Automotive Electrical Systems and Systems Integration of Powertrain

In this article, I will look into systems integration of powertrain controls, transmission controls, calibration and driveability. This will look further in depth into engine fuelling requirements, air fuel ratio and the stoichiometric value, what a rich/lean air/fuel mixture means in terms of NOx and CO and a closed loop emission control system. Please feel free to skip to any part most relevant to you.

System Integration of Powertrain Controls

The main objectives of the systems integration of powertrain controls are to:
  • Interface with other systems on the vehicle such as the transmission, driver interface and traction control to enhance the vehicle activity, safety and efficiency.
  • Control the fuelling of the engine, ignition, emissions and cam phase under any sort of operating conditions while making sure it is done to provide a smooth, predictable and reliable driving experience.

Engine Fuelling Requirements

Engine fuelling requirements consist of a fuel map which designates the fuel required at each engine operating  point for speed and load. The x-axis is the speed of the engine (most commonly shown in RPM) and the y-axis is the power at that given point for speed of the engine (either in kilowatts or horsepower). This type of fuel map is 2D.

There are such things as 3D fuel maps too, which look at the fuel required for an engine at a certain speed at different loads (this will be as a percentage of full divided by torque).

Air Fuel Ratio

The air fuel ratio is an important ratio when it comes to engine performance and efficiency. It is ratio of mass of air to to mass of fuel in the combustion chamber of an ICE. To measure the air mass, a mass flow sensor (MAF sensor) is used during induction and a the fuel mass is calculated based on the fuel injection volume into the combustion chamber.
  • A mass flow air sensor is a hot wire anemometer. It uses the cooling effect of the air flow over a heated platinum wire to calculate the mass air flow per second that is flowing through the device. A MAF sensor would typically operate at 5V DC using the same electrical feeds as a ECU (it is important to bear in mind this is one of several types of mass air flow sensors that can be used).
  • The fuel to the combustion chamber is supplied to the injector at a constant pressure from the fuel rail/pump system. The ECU controls the fuel timing and injection (so that it controls the mass of fuel into the combustion chamber). The volume of the fuel into the chamber is computed by the length of time the the actuation voltage of +5 Volts is applied to the injector coil.
Air fuel ratio = mass of air / mass of fuel

Established when the engine is tested using a dynamometer, the mass fuel into the combustion chamber can be worked out for optimum power/torque at a given engine load or speed. This, of course, will also depend on the spark ignition time in line with the crankshaft rotation. Therefore, the optimum power of an engine is dependent on how rich (high) the air fuel ratio is as well as the timing of the spark ignition.

Stoichiometric Value

The stoichiometric value is the the air fuel ratio of perfect chemistry for complete combustion. This means that for every hydrocarbon that is burnt, it has exactly the right amount of air to burn it perfectly. For engines, the stoichiometric value is 14.7:1:
  • Rich - As the air fuel ratio increases past 14.7:1, the number of hydrocarbons that cannot burn increases since there is more than enough air in the combustion chamber to burn the hydrocarbons. As well as this, the number of nitrogen oxides (NOx) decreases which is a good thing since NOx are poisonous.
  • Lean - As the air fuel ratio decreases below 14.7:1, the amount of air needed to burn all of the hydrocarbons in the combustion chamber is less than necessary. This causes the amount of carbon monoxide to increase dramatically (as there is not enough oxygen to create carbon dioxide). As well as this, the nitrogen oxides level decreases again.
  • At the stoichiometric value for air fuel ratio, carbon monide levels are low and unburnt hydrocarbons are low. However, the amount of NOx produced is much higher than rich/lean ratios.

Closed Loop Emissions Control System

Diagram of a typical closed loop emissions control system for an engine. Note the two catalytic converted to clean the exhaust gases up as much as possible.

Fuel Control with Closed Loop Feedback

The heated exhaust gas oxygen sensor (HEGO sensor which is also known as a lambda sensor) determines the amount of oxygen in the exhaust gas. This data is then fed back into the ECU which helps the ECU decide whether the air fuel mixture is too lean or rich. By knowing this, the ECU can then change the length of the voltage to the injector coil to change the air fuel ratio into the combustion so that it maintains the stoichiometric value (the stoichiometric value will have the best emissions and optimum efficiency).