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Old 30th December 2005, 22:49   #16
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Quote:
Originally Posted by pratheesh
there are many ways to improve scavenging efficiency....like using reed valves (a project done at my college on 2S engine, using reed valve proved to be very successful)...
using turbo/super charging might also help (i am not so sure about that, well that was one of my thought experiment product), and also redesigning engine.... plz do reply
Actually, a properly designed expansion chamber will do the trick. you don't need a turbo. Trust me. Chambers when properly designed, make a world of difference.

There were several two stroke cars (some french manufacturers i think) that were quite popular in racing cos of their awesome power to engine weight ratio... theoretically, the biggest advantage with 2 strokes is power on every alternate stroke and lesser moving parts..

Heh heh. And anyone who doubts the power outputs of a 2 stoke engine has obviously never ridden a properly tuned 2-stroke monster

Anyone try getting their hands on Rossi's favorite bike (which happens to be a 2-stroke 500 cc monster)...
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Old 8th January 2006, 00:18   #17
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Originally Posted by pratheesh
Why did Two stroke engines became obsolete? I think the only problem they had was the fuel efficiency problem. Now, if we use a fuel injector instead of a carb, it should give fantastic fuel efficiency because of better scavenging. Use a turbocharger, more better efficiency?
i have'nt read about it anywhere, but it came into my thoughts n i dont know whether they are in some text books; are they?)

the turbocharging could also increase the thermal efficiency,... add a catalytic converter, you will get rid of all the NOXs....

SO, WHY NOT TWO STROKES???

Please read the thread started by GTO - Most powerful diesel engine in the world. It is a two stroke engine and it has one of the highest thermal efficiencies in the world.

Please note that supercharging (turbocharging is one form of supercharging) is used to increase the power of the engine while keeping the displacement same. Theoritically, it has got nothing to do with thermal efficiency.

So what is the problem?
The main problem is the lubrication of gudgeon pin. The lubrication of gudgeon pin is not purely hydro-dyanamic but more so of boundary layer lubrication. In 4-stroke engines, while the suction stroke is taking place, the load on gudgeon pin is reversed or is minimum. The lubricating oil has a chance to fill up the top space of gudgeon pin. In 2-stroke engines, the load on gudgeon pin is never reversed, so their is no chance of oil to reach the top space of gudgeon pin and hence gudgeon pin bearing is liable to fail.

In large engines, the one described in above mentioned thread, separate gudgeon pin in form of crosshead. These are made large to increase the area under lubrication and hence reduce the bearing load. Also there is a provision of special supply of lubricating oil under pressure to the top of crosshead. But becuse of their size they are not suitable for cars. They are mainly used in shore specialised powerplants etc.
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Old 8th January 2006, 10:38   #18
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Quote:
Originally Posted by jat
Please read the thread started by GTO - Most powerful diesel engine in the world. It is a two stroke engine and it has one of the highest thermal efficiencies in the world.

Please note that supercharging (turbocharging is one form of supercharging) is used to increase the power of the engine while keeping the displacement same. Theoritically, it has got nothing to do with thermal efficiency.

So what is the problem?
The main problem is the lubrication of gudgeon pin. The lubrication of gudgeon pin is not purely hydro-dyanamic but more so of boundary layer lubrication. In 4-stroke engines, while the suction stroke is taking place, the load on gudgeon pin is reversed or is minimum. The lubricating oil has a chance to fill up the top space of gudgeon pin. In 2-stroke engines, the load on gudgeon pin is never reversed, so their is no chance of oil to reach the top space of gudgeon pin and hence gudgeon pin bearing is liable to fail.

In large engines, the one described in above mentioned thread, separate gudgeon pin in form of crosshead. These are made large to increase the area under lubrication and hence reduce the bearing load. Also there is a provision of special supply of lubricating oil under pressure to the top of crosshead. But becuse of their size they are not suitable for cars. They are mainly used in shore specialised powerplants etc.
hey buddie, which ever books i referred said the thermal efficiency of a 2S is less than a comparable 4S engine. I think it is because you are talking about a diesel engine, which has higher compression ratios and thus higher efficiency.... And, since only air is inducted in a disel engine and fuel is injected only during beginning of exapnsion stroke, fuel loss also becomes negligibly low....
And this scenario threw two more questions,
1) Though it is proved that a 2S diesel engine is much compact, provides better power/unit voulme, highly efficient, why is it that the conventional diesel engines are 4S?
2) Why petrol engines cant employ same injection principle?

And about the type of luburication you specified, just read this post

Quote:
Originally Posted by newbie
Well i remember toyota filled a (US) patent once for new 2 stroke engines that (they claim) is atleast equal to current 4 stroke engines.Also prateesh the lube/oil mix was done not as neccesity but to save costs and a conventional lube system like a 4 stroke would work (like in the toyota patent). I will try to look for it but meanwhile try: www.uspat.org for Us patents and ofcourse google.
we could solve the problem of lubrication by using your method, pressurised system.

And one more thing, before I thought turbocharging/supercharging will increase compression ratio, but just now I recalled that turbo/super only increases the cylinder pressure, not the compression ratio (the displacement is constant), in fact the turbo engines have relatively lower compression ratio...OOPS!!
But still new questions aroused and remains unsolved!!!
Please reply!!!

Pratheesh Prakash

Last edited by pratheesh : 8th January 2006 at 10:41.
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Old 9th January 2006, 00:33   #19
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Originally Posted by pratheesh
hey buddie, which ever books i referred said the thermal efficiency of a 2S is less than a comparable 4S engine. I think it is because you are talking about a diesel engine, which has higher compression ratios and thus higher efficiency.... And, since only air is inducted in a disel engine and fuel is injected only during beginning of exapnsion stroke, fuel loss also becomes negligibly low....
I mentioned an example about the two stroke thermal efficiency - that it not the thermal efficiency factor for two stroke being out of market. If you compare the 2S and 4S engines of same era, you will find that the therma efficiencies is almost same.

Thermal efficiency depends on lot of other factors and two stroke or four stroke plays a less important role. People were happy with around 45 % thermal efficiencies (car engines thermal efficiencies were much lower - in 90s they were able to get more that 30 %) till the Israel-Egypt war created an oil price shock and research for higher thermal efficiencies started. All these factors are mainly dependant upon the economic factors. You can see that in India also. Suddenly there are more diesel variants in the market and Honda city has also developed I-DSi engine. Fuel loss etc are tolerated when leagal system allows, combined with the affordability of fuel cost. People who increase power of their vehicle engines to extremes are not bothered by fuel loss - their vehicle thermal efficiency is lower than the stock car engines. Please note that fuel loss is dependant on how the fuel is burnt and more of less independant of how the fuel is injected. You would have noted the two numbers of spark plug in Honda I-DSi engine to reduce fuel loss - and it is a four stroke engine. Also please note that in diesel engines, fuel is injected near the end of compression stroke and injection is comleted during expansion stroke. If beginning of fuel injection is during expansion stroke, there won't be enough time to burn the fuel completely and you may find the flame coming out of the exhaust valves which will get burnt out in no time (metallurgical limitations)

Quote:
Originally Posted by pratheesh
And this scenario threw two more questions,
1) Though it is proved that a 2S diesel engine is much compact, provides better power/unit voulme, highly efficient, why is it that the conventional diesel engines are 4S?
2) Why petrol engines cant employ same injection principle?
1) Initially 2S engines were in market, but as supercharging ratios increased, so did the bearing loads and due to lubrication problems involved - all trunk type engines converted to 4S as 2S engines did not offer much advantage due to cost factor involved. But for crosshead type engines (the types mentioned by GTO), the lubrication difficulties were easily solved by a) increaseing the bearing area and b) boosting the lubrication oil pressure - upto around 16 bar!!!. If we add a crosshead to our car engines, then the weight and space occupied by those engines is just going to be double - which I think nobody is going buy.

2) Petrol engines can employ same injection principles, but it means that same kind of high pressure fuel pump and high pressure injectors, which means the increase in cost by a huge amount without giving any significant advantage - and I think no body is going to pay for that just for fashion which nobody can see!!!



Quote:
Originally Posted by pratheesh
And about the type of luburication you specified, just read this post



we could solve the problem of lubrication by using your method, pressurised system.
Yes, you are right. Pressurised system has been in use in industrial and offshore applications for a long time. Only the car makers are going to use it now ( Please note that crdi system has been there for long time but car makers are using it now only).


Quote:
Originally Posted by pratheesh
And one more thing, before I thought turbocharging/supercharging will increase compression ratio, but just now I recalled that turbo/super only increases the cylinder pressure, not the compression ratio (the displacement is constant), in fact the turbo engines have relatively lower compression ratio...OOPS!!
But still new questions aroused and remains unsolved!!!
Whenever an engine is supercharged/turbocharged or supercharging ratio is increased, the max cylinder pressure is maintained otherwise there will be mechanical failure of engine. The compression ratio is lowered (highly rated engines have compression ratios around 10~12!!!). You will wonder then how those engines are started with such low compression ratios. Well they are assisted by many different means such as electric blowers, accelarating turbocharger by compressed air etc.

RK
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Old 9th January 2006, 00:43   #20
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Hey Goldie i do agree with you earlier i had pulser but sold it as it was not fun to ride n now i ride a YAMAHA RX100 its an 1988 model its 17yrs old n still has the power n very low on maintenance today also i can beat a ZMA n P180 on the road on innitial pickup i just LOVE my bike
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