Why does warming up use so much gas?

I believe the ICE coolant operating temp is 185F, so even at 70F the car needs to warm up the engine.

 

This is my theory:

While owners buy a Prius to save gas, Toyota designed the Prius to minimize pollution.

One way to emit fewer hydrocarbons is to consume fewer hydrocarbons, so once the car is fully warmed up, (stage 4 in PriusChat lingo) it focuses on improving gas mileage. Before that, however, it focuses on other aspects of reduced emissions, such as warming the catalytic converter. During this warm up time (Phase 0 to 2) the Prius does not get phenomenal gas mileage as the goal is to emit the smallest amount of pollution.

 

The Prius doesn't -need- to warm up. You can drive away right after putting it in "ready". It gets worse mileage because it needs to enrich the mixture to run properly, like any internal combustion engine. Now, we Prius owners are often "shocked" to see how "poor" the mileage is in the first 10 min or so. That is because the mileage -after- that is so much better.

Further, the car needs to warm up the cat., and if you have the A/C on demanding heat, the car won't let the engine shut down when it's "not needed for power". So that "wastes" fuel.

If you think this increased fuel consumption is different than "normal" non-hybrid cars, you're dead wrong.
My FJ Cruiser has a Scangauge II installed, just like Pearl S. It shows clearly that the FJ guzzles fuel until it warms up as well, it just never allows the engine to shut down when not needed. Well, the FJ guzzles fuel all the time, just more so when cold.

 

All cars with internal cumbustion engines have substantially poorer mileage when warming up. We just don't notice since there isn't instantaneous fuel economy displayed. I get maybe 30mpg for the first couple miles on a 32 degree F morning. I'll bet very few ICE cars on the road would do that well.

 

The catalytic converter must reach a high temperature before it functions properly and as Dave mentioned, Toyota primary effort was to reduce emissions.

 

BINGO, we have a winner!


Also, engine efficiency is related to temperature, too. Fortunately, once the engine is warmed up, the Prius has an insulated coolant tank, so it can KEEP the engine warmed longer. Overall, it's much easier to "eat" this cost up-front, by running the ICE until it's warmed up.

 

There is a 45-55 second period when the gas engine first runs that the control laws attempt to keep the engine at idle while using traction battery power to move the car. With instrumentation and parking close to a street where modest speeds, 35-40 mph (55-65 km/h) can be maintained for 1-2 miles (1.5-3.0 km), one can accelerate and then shift into "N" to coast at traffic speed. This will give very good starting MPG and even if it has to run in "D" (shifting back and forth between "D" and "N" can be done at just about any speed) the car will show excellent fuel efficiency in this initial 45-55 seconds.

Once the catalytic converters begin operating, they provide the feedback mechanism needed to run an optimum fuel-air ratio. After about 1-2 miles (1.5-3.0 km), the engine coolant will be warm enough that normal operation in "D" works just fine and any traction battery charge is replaced. However, this initial operation is not easy without practice and using instrumentation showing engine rpm.

In a perfect world, we always park at the top of a hill and use the downgrade when warming the engine. But this is not always possible. If on a flat, one can shift between "D" and "N" to achieve reasonable, warm-up efficiency. However, Murphy's Law says most exits from a parking lot will be up-hill (and into the wind, with blowing snow, and bare-foot . . . opps, another old man story about going to school.)

I tend to use EV to reach the street from the parking lot. Once I turn on to the cross-street the 45-55 seconds begins. I'll accelerate so as to keep the engine rpm at a low value, drawing on traction battery power, until I reach a desired cruise/coasting velocity. Then I shift into "N" until the speed drops too much and back to "D" long enough to reach the targeted speed again. After the catalytic converters 'light off', I tend to just drop into "D" and not worry about it.

Bob Wilson

 

how much fuel per second does it use during warm up?

 

I only have data for the 1.5L Prius:

Thanks to Ken@Japan, we have this data for fuel consumption of a Japanese NHW20.

I don't have data for our 1.8L Prius, yet, but I would expect it to have NHW20 warm-up characteristics which are consistent with warm-up of just the catalytic converter. The less than one minute to engine-off is what it takes to heat the catalytic converter. Once the O{2} sensors of the catalytic converter are working, the engine controller self-tunes the fuel-air mixture for an optimum burn rate (i.e., stoichiometric ratio.)

This is data from my NHW11:


In contrast, the NHW11 warms up the engine to 70C before it can shutdown. So this warm-up incorporates both the catalytic converter warm-up followed by heating the engine block. You can see the rate of fuel consumption is much steeper in the first minute and then follows a steady fuel consumption until the coolant reaches 70C.

Measuring the 1.8L catalytic warm-up is easy but measuring the fuel consumption vs engine coolant will take some thought. Does anyone know a way to get a parked, ZVW30 to keep the engine on from a cold start?

Bob Wilson

 

In terms of engine operating temperature, 20C is stone cold. 'Warm' is about 90C.

A Prius can, and should, drive off just as soon as a regular ICE car.

My non-hybrid equipped with a ScanGauge starts up by guzzling 0.8 - 1.2 gallons/hour (GPH) during idle, depending on outside temperature, then gradually falls to 0.3 GPH when fully warmed. I haven't watched the Prius GPH very close, but it seems to start a bit lower, falls faster to 0.2 GPH, then auto-stops. The old guzzler never auto-stops.

 

It is a peculiar inherent characteristic of the prius gen2 or gen3, etc.
I tried to time and measure stage 1a to 2 before on my gen2. On P mode, from 22-50 degC, it took 1" 38sec for the engine to shut down. It used .06 liters which equates to A$ .09 for that warm-up time (stage 1a to 2). It took the HV battery from 50.5% to 53.5% state-of-charge.

Others would suggest to remain in Park and just let the car do the warm-up stage and get all the charge you can. This technique would be best if you're starting at the bottom of a hill. If you start on top of a hill or if relatively flat with not much traffic during the first 2 mins, I suggest crawling out of the driveway without touching the accelerator as the car can still move forward and help cover some ground which helps in the initial MPG/FE calculation while still getting enough juice into the HV battery @ around -1.5 to -2 amps. If you still have lots of juice, you can feather the accelerator just a bit and creep a little faster to just use a bit of the HV battery while the ICE warms up. Soon you'll realize your at the end of stage 2 and you've already covered some ground and watch as your initial FE has improved from your previous observations.

Enter stage 3, but that's another topic altogether.

hope this helps mate

 

I was doing some 40 mph commuting into a cold, freezing head-wind the other night and it looked like the coolant was staying at 70-72 C. I strongly suspect 70 C is the cut-off but I could not find in my older version of AutoEnginuity a metric for the exhaust heat recovery valve. There may be one but I don't know what it is.

Bob Wilson

 

That was not mine but I've found with our ScanGauge the ZVW30 fan has this range:

• ~96C - radiator fan comes on
• ~92C - radiator fan goes off
• ~88C - where the coolant temperature falls to before climbing again

It may be possible to roll down the windows, set the heater to max, and let the cabin dump the extra heat that escapes via the windows. But it isn't clear if this strategy is more or less efficient than letting the radiator fans cycle. I can testify it is mighty uncomfortable.

Bob Wilson

 

i can't put any numbers to it, but the pip just dropped the first bar on the gas gauge which is about 1 gallon. i've travelled 60 miles in hv vs. 140 ev. so it has mostly been warming up or providing heat. seems pretty efficient!

 

Like it has been stated, the Prius isn't the only vehicle that suffers a warmup fuel economy penalty. Most vehicles will including electric cars!

In the standard Prius you are also heating up the various fluids and drivetrain components. This require energy. You are heating up the tires as you drive. This requires energy.