Holy Carp! The regular block heater takes about 400 W. Your 1,500 W. tank heater is about 4x that amount!
Ok the
temperature hack link. The engine ECU is located between the glove compartment and the passenger side, outer shell. Some unpublished additions.
Part of the development was not only the early warm-up spoof but also handling the cool down:
For example, as soon at the coolant temperature reached 40C, the hack spoofed 70C so the car could go into Stage-4. As the engine warmed up, the PWM duration dropped off.
Around seconds 800, the ICE was off and started cooling down. So when it dropped below 70 C, the PWM cut in to keep the indicated temperature above 70C. Eventually around seconds 1200, it was so cool that the PWM pulses were not quite able to keep the temperature above 70C. Around seconds 1400, it failed to continue the spoof but the coolant temperature was too low and the ICE choked.
This detailed chart shows what I mean by PWM during cool down:
You can see how the hack kept the apparent ICE coolant high enough that the ICE stayed off. It turns out the ICE will start on its own in the 60-63 degree region. So by spoofing the ICE temperature, I could continue in EV mode, headed home. It was a brisk evening.
One of the earlier tests used a fixed resistor and diode:
The problem is it needs to trigger at a starting coolant temperature. Experimentally I found 30C was about as low as it could go w/o the ICE choking. Pragmatically, I used 40C to make sure there was a 'guard band.'
Here are some parametric curves that assumed a simple resistor, no diode, is used for biasing the coolant thermistor:
The problem with a fixed resistor is it doesn't shorten the 70C point that much but as the resistance goes down, it risks triggering a false, over temperature condition. This is why we use a silicon diode to gain the 1.2 V forward bias. As the coolant warms up, the voltage drops below 1.2 V and the diode-resistor network has an apparent rise in resistance. ... Kinda of stupid but effective hack.
When we were still looking at fixed resistors and hadn't considered the diode:
Ok, this was probably more information than you'd expected. But I had it handy.
WHY?
I warmed up our NHW11 in the driveway normally, until the first autostop:
From this systems analysis, we find:
- 0.100 gallons burned just to reach warm-up temperature.
- 0.320 gallons burned for 10 mile commute including warm-up
- 1/3d of the gas just to reach Stage-4 temperature
The thermistor hack offers a potential savings of ~1/3d. So warm-up is a critical part of efficient, NHW11 operation.
I subsequently learned some 'tricks' that I can use due to the lucky geography of our home in Huntsville so I don't have to use the thermistor hack. But someone in a colder climate with less opportunity for coasting in "N" through their neighborhood needs the hack.
Bob Wilson