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Old 02-25-2007, 03:33 PM   #1
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Idea for improving fuel economy on short daily commutes

The problem with short commutes is that the engine cools down a lot before each journey, wasting a lot of fuel at the start of each journey. Even if you have a block heater, the return journey home will often consume a lot more fuel than the outgoing journey.

I have had an idea to improve this - as follows (assuming that your engine gets up to temperature at some point during the commute - it would happen after 3-4 miles, usually).

(1) In between the radiator and engine thermostat, insert an insulated water container, of about 10 litres capacity. Then insulate the pipe leading from the engine into this container (which will contain engine coolant). Once the thermostat opens, the hot water will flow out, at full temperature, into this container. If you design baffles inside this container, then the water will flow through the circuit fairly well. Once going through the box, it will then go to the radiator, cool down, and go back into the engine. After maybe 5 miles, you will have 10 litres engine coolant, in an insulated container.

(2) Have two pipes coming from this box, and inserted into a place in the cooling circuit, together with a small 12v pump in one of the pipes, and 12v solenoids in each pipe (such that, when there is no power to the solenoids, the solenoids are closed). They should be set up so that, when water flows out through one pipe, it will then go through the engine, before going back into the other pipe. Hint : the air heater circuit is a good place for this, if the heater is set to cold.

(3) To use the system - when you get back into your car, flick a switch in the car, which will open the two 12v solenoids, and begin circulating the water. The baffles in the 10 litre tank will mean that the cold water will enter 'at the back' while the hot water flows into the engine.

This should make the engine reach near operating-temperature, prior to starting up and driving off, in a few minutes.

The concerns would be heat shock - however, if a slow 12v pump is used, the heating would occur over e.g. 3 minutes, and, the thermal shock is going to be much less than with a cold engine, which warms up to full temperature in a few minutes of driving, and always has a huge temperature gradient between the coolant and the combustion chamber anyway.

Any thoughts on this? It is something I intend to make soon, particularly as it will be quite cheap to make, and could give a real benefit to fuel consumption
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Old 02-25-2007, 03:42 PM   #2
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The Prius does something similar, so you're definitely on target...whether or not it can be done easily and economically is another question. I'll definitely be interested to see what develops, since I have a short commute (~16 miles) and take quite a hit from the warm-up.
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Old 02-25-2007, 03:47 PM   #3
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I think you are on the right track. The Toyota Prius uses an insulated container to hold some coolant at around 130F for quite some time. I'm not sure how the Prius is plumbed or regulated but it sounds like your basic idea is at least in the ballpark. If you decide to carry out this experiment, be sure to keep us posted.
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Old 02-25-2007, 05:48 PM   #4
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Another option: heated, hyperinsulated, fully enclosed mini "doghouses" for our cars, to heat them while parked (at home anyway).

Actually this would have the added benefit of warming the entire car (drivetrain, bearings, tires, etc...) instead of only the engine.
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Old 02-25-2007, 05:55 PM   #5
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The Prius pumps the coolant with a separate pump that runs before the engine starts up and after the engine is shut down saving the coolant in the engine at whatever temp it is at. Not sure if it is smart enough to bother if the coolant is at different temperatures between the bottle and engine. Something like that could easily be used with a small pipe connection into the engine and a tank using a positive displacement pump that would prevent coolant flow unless the pump is running. You would run the pump to heat up the storage coolant while the engine is hot and running before shutting down that way you get all the coolant up to max temperature... then pump it back into the engine when you go to take off. You can also use it in the summer to cool the engine when needed.
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Old 02-25-2007, 07:54 PM   #6
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MetroMPG -

Quote:
Originally Posted by MetroMPG View Post
Another option: heated, hyperinsulated, fully enclosed mini "doghouses" for our cars, to heat them while parked (at home anyway).

Actually this would have the added benefit of warming the entire car (drivetrain, bearings, tires, etc...) instead of only the engine.
A black "cooking tarp" for the car? Kind of like a dark plastic mulch box?

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Old 02-26-2007, 09:49 AM   #7
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The reason I thought up this idea is that it would be easier to construct compared to the Prius - also, another concern with trying to install something similiar to the Prius method is that, if you get it wrong, you get airlocks, or even no coolant, in your engine

I'm thinking the 10 litres of coolant would be enough to heat up the engine most of the way to full operating temperature - there should be at most 2 litres in the engine itself (possibly less in my car), so you will lose about 20% of the temperature to that. The specific heat capacity of coolant is so much more than the engine, so, overall, the engine should warm up to 70% of full operating temperature - better than a block heater!. The reason I want to use the water, just after it flows out past the thermostat, is that this water is the hottest water you will find in the engine, and once you stop the engine, you will have a 10 litre container full of hot coolant ready for use later. The challenges will be to stop convection wasting lots of the heat into the pipe, and to insulate the box well enough to prevent it losing all the heat overnight or during the day. If you put the main pipe (to engine + rad) at the bottom of the container, this would help... although it would open up the risk of airlocks in the container - a way to prevent this would have to be found.
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Old 02-26-2007, 12:14 PM   #8
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Wouldn't coolant still flow due to the thermostat remaining open after you shut down?
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Old 02-26-2007, 12:26 PM   #9
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Cool! I installed a cardboard sheet in front of the radiator and an intake hose that draws near the catalytic converter. These two simple mods increase the car to temp in under 2 miles. Unmodified it takes 6 miles.

At temp, the mpg goes from 30-40 to 50-60. So, it really adds 5mpg or more to my short trips around town too.

On another note, have you considered a block heater like they use in Alaska to keep the cars warm and ready to start? You should flip it on before driving. It might even be able to run off a storage battery somehow.
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Old 02-26-2007, 12:29 PM   #10
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PS: I find myself wondering if the thermal mass of the water is enough to make a huge difference. Perhaps it would make sense to also insulate the engine bay. There is alot of metal in there that needs to get hot. Perhaps you could also have something that drapes over the radiator at a stop to keep it really warm.

PPS: I have also heard of people "tricking" the temperature sensor into reading higher than it really is. This of course messes up the mixture and probably dumps pollution and runs rough.
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