[Sutripta]

Originally Posted by vina

Well, I hadn't seen this thread earlier, otherwise I would have posted earlier.

I think this thread died before you became a member!

Originally Posted by pranavt

Too much has already been said so don't know where I can chime in. Pity, a lot could've been added and a lot of knowledge could've been gained in the process lol.

No timelimit to learning. We are all ears.

Originally Posted by pranavt

This doesn't really disregard theoretical proofs as much as it shows that you can make freak motors that go against conventional logic and still be reliable.

Yes, for every generalisation, one can find a couple of counterexamples.

Any thoughts on the Honda statement?

Regards
Sutripta

[pranavt]

Which Honda statement? Can you quote it?

[Sutripta]

^^^
Honda Worldwide | CBR250R

To fulfill basic performance requirements as a sport bike while setting higher targets for environmental performance, a DOHC was chosen as the valve system. DOHC improves combustion efficiency by reducing the weight of the reciprocating portion of the valves. This selection also allows us to freely choose the included valve angle, the port shape, and the shape of the combustion chamber. The choice of DOHC contributes to improved product appeal as a sports bike as well as to performance.

Regards
Sutripta

[pranavt]

This is just an educated guess as I have not seen the engine construction, but off the top of my head, here is what I believe. With seperate camshafts for the intake and exhaust, the rocker arms can be made shorter, the valve stems can be made shorter. With the valves being lighter, the you can go softer on the valvesprings which reduces friction. Further, roller rockers are much more efficient and reliable than rocker pads which slide against the cam lobes. Less friction, less wear. Will know more once I have the bike with me

[vina]

Originally Posted by pranavt

The spring rates of the valvesprings affect the resistance faced by the cam in turning more than the weight of the valves. But stiffer valvesprings are a necessity if you're going for higher revlimits or higher-lift cams. Lighter valves would allow you to get away with springs which are a little less stiffer, ultimately making the rotation of the cam easier, which I believe you are pointing at in your question.

Just by considering stroke between 2 different engines, you cannot conclude that one will make more power than the other, or where it will make more power reliably. The bore diameter matters, as does the rod-stroke ratio which determines dwell, side-loading force on the piston (which would impact combustion characteristics), etc.

Just to discuss the theory about shorter stroke = higher RPM reliably, the F20C motor by Honda which was made for the Honda S2000 had a long stroke (when compared to other high-revving motors) because of which set the record for the highest mean-piston-speed when revving at 9000 rpm, also being one of the highest-revving production engines in the world. This doesn't really disregard theoretical proofs as much as it shows that you can make freak motors that go against conventional logic and still be reliable.

hi Pranav

I think I mentioned earlier that bore diameter is the one that matters for specific power, rather than the stroke, and stroke is what matters for torque - so I think I already agree with your second paragraph.

For every general rule in a scenario where there are trades-off you can always find exceptions (and sometimes challenging the conventional wisdom can yield very nice results).

Regarding Sutripta's question of CBR250R I have a question - if you are a cam designer then the cam profile will be limited by a lot of factors including stresses generated when the valves open/close. Ideally you would want the time taken to open/close the valves to be a minimum.

Now by reducing the valve weight (and hence spring rates) if you can reduce the stresses, then isn't it possible that you might decide to use this benefit in improving the speed with which the valves open/close (i.e. keep the springs stiffer and/or go for an agressive cam profile to improve time available for combustion). I'm just shooting blind, let me know if this can make sense.

also I would presume the lighter valves will also have less heat capacity - can that effect combustion in the cylinder significantly?

[Sutripta]

Originally Posted by pranavt

This is just an educated guess as I have not seen the engine construction, but off the top of my head, here is what I believe.
.....
Will know more once I have the bike with me

Agreed with all that.
But I'm puzzled by lighter valves = improved combustion efficiency? And which portion of the valve is not reciprocating?

@Vina ^^^^
Check out valve float.
Valves are opened by the cam. And closed (except in desmos) by the spring.
Lighter valves mean higher rpms before valve float sets in, for the same spring.
There are two common cam profiles, const vel, and const acc. Also keep in mind you have to factor in wear/ life, and manufacturability.

Valves do have an effect on combustion, in an adverse way. If the edge is too thin, it might form a hotspot, leading to preignition.

Anyway, valves not the focus of this thread!

Regards
Sutripta

[Mpower]

Originally Posted by pranavt

This is just an educated guess as I have not seen the engine construction, but off the top of my head, here is what I believe. With seperate camshafts for the intake and exhaust, the rocker arms can be made shorter, the valve stems can be made shorter. With the valves being lighter, the you can go softer on the valvesprings which reduces friction.

With DOHC, you don't really need rockers anymore.
Why would the stem get shorter?
Spring tension depends more on rev celling than anything else..

DOHC improves combustion efficiency by reducing the weight of the reciprocating portion of the valves. This selection also allows us to freely choose the included valve angle, the port shape, and the shape of the combustion chamber. The choice of DOHC contributes to improved product appeal as a sports bike as well as to performance

I have a feeling something is lost in translation (from Japanese). I have seen some translated stuff like this that doesn't make total sense.

And which portion of the valve is not reciprocating?

LOL, good question..

[gsferrari]

Sutripta...by any chance did you take exception to a comment I made in the CBR250R ride report about the new CBR being a over-square and the karizma being under-square and therefore drawing certain inferences about the CBR's lack of low-end grunt?

[vina]

Originally Posted by Sutripta

Agreed with all that.
But I'm puzzled by lighter valves = improved combustion efficiency? And which portion of the valve is not reciprocating?

@Vina ^^^^
Check out valve float.
Valves are opened by the cam. And closed (except in desmos) by the spring.
Lighter valves mean higher rpms before valve float sets in, for the same spring.
There are two common cam profiles, const vel, and const acc. Also keep in mind you have to factor in wear/ life, and manufacturability.

Valves do have an effect on combustion, in an adverse way. If the edge is too thin, it might form a hotspot, leading to preignition.

Anyway, valves not the focus of this thread!

Regards
Sutripta

Hi Sutripta

I didn't know the terminology, but the effect is obvious - if the spring is not fast enough then the valve will not close. Thanks for giving me the correct name - I could search more on cam and learn more .

However here's the thing:
(Refer to Chapter 6. Cams )

Constant acceleration design is used to because by keeping acceleration constant it also keeps the stresses minimised (const acceleration = average acceleration over one cycle. If acceleration is not constant then peak acceleration> average acceleration) by keeping the peak acceleration minimum.



Now stress comes not only because of acceleration but because of acceleration and mass. So for similar kind of materials used, if mass is reduced then acceleration can be increased - leading to better rpm - for similar stresses and reliability. This of course can potentially increase power but has nothing to do with better combustion.

Also while constant velocity (the article says it is never used - it should be obvious why - peak acceleration is infinity when velocity changes direction) and constant acceleration are idealizations, I'm sure in almost every engine something in between will be used, some sort of a constantly variable acceleration.

My point regarding the better combustion part is this - suppose in your design the rpm was limited due to reasons other than CAM, then you now have low-mass valves due to which you could achieve better acceleration, but for a constant-acceleration CAM that doesn't buy you much any more.

What you can do in such a case is - go for a something in between the two extremes of CAM design and get better acceleration when you need it (i.e. while opening or closing the valves.) and none when you don't need it (i.e. while the valves are fully open or fully closed).

That'll improve the time available for combustion.

[Sutripta]

@GSF
Hi,
Of all things, why should I take exception to an observation. Any particular reason for thinking so? Considering I've not posted on that thread for quite some time.

If the cbr250r lacks low end torque, it lacks low end torque. Thats a measurable fact, not an opinion or value judgment. (But I must add that given its role:- ASEAN one size fits all commuter with feel good factor, this lack of LET is puzzling)

Actually, the choice of stroke has puzzled me (and others). Check out http://www.team-bhp.com/forum/motorb...ml#post2308419

The 'raise the bar' thread is less passionate than the other thread. So I post on that, and not on the other one. The same reason you'll hardly find any post of mine in the Thar threads!

Regards
Sutripta

[gsferrari]

The timing of this thread had me wondering...thats all.

The choice of stroke is indeed bizarre for the riding conditions in India and all I can do is dream of "what if" because now I have to save up for the more expensive Ninja 650R for my Ladakh steed. I was hoping the CBR would do the job but not with this engine and gear ratios :(

[Sutripta]

Originally Posted by Mpower

I have a feeling something is lost in translation (from Japanese)

Best explanation so far! (And thanks for cluing me in Re: the other matter)

Originally Posted by gsferrari

The timing of this thread had me wondering...thats all.

See post @26. http://www.team-bhp.com/forum/techni...ml#post2325560
I rest my case!

Originally Posted by vina

Hi Sutripta
....
That'll improve the time available for combustion.

Hi,
Can I reply later in a PM. Lots of ground to cover, and will seriously clutter up this thread.

Regards
Sutripta

[vina]

Originally Posted by Sutripta

Best explanation so far!


See post @26. http://www.team-bhp.com/forum/techni...ml#post2325560
I rest my case!



Hi,
Can I reply later in a PM. Lots of ground to cover, and will seriously clutter up this thread.

Regards
Sutripta

Sure, better still, send some links - I'll read up.

[pranavt]

Originally Posted by Mpower

With DOHC, you don't really need rockers anymore.
Why would the stem get shorter?
Spring tension depends more on rev celling than anything else..

The CBR has roller rockers, which is why I talked about them. Wouldn't be talking about them if they weren't there.

With DOHC, you can push both the cams quite a lot lower towards the combustion chamber and still have the desired valve angle. Hence the shorter stem. With a SOHC, there is a minimum limit for shortening the rockers or the valve stems before the geometry becomes unreliable

Originally Posted by Mpower

I have a feeling something is lost in translation (from Japanese). I have seen some translated stuff like this that doesn't make total sense.

Probably. You can either assume that the marketing people are fools and they've made a mistake. Or assume that they know things that are beyond your current knowledge. I chose to assume the second option rather than make a complete *** of myself.


Finally, if there is still more discussion left, can we move it towards a different and a better understood engine than one which hasn't been opened by anyone yet?

[Sutripta]

Originally Posted by vina

Sure, better still, send some links - I'll read up.

OK. Not much of an internet guy, so I'll rattle off a few terms and you can do your own searching.
Mushroom and Tangential cams. Geneva mechanisms.
Valve overlap. Relation to powerbands.
Advantage of OHC to pushrod implementations. (Maybe even an debate of this Re: V8s)
DOHC vs SOHC.
Slightly OT - dual springs, pneumatic springs, beehive springs. Desmodromic setups.

Regards
Sutripta

PS. Gosh. This is the worst case of jargon dropping I've seen in a long time.