Debunking fuel-economy myths

[Alchemy]

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.

[RichardM]

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.

[Psihala]

When I read the baking soda one, I very much got the same feeling I got as a child when the big kids would knowingly tell us "gullible little kids that would believe anything" that something great would happen if we did something obviously stupid.

As far as I can tell, this just the 'internet playground' equivalent; something someone posted knowing full well it isn't true to get the less technology-inclined and gullible to do something stupid "for the lulz".

~Psihala

[KirkMcD]

Is gasoline even acidic enough for the baking soda to react and doesn't it create carbon dioxide anyway which would be bad for combustion?

[GenYus234]

Acids are solids dissolved in water, so unless the gasoline is contaminated with water, it can't be acidic. If it did create CO2, that would probably escape through the fuel tank ventilation system and wouldn't be pumped into the cylinders anyway as a fuel pump can't pump a gas.

[jimmy101_again]

Originally Posted by GenYus234

Acids are solids dissolved in water, so unless the gasoline is contaminated with water, it can't be acidic.

"Acids" are compounds that easily transfer a proton (a hydrogen atom without its electron) to another compound. And, relatively few acids are solids. The "seven strong acids" are all liquids under normal conditions.

Baking soda is sodium bicarbonate which is the mono-sodium salt of carbonic acid. It is a very weak acid and a moderately strong base. Carbonic acid is one of the compounds formed when carbon dioxide is dissolved in water. Baking soda plus an acid gives carbonic acid, which generally spontaneously decomposes into water and bubble of CO2.

Baking soda won't dissolve to any appreciable extent in gasoline and will just muck up the pump, filters, lines and injectors (like others have said).

[Alchemy]

Originally Posted by KirkMcD

Is gasoline even acidic enough for the baking soda to react

Nope!

and doesn't it create carbon dioxide anyway

Yep!

which would be bad for combustion?

Kinda!

Obviously, this is a completely unworkable delivery method of getting CO2 into the cylinders to begin with.

But engines actually use their own exhaust (which is mostly water and CO2 in nitrogen) to displace some air/fuel by just sucking exhaust back into the cylinder (exhaust gas recirculation or EGR). This alters the thermodynamics of combustion and can reduce nitrogen oxides formation, and reduce power loss due to throttle pressure drop, among other benefits.

[RichardM]

EGR is primarily to reduce pollution. It doesn't improve power output at all.

[Alchemy]

Originally Posted by RichardM

EGR is primarily to reduce pollution. It doesn't improve power output at all.

"Reduced power loss" as in "Less of the engine's power output is spent pulling air across a high-drag-coefficient obstruction, i.e., a mostly-closed throttle plate."

I should have just said "improved fuel efficiency at low power demand."

EGR always reduces engine power output because it's cutting down how much fuel and air you can get into the cylinder. (Which is why EGR is not active during high power demand.)

But when the engine is at low load and low throttle angle, a lot of the power being produced by the engine is being spent sucking air across the throttle body. Recirculating exhaust back into the cylinder reduces the amount of fresh air you need to draw through the throttle body, and increases manifold pressure so that the throttle can be opened further for the same flowrate. Both of these things reduce the amount of energy the engine spends on sucking air from outside.

While EGR will in some cases reduce the efficiency of combustion (the charge still burns completely but the energy isn't as easily extracted), there are a lot of low-load conditions where the reduced throttle loss more than makes up for it, giving you improved fuel economy overall.

Since diesels don't control power by throttling, and can handle a wide range of air-fuel ratios, there's no real benefit, fuel-efficiency-wise, in displacing the charge with exhaust. So while EGR also reduces NOX in diesels, it does so at a significant fuel economy cost.

[urbanlegendfanatic]

Originally Posted by Elkhound

How about switching to a motor scooter? That'll save you money. (Yes, I know it isn't reasonable for people who have to haul kids or goods.)

Motor scooters don't do very well in below zero weather with close to two feet of snow.

[urbanlegendfanatic]

Originally Posted by Psihala

When I read the baking soda one, I very much got the same feeling I got as a child when the big kids would knowingly tell us "gullible little kids that would believe anything" that something great would happen if we did something obviously stupid.

As far as I can tell, this just the 'internet playground' equivalent; something someone posted knowing full well it isn't true to get the less technology-inclined and gullible to do something stupid "for the lulz".

~Psihala

ThinkGeek were the ones who posted it though I don't know if they were the ones to start the rumor.

[Elkhound]

Originally Posted by urbanlegendfanatic

Motor scooters don't do very well in below zero weather with close to two feet of snow.

True. But my friends in Minnesota say that the scooter dealerships there can't keep them on the lot, so they must be SOME use.

Here in WV, I'll take mine out no matter how cold the weather (just bundle up more) as long as the roads are clear. Back in January I rode up to Buckhannon for my aunt's funeral (and in the previous January I rode over to Lexington, KY for my other aunt's funeral.)

[niner]

Scooter for the summer, snowmobile for the winter?

Baking soda (sodium bicarbonate) is used under high pressure to clean carbon deposits from wood in houses that have had minor house fires (that is, minor enough to not burn the house completely down). It's kind of like sandblasting, except it's soda blasting. I wonder if someone heard that, combined with "carbon deposits" in the engine, and created this idea?

[GenYus234]

Originally Posted by Elkhound

Here in WV, I'll take mine out no matter how cold the weather (just bundle up more) as long as the roads are clear.

You must have better roads down there than in Monongalia county. Whenever we visit my wife's family I see potholes that would swallow a scooter.

[Elkhound]

Originally Posted by GenYus234

You must have better roads down there than in Monongalia county. Whenever we visit my wife's family I see potholes that would swallow a scooter.

Mine isn't one of those put-put Vespas; it is a 400cc Suzuki. It is really a motorcycle, and is only considered a scooter technically because it has a step-through frame, enclosed engine, and automatic transmission.

[Elkhound]

Here's a (Spanish) promo for what I ride.

[ganzfeld]

Ah, I see why it does OK when there's snow. It's a snowmobile.

[42liter]

Originally Posted by Singing in the Drizzle

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.

ef·fer·ent


/ˈefərənt/


adjective
Physiology

adjective: efferent




conducted or conducting outward or away from something (for nerves, the central nervous system; for blood vessels, the organ supplied

[Malmensa]

Originally Posted by RichardM

"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.

[overyonder]

Originally Posted by Malmensa

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