Making Sense!

In ThIs Issue■■ Outline■■ Construction■■ Principles■■ Operation

nexT Issue■■ Smoke Ventilation Sensor

Outline

The Mass Air Flow (MAF) meter is installed in the intake duct between the air cleaner and the throttle body. The MAF meter is vital in determining the fuel injection quantity through the use

of a sensor that directly measures intake air mass.

Beginning as a vane type (mechanical type) meter, the DENSO MAF meter achieved a compact and lightweight design, broader measuring range, and improved measuring accuracy in line with intensifying fuel consumption and emission controls. Currently the most common MAF meter is the hot wire plug-in type, which is inserted in an installation orifice in the intake duct.

Mass Air Flow (MAF) Meter

2010sensor Fundamentals #012

Reference: Intake air mass detection methodsIntake air mass detection methods fall into the following three types. The MAF meter utilizes the mass flow system.

• Mass flow system: A method in which the intake air mass is measured directly.

• Speed density system: A method in which the intake air mass is indirectly measured from the pressure.

• Throttle speed system: A method in which the intake air mass is indirectly measured from the throttle position.

200019901980 2010

Air Cleaner

Throttle Body

Mass Air Flow (MAF) Meter Injector

Intake Duct

Air

Vane Type (Mechanical Type)Hot Wire Type

Plug-In Type

Changes in DENSO MAF Meters

LA990009-2421

© 2010 DENSO Sales California, Inc. All rights reserved. This document may not be reproduced or copied, in whole or in part, without the written permission of the publisher. DENSO Sales California, Inc. reserves the right to make changes without prior notice. 5/2010

Construction

Mass Air Flow meter construction is explained using the hot wire plug-in type as an example.

The MAF meter consists primarily of a housing, two wires which are called hot wire (sensor portion) and cold wire1, intake air temperature sensor2, and a hybrid integrated circuit (IC). The housing contains a bypass passage allowing intake air to pass through the sensor. Above this passage are the hot and cold wires, which act as resistors. These two resistors are made from platinum wrapped in aluminum, which is then coated with glass. This construction is resistant to deterioration caused by airborne dust colliding and adhering to the resistors. In addition, a circuit is integrated with the hybrid IC to detect the intake air mass and then output intake air mass signals.

1. A standard temperature sensor used to detect the air mass.2. A sensor that measures the intake air temperature for use in engine control.

Hot Wire and Cold Wire ConstructionHot Wire and Cold Wire Construction

Glass Coating Aluminum Bobbin

Connector Hybrid IC Housing Cover

Housing

Cold WireFeederStream

MainStream

Hot Wire(Sensor Portion)

Intake AirTemperatureSensor

Platinum/Iridium WirePlatinum Wire

Terminal

Principles

When a heating element is placed in a flow of air, heat is taken by the air, cooling the element.

Since there is a correlation between the heat quantity taken by the air and the air flow mass, air flow mass can be measured based on changes in the heat quantity. Additionally, a Wheatstone Bridge circuit is used to detect variations in heat quantity as an electrical change.

Wheatstone Bridge circuitIn the bridge circuit to the right, if the ratio of the two resistances “R1/R2” is equal to the ratio of the two resistances “R4/R3”, then the voltage reading for the galvanometer “Z” set at the midpoint between A and B will be zero. As such, no current will flow through the midpoint (galvanometer). Conversely, if the balance is lost due to a slight change in any of the resistance values current will flow through the midpoint. Therefore, the resistance value change can be detected from the current value.

Operation

In the MAF meter, heat absorbed by the hot wire from the air

is converted into a resistance value. The power transistor in the hybrid IC controls the circuit current value such that the hot wire temperature always increases to a fixed temperature (approximately 392°F, 200°C) in relation to the cold wire temperature. As a result, the voltage at point B in the circuit changes. This voltage is then sent to the engine ECU through the output circuit.

Operating CircuitOperating Circuit Output CharacteristicsOutput Characteristics

Current

+B

Hot Wire

Power Transistor

Cold Wire

BA OutputCircuit

EngineECU

Intake Air mass

OutputVoltage

The radiated heat quantity isproportional to the square rootof the air flow mass G.

Quantity of Radiated Heat W

Air Flow mass G

Heating Element Temperature TH

Intake AirTemperatureTK

Here, : ConstantTH : Heating Element TemperatureTK : Intake Air Temperature

W = ∗ G∗ (TH - TK)

A+

BZR3

R4

R1

R2