A Simulation of An Engine Start-Stop System with An Integrated Starter Generator (ISG) by Decreasing Damping Effects and Developing a Control Technique
Introduction:
As we are consuming our natural resources, which are depleting day by day, a day will come when our natural resources will be hard to find or eventually disappear. The increasing demand and overuse of natural resources, especially natural petroleum such as diesel and petrol, leads to a rise in their prices on a daily basis. Consequently, it causes an increase in cost and expenses in the automobile industry as we speak. In order to avoid any further fuel cost increase, we require such methods in the automobile systems which would be able to optimize the automobile fuel.
The fuel economy standard remains a challenge while it continues to promote optimization and sometimes (although in rare occasions) novel architecture that combines internal combustion engines (ICE) with various forms of electric vehicles. Different types of electric vehicles, such as a hybrid electric vehicle (HEV), are the best fit for the ICE in the coming future. Therefore, it appears that there is a trend of ample availability in hybridized powertrains, which are used as the Integrated Starter Generator (ISG).
IGS tends to replace the conventional starter alternator (generator) and motor. Beside its basic function (motor and alternator), IGS provides an automatic start-stop system for fuel-efficient vehicles.
Another problem to be considered in vehicles is the transfer of oscillation from car to passenger during the “start and stop” process. The IGS is also used for damping these oscillations.
Integrated Starter Generator (ISG)
The electronically controlled integrated starter generator, as its name implies, replaces both the conventional generator and the starter of a vehicle.
Advantages of combining the alternator and starter into single units are:
- Elimination of the starter which is passive only at the start of the engine
- Ability to replace the pulley between the crankshaft and the alternator
- Ability to eliminate slip rings and brushes present in some wound rotor alternators.
The ISG fundamentally works as a bi-directional converter for converting mechanical energy to electrical energy and vice versa.
In many cases, ISG is located (sandwiched) between the engine and the transmission.
Three main and major functions of ISG are:
- Start-stop
- Electricity generation
- Power assistance.
The ICE is turned off through the ISG in order to conserve energy at stops and then starts immediately by pressing the gas pedal. So we can say that after stopping for a long time, when the vehicle is no longer in motion, the car engine completely shuts off with the help of the ISG,And when the driver starts the accelerator, the ISG starts immediately without any oscillation.
In the “Engine Cranking” mode the starting speed is provided through the battery with the help of the ISG. Once the maximum speed is achieved, ISG power supply is turned off. However, another great advantage is the fact that the driver would not feel any difference, mainly because of the ICE starting independently by the ISG.
Electricity is generated by the ISG by spinning the crankshaft of the vehicle. Advantages of the ISG are listed below:
- When the ICE is combined with the electrical motor, the electric motor power is assisted by the ISG by enabling the start-stop feature
- Its start-stop function and breaking capability can reduce the fuel consumption by 20%
- No fuel is needed to power ISG, therefore a lower start emission
- The engine completely goes off instead of idling, by using the ISG, which is a very helpful feature while driving in urban areas.
- Noise and vibration free operation
- No wear and tear components in the ISG.
The main drawback of the ISG is that it requires a specialized power system. The ISG design is quite complex, so it requires a professional to design it. Some of the complex requirements for designing the ISG are:
- The starting torque required for the ISG is high, which is an unfavorable condition
- Speed range is high in the generator mode
- Life cycle for 10 years is 250,000/star-stop
- Working temperature of -300C to +1150C
- Good serviceability, reliability, acceptable cost, etc.
Types of ISG
Two types if ISG which are currently used:
- Directly connected to the crankshaft between the engine and the gearbox.
- Belt driven.
Directly connected to the crankshaft between the engine and the gearbox:
An ISG directly connected to the crankshaft between the engine and the gearbox experiences direct heat from the engine, which might affect the ISG. The whole setup is shown in the figure below:
Figure 1: ISG directly connected to the crankshaft between the engine and the gearbox
Belt driven
This type of the ISG is mounted on the conventional location of the starter motor or the generator in order to couple with the flywheel via a belt. It is simple and inexpensive. Its basic assembly is shown in the figure below:
Figure 2: Belt driven ISG
Design of Model in MATLAB:
MATLAB/Simulink model for studying the ISG system consists of a different subsystem, which includes a specification of vehicles.
The different subsystems are:
- Engine torque block
- ISG block
- Crankshaft block.
Engine torque block
It is made by studying variously related literature. It further includes three different types of the subsystem, i.e. indicated torque, fraction torque, and inertia torque. The torque equation for the engine model is:
Indicated torque is mainly done inside the cylinder due to the combustion. Inertia torque is used to approximate forces generated by masses in reciprocating motion. Friction torque is the torque responsible for affecting the piston during sliding motion
Simulink subsystem of the engine model is shown below:
Figure 3: Engine model
Indicating torque model
It is obtained by the following equations:
Indicated torque submodel is shown in Figure 4.
Figure 4: Indicated torque model for one cylinder
Inertia torque model
Inertia torque is calculated for every cylinder separately and is obtained with the following equations:
The above equations can be used in different combinations of engine parameters in order to define the indicated torque and inertia torque. The inertia torque model is shown in Figure 5:
Figure 5: Inertia torque for single cylinder
Friction torque model
The friction torque model is not calculated for every cylinder but only for the whole engine at once. Its model is quite different from the other two (inertia and indicated) torque models. The Simulink sub-model of the friction torque is shown in the figure below:
Figure 6: Friction torque for the whole engine
Integrated Starter Generator (ISG) model
The basic purpose of the ISG model is to provide sufficient starting torque to the system and to rotate the crankshaft to the required speed. The main equation describing the working of the ISG is given as:
At the start, the torque from the ISG is at maximum in order to provide sufficient speed to the crankshaft. As the speed for the crankshaft is increased, the torque of the ISG decreases continuously. The thing which differentiates the ISG from other starters is that the ISG depends on the angular speed. Conventional starters relate to the gearbox through a flywheel because the speed is increased much more than with the ISG starter.
The ISG starter is shown in the following figure:
Figure 7: ISG block in Simulink
Crankshaft Block
For the conversion of the torque from cylinder and ISG system to a rotational motion of the crankshaft, a dynamic crankshaft model is required. The total inertia provided to the crankshaft is the sum of the flywheel inertia, engine inertia, and damper inertia. It is obtained by the following equation:
A developed design of the crankshaft is shown below:
Figure 8: Crankshaft model
Simulation and Results
With a developed design, we can see and verify our results. The results are obtained by simulating the design model in the MATLAB/Simulink.
Torque comparison
The torque of the system is taken with and without the ISG system. Results verify that the actual torque without the ISG is vibrating, therefore it is not stable. The torque graph is shown in the figure below:
Figure 9: Comparison of torque
The blue line shows that torque without the ISG system becomes stable after reaching a peak but cannot come down to zero, whereas the pink line shows the torque with the ISG system, which is more stable and which comes to zero in a matter of a few seconds.
Speed of crankshaft
The speed of the crankshaft is low at the start, but it will continue increasing as the torque of the ISG is decreased. The torque of the ISG is inversely related to the speed of the crankshaft.
Figure 10: Speed of the crankshaft rps
Figure 111: Speed of the crankshaft rpm
Figure 12: Comparison of the crankshaft starting alone and with the ISG
FFT of crankshaft original signal
FFT signal for the speed of the crankshaft is obtained, shown in the figure below:
Figure 13: FFT of the crankshaft speed
We can see that after the FFT we have specific amplitudes on specific frequencies – 5, 27, 31, 6 5Hz respectively. This is shown in the graph below. The average speed of the crankshaft is 698 rpm and the frequency is 35.9Hz.
The FFT of the crankshaft with the ISG is as shown below:
Conclusion
The main aim of the project was to simulate an engine start-stop system with the ISG in order to reduce the vibrations (oscillations) during the start and turning-off of the vehicle. The ISG system is the most effective in controlling these vibrations and damps them very quickly, in a matter of a few seconds.
