Obviously A/C alters the fuel economy of vehicles, but does using heat at all?

I've never heard anything about heat having any effect on fuel economy, but anyways I'm just curious.

 

If it does it's very minor. You have to run a fan which uses some power. In a conventional car you use the heat the engine is already generated and being removed via the cooling system. A hybrid would be the same. I'd assume that even if the ICE is off for a while it would not cool down significantly and have no effect on the heat produced.

A/C needs to power a compressor which does suck down some power.

 

For heating your just running the warm water through a smaller radiator (heater core). The milaage drop is minor as you are only running a small fan to push the air through your heater core. Except however, when you use your defroster and that runs you A/C compressor. The A/C compressor, except in the late model Prius/Highlander, runs off a belt and this saps power and fuel mileage but even withthe electric driven compressor it takes so much to run it your ICE has to run more often to keep the battery charged.

 

Using heat does accelerate the cooling of the ICE when you are running on electric. The ICE will turn on more often to keep at temperature and slightly reduce your MPG.

JeffD

 

This effect can be mitigated by installing a radiator block in the air grill . . . in 50F or cooler weather. This not only minimizes radiator heat losses but it also reduces the cold-air drag at high speeds.

Bob Wilson

 

A radiator block is straight forward and does work as advertised, but also add a facility to track coolant (two loops, ICE and Tranny) temperature and alarm on overheating. Too much heat can be dangerous to you pocketbook.

JeffD

 

Here's a tricky question about effect of heat, specific to the Prius:

The Prius ICE runs in very rich mode for the forst two minutes after a cold start to warmp up rapidly. AFAIKleaving this rich mode is in part dependent on mesured enine temperature. FE is usually abysmal during this time, which is why many short trips in the Prius tank the mileage. A block heater would help.

The Prius also uses an electric heating element to help heat the cabin, if the ICE is not yet warmed up sufficiently. Thus, if you use heat during the first 2 minutes, you will be using the electric heater element, which should suck up energy via the electric generator/motor.

However, this same process (electric heating) should increase the load for the ICE during startup, and may shorten the warmup period of rich running at low FE, saving fuel. The question is: how does this balance out.
Any thoughts?

 

I doubt the elecrtic heating would improve efficiency compared to not running heat (though it would be better than using engine coolant before it's warmed up). I suspect the main idea there is not to have fluctuation in cabin temp if the engine is off. Though the load would decrease the warmup time, that elecrticity will need to be paid pack with engine power eventually, and will be compounded with the inefficiency of producing that electricity.

From what I understand the relevant system is the catalytic converter -- if that runs too cold, it is unable to breakdown NOx, CO, and other harmful gases. As far as I know, this is the same reason why the Insight with Lean Burn has a poorer emissions rating than the CVT version. Running the engine lean at partial load, rather than with a partial vacuum, will increase the thermal efficiency of the engine by converting more heat into power and throwing less out the tailpipe, but this has the tradeoff of worse emissions. This is the major drawback of proposed heat-capture mechanisms that could potentially offer greater mileage.*

* The BMW system though is pretty clever, and seems to be an exception, it uses the catalytic converter as a heat exchanger once it reaches operating temp, to power a steam motor.

 

Without pre-heating, my approach is to keep the ICE power load as low as possible. I do this by driving through the neighborhood at speeds of 20-25 mph and traffic permitting, with the ICE in idle and the vehicle coasting in neutral. The instantanious MPG often doubles during these coasts. The distance varies but generally, it ranges from 1-2 miles. Still, there is no free lunch and the first 5 minutes are pretty poor.

I have yet to find an easily detectable indicator of the transition from poor to good MPG. The 'blue' engine cold light goes out too early but is better than nothing. Experiments using the access road on my morning commute indicates warm-up continues for several minutes. But my approach is to add a block heater and transaxle oil pan heater ("Don't curse the darkness, light a candle.")

Bob Wilson