© Robert Bosch GmbH, 2000
Postfach 30 02 20,
Automotive Equipment Business Sector,
Department for Automotive Services,
Technical Publications (KH/PDI2).
Dipl.-Ing. (FH) Horst Bauer.
Dipl.-Ing. Karl-Heinz Dietsche,
Dipl.-Ing. (BA) Jürgen Crepin.
Dipl.-Ing. (FH) Ulrich Adler,
Berthold Gauder, Leinfelden-Echterdingen.
Bauer & Partner, Stuttgart.
Unless otherwise stated, the above are all
employees of Robert Bosch GmbH, Stuttgart.
Reproduction, copying, or translation of this
publication, including excerpts therefrom, is only to
ensue with our previous written consent and with
Illustrations, descriptions, schematic diagrams,
and other data only serve for explanatory purposes
and for presentation of the text. They cannot be
used as the basis for design, installation, or scope
of delivery. We assume no liability for conformity of
the contents with national or local legal regulations.
We are exempt from liability.
We reserve the right to make changes at any time.
Printed in Germany.
Imprimé en Allemagne.
4th Edition, February 2000.
English translation of the German edition dated:
Combustion in the gasoline engine
Since its introduction, the K-Jetronic
The spark-ignition or
gasoline-injection system has pro-
ved itself in millions of vehicles.
This development was a direct result
of the advantages which are inherent
in the injection of gasoline with
regard to demands for economy of
operation, high output power, and
last but not least improvements to
the quality of the exhaust gases
emitted by the vehicle. Whereas the
call for higher engine output was the
foremost consideration at the start of
Adapting to operating conditions
the development work on gasoline
injection, today the target is to
achieve higher fuel economy and
lower toxic emissions.
Workshop testing techniques
Between the years 1973 and 1995,
the highly reliable, mechanical multi-
point injection system K-Jetronic
was installed as Original Equipment
in series-production vehicles. Today,
it has been superseded by gasoline
injection systems which thanks to
electronics have been vastly im-
proved and expanded in their func-
tions. Since this point, the K-Jetronic
has now become particularly impor-
tant with regard to maintenance and
This manual will describe the
K-Jetronic’s function and its particu-
the gasoline engine
combustion process pressurizes the
cylinder, propelling the piston back down,
or Otto-cycle engine
exerting force against the crankshaft and
performing work. After each combustion
stroke the spent gases are expelled from
the cylinder in preparation for ingestion of
The spark-ignition or Otto-cycle1)
a fresh charge of air/fuel mixture. The
powerplant is an internal-combustion (IC)
primary design concept used to govern
engine that relies on an externally-
this gas transfer in powerplants for
generated ignition spark to transform the
automotive applications is the four-stroke
chemical energy contained in fuel into
principle, with two crankshaft revolutions
being required for each complete cycle.
Today’s standard spark-ignition engines
employ manifold injection for mixture
The four-stroke principle
formation outside the combustion
chamber. The mixture formation system
The four-stroke engine employs flow-
produces an air/fuel mixture (based on
control valves to govern gas transfer
gasoline or a gaseous fuel), which is (charge control). These valves open and
then drawn into the engine by the suction
close the intake and exhaust tracts
generated as the pistons descend. The
leading to and from the cylinder:
future will see increasing application of
systems that inject the fuel directly into the
1st stroke: Induction,
combustion chamber as an alternate
2nd stroke: Compression and ignition,
concept. As the piston rises, it compresses
3rd stroke: Combustion and work,
the mixture in preparation for the timed
4th stroke: Exhaust.
ignition process, in which externally-
generated energy initiates combustion via
the spark plug. The heat released in the
Intake valve: open,
Exhaust valve: closed,
Piston travel: downward,
Reciprocating piston-engine design concept
OT = TDC (Top Dead Center); UT = BDC (Bottom
Dead Center), Vh Swept volume, VC Compressed
volume, s Piston stroke.
The piston’s downward motion increases
the cylinder’s effective volume to draw
fresh air/fuel mixture through the passage
exposed by the open intake valve.
Intake valve: closed,
Exhaust valve: closed,
Piston travel: upward,
Combustion: initial ignition phase.
1) After Nikolaus August Otto (1832 –1891), who
unveiled the first four-stroke gas-compression engine
at the Paris World Exhibition in 1876.
As the piston travels upward it reduces
The ignition spark at the spark plug
the cylinder’s effective volume to
ignites the compressed air/fuel mixture,
compress the air/fuel mixture. Just before
thus initiating combustion and the
the piston reaches top dead center (TDC)
attendant temperature rise.
the spark plug ignites the concentrated
This raises pressure levels within the
air/fuel mixture to initiate combustion.
cylinder to propel the piston downward.
Stroke volume Vh
The piston, in turn, exerts force against
and compression volume VC
the crankshaft to perform work; this
provide the basis for calculating the
process is the source of the engine’s
ε = (Vh+VC)/VC.
Power rises as a function of engine speed
Compression ratios ε range from 7...13,
and torque (P = M⋅ω).
depending upon specific engine design.
A transmission incorporating various
Raising an IC engine’s compression ratio
conversion ratios is required to adapt the
increases its thermal efficiency, allowing
combustion engine’s power and torque
more efficient use of the fuel. As an
curves to the demands of automotive
example, increasing the compression ratio
operation under real-world conditions.
from 6:1 to 8:1 enhances thermal
efficiency by a factor of 12 %. The latitude
for increasing compression ratio is
Intake valve: closed,
restricted by knock. This term refers to
Exhaust valve: open,
uncontrolled mixture inflammation charac-
Piston travel: upward,
terized by radical pressure peaks.
Combustion knock leads to engine
damage. Suitable fuels and favorable
As the piston travels upward it forces the
combustion-chamber configurations can
spent gases (exhaust) out through the
be applied to shift the knock threshold into
passage exposed by the open exhaust
higher compression ranges.
valve. The entire cycle then recommences
with a new intake stroke. The intake and
exhaust valves are open simultaneously
Intake valve: closed,
during part of the cycle. This overlap
Exhaust valve: closed,
exploits gas-flow and resonance patterns
Piston travel: upward,
to promote cylinder charging and
Operating cycle of the 4-stroke spark-ignition engine
Stroke 1: Induction
Stroke 2: Compression
Stroke 3: Combustion
Stroke 4: Exhaust
Technical requirements Primary engine-
The engine-management system’s first
and foremost task is to regulate the
engine’s torque generation by controlling
all of those functions and factors in the
The power P furnished by the spark-
various engine-management subsystems
ignition engine is determined by the
that determine how much torque is
available net flywheel torque and the
The net flywheel torque consists of the
force generated in the combustion
In Bosch engine-management systems
process minus frictional losses (internal
featuring electronic throttle control (ETC),
friction within the engine), the gas-
the “cylinder-charge control” subsystem
exchange losses and the torque required
determines the required induction-air
to drive the engine ancillaries (Figure 1).
mass and adjusts the throttle-valve
The combustion force is generated
opening accordingly. The driver exercises
during the power stroke and is defined by
direct control over throttle-valve opening
the following factors:
on conventional injection systems via the
– The mass of the air available for
physical link with the accelerator pedal.
combustion once the intake valves
– The mass of the simultaneously
The “mixture formation” subsystem cal-
available fuel, and
culates the instantaneous mass fuel
– The point at which the ignition spark
requirement as the basis for determining
initiates combustion of the air/fuel
the correct injection duration and optimal
Driveline torque factors
1 Ancillary equipment
a/c compressor, etc.),
Air mass (fresh induction charge)
output torque torque
Ignition angle (firing point)
Gas-transfer and friction
Clutch/converter losses and conversion ratios
Transmission losses and conversion ratios
emissions control system (Figure 2). The
Finally, the “ignition” subsystem de-
air entering through the throttle-valve and
termines the crankshaft angle that
remaining in the cylinder after intake-
corresponds to precisely the ideal instant
valve closure is the decisive factor
for the spark to ignite the mixture.
defining the amount of work transferred
through the piston during combustion,
The purpose of this closed-loop control
and thus the prime determinant for the
system is to provide the torque
amount of torque generated by the
demanded by the driver while at the
engine. In consequence, modifications to
same time satisfying strict criteria in the
enhance maximum engine power and
torque almost always entail increasing
– Exhaust emissions,
the maximum possible cylinder charge.
– Fuel consumption,
The theoretical maximum charge is
defined by the volumetric capacity.
– Comfort and convenience, and
The portion of the charge consisting of
residual gases is composed of
– The exhaust-gas mass that is not
discharged while the exhaust valve is
open and thus remains in the cylinder,
The gas mixture found in the cylinder
– The mass of recirculated exhaust gas
once the intake valve closes is referred to
(on systems with exhaust-gas recircu-
as the cylinder charge, and consists of
lation, Figure 2).
the inducted fresh air-fuel mixture along
The proportion of residual gas is de-
with residual gases.
termined by the gas-exchange process.
Although the residual gas does not
participate directly in combustion, it does
The fresh mixture drawn into the cylinder
influence ignition patterns and the actual
is a combination of fresh air and the fuel
combustion sequence. The effects of this
entrained with it. While most of the fresh
residual-gas component may be thoroughly
air enters through the throttle valve,
desirable under part-throttle operation.
supplementary fresh gas can also be
Larger throttle-valve openings to com-
drawn in through the evaporative-
pensate for reductions in fresh-gas filling
Cylinder charge in the spark-ignition engine
1 Air and fuel vapor,
2 Purge valve
with variable aperture,
3 Link to evaporative-emissions
4 Exhaust gas,
5 EGR valve with
6 Mass airflow (barometric pressure pU),
7 Mass airflow
(intake-manifold pressure p
8 Fresh air charge
(combustion-chamber pressure pB),
9 Residual gas charge
(combustion-chamber pressure pB),
10 Exhaust gas (back-pressure pA),
11 Intake valve,
12 Exhaust valve,
α Throttle-valve angle.
are needed to meet higher torque
on a supplementary EGR valve linking
engine demand. These higher angles reduce the
the intake and exhaust manifolds. The
management engine’s pumping losses, leading to
engine ingests a mixture of fresh air and
lower fuel consumption. Precisely reg-
exhaust gas when this valve is open.
ulated injection of residual gases can
also modify the combustion process to
reduce emissions of nitrous oxides (NOx)
Because maximum possible torque is
and unburned hydrocarbons (HC).
proportional to fresh-air charge density, it
is possible to raise power output by
compressing the air before it enters the
Dynamic pressure charging
The power produced by the spark-
A supercharging (or boost) effect can be
ignition engine is directly proportional to
obtained by exploiting dynamics within
the mass airflow entering it. Control of
the intake manifold. The actual degree of
engine output and the corresponding
boost will depend upon the manifold’s
torque at each engine speed is regulated
configuration as well as the engine’s
by governing the amount of air being
instantaneous operating point
inducted via the throttle valve. Leaving
(essentially a function of the engine’s
the throttle valve partially closed restricts
speed, but also affected by load factor).
the amount of air being drawn into the
The option of varying intake-manifold
engine and reduces torque generation.
geometry while the vehicle is actually
The extent of this throttling effect
being driven, makes it possible to employ
depends on the throttle valve’s position
dynamic precharging to increase the
and the size of the resulting aperture.
maximum available charge mass through
The engine produces maximum power
a wide operational range.
when the throttle valve is fully open
(WOT, or wide open throttle).
Figure 3 illustrates the conceptual
Further increases in air mass are
correlation between fresh-air charge
available through the agency of
density and engine speed as a function
of throttle-valve aperture.
Throttle-valve map for spark-ignition engine
Throttle valve at intermediate aperture
The intake and exhaust valves open and
close at specific points to control the
transfer of fresh and residual gases. The
ramps on the camshaft lobes determine
both the points and the rates at which the
valves open and close (valve timing) to
define the gas-exchange process, and
with it the amount of fresh gas available
Valve overlap defines the phase in which
the intake and exhaust valves are open
resh gas charge
simultaneously, and is the prime factor in
determining the amount of residual gas
remaining in the cylinder. This process is
known as "internal" exhaust-gas
recirculation. The mass of residual gas
can also be increased using "external"
exhaust-gas recirculation, which relies
mechanically driven compressors pow-
ered by the engine’s crankshaft, with the
two elements usually rotating at an in-
variable relative ratio. Clutches are often
used to control compressor activation.
Here the energy employed to power the
Operation of the spark-ignition engine is
compressor is extracted from the exhaust
contingent upon availability of a mixture
gas. This process uses the energy that
with a specific air/fuel (A/F) ratio. The
naturally-aspirated engines cannot
theoretical ideal for complete combustion
exploit directly owing to the inherent
is a mass ratio of 14.7:1, referred to as
restrictions imposed by the gas ex-
the stoichiometric ratio. In concrete terms
pansion characteristics resulting from the
this translates into a mass relationship of
crankshaft concept. One disadvantage is
14.7 kg of air to burn 1 kg of fuel, while
the higher back-pressure in the exhaust
the corresponding volumetric ratio is
gas exiting the engine. This back-
roughly 9,500 litres of air for complete
pressure stems from the force needed to
combustion of 1 litre of fuel.
maintain compressor output.
The exhaust turbine converts the
The air-fuel mixture is a major factor in
exhaust-gas energy into mechanical
determining the spark-ignition engine’s
energy, making it possible to employ an
rate of specific fuel consumption.
impeller to precompress the incoming
Genuine complete combustion and
fresh air. The turbocharger is thus a
absolutely minimal fuel consumption
combination of the turbine in the exhaust-
would be possible only with excess air,
fas flow and the impeller that compresses
but here limits are imposed by such
the intake air.
considerations as mixture flammability
Figure 4 illustrates the differences in the
and the time available for combustion.
torque curves of a naturally-aspirated
engine and a turbocharged engine.
The air-fuel mixture is also vital in
determining the efficiency of exhaust-gas
treatment system. The current state-of-
the-art features a 3-way catalytic
Torque curves for turbocharged
and atmospheric-induction engines
converter, a device which relies on a
with equal power outputs
stoichiometric A/F ratio to operate at
1 Engine with turbocharger,
maximum efficiency and reduce un-
2 Atmospheric-induction engine.
desirable exhaust-gas components by
more than 98 %.
Current engines therefore operate with a
stoichiometric A/F ratio as soon as the
engine’s operating status permits
Certain engine operating conditions
make mixture adjustments to non-
stoichiometric ratios essential. With a
cold engine for instance, where specific
adjustments to the A/F ratio are required.
As this implies, the mixture-formation
system must be capable of responding to
a range of variable requirements.
Engine rpm nn
deficiencies of 5...15 % (λ = 0.95...0.85),
engine The designation l (lambda) has been
but maximum fuel economy comes in at
management selected to identify the excess-air factor
10...20 % excess air (λ = 1.1...1.2).
(or air ratio) used to quantify the spread
Figures 1 and 2 illustrate the effect of the
between the actual current mass A/F ratio
excess-air factor on power, specific fuel
and the theoretical optimum (14.7:1):
consumption and generation of toxic
λ = Ratio of induction air mass to air emissions. As can be seen, there is no
requirement for stoichiometric com-
single excess-air factor which can
simultaneously generate the most
λ = 1: The inducted air mass corresponds favorable levels for all three factors. Air
to the theoretical requirement.
factors of λ
= 0.9...1.1 produce
λ < 1: Indicates an air deficiency, “conditionally optimal” fuel economy with
producing a corresponding rich mixture.
“conditionally optimal” power generation
Maximum power is derived from λ =
in actual practice.
Once the engine warms to its normal
λ > 1: This range is characterized by operating temperature, precise and
excess air and lean mixture, leading to
consistent maintenance of λ = 1 is vital
lower fuel consumption and reduced
for the 3-way catalytic treatment of
power. The potential maximum value for λ
exhaust gases. Satisfying this re-
– called the “lean-burn limit (LML)” – is
quirement entails exact monitoring of
essentially defined by the design of the
induction-air mass and precise metering
engine and of its mixture for-
of fuel mass.
mation/induction system. Beyond the
Optimal combustion from current en-
lean-burn limit the mixture ceases to be
gines equipped with manifold injection
ignitable and combustion miss sets in,
relies on formation of a homogenous
accompanied by substantial degener-
mixture as well as precise metering of the
ation of operating smoothness.
injected fuel quantity. This makes
In engines featuring systems to inject fuel
effective atomization essential. Failure to
directly into the chamber, these operate
satisfy this requirement will foster the
with substantially higher excess-air
formation of large droplets of condensed
factors (extending to λ = 4) since com-
fuel on the walls of the intake tract and in
bustion proceeds according to different
the combustion chamber. These droplets
will fail to combust completely and the
Spark-ignition engines with manifold
ultimate result will be higher HC
injection produce maximum power at air
Effects of excess-air factor λ on power P and
Effect of excess-air factor λ on untreated
specific fuel consumption be.
a Rich mixture (air deficiency),
b Lean mixture (excess air).
Specific fuel consumption
Relative quantities of
Excess-air factor λ
Excess-air factor λ