Friday, August 14, 2015

The Carburetor: Elegance and the Real World

The carburetor is a simple, elegant device for providing fuel to a gasoline engine. So simple and elegant that it was the primary mechanism for doing so for approximately 100 years, and remains the primary mechanism for small engines such as lawnmowers.

Here's a simplified explanation of how it works: all air entering the engine flows through the carburetor; the “jet,” a small tube connected to the supply of gasoline, is exposed to the airflow; as air flows through the carburetor body, it pulls gasoline out of the jet and vaporizes it. The more air flowing past the jet, the more fuel is drawn into it. By properly sizing the jet, you can approach a near-optimal fuel-air mixture — although more on that later.

The airflow through a carburetor is caused by the intake stroke of the engine: as the piston moves down in the cylinder, it creates a vacuum that pulls air from the intake manifold; the faster the engine turns, the more air it wants (and the more fuel it needs). Now here's the interesting part: there's a valve inside the carburetor, attached to the accelerator pedal: when your foot is off the pedal, that valve is (almost) closed; the engine can only get enough air (and fuel) to run at idle speed. When you push the pedal to the floor, the valve opens wide, and the engine gets all the air and fuel that it wants.

But that leads to the first problem with a carbureted engine: the feedback loop (and lag) between opening the throttle and actually going faster. At a normal cruise speed, the butterfly valve is only partway open; the engine doesn't get all the air that it can handle. When you open the throttle, that lets more air into the system, and indirectly, more fuel. More fuel means more power generated by the engine, which (given constant load) means that the engine will speed up. Which increases the vacuum, which means that more air is drawn into the system, which means that more fuel is provided with it, which …

But, as I said, there's lag: when you first allow more air into the system, the flow of gasoline doesn't keep up. So at some point carburetor designers added an “accelerator pump”: an actual pump that squirts extra gasoline into the airflow. Most of the time it does nothing, except when you push the pedal to the floor; then it sends that extra gasoline to the engine, to compensate for the flow through the main jet.

While the single carburetor works fairly well for small engines, it doesn't work so well for large engines that have widely differing airflow requirements between idle and full throttle. So, carburetor designers compensated with multiple barrels: separate paths for the air to follow. At idle the “primaries” provide all the air and fuel; under full acceleration, the “secondaries” open to provide vastly more air and fuel.

All of which works fairly well as long as the altitude doesn't change. A typical automotive carburetor is sized for its market, on the fairly reasonable assumption that drivers stay close to home. But take an east coast car, tuned for driving at sea level, up into the mountains of Colorado, and the mixture becomes excessively rich: the carburetor allows too much fuel to mix with the air, and (perhaps surprisingly) performance suffers. Eventually the spark plugs will get coated with a layer of soot. A worse fate meets the Colorado car driven to the shore: it doesn't provide enough fuel, which causes the engine to run hot, which eventually causes extensive (and expensive) damage to the engine's valves and pistons.

Light aircraft, which cover a similar altitude range on every flight, have a solution: the mixture control. After every significant change in altitude, the pilot has to re-adjust the mixture to match the air density at that altitude. In a typical long-distance flight, the pilot might change throttle settings three times (takeoff, cruise, landing) but adjust mixture a half-dozen times or more. Not something that the typical automobile driver would want to do (or do particularly well; all teenagers' cars would be overly rich “for more power”).

The simple, elegant carburetor is no longer so simple or elegant: to meet the needs of the real world, it's grown a bunch of features. These features expand what I'll call the “conceptual model” of the carburetor. A simple function relating airflow velocity and fuel delivery is not sufficient to implement the real-world model.

Today we use fuel injection in almost every car (and in most high-performance light aircraft). Fuel injection systems are certainly not simple: a computer decides how much fuel to inject based on inputs from a plethora of sensors, ranging from ambient temperature to position of the accelerator pedal. But fuel injectors do a much better job of providing exactly the right amount of fuel at exactly the right time.

And, although a fuel injection system is more complex than a carburetor, its conceptual model is actually simpler: there's a single sensor that measures the amount of residual oxygen in the exhaust, and the computer attempts to optimize this value.

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