Physics - Air 2
Air has miraculous properties. Anyone who has ever made an injector or nozzle illegally spraying fuel, whether petrol or Diesel, outside the combustion engine and then also ignited it, could see the devastating power of this
combustion at work, burning completely evenly at a certain point in space, almost without leaving any residue.
This is somewhat disdainfully called an air-fuel mixture, perhaps forgetting how often and desperately technicians have tried to establish this carefully prepared mixture in the combustion chamber in as many operating
conditions as possible. Presumably this is hopeless because, at least in the case of petrol engines, the mixture finds this space in too many different sizes.
To make the possibilities of generating such a mixture a little clearer, the nearly forgotten carburetor technology is probably the most suitable. We want to try to make the experiences and design solutions there usable once
again on the subject of 'air'. To do so, we return to a very old principle of carburetor technology. But don't worry, this will not be a special chapter for vintage car fans.
Bernoulli's principle (picture above) is also called the 'Venturi effect'. The venturi is the constriction in the top right-hand corner of the air duct. In the case of Wilhelm Maybach, it was initially simply intended to create negative
pressure in a fuel line that opened out there. Here the whole thing looks a bit more uniform. Important about the diameter of the constriction is that, coming from above, it decreases relatively quickly and then slowly again.
The crucial thing is to avoid turbulence. In principle, any constriction in an air duct causes a higher air velocity and thus negative pressure. This increases with the flow rate. And this is where the beauty of the above design
comes into play. The nozzle at the top is called an 'air correction nozzle'. In other words, it is precisely at this point, or below it, that any excess fuel sucked out may be corrected.
Have you ever heard of 'brake air'? This is precisely what rushes through the nozzle as a precisely measured proportion of the air flowing into the carburetor onto the fuel standing in the tube. The higher its speed, the lower it
comes. There it increases the proportion of air and thus causes the mixture, which would otherwise become too rich with greater underpressure, to become leaner. Together with the fuel, it then passes through the holes in
the tube to the outside and then into the air funnel at the top.
Result: Even with more air flow, the mixture ratio can remain the same. This was a bit more complicated in the carburetors of that time, because they produced a rather lean mixture at partial load and only really got rich at
full load in order to achieve the catalogue performance, but this actually shows even more what can be achieved with such a relatively simple design. The prerequisite, however, is a precisely adjusted float that sets a very
specific fuel level.
And here the same task, only with a variable air funnel. It is also called an 'equal-pressure' or SU.carburetor, meaning equal pressure conditions between the piston and the throttle flap and at the very top beyond the
diaphragm. The air controls itself, pulling the piston upwards along with its seats stuck in a ring nozzle and providing the corresponding increase in fuel when more air is drawn in by the engine.
Here, too, the ratio between air and fuel is always the same. A special shape of the needle even allows for enrichment or leaning in certain operating ranges, all without electronics. The system is so sensitive that a kind of
shock absorber with a fixed piston and damper fluid is needed in the middle to prevent vibrations. Despite the different design, the functions are completely comparable to the above system.
Do you have a moment? Then please take a look at what the engineers did with the idling system when the first exhaust regulations were issued. The correspondingly outlined is all necessary to prepare the idle mixture as
well as possible and at the same time to make it precisely adjustable in terms of mixture (small screw) and idle speed (large screw). With a low flow rate, it is more difficult to mix fuel and air well.
It gets really exciting, even for performance-hungry fans, when it comes to systems with several carburetors on one engine. There are even those who, not exactly sensibly, were so taken with two twin carburetors that they
installed them instead of an existing injection system. Above you see such a system, perfectly suited to the four-cylinder in-line engine.
But there is also the difficulty of synchronisation involved. All four air funnels should provide the same mixture at the same throttle position. If you have just achieved this for one operating point and then select another, the
clocks show different values again. A lot of experience and intuition is required. It shows the moody side of the air and the difficulty of dealing with it. More about this in the next chapter . . .
|Imagine having to synchronise the six twin carburetors of an old V12 . . .|