Direct injection, as the name suggests, refers to a type of fuel system which introduces fuel directly into an engine's combustion chambers. This differentiates it from
conventional injection systems
, which typically introduce fuel into the engine at points in the intake system or inlet manifold.
How does direct fuel injection work?
Instead of having injectors in a throttle body, or in the intake manifold, a direct injection system makes use of an injector that fires into the combustion chamber.
Virtually all diesels use direct injection, as it's the introduction of the high-pressure spray of fuel into the engine's combustion chamber that starts the combustion cycle. The injectors are typically electronically triggered and fed by a 'common rail' fuel system, which supplies them with fuel at the required pressure.
Older or industrial diesels, however, may use 'unit injectors' - which are mechanically compressed by a camshaft to generate the high pressures required - or in-line pump set-ups. These aren't as flexible as common-rail systems, however.
Common-rail direct injection is also often found in petrol engines, particularly high-performance ones. Like modern diesels, the injectors rely on a common fuel supply and are electronically operated. However, some engines - like the V10 found in the Audi R8 - blend direct and indirect port injection to combat some of the issues that can arise when using only direct injection.
Both petrol and diesel direct injection systems operate at far higher pressures than conventional systems, however; this fine high-pressure spray helps the fuel to atomise and mix with the air in the chamber in the short window available.
What are the key benefits of common-rail direct injection?
Modern common-rail direct injection systems offer several benefits when compared to indirect injection. Primarily, they allow for more accurate fuel delivery at very specific times, improving mixture and combustion control. This can result in improved power output, reduced emissions and lower fuel consumption.
Compression ratios can be increased in petrol engines with direct injection, too, as there is no fuel in the combustion chamber until just before the power stroke; this reduces the chance of pre-ignition occurring. The cooling effect of the fuel being injected into the combustion chamber reduces the chance of engine-damaging knock, too, again permitting safe higher compression ratios - and additional power and efficiency gains.
Common-rail direct injection also allows for multiple injections during the combustion cycle. These can be used to more precisely control combustion in the chamber. In particular, multiple injections are used in diesel engines to reduce the clatter typically associated with diesel combustion.
Are there any drawbacks to direct injection?
It is always more complicated and more expensive, for starters. The increased complexity can cause reliability issues and servicing direct injection systems can be more challenging due to the high fuel pressures involved.
The direct injection systems used in many a modern petrol engines can suffer from other issues, too. In a conventional injection set-up, fuel will pass through the intake port and over the valve. This washes combustion by-products away. This doesn't happen in a direct-injection engine, however, and the carbon deposits that build up can reduce performance or, in extreme cases, be ingested into the engine and cause damage.
Direct petrol injection, while cleaner overall, can also cause excess nitrous oxide and soot emissions due to different combustion temperatures and mixtures that result compared to normal injection systems. As mentioned previously, some engines use both direct and port injection to reduce these emissions and the chance of carbon build-up.
A brief history of direct injection
Direct injection has been used in diesels since the early 1800s, in various forms. The more advanced common-rail diesel set-ups, in automotive applications, arose in the late 1990s.
Direct petrol injection appeared in the early 1900s but became far more prominent during World War II, as the system was used in myriad high-performance aviation engines. The first production direct-injection petrol car, which used a diesel-style in-line fuel pump, was the 1955 Mercedes-Benz 300SL
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