inside car heater for improved mpg

Don't think so.

Heat from car:

Engine --> heat --> cabin

Heat from 12 volt

Engine -1-> motor-generator -2-> battery -3-> heater -4-> heat -5-> cabin

Significant losses at steps 1,2,3, and enginge is still producing more heat than it needs.

My logic may be flawed, but I would like to see some data before being convinced.

 

Hi lIon and Doc,


lIon might be right. Doc you are right in your efficiency evaluation, but you did not do a comparitive analysis of what the efficiency is with the alternative. The alternative being sitting in traffic with the engine running and 60 to 70 percent of the heat going out of the tail pipe, and 20 percent into the engine bay, and 10 percent into the cabin. With an electric heater one gets a hybrid heating action. The engine runs less, but at a higher output to charge the traction battery. Even with a 20 percent electric heater system efficiency its a 100 percent improvement in heating system efficiency. One should set the Prius to climate control off, run the electric heater to get the cabin to 65 F, then set the Prius climate control to 65 F while thermostatically controlling the electric heater.

Evidence that an electric heater might be better is that there is not as big fuel economy reduction (like 58 to 53 mpg) with the Air Conditoning in traffic as we do with the heater (50 to 42 mpg). Although the amount of heat moved with the compressor is less than the electric energy it uses. And warm temperature results in less rolling resistance and air drag and less traffic jamns (at least around here).

 

Hi Evan,

Electric heaters themselves are very efficient. Electric energy in for heat energy out is nearly 1:1. The only inefficiency is in the interconects, which the worst of them are in the cabin. Where one wants the heat anyway.

 

Hi All,

Another way to help the situation is to insulate the engine like Evan has done, too. Now you get maybe 10 percent loss in the engine bay, and 20 percent into the cabin.

 

Hi All,

A real slick solution would be Thermo-electric devices on the tail pipe run to heaters in the cabin. But the Thermo-electric devices need better effiiciencies than present commercial ones. There has been some lab work recently that has doubled the efficiencies over the commercial ones.

If your wondering why I am really interested in this, it might be the 2 1/2 hour drive home on Thursday night in 20 F degree weather. Not too bad, but I have had similar drives in 5 F degree weather too. Traffic on the main road, 1 mile from the office is stop and go, and the interstate is stop and go too. So, its 23 miles of stop and go for 2 1/2 hours. Needless to say, the engine has to run all the time to stay warm. With a tank average dropping from 47 to 42 mpg (finished the tank at 45 mpg a day later) during that trip. The 47 mpg included 3 sub zero morning starts over a week of driving. So its apparent that the non-hybrid nature of the heating is a more sever problem than even driving in -4 F. As to the mileage on the trip home was on the order of 19.3 mpg as I had 275 miles on the tank when it started. The only bright spot is that the mileage for the last 50 miles on the tank was in the 70's (due to the slow but steady driving at near 40 mph due to conditions, which about 30 F and included bare pavement where the wheels run, but not elsewhere. Which is why everybody elses was doing 40 mph too.

 

Only run the heater when the coolant temperature is above 145°F.

That is by far the most efficient choice, since the engine will remain off while above that threshold... even when set at MAX HOT with the fan blowing hard. It's heat that would otherwise be wasted.
.

 

Hi john... and lIon,

I left out that the 2 1/2 hour drive home was in the middle of a snow storm during rush hour(sss). So it was neccassary to bump up the climate control temp to 70 F and even so run the defrosters at time to be able to see, out of the car. The wipers were going in highest intermitant setting all the time. The rear wiper really needs an indepdant intermitancy setting. I was very important to be able to see out of the car considering all the other cars moving all the time in relation to my car. There was 3 inches of snow on the hood of the car when I got home, even though I cleared it all off before leaving work.

Yes, I have 100 % top grill blocking, and 75 % lower (all but the lower slit) blocked. With the snow, the grill was 100 % blocked by the time I got home.

 

Hi docwillie,,

To clarify,

Stock Prius - sitting in traffic with 65 F set on climate control - Engine runs to heat car, no motive or battery charging occurs most of this time. Engine runs to make heat. Efficiency about 10 % (best guestimate) fuel to heat.

Prius with auxiliary cabin electric heater - sitting in traffic with climate control off, and aux electric heater thermostat set to 65 F - engine off most of the time, but when its on for heating purpose, its running to charge the main traction battery. 12 V converter efficiency about 90 %, heater efficency problaby better than 95 %, efficiency around 25 % fuel to electric energy - total 23 %.

The best scenario would be an auxiliary furnace that burned gasoline and got 75 % cabin heating efficiency.

Another alternative is a exhaust heat recovery for cabin heat. Say we get a system that is just 30 % efficiency. Then the fuel to cabin heat efficiency is 10 % plus 70 % (gas to exhaust heat) times 30 %, or 31 % overall efficient.