbo certainly makes it easy to do that, but if you increase the effective compression ratio on a gasoline engine, the fuel pre-ignites.
Ford claims to have solved the pre-ignition problem using direct fuel injection. I think the fuel spray cools down hot spots in the combustion chamber and they probably time the delivery of fuel very carefully. It's possibly more like a diesel in some respects. Ford is claiming "hybrid" type fuel economy levels, so this could be a rather significant development.
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Yes. It's new...Ford calls it EcoBoost. Should be available on their 2013 models.
From I've (quickly) found on the web, it looks like Ford are only going to get a 2mpg improvement of vehicles of ~18mpg(US). By 2013! - I despair!
I can see that the technology has real advantages over the older injection techniques, especially when combined with high boost.
I still expect hybrids (allowing peak the engine to operate in its peak efficiency range for much longer) to continue outstrip this slight mod.
BTW - how much horsepower does a high pressure fuel injection system eat up?
The cylinder cooling effect of DI just doesn't make sense to me. What also doesn't make sense is that engines running lean get hotter (even when compared to stoichiometric) - I think it might be to do with a slower flame-front speed relative to rate of expansion.
Anyway, my thought is if the engine is at part-load then the turbo is going to have to be bypassed or the cylinder walls will get too hot - putting the engine back into standard non-boosted configuration for much of the time.
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Yes. It's new...Ford calls it EcoBoost. Should be available on their 2013 models.
From I've (quickly) found on the web, it looks like Ford are only going to get a 2mpg improvement of vehicles of ~18mpg(US). By 2013! - I despair!
I can see that the technology has real advantages over the older injection techniques, especially when combined with high boost.
I still expect hybrids (allowing peak the engine to operate in its peak efficiency range for much longer) to continue outstrip this slight mod.
BTW - how much horsepower does a high pressure fuel injection system eat up?
The cylinder cooling effect of DI just doesn't make sense to me. What also doesn't make sense is that engines running lean get hotter (even when compared to stoichiometric) - I think it might be to do with a slower flame-front speed relative to rate of expansion.
Anyway, my thought is if the engine is at part-load then the turbo is going to have to be bypassed or the cylinder walls will get too hot - putting the engine back into standard non-boosted configuration for much of the time.
2 mpg - That's not much good!
Re power consumed to pump the fuel. I don't know, but probably not too much. Being liquid, the fuel does not compress significantly, so it will not store energy. It's more a case of forcing it into the combustion chamber, and because the volume of fuel is relatively small that should not require too much work.
I think the mixture/temperature relationship is mainly due to the heat that's used in converting the liquid fuel into a gas. Converting liquids into gas consumes a lot of energy. If the mixture is rich, some of the fuel is not oxidised, so it acts as a coolant then gets ejected out the exhaust.
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If the mixture is rich, some of the fuel is not oxidised, so it acts as a coolant then gets ejected out the exhaust.
Yes, but I think it's more than just the that. Even on engines running at stoichiometric ratio (so that all the O2 in the cylinder is used up). This article would seem to support this idea for engines at higher revs:
This lean-burn ability by the necessity of the limits of physics, and the chemistry of combustion as it applies to a current gasoline engine must be limited to light load and lower RPM conditions. A "top" speed cut-off point is required since leaner gasoline fuel mixtures burn slower and for power to be produced combustion must be "complete" by the time the exhaust valve opens.
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If the mixture is rich, some of the fuel is not oxidised, so it acts as a coolant then gets ejected out the exhaust.
Yes, but I think it's more than just that. Even compared to engines running at stoichiometric ratio (so that all the O2 in the cylinder is used up) lean mixture makes the engine hotter. This article would seem to support this idea for engines at higher revs:
This lean-burn ability by the necessity of the limits of physics, and the chemistry of combustion as it applies to a current gasoline engine must be limited to light load and lower RPM conditions. A "top" speed cut-off point is required since leaner gasoline fuel mixtures burn slower and for power to be produced combustion must be "complete" by the time the exhaust valve opens.
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Could be. I suppose leaner will burn slower. If I remember correctly, lean tends to burn the exhaust valves and seats. Is that because the mixture is still burning as it passes through the exhaust valves?
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Yes turbines are more efficient than internal combustion engines. BTW if we are talking about efficiency, maximum efficiency of any machine can be obtained at its full load.