Page 230 - 05. Subyek Teknik Mesin - Automobile Mechanical and Electrical Systems Automotive Technology Vehicle Maintenance and Repair (Vehicle Maintenance Repr Nv2) by Tom Denton

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214 Automobile mechanical and electrical systems

Figure 2.277 Fuel droplet size is important. (Source: Bosch Media)

engine control unit regulates the operating modes of the engine with gasoline
direct injection in three ways:
stratifi ed charge operation – with λ values greater than 1
●
homogeneous operation – at λ 1
●
rich homogeneous operation – with λ 0.8.
●
Defi nition Compared to the traditional manifold injection system, the entire fuel amount
must be injected in full-load operation in a quarter of the time. The available time
Micrometre (μm)
is signifi cantly shorter during stratifi ed charge operation in part-load. Especially
One micrometre (1 μm) is a millionth
of a metre. at idle, injection times of less than 0.5 milliseconds are required owing to the
lower fuel consumption. This is only one-fi fth of the available time for manifold
injection.
The fuel must be atomized very fi nely to create an optimal mixture in the brief
moment between injection and ignition ( Fig. 2.277 ). The fuel droplets for direct
injection are on average smaller than 20 μm (micrometres, i.e. a millionth of a

metre). This is one-fi fth of the droplet size reached with the traditional manifold
Key fact
injection and one-third of the diameter of a single human hair. It improves
The fuel droplets for direct injection effi ciency considerably. However, even more important than fi ne atomization
are on average smaller than 20 μm.
is even fuel distribution in the injection beam. This is done to achieve fast and
uniform combustion.
Conventional spark ignition engines have a homogeneous (well mixed up!) air/
fuel mixture at a 14.7:1 ratio, corresponding to a value of λ 1. Direct injection
engines, however, operate according to the stratifi ed charge concept in the part-
load range and function with high excess air. In return, very low fuel consumption
is achieved.
Fuel injection just before the ignition point and injection directly into the
Defi nition combustion chamber is to create a stratifi ed (layered) mode ( Fig. 2.278 ). The
Stratifi ed result is a combustible air/fuel mixture cloud on the spark plug, cushioned in
Arranged in approximately horizontal a thermally insulated layer, composed of air and residual gas. This raises the
layers. effi ciency level because heat loss is avoided on the combustion chamber walls.
The engine operates with an almost completely opened throttle valve, which
avoids additional charge losses.

With stratifi ed charge operation, the lambda value in the combustion chamber
is between about 1.5 and 3. In the part-load range, gasoline direct injection
achieves the greatest fuel savings, with up to 40% at idle compared to
conventional petrol injection processes.