June 04, 2019
| By: Bonnie Baker
Blogger, Maxim Integrated
As vehicle emissions standards become more strict, automotive design engineers are striving to achieve the proper fuel-oxygen ratio in their designs. This ratio determines the performance and the exhaust toxicity of a vehicle, so it’s critical for engineers to not only strike the right balance but also to maintain it over time.
A proper fuel-oxygen mixture calls for measurements from two oxygen sensors and physical parameters like crank speed, temperature, and fuel pressure. One of the oxygen sensors is used to detect the engine’s catalytic converter input oxygen concentration, while the other measures output exhaust oxygen concentration. These measurements help in the production of a continuous fuel-injector duty cycle. The vehicle’s powertrain control module (PCM) takes readings from the oxygen sensors, the engine, and other sensor data to calculate and dynamically adjust the engine’s fuel-oxygen ratio.
It’s important to closely monitor the power rails and execution timing of the PCM internal microcontroller and powertrain sensing electronics to ensure that all is working well. This is where supervisory and watchdog-timer ICs can be useful in ensuring that the car’s engine continues to run safely, smoothly, and reliably.
Supervisory and watchdog-timer ICs can help ensure that a car’s engine runs safely and smoothly—and help the vehicle meet strict emissions standards.
A vehicle’s catalytic converter plays an important role in the vehicle’s ability to meet emissions standards. The converter turns toxic gases and pollutants from an internal combustion engine into less toxic waste. When it comes to controlling the car’s ability to eliminate pollutants, the fuel-oxygen ratio in the engine’s combustion chambers is the key variable to consider. Stoichiometry, the study of ratios or the quantitative relationships between two or more substances undergoing a physical change or chemical reaction, quantifies the ratio of fuel-to-oxygen in the combustion chamber. In vehicles, a stoichiometric air-fuel ratio (AFR) or target range has the right amount of fuel and air that is as close as possible to a chemically complete combustion event (0% fuel, 0% oxygen, minimal pollutants).
A gas engine has 14.7 parts of air to one part of fuel (for a stoichiometric AFR of 14.7:1). Alcohol and diesel fuel engineers have stoichiometric AFRs that are 6.4:1 and 14.5:1, respectively. A vehicle with the best fuel economy typically operates with high levels of nitrogen oxides that get expelled through the tailpipe. On the other end of the spectrum, a vehicle in the fuel-rich zone operates with the most power, which, in turn, produces a high output level of CO and VOC pollutants.
The catalytic converter, along with the fuel injector, PCM, sparkplug, and the two oxygen sensors, make up a vehicle’s fuel-injection system. This system monitors and controls the fuel-oxygen mixture throughout the combustion process. For fuel-injection events, the fuel-injection feedback loop controls the pulse-width algorithm. Stable engine operation—in the face of varying conditions—requires the contribution of more than 100 sensed events and parameters throughout the engine to the fuel-injection PCM algorithm. When there’s a pulse-width signal from the PCM, the fuel injector’s pressurized fuel squirts into the combustion chamber. The upstream oxygen sensor takes an initial measurement of the exhaust’s oxygen concentration. Next, the fumes travel through the catalytic converter, which filters many of the pollutants. Following this step, the second oxygen sensor measures the oxygen concentration.
Given the variety of operating conditions for a vehicle, the engine control unit relies on many lookup tables and a formula to determine the pulse-width. While proper fuel-injection system operation depends on various factors, having a “health check” in place can also be beneficial. This is where supervisory and watchdog-timer ICs come into play, monitoring and verifying power and microcontroller computations over the life of the vehicle. To learn more about how these analog ICs help ensure circuit reliability and stability to keep vehicles moving, read my article, “Build in Automotive Insurance with Supervisor and Watchdog Policies,” in Electronic Design.