M20100155
Two Wheeler Engine Control Unit – Development, Challenges and
Solutions
Aravind Aithal, Ramakrishna Donakonda
Robert Bosch Engineering and business Solutions Limited, Bangalore, India
Copyright © 2011 SAE INDIA
ABSTRACT
In motorcycles, Engine Management systems (EMS) are
used mostly in high end models. However, increasing
demands for improved fuel efficiency and tighter
legislations for reduced emissions have made it
inevitable to adopt EMS, instead of the carburetors even
in small gasoline engines motorcycles which are already
quite fuel-efficient.
Today’s system cost is significantly high and comparable
with passenger car applications and success in
emerging markets due to fun-to-drive aspects is likely to
be limited to premium segment. So there is an obvious
requirement for minimized system cost along with smart
system solutions. For reducing the number of sensors
and cost, highly accurate methods have been developed
to eliminate sensors like barometric sensor and cam
timing sensor. An engine management system has been
developed for the unique problems of one cylinder, high
performance engines.
areas. Figure 1 and figure 2 explain the legislation,
emission limits and test cycles in different countries for
two wheelers.
Figure 1 – Two wheeler emission legislations
INTRODUCTION
In automobiles, hydrocarbons, carbon monoxide and
oxides of nitrogen are created during the combustion
process and are emitted into the atmosphere from the
tail pipe. Over the last 30 years, reductions in tailpipe
exhaust emissions of more than 90% have been
demanded of and have been achieved by the automobile
industry. The need to control these emissions gave rise
to the computerization of the automobile.
To achieve improved combustion efficiency and
performance, as well as the reduction of exhaust
emissions, an EMS is used is to control air fuel ratio
(AFR) and spark timing in response to various operating
conditions like engine speed, temperature, load, altitude,
and battery voltages. The EMS comprises of an
Electronic control unit (ECU), related sensors and
actuators.
In countries like India a large number of two and three
wheelers are used and the numbers continue to
increase, leading to high levels of pollution in urban
Figure 2 – Emission limits and driving cycles
Future legislations for motorcycles with small engines
will make it necessary for manufacturers to follow the
same path in engine development as car manufacturers
have already taken.
The figure 3 shows the evolution of motorcycle control
technologies -present and future.
SMALL ENGINE EMS
The engine management system developed for small
engine applications as shown in figure 4
Figure 3 - Different stages of motor cycle control
technologies
Adopting EMS for 2 wheeler application is quite
challenging. There are many technical challenges and
challenges related to cost. Approach of directly taking
over a passenger car EMS to a small engine application
will result in severe disadvantages due to the large
differences in physical behavior between the engine
types, different technologies, market requirements,
functional differences and system complexities. Hence
EMS of passenger cars cannot be directly applied to
small engines.
Figure 4 - System schematic - 4-stroke electronic
injection system
Test Engine - The engine used for this work was a 1cylinder 125 cc. Specifications are given in figure 5
When considering the small engine motorcycle market
relative to the automobile market, one key difference
immediately becomes apparent, namely; cost. The need
for cost-effective emission solutions in the small vehicle
industry is therefore widely recognized and as outlined in
the following section, a number of alternative low-cost
strategies are currently being pursued.
One of the main problems to solve has been accurate
load detection at different throttle openings. Depending
on the inlet pressure for load determination is difficult
due to its very non-linear behavior. On the other hand,
using only throttle potentiometer information will result in
poor load signal calculation at part loads which often
makes it necessary to run rich at low loads.
This paper describes an investigation into an EMS
applied to small gasoline-fuelled, spark-ignited internalcombustion engines used in two and three wheelers, to
meet the challenges of fuel economy, emissions and
also capable of some diagnostics to assist in
troubleshooting faults as the system becomes more
complex. The principles of the advanced algorithms and
their effects are discussed in the paper. The paper will
also show emissions, and engine operating data that
verify system performance. This paper also discusses
the system and the software developed which is able to
combine good response, improved cold start, good idle
stability, improved transient behavior and better
drivability for motorcycle applications and is also costefficient, compact, and state of art for the defined
segment.
Figure 5 - Test engine and motorcycle specification
A one-cylinder engine has high pressure oscillations in
the intake manifold compared to a car engine. This
makes it difficult to accurately estimate the air mass in
the combustion chamber. The oscillations also result in
sensitivity to wall wetting effects, which in turn affects the
drivability when the mixture is set toward stoichiometric
ratio. Fuel film compensation algorithms are used in the
EMS to handle these phenomena.
For measuring the engine control data, the following
methods are investigated from a small engine point of
view

Intake air mass calculation

Atmospheric pressure estimation

Phase detection

Dynamic correction
INTAKE AIR MASS CALCULATION
Study was made as to the applicability of the
conventional intake air mass measurement methods speed density, throttle speed and the method combining
both of these - with 4-stroke single-cylinder engines
generally used in small engine applications.
Speed density method
In the 4-cylinder engine, there is a relatively large
volume called surge volume connected to the manifold
for each cylinder in the throttle valve downstream. This
gives averaged intake manifold pressure automatically
and in case of steady operation, an intake air mass can
be calculated from the pressure value independent of
sampling timing as well as from engine speed (speed
density method). On the other hand, the volume of the
throttle valve downstream in the single-cylinder engine is
far smaller than that of the above-mentioned multicylinder engine and thus will be subjected to great
fluctuations in one cycle of engine operation as figure 6
shows. If these pressure fluctuations are to be
smoothened, signal may be delayed, which would be
inappropriate for motorcycles requiring high response.
Furthermore even in a steady state, not much
measurement accuracy can be achieved because of
small change in intake manifold pressure between
medium and heavy load, thereby making it difficult to
adopt this method.
Figure 7 - Intake manifold pressure of single cylinder
engine
The relationship between intake air mass and manifold
pressure in multi cylinder engines at different engine
speeds is shown in figure 8
Compared to multi-cylinder passenger engines, small
motorcycle engines have a relative small intake manifold
volume between throttle body and intake valve strongly
pulsating air mass flow, highly dynamic behavior as
shown in the figure 6
Figure 8 - Averaged intake manifold pressure with
respect to intake air mass
In single cylinder engines, the relationship between
intake air mass and manifold pressure is shown in figure
9. A non linear behavior is observed at high engine load
due to pressure pulsations in the intake manifold. The
difficulty to correlate this pressure pulsation with the
actual airflow to the engine is the main reason that a
manifold air pressure (MAP) sensor is often not used as
primary sensor for air charge determination in single
cylinder applications.
Figure 6 - Difference of intake manifold behavior
between 4 cylinder engine and single cylinder engine [1]
The screenshot of the measurement of the above
behavior on actual engine is shown in figure 7
Figure 9 - Air charge calculation based on speed density
method
Throttle speed method - This method, which is employed
in racing cars etc., produces a high response. However,
because of a very sharp initial rise in intake air mass
against the change at narrow opening and non-linear
character (figure 10), it would be difficult for this method
to provide the intake air mass measuring accuracy that
could satisfy today’s rigorous emission regulations.
very small for small engines, it will result in quick
recovery towards the ambient pressure once the suction
phase is over even when the throttle is not wide open.
As shown in figure 7, a point free from increase in
pressure (saturation point) is detected and is set for the
atmospheric pressure and during full load; the filtered
intake manifold pressure can be used as the primary
source of ambient pressure.
Figure 12 shows the test results for the atmospheric
pressure
estimation
during
positive
gradients.
Measurements on a separately installed barometer are
given for comparison plotted as error level. Results
prove that this method gives a fairly accurate estimation
of the atmospheric pressure.
Engine
speed
Throttle
opening
Figure 10 - Air charge calculation based on throttle
speed method
Combined speed density and throttle speed method - To
compensate for drawbacks of these two methods, a
combined method could be considered – the speed
density method at low load range and the throttle speed
method at the remaining operation range. This method
has potential for sufficient accuracy; however
complications in switching over between the two
methods exist. For a fixed engine speed, the figure 11
shows the relation between the throttle opening and
estimated air charge in the combustion chamber for
different load ranges. Using this approach, the deviation
between the estimated and actual air quantity is within a
tolerance band of AFR. This variation is shown in upper
part of figure 11.
Figure 11 - Variation of estimated air charge w.r.t.
throttle angle using combined method. The error in the
calculation is measured by AFR variation.
ATMOSPHERIC PRESSURE ESTIMATION
During engine stop state, the atmospheric pressure can
easily be measured because it is equal to the intake
manifold pressure. While the engine is running, during
the suction phase there is a large drop in intake manifold
pressure, however since the intake manifold volume is
Figure 12 - Tolerances of atmospheric pressure
estimation based on an up hill test trip
PHASE DETECTION
Early engine control used to provide ignition once in
each engine revolution without distinction of
compression and exhaust stroke. In modern engines, the
fuel injection and ignition occur at the correct phase of
the engine cycle. For distinction of the stroke in 4-stroke
multi cylinder engines, a cam position sensor is in
common use. In single cylinder engines however it is not
often used for cost reasons. To detect phase, the
existing methods (without a cam position sensor) have
disadvantages like start emissions (depending on initial
piston position at start). To tackle this, the following
method of phase detection was implemented which uses
variations in the intake manifold pressure. It was also
found to be a very reliable method of phase detection.
As described so far, the intake manifold pressure in
single-cylinder engines are deeply affected by the intake
stroke taking place during the intake valve opening.
From this characteristic, it may be considered that
observing the intake manifold pressure affords an
understanding of the intake stroke. P0 and P1 in figure
13 indicate the crank timing pressures at the BDC. At the
BDC in one cycle, the intake valve is open at one end
and closed at the other, obviously making a great
change in the intake manifold pressure. Thus, comparing
the intake manifold pressures sampled in this timing
should make the distinction of engine stroke possible.
Figure 13 - Wave form for engine phase distinction
The BDC is detected by crank pulse and the intake
manifold pressure is sampled and stored in memory
every 360°CA. The value thus memorized is retained
until the next sampling. The difference between the new
and the previous pressure data is checked as to whether
it is above the set value. When a difference with a higher
value than set is repeated more than several times, the
program confirms the engine stroke and switches over
from one-revolution to one-cycle control.
Figure 14 Transients without wall wetting compensation
Figure 15 shows the results when the wall wetting
control is activated and calibrated with a steady state
lambda target of 1.0. It can be seen that big
improvements in lambda behavior under very fast load
and speed changes are realized. This functionality
makes it possible to achieve a vast reduction in engine
out exhaust emissions in real driving conditions and
should therefore make it possible to reduce costs on the
catalyst.
DYNAMIC CORRECTION
Today's fuel injection systems usually inject the fuel late
at closed inlet valves. This is a compromise which may
result in either that too much fuel already has been
injected at decelerations due to early injection timing or
that too little fuel injected at fast accelerations, which will
result in engine misfire. This may be acceptable on a
multi cylinder engine, but absolutely not on a onecylinder engine that will lose its power during a whole
cycle. This makes the engine very sensitive to engine
stall at fast throttle opening from idling (throttle blipping).
Immediate response control is required in order to
ensure that no combustion misfire may occur even
during the fastest throttle opening. Engine performance
is strongly dependent on gas dynamic phenomena in
intake and exhaust systems.
Another major problem in driving an engine with extreme
transients, such as in motorcycle driving, is that the
effect from changes in the wall wetting makes it
necessary to enrich the overall air fuel mixture. This can
result in drivability problems due to over fueling and
fouled sparkplugs. This is not a problem with the wall
wetting compensation algorithm used. Figure 14 shows
a typical behavior in a fuel injection system which has a
target steady state lambda at 1.0. Without the wall
wetting function, a lambda variation of more than ± 20%
is possible and hence results in drivability problems and
the need of more overall enrichment or enleanment,
which will produce more exhaust emissions.
Figure 15 Transients with wall wetting compensation
TEST RESULTS – AFR CONTROL
The vehicle emissions are within EURO 4 limits including
cold start mode. It has been proven that the EMS is
effective in AFR control and hence emission control
compared to conventional carburetor system.
The figure 16 shows the comparison in the AFR control
across the engine speed range for ECU based systems
versus conventional carbureted systems. It is observed
that lambda (λ) i.e. ratio of actual AFR to stoichiometric
AFR) is controlled closer to the target value of λ = 1
with the EMS.




Figure 16 Exhaust improvement
EMS COMPONENTS (SENSORS)
Due to price sensitivity in this market segment, there is
an obvious requirement for minimizing system costs with
system solutions. For reducing the number of sensors
and hence cost, accurate software methods have been
developed to eliminate sensors like barometric sensor,
cam timing sensor which has been discussed above.
Smart system solutions, combination modules for
sensors and actuators and value engineered
components are also required to reduce the overall
system cost.
For small motorcycles, the EMS system has to be
simplified and down-sized considerably from the
conventional one. The goal is to pack various functions
into a compact and yet simple system that could be
installed in the place of a conventional carburetor.
For example, by modularizing the throttle body, the
throttle position sensor, intake air temperature sensor
and manifold pressure sensor into one combined unit,
packaging effort and interfaces (wiring harness) are
minimized and also resulting in a unit as compact as a
carburetor.
CONCLUSION
It has been shown that a high-speed, one-cylinder
engine shows a very different physical behavior
compared to a multi cylinder engine. ECU control
software has been developed to tackle the unique
problems with small engines.
By using a fast and accurate airflow detection method
together with advanced fuel wall-wetting model it is
possible to keep accurate air fuel mixture and ensure
lowest possible emissions. Measurements of the intake
manifold behavior of single-cylinder engines w.r.t. crank
angle have been taken and analyzed for the combustion
phase, and following conclusions have been derived.

This system permits the measurement of intake
air mass by use of the combination of intake
manifold pressure along with throttle position
sensing, suited for intake air mass measurement
in small engine applications.
This system makes it possible to measure the
engine load with high accuracy and response.
This system is capable of estimating the
atmospheric pressure through intake manifold
pressures without the use of a barometric
sensor.
This system makes the detection of the phase of
the engine cycle through intake manifold
pressures without use of a cam phase sensor.
It is possible to create the low cost EMS system
for small engine applications without cam-timing
sensor, barometric sensor etc. along with value
components usage.
Engine management systems for the commuter segment
of the two wheeler market will see many more
innovations for low cost solutions in the fields of
miniaturization of components and reduction of system
costs by replacing components with algorithms/models.
The goal would be to bridge the costs and system
simplicity as close as possible to the existing carbureted
system.
ACKNOWLEDGMENTS
The efforts for development for this system and the
support for writing this paper by members in ROBERT
BOSCH
ENGINEERING
AND
BUSINESS
SOLUTIONS, ROBERT BOSCH INDIA LIMITED
and our principals ROBERT BOSCH GmbH are
gratefully acknowledged.
REFERENCES
[1] Motorad Vergaser und Einspritzsysteme – By John
Robinson
[2] CONCAWE, “Motor Vehicle Emission Regulations
and Fuel Specifications – Part 2 – Detailed Information
and Historic Review (1970 –1999)”, 2000.
[3] Charles Fayette Taylor, “The Internal Combustion
Engine in Theory and Practice”, Massachusetts
Institute of Technology
CONTACT
Aravind.aithal@in.bosch.com
Ramakrishna.d@in.bosch.com
DEFINITIONS, ACRONYMS, ABBREVIATIONS
AFR – Air fuel ratio
ATDC – After top dead centre
ABDC - After bottom dead centre
BTDC – Before top dead centre
BBDC – Before bottom dead centre
BDC – Bottom dead centre
CA – Crank angle
ECU – Electronic control unit
EMS – Engine Management system
MAP – Manifold absolute pressure sensor
OBD – On Board Diagnostics
SOD – Start of Development
SOHC – Single overhead camshaft
SOP – Start of production
TDC – Top dead centre