Hi Paul

Could you please advise me how the "Relative Mixture" graph was derived (Part 9). It seems a bit strange to me that the shape of the blue spring curve does not sit roughly half way between the fixed weight curve and the red spring curve. I am obviously missing something!

Regards

Bill

Bill,

A fixed weight piston provides the same downward force independent of piston height. The effective “weight” of a lighter piston fitted with a spring increases as the spring is compressed.

The springs are already partially compressed with the piston at the bottom of its travel. This means the effective weight of the piston at the bottom of its travel depends on the spring rate.

Considering the RED spring (rated at 4.5oz), With the piston at the bottom of its travel, the effective weight is the same as the fixed weight piston. Hence at low piston heights, the fixed weight and red spring give virtually the same mixture. As the piston height increases, so does the effective weight of the red spring making the mixture RICHER.

The blue spring is not as strong as the red spring (rated at 2.5oz). This makes the effective weight of the piston at the bottom of travel less than that of the fixed weight piston. Hence as the airflow through the carburettor increases, the piston rises faster than the fixed weight or red spring pistons, making the mixture WEAKER.

The graph shows what you would expect the FIXED weight curve lies between that of the BLUE (weaker) and RED (richer) springs.

Hope this makes things clearer.

Regards Paul

Thanks Paul

I obviously didn't phrase my questions clearly. Firstly can you point me at the calculations behind the graph. As I understand it, increasing effective weight increases the 'constant depression' hence more suck so richer mixture. Against that, the piston is lower so the needle is deeper into the jet so weaker mixture. I gather that the first factor wins and have seen references to the Darcy-Weisbach equation. However, that equation seems to apply to incompressible fluids which our mixture certainly isn't. I am therefore confused!

Secondly it is the shape of the blue spring graph that I don't understand. If a fixed piston of 256gm is replaced by a 128 gm piston and a red spring (4.5oz = 128gm) then the mixtures at Station 1 will be identical as shown on the graph. Now consider the same piston with a blue spring (2.5oz = 71gm) and an extra 57gm of fixed weight added. The mixture will be the same at Station 1 but, as the rate of the blue spring will be less than that of the red one, there will be less enrichment as the throttle is opened. Thus, I would expect the blue curve to sit between the black and red curves. Removing the extra 57gm would just shift the blue curve downwards with its shape unchanged. What is wrong with my thinking?

I am sorry if I am being unduly thick - it goes with my advancing years!

Regards

Bill

Bill,

I think I understand why you are confused. The effects of the spring on the mixture is complex and non-intuitive. There are two effects and I think you may be missing one of these. To try to explain, imagine the engine is running at half throttle.

With the RED spring the effective weight of the piston is GREATER than the fixed weight piston. As a result it does not rise as high in the suction chamber. This has TWO effects it:

1. INCREASES the depression in the choke, i.e. increases the pressure difference forcing more fuel out of the jet.
2. DECREASES the size of the annulus between the jet and needle reducing the amount of fuel that can flow out of the jet.

With the BLUE spring the effective weight of the piston is LESS than the fixed weight piston. As a result it rises higher in the suction chamber than the fixed weight piston. This again has TWO effects it:

1. DECREASES the depression in the choke, i.e. decreasing the pressure difference forcing less fuel out of the jet.
2. INCREASES the size of the annulus between the jet, allowing more to flow out of the jet.

As the flow of the fuel through the jet depends on both the pressure difference AND the size of the annulus, (which in turn depends on the needle profile) the effect on the mixture depends on the balance between these two effects. The first is calculated using the flow of a compressible gas through a rectangular orifice. The second the flow of a liquid through an annulus using a parameterised curve fitted to the profile of the needle.

With the ES needle and the red spring, the pressure effect is greater than the reduction in area of the jet making the mixture richer with increasing airflow. With the blue spring the reverse applies making the mixture weaker. Obviously, the effect on mixture will be different with different needle profiles. It is possible there will be a needle profile where the blue spring also makes the mixture richer. In this case, as you suggest, the blue curve will lie between the red and fixed weight curves.

Hope this helps,

Paul

Thanks again Paul

I thought I had described both effects but what I didn't understand is that both effects are quite large but similar though different in sign. Thus, a small variation in one (i.e. the needle profile) can send the difference either way. My interest is in a different carburettor (HV2) with different needles (no.2 and later R3). The car is a Lagonda Rapier, early cars having a heavy piston and later cars having a lighter piston with a red spring. I am trying to estimate the difference between the two set-ups.

Anyway, I think I understand things a bit better now and will have a go at the calculations involved. Thank you very much for your patience.

Regards

Bill

Bill,

I used Excel for the calculations. You would be most welcome to a copy of the worksheet if you would like. Please let me know.

Kind regards

Paul

Hi Paul

That would be fantastic! Thank you. As a linux user I use Libreofice Calc but most spreadsheets import from Excel OK.

Regards

Bill