Bosch
K-Jetronic
|
 |
Part 1
|
The Engine's Fuel Requirements
A spark-ignition engine needs a particular air-fuel ratio in
order to operate. The ideal air-fuel ratio is 14.7:1. Certain
operating conditions make it necessary to correct the mixture
accordingly.
The air-fuel ratio
Essentially, the power, the fuel consumption and the
exhaust-gas composition of a spark-ignition engine depend upon the
air-fuel ratio. Perfect ignition and perfect combustion only take
place within particular air-fuel ratios. In the case of gasoline
(petrol), the ideal air-fuel ratio is about 15:1. In other words,
15 kg of air are required for complete combustion of 1 kg of
gasoline (stoichiometric ratio). Deviations from this ratio affect
engine operation. The amount of fuel to be injected depends upon
load, engine speed and the particular exhaust-gas regulations in
force at the time. Depending upon the mode of operation, i.e.
idle, part load or full load, a different air-fuel ratio is
optimal in each case. Of decisive importance is the strict
adherence to the particular most favourable air-fuel ratio at any
one time.
The excess-air factor
The excess-air factor is identified by the symbol for Lambda.
Lambda = amount of air supplied ÷ theoretical air requirement
Lambda=1
This means that the amount of air supplied to the engine
corresponds to the theoretical amount of air required
(stoichiometric air-fuel ratio).
Lambda<1
This means air deficiency, or a rich mixture, and increased
power.
Lambda>1
This means air excess, or lean mixture, lower fuel consumption,
less power.
Lambda> 1.3
This means that the mixture will no longer ignite, the lean
misfire limit (LML) has been exceeded.
Fuel-management systems
Fuel-management systems, whether of the carburettor or
injection types, have the task of preparing an optimum air-fuel
mixture. Fuel management by means of manifold injection permits
the optimum adaptation of the air-fuel mixture to every operating
phase of the engine. It also ensures a lower level of pollutants
in the exhaust gas.
In spark-ignition systems, fuel management is by means of
either a carburettor or a fuel-injection system. Although, up to
now, the carburettor has been the most commonly used method, there
has been a distinct trend in the last couple of years towards
manifold fuel injection. This trend came about due to the
advantages offered by fuel injection as regards the demands for
fuel economy, high performance and, last but not least, a lower
level of pollutants in the exhaust gas. These advantages are based
on the fact that manifold fuel injection permits extremely precise
metering of the fuel depending upon the operating conditions of
the engine and its load, and taking environmental effects into
account. With manifold fuel injection, the correct air-fuel ratio
is maintained so precisely that the pollutant level in the exhaust
gas is considerably lower. Since with this system, the carburettor
is no longer required, the intake paths can be optimally designed
and laid out. This results in better cylinder charge which in turn
leads to a more favourable torque characteristic.
What types of mixture formation are available using fuel
injection?
There are both mechanically and electronically controlled
systems available. The K-Jetronic is a mechanical fuel injection
system which injects continuously and which needs no form of drive
whatsoever.
Electronically controlled systems
The fuel is supplied by an electrically driven fuel pump which
develops the pressure necessary for injection. The fuel is
injected by solenoid-operated fuel-injection valves into the
cylinder intake ports. The injection valves are controlled by an
electronic control unit (ECU) and the amount of fuel injected
depends upon the length of time that they stay open. By means of
sensors, the ECU is provided with information about the operating
conditions of the engine and about the ambient conditions around
the vehicle. The basis for assessing the amount of fuel to be
injected is the amount of air drawn in by the engine. The
L-Jetronic is an electronically controlled fuel-injection system.
In the case of the L-Jetronic, the amount of air drawn in by the
engine is directly measured by an air-flow sensor. Electronically
controlled fuel-injection systems are dealt with in detail in the
Publication "Electronically Controlled Fuel Injection"
in the Bosch Technical Instruction series.
Mechanical systems
With mechanical fuel-injection systems, one differentiates
between those which require a drive from the engine and those
which do not. The engine-driven systems comprise a fuel-injection
pump with an integrated governor. Their principle of operation is
the same as that of the fuel-injection systems for Diesel engines.
The other variation of the mechanical system is one which needs no
drive and which injects continuously. This system, the K-Jetronic,
is described in the following.
The K-Jetronic
The K-Jetronic is a mechanical fuel injection system from
Bosch. It is divided into three main functional areas:
- Air-flow measurement
- Fuel supply
- Fuel induction
Air-flow measurement
The amount of air sucked in by the engine is controlled by a
throttle valve and measured by an air-flow sensor.
Fuel supply
An electrically driven fuel pump delivers the fuel to the fuel
distributor via a fuel accumulator and a filter. The fuel
distributor allocates this fuel to the injection valves in the
cylinder intake tubes.
Fuel induction
The amount of air, corresponding to the position of the
throttle plate, sucked in by the engine serves as the criteria for
the metering of the fuel to the individual cylinders. The amount
of air sucked in by the engine is measured by the air-flow sensor
which, in turn, controls the fuel distributor. The air-f low
sensor and the fuel distributor are assemblies which form Part of
the mixture control unit. Injection takes place continuously, that
is, without regard to the position of the intake valve. During the
intake-valve closed phase, the fuel is "stored" in the
intake tubes.
Fuel supply
Outline of system
The fuel is drawn out of the fuel thank by an electrically
driven fuel pump. It is then forced, under pressure, through a
pressure accumulator and a fine filter to the fuel distributor,
which is located in the mixture control unit. The pressure is held
constant by a pressure regulator in the mixture control unit from
where it flows to the fuel-injection valves. The injection valves
inject fuel continuously into the intake ports of the engine
cylinders. The designation K-Jetronic stems from his fact
("K' stands for the German word for "continuous").
When the intake valves open, the air-fuel mixture is drawn into
the cylinders. The individual subassemblies of the fuel-supply
system are described in the following
Electric fuel pump
The electric fuel pump is a roller-cell pump the electric motor
of which is permanently surrounded by fuel. The fuel pump is
driven by a permanent magnet electric motor. The rotor disc which
is eccentrically mounted in the pump housing is fitted with metal
rollers in notches around its circumference which are pressed
against the thrust ring of the pump housing by centrifugal force
and act as seals. The fuel is carried in the cavities, which form
between the rollers. The fuel flows directly around the electric
motor. There is no danger of explosion, however, because there is
never an ignitable mixture in the pump housing. The pump delivers
more fuel than the maximum requirement of the engine so that the
pressure in the fuel system can be maintained under all operating
conditions. During starting, the pump runs as long as the ignition
key is operated. The pump continues to run when the engine has
started. A safety circuit is incorporated to stop the pump running
and fuel being delivered if the ignition is switched on but the
engine has stopped turning (for instance in the case of an
accident).
Fuel accumulator
The fuel accumulator maintains the pressure in the fuel system
for a certain time after the engine has been switched off. When
the engine is running it serves to deaden the noise of the
electric fuel pump. After the engine has been switched off, the
fuel accumulator maintains the pressure in the fuel system in
order to facilitate re-starting, particularly when the engine is
hot. The design of the accumulator housing is such that it deadens
the noise from the fuel pump when the engine is running. The
interior of the fuel accumulator is divided into two chambers by
means of a diaphragm. One chamber serves as the accumulator volume
for the fuel, the other chamber contains a spring. During
operation the accumulator chamber is filled with fuel. This causes
the diaphragm to bend back against the force of the spring until
it is halted by the stops in the spring chamber. The diaphragm
remains in this position, which corresponds to the maximum
accumulator volume, as long as the engine is running.
Fuel filter
Due to the extremely close tolerances of various components in
the system, it is necessary to fit a special fine filter for the
fuel in order to guarantee faultless performance of the
K-Jetronic. The fuel filter retains particles of dirt which are
present in the fuel and which would otherwise adversely affect the
functioning of the injection system. The fuel filter contains a
paper filter element, which is backed up by a strainer. This
combination results in a high degree of cleaning being achieved. A
supporting plate is used to hold the filtering elements in place
in the filter housing. It is of utmost importance that the
direction of flow indicated on the housing is complied with. The
filter is fitted in the fuel line downstream of the fuel
accumulator.
Primary-pressure regulator
The primary-pressure regulator maintains the pressure in the
fuel system constant. The pressure regulator incorporated in the
fuel-distributor housing maintains the delivery pressure (=
primary pressure) at about 5 bar. Due to the fact that the fuel
pump delivers more fuel than the engine needs, a plunger shifts in
the pressure regulator and opens a port through which excess fuel
can return to the fuel tank. The pressure in the fuel system and
the force exerted by the spring on the plunger in the pressure
regulator balance each other out. If for instance, the fuel pump
delivers slightly less fuel, the plunger is shifted by the spring
into the corresponding new position and in doing so reduces the
open section of the port through which excess fuel flows back to
the tank. This means that less fuel leaves the system at this
point, and as a result the primary pressure in the system
increases to the specified value. When the engine is switched off,
the fuel pump also stops running. The primary pressure drops to
below the injection-valve opening pressure. The pressure regulator
closes the return-flow port and prevents further pressure
reduction in the fuel system.
Fuel-injection valve
The fuel-injection valves open at a certain pressure and inject
fuel into the intake tubes. The fuel is atomised by the
oscillation of the valve needle. The injection valves inject the
fuel allocated by the fuel distributor into the intake ports
directly in front of the intake valves of the cylinders. The
injection valves are secured in a special holder in order to
insulate them from engine heat. The insulation prevents vapour
bubbles forming in the fuel injection lines which would lead to
poor starting behaviour when the engine is hot. The injection
valves have no metering function. They open of their own accord
when the opening pressure of 3.3 bar is exceeded. They are fitted
with a valve needle which vibrates ("chatters") audibly
at high frequency when fuel is injected. This means that excellent
fuel atomisation is achieved, even with the smallest of injected
quantities. When the engine is switched off, the injection valve
closes tightly and forms a seal when the fuel-system pressure has
dropped below the injection-valve opening pressure. As a result,
no more fuel can drip into the intake ports after the engine has
been switched off.
|
