Engine performance systems (Part one)

Engine performance systems (Part one)

Engine performance systems are those responsible for how an engine runs. How well an engine runs depends on the combustion process. Today’s systems are designed to achieve as close to complete combustion as possible. Basically if all of the fuel that enters an engine’s cylinder is burned, combustion is complete. Complete combustion will occur when the correct amount of air is mixed, in a sealed container, with the correct amount of fuel and shocked by the correct amount of heat at the correct time.

Emission control devices are added to the vehicle because complete combustion at all times has not been achieved. These devices reduce the amount of bad vehicle emissions. They also affect the operation of the engine and are therefore, an engine performance system.

The ignition systems:

For each cylinder, the ignition system has three roles:

  • It generates an electrical spark with enough heat to ignite the air fuel mixture in the combustion chamber.
  • It maintains that spark long enough to allow total combustion in the chamber.
  • It delivers the spark to each cylinder to allow combustion to begin at the right time during the combustion stroke.

Because the combustion process requires a short period to complete, normally measured in thousandths of a second, combustion must begin before the piston on its power stroke. Therefore, the delivery of the spark must be timed to arrive at some point before the piston reaches TDC.

Determining how much before TDC is complicated. The speed of the piston moving from its compression stroke to the power stroke changes, whereas the time required for combustion remains the same. This means the spark should be delivered earlier as the engine’s speed increases. However, when the engine is doing much work, the load on the crankshaft tends to slow down the acceleration of the piston, and spark delivery should be somewhat delayed.

There are also other factors that can affect combustion times. Higher compression ratios tend to speed up combustion. Higher octane fuels ignite less easily and require more burning time. Increased vaporization and turbulence tend to decrease combustion times. Other factors, including intake air temperature, humidity, and barometric pressure, also affect combustion. Because of all of these, delivering the spark at the right time is a difficult task.

Engine speed: At higher engine speeds, the crankshaft rotates through more degrees in a given period. If combustion is to be completed by 10 degrees ATDC (After Top Dead Center), ignition timing must begin sooner. When the engine is cranked to start it, the ignition timing is retarded.

Engine load:

Under light loads a high vacuum exists in the intake manifold. The density of the air-fuel mixture drawn into the cylinders is low. On compression this thin mixture produces less combustion pressure and combustion time is slow. To complete combustion by 10 degrees ATDC, ignition timing must be advanced.

Under heavy loads, the vacuum in the cylinders is low and a very dense mass of air and fuel is drawn into the cylinders. High combustion pressure and rapid burning occur. In this case, the ignition timing must be retarded to prevent the combustion process from ending before 10 degrees ATDC.

Firing Order:

   Each cylinder must produce power once every 720 degrees of crankshaft rotation. Therefore, the ignition system must provide a spark at the right time so that each cylinder can have a power stroke at its own appropriate time. To do this, the ignition system must monitor the rotation of the crankshaft and the relative position of each piston to determine which piston should be delivred the spark.

Computer controlled systems:

With computerized ignition systems, inputs usually consist of engine temperature, engine speed, and manifold vacuum. There may be other sensors for throttle position, incoming air temperature, or engine knocking (detonation). The computer processes these inputs and advances or retards spark timing as required. This causes changes in engine operation, which sends new messages to the computer. The computer can constantly adjust timing for maximum efficiency.

The advantage of the electronic spark control system is threefold. It compensates for changes in engine temperature. It makes changes at a rate many times faster than older systems. And , finally, it has a feedback mechanism in which sensor readings allow it to constantly compensate for changing conditions.


Fuel system:

The fuel delivery system is delivering a clean fuel to the fuel injection system in the right quantities at the right pressure.

A typical fuel delivery system includes a fuel tank, fuel lines, fuel filters and a pump. The system works by using a pump to draw fuel from the fuel tank and passing it under pressure through fuel lines and filters to the fuel injection system. The filter removes dirt and other harmful impurities from the fuel. A fuel line pressure regulator maintains a constant high fuel pressure. This pressure generates the spraying force needed to inject the fuel. Excess fuel not required by the engine returns to the fuel tank through a fuel return line.

Electronic fuel injection (EFI) has proven to be the most precise, reliable, and cost effective method of delivering fuel to the combustion chambers of today’s engines. EFI are computer controlled and designed to provide the correct air-fuel ratio for all engine loads, speeds, and temperature conditions.

Most EFI systems only inject fuel during the engine’s intake cycle. The engine’s fuel needs are measured by intake airflow past a sensor. The airflow sensor converts its reading to an electrical signal and sends it to the engine control computer. The computer processes the signal (and others) and calculates the fuel needs of the engine. The computer then sends an electrical signal to the fuel injector. This signal determines the amount of time the injector opens and sprays fuel. This interval is known as the injector pulse width.







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