I understand that direct injection puts the air/fuel mix into the cylinder body directly vs the intake valve like the sequential does. What are the advantages of doing so? E.g. does it yield a better gas mileage?

  • I would assume lesser wastage in case of direct injection? Interested to see the answers – chilljeet Jun 5 '15 at 20:52

Direct Injection has a lot of advantages over the MPFI or port injection and its the way of the future.

  • It improves mileage through a stratified charge engine process(basically staying as close to the stochiometric ratio as possible this achieving better burn.
  • Lower emissions.
  • Prevents engine knocking/detonation.
  • Better control of the engine since before GDI it was only possible to manipulate valve timings in real time operation to change the efficiency/power characteristics of the engine, now since you can vary the fuel pressure,amount. there are a whole lot of possibilities to explore.
  • GDI engines are built sturdier compared to MPFI engines and last longer.
  • Better power output compared to mpfi.

The ultimate advantage of GDI is that through this technology everything happening inside the engine can be precisely controlled by the electronics and nothing is left to chances. For example in mpfi some amount of fuel can be left over inside the injectors though not causing an issue , causes slight variation in every stroke but when you move over to GDI , its like a German train time table .it will be Perfect.

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    There is a noted downside, which they are just recently seeing on the GM cars. With the lack of fuel hitting the back of the intake valve, they are seeing buildup (of what I don't know) there. I guess we need to do a regular Seafoam treatment on them, lol! – Pᴀᴜʟsᴛᴇʀ2 Jun 6 '15 at 2:21
  • Its actually not specific to GM cars :-) – Shobin P Jun 6 '15 at 3:51
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    I get that. I know GM isn't the only one using DI. I was just using it as an example. – Pᴀᴜʟsᴛᴇʀ2 Jun 6 '15 at 3:58
  • Mercedes was the first to use on the gullwing way back in the 60s .. – Shobin P Jun 6 '15 at 3:58

Gasoline direct injection can improve both fuel efficiency and power in an engine. Port fuel injection engines need to run near stoichiometric fuel/air mixtures (slightly lean is possible), in part because it is difficult to ignite very lean fuel/air mixtures.

By design, port fuel injection creates a homogeneous charge (the fuel/air ratio is the same everywhere in the cylinder when the spark plug fires). With direct injection, you can create a stratified charge, where the fuel/air mixture is different depending on location in the cylinder. By injecting the fuel near the spark plug just before it fires, you can have a stoichiometric fuel/air mixture in the center of the cylinder for proper ignition, while there is an ultra lean (very low fuel/air ratio) near the cylinder walls. In addition to using less fuel, it keeps the cylinder walls cooler, leading to longer engine life.

To produce more power, the same tactic is used as ultra lean burning, but with a stoichiometric fuel/air mixture. By injecting the fuel right before the spark plug fires, you can avoid detonation. Because of reduced detonation (engine knock), engines can be built with a higher compression ratio - improving power output, while using the same octane gasoline.

Direct injection systems do cost more as the injectors require much higher pressures, and are exposed to the rigors of combustion. The timing and data processing requirements are made trivial with modern computers - this is also true of port injection schemes.

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While most of these statements may be true there is also some major down sides to DI. Such as since the injector nozzel sits in the cylinder head, carbon build up is a problem. After 30k miles (which isn’t that much) you can have injectors looking like they came out of a 90k mile car just from all the carbon it gets from the combustion process. Also you have more possibilities of misfires in the cylinders as the nozzles continue to gain carbon build up and begin to clog. Another note is PFI tends to clean the intake valve before going into the cylinder which is not the case on DI engines. This leads to a possible cost for the DI engines in having to get your intake valves and runners walnut basted to take away all the carbon build up as a result. There’s pros and cons to every fuel systems and DI isn’t without some major flaws. You may think using products like sea foam will help but this is not the case. The best and practically only way is the pricey maintenance of walnut blasting.

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There is another advantage in some iterations. Injection timing is only limited to the injector capability, which can be precisely controlled (as mentioned).

However, there's even the possibility of multiple injections, perhaps during some part of intake and/or compression stroke.

One example injects a very lean mixture that is NOT stochiometric; in fact, so lean as to be nearly impossible to ignite. However, a small thimble depression in the top of the piston, near the spark plug, receives a "seed" injection just before ignition. This "seed" will be stochiometric, and acts as a "fuse" to get all of the fuel combusting and oxidized. More power, less fuel, efficiency, better atomization, no fuel condensing on an intake valve or plenum runner, etc.

The downside is the added BMEP, intense heat, and NOX emissions which have to be dealt with. Engines need to be made stronger, cylinders must be properly cooled, and NOX needs to be catalyzed in the exhaust. And of course buildup in the air intake system as previously stated.

From my racing experience, "Lean is Mean" ... but you have to be very careful how far you can go - especially on my 1972 BMW 2002tii, which has virtually none of this modern technology. Upgraded water and oil cooling, and a realtime digital A/F gauge is a must.

As others mentioned, this is the Cat's Meow and GDI is here to stay. It's such an incredible advance that it's one of the things that will secure the future of the IC engine for many years.

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