Function of an engine
Some important background to understand with all of this is the function of a car's engine. An engine, in the most basic sense, is a machine that converts chemical energy into mechanical work which, in a car, drives the wheels, usually via a thermodynamic process. The majority of cars from the early days of motor vehicles through the present (though I predict will end soon), including Bugs, use some form of a four-stroke spark-ignited internal combustion reciprocating piston engine, commonly referred to as the otto cycle engine, or gasoline engine (for the fuel used.) The basic operation is as follows. The cycle starts with the piston at the top of the cylinder – note that "top" refers to the piston being closest to the cylinder head, the smallest cylinder volume, and "bottom" is opposite this even in engines where the cylinders are not arranged upward (in bugs they are oriented horizontally), it is just a terminology convention. The intake valve opens and the piston moves down, drawing in a previously mixed flammable mixture of fuel and air – the process of mixing this fuel and air is the main point I'm going for, and shall be elaborated on later. Once the piston has reached the bottom of the cylinder, the intake valve closes and the piston moves back upward. This compresses the mixture of fuel and air on the compression stroke, a process that requires energy input – this comes from the momentum of a flywheel on the crankshaft, power strokes of other cylinders, or a combination of those two. With the piston near the top, the spark plug fires, a low-current high-voltage jolt of electricity jumping across a small gap, igniting the fuel-air mixture. The fuel air mixture burns rapidly, contrary to popular belief it does not explode. It is possible for it to explode under certain conditions, a situation called Detonation, and often referred to as "knocking" or "pinging" for the sound it makes, this is very damaging to the engine and must be avoided. The burning causes a large increase in temperature and pressure inside the cylinder, driving down the piston with great force, putting out far more energy than was required on the compression stroke for a net output of work – this is the power stroke. Then the exhaust valve opens and the piston moves upward for it's fourth stroke, the exhaust stroke, to exhale the burned combustion products out of the cylinder. The cycle then repeats with the intake valve opening again, all having occurred in four movements of the piston, or "strokes," with the crankshaft having rotated twice. In most car engines this entire cycle occurs several thousand times per minute, the maximum for a Bug engine is around 5,000 RPM, thus 2,500 cycles per minute or about 42 times per second – mind-bogglingly fast. This process occurs in all the cylinders simultaneously, each in a different part of the cycle at any given moment.
A very important factor in all of this functioning properly is the aforementioned mixing of fuel and air prior to it's entry into the cylinder. This is a very important function for an engine to run properly, produce good power, have decent fuel economy, last a long time, and minimize harmful pollution in the exhaust. Gasoline consists mainly of hydrocarbons, chains of carbon atoms with attached hydrogen atoms. These react with the oxygen in air, the carbon atoms bonding with oxygen to form carbon dioxide, the hydrogen atoms reacting with oxygen to create water – the latter is the cause of steam being visible from the exhaust on a cold day, water sometimes dripping from tailpipes, mufflers rusting far more rapidly than other parts, and the contrails trailing behind jet airliners (they have different types of engines, but they also burn hydrocarbon fuels and have similar exhaust composition). In theory the optimal is a "Stoichiometric" ratio, which means that there is the exact right ratio of gasoline to oxygen from the air so that all of both are consumed, no unburned hydrocarbons, carbon monoxide, or hydrocarbons left over after combustion. This ideal ratio is 14.7 units air to one unit gasoline, by weight. In reality, this "optimal" situation isn't always used, even if it is possible to deliver the correct ratio, because it could cause the engine to run too hot (basically by being too efficient), especially in an air-cooled engine, or could be difficult to reliably ignite, so in many cases a slightly "richer" mixture, a mixture containing a small excess of gasoline, is used for best results.
It is thus necessary to somehow mix the fuel and air in the right proportions. This can be a fairly complicated task, especially in a car, as the correct ratio must be delivered under a wide range of conditions – a range of engine speeds, power, throttle position, air pressure, air temperature, engine temperature, and so on, with some of these parameters rapidly changing. Throughout history, there have been two main methods in use for metering fuel into the passing air. The only practical way of meeting this need from the early history of automobiles through the mid 20th century was a mechanical device, the Carburetor. Starting in the mid 1960s, and gradually gaining dominance until it had become universal by the mid 1980s, as it remains today, is the more technologically advanced computer-controlled Fuel Injection.
It is thus necessary to somehow mix the fuel and air in the right proportions. This can be a fairly complicated task, especially in a car, as the correct ratio must be delivered under a wide range of conditions – a range of engine speeds, power, throttle position, air pressure, air temperature, engine temperature, and so on, with some of these parameters rapidly changing. Throughout history, there have been two main methods in use for metering fuel into the passing air. The only practical way of meeting this need from the early history of automobiles through the mid 20th century was a mechanical device, the Carburetor. Starting in the mid 1960s, and gradually gaining dominance until it had become universal by the mid 1980s, as it remains today, is the more technologically advanced computer-controlled Fuel Injection.
Carburetor
The earlier of these technologies was the carburetor. The carburetor works on a fairly simple concept, and in it's most basic form is a pleasingly elegant system. The main idea is that of a venturi. Air headed into the engine is drawn through a pipe that has a somewhat narrower throat inside it, called a venturi. As the diameter of this throat is narrower while the volume of air passing through is constant, the air must speed up as it goes through the throat. To speed up, the pressure energy of the air is converted into kinetic energy, resulting in a reduction of pressure inside the throat, after which the air slows down and returns to it's original pressure. This difference in pressure between the air in the throat and elsewhere in the carburetor is proportional to the mass flow rate of air through the carburetor. Fuel is held in a float bowl alongside this, the level in which is controlled by a float valve very similar to that of a toilet. The surface of this liquid is connected to the inlet of the carburetor, so pressure inside it is equal to that of the air, while it is drawn out through a "jet," or nozzle of a very precise size, by the lower pressure in the throat. The rate of fuel being drawn through the jet is proportional to the pressure difference in the throat, which is proportional to the flow rate of air, so fuel and air are mixed at the desired ratio regardless of flow rate. The throttle valve, usually a butterfly valve, is downstream of this, to control the rate of fuel and air admitted into the engine. It is connected to the accelerator pedal used by the driver to control engine power.
At least that's the theory. In reality, this doesn't work so well as simply as described. There are a lot of challenges that make it more difficult, especially in a car engine. Car engines work under a wide variety of conditions, which the carburetor must accommodate. The engine must work well at idle, when the flow rate of air is very low, as well as at full throttle and high speed when the flow rate is very high. It must be able to contend with the engine running fast but the throttle hardly open, as when descending a hill, as well as full throttle with the engine running slowly, as when climbing or accelerating away from a traffic light. All these varied situations cause very different rates of flow through the venturi, the design of which can only be optimized for a certain condition. There are also variations in temperature, both of the air and of the engine, which the carburetor has a hard time adjusting for. Same goes for changes in outside air pressure, particularly caused by changes in altitude. And all this works best when everything is "steady state," which often isn't the case, as the driver must be able to change throttle setting rapidly, and, especially when driving in traffic, having power right away when "stepping on it" is very important. In order to make it function in all these situations, various other devices must be added to the carburetor. Typically a throttle bypass that is essentially a miniature auxiliary carburetor is used when the throttle is closed and the engine is at idle, as the main venturi is not very effective with such low airflow. A small pump, the "accelerator pump," connected to the throttle linkage squirts fuel straight into the throat when the throttle is opened rapidly, as the flow rate of fuel via the venturi cannot respond instantaneously to changes in airflow. To provide a richer mixture necessary for a cold engine to run, a choke in the main inlet is used to further reduce pressure and draw in more fuel – this can be manually controlled by the driver, or in many cases automatically by an electrically heated bi-metallic spring. In Fenix this Auto-Choke was probably the most troublesome component on the entire car. Some more complicated (thus expensive) carburetors employ multiple barrels, sometimes having a smaller one that opens first then another that opens when more power is needed, in order to optimize efficiency in all situations, at the expense of even more complexity. Thus this seemingly simple idea results in a very complicated device, with many manual adjustments that must be kept in order and adjusted for different operational conditions, and plenty of opportunities for malfunctions. The problems don't end there – another problem is that between the carburetor and engine the mixed fuel and air must pass through some lengths of pipe in the intake manifold – in the Volkswagen, this is fairly long. Especially in cold weather the fuel can condense out of the air and not reach the engine, causing very poor running. In many water-cooled cars this was solved by warming the manifold with a water jacket that passes warm engine coolant around it, in the air-cooled Volkswagen (which has no coolant) a pipe carries exhaust alongside the manifold, known as the “heat riser” – a pipe that has a tendency to clog, and isn't adequate on the coldest days. Given all these situations, carburetors are a series of compromises. Between cost and complexity. Between good running, fuel economy, and clean emissions. Between working best under a wide variety of circumstances. It is basically never ideal or optimal.
Fuel injection
Due to these drawbacks, carburetors are basically obsolete except for a few specialty applications such as small engines (lawn mowers) and some airplanes (reluctant to adopt new technology.) Motivated by increased gasoline prices, new pollution regulations, and increasingly capable and cheap computer devices, a new design was developed starting in the late 1960s and maturing, fully dominating the market, in the late 1970s and early 1980s. This is fuel injection, a computerized system that uses a range of sensors to calculate the required amount of fuel, then delivering it via electric-controlled injectors. On VW Bugs, a fuel injection system, the Bosch "L-Jettronic" was introduced in 1975 (two years after Fenix was built), though it was crude and temperamental by modern standards. Later on, after aircooled VWs were no longer offered in the US market, but were still sold in other countries, particularly Mexico, a newer, more advanced "Digifant" system was developed.
This is a much more precise way of metering fuel into the engine, which has several benefits. The car handles much better, responding much more rapidly to changes in throttle position. Though more complicated in concept, the system has fewer moving parts and often no mechanical adjustments, meaning it typically requires far less maintenance and is more reliable. It is better able to adapt to different circumstances, particularly cold weather. (This was one of the bigger motives to make this conversion in Fenix, as with the carburetor she often ran rather poorly in extremely cold weather – it has yet to be seen if the fuel injection will help with this though as the weather has been relatively warm since the conversion.) Emissions are significantly improved, as the engine can run far closer to the optimal stoichiometric ratio, meaning far less unburned hydrocarbons and other pollutants are leftover in the exhaust. This can further be improved using a catalytic converter, which all modern cars are equipped with. Power is improved somewhat, especially when the engine is running slowly (low-end torque) as carburetors don't perform well in that situation. And fuel economy is substantially improved. Overall, the benefit is that fuel injection allows for everything to be optimized in every situation, while carburetors are always a series of compromises to work well enough in all required situations. The biggest drawback, is that, in general, the system is more expensive, though in all modern cars this is considered worthwhile.
The main component is the Engine Control Unit, henceforth termed the ECU, which is a computer that controls everything. The ECU receives inputs from various sensors attached to the engine that provide it with various parameters. These typically include:
- Engine speed and position (sometimes encoded at individual angles by a toothed wheel, some cases, such as Fenix, simply an electronic signal sent when the engine is at a set position). This is necessary because the ECU will fire the ignition at a very specific time based on it's decided advance. By counting the frequency of this signal the ECU can determine the engine revolutions per minute, which is important for some parameters such as spark advance and fuel quantity.
- Manifold Absolute Pressure, often termed MAP. This sensor tells the ECU how much air pressure is going into the engine, as it must mix the correct amount of fuel relative to this. In some systems this is an electronic sensor mounted on the engine, but in many cases it is a hose connected to the ECU, with the electronic sensor inside.
- Throttle Position Sensor: this uses a potentiometer to determine the throttle setting, particularly for use when it is rapidly changed (the driver “steps on it”) so that the ECU can respond quicker than the MAP sensor will detect the change.
- Air Temperature: This detects how cold or warm the air going into the engine is, in order to increase fuel flow into the engine when it is cold as colder air is more dense, and fuel vaporizes less readily.
- Coolant temperature, or in Fenix's case, cylinder head temperature. This sensor detects how warm the engine is, adding more fuel when the engine is cold.
- Oxygen sensor, often referred to as the “O2 sensor” and the signal coming from it is sometimes referred to with the Greek letter Lambda. This is a fairly complicated sensor that measures the quantity of leftover oxygen in the exhaust. It can provide direct feedback to the ECU to know if the mixture is too rich or too lean. These come in different types, many cars use “narrowband” sensors, while Fenix uses a more complicated and expensive “Wideband” sensor system that can detect a wider range of mixtures further from stoichiometric.
Using signals from these sensors, the ECU decides how much fuel is necessary for the engine to run optimally, based on internal programming. It triggers injectors, in most cases one per cylinder, by sending an electric current through an electromagnet that opens a plunger valve. This squirts fuel into the manifold, in most cases immediately upstream of the intake valve. The injectors are either open or closed, so the amount of fuel is controlled by how long the injector is open.
This is a much more precise way of metering fuel into the engine, which has several benefits. The car handles much better, responding much more rapidly to changes in throttle position. Though more complicated in concept, the system has fewer moving parts and often no mechanical adjustments, meaning it typically requires far less maintenance and is more reliable. It is better able to adapt to different circumstances, particularly cold weather. (This was one of the bigger motives to make this conversion in Fenix, as with the carburetor she often ran rather poorly in extremely cold weather – it has yet to be seen if the fuel injection will help with this though as the weather has been relatively warm since the conversion.) Emissions are significantly improved, as the engine can run far closer to the optimal stoichiometric ratio, meaning far less unburned hydrocarbons and other pollutants are leftover in the exhaust. This can further be improved using a catalytic converter, which all modern cars are equipped with. Power is improved somewhat, especially when the engine is running slowly (low-end torque) as carburetors don't perform well in that situation. And fuel economy is substantially improved. Overall, the benefit is that fuel injection allows for everything to be optimized in every situation, while carburetors are always a series of compromises to work well enough in all required situations. The biggest drawback, is that, in general, the system is more expensive, though in all modern cars this is considered worthwhile.
The main component is the Engine Control Unit, henceforth termed the ECU, which is a computer that controls everything. The ECU receives inputs from various sensors attached to the engine that provide it with various parameters. These typically include:
- Engine speed and position (sometimes encoded at individual angles by a toothed wheel, some cases, such as Fenix, simply an electronic signal sent when the engine is at a set position). This is necessary because the ECU will fire the ignition at a very specific time based on it's decided advance. By counting the frequency of this signal the ECU can determine the engine revolutions per minute, which is important for some parameters such as spark advance and fuel quantity.
- Manifold Absolute Pressure, often termed MAP. This sensor tells the ECU how much air pressure is going into the engine, as it must mix the correct amount of fuel relative to this. In some systems this is an electronic sensor mounted on the engine, but in many cases it is a hose connected to the ECU, with the electronic sensor inside.
- Throttle Position Sensor: this uses a potentiometer to determine the throttle setting, particularly for use when it is rapidly changed (the driver “steps on it”) so that the ECU can respond quicker than the MAP sensor will detect the change.
- Air Temperature: This detects how cold or warm the air going into the engine is, in order to increase fuel flow into the engine when it is cold as colder air is more dense, and fuel vaporizes less readily.
- Coolant temperature, or in Fenix's case, cylinder head temperature. This sensor detects how warm the engine is, adding more fuel when the engine is cold.
- Oxygen sensor, often referred to as the “O2 sensor” and the signal coming from it is sometimes referred to with the Greek letter Lambda. This is a fairly complicated sensor that measures the quantity of leftover oxygen in the exhaust. It can provide direct feedback to the ECU to know if the mixture is too rich or too lean. These come in different types, many cars use “narrowband” sensors, while Fenix uses a more complicated and expensive “Wideband” sensor system that can detect a wider range of mixtures further from stoichiometric.
Using signals from these sensors, the ECU decides how much fuel is necessary for the engine to run optimally, based on internal programming. It triggers injectors, in most cases one per cylinder, by sending an electric current through an electromagnet that opens a plunger valve. This squirts fuel into the manifold, in most cases immediately upstream of the intake valve. The injectors are either open or closed, so the amount of fuel is controlled by how long the injector is open.