Reduced winter MPG

Thanks.

For regen you basically just need to be either not pressing the accelerator pedal at all (small amount of regen to simulate engine braking), or you have to be actually pressing the brake. The gentler the braking the higher your MPG will become because you're travelling a further distance in that minute. So you're likely braking quite hard.

For a low MPG, the ICE needs to be running (using fuel) but not travelling very far in that time (high fuel, short distance). This would typically happen when accelerating hard.

So maybe in that minute you were accelerating hard for part of it (low MPG), and braking hard (high regen).

You can get a bar like that by flooring the accelerator until you reach 70 (or whatever), then braking at max regen until you stop.

 

That driving scenario sounds like the exact opposite of what a hypermiler would want to see on that gauge and exactly what happens sometimes on the hwy / freeway when there's a bottleneck ahead and traffic flow gets all wonky. Excellent, I may even give it a try once of twice (without any traffic around) to see if I can recreate it on my gauge.

 

I have seen that pattern a couple of times and I was probably in the DC area so hard acceleration and deceleration is the norm.

And thanks for the explaination!

I bought my Prius in mid Feb so it is still new to me at 5600 miles and I am tivht with money. So I have been watching it.

Or it could have been coming to a red light and sitting there for the majority of that minute. For long lights I have seen zero mpg.

 

The cars computers will do there best to adjust to the driving patterns you have. If the gas engine shuts off at traffic lights, that's a good sign, but it doesn't always happen that way. I've driven I-495 395 and 95 and the George Washington from 495 to Airlington in DC so I kinda understand what your dealing with, traffic wise.
Not to many ways around having to drive really inefficient patterns for the prius and there are not many ways that I know of to get better mileage while in heavy traffic with lots of fast starts and slowdowns or stops. But the prius will still do it's best to get you as much mileage as it can.

 

With summer time approaching I was sindering if these cars take a hit when running the ac. I have used the ac on several trips but haven't noticed any mpg drop.

 

AC does eat gas especially around town. On the highway in eco mode the conventional wisdom is windows up aero balances out ac use. Eco mode does manipulate the ac power consumption for economy which translates to slightly reduced btus.

The ac is sized for extreme loads like Texas and Florida where high temps are combined with high humidity much of the time. Removing the latent component takes power without reducing temperature by itself. Finally the battery life benefits from lower cabin temperatures so ac has a long term benefit.

 

The size of the AC hit isn't fixed, but is quite variable, from negligible to very noticeable, depending on just how hot and humid conditions are.

Note also that the observed hit also depends on average trip speed. The human need for AC depends on time spent in the car, not distance traveled, but MPG is computed on distance, not time. So around town with low speeds, lots of stops, and shorter distances, the hit expressed in MPG appears worse, even though the actual fuel cost is the same as on a longer highway trip taking the same amount to time.

Note also that the apparent hit is magnified by the math of the U.S. MPG scale, making identical AC fuel use look a lot worse on high-MPG cars than on gas guzzlers. When comparing different vehicles, it is much more realistic to express fuel economy on a gallons-per-100 miles scale, equivalent to the liters-per-km scale that much of the rest of the world uses. On that scale, the Prius AC hit doesn't look so big.

Humans need AC in many climates and weather conditions, so don't be shy about using it when needed. And as mentioned previously, the big battery may be happier too.

 

The AC in Prius is all electric. No effect on ICE MPG.

 

When the electricity for the AC comes from something like a roof solar panel that could be true. Until then…

 

That simply isn't true. While the AC is all-electric, that electric energy comes from the ICE, which requires gasoline.

 

Yeah I think people always forget that all energy has to initially come from somewhere, and the only source of that in a Prius is the fuel that the ICE consumes, which is then converted into other forms of energy, be it heat, electricity, or kinetic, and all of it, in one way or another, always ends up eventually just being heat.

That being said... I thin the spirit of what was said was correct. My AC was broken (and I didn't realise it) for a long time. I just can't spot any noticeable difference in MPG with it on or off. Other things play a WAY bigger role in fuel use.

 

AC takes far less energy than heat. In general, AC use (unless you live somewhere in crazy hot temps) should have minimal effects on your mpg.

 

I go along with that too, I do not notice any change in consumption whether its on or off. I leave mine on Auto 21oC year round, the only time I turn off is if I fancy having the windows open, then its just wasteful.

 

For those of us in temperate northern climates such as WA, MN, and UK, yes, the AC can be minimal. But drivers in more hot humid southern climates, where the AC must be run stronger and longer, have reported quite noticeable effects.

And while we northerners tend to get our best MPG in or near summer, southerners seem to tend towards getting their best MPG earlier in spring and later in autumn, shoulder seasons where they don't need any or as much AC as the core summer.

 

Not quite, it depends on the delta t and humidity you are facing. If you are trying to heat from 55f to 75f, the number of base btus needed are the same as dry climate cooling from 95f to 75f.

In humid areas, extra btus are used to handle latent heat, eg the heat required to condense water vapor to liquid. So a humid cooling 20f delta t takes far more energy than a heating 20f.

So down south where the average dewpoints are high from spring to fall, ac are used eight or nine months a year, some months primarily to reduce humidity. Obviously if you are below freezing for weeks at a time, like in Maine, heating delta t will be 40 to 60f.

 

Both you and Fuzzy bring up good points.
I am not saying AC takes no energy.
I am saying that cooling takes less energy barring extreme conditions.

In winter it is not unusual to have a 40 degree delta T. In northern climates, a 70 degree delta T is not unusual.
I would be willing to bet the average delta T for cooling, in the USA is less than the average delta T for heat.

The original poster is in PA. I don’t see cooling taking nearly as much energy as heating.

 

Hang on - this isn't true in the context of a ICE powered car because you're simply diverting excess heat from the ICE into the cabin, instead of venting it directly outside. So this costs nothing (other than the fan). Sure, if you're using things like the heated seats, heated rear window / mirrors, that's different, but the heat coming out the air vents is generally free.

Sure, in some vehicles (e.g. purely electric vehicles) then there is a cost for heating, but not in most ICE fitted vehicles.

 

Excellent point! I am so used to thinking about the hit EVs take for heating.
Rather than ‘free’ I consider ICE cars always loosing efficiency. It is just that in the winter, some of that heat is useful.

So, for the OP, the mpg loss in the winter is due to increased air resistance (colder air is tougher to move through), the fact that driving on snow/ice takes more energy, and if you use winter tires, they are most likely heavier.

 

Do note that hypermilers have found that winter cabin heat in ICE vehicles can reduce MPG, but like AC, it varies with conditions. And the situation can be improved.

Unlike AC, it is not a theoretical requirement, as the ICE system sheds plenty of heat. But in practice, ICE vehicles are built to mostly shed it, and aren't highly efficient at scavenging it for the cabin. So it is possible, sometimes common, for the cabin heat system to draw off more engine heat than it really has surplus for that purpose, so it must burn a bit more fuel to keep the block up to desired temperature. If we could see the cooling system thermostat status, to know when it is open to send surplus heat to the radiator vs closed because it doesn't have surplus, this would be more clear. Reducing wind chill on the engine block, typically by blocking the radiator, will help make surplus heat available in more conditions.

Hybrid cars, which have more efficient engines / less waste heat, and may run in ICE-Off mode for a while, tend to run into this no-surplus-heat situation a lot more than non-hybrids, so see more MPG loss from using cabin heat.

 

When our 2010’s partially warmed, turning the cabin temp down, or sometimes right off, will often shut the engine off. Once fully warmed it doesn’t matter.