Thermal Anemometer Air Flow Sensors

This is a preview of the paper, limited to some initial content. Full access requires DieselNet subscription.
Please log in to view the complete version of this paper.

Abstract: Sensors based on the thermal anemometer principle are commonly used to provide a measurement of the engine intake air mass flow rate. The early sensors utilized a hot wire anemometer, which was later replaced by a flat heating resistor. The newest sensors—called hot film anemometers—integrate several heating and temperature measurement resistors on a wafer-thin membrane to provide a range of capabilities, such as the detection of flow pulsation. Hot film air flow sensors have also been used in exhaust gas applications.

Introduction

Most internal combustion engines require a reliable flow rate measurement of the incoming fresh air. In the diesel engine, an air flow sensor allows to accurately control the injected fuel quantity and to optimally and reproducibly adjust the fuel-air ratio at any load point. This is particularly important for meeting strict NOx and PM emission limits. In modern diesel engines—such as those that use a combination of high and low pressure loop exhaust gas recirculation (EGR)—the air flow meter can also be used to control the EGR rate.

Through the 1970s, the incoming air quantity was determined using a dynamic pressure flap. As technology advanced, it became apparent that this approach had several disadvantages:

In the early 1980s, an increasing number of sensor systems were introduced that utilized a new measuring principle: thermal anemometry. Thermal anemometers utilize at least one electrically heated sensor element—typically made of nickel or platinum—the ohmic resistance of which is dependent on the temperature. When gas flows around the measuring element, heat is transferred into the flow medium. A correlation exists between the gas velocity and heat dissipation—the higher the flow velocity, the higher the rate of heat dissipation—which allows to determine the gas flow rate through measurement of the electrical resistance.

Thermal anemometer air flow sensors are also known as mass air flow (MAF) sensors, air flow sensors (AFS) or air mass-flow sensors (AMS).

Generation 1: Hot Wire Air Flow Meter

The hot wire air flow meter is the oldest type of anemometer, introduced into commercial automotive technology around 1985. The hot wire air flow meter (HLM) by Bosch proved highly popular. While no longer used in automotive applications, hot wire sensors are still installed in aeroplanes with piston engine drives.

The hot wire sensor has a simple functional principle and can operate without any moving parts. An electrically-heated wire (hot wire) is placed in the intake pipe after the air filter, transverse to the flow direction, Figure 1. A temperature sensor is installed upstream of the hot wire. The hot wire is then heated to a temperature higher than the air temperature determined by the temperature sensor. As the hot wire is cooled by the incoming air, the heating current must be increased to maintain a constant wire temperature. In this method, the heating current represents a direct measurement for the incoming air mass, independent of the air density. Thus, the reduced air density at high altitudes is elegantly compensated for.

The hot wire, made of platinum, can withstand temperatures of over 1000°C. As platinum increases its electrical resistance value linear to the increasing temperature, it is possible to use the hot wire as a variable resistor within a bridge circuit. The wire must be sturdy enough to withstand load variations and fouling by deposited dirt particles. On the other hand, a low thermal mass is essential for the response time of the entire system. The wire is operated at approximately 100°C over the ambient air temperature. The resulting thermal length expansion is adsorbed through a flexible bearing. The U-shaped, clamped hot wire has a diameter of only 70 µm in order to keep the response time as short as possible.