Does Gas Burn Faster At High Speeds..?

[solaris=amazing]

Lately i've been hitting like 90-95 mph everyday going to work, yeah i know that is pretty fast considering the highway is 55. Buuuuutt once i get on the 135 it's like a dead highway....and nowhere for cops to hang out.

To the point, i notice that along with this driving habit, my gas gauge goes down very quickly, does that mean that higher speeds, say a steady 90 burn gas faster then a steady 55..? I know the worst gas eater is floor it from a stop, but i didn't realize steady speeds at high speeds could burn gas this fast, i gotta put in $15 every other day.

BTW, everything is fine on my baby also, filters/hoses/pcv/plugs/wireset/ba-na-na-nana..

[solaris=amazing]

BTW, my tranny is a 3speed with overdrive, so when i'm at past 90, my trans is in overdrive, and at about 2100-2300 rpms i believe, not exact. So how would that burn gas that fast..?

[BeEfCaKe]

Originally Posted by solaris=amazing

Lately i've been hitting like 90-95 mph everyday going to work, yeah i know that is pretty fast considering the highway is 55. Buuuuutt once i get on the 135 it's like a dead highway....and nowhere for cops to hang out.

To the point, i notice that along with this driving habit, my gas gauge goes down very quickly, does that mean that higher speeds, say a steady 90 burn gas faster then a steady 55..? I know the worst gas eater is floor it from a stop, but i didn't realize steady speeds at high speeds could burn gas this fast, i gotta put in $15 every other day.

BTW, everything is fine on my baby also, filters/hoses/pcv/plugs/wireset/ba-na-na-nana..

To go at those speeds, your engine will be revving higher than it would if you were going 65. Also, tHe faster you go, the more aerodynamics come into play, more power is needed to overcome the drag.

[curtis73]

Aerodynamic drag increases by the square of speed and coefficient of drag. Going twice as fast increases your drag by something like 7.4 times.

[Alastor187]

Originally Posted by curtis73

Aerodynamic drag increases by the square of speed and coefficient of drag. Going twice as fast increases your drag by something like 7.4 times.

Wouldn't the speeds the original poster is referring to be regarded as turbulent flow, so a constant coefficient of drag is applicable?

[curtis73]

You may know more about it than I, but I didn't think coefficient of drag had any bearing on turbulence since CD only refers to frontal area. You are right, though, laminant airflow can only be maintained to a certain speed which I'm sure is different when comparing a Tbird to a Van. Once that delamination happens, drag increases exponentially.

[Alastor187]

Originally Posted by curtis73

You may know more about it than I, but I didn't think coefficient of drag had any bearing on turbulence since CD only refers to frontal area. You are right, though, laminant airflow can only be maintained to a certain speed which I'm sure is different when comparing a Tbird to a Van. Once that delamination happens, drag increases exponentially.

The coefficient of drag (Cd) is a dimensionless value that approximately describes the resistance of a particular shape as it moves through a fluid median. The Cd is a function of frontal area but also, Reynolds Number (Re), Mach Number (Ma), and surface roughness.

Generally we are not interested in calculating the Cd but the actual drag force. In this case the Cd (usually experimentally determined) is a dimensionless value. When multiplied by the frontal area (A), the result is CdA (sometimes called the “Drag Area”) and it has the units of length ^2.

I was asking because in order to get 7.4 times the drag at twice the speed the coefficient of drag would have to increase with velocity. I have always thought the Cd would not be constant with velocity even though it is usually treated that way.

After doing some research it would seem it is not constant for most shapes. However, due to the complexity and cost of determining the Cd over a range of velocities a single approximate Cd is used.

I thought this was a good diagram depicting the unpredictability of the Cd:

[curtis73]

Great stuff, Alastor!!! That graph explains a lot. The 7.4 number was one that I remembered from flight training as a number that we calculated for a certain exercise. I didn't recall that it was dependent on shape, but it could have been once delamination occurs or something.

I just remembered that drag increases considerably more than twice when speed doubles. Hence why a car with 200 hp can go, lets say, 130 mph. If you double the hp to 400, the car can't suddenly go 260 mph.

[solaris=amazing]

Jeez guys, thanks....man you mofo's are smart. I gotta say though, there is a certain peace in going fast...until ofcoarse sh*t happens....lol..

[AlmostStock]

When your explaining to the cop why you were speeding just tell him your almost out of gas and want to get to the station before you run out!
Sorry, I couldn't resist.

[stoned_pimp420]

I've tried that one before, believe me it didn't work! He just looked at me like I was stupid.

[SaabJohan]

Going twice the speed means that the aerodynamic drag will be four times as high since the drag increase on the square of the velocity. This also means that going twice the speed will require about four times the power. The engine will however consume less fuel per kWh produced so fuel consumption will therefore be less than four times as high, but it will still be higher.

As for acceleration you will actually save fuel by accelerating quick and hard up to speed (since otto engines are most efficient at high load).

[AlmostStock]

Originally Posted by SaabJohan

As for acceleration you will actually save fuel by accelerating quick and hard up to speed (since otto engines are most efficient at high load).

More throttle opening means more fuel. If you throttle it hard enough you are also kicking in accelerator pumps and secondary barrels. How can that use less fuel than accelerating gently with light throttle? Have you ever seen MPG (Edit: fuel economy) contests? The winners aren't pulling hole shots off the line!

[v10_viper]

Originally Posted by SaabJohan

As for acceleration you will actually save fuel by accelerating quick and hard up to speed (since otto engines are most efficient at high load).

I once had a theory like this and everyone called me a dumbass for it. But I agree with Saab because if you think about it it makes sense. Now I'm not saying go out there and just floor it. But I think if you only hit the gas at 1/3 throttle and take's you 40 seconds to get to 60 mph, versus you hitting it at 2/3 throttle and it taking you oh...25 seconds to get to 60 mph. Times are kind of slow but I'm just using them. You have to stay in each gear longer, travel farther and I think this uses more gas to accelerate up to speed. But if your going with the 2/3 throttle it takes less time to get up to speed while traveling a shorter distance. Now on some engines are going to be different, such as ones with a 4 barrel carb, but on some fuel injected engines it might be true.

Have you heard of that Venom 400 control unit thingy.(haha) It supposedly will add up to 25% power, "supposedly", during acceleration, because it remaps all that engine shit, but negatively effects your fuel mileage, something I just thought I'd add, I dunno why.

[SaabJohan]

Originally Posted by AlmostStock

More throttle opening means more fuel. If you throttle it hard enough you are also kicking in accelerator pumps and secondary barrels. How can that use less fuel than accelerating gently with light throttle? Have you ever seen MPG (Edit: fuel economy) contests? The winners aren't pulling hole shots off the line!

A better question is "have you seen any fuel economy contests?".

If you have seen, or read about them you would know that they use very small engines, and they use them at or close to full throttle. Sometimes they even use them at full throttle, after which they shut the engine off and lets the car roll (this is similar to how a hybrid uses its engine).

To accelerate a car up to a given speed will take a certain amount of energy, so how can this energy be produced most efficient? At part throttle with a consumption of 600 g/kWh or at about 70% of full throttle where the consumption is around 250 g/kWh?

A modern engine management system gives a little richer mixture when full throttle is given; this is because at the higher manifold pressure "wall wetting" will occur. If the mixture wasn't made richer the engine will run lean and most likely also misfire. When going off throttle the fuel on the walls will vaporize (due to the lower pressure) and a leaner mixture can be given.
A second reason for a richer mixture at high loads is because you need a mixture around 12.5:1 to get maximum power with gasoline, even richer mixtures can sometimes be needed for additional cooling. This is the reason why consumption increase over about 70% throttle.

The reason why otto engines are so inefficient at part throttle is mainly because of the high pumping losses which occur when there is a vacuum in the inlet manifold. Another reason is that the engines internal friction is high compared to the output of the engine. This can cause the fuel consumption to increase several times, as I mentioned above 250 g/kWh is common at high load, where the consumption can be as high as 600 to 800 g/kWh at part load.