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is a 5 port self supercharging ?
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Author:  ROBERTXX [ Fri Mar 18, 2011 12:19 am ]
Post subject:  is a 5 port self supercharging ?
using my tiny pea brain ,iv been contemplating a theory ,and thought i'd stick it on here as well,,

theres been a lot written about charge robbing and the imbalance between inner and outer cylinder mixture ,and the problems that creates .

general thinking appears to be ,that the inner cylinder pulls in a charge of air fuel mix ,and then just before in closes ,the other cylinder in the pair opens up and asks for its share .the fuel ,being heavy ,wont change direction so easily ,and so the outer cyl gets a bit more air than fuel ,and so runs lean .

my thought was ,if you run a tube full of gas (eg , inlet tract) at a certain speed ,and close it ,theres a continuing ram of gas compacting on the closure point,for a moment .

now , imagine if almost concurrently another opening occurs , in this case the outer cylinder inlet valve ., its now opening on a higher pressure gas column ,carrying with it the inertia it built up feeding no3 . so its volumetric efficiency is raised artificially compared to no 3 .its fuel delivery may be similar to no3 , but its air delivery is way higher ...in effect , its been supercharged by no 3 initiating the flow speed ,and resulting inertia.

this would tend to explain the 100 bhp/litre + outputs of some 5 ports ,where on the face of it theoretical modelling such engines with a ve of 85 % simply doesnt replicate reality .

if this theory holds water ,then its a feature non siamese port engines dont share .and maybe the only type of engine with a built in supercharger ,if only on 2 of the 4 cylinders .

ps, im sure this must have been written about ages ago .or its total nonsense lol.

regards
robert .
Author:  awdmoke [ Fri Mar 18, 2011 9:46 am ]
Post subject: 
Yes, the pulse wave is "supercharging" for a few microseconds, but the charge robbing phase is longer so the siamesed ports actually fair worse than the two outer ports.
Author:  ROBERTXX [ Fri Mar 18, 2011 5:59 pm ]
Post subject: 
awdmoke wrote:
Yes, the pulse wave is "supercharging" for a few microseconds, but the charge robbing phase is longer so the siamesed ports actually fair worse than the two outer ports.


i dont understand that at all ! :D

the inner ports open first ,so get a clean shot .the outer ports get the inertia of the inner ports column of air as they open ,so get a substantial increase in flow , over what a single port would .

imagine a single port opening , it has to accelerate a stalled mass of air in a tube ,up to speed ,then maintian that speed then close .

now imagine that same valve opening ,but the air allready doing umpteen fps in its direction ,so it flow faster much sooner ,so the average port speed is way higher in a port when the piston speed is quite low ,this would give a very high percentage increase in flow on that first 50 odd degrees of lift ,and mean that port speed maximum is reached far sooner ..

its a bit like drag racing from a 30 mph speed ,rather than a standing start ,,, it would affect the whole run in a cumulative way ,,not just the launch .
Author:  simon k [ Fri Mar 18, 2011 8:11 pm ]
Post subject: 
you are correct, but it can't really be called supercharging

The effect you're talking about is why twin carbs aren't especially ideal, why it's apparently hard to do multi-point fuel injection, and why the inner pair of cylinders get a different mixture to the outside pair.

The reason twins aren't ideal is because the air is doing it's stop start thing, and the pistons will flutter - the firing order is 1-3-4-2, which as far as the carbs are concerned is 2-1-3-4. Each carb gets two pulses, then the air almost stops while the other one gets two pulses, then back again. So the air needs to be accelerated. With a big single, the air speed is constant. This is why the balance tube between the carbs is so important, and Vizard did a lot of research into ideal balance tubes
Author:  drmini in aust [ Fri Mar 18, 2011 8:25 pm ]
Post subject: 
I would add that for a while, both valves are open at the same time and the outers can rob charge from the inners.
Author:  david rosenthal [ Fri Mar 18, 2011 10:26 pm ]
Post subject: 
drmini in aust wrote:
I would add that for a while, both valves are open at the same time and the outers can rob charge from the inners.


Because the first cylinder is almost fully charged and the air velocity has decreased ,then the next cylinder is starting to charge and with a greater piston acceleration and higher vaccume ,so it gets all the air.

So air flows from higher pressure to a lower pressure and as the first cylinder is on the up stroke before the valve shuts then the air flow stops or even reverses to the lower pressure cylinder.

The problem with fuel condensation in the system is that as the air/fuel mixture is travelling at a velocity from the carby and thru the manifold, it suddenly enters a big volume[ the head port chamber] and the air velocity drops, and the pressure drops and so the fuel "condensates" out.

ultimately the best flow is to have a even small increase in velocity from the carby right to the valve. The valve seat area x the opening height is the final restriction in the whole system.
Look at the cylinder heads on bikes like the honda head, the port opening is then broken into 2 throats and the taper right to the valve. This is to get the maximum velocity for a short stroke .
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