how much would a block heater improve MPG

I think some folks have done this. The improvement would be greater in a cold climate, where the car starts out colder without it. I think the results were positive. Try a search for block heater or engine block heater. There have been threads on this before.

 

P.S. Note, however, that at 44 mpg and $3 gas, a 7-mile trip costs about 47 cents. At 50 mpg it costs 42 cents. So you'd be spending 70 cents on electricity to save a nickel on gas. Would your wife want to have to plug in and unplug the car all the time, just so you'd have the bragging rights of 50 mpg instead of 44?

Back in the old days in ND I plugged in my car so it would start. And in that cold a climate, it may be worth plugging in so you'd get cabin heat a minute sooner. But from a strict save-money-on-gas calculation, it looks to me like a loser.

But people have raised their mpg that way, and maybe the bragging rights are worth it to you.

 

P.P.S. The calculation may look different if you run the block heater for just 2 hours immediately before a daily commute, rather than leaving it on all day for unplanned trips. Maybe you'd come close to break-even.

 

The posts I have read about this suggest that a much shorter preheat time is suitable; 3 or 4 hours on your timer.

For 2004 and later Prius, an ebay auction 8044230976 ended recently with the part unsold. Possibly still available? Anyway, it tells us that the Toyota part # is CO140-00889 (this is a made-in-Canada part).

Prius that never really get warmed up in operation concern me because of water accumulaton in the engine oil. I do not believe that a block heater can cure this, so please try to schedule a longer drive monthly during winter.

 

Hello

Cost/benefit is one way to look at it. That is not my priority. I have a block heater but I have not done such a comparison. I have not noticed a substantial increase in my electric bill. I have noticed that the heat is avalable right away when I start. I was averaging about 42 mpg before the block heater. I am now at 52 mpg but I am still on my third tank so there could be other factors affecting the change. I must add that I seldom have trips of over ten miles so a full warmup is an issue with me. I also suspect that it is a little better for the ICE to have the warmer coolant at the start. I do not find the inconvenience of plugging the car in to be a problem. I have never driven off wit the car plugged in. I loop the extension cord over the outside rear view mirror where I can't miss seeing it when I get into the car. I am quite happy with my decision. But I freely admit that this is not a requirement.

 

The last couple of months, I've been experimenting with a "warm-up mode bypass" circuit on my Classic Prius (I suppose you could call it a "virtual block heater"), and my conclusion is that the Prius warm-up mode is responsible for wasting a lot of gas and making the car behave like a slug for the first few minutes, but does not significantly improve actual engine warm up time.

In below -10C temperatures, the warm-up mode gets the car to 70C an average of about 1.5 minutes faster than if I bypass it (cabin heat turned OFF), but on the other hand, I get engine-stop availability after the car has only been running 2 minutes (it still needs the first couple of minutes to warm up the cat and the oxygen sensors).. Without the bypass, my first 5 minute fuel bar was *always* greater than 7.5 L/100km, most often in the 8.5-9.5 range and off-the-scale (>10) if I got stuck at a traffic light and had to idle. With the bypass, it still gets a worse first bar, but it's in the 6.5 to 7.5 range due to operation in a more fuel efficient mode and earlier availability of idle-stop.

The biggest impact is actually in the case where the car has been driven within the last hour or so and the engine temp is still around 20C or better- the warm up mode still runs in this case and you get a really poor 1st 5 minute bar (average 7.5 L/100km in my case), when in fact, if you bypass the warm-up when the car still has some heat in it, the car gets to temperature just as quickly and you get normal consumption (in the 5.0 L/100km range) for the first 5 minute bar.

These may not be applicable to the HSD Prius as it has slightly different engine-management software and the addition of the thermos bottle, but I've long argued against the special warm-up procedures that the Prius uses.. They claim better start-up emissions as the rationale for the warm-up mode, and they're probably right if you can get the car moving continuously onto a highway and the car is doing useful motive work while it is warming up, but if you're in the city, chances are that you're doing a lot of stop-and-go and idling at traffic lights before you get out to the highway for your commute.. Based on what I'm seeing in terms of fuel consumption numbers, I'd argue that I'm probably better off in the emissions department by allowing the engine to shut down and not emit anything at all, than by allowing the car to idle in an inefficient mode for 3 minutes at a traffic light..

 

Would you tell us more about how your "warm-up mode bypass circuit" works? Did you somehow hack the ECU?

I tend to agree with your conclusions, and I use the EV switch on my '04 to stop the engine at stoplights during the warm-up (when it's slow to shut down on its own), though I've never seen it run for 3 minutes at a stoplight. (Maybe a difference between the Classic and the '04.)

I also agree that burning all that gas during warm-up, when the cat conv is cold, pollutes more, or at least as much, as letting the car run efficiently.

 

I built a little circuit with a microcontroller and utilize a 1024 step digital potentiometer and the uC A-to-Ds to monitor and spoof the coolant temperature input.. There are a few little hoops you have to jump through as the Prius does some internal cross-checks on vehicle start up, so you can't just feed any aribrary temperature reading into the input or you'll get a MIL..

The premise behind the bypass circuit is that the Prius follows a set warm-up sequence and enters various "stages" (see the 5 stages of Prius operation in the knowledge base- this is for the HSD- the Classic behaves similarly, but with more restrictions) depending on the perceived engine temperature, so a circuit that can carefully control the temperature input can fool the engine into going into its various stages early.. There's also logic in there to handle various starting temperatures and ramp rates and logic to keep the temp reading as close to actual engine temp as possible to reduce impact to the normal engine management by taking advantage of the built-in hysteresis of the Prius programming (ie, boost the temp reading to value required for next stage, then reduce down to the lowest possible value to still maintain in that stage, or actual engine temp, whichever is higher).. What I've found is that the ECU takes a lot of pains to get those emissions components warmed up- even with the bypass circuit, the engine will always at least warm up the cat and the O2 sensors, so as I said, it always takes the first two minutes or so before you have engine shut-down available unless the emissions components are already hot, but two minutes is still far, far better than the 5-10 minutes it normally takes in -20 winter temperatures..

From what I can tell, the HSD Prius does allow the engine to shut down at more occasions, and the thermos helps speed up engine warm-up, so as I said, a bypass is probably not as effective on the HSD, but on the Classic, it absolutely will not shut down until the engine has reached 70C.. It doesn't matter if it takes you 2 minutes or 2 hours-on the Classic, you must get to 70C before you have any stealth or engine shutdown, which wastes an inordinate amount of gas, especially in the winter.. A block heater also helps a great deal, but I only have that option at home, where the car is garaged anyways; at work and everywhere else, I'm stuck outside in the cold with no plug ins, hence the motivation to create a bypass circuit that works everywhere- if you have a block heater, it takes advantage of that too by speeding up the bypass that much more..