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Old 07-20-2011, 08:06 AM   #19 (permalink)
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Originally Posted by HannahWCU View Post
Sorry for the long post.

I do fairly well hypermiling, but I have a question about coasting. My 2004 Neon is a 5-spd and depending on the steepness of the hill I am descending, I will either take the transmission out of gear (if I don't need the engine breaking) or leave it in gear so that the fuel will cut off and increase my mileage.

My concern is that I was recently traveling from Asheville, NC to Hickory on I-40. Anyone who has traveled that road knows how long and steep I-40 gets on Old Fort. It is about a 4-5% grade for about 5 miles. I left my Neon in gear and the fuel cut out (I monitor this on my scangauge). For the first time I had the Water Temp on the scangauge because of the heat outside and I noticed something strange. The water temperature of the engine dropped to 196 after about 1/2 mile and stayed there (didn't go any lower).

Now I would think that if the fuel was shut off for the 5 minutes it takes to coast to the bottom that the engine would continue to cool the whole way to the bottom. I am traveling between 55-70 MPH, so air is flowing over the radiator (and the engine) the whole time.

I also would think that the thermostat is still open at this temperature because during the winter my car runs 190-192 and when coasting (engine on) I have seen as low as 185.

The reason I am wondering about this is when I checked my mileage for the tank coming from Asheville there was a BIG difference between what the scangauge read and what the actual calculated mileage was (~10%). I have to wonder if the engine is actually burning fuel but the scangauge doesn't see it.

Any thoughts?
The reason your coolant water temperature didn't drop below 196 while engine braking down the Old Fort grade was because you were continually putting energy into the engine. Just as using the friction brakes heats up your brake pads, using engine braking heats up the engine via friction and compressing air in the cylinders. All of that external rotational energy entering the engine has to go somewhere and it eventually gets converted into heat.
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