City MPG Casual Driving

Kaps said:

Was reading an article on alternator impact on mpg - if you completely get rid of it and the weight gain + less engine draig helps you get 7-11% more in mpg. Ofcourse it was extreme like the guys who manage 60 mpg on civics etc.- I think there are easier mpg gains to be had by replacing wheels with lighter ones and efficient hubcaps or lowering it a bit.

Other day saw a super hyper miler during office hour rush traffic on an internal road - was doing 32 in a 45 zone .. people were pissed.

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banjos-n-beer said:

Out of curiosity, how did you know they were a hyper miler and not just your average idiot? Anyone doing ~10 under the speed limit around here is either on the phone, or incredibly elderly.

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Unobtanium said:

My average speed is 32-34mph, and average mpg is 22.5-23.5 typically for any given tank. Car works great, all these "lemon theories" crack me up. I just live in an area with looonnnng inclines, and drive 75ish quite often. MPG at 95mph is a hair under 15, at 85mph, it's around 16-17, also as datapoints, tested over a 25 mile span each. This car just isn't meant to be driven like a tourer. It's a citycar.

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To figure that out, we need to start by estimating how much electrical energy is required to run the engine and accessories. Fortunately for me, Randy has already done the work. He measured the electrical power required to run his '94 Civic EX:


Item...................@ 14.4v
Idling ............... 128w
Rev up to 3500 rpm: .. +35w
Heater fan setting 1: . 55w
.. Setting 2: ......... 98w
.. Setting 3: ........ 141w
.. Setting 4: ........ 189w
Marker lights: ........ 95w
Full lights: ......... 240w
A/C (heater fan
subtracted, but
incl. condenser fan):. 163w
Defroster: ........... 104w
Wipers 1: ............. 48w
Wipers 2: ............. 76w
Radiator fan: ........ 144w

With that info in hand, a back of the envelope calculation begins: around 163 watts for running the engine, ECU & fuel pump; marker lights @ 95 watts; cruise control at 30 watts (just a guess). For a total of about 288 watts.

Estimating fuel needed to run the alternator ...

The alternator isn't 100% efficient. In other words, the engine has to contribute more than 288 watts to generate 288 watts. According to Wikipedia, a modern alternator is between 50-62% efficient.

At 60% efficiency, it would take 480 watts of work from the engine to generate 288 watts of electricity.

Back of the envelope calculations
Back of the envelope calculations: 530 watts (0.64 hp) of energy from gasoline to run the alternator under light load.
But wait, there's more!

Let's not forget losses to belt friction and aerodynamic drag at the alternator pulley fan (which, being smaller than the crank pulley, spins much faster than engine RPM). I'm not sure if pulley drag is figured in the Wikipedia efficiency calculation or not, but since this is a back-of-the-envelope exercise, we'll say it isn't. Let's say belt & aero losses consume an additional 50 watts.

So the back of the envelope tells us that my car requires around 530 watts, or nearly two thirds of a horsepower (745.7 watts = 1 hp) to run its alternator (for the load specified).

I've calculated elsewhere that the Firefly has to generate 5.4 hp to maintain 70 km/h in ideal circumstances. So reducing that power requirement by 0.64 hp should reduce fuel consumption by a similar amount: which is 11.9% (at that speed).

Worth trying? Sure!

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