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Electronic fuel injection was primarily intended to address initial emission regulations. (Andrzej Thiel/photos.com)
Electronic fuel injection was primarily intended to address initial emission regulations. (Andrzej Thiel/photos.com)

How It Works

How direct injection works Add to ...

Direct injection is all the rage these days. It has become the most significant development in internal combustion engines since, well, fuel injection. But the difference between the two is more complex than it would appear.Electronic fuel injection was primarily intended to address initial emission regulations. One role was to ensure cleaner, more consistent combustion and the other to ensure no major tune-ups for 100,000 miles.

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Fuel injection did away with the carburetor, which required frequent (in terms of mileage) adjustment and was especially “dirty” during cold starts. Carburetors rely on fuel and air being sucked into the engine by the down-stroke of the pistons on the intake stroke. Fuel injection involves atomizing the fuel by forcing it under pressure through a small opening, forcing the fuel into the intake stream. Fuel injection brought cleaner combustion and, thanks to cleaner unleaded fuel, spark plugs lasted much longer and the combustion chamber remained cleaner. By the late 1980s, carburetors were a memory.

Initially, fuel injection was done through a single point in the bottom of what had previously been a carburetor, throttle body injection squirted fuel into the airstream above the intake manifold. Constant development gradually saw an increase in the number of injectors to one per cylinder and they were placed in closer proximity to the intake port of the engine.

As engine control computers became faster and more powerful, and injectors themselves developed further, engineers were able to more accurately control the amount of fuel injected and the timing of that injection of that fuel into the combustion chamber atop the piston and adjacent to the spark plug. Electronic fuel injection as used across the industry today had reached its zenith.

The ongoing battle to squeeze more efficiency out of the internal combustion engine centres around the fact that up to 70 per cent of the energy of the fuel placed in an engine is wasted through heat or pumping losses. Controlling the combustion process became a mantra in every engine R&D lab around the globe.

Modern developments in computer simulation and the ability to actually see inside a combustion chamber have led to the knowledge that, even with the latest version of electronic fuel injection, the actual combustion is a haphazard affair in terms of what happens when the spark ignites the fuel/air mixture. In this incredibly short time frame, the resulting flame has hot and cold points, and goes in a variety of directions within that small chamber.

Researchers discovered that controlling the direction and location of the fuel/air mixture prior to and following combustion allowed them to squeeze more energy from the event, to get more bang per unit of fuel. But this required even greater control of the injection of the fuel and the timing and location of the spark.

Direct injection to the rescue.

When a conventional fuel injection systems squirt atomized fuel into the combustion chamber, it does so at a pressure of about 120 pounds psi (per square inch); high-pressure direct injection does it at 2,200-2,600 psi through a series of tiny holes.

As a result, the fuel is broken into millions of tiny particles before combustion. In conjunction with sophisticated injectors, redesigned piston tops and extremely accurate ignition systems, there are actually multiple spark/ignition events on each stroke of the piston.

A tiny amount of fuel may be introduced and ignited initially to get things started, directing the fuel/air mixture in a certain direction before the main event. The main spark can thus result in a more controlled and thus more efficient explosion. A following spark event can ensure all the spent gasses are expelled before the next batch is brought in.

All of this is thanks to modern and powerful computers and fast-acting injectors and ignition systems. The force of the injection and the fast injectors are the source of the “rattle” or noise from an engine with direct injection. Direct injection lowers the combustion chamber temperature allowing the use of lower-octane fuel – or more power with higher octane – and the ability to adjust an engine automatically to either one.


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