Heating the Car

Of course, 8 Amps at 12V is 96 Watts; I question how effectively that can actually heat the car. From what I've Googled, electric car resistance heaters (presumably including the Clarity) are in the range of 4000-8000 Watts. Running the car's climate control such that it heats for 1/40th of your drive would be equivalent. Now, if the low wattage heater provides enough comfort that the driver doesn't feel the need to raise the car's ambient temp to, say, 65-70 degrees, then it is indeed a major savings. Similarly, seat heaters seem to be around 50 watts max, so if they reduce the desire to heat the entire car it's a win.

Of course, the "automatic" climate control systems that focus on ambient temperature make this harder to manage, but that's for another thread...

 

I had the same question, until I realized it is a diesel fired heater. The 96 watts is only the air blower: The heater is 5 KW.

 

If you're running defrost, you're running the AC for dehumidification (as well as using the engine heat). I doubt, however, that any of this has to do with why the EV range drops even with hold--the system seems to habitually slightly underestimate the load, even without climate control, and requires occassional HV Charge mode to get back to the initial range, at least in my experience.

As to the waste heat, it's a very efficient engine, but still only reaches ~40% efficiency. The climate control logic (per the AC system description doc) takes into account the engine coolant temp, which implies to me that it uses the engine heat when it's available. Given the on/off cycle of a modern hybrid drive cycle, this may be significantly less time than a normal ICE car, however.

 

Ah, I missed that too, that makes more sense. Yeah, I think I'll pass on adding another fuel-filled box to my car

 

I am not convinced the car uses engine heat as much as I would like it to. You mention the on/off cycle of the hybrid drive: This would make cabin temperature control difficult if the engineers used engine heat when it was available, then had to kick on the resistance heater when the engine shut off. It would be difficult to make this transition without affecting passenger comfort.

I believe the Honda engineers have chosen an easier method, and only use engine heat when it is going to be consistently available. I believe your assumption / implication is wrong (that it uses the engine heat when it's available). This means they use resistance heat for the cabin, even when the engine is running: This adds load to the main generator, that is powered by the output of the gas engine (not 'waste heat').

The only time that waste heat will be consistently available is when the gas engine is 'locked on'. It is interesting that the gas engine is 'locked on' at temperatures below 15F to assure the battery discharge is not excessive. I had also heard the climate control only uses waste engine heat at temperatures below 15F: These two temperatures are an interesting coincidence.

 

I would agree if they were heating air with the resistance heater. Since the heat [for the PHEV] uses the coolant loop heated with resistance heating, temp control is much easier since coolant has a much higher specific heat capacity than air. Anyway, I believe your assumption is wrong, but I doubt any of us will be able to prove it one way or the other unless someone puts a meter on the heater element and records its use in normal operation.

 

It seems it would be easy to use engine heat by opening a valve as soon as the water reaches a temperature that can warm the cabin. After the engine is providing heat, the Clarity can turn off the resistance heater. Then, when the engine shuts off and the water temperature falls below the cabin-warming temperature, the Clarity can close the valve and turn on the resistance heater again. Thermostat technology is well-proven at this point.

 

This can be more complicated than you may realize. I will only go into the problem when the engine shuts off.

A step input to a control system causes the most disruption.

The engine is providing heat, and shuts down, causing a step loss of heat input to the system. The resistance heating element is 'cold'. Activating the thermostat for the resistance heater will allow the heating loop temperature to drop until the temperature is low enough to turn on the heater. The element will begin to warm, and the loop temperature will continue to drop, until the element gets hotter than the loop. The loop temperature has dropped more than 'normal', so the heater will go to higher output than normal, and will overshoot the loop temperature desired. Passengers will feel this as inconsistent heat in the cabin.

Every time the gas engine starts or stops would cause a step change in the heat supply, if they were using the maximum waste heat available.

Fixing this issue is called tuning, and control engineers can diddle for days on this stuff... It does not appear that Honda has put that much effort in the control systems on this car. IMHO, Honda has ignored some energy-saving programming to achieve a smoother feel in operation.

Absolute proof would be an amp meter on the heater. I have seen indirect proof in the rpm response when I turn on and off the heater: It is not immediate, but the same hill is much harder with the heat turned on. (Gas engine running the whole time).

 

You don’t want to end up in a situation where your electric heating element is heating coolant that is returned to the engine. This would be a giant waste of energy.
I wonder if an engine warmed but separate heater core as a pre-heater to the electric heater would be practical.

 

BTW the Chevy Volt has a nice user adjustable feature. The engine turning on for heat can be adjusted to either 35F or 15F. So for those that make shorter trips and want to stay all electric as long as possible select 15F. For those making longer trips beyond the electric range can leverage the engine heat and preserve electric range by setting to 35F. I am curious how these two heats sources are blended or is it a hard cutover.
Most standard hybrids (not plug-ins) just run the engine for heat which is a hit to MPG. They don’t have a separate electric heating source.

 

It's clear you've given it more thought than I have so I accept that it's more complicated than I thought it needed to be to provide the performance I experience. The heating system is not as mission-critical as, say, the braking system, so I didn't believe it needed to be as sophisticated. I didn't consider the engine shut-off scenario.

 

Ive been able to turn on the ac while using a 120v without a full battery.

Ive tried it 5 or 6 times and it did it twice. The battery was 60-70% full.

 

Insightman sums it up pretty well. There is a technical write up on the Honda Service Bulletins post in the FAQ.

It’s pretty simple to observe part of the behavior. Engine Coolant Temperature is a standard PID that can be pulled by any OBD II.

I have observed the following behavior many times and think I have found the sweet spot. From a cold engine start the ECT increases at a steady pace. If I turn the heat on before the ECT has reached 95 to 110 degrees the temperature climb stops and suddenly drops 3 to 5 degrees. This suggests that the electric water valve that connects the 2 coolant loops opens and the cold coolant from the HVAC loop floods the coolant in the engine loop. If I wait until the ECT has reach 95 to 110 there
is no temp drop-a pause-but no drop. This suggests to me that the heat contained in the engine coolant volume along with the ICE increasing temp is great enough to absorb the flood of cold coolant from the HVAC loop. I can’t prove it but above this temp I believe that I am not using heat from the the resistance heater. The 95 to 110 ECT range target is driven by how cold the coolant is.

So I generally wait till I hit my target ECT to turn the heat on. If I notice an immediate drop in coolant temp I turn the heat off and wait another 60 seconds or so and try again. The ICE does not start its start/stop operation until the ECT reaches 140 or so. The ECT maxes out and does not go above 177. The rate of temp drop is pretty slow when the ICE is off so I think the HVAC continues to use the hot coolant until it reaches a certain point and the the resistance heat turns back on.


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This would imply if the ICE is already running & coolant is hot then I can use as much heat as I want without affecting range or MPG.

 

Like I said I can’t prove it definitively, but based on the information I do have, that is my assumption also.


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