If you are under thirty years old, you have likely never had to resolve excessive fuel consumption and/or drivability issues on cars that were caused by worn, poorly adjusted, incorrectly adjusted, or tired carburettors. This is not necessarily a bad thing, but if you were thinking that carburettors have completely disappeared from the motoring scene, you would be wrong.
Of course, carburettors are still almost universally used on things like chain saws, small motorbikes, mopeds/scooters, and lawnmowers but they have largely disappeared from standard, road-going vehicles. This is also not necessarily a bad thing, but in some applications, an appropriate carburettor is more effective at fueling an engine than an electronic fuel injection system. Moreover, carburettors are cheaper than any fuel injection system, and they can often be set up and calibrated for specific applications more easily than most injection systems.
On a purely practical level, however, electronic fuel injection systems have many distinct advantages over carburettors, not the least of which is greatly reduced exhaust emission levels, but at its core, any typical standard fuel injection system in use today is just a somewhat intelligent version of an old-style carburettor. In fact, many of the features of modern fuel injection systems are just upgraded versions of what it was that made carburettors work, and in this article, we will discuss these similarities in some detail. Before we get to the specifics, though, let us take a step back into history to see how carburettors work, and still do on many high-performance engines. Let us start with-
NOTE: For the purposes of this section, we will ignore the differences between "down draught" and "side draught" carburettors. Briefly, with down draught carburettors, the intake air enters the engine at the top of the unit and flows vertically (downward) into the intake manifold. By contrast, the intake air flows through a "side draught" carburettor from the side and into the engine in a horizontal plane. As a practical matter, though, all automotive carburettors contain largely the same parts, all follow the same operating principles, and all work in the same way despite the significant design differences between the two main types of carburettors.
In carburettor terminology, the concept of "carburetion" refers to how well (or otherwise) a carburettor atomizes the fuel before it enters the cylinders as a part of the air/fuel mixture.
Today, we talk of “atomization”, which refers to the same thing but in the case of a fuel injection system, this is determined by a combination of factors that include (among others), the fuel pressure, injector design, cylinder head design, the number of injection events per cylinder per engine cycle, and the injector’s pulse width at any given moment. In addition, whether an injection system is of the port injection type or direct injection has a major effect on fuel atomization. In fact, since the air and fuel largely mix outside of the cylinder in port injection designs, the uniformity of air/fuel mixtures is often superior to air/fuel mixtures in direct-injection designs.
So how do carburettors work exactly? If you have not had any exposure to carburettors, their operating principles might seem complicated because the fuel is not pumped into the engine, but in reality, the basic function of a carburettor is based on a simple law of physics. In short, this law says that if you have a moving stream of air (or any fluid, for that matter), and you partially restrict the flow of that stream of air, the speed of the air stream will increase after the point of restriction. So here is the short version of how carburettors work but refer to the image below for more clarity-
We need not delve into the specifics of this particular example of a simple carburettor, beyond saying that this design is representative of the two-stage carburettors that saw service on hundreds of millions of four-cylinder engines of most vehicle brands during the 1970s and 1980s and even up to the point when fuel injection became the norm.
Having said the above, let us look at what the arrows mean because this will help you understand how carburettors work-
Red arrow
This is the main, or primary barrel, or throat through which air flows into the engine. This barrel is partially restricted by a barrel-shaped venture that causes the speed of the airflow to increase as it is forced to flow past the restriction. On all multi-barrel carburettors, the primary barrel is the only one in operation at low to mid-range engine speeds.
Orange arrow
This is the secondary barrel, which is slightly smaller than the primary. Like the primary barrel, the secondary is also fitted with a venturi to increase the speed of the air flowing through it. As mentioned above, this barrel remains typically closed off by a throttle plate until the engine speed, and hence, the intake manifold vacuum exceeds a certain value.
Note that on some carburettor designs, the secondary barrel can be bought into operation by engine vacuum with a vacuum-operated actuator, while on other designs this barrel can be brought into operation mechanically by a linkage that is connected to the primary barrel’s throttle plate. In a few cases, however, both barrels are brought into operation simultaneously through drive gears or mechanical linkages. One example of this arrangement is the twin-barrel Weber carburettors that were fitted to Ford's V6 Essex engine and some of its variants. Note that the "throttle blade" shown in the secondary barrel in this example is a moveable valve that can close off the barrel almost completely when the choke is in operation.
Yellow arrow
This is the acceleration pump nozzle, which squirts small quantities of raw and fuel into the primary barrel to enrich the air/fuel temporarily under some operating conditions.
Black arrow
This is the choke's mechanism. In this example, the unit is controlled by a bi-metal strip that deforms as the engine coolant's temperature rises. In other designs, the bi-metal strip is controlled by an electrically operated heater element, but in both cases, the deformation of the bi-metal strip controls the movement of the throttle blade in the secondary barrel.
Blue arrow
This is the attachment point for a fuel line from a low volume, low-pressure fuel pump. On the vast majority of vehicles, the fuel pump was mechanically driven off a cam on the camshaft, while other vehicles, mainly pre-1980 English-made vehicles, used a low-pressure electrical pump.
Purple arrow
This is an outlet to divert excess fuel back to the fuel tank through a dedicated return line. Note that for the most part, this feature only appeared on carburettors from about the early 1980s.
Green arrow
This could be one of two things, depending on the carburettor brand. In some cases, this is an electrically controlled solenoid that allows fuel to pass into the idling circuit; if this solenoid is defective the engine will not idle.
In other cases, this is a solenoid that controls the flow of fuel into the primary fuel circuits through a jet or metered orifice; if this solenoid fails, the supply of fuel to the engine is effectively cut off and the engine dies, and will not start again.
The image above shows most of the principal components of a typical carburettor as seen from the outside, but there are some critically important parts and components that are not shown. Below are some details of these-
The float chamber is built into the casing of the carburettor, which is why it is not visible in this example. The bowl is fitted with a metal float that rises as the bowl fills up with fuel, but to prevent the bowl from overflowing, the float acts on a small moveable valve to shut off the fuel flow. In practice, this valve works just like the float valve in a flushing toilet- it allows the fluid level to rise to a certain level in the bowl/cistern, but then shuts off the inlet orifice when that level is reached.
Note that in all carburettor designs, the tab on the float that acts on the small valve is adjustable, because this adjustment is arguably the single most important setting when tuning or servicing a carburettor. If this setting is too high the float chamber overflows, and this usually has serious negative consequences for fuel economy and engine performance. If, on the other hand, the setting is too low, the engine can be starved of fuel at high(er) engine speeds, which could cause the engine to misfire, stumble, or even shut off completely.
This image shows a random selection of carburettor parts, with a selection of jets in the foreground. However, the topic of jets or metered orifices is perhaps the single biggest subject within the overall discussion on carburettors, and therefore, we will not discuss this in any great detail, beyond saying that both fuel and air jets are typically available in various sizes, which makes it relatively easy to change the fuel delivery characteristics of any carburettor. However, to keep things simple and to the point, we will discuss jets within the context of the various circuits they control. Let us start with-
Primary (fuel) jets
These jets control and manage the flow of fuel through the primary channels that are cast or machined into the carburettor body, and which terminate in the barrels just below the venturi where the airflow is moving fastest.
As the intake air moves through the barrel, the air pressure in the immediate area around the jet or circuit termination is slightly lower than elsewhere in the barrel. Thus, while it might appear as if the airflow is "sucking" fuel through the jets, it is, in fact, ambient atmospheric pressure that pushes fuel through the jets from the float chamber because of the (atmospheric) pressure differential between the inside of the float chamber and the inside of the barrel.
Secondary (air) jets
These jets work on the same principle as the fuel jets, but their primary purpose is to “inject” additional air into the barrels to help with the atomization of the fuel. In all cases, the fuel and secondary air circuits are separate, and never intersect.
Idling jets
As the name suggests, the function of the idling jet is to allow just enough fuel to enter the primary barrel to allow the engine to run at idling speeds. In practice, the idling circuit typically terminates near the bottom of the barrel just above, or just below the throttle plate, which is typically only about 10 to 15 per cent open at idling speeds.
In practice, this circuit is effectively shut off by the higher airflow that rushes through the barrel at engine speeds above idling, although there is usually some overlap between the two circuits as the engine speed increases until the idling circuit is shut off completely, which brings us to-
Carburettors might seem a bit primitive today when we compare them to systems like GDI, Sequential Injection, Multiple Injection, and particularly Homogeneous Charge Compression, but when judged by the standards of their day, carburettors were perfectly efficient, and provided they were maintained properly, they delivered perfectly acceptable fuel consumption rates.
Moreover, when carburettors were supplied with clean fuel, they provided many years of reliable service, and when they did go wrong, fixing them was often as easy as making a few small adjustments with a small screwdriver. Of course, nothing is always easy, and in some cases, such as when the bushes on a carburettor's throttle plate spindles wore out, it was often more cost-effective to replace the carburettor than it was to fix the worn spindles.
But we digress; we were talking about efficient carburetion. As mentioned elsewhere, this refers to how well (or otherwise) any given carburettor could atomize fuel, but we cannot judge this by the standards of modern fuel injection, despite the many similarities between how carburettors and modern fuel injection systems work, which we will discuss shortly.
While carburettors did an amazing job of supplying fuel to billions of engines in all conceivable operating conditions, there is no denying the fact that by today’s standards, they were not at all good at fuelling engines efficiently. Nonetheless, in the olden days, it was possible to “tune” an engine purely by ear; if it did not misfire, run roughly or use excessive amounts of fuel (judged by the standards of the time) the carburettor was fine. Moreover, one could verify this simply by touching the carburettor when the engine was hot; if the carburettor was cool or even cold to the touch it was working fine, since vaporizing fuel efficiently greatly reduces the overall temperature of the air/fuel mixture. Note though that this test was entirely subjective because what felt like "cold" to one mechanic, was often just "cool" to another.
However, to achieve a cold, and hence, efficient carburettor on a hot engine was not always easy, because many things had to be present at the same time to produce efficient carburetion. Chief among these was the absence of air leaks through gaskets and manifolds, as well as intake valves that did not leak. Then, the carburettor had to have exactly the right jets for that particular engine and ambient atmospheric pressure, and most importantly, all adjustments, such as the ignition timing, idling speed, and particularly, the air correction on the carburettor had to be absolutely spot on.
The problem was of course that except for carbon dioxide emissions, there were no objective tests one could perform to evaluate or assess the overall efficiency of any carburettor. One could perhaps measure sulphur and carbon monoxide levels in the exhaust stream but combustion was so poor (relative to modern standards) in carburettor engines that such readings were useless for practical purposes.
Of course, the advent of fuel injection, emissions regulations, OBD II standards, and the first diagnostic scan tools brought improved combustion and several objective tests with which to test and verify the efficiency of combustion processes. This was good for both the environment and car owners, who used less fuel, but at their cores, fuel injection systems are simply glorified carburettors, and in the following section we will explain this relationship, starting with-
In their simplest form, carburettors are fuel-metering devices, although to make them work, the amount of fuel they allow into the engine can not be controlled with any degree of accuracy. Regardless of the size of the hole in main jets, the amount of fuel that can pass through the jet remains relatively constant, even though the speed of the airflow through the carburettor can, and does affect this on some carburettor designs.
Moreover, fuel pressure plays no role in the amount of fuel a carburettor lets into an engine, although abnormally high fuel pressure can overcome the fuel shut-off valve in the float chamber, thus causing the engine to flood.
By way of contrast, the diameter and number of injection orifices in a fuel injector is roughly analogous to the diameter of the hole in a carburettor's main jet(s), while the fuel pressure and injector's pulse width are roughly analogous to how a carburettor venturi speeds up the airflow through a barrel. Put differently, this means that while the size of the jet and the airflow through a carburettor determines how much fuel enters an engine, an injector's pulse width and fuel pressure determine this on a fuel-injected engine.
Therefore, on modern injection systems that do not allow for the individual control of injector pulse widths, the injectors are nothing more than highly engineered carburettor main jets, albeit jets that deliver fuel far more accurately than any old-style jet can.
While the concept of “air metering” on carburettors is a bit nebulous, there are nevertheless some standards and guidelines one should follow when considering fitting a multi-barrel carburettor on a high-performance engine. Chief among these guidelines involves how much air can pass through the carburettor, which is always expressed as "CFM", or Cubic Feet per Minute. As a practical matter, this is based on the engine's displacement, but it is worth remembering that any carburettor's CFM rating refers to the maximum amount of air that can pass through all the barrels at the same time when the engine runs at its nominal maximum speed.
In the performance world, this value is closely related to the number and size of the main jets in a given carburettor and while this relationship is also important on standard road-going vehicles, it is less so because a slight mismatch will not produce the same dramatic results as a similar mismatch would on a large, highly modified V8 engine.
In fuel injection systems, the term “CFM” is replaced by “GPS”, or Gram per Second. While this allows for more precise control of the ultimate air/fuel mixture than is possible to do on carburettors, the various strategies modern systems use to measure and monitor the volume of air that enters an engine is cumbersome, finicky, and expensive to implement. Nonetheless, being able to monitor the volume of intake air from one second to the next allows for improved control over combustion processes, although no fuel injection system in use today can deliver complete combustion, much less perfect combustion.
Under steady cruising conditions, neither carbureted engines, nor fuel-injected engines require fuel mixture enrichment, and in both cases, it is under transient conditions such as large throttle inputs that could starve the engine of fuel, that enrichment is required.
With carburettors, this is accomplished by using a dedicated acceleration pump that squirts a small quantity of raw fuel into the barrels in operation, and in a properly tuned carburettor, this raw fuel is immediately incorporated into the air/fuel mixture below the venturi. In practice, this method of enrichment means that the air/fuel mixture can reach ratios of as high as 18 parts of fuel to one part of air for brief periods, but the upside was a smooth acceleration.
By way of contrast, fuel injection systems enrich the air/fuel mixture by briefly increasing the injectors' pulse widths, which allows more fuel to enter the cylinders. While this also greatly enriches the air/fuel mixture on early iterations of injection systems, modern injection systems match the increased fuel more closely with the available air, meaning that it is almost unheard of for modern engines to ever see mixtures as rich as 18:1.
Nonetheless, pulse width adaptations are the exact analogues to mechanical acceleration pumps that squirt raw fuel into carburettors, the level of control in injection systems is several orders of magnitude higher than in carburettors.
The above outline the three main similarities between carburettors and fuel injection. There are several others, but these three are what make modern engines work in the same way that carburettors do, which leaves us with this-
So, to answer the question, are fuel injection systems just glorified carburettors? This depends on both your point of view and your experience with carburettors, but in general terms, one could answer both Yes and No, and be right both times.
Confused? Well, one could look at this issue in this way; if you have extensive experience with carburettors, then it becomes difficult to see meaningful differences between carburettors and fuel injection beyond their different efficiencies since the operating principles of fuel injection systems are just refined versions of the operating principles of carburettors.
If, however, you have limited or no experience with carburettors, you might be inclined to see them as arcane, anachronistic devices that have no place in the modern motoring world. However, and overall, there is no denying that if one ignores or discounts emissions regulations, a properly calibrated carburettor can extract the same power out of any given engine as any fuel injection system fitted to the same engine.
One final thing to consider is this: regardless of whether you are faced with a defective fuel injection system or a poorly performing carburettor, you need at least a basic understanding not only of whatever system you are faced with but also of the operating principles of each delivery system. Thus, given that even modern fuel injection systems have evolved directly from carburettors and, therefore, work in much the same way, you should not have undue difficulties in diagnosing and fixing either fuel delivery system.
