Keywords

1 Introduction

1.1 Trends of Growing Need for Electric Power

Today’s vehicles require much more electric energy than older ones, due to the much wider array of electrical and electronic on-board comfort and safety systems. Electric loads in a conventional ICE vehicle are increased up to 2–3 kW. In general, in the case of power 1 kW, driving 100 km each needs to consume 0.7–1.2 L of petrol. Fuel consumption and exhaust emissions pose serious challenges due to the increase of electric power. Also the reliability of power supply is strongly required, when electric security systems such as x-by-wire systems (e.g. steering-by-wire, brake-by-wire) are introduced in the vehicles [1, 2]. The balance of power delivery to different systems is becoming more and more difficult.

1.2 Solutions to Improve Fuel Economy

For a conventional internal combustion engine (ICE) vehicle, there are several ways to improve fuel economy by improving the electric energy system. Charging management is one technique. Regeneration is converting a vehicle’s kinetic energy to electric energy for battery recharging during deceleration. Regeneration improves fuel efficiency by up to two percent, though this result is influenced by the engine capacity and electric load. But overcharge or insufficient battery SOC can still be problems if there is no battery SOC monitoring.

The second way is the auto start stop system which turns off the engine each time the vehicle comes to a complete halt—such as at traffic lights—and restarts it automatically. This is an effective way for drivers living in urban areas to reduce fuel consumption by an estimated 5 % [3]. But inadequate battery SOC level can be caused by frequent engine stop, which limits fuel reduction effect.

1.3 Introduction of Electric Energy Management System

This paper presents an electric energy management system which integrates charging management, auto start stop function, battery monitoring and electrical load management. This system shows the following features:

  • Strategy for energy management is based on the battery state of charge (SOC).

  • Charging voltage is increased to stimulate battery charging when the battery soc is low, but when soc is in a normal range, charging voltage is adjusted according to vehicle motion to improve fuel economy.

  • Start stop function turns off and restarts engine automatically, when the battery SOC is in an appropriate status.

  • Power supply priority is applied to preferentially delivery electricity to the important loads, when battery SOC level is low.

2 Analysis of Electric Energy Management

2.1 Electric Power Flow in a Conventional Vehicle

In a conventional ICE vehicle, usually the electric energy system consists of an alternator that generates electric power, an electric storage device, such as a lead-acid battery, and various electric loads [4]. The alternator tries to maintain a fixed voltage level on the power net. A traditional lead-acid battery is present for supplying IG-off loads and for making the power net more robust against peak-power demands. The power flow is shown in Fig. 1.

Fig. 1
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Power flow in a conventional ICE vehicle

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The power flow starts with fuel which is injected into the engine. The output power of engine splits up into several directions: one part goes to the transmission for vehicle propulsion, while other part goes to the alternator. The alternator generates electric power for various electric loads, and also charges the battery. Contrary to the electric loads, the power flow of the battery can be positive as well as negative. In the end, all power, except for losses, is used for vehicle propulsion and for electric devices connected to the power net.

2.2 Problems of Common Electric Energy System

Electric energy system in a conventional ICE vehicle has several problems as follows:

  • Bad battery maintenance: There is no monitoring function for battery SOC, both charging side and consuming side are out of control from the electric energy system. Insufficient SOC and overcharge both can be caused by the imbalance of generated power and load power.

  • Lack of protection for important loads: There is no priority distinction for different electric loads. Important loads can also be shut down when the SOC is low.

  • Fuel economy degradation: The alternator is connected to the engine by a belt with a fixed gear ratio, and output voltage is fixed. This means that electric power is going to be generated even though efficiency of the engine is worse. In such a case, more fuel is required to generate electricity and fuel economy deteriorates.

2.3 Model of Electric Energy Controller

The defects of the electric energy system in a conventional ICE vehicle are caused by the fact that there is no vehicle level control of electric power flow. Therefore, an electric energy controller is needed. The electric energy system should be restructured as Fig. 2.

Fig. 2
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Model of electric energy management controller

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The electric energy management system consists of three subsystems: generation, storage and distribution. In such a system, generation can be adjusted according to the vehicle motion status, battery sensor is present for monitoring the SOC to keep good battery maintenance, and power supply priority is introduced to guarantee the safety of important loads. Similar like the idea of “torque based control”, SOC of battery is the core parameter to coordinate three subsystems. Based on the battery state of charge (SOC), system determines the strategy for energy management.

3 Design of an Integrated Electric Energy Management System

3.1 Structure of an Integrated Electric Energy Management System

This chapter presents the implementation of an integrated electric energy management system, which is shown in Fig. 3. An energy management unit (EMU) is the brain of whole system. Based on the battery state of charge (SOC), EMU determines the strategy for energy management. Charging management, auto start stop function, battery monitoring and electric load management are all controlled by EMU.

Fig. 3
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Implementation of electric energy management system

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A battery sensor computes the SOC of battery from voltage current and temperature signals, and sends the info to EMU by LIN bus. EMU controls charging voltage by a LIN connected alternator regulator to maintain SOC. When the battery soc is low, EMU increases the charging voltage to stimulate battery charging. But when soc is in a normal range, charging voltage is adjusted according to vehicle motion to improve fuel economy. Producing electricity in this highly efficient way delivers an additional advantage: when accelerating, the alternator is restrained; more power of the engine can be directed to the drive wheels. Regeneration increases fuel efficiency while simultaneously enhancing driving dynamics.

The status of power train can be derived from clutch sensor and neutral sensor (for manual transmission) or TCU (Transmission Control Unit for automatic transmission). Also engine status signals from EMS (Engine management system) are also necessary for the auto start stop function. The auto start stop function turns off and restarts engine automatically, when the battery SOC is in an appropriate status. If necessary for comfort or safety reasons, EMU will restart the engine: for example, if the vehicle begins to roll or refrigeration is needed for a comfortable temperature. Because of good battery maintenance, battery SOC never falls too low, the auto start stop function is available in most cases, makes more fuel reduction.

EMU determines which electric load the power should be preferentially supplied. When the battery SOC is low or when the electric system malfunctions, EMU shut down the electric loads with low supply priority such as a seat heater by a CAN connected digital relay controller.

Fuel reduction is obtained from regenerative braking and engine shut-off. For regeneration, the alternator absorbs energy from the drive train and stores it into the battery in terms of electric energy. This is the most economical way to charge the battery, since it requires no additional fuel. When used consistently, auto start stop function can deliver significant reductions in fuel consumption and emissions.

3.2 Torque Control Improvement for Regeneration

For most regeneration cases, charge voltage is stetted in only three fixed steps [3]. As showed in Fig. 4, system increases the alternator’s adjustment voltage during accelerating phase while decreases the voltage when vehicle is decelerating. Adjustment voltage is set to be a default value if cruise driving is recognized or in a malfunction mode. By controlling the alternator, the engine torque output can be influenced. Discontinuous torque output to wheels can be felt due to discrete voltage setting steps.

Fig. 4
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Charge voltage adjustment by estimating vehicle motion

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A better way for charging voltage adjustment is to set the voltage in continuous steps. Setting voltage is dependent on vehicle’s acceleration which is the first derivative of vehicle speed. The relationship between setting voltage and acceleration can be expressed as a linear function which is shown in Fig. 5. A nonlinear function can also be adopted for accelerating the response.

Fig. 5
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Relationship between setting voltage and acceleration

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The setting voltage V can be represented as follow:

$$ {\text{V}} = {\text{V}}\;{\text{default}} + {\text{k}}\; \times \;{\text{a}} $$

Where V default is the default voltage, a is acceleration of vehicle and k is the coefficient. The value of acceleration becomes positive during accelerating while minus if vehicle is decelerating. This brings the same effect as above fixed voltage adjustment but avoids the abrupt torque output, and improves NVH performance.

4 Conclusion

Improving fuel economy and restricting emissions has always been a major challenge to the automotive industry. Historically, the research was focused on improving the mechanical side of the vehicle. Because of the growing need for more electric power, electric energy management becomes a promising resolution.

This paper presents a design of electric energy management system which integrates charging management, auto start stop function, battery monitoring and electrical load management. A closed-loop control of the battery SOC improves stability of electric power net. Auto start stop function combined with charging management delivers more reductions in fuel consumption and CO2 emissions. A continuous charging voltage adjustment way is introduced. This avoids the abrupt torque output, and improves NVH performance.