Friday, June 4, 2010

Mass Air Flow meter - Critical equipment in older hybrid vehicles

The ideal fuel mixture entering the combustion chamber is controlled by the onboard computer (ECU).  Depending on the power requirement, the amount of fuel injected is regulated taking into consideration

The number of air molecules entering the chamber ( Its Mass)
The temperature
The Presssure
The Burn ( Oxygen molecules left in the exhaust)

The mass air flow meter provides information on the first 3 items to the ECU.

The Prius uses the hot wire design type air flow meter to determine the air mass flow.  This equipment senses the amount of air flow by measuring the amount of current needed to maintain a constant temperature through very thin platinum wires. 

When air flows pass the wires the wire cools decreasing it's resistance and allows more current to flow through the wires. As more current flow through the wires the wire heats up increasing its resistance which in turn reduces the amount of current flowing through the wires. By measuring the current, the amount of air flowing through the engine is determined.

The  two  platinum wire sensors are set in parallel in the Mass air flow meter. This is important to measure laminar or streamline layered air flows.  If these wires are no longer parallel the laminar flow is disrupted which leads to inaccurate measurements, incorrect fuel mixture, poor engine performance.


Notice in this picture that the bottom wire is bent towards the right side. Air flow is no longer laminar, This MAF should be replaced.

 The paradox is that this MAF still works. It may have poor performance in accleration but is not noticeable in street driving. Fuel consumption is still good because working in conunction with the oxygen sensor the fuel mixture is regulated. 

In a normal petrol vehicle one could drive the car happily, putting up with the drop in performance. However in a hybrid vehicle any drop in performance in the ICE is compensated for in the Electric engine. This added demand and completely warp charging regime to the HEV battery quickly cause the battery to fail. 
In a hybrid vehicle, especially one where the HEV battery ia already 10 years old, it is imperative that you keep the air flow meter in perfect condition. It is a lot cheaper than reconditioning or replacing the battery.

 It is part of our warranty conditions that you replace the air flow meter after battery reconditioning or the one year warranty will not apply.

Another problem is that dirt and oil can contaminate the hot wires deteriorating it's accuracy. As part of normal service the air flow meter should be cleaned with a commercial Mass Air Flow Meter cleaner product, and the air filter changed.


chicago toyota prius said...

Wonderful post about internal subjects of a four wheeler. I must follow your suggestions on my car. Others may get benefit from this post. Keep on giving more useful information. Thanks...

Flow Meter said...

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