Question for people who know something about cars

[NW-Bound]

Didn't consider air density. Found this:

https://www.gribble.org/cycling/air_density.html

and the air @ 20F is about 6% more dense than at 50F, so 6% higher drag per your formula, and drag is a fairly high component at highway speeds (you still have engine losses)...

Pilots know the effect of air density very well. They may run out of runway for take off on a hot and humid day; not enough lift due to thin air and perhaps the engine power is also down. Water vapor is lighter than air, so humid air is less dense.

Lift and drag both increase with higher air density. And they have the same equation. Just replace Cd with Cl, and A with S for wing surface.

 

[NW-Bound]

Drawing air from the manifold area was done to prevent carburetor icing. It decreased HP back then, too.

Thanks. I was not an automobile engineer.

About towing, yes, a truck or car gas mileage is really shot when pulling a trailer which is as aerodynamic as a brick.

On the other hand, my class C gas mileage does not budge when I tow a car. I figure that the toad is so much smaller and hidden in the slipstream of the RV so that it becomes insignificant. I do notice the difference when going up hill though, and the weight of the toad really slows me down. On flat land at cruising speed, no problem.

 

[38Chevy454]

As many have said, the primary difference is likely wind and second engine efficiency. All new fuel injected engines have an ambient air temp sensor, and that (combined with the various other sensors on the engine, with the fuel map programming) can cause a slight increase in fuel usage.

Since your trip was within the same week, not due to the winter fuel blend, which is known to decrease fuel mileage. I also don't think the 30 degree difference in air temp caused enough change in tire air pressure to be measurable.

 

[harley]

Thanks for all the responses. Way more information than I expected. And I still don't really know the cause. It was much windier on the first day, when I got the better mileage. Maybe I was getting a tail wind, at least in one direction. The wind had died down a bit on the return trip. Maybe the denser cold air, maybe tire pressure, maybe Marge (our car) was just having a bad day. We might be doing the same trip again in a couple of days. I'll reset and check it again.

 

[Winemaker]

The other thing we failed to mention was weight of the vehicle. Did you deliver or transport anything that increased the weight of the vehicle. I have been known to transport copious amounts of old bottled grape/fruit juice to various areas of the US to family, and it does make the vehicle hug the ground a little.

 

[harley]

Lol. No, same weight, just the two of us, and although I was fasting during the first trip, I don't think that would account for it. I did some calculations and while the temp difference would probably have lowered the tire pressure by about 3 psi, that wouldn't account for but about a sixth of the difference. So I'm puzzled, but it's truly not that important. As I said, I'm probably making the trip again soon, so I'll check the conditions and mpg and see what I see.

 

[ERD50]

Pretty sure wind is the Big Red X. Many other factors, but that is very likely the main one.

...
About towing, yes, a truck or car gas mileage is really shot when pulling a trailer which is as aerodynamic as a brick.

On the other hand, my class C gas mileage does not budge when I tow a car. I figure that the toad is so much smaller and hidden in the slipstream of the RV so that it becomes insignificant. ...

I recall some forums discussing aerodynamics of vehicles. Surprising to me, but the back of the vehicle was very important. A sort of 'duck tail' shape really helped reduce drag. Maybe having the car in back of your RV effectively smooths out the air flow in the rear, and is actually helping?

-ERD50

 

[braumeister]

I've tracked my mileage for many years. I put a lot of miles on my cars (average about 24K a year) and it has always been consistent. My cold weather efficiency is always about 2 mpg less than summer. The in-between seasons are proportional.

 

[NW-Bound]

...I recall some forums discussing aerodynamics of vehicles. Surprising to me, but the back of the vehicle was very important. A sort of 'duck tail' shape really helped reduce drag. Maybe having the car in back of your RV effectively smooths out the air flow in the rear, and is actually helping?

Yes, the square end of the RV certainly hurts, though not to the same extent as a flat front of a trailer does.
Some semi-trailers sport this to reduce drag. It is said that drag is reduced by 5%.

And even bullets have a boat tail.

I don't think a car towed behind an RV has the same effect because there's a big gap between the two vehicles. See photo of a typical configuration.

 

[youbet]

Our new car has one of those fancy computer displays that keeps track of your mileage for you.

What accuracy does the manufacturer state for this "fancy computer" device? +/- 5%? +/-10%?

I suspect the 5% - 6% difference in mileage your instrument read between the two nearly identical trips can be substantially explained by the accuracy and repeatability of the instrument itself. The rest would be easily explained by minor differences in conditions such as how many red lights you experienced, wind speed and direction, number of times you changed lanes to pass, a small difference in tire pressure from one trip to the next, your vehicle's relative position vs. other vehicles and for how long, etc.

I don't think your small variation in mileage readings from trip to trip is significant.

 

[jazz4cash]

Actually the toad might help quite a bit from an aero standpoint. When race cars draft it improves drag on both vehicles. This of course is offset by added weight. At higher speed the aero drag dominates.
And I agree the variation is well within typical range considering all the possible factors affecting the result.

 

[brucethebroker]

Yes, the square end of the RV certainly hurts, though not to the same extent as a flat front of a trailer does.
Some semi-trailers sport this to reduce drag. It is said that drag is reduced by 5%.

And even bullets have a boat tail.

I don't think a car towed behind an RV has the same effect because there's a big gap between the two vehicles. See photo of a typical configuration.

Huh........and I thought that thingy on the back of the semi was where they packed the drag chute. Who knew.....

 

[marko]

Only drive west in the morning and east in the evening so the solar wind will help push you along and improve your mileage!

And driving south is all downhill! Just look at any map.

Seriously, a rainy day might also change your mileage slightly. A little extra water gooses the HP. At least in old carburetor cars anyway.

 

[Surewhitey]

We flew to Hawaii from Dallas. Going took 30 mins longer than returning, so wind always plays a difference in mileage and time.

We drafted behind a truck once to do a little test. For appx. 30 mins, we got 45 mpg vs. 27 mpg when not...
my $.02

 

[kombat]

Another factor is that cold air is denser than warm air, hence causes more wind drag.

I have made many trips with my RV, and try to maintain a constant speed for the best fuel economy. I invariably observe better gas mileages at high elevation vs at sea level.

But denser air would also result in more efficient combustion, allowing the engine to produce more work with the same amount of fuel, or the same work with less fuel.

In your example, you cite your RV, which I presume is diesel, meaning it has a turbocharger that erases any effect of ambient air pressure (since the turbocharger pressurizes the incoming air to the same pressure regardless of its initial pressure). However, in a naturally-aspirated engine, the engine is fed air at whatever the current atmospheric pressure is, without the benefit of being "compacted" by a turbocharger.

 

[NW-Bound]

No, my RV has a gas engine.

About the air being thinner, I think that it would be compensated by the mass airflow sensor to reduce the injected gasoline proportionally. Else, you would not have enough air to burn the fuel, and the emission would be shot. Don't they try to maintain the proper fuel/air ratio?

Thus, the maximum available power is reduced with thin air, but not the efficiency when you are just cruising.

 

[Winemaker]

If you own a car with a carburetor, when you "step on it and give it the gas", you are actually giving the engine more air. The venturi effect then pulls more fuel into the intake.

 

[NW-Bound]

In modern gas engines, you still control the air intake with an old-fashioned throttle plate. Instead of a venturi, a mass airflow sensor measures the amount of air intake, and a computer injects the proportional fuel amount. For finer adjustments, the computer uses the O2 sensor in the exhaust manifold as a feedback to vary the fuel amount slightly from the nominal. I guess they want to see the right amount of O2 left over from combustion to make sure the mixture of fuel/air is not too lean nor too rich.

Talk about carburetor reminds me of this. The Onan genset in my RV is carbureted and not an injection engine. Its carburetor has a knob to adjust the mixture to lean out at high altitudes. The dial marking is in 1,000-ft increments.

So, yes, carbureted cars would get poorer gas mileage at high altitudes, unless some adjustment is made. Fuel injection cars would auto-compensate with a computer, I believe.

 

[ERD50]

If you own a car with a carburetor, when you "step on it and give it the gas", you are actually giving the engine more air. The venturi effect then pulls more fuel into the intake.

Good point, I never really thought about it that way. But 'giving it the air' just doesn't sound the same!

But when you really 'punch it', as I recall, there was a little pump that would inject gas into the carb to give extra richness for acceleration. I think it was called a 'dashpot' mechanism? It wouldn't respond to a slow change in pedal position, only a fast pump. Now I recall, on a cold day, in addition to the choke, you might pump the gas pedal like that to get a richer mixture.

Man, these computer controlled injectors, sensors, and smarts do a remarkable job.

Maybe later, I'll add to my 'good environmental ideas' thread (if I can find it -search for 'Wrightspeed'), I recently saw some info on a car engine with computer controlled intake/exhaust valves. Some of the improvements were things I never would have considered.

-ERD50

 

[NW-Bound]

But when you really 'punch it', as I recall, there was a little pump that would inject gas into the carb to give extra richness for acceleration. I think it was called a 'dashpot' mechanism? It wouldn't respond to a slow change in pedal position, only a fast pump. Now I recall, on a cold day, in addition to the choke, you might pump the gas pedal like that to get a richer mixture...

Yes. When you stomp on the gas pedal, the air intake suddenly increases, but initially without the corresponding increase in fuel. The engine stalls. This effect would be seen very clearly when that pump you describe failed.

I think many modern engines do the same with a throttle position sensor. This is used to anticipate fuel demand when the driver steps on the pedal. In steady state, the fuel injection is computed from the MAF (mass airflow) sensor and the O2 sensor as described earlier.

 

[NW-Bound]

I recall rebuilding a carburetor when young, and examined that integral little pump that you describe. It's just a piston loosely fit inside a small cylinder. The loose piston did not seal tight against the wall of the cylinder, such that the high leakage around it would not cause any fuel to be pumped if the throttle was opened slowly. It was only under fast actuation that the little pump would inject any fuel.

 

[ChiliPepr]

https://www.fueleconomy.gov/feg/coldweather.shtml

Why is winter fuel economy lower?

Cold weather affects your vehicle in more ways than you might expect:

• Engine and transmission friction increases in cold temperatures due to cold engine oil and other drive-line fluids.
• It takes longer for your engine to reach its most fuel-efficient temperature. This affects shorter trips more, since your car spends more of your trip at less-than-optimal temperatures.
• Heated seats, window defrosters, and heater fans use additional power.
• Warming up your vehicle before you start your trip lowers your fuel economy—idling gets 0 miles per gallon.
• Colder air is denser, increasing aerodynamic drag on your vehicle, especially at highway speeds.
• Tire pressure decreases in colder temperatures, increasing rolling resistance.
• Winter grades of gasoline can have slightly less energy per gallon than summer blends.
• Battery performance decreases in cold weather, making it harder for your alternator to keep your battery charged. This also affects the performance of the regenerative braking system on hybrids.

In severe winter weather, your mpg can drop even further.

• Icy or snow-covered roads decrease your tires' grip on the road, wasting energy.
• Safe driving speeds on slick roads can be much lower than normal, further reducing fuel economy, especially at speeds below 30 to 40 mph.
• Using four-wheel drive uses more fuel.

Although, I am not sure I believe that "Tire pressure decreases in colder temperatures." I thought it was proven that pressure does not go down when it gets cold?

 

[REWahoo]

Although, I am not sure I believe that "Tire pressure decreases in colder temperatures." I thought it was proven that pressure does not go down when it gets cold?

Google Gay-Lussac's Law.

 

[NW-Bound]

"Tire pressure decreases in colder temperatures."

Conversely, air and gases expand when hot, increasing pressure if constraint in the same volume.

 

[NW-Bound]

I never heard of Gay-Lussac's law.

It turns out to be just a special case of the "Ideal Gas Law" where volume is held constant instead of allowing to be arbitrary.

P x V = n x R x T

Where:
P = pressure
V = volume
n = amount of gas (in moles)
R = Avogrado's constant
T = absolute temperature

If V is held constant, then pressure P is directly proportional to T. And that's Gay-Lussac's law.