A car engine is a machine whose basic purpose is to convert fuel into motion. An internal combustion engine accomplishes this conversion by isolating a small amount of fuel in a contained space and igniting it, converting it to an expanding gas that releases energy. That energy gets translated into motion by the many different parts of the engine working together in a precise and continuous cycle of activity. Understanding the various parts of the engine and how they work together is an important first step in being able to improve the efficiency and performance of the engine.
Parts of the Engine and What They Do
Inside the main body of the engine (the engine block) are cylinders. Modern cars have engines that have four, six or eight cylinders. Inside each cylinder is a piston that moves up and down. When fuel is injected into the cylinder, the movement of the piston controls the volume of air within the cylinder, providing space for the fuel and air to mix and then compressing it to aid in combustion. The pistons are connected to the crankshaft by a connecting rod. The crankshaft translates the up and down movement of the pistons into the rotational motion that moves the car. On the crankshaft there is a gear that attaches to the camshaft. The camshaft controls the intake and exhaust valves. The camshaft has egg shaped structures on them (cams) that push the valves open and closed, allowing the fuel to enter the cylinder and expelling the products of combustion after ignition.
The ECU controls the processes of the engine. It first sends signals to the injectors controlling the amount of fuel going into the cylinder by indicating how long the injectors should stay open. The injectors spray the fuel drops into the cylinder. There are many different sized injectors for use with different engines. It is important to have injectors that are the correct size for the engine being used. If injectors are too large or too small the car will not perform at maximum efficiency.
The ECU also sends a signal to the spark coil so that it sparks at exactly the right time to ignite the fuel (when compression within the cylinder is greatest). The spark coil generates the 35,000 volts needed for the spark to initiate combustion from the 12 volt battery. The spark coil is a transistor so it amplifies the voltage, causing the spark in the spark plugs, which in turn ignite the fuel-air mixture.
The Four-Stroke Combustion Cycle
Most automobile engines today operate using a four-stoke combustion cycle. A four-stroke engine goes through four distinct stages. The first stage is intake. In this stage the piston moves down, and the intake valve opens allowing the air/fuel mixture that was combined by fuel injection to be pushed though the intake valve into the cylinder. The next step is compression.
In this stage the piston moves up causing the mixture that was just sprayed into the chamber to be compressed. Before top dead center is reached, the spark plug goes off. The third stage of the 4 stage cycle is combustion (or power). The air/fuel mixture ignites and the piston is forced down. For ideal torque (for the most power) the combustion should be completed when the crankshaft is 90 degrees beyond top dead center. The last step is exhaust. In this stage the piston is moving up and pushing out the products of the combustion reaction out of the car through the exhaust pipe. The cycle then starts over again.
Optimizing Engine Performance Using an Electronic Fuel Injection System
Tuning the engine to achieve maximum efficiency is a huge part of getting the best performance out of the race cars. Making adjustments to the fuel injection system to change the amount and release time of fuel, changing the size or flow of fuel injectors, and utilizing the many available engine sensors to make other modifications can all have a great impact on engine horsepower and efficiency.
All of the above sensors and parameters are controlled by the car’s engine control unit, or ECU. The ECU in an electronic fuel injection systems sends signals to the fuel injectors, indicating how long to remain open in order to reach the maximum efficiency. The fuel is added to the air mixture at the latest possible time (right before combustion). The ideal air to fuel ratio, called the stoichiometric value, is 14.68 parts air to one part fuel (AFR is 14.68:1 by mass).
This is the ratio of air to fuel that allows the oxygen and fuel to burn completely. However, conditions are never ideal so the most efficient AFR depends on the temperature, the rotations per minute of the engine (the engine’s rpm), and the load on the engine (how much work the engine is doing).
Different-sized injectors give different amounts of fuel per certain time. The amount of fuel an injector sprays is the injector flow which is measured in pounds per hour at a certain pressure. Every injector has an upper limit of fuel and a lower limit of fuel. The upper limit is when the injector stays completely open or at full throttle. The lower limit is when the time it takes for the injector to open is longer than the time the injector needs to be open for (it starts to close before it finishes opening). The size of the injector valve also has an impact on the injector flow rate. The smaller the valve opening, the less fuel will be sprayed. Pressure also affects the injector flow rate. When there is lower pressure there is less fuel sprayed into the engine than if there were higher pressure.
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