energetics of autos

[Norman Albers]

When temperatures outside are frigid we really appreciate the heat available from our internal combustion engines. It takes notably longer to heat the engine to where the thermostat opens excess to the radiator (not the heater core). How much more relatively efficient are we now? Figure output of horsepower, take a low cruising output of maybe 40 horsepower. That is like 30 kilowatts. I figure waste heat is of a similar magnitude, and that is a lot of heat, so it seems still wasteful. Is this correct, and, how do electric or hybrids deal with the notable loads of heating and AC cooling in summer? These will be proportionally higher loads to a smaller power plant.

[K@meleon]

Hi there,

 

Dunno if understood the questions correctly, but I heard that nowadays diesel engines are getting towards the 60% max efficiency a combustion engine can reach. The rest is lost in heat and friction. So yes, theoritically, it's a lot of heat..

I also know that an AC in the summer will "cost" you approximately 5 to 10 hp. So an electric car will just have to provide the extra energy from its batteries/power source. A hybrid is able to use its engine for this goal I guess. At least for the heat of the heating. And maybe plug the compressor of the AC in mixed mode. On the dynamo of the engine with a switch if the car is in "battery" mode. Dunno exactly. But it ougth to be a simple solution.

[Norman Albers]

Thanks, k@meleon, I am trying to get a grip on the magnitudes to see how much more efficient my Subaru 1.8liter wagon is when I pull heat out rapidly. The opposite question pertains to AC. Both have to be dealt with in smaller machines. Insulation and sunblock become more important. Simple, yes, but a notable change to any high mileage rating I suspect. A lighter vehicle will cruise at what, twenty HP, I'm stabbing here.

[K@meleon]

Hum.. I'm afraid your subaru (or any conventional car) won't get more efficient if you pull heat rapidly.. The heat you use for your heater comes from the coolant of your engine, mixed through an air exchanger with cool air from outside. The warm coolant just continues after the heat exchanger back to the radiator. So in fact, by pulling the heat out of the coolant going to the radiator, you don't really change much in the process, if you see what I mean. It just allows you to take a bit more heat out than with just the radiator, which only plays a role when the radiator is saturated. And this is hardly the case with normal engines and normal oustide temperatures. The engine coolant itself stays at around 80°C. (Engines do like to run a bit hot)

 

But the trick I just described is sometimes used in rallying (WRC) where the engines are pushed really hard, and the radiators can get a hard time dumping all the heat away by outside temp of 40-50°C. So they sometimes have to put the heating on full blast.. ..with resulting inside car temperatures of 60-70°C.. Poor fellows.

 

For the horse power needed for stable cruise, I'm not really sure anymore. I think you can start with:

[math]

D=\frac{1}{2} \rho V^2 C_d S

[/math]

And then, if I'm not mistaken:

[math]

P = D\cdot V

[/math]

This neglecting friction drag on the road. As you can see, the power needed is proportional to the velocity cubed.. If I'm very wrong, can somebody correct me?

If I'm right, well, you can look up the drag coefficient of your car, and do the maths.

[Norman Albers]

I think you're lost in process consideration. Heat which would have been thrown away is needed in the cold. If you did not exchange it you would have to produce it otherwise. . . . . .Yup, I once nursed an old truck with inadequate compression out of Death Valley this way. Warm ride.

[K@meleon]

I think you're lost in process consideration. Heat which would have been thrown away is needed in the cold. If you did not exchange it you would have to produce it otherwise. . . . . .

 

How do you mean?

[Norman Albers]

I don't even have to take in outside air; I can run the AC on RECIRC at high temp., and it does a very short duty cycle on the AC compressor, but uses full heater core heat. So when it is freezing as it has been here for a month, the engine takes notably longer to reach temperature if I am sucking heat from the heater core. Some fraction of what would have been waste energy on a warmer day is now needed. On the other side of the discussion, hybrids must have some small radiator, no? Would an electric motor doing most or all the automotive work have waste heat that could be captured?