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Author: devoish Big red star, 1000 posts Feste Award Nominee! Old School Fool CAPS All Star Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: of 71394  
Subject: Re: Gas mileage dropped lately Date: 2/2/2008 9:22 PM
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now if only we could account for normal cars also doing worse in winter
Travel with me if you will to a time gone by... A time before muscle cars, when steel engines, mechanical ignition and carbureteurs ruled. A time when a gasket was copper, and oil was only one weight (SAE 30 as opposed to SAE 10-40). To a time when my grandfather could drive 30 miles on "The Jericho" from Queens NY to Northport Long Island and encounter one lonely traffic light at the intersection of Rt 110.
And make the trip he did, with a spare fan belt and tire, and more likely than not to need them.
On those cold winter nights his car was parked in an unheated garage. On the very cold ones he would drain the oil from the engine in the evening. It would keep on a steam radiator all night long, and he would pour it warm, back into the engine before trying the ignition. The warm oil would help the reluctant engine turn, but the cold steel was tight and the oil only warm, not hot. As the frostbit engine turned it pulled a mixture of air and fuel down to the combustion chamber, but it did not fire. For before the mix could arrive to the spark, the cold metal condensed the fuel, robbing it from the air to form beads of liquid on the cold steel walls. My Grandfather applied the choke, cutting of the air supply so the mixture was 90% gasoline, and then turned the cold engine again. Once again the fuel condensed, but enough remained mixed in the air, and the engine coughed and sputtered and ran and then stalled. He turned the engine again and it ran, deftly he opened the choke a crack, and at 80% fuel it continued to run. And there in the cold garage he sat with gloves on and a wool coat huddled in front of the steering wheel. With no one to keep him company he listened to his engine, and after a minute or two it asked for more air. Grandpa opened the choke a little more, 70% and the engine asked again and again for a little more air until it was ready to run. But the choke never fully opened in those days, for the cold intake air would ice the throttle and extra fuel was always needed.
But like everything else, cars evolved. Multi-weight oils arrived and automatic chokes. And when my Father started a cold engine, he could go back into the house and finish a cup of coffee, to warm him while the engine warmed, before he left for his winter drive to work.
Time passed, automobile evolution continued, and I went to work in a repair shop. Air pollution came, then emissions controls, but still the air worsened. One day New York State told us about an emissions inspection program. Cars would be checked and made to pass. No longer could I just diasble any parts I did not understand. I would need a teacher and knowledge, and more tools.
I learned from the teacher that the perfect fuel mixture was 14.7 to 1.
"Stochiometric" the Europeans called it, like I gave a sh*t. And from the emissions machine I knew when I was there, when the CO2 reading was 14.7 But I also learned from the emissions machine that many engines did not achieve 14.7 and still ran fine. 13.0 was common, 15.2 was rare. But a range existed. 13 was leaner, and less fuel gave better MPG. At 14.7 it was quicker to get up to highway speeds.
From school I was told that I needed 1 molecule of HC to burn 14.7 molecules of oxygen. And colder denser air with more molecules of 0xygen needed more molecules of HC. And while the denser air came with more power, some additional fuel was still wasted, lost to a design that was built to work at 70 degrees farenheit and 210 degree engine temperature and 2000 degree combustion, and adapted for other conditions, of which cold was more demanding than hot.
And evolution continued, fuel injectors were added, and the choke was replaced by injectors that simply stayed open longer in the cold to supply more fuel instead of choking off the air. So much additional fuel that some cars even added an extra injector for cold starts and warm up. Then the fuel injectors were moved nearer the valves, one for each cylinder. And condensation took less fuel from the air. Today Direct Injection has arrived in some cars, putting the fuel directly into the combustion chamber.
But Grandpas problems did not go away. Physics are the same, though we deal with them better.
I was taught that cars were designed to perform at 2000rpm, at 70 degrees ambient air temp, and engine temp of 210 degrees because that is warmed up and how they spend most of their running time. All other conditions require compensation, and are less efficient.
I know from reading the computers values, that most fuel injectors stay open 1.0- 1.3 miliseconds when the engine is warmed up and at idle. I know that an engine just started at 32 degrees is programmed to keep those same injectors open for 20 miliseconds and if it doesn't the car will stall. I presume that is the least amount of fuel that will work. As the engine warms, the on-time will slowly decrease until operating temperature and 1.0 miliseconds is achieved.
Hopefully this will explain why cars do indeed get poorer MPG when it is cold outside and help our OP. How much of that loss is due to warm-up times and cold starts as compared to just less than ideal cold temperatures I cannot tell you, and the length of your trip makes a big difference to your results. So does how long you let your car warm up or if you do at all. (In todays cars you need not warm it up for more than 30 seconds, just drive it gently until it is warmed up). Turn off the defroster and switch it to heat as soon as the windows are cleared. The A/C compressor runs in defrost mode and robs additional MPG.
As to the question of whether cold air, by itself and with no other factors involved adversly affects MPG, I am opining "yes it does". I believe the design of an engine to perform at 70 degrees, and more fuel being needed for more denser molecules of air outweigh the other arguments.
As to the question of why cold air gets you better performance in the 1/4 mile, so why shouldn't the extra power convert to better MPG? It doesn't because the best fuel mixture range for MPG with acceptable performance, is not the same as the fuel mixture for speed with acceptable MPG. Also I should have added that the 14.7 to 1 ratio is for neither best mpg or fastest acceleration. It is for lowest emissions by completely burning everything in the chamber at a perfect ratio. Slightly better acceleration is achieved by wasting some fuel and burning the fuel mixture more quickly. Slightly better MPG is achieved by wasting some air and burning all the fuel.

Regards,
Devoish
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