# Massaging Seat Cushion

#1

A caller in this weekend’s show asked if her fianc??s use of a massaging seat cushion was hurting the fuel mileage of the vehicle and they quickly dismissed her question as ridiculous. The caller made mention that the extra electrical load was probably increasing the load on the alternator and that it in turn was loading the engine.

I want to say the caller is exactly right. She was dead on with the mechanism, but she may have overestimated the scale a bit. Increasing electrical load does increase the load on the alternator, which increases the load on the engine, and requires more fuel to keep it moving. However, Tom and Ray were correct in the significance of the effect. I took some time to calculate it, making a lot of assumptions and this is what I came up with.

Assume the seat cushion uses 12W.

Assume a 70% efficient alternator.

This increases the power demand on the engine by 17.14W.

Assume 26% thermal efficiency in the engine.

This increases the thermal requirement on the fuel by 65.93W.

In Joules per hour it?s 2.3710^5.

A gallon of fuel provides 1.27
10^8 Joules.

The extra energy required by the seat cushion burns an extra gallon of fuel every 536 hours.

That?s probably not reducing the fuel mileage by 6 miles per gallon, but it does reduce fuel mileage.

#2

AAAaaa!! Piece of junk was supposed to post in the 2nd opinion section.