What is FSI and enhancement of FSI

Direct petrol injection - advantages of FSI technology

  Gasoline engine 

Fuel Stratified Injection (FSI) - gasoline direct injection stratified charge - from Audi and VW

Description of the fuel direct injection system FSI from VW and Audi. Structure, function, operating modes, exhaust gas aftertreatment

At VW, the era of petrol direct injection began in 2000 with the Lupo FSI. The Audi A4 followed suit in 2001 and has also been available as a DI in the VW Golf 1.6 since 2002. Further models followed in 2003: AUDI A3, the new Golf in several versions.

System picture FSI (all pictures VW)


  • In order to generate the fuel pressure of up to 100 bar required for direct injection, a high-pressure pump is used, which is driven directly via the intake camshaft.

  • This supplies the high-pressure injection valves located under the intake port in the cylinder head via a fuel rail (common rail).

  • The electromagnetic actuation of the valves enables the fuel to be injected precisely into the combustion chamber at specific times.

  • The essential component for the new type of mixture formation is a flange arranged between the cylinder head and the aluminum intake manifold, which, in addition to the "common rail", contains the injection components and a so-called tumble circuit for targeted influencing of the internal cylinder flow (see also combustion process).

  • An intake camshaft that can be adjusted by up to 40 ° not only ensures improved pulling power at low and medium speeds, but also reduces emissions via the internal exhaust gas recirculation.

  • At partial load, the direct-injection gasoline engine from Volkswagen is operated with a very high excess of air, in which homogeneous air-fuel mixtures - as generated with conventional mixture formation systems - are no longer ignitable.

  • The reduced gas exchange and the minimized heat losses as a result of the strong dethrottling are the essential features for the consumption savings potential of the direct-injection gasoline engine of up to 15%.

  • The evaporation of the fuel in the combustion chamber instead of in the intake manifold removes heat of evaporation from the surroundings. This leads to a stronger internal cooling.

  • The result: the compression ratio can be increased. For the AUDI FSI it is 11.5: 1. This increases the efficiency even in homogeneous operation.

FSI in shift mode and homogeneous operation

Operating modes

In order to be able to realize this high excess air, which has a beneficial effect on fuel consumption, the engine has to work in the so-called

Stratified charge mode


In this operating mode, the mixture is concentrated around the spark plug, which is centrally positioned in the combustion chamber; there is clean air in the edge areas of the combustion chamber. The associated consumption savings result on the one hand from the dethrottling of the engine and, on the other hand, to a not inconsiderable extent from the minimized heat losses that are made possible by the combustion taking place centrally in the combustion chamber with the surrounding insulating air envelope.

With the Volkswagen FSI engine, a switchable is used to display a targeted air flowTumble system used. The so-called "tumble plate" divides the air inlet duct into an upper and a lower half. An upstream flap (see below) allows the air to be routed either only over the upper part or over the entire cross-section:

In stratified charge mode, the flap is closed, creating an intense, cylindrical flow in the combustion chamber. In this mode, the fuel is only injected in the last third of the piston upward movement. Due to the combination of targeted air flow and the special geometry of the piston, which has a pronounced fuel and flow trough, the particularly finely atomized fuel is optimally concentrated around the spark plug and safely ignited in a so-called "mixture ball". The engine management system ensures that the injection parameters are optimally adjusted (injection timing, fuel pressure).

Homogeneous mode

At very high speeds or loads, the FSI engine works in the same way as a conventional engine with manifold injection, but with better efficiency due to the higher compression. The fuel is injected into the intake stroke so that the fuel-air mixture can be prepared in the best possible way. In order to be able to offer the engine the maximum amount of air, the flap is opened - the entire cross-section of the air inlet duct is available.

The switchable tumble system thus allows the combination of the low-consumption stratified charge mode with very good power and torque values.

FSI diagram operating modes

The currently determined consumption values ​​are less than five liters for the Lupo FSI with a lightweight body and direct shift gearbox and approx. 5.6 liters for the Lupo FSI in the normal version. The CO2 values ​​are very low at 119 g / km and 134 g / km (1st generation). Unfortunately, it should also be noted that economical consumption is only achieved if the vehicle is mainly driven in stratified charge mode. Often the entire potential for savings is used up if you drive briskly. A lot of development work is certainly necessary here in order to achieve a savings miracle from the gasoline DI, as with the diesel direct injection engine.

Exhaust aftertreatment

This includes

  • the exhaust gas recirculation
  • an elaborate exhaust system with a pre-catalytic converter and a
  • NOx storage cat

In order to ensure optimal pollutant conversion even in the operating phases with high excess air, Volkswagen uses a NOx storage catalytic converter in combination with a NOx sensor in the Lupo FSI.

FSI exhaust system

Description of the exhaust system

  • During lean operation, the critical nitrogen oxides are temporarily stored in the catalytic converter and converted back into harmless nitrogen during conventional engine operation.
  • In addition, the NOx storage catalytic converter has the usual three-way properties for efficient HC or CO oxidation in both stratified and homogeneous operation. Thanks to a pre-catalytic converter, which is positioned close to the exhaust tract and heated by engine measures during the warm-up phase, the exhaust gas cleaning system responds quickly. (Working temperature 250-500 ° C)
  • For the first time, the Lupo FSI uses a NOx sensor to optimize exhaust gas cleaning.
  • This recognizes the storage status of the NOx-Kats and this is regenerated accordingly:
  • by switching from lean to homogeneous operation or to rich operation (8 = 0.8).
  • This happens at intervals of 30 - 60 s,
  • regeneration takes about two seconds
  • the control takes place without any noticeable effects for the driver
  • The additional consumption associated with this "cleaning" process is kept to a minimum.


FSI regeneration

Regeneration and sulfur

  • A problem with FSI engines would be sulfur contained in the fuel (e.g. USA), which would increasingly reduce the activity of the NOx storage catalytic converter. In Europe, the sulfur content was significantly reduced years ago.

  • This poisoning of the catalytic converter can be reversed at temperatures above 650 ° C, which often occur in mixed driving. However, this temperature level is not reached in pure city traffic; here, sulfur in the fuel would become a problem.

  • Thanks to the NOx sensor used for the first time, the sulfur content in the storage catalytic converter can be precisely determined and targeted desulphurisation can be initiated.

Special features of the Audi FSI

In the Audi FSI, the tumble flap is infinitely adjustable, the pump is regulated like in the Golf.


FSI at Audi

What is different about the Golf FSI?

It works with 3 operating modes:

  • Stratified charge operation

  • Homogeneous lean operation

  • Homogeneous operation

in the Stratified charge operation At low load and speed, the throttle valve is fully open to reduce gas exchange losses, the tumble valve in the intake duct is closed, the air is accelerated and swirled. The injection takes place shortly before the ignition, an ignitable mixture is created around the spark plug, while an insulating air cushion (reduction of heat loss) lies around the outside (cloud lambda = 3). The piston crown helps with the turbulence.

In the transition areaHomogeneous lean operation

the injection occurs earlier, the switching flap is closed, lambda = 1.4. There is no EGR.

in the Homogeneous operation from 3000 l / min combustion takes place at lLambda = 1 (homogeneous). The behavior is similar to the naturally aspirated engine, injection already takes place in the intake stroke, the switching flap is open. The regulation takes place through the throttle valve. The better internal cooling results in a higher compression (approx. 12: 1). The pressure of the radial piston pump is between 50 and 110 bar.


FSI model year 2002

The information and pictures about the FSI come from VW and Audi

Author: Johannes Wiesinger

edited: 10/30/2018

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