Indirect injection and direct injection

Direct Injection

As we all know, engines work thanks to the fuel they burn to produce movement. Decades ago, it was introduced inside the cylinders thanks to the carburetor. However, this has become obsolete to make way for another more sophisticated and efficient system: the injection.

Depending on its operation, location, and other factors, there are several injection types of an engine. Although at present there is a particularly notable difference: that it is direct or indirect. Each with their advantages and disadvantages and the possibility of being mounted on some motors yes and on others no, depending on their architecture.

indirect injection

First of all, we have indirect injection, which was used in old diesel vehicles and which some gasoline models still have. With her, fuel is supplied outside the combustion chamber. In gasoline vehicles, it is located in the intake manifold, so that it mixes well with the moving air and then reaches the interior of the cylinder.

The Toyota Prius uses the indirect type

To ensure that the fuel is mixed as well as possible with the air, engines with this type of injection have the rough manifold interior. so they get create a turbulence which contributes to a better spraying of the fuel. An effect that is even more accentuated in the swirl chamber of the cylinder head, to be added if necessary.

On the contrary, in diesel vehicles with indirect injection, the cylinder has a combustion prechamber located in the cylinder head, where the fuel is injected. Some of it is burned in the prechamber to increase pressure, and the unburned fuel is sent to the cylinder, where it burns completely.

Disadvantages of indirect injection

Indirect injection tends to consume more fuel

Traditionally this type of injection had a series of weak points, which with current knowledge and technology have been corrected by other means. Without going any further, the Toyota Prius uses indirect injection and its combustion engine is one of the most efficient. Even so, here we indicate all of them knowing that brands have found solutions to many of them or at least have minimized them:

  • Higher consumption made out of fuel:
    • Has more heat losses transferred to the cylinder walls, because the fuel is distributed throughout the combustion chamber and when turned on it heats it up more, reducing the thermal performance the motor.
    • It allows a lower compression ratio.
    • La temperature in the collector is slightly older
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  • Operation less stable:
    • El cold start is more difficult because the fuel sticks to the walls of the combustion chamber.
    • El fuel delivery is not as accurate as with direct injection, so it is less stable at low idle speeds.

In addition, indirect injection cars had less ability to suck in air by the turbulence necessary to mix the fuel in the intake manifold and a smaller butterfly opening.

Advantages of indirect injection

Indirect injection allows less emissions

The main advantages of this type of motors are:

  • La emissions they are easily neutralized in the exhaust by the catalyst. I know produces less NOx in combustion.
  • Allows the use of Atkinson cycle, more efficient than the Otto cycle, albeit with lower performance.
  • More or economic sector. to build
  • The injectors are far from the combustion so they get less dirty and have less chance of obstruction

Direct Injection

Another type of injection is known as direct. For decades, it has been installed in all diesel vehicles and today they have also spread massively throughout most gasoline vehicles. For example: the TSI of the Volkswagen group, the Ford EcoBoost, the PureTech of the PSA group, those of BMW and Mercedes, etc.

Direct injection introduces fuel into the combustion chamber

In this case, injectors work directly in the cylinderboth in diesel and gasoline engines. In this way, the fuel enters directly into the combustion chamber of the engine and the quantity that enters is controlled exactly.

Disadvantages of direct injection

  • As the NOx emissions are higher they require more effective and expensive anti-pollution systems to comply with environmental pollution regulations.
  • No allows the use of Atkinson cycle.
  • the engine is something more expensive to manufacture because it requires an injection system that withstands combustion and a slightly more complex construction.
  • Requires clean injectors regularly to keep the car in good condition. Otherwise it will increase consumption and may even cause a power loss.

The direct has been extended to a large number of gasoline engines such as the TSI of the Volkswagen group

Advantages of direct injection

  • El fuel consumption is lower because:
    • Unlike indirect fuel injection, most of the combustion occurs near the spark plug so it has less heat loss yielded to the cylinder walls. In addition, the temperature in the intake manifold is lower.
    • Allows compression ratio highest without falling into self-detonation.
  • Es more stable because:
    • The possibility of pistoning is reduced. That is, combustion failures.
    • Allows easier cold starts because the fuel sticks less to the walls of the combustion chamber.
    • Allows stability at lower idle because the fuel supply is more precise and restrained.

Since the intake manifold does not need to generate more turbulence and the throttle can be more open, air passes more easily, so it is able to get more air.

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Direct injection allows you to consume less

Which is better, direct or indirect injection in gasoline?

Injection is just one element of the engine, so Results what do you get with it depend more on the combination with the rest of engine components, that of its operation by itself.

Having said that, the advantages and disadvantages of each type of injection do exist when analyzed in isolation. So if you are looking low emissions and lower cost it will be easier to get with the injection hint. Instead, if you search efficiency and stability, it will be easier to get it in cars with injection direct.

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Direct and indirect dual injection

Given the advantages and disadvantages offered by each type of injection, there are brands that have opted to install dual systems that combine the best of both worlds. For example, him 5.0-liter naturally aspirated engine from the Ford Mustang or Audi's 2.5 TFSI engine.

The Ford Mustang uses dual direct and indirect injection

El operation of dual injection is very simple: on the one hand, it uses the indirect injection at low or medium load because, as we have indicated before, it achieves a better carburation that reduces emissions. On the other, use the direct injection at ignition and at high loads, because it improves starting and prevents self-detonation when there is a lot of fuel in the chamber.

The result in these cases has been the improvement of the operation of the engines, which has been worth a lot to both brands. On the one hand has managed to sufficiently reduce the emissions of its huge engine, so as not to have to dispose of it for not complying with anti-pollution regulations. So for the moment they have managed to maintain the essence of your engine typeworthy of a whole muscle car. Audi, for its part, has managed to get the best out of a five-cylinder block with which part of its tradition is also at stake.

injection engine
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Images 3, 4 and 7 – Eli Duke, Javier Prazak, Images Money


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  1.   Ivan said

    Hello good, I have contacted you because I have two cars; Both of large displacement and cv. And I have a series of doubts and questions… My Honda Civic Type R year 2004
    With a 2.0 and 215hp engine, revving to the maximum, it cuts what we all understand by popo popo popo... Then I have a Volkswagen Golf R year 2015 with a 2.0 TSI 300cv engine, I speed it up to the maximum and it cuts like the Type R , makes the threat as if to collapse. What is the why? Thanks and regards

  2.   build said

    Hello, I have a Mercedes Benz 608 D motorhome from 1972. It is direct injection. So as we can see, the topic is not so new.
    As for the carburetor, until recently I had a Daewoo TICO that with its carburetor did more km than the injection that are the Spark and the Matiz.

    At this moment one of my cars is in the workshop because the injectors were clogged, the most absurd thing is I run on CNG gas.
    The carburettor is still a very interesting option, you can even carry a spare one.
    regards