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Direct injection explained

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Khatir Soltani
Technically similar to a diesel engine's injection, direct gasoline injection technology promises an improvement in fuel efficiency as well as a reduction in emissions.

For any gasoline engine to operate properly, it needs the right combination of air and fuel for combustion. The ideal mixture is 14.8 grams of air for each gram of fuel. The role of the injector -- and, before that, the carburetor (still used in NASCAR) -- is to properly calculate and mix air with fuel.

If the mixture is too lean (not enough gasoline), the emission of nitrous oxides (NOx) will increase, engine knocking will likely ensue, fuel consumption will decrease, ignition will be hesitant and finally power will be affected. Ultimately, the engine will stop working. On the other hand, if the mixture is too rich (too much gasoline), fuel consumption will increase, pollution caused by carbon monoxide (CO) and unburned hydrocarbons (HC) will also increase and power will enjoy a short-lived boost before dropping significantly. In the end, the engine will stop working once again.

Using a pneumatic system, carburetors measure the volume of air swallowed by the engine and then allow a similar volume of gasoline to enter. However, they present three major problems. First, measuring air by volume instead of weight means no corrections are made to take air density changes into account. Carburetors have to be constantly readjusted based on ambient temperature and altitude.

Second, the pneumatic system is slow to react, which leads to a continuous variation in fuel delivery. And third, when using a single carburetor connected to the cylinders by a relatively long intake manifold, the engine's mixture is not uniform -- rich near the carburetor, it becomes lean away from it. To prevent the lean extremities from causing engine knocking, the mixture must always be enriched.

The 4 cycles of operation

Indirect injection
Universally used in modern automobiles, indirect injection is fairly simple: each cylinder has its own injector, located inside the intake manifold, as close as possible to the cylinder's intake valve. As a result, an indirect fuel injection system delivers the same, precisely-calculated mixture to all cylinders. The advent of indirect injection and electronic control devices measuring several parameters such as intake air weight has not only allowed a significant reduction in emissions and fuel consumption but also an increase in power.

Direct injection now finds its way into a growing number of gasoline-powered cars. Injection of fuel into air is performed inside the combustion chamber, similar to a diesel engine except for timing. By injecting fuel directly into the combustion chamber instead of the intake manifold and by maintaining ultra-tight control over the timing of gasoline delivery, a direct injection system is able to vary the air-fuel mixture from rich to lean or vice versa.

It favors a rich mixture to provide more power under acceleration or when going uphill. On the other hand, at low or cruising speeds (read: most of the time), the system favors a lean mixture to save fuel. The use of a three-way catalytic converter reduces emissions to a level deemed acceptable, whether the mixture is rich or lean.

Stratified injection
Here, the engine runs on a lean, efficient mixture and, with precisely-timed distribution, enables a stratified fuel charge. This means that the mixture is rich near the spark plugs to ensure proper ignition and gradually leaner as it moves away from the plugs to save fuel. A stratified fuel charge allows an average mixture that's lean and efficient while ensuring dynamic ignition to prevent engine knocking.

Direct injection = more power, more economy and fewer emissions.

P.S.: Direct injection on gasoline engines is not really new technology. It was used by several planes that took part in World War II.
photo:Honda, General Motors
Khatir Soltani
Khatir Soltani
Automotive expert
  • Over 6 years experience as a car reviewer
  • Over 50 test drives in the last year
  • Involved in discussions with virtually every auto manufacturer in Canada