A Thermos filled with coolant sounds like it could be part of a really cheesy murder plot. Some guy goes out on a mountain, takes a swig from the Thermos, dies of poisoning before he can get back to the ski lodge. Soap opera stuff. But what if the entity taking a swig from the coolant Thermos was a 2004 to 2008 Toyota Prius? No really, bear with me on this.
It’s no secret that starting an engine from cold isn’t great for emissions. While there are a litany of reasons why cold starts are bad for the environment (high losses from thick lubricating fluids, high losses to metals due to high deltaT between combustion reaction and surrounding metals, etc), but let’s focus on how fuel behaves when cold. Gasoline from the pump is generally (unless you’re doing something really wrong) a liquid, but before it’s injected into a combustion chamber, it’s atomized to ensure nice mixing with air and therefore efficient combustion. Unfortunately, cold fuel is hard to get to vaporize; it tends to condense on cold engine components much like water vapor in the air in your room will condense against the outside of your cold Coke can. The result of the condensed liquid fuel dribbling down your cylinder walls is a much less effective burn than if the gasoline had been properly-atomized. Think lean conditions, unstable flame fronts, that sort of stuff. To combat fuel condensation, engines tend to just add more fuel to the fuel/air mixture when the coolant, and thus the engine, is cold.
Coolant temperature enrichment seems to work. Yes, it can stink like the aftermath of a Taco Bell binge sometimes and it often requires secondary air injection to get a complete burn prior to reaching the catalytic converter, but hey. It’s cheap to tune and easy to implement. However, coolant temperature enrichment is not necessarily the best solution. Ideally, coolant temperature would never get properly frigid, and warm coolant would cycle through the engine upon cranking — this way you don’t have to enrich your fuel mixture. But how do you keep coolant warm when cars are switched off overnight? Enter the vacuum flask, more commonly known as the Thermos.
If you like the soup in your packed lunch to be piping hot or have ever enjoyed hot cocoa in the snow, you’ve likely used a vacuum flask. It’s a pretty simple bit of kit, essentially two concentric flasks joined together at the neck with a near-vacuum in between them. Conduction and convection don’t work very well in a vacuum, so anything in a vacuum flask typically retains temperature very well. It’s perfect for storing cocoa, chicken noodle soup or coolant. Maybe not all in the same vacuum flask, but still.
Toyota saw fit to equip North American examples of the 2004 to 2008 Toyota Prius with a coolant vacuum flask. It’s a common trope that overseas manufacturers save their best tech for their home market, so a North American exclusive is a cool thing to see. So how does the vacuum flask work? Well, hot coolant gets stored in the vacuum flask upon engine shutdown, thanks to the closing of a valve. Upon cold start, there’s a brief delay between when a Prius powers on and when its engine fires up. In those few precious seconds, an electric water pump shoots warm coolant from the “coolant storage tank” into the cylinder head. The vacuum flask can’t store much coolant, but Toyota said in a slide deck that it’s effective enough to raise intake port temperature to 104 °F (40 °C) upon startup, a temperature that aids fuel atomization and cuts cold start emissions.
[Editor’s Note: As I was once a cooling system engineer, I’ve got to hop in here. Prepare for the longest Editor’s Note in the history of journalism.
Let’s take a closer look at the cooling loops for the Prius. Out front there’s a single radiator with two distinct circuits (I can’t for the life of me remember what we used to call such a shared heat exchanger at Chrysler). The bottom circuit part of the heat exchanger cools the two motors in the Prius’s transaxle as well as the inverter, and receives water via an electric pump. The top section of the radiator is the engine radiator, and is fed water via a mechanical water pump on the car’s accessory drive. Let’s have a look at some slides I found on some random website:
Here’s a quick look at the transaxle cooling system that feeds the bottom of that rad. MG1/MG2 refers to the motors in the hybrid transaxle:
Obviously, we’re more interested in the engine cooling circuit. Here’s a high-level overview via YouTuber EyeOnAiman:
You can see there’s a mechanical water pump circulating coolant through the engine, out of a conventional thermostat, and through a throttle body and heater core. In orange is our storage tank, an accessory water pump, and a water valve.
Let’s focus on that orange section:
Let’s look at how this works. When the engine is running, and the storage operation is off, no coolant needs to go to the storage tank. And since there’s a mechanical water pump running off the front-end accessory drive, the electric water pump can shut off, too. So this means hot coolant comes out of the top of the engine (where it says “Engine Coolant Temp Sensor” in the image above), and then heads straight into a water valve that looks like this:
From there, coolant goes through an electric water pump (which again, is off) into the heater core and back into the engine, where it picks up more heat to deliver to your cabin for maximum comfort. Here’s a diagram showing that “engine running (w/o storage operation)” condition. You’ll see that coolant is bypassing the thermos:
I’m not entirely sure I understand the conditions under which this next condition would occur, but it seems like coolant storage can happen when the engine is running, per the diagram below. In this instance, the electric pump would remain off, since the mechanical pump is still circulating coolant, and the three-way valve would just open both outlets, sending coolant to both the heater core and storage tank:
Here, the document talks about why you’d have the engine on and still send coolant to the thermos. I don’t really get it, though it seems the “max.: 4 times” is done to preserve the life of the coolant valve:
Speaking of the valve, here are the signals that go to the valve to determine where to send flow. As you can see, coolant can go from the engine directly to the thermos, it can go to the thermos and heater core, or it can go just to the heater core:
Oddly, I don’t have diagrams of engine-off operation, but those diagrams above would be largely unchanged, except the electric water pump would be on. Coolant would come out of the top of the motor, it’d be circulated via that little electric pump, and it’d be diverted to the heater core and/or the storage tank.
Other fun stuff from this presentation I found online includes this explanation for why this storage tank exists in the first place — again, it comes down to the vaporized fuel mixture condensing on the cold metal::
And there’s also a good explanation from the publication Toyotatech:
As part of its PZEV emissions rating, the system includes a thermos (called the “Coolant Heat Storage” or CHS tank) that can store coolant at 180 degrees F for up to 3 days.
When readied on, the driver may hear a faint whine while a third electric pump (at the thermos) runs for several seconds (prior to engine crank) to push hot coolant to the block, heating it to reduce fuel condensation, therefore lowering hydrocarbon emissions during startup.
Likewise the whine may be noticed when the car is turned off, pumping hot coolant from the engine into the thermos for storage.
To manage both exceptional cases — cabin heating and engine warm up — the Gen 2 has yet another exceptional component: the so-called “water valve” (a.k.a. “three way valve”). This valve determines when hot coolant is circulated to the heater core, the thermos, or both. The three way valve is the gatekeeper to make sure heat isn’t wasted, but its existence in the system adds yet another layer of complexity regarding diagnosis and bleeding.
Anyway, back to Thomas. -DT]
Are there any drawbacks to the vacuum flask system? Of course, nothing is perfect. The first issue is a matter of cost. The coolant vacuum flask on a 2004 to 2008 Prius retails for around $1,700, with a wholesale value closer to $1,200. Even if there could hypothetically be a 100 percent profit margin on the wholesale price of each coolant vacuum flask, that’s still one seriously expensive part. It’s also a fairly hefty part to package. We’re talking a cylinder roughly the size of a rotisserie chicken. As such, Toyota tucked it under the front bumper cover just in front of the left front wheel. If you’re thinking that this placement sounds a bit vulnerable in a crash, you’d be right.
In the end, Toyota abandoned the vacuum flask tech for the third-generation Prius. Still, the drawbacks of vacuum flask tech don’t hugely detract from its impressive potential. Imagine a cost-no-object scenario, like on a BMW 7-Series or Mercedes-Benz S-Class or any bewilderingly complex six-figure, 155 mph land barge for plutocrats that you’d never want to service when out of warranty. One of these tire-frying gin palaces could have two vacuum flasks, one for emissions and one for piping-hot heat as soon as the owner pushes the starter button before an electric resistive heater has any chance to get going. Doesn’t that sound marvelous?
Lead photo credit: Toyota