Does anyone know how spark ignition ICE efficiency would be expected to vary with ambient temperature?

I have a Toyota Prius and notice the MPG suffers  ~10% drop in cold weather even after warm-up. This could be due to many factors, but just wondered if the above could be one of them.

[My own guess, not being an expert, was that colder air having higher density would need more heat input to achieve the same pressure rise. Also, since the same volume of colder air contains more oxygen than warmer air, perhaps higher pumping loss in the throttle is inevitable at low output powers]


Geoff

The main efficiency loss in colder weather occurs during the time the engine is warming up to its normal operating temperature, after which, in a well-designed system, any loss should be minimal.

As a tangent or partial answer or my observation goes along this line.  The air meter compensates for temperature and the computer adds more fuel FOR THE SAME air flow when the temperature is low so the engine should see the same O2 in the exhaust (in percent) as when it is hot out, all this after the engine is up to temperature.  So colder air, means more O2 and more fuel and more energy (HP). So at any throttle postion you'll have more HP and more acceleration.  As a male, I like the more HP and I use it, so guess what, I use more gas when ikts cold outside.

To compund the problem with the Hybred, the batteries do not work as efficient and you do not have the full potential for work and your engine will make up for it.  I'll stop short on the physics of hybred's and effiency because the facts get in the way.  I drive a 1.8 liter 2ggze Toyota engine and it gets better overall milage than their Hybred.

Many thanks for the information. I looks then like effect I see must be more down to other factors, like the battery temperature as mentioned (though this too should not be a factor after warm-up, one would think), and perhpas other factors like tyre temperature, and increase drag due to denser air.

Contrary to the analysis provided, I must say that subjectively, I always feel like I have more performance during the summer months compared to winter. This could be either

1) In summer I have less HP but this more than outweighed by even less drag

or

2) It's sunny outside, so everything seems better anyway.  

Thanks,

Geoff

Hello everybody:

harveygp,
Take a look to thread108-138154 and thread71-72284.
There you can find answers to your questions from another angles.