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  #1  
Old 09 March 2013, 06:02 PM
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Driver Debunking fuel-economy myths

Like all drivers, you want to save gas and do what’s right for your car. But along with the tried-and-true advice, there are some well-intentioned — if off-the-mark — tips that can lead you astray. Below are several common myths about fuel use and gas mileage, and the real stories behind them.

http://www.consumerreports.org/cro/m...yths/index.htm
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  #2  
Old 09 March 2013, 06:13 PM
RichardM RichardM is offline
 
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"A dirty air filter drops gas mileage"

CR says this isn't true. I disagree, it is true although maybe to a negligble point. Pulling the air through a dirty filter takes more energy that pulling it through a clean filter. That can be proven in the lab although apparently not on the road.
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Old 09 March 2013, 10:08 PM
Alchemy Alchemy is offline
 
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Yeah, the filter thing seems a little odd.

Catalytic converter technology has required careful control of the fuel air mixture on practically every gasoline-fueled passenger vehicle in the US for the past 30 years. If the mixture was so badly controlled that a slight increase in filter pressure drop due to particulate buildup could continuously enrich the mixture enough to affect fuel economy, that would produce an absolutely massive increase in post-catalyst CO and hydrocarbons. Such a car would never pass emissions. I don't think anyone very familiar with engines would think this a serious concern for a light passenger vehicle built after (or perhaps even during) the 1980s.

The actual issue, as you state, is the pressure drop increasing the (effective) throttle loss. I'm curious how thick the filter cake on the "dirty" filter got.

Still, filter blockage only adds to the pumping loss already generated by the throttle, and it's entirely possible that, to a limit, a dirty filter simply offsets the throttle setting.
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Old 10 March 2013, 01:55 AM
Meka Meka is offline
 
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I'd have to question the "Windows down vs. A/C" one as well, as I suspect that may vary depending on speed and the type of car. The Mythbusters, for example, found windows down to be more fuel efficient on an SUV at 45 mph, but air conditioning to be more efficient at 55 mph.
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Old 11 March 2013, 08:30 PM
RocMills RocMills is offline
 
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Quote:
Originally Posted by Meka View Post
I'd have to question the "Windows down vs. A/C" one as well...
Definitely depends on the type of car being driven. My hybrid gets much worse MPGs if the A/C is cranked up.
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Old 11 March 2013, 09:12 PM
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I would also expect a car that's designed to be very aerodynamic like a Prius to suffer more from the affects of wind resistance when the windows are rolled down, as well as the drain from running the AC.

I usually roll down the windows when I'm driving at low speeds, say less than 45 mph, and roll them up and turn on the AC on the highway. That's not just because of the fuel economy thing, but also because the wind noise just starts to become bothersome at higher speeds.
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Old 07 June 2016, 05:20 AM
Malmensa Malmensa is offline
 
 
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Quote:
Originally Posted by RichardM View Post
"A dirty air filter drops gas mileage"

CR says this isn't true. I disagree, it is true although maybe to a negligble point. Pulling the air through a dirty filter takes more energy that pulling it through a clean filter. That can be proven in the lab although apparently not on the road.
Pretty sure this would depend upon the type of fuel injection system, how metering is implemented, type of transmission etc.

A partially blocked air filter restricts incoming air flow, a similar effect to limiting throttle opening. A vehicle with a very dirty filter at full throttle might have a similar air flow to one with a clean filter and 3/4 throttle. Thus it wouod just preform poorly. HOWEVER, most engines measure throttle position, and make some use of that information to determine how much fuel to inject, or what gear to be in. Most modern cars are closed loop and will determine from O2 sensor info how much fuel to meter, but some older cars have no O2 sensor. In a worse case scenario fuel requirements would be determined just via throttle position, so with a blocked filter, way too much fuel would be delivered. This, along with being in a sub-optimal gear could mean a noticeable increase in fuel consumption.
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Old 20 June 2016, 02:12 PM
overyonder overyonder is offline
 
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Quote:
Originally Posted by Malmensa View Post
Most modern cars are closed loop and will determine from O2 sensor info how much fuel to meter, but some older cars have no O2 sensor.
I think you're mixing O2 sensor versus MAF (mass-airflow sensor). The MAF measures the amount of air flow into the engine (and sometimes the temperature of the air charge) and adjust the amount of fuel based on that. The O2 sensor is more to verify that the fuel charge is burnt cleanly to make sure the engine is not running lean or rich. Yes the O2 sensor can help the PCM adjust fuel amounts, but it's mostly to adjust the timing.

It's really a "Big Dance" that the PCM takes inputs and determines what to do from there.

OY
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Old 21 June 2016, 12:55 AM
jimmy101_again jimmy101_again is offline
 
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Quote:
Originally Posted by Malmensa View Post
Pretty sure this would depend upon the type of fuel injection system, how metering is implemented, type of transmission etc.

A partially blocked air filter restricts incoming air flow, a similar effect to limiting throttle opening. A vehicle with a very dirty filter at full throttle might have a similar air flow to one with a clean filter and 3/4 throttle. Thus it wouod just preform poorly. HOWEVER, most engines measure throttle position, and make some use of that information to determine how much fuel to inject, or what gear to be in. Most modern cars are closed loop and will determine from O2 sensor info how much fuel to meter, but some older cars have no O2 sensor. In a worse case scenario fuel requirements would be determined just via throttle position, so with a blocked filter, way too much fuel would be delivered. This, along with being in a sub-optimal gear could mean a noticeable increase in fuel consumption.
A partially clogged air filter will act as an obstruction in the air flow requiring more energy from the engine to suck air through the clogged filter. This would be independent of whatever the engine control system does to keep the fuel and air properly balanced and will (probably very slightly) reduce engine power per unit of fuel. (A car engine is basically an air pump that pumps air from the front of the car to the back. )
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  #10  
Old 02 November 2014, 03:11 PM
urbanlegendfanatic urbanlegendfanatic is offline
 
 
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Here's the latest myth about saving on oil going around - I don't know if there's any truth to it and I'm not about to try it out.
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  #11  
Old 02 November 2014, 11:46 PM
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Roll eyes

If your car is powered by a hydrogen fuel cell, maybe. Otherwise, it's about as effective as dumping sand in your fuel tank.
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  #12  
Old 03 November 2014, 03:40 AM
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That and even if it did "aerate the fuel", I don't see how "operating on fuel vapor instead of liquid" would increase you mileage. I'm not sure if the car would even run. The fuel injectors require fuel at a certain pressure in order to operate. I don't think the pump would be capable of getting fuel vapors up to the right pressure and to the injectors. And on an old carbonated car it would be even less likely to work as the carburetor requires a supply of liquid fuel in the float chamber.
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  #13  
Old 03 November 2014, 05:39 AM
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If you add 7 cups of baking soda to your vehicle's fuel tank, the baking soda will clog the fuel line and your vehicle will be shot to s*it. But hey, maybe it will smell better. Who knows?

This has been a Public Service Announcement.
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  #14  
Old 03 November 2014, 12:30 PM
Singing in the Drizzle Singing in the Drizzle is offline
 
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It is more efferent to burn gas as vapor instead of liquid, but there are many problems with it. The first one is safely converting the gas to vapor fast, safely and delivery to the engine. It tends to burn a lot hotter so you need to have a much improved cooling system on the motor. Then you need an efferent delivery and mixing system designed to work with vapor instead of liquid. In short it can work but you will have to redesign your engine for it to work.

Just adding baking soda to a gas take does sound like it might be good for the environment because your car may never pollute the environment again with CO2. This do to engine no longer functioning.
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  #15  
Old 03 November 2014, 05:16 PM
Alchemy Alchemy is offline
 
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In an automobile, liquid fuel from the fuel tank gets pumped to the fuel rail at anywhere from 15 to 3000 psi depending on the type of engine and injectors. The fuel injectors have tiny solenoid valves in them that are made to inject fuel in microseconds-long bursts. (Higher rail pressure means more fuel delivered faster; additionally, you need extra pressure to deliver fuel to an already-pressurized post-turbo manifold or cylinder.)

In a gasoline engine, when the injected fuel enters the intake manifold or the cylinder (depending on the injector location) it aerosolizes and evaporates and mixes with the air to form a fuel-air charge. There is a huge body of research on there as to how exactly to get the fuel to vaporize and mix rapidly with intake air, and lots of engines have really off-the-wall strategies to mesh airflow and flame characteristics to get efficient burning in the cylinder.

"Aerating" the fuel while still in the gas tank doesn't help anything; if you could actually get a mixture of air and fuel going on in the tank you'd have fuel foaming, which is quite detrimental to the pump life, and assuming the pump functions it's still going to compress that foamed fuel into a high-pressure liquid in the fuel rail.

Sodium bicarbonate (baking soda) isn't very soluble in gasoline or diesel, so it's going to just form a film atop the fuel for a while (surface tension) and then get wet enough to sink down to the fuel filter and plug it up if there's enough crap in there. It won't decompose so it's not going to make little CO2 bubbles, and decomposition of sodium bicarbonate doesn't release any oxygen or hydrogen that would influence combustion.
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  #16  
Old 03 November 2014, 06:04 PM
RichardM RichardM is offline
 
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But it would work if alchemy was factual. That is to say, if alchemy the turning of lead into gold was factual rather than Alchemy the person.
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