Volumetric efficiency

[TwoSeven]

Anyone know how to calculate this for an inline 4 engine. If not is there a rule of thumb that they operate at. I'm figuring between 80-90%, but i'd like to have a better go at being accurate.

[FlyingDutchMan]

I'm not a mech enginer, but I know some rough rules of thumb.
With carbs you can never get %100 v.e. (I don't think more than 90% is possible)
Most good motor/carbs combo seem to have about 85%.
F.I. has a higher v.e.
Direct cyclinder F.I. has an even higher v.e. (as the petrol doesn't take the place of air on the way in).

That probably doesn't help, but what can you expect from an elec enginer?

[TwoSeven]

Thats great - it sort of ties in with my wild guess.

I'm doing some cylinder design for my race bike and I need to work out what the real compression ratio will be if I bore it out 2mm. I can do the math to work out what the theoretical is, but I prefer to tune it to actual

And I'm a software engineer - after 20 years I've only just figured out what the number means that Honda put on the end of their bolts.

[pete376403]

VE of over 100% CAN be done using very carefully tuned length intake and either megaphone (4 stroke) or expansion chamber (2 stroke) but it will within a very narrow rev range and run like a bag of shit either side of these revs.

This page http://www.epi-eng.com/ET-VolEff.htm has a calculator but its all theoretical

[F5 Dave]

Real compression ratio?

Vs volumetric efficiency?

Um what are you really trying to do? Load info into a SW program?

A few points which may or may not be obvious.

The stated compression ratio for a bike is a target the manufacturer may be able to meet if all the tolerances are in the tight direction (long conrod, thin head gasket etc).

You have to measure the volume of your engine to find out. Then the head volume + swept area divided by the head area is compression ie: 12:1 or whatever.

As for BMEP it is a little different & not necessarily proportional.

As far as measured compression (with a gauge) that is affected by comp ratio & engine condition but a hot cam will reduce that reading so is irrelevant.

The valves open so a ‘true’ compression ratio is dependant on the cam so measured static is the only sensible guess. How high you can go is another thing again.

[Blackbird]

Have a read of this - helps with context.

http://www.pumaracing.co.uk/power2.htm

[TwoSeven]

Real compression ratio?

Vs volumetric efficiency?

Um what are you really trying to do? Load info into a SW program?

A few points which may or may not be obvious.

The stated compression ratio for a bike is a target the manufacturer may be able to meet if all the tolerances are in the tight direction (long conrod, thin head gasket etc).

You have to measure the volume of your engine to find out. Then the head volume + swept area divided by the head area is compression ie: 12:1 or whatever.

As for BMEP it is a little different & not necessarily proportional.

As far as measured compression (with a gauge) that is affected by comp ratio & engine condition but a hot cam will reduce that reading so is irrelevant.

The valves open so a ‘true’ compression ratio is dependant on the cam so measured static is the only sensible guess. How high you can go is another thing again.

I think I have what you are saying, although I havnt factored in gas expansion under temp yet. Will do later.

Quoted compression ration is simply a ratio between the area of the cylinder at BDC compared with the area of the cylinder at TDC. The forumla is simply (cylinder volume + combustion chamber volume)/combustion chamber volume - where the combustion chamber volume also includes the head gasket minus any intrusion by the piston crown.

eg. for a cbr600f2 its 598/4 = (149.5 + 14.01)/14.01 = 11.6:1 in theory.

But the problem is, the gas flow is restricted due to poor intake design and other factors, so on the intake stroke the cylinder might only fill by 90% (what is meant by VE). This means that the real compression ratio is 11.6 * 0.9 = 10.44:1.

Knowing this figure I can calculate the air flow in litres/min then work out what the fuel flow needs to be in litres/min. This means I can set the jetting to flow that amount of fuel for the fuel ratio I want at given rpms - eg 1:12.5 at 10.5k rpm.

That means in rough terms I want to set my jetting to flow about 0.1 litres per min at peak power (or about 100cc/min). Since 10.5k rpm is over 3/4 throttle we are looking at the main jet and that works out in mikuni terms as a #150 main jet (rough guessish since the bike uses a #130. [about 78cc/min or a VE of 70%]).


Thats about as far as I've got at the moment, will test it some stage when I get some money together to try new jets and a dyno run - hopefully by then i'll have worked in some more math to give a more accurate figure.

[XTC]

Geez just get out there and ride the thing......

[Motu]

Geez just get out there and ride the thing......

He's gotta wash his hands first...again,drying them on a nice clean white towel,arranged on the towel rail perfectly inline with the (white) cupboard door.When he's got his bike out,polished and ready to go,he'll have to wash his hands again.Perhaps he should take up Superbike racing for Honda?....

[TwoSeven]

Geez just get out there and ride the thing......

haha.. I do, well up and down the street at the moment.

Yesterday I had a go at polishing and lapping valves on the spare cylinderhead.. Tomorrow I start making the moulds for the fairing.

http://203.79.122.87/motorcycle/Assets/cbr600_pics/race_bikeA_thumb.jpg

[F5 Dave]

I think I have what you are saying, although I havnt factored in gas expansion under temp yet. Will do later.

Quoted compression ration is simply a ratio between the area of the cylinder at BDC compared with the area of the cylinder at TDC. The forumla is simply (cylinder volume + combustion chamber volume)/combustion chamber volume - where the combustion chamber volume also includes the head gasket minus any intrusion by the piston crown.

eg. for a cbr600f2 its 598/4 = (149.5 + 14.01)/14.01 = 11.6:1 in theory.

But the problem is, the gas flow is restricted due to poor intake design and other factors, so on the intake stroke the cylinder might only fill by 90% (what is meant by VE). This means that the real compression ratio is 11.6 * 0.9 = 10.44:1.

Knowing this figure I can calculate the air flow in litres/min then work out what the fuel flow needs to be in litres/min. This means I can set the jetting to flow that amount of fuel for the fuel ratio I want at given rpms - eg 1:12.5 at 10.5k rpm.

That means in rough terms I want to set my jetting to flow about 0.1 litres per min at peak power (or about 100cc/min). Since 10.5k rpm is over 3/4 throttle we are looking at the main jet and that works out in mikuni terms as a #150 main jet (rough guessish since the bike uses a #130. [about 78cc/min or a VE of 70%]).


Thats about as far as I've got at the moment, will test it some stage when I get some money together to try new jets and a dyno run - hopefully by then i'll have worked in some more math to give a more accurate figure.


Sorry you clinical analysis is oversimplified & fundamentally flawed.

Any sort of attempt to work out the flow rate assumes the gas is unimpeded & valves are open all the time. Unless you can make the piston keep descending in one direction & producing the same suck then you will come unstuck.

This may make the engine a bit tall. :cool:

As it is with valves that open & close the flow is not continuous, consistent, or even one direction. When the valve closes the gas can flow in the other direction even back through the carbs picking up another charge of gas before returning through again.

At a particular rev range the intake flow is fast enough to continue (in one direction compressing when valve closed) & avoid this, but at lower revs the cams are open at the wrong time (overlap) & the resonance of the intake/airbox etc is not matched.

Concise version: Bigger ports look better but often work worse. Try to think of the flow in a dynamic rather than static position. Intake inertia is very important to keep the flow going when the piston is rising & the cam (valve) still open. You may even want to look at reducing the size of the ports in any ‘dead areas’.

[bluninja]

This link may be useful...http://www.mototuneusa.com/thanx.htm

[TwoSeven]

Sorry you clinical analysis is oversimplified & fundamentally flawed.

Any sort of attempt to work out the flow rate assumes the gas is unimpeded & valves are open all the time. Unless you can make the piston keep descending in one direction & producing the same suck then you will come unstuck.

This may make the engine a bit tall. :cool:

As it is with valves that open & close the flow is not continuous, consistent, or even one direction. When the valve closes the gas can flow in the other direction even back through the carbs picking up another charge of gas before returning through again.

At a particular rev range the intake flow is fast enough to continue (in one direction compressing when valve closed) & avoid this, but at lower revs the cams are open at the wrong time (overlap) & the resonance of the intake/airbox etc is not matched.

Concise version: Bigger ports look better but often work worse. Try to think of the flow in a dynamic rather than static position. Intake inertia is very important to keep the flow going when the piston is rising & the cam (valve) still open. You may even want to look at reducing the size of the ports in any ‘dead areas’.

I dont think so - your talking about intake efficiency, which is what VE is all about. I've taken a random guess at VE and my original question was how to calculate it more accurately which you have supplied data for :)

However, flow rate of the ports is fairly simple to calculate in a general way since you just need to work out how much fuel the bike uses in a period of time and the given fuel air ratio. A dyno and a measuring bottle will give you both. You can then arrange the formula to calculate the air flow, and then again to work out the porting information.

It seems to me at the moment that the cbr600f2 engine isnt restricted via any bad porting - indicators at the moment point to the carb setup. Although I seen in the F3 they straightned the ports up slightly and made them 5mm shorter which must have been done for some reason. However, I need to learn the math for ports first so I can make sure I understand exactly how it works. I can do most of it, but calculating the impact of the bends will be tough I think. :)

Also, I'm treating each part of the engine (cylinder, ports, carbs, airbox, exhaust) as different systems and reversion pulses (reversed charge) i've put currently in airbox as thats where it has the most impact (where you do the tuning for them) - so will be doing that at a later date :)
-----------

thanks for posting the links - happily reading thru them :)

[vifferman]

Where does 'overfilling' figure in all this VE mumbo jumbo? Or doesn't it?

[F5 Dave]

. . . , flow rate of the ports is fairly simple to calculate in a general way since you just need to work out how much fuel the bike uses in a period of time and the given fuel air ratio. A dyno and a measuring bottle will give you both. . .

Yeah sure it will.

Treating the items separately is a good idea & much more simple than having to consider that the engine rotates or interacts in any way.

I expect you will find the answer is 41.9 reoccurring & we’ll all slap our heads & wonder why we didn’t think of that before.

[TwoSeven]

You have to break things down to simple components and understand them first, before trying to understand the more complex interactions of the complete systems. This is basic human logic.

Also, i've never believed in deliberatly making something more confusing than it has to be. The trick of measuring things in the real world first before working out the math was one that was shown to me many years ago and as worked quite well over the years. :)

[F5 Dave]

Yeah sure it will.

Treating the items separately is a good idea & much more simple than having to consider that the engine rotates or interacts in any way.

I expect you will find the answer is 41.9 reoccurring & we’ll all slap our heads & wonder why we didn’t think of that before.

When I said that in my head it sounded so much more sarcastic.
If the effect was missed I’m sorry.

The problem with isolation is that you convince yourself that you have found the perfect way to measure something & believe it. People created the flow bench & then ported & polish heads up to big numbers & then found that the answers all change when you replicate a bore edge next to the valve or add a carburettor. Previously they had a simple measurement.

Then there are the stories of (think is was Austin car) factory team who would flow test heads & sell off the bad ones only to be beaten by privateers using ‘dud’ heads.

In simple isolation it was obvious they were flowing better. Just as simple human logic ascertained the world was flat. It was obvious in isolation. Just not correct.

My background is more 2 strokes, but as an example I was stuck for many months changing an engine one step at a time. I’d just put on a new expansion chamber & no amount of jetting would stop it plateauing. The problem was the extra suction was causing the intake reeds to flutter.

I did have some more reeds to try all those months but I didn’t want to change 2 things at once until it dawned on me the external effect the exhaust was having on the intake. When I changed them the power curve virtually doubled.

Total package.

[Sniper]

haha.. I do, well up and down the street at the moment.

Yesterday I had a go at polishing and lapping valves on the spare cylinderhead.. Tomorrow I start making the moulds for the fairing.

http://203.79.122.87/motorcycle/Assets/cbr600_pics/race_bikeA_thumb.jpg

Hmmmm, yummy :niceone: :apint:

[TwoSeven]

he he .. kiwi sarcasm is lost on me as being one of those wierd native things where you just nod your head and smile to keep em happy :)

Just let me know when its time to get out the shiny beads - I sometimes forget.

[FlyingDutchMan]

Where does 'overfilling' figure in all this VE mumbo jumbo? Or doesn't it?

Thats what a turbo does! More air means you can have more petrol means more power!

[F5 Dave]

As does pressurised airboxes, airbox resonance & on 2 strokes, expansion chambers.

[Motu]

And the superdupercharger,not often seen on bikes,but are making a comeback on cars - such a linier (sp) boost,it's like a capacity increase.Saw a really nice supercharger on a HD at Paeroa last year,a vertical roots off the timing cover,a real factory looking conversion.

[vifferman]

And the superdupercharger,not often seen on bikes,but are making a comeback on cars - such a linear boost,it's like a capacity increase.Saw a really nice supercharger on a HD at Paeroa last year,a vertical roots off the timing cover,a real factory looking conversion.
I saw a BMW 5(?) or 7 series a few weeks ago with a blower stack sticking out of the bonnet.