In the previous article of this series, we have told the story of the carburettors invention and development. Now, it’s time to take a closer look at how it actually works. How does a carburettor, a purely mechanical device, deal with the two tasks – to mix fuel and air in the correct ratio and combine them into combustible mixture.
The air is sucked in by the vacuum created by the piston in the intake stroke, when the intake valves are open. The carburettor is placed on the cylinder head, between the air filter on one side and intake manifold on the other. The air goes through the air filter into the carburettor, where it goes through choke valve into a venturi, a narrowed part that utilizes the Bernoulli’s principle, accelerating the flow of air and creating a vacuum.
At this point, the fuel is sucked in from the float chamber into the jet, which sprays it into small droplets. The fuel level at the float chamber drops, moving the float and opening the float valve, which lets more fuel into the chamber. The carburettor is now ready for another cycle.
Sounds quite simple, doesn’t it? The stoichiometric ratio – the ideal ratio of fuel and air – is a given thing, so what’s the problem with building an “ideal” engine running on optimal ratio of fuel and air? The first problem comes with the fluctuating engine speed. It’s true that a carburettor for an engine running at constant revolutions per minute would not be particularly difficult and would easily prepare the perfect mix of fuel and air.
However, when you have to work with factors like changing engine speed, varying amount of the fuel/air mixture needed or seemingly marginal things like temperature of both the fuel and air, changing fuel quality or the need to get the fuel mixture from the carburettor to the cylinder in the shortest possible way, so it doesn’t condensate or self-ignite in the intake manifold, you will end up with a really complicated device. Or, quite likely, more than one carburettor.
For a simple engine, like in-line four-cylinder, which was probably the most common engine layout of 20thcentury, it is possible to make do with just a single carburettor. For larger displacements, though, it is necessary to use a two- or even four-barrel carburettor (basically two or four carburettors in one), or two-stage carburettor (two different-sized carburettors in a single device), or even a dual two-stage carburettor (two and two different sized carburettors in one device).
We can illustrate the whole variety of them, from the simple, economy-minded carburettors to high-performance racing setups or advanced electronic carburettors that came right before the advent of fuel injection, on the classic ŠKODA models from the latter half of the 20thcentury.
Even the most basic carburettors, present a tough challenge to design and manufacture with the tight engineering tolerances that are necessary for the proper function. That’s one of the reasons why carburettors were usually manufactured by specialist companies, instead of carmakers themselves.
Most post-war ŠKODA models used carburettors manufactured by a traditional Czechoslovak manufacturer Jikov, one of the very few companies in the former Eastern Bloc that was capable of fine engineering that was necessary to produce high quality carburettors. They were quite simple single-barrel downdraft units, but they provided a good compromise between the performance and fuel economy.
Some models, though, were equipped with a more exotic hardware. The most basic upgrade was fitting two Jikov carburettors instead of just one – this solution was used on sportier versions of various ŠKODA cars, like OCTAVIA TOURING SPORT, FELICIA and 1000MBX and MBG. The use of twin carburettors allowed for more even distribution of the fuel/air mixture into the cylinders and of course made it possible to deliver more of it, increasing performance.
For racing and rallying, a more radical solution was necessary. It came in the form of probably the most famous carburettor of all time – the side-draft, dual DCOE carburettor made by Italian brand Weber. Mounted in pairs, these dual carburettors effectively provide each cylinder with its own little carburettor. This means the most even distribution of the fuel/air mixture, as well as shortest route from the carburettor to the cylinder, which is why DCOE Webers are popular on all kinds of sporty and performance cars.
The downside? Of course, there’s the cost of the high-quality equipment, but even more important is the effect such a setup has on running costs. Optimized for the maximum performance, the engine is not very efficient in gentle driving. While cars like ŠKODA 180 RS offered lots of power – 180 RS had 1,771 cc engine with 11:1 compression ratio and 154 horespower – but the fuel economy was, of course, terrible.
How to combine a great performance with supreme fuel economy? The answer to this question came with modern technology, namely electronics. Of course, the ultimate solution was to replace carburettors with fuel injection, but that is something we will be interested in later. Before the carburettors were consigned to the dustbin of history (and garages of classic car enthusiasts), their last hurrah came in the form of electronic carburettor.
With the electronic control unit using sensors to determine the ideal fuel to air ratio, the electronic carburettors were able to provide much better compromise between performance and fuel economy. This solution was used on the first modern hatchback in the brand’s history, a ŠKODA FAVORIT that was introduced in 1987. It was initially equipped with an electronic carburettor made by German company Pierburg, before it was replaced with a Bosch MonoMotronic fuel injection that ended the era of carburetted ŠKODA cars.
In the next instalment, we’ll look at the history, principles and advantages of fuel injection. Keep watching us!