Hey all,

This might seem like an odd question, but I have been driving a lot lately with the HVAC completely off to try to maximize fuel economy (and got my best tank to date).

It got too hot to go without AC recently, so I started using it again, and I started thinking about the fact that the AC compressor is driven by an electric motor, at the HV battery voltage.

So if this is the case, it makes sense that the depletion of the HV battery will reduce overall fuel economy, but should running the AC compressor show a reduction in the instantaneous MPG meter? Does the ICE run at a higher torque or power level when the AC compressor is drawing current off the battery? Or does the AC run off the generator output sometimes too? This scenario seem like it might show a drop in the instantaneous MPG reading.

Just thought I would pose this question for the gurus out there...

Thanks,
Jeff

 

It would seem that only when the battery is being recharged by the ICE that the AC electricity use would be noticed. MPG only shows gasoline usage, not electric. Eventually the usage will be "charged" against gas usage unless you always go downhill and thus keep the battery charged.

 

Think of it this way: you stop your car in traffic and battery is full so the engine shuts down. If AC running, your battery will be depleted in 5 min and engine will start to recharge it, if AC not running your battery will last 10 min (numbers are not real, but close enough) until engine kicks in. Same when driving, using AC will draw more electricity from drive train for charging, also you'll get less boost from electric during hard acceleration. So yes, MPG gauge will reflect higher usage of gas when using AC. Some simple ways to save AC energy use:
use ECO button, try to park in a shade so car doesn't get too hot, leave the window tiny bit open and you can lift the roof up if you have one (watch for rain in forecast), close other vents so most of cooling goes for the driver if alone in a car, same for back seats, use recirculation if really hot outside and outside air if car get's hot mostly from Sun etc. Also when close to your destination it is good to shut down AC and let the air circulate without compressor running. Main reason to do so is to dry out and warm up all the ducts, so the moisture doesn't build up inside and cause mildew, bad smell etc. etc. could save you much more in cleaning and repair costs than gas savings alone.