Maximum efficiency of an engine
Discussion
Hi all,
I hope I've done this correctly. Love the site and wanted to ask something of the technical elves that must haunt Pistonheads Towers!
Anyway, on with the question:
What would be the maximum fuel economy of a car without any hybrid technology? To qualify, assuming a 1.5 ton vehicle and zero drag etc.. What is the theoretical maximum economy you can get from 1 gallon of petrol? I assume there's some kind of thermodynamic law whereby there is a maximum amount of energy contained within a certain amount of fuel applies?
I therefore think that with the laws of diminishing returns and the aforementioned drag, weight, heat, sound, friction, gearing(?) we can work out how far along the development path of the internal combustion engine we are.
Apologies if the above is a bit of a ramble but this isn't my day job.
I hope I've done this correctly. Love the site and wanted to ask something of the technical elves that must haunt Pistonheads Towers!
Anyway, on with the question:
What would be the maximum fuel economy of a car without any hybrid technology? To qualify, assuming a 1.5 ton vehicle and zero drag etc.. What is the theoretical maximum economy you can get from 1 gallon of petrol? I assume there's some kind of thermodynamic law whereby there is a maximum amount of energy contained within a certain amount of fuel applies?
I therefore think that with the laws of diminishing returns and the aforementioned drag, weight, heat, sound, friction, gearing(?) we can work out how far along the development path of the internal combustion engine we are.
Apologies if the above is a bit of a ramble but this isn't my day job.
I litre of petrol contains the equivalent of 8.6 kilowatt hours of energy.
I had a 250cc petrol Vespa scooter which produced about 22bhp and did 80mpg with a modern 4 valve 4 stroke fuel injected engine. I have replaced it with a 27bhp electric scooter which does the equivalent of 380mpg in terms of miles travelled per kilowatt hour. So there's some room for improvement yet with internal combustion, but I think reciprocating piston engines are going to struggle to improve past 40% efficiency.
I had a 250cc petrol Vespa scooter which produced about 22bhp and did 80mpg with a modern 4 valve 4 stroke fuel injected engine. I have replaced it with a 27bhp electric scooter which does the equivalent of 380mpg in terms of miles travelled per kilowatt hour. So there's some room for improvement yet with internal combustion, but I think reciprocating piston engines are going to struggle to improve past 40% efficiency.
Zero drag, no hybrid system? In that case your economy depends on how far you go... but yes a gallon of petrol has a certain amount of energy, for a typical-ish (petrol) engine maximum theoretical efficiency is about 40%, higher for diesels... the way to increase maximum thermodynamic efficiency is to increase the compression ratio (how much the air is squashed), but this is difficult on a petrol engine as high compression ratios make the fuel burn too early and too quickly. Diesels tolerate, even need, higher compression ratios so are more efficient, but you still have friction etc to consider.
For the record, real cars have been built that have done thousands of miles to the gallon, but they seat one person and are driven round and round a bowl at 15mph... not very practical, but shows what is theoretically possible
For the record, real cars have been built that have done thousands of miles to the gallon, but they seat one person and are driven round and round a bowl at 15mph... not very practical, but shows what is theoretically possible
^Yes a flippant answer, but you can't start a sensible discussion on economy of efficiency and start with the assumtion of 'zero drag' it's rediculous.
PS - have you heard of the spherical cows?
http://en.wikipedia.org/wiki/Spherical_cow
PS - have you heard of the spherical cows?
http://en.wikipedia.org/wiki/Spherical_cow
thinfourth2 said:
I'm sure modern supertanker engines are pushing 52% including waste heat recovery
Yep thats pretty damn amazing! Otto cycles (gasoline engines) are more like 40%. The massive size of diesel ship engines helps the thermal efficiency I believe.Why go for zero drag? Why not just say Cd 0.2 or something low but achievable? If all your drag forces were 0, efficiency would be infinite, as you could just turn off the engine and keep going
Unless you had the radio on ofcourse...Theoretical maximum efficiency of a simple heat engine with a metal block is a little over 40%, although exhaust gas heat recovery can theoretically raise that significantly.
There is no theoretical maximum economy and steady speed, because there is no theoretical minimum drag. In practice I suppose you could work out a rough minimum CDa that a vehicle containing a person could achieve and use that, which would probably put you at a few thousand mpg?
There is no theoretical maximum economy and steady speed, because there is no theoretical minimum drag. In practice I suppose you could work out a rough minimum CDa that a vehicle containing a person could achieve and use that, which would probably put you at a few thousand mpg?
Edited by kambites on Thursday 2nd June 08:38
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