# Speed and rate of combustion (in car)

1. Jan 15, 2004

### thunderfvck

Hi.
Let's suppose you have to drive 100 km, you would be wasting more gas by:
a)driving at a slower speed
b)driving at a faster speed
.
Someone who adores cars told me that when you press on the accelerator all you're really doing is allowing more air into the combustion chamber/cylinder/whatever it is, and thereby eating more gas to build and/or maintain speed. So, if you're eating less gas/time but taking your sweet time getting to point b and your friend is burning a whole lot of gas and getting there in less time, who will have burned more gas?

2. Jan 15, 2004

### chroot

Staff Emeritus
Cars actually are most efficient at one specific speed. Most automakers design their vehicles to be optimized for 55 mph.

- Warren

3. Jan 15, 2004

### Chemicalsuperfreak

It depends on what gear you're in. You burn the most gas when you're accelerating or going up the hill, basically when you've got the most rpms. That's why mileage in the city goes way down. You have to accelerate at the stop lights everytime the light turns green. When you're driving on the freeway you're in a high gear with low rpms. You can maintain your speed without steping on the gas to much. That said, if you're speeding and you've got the pedal to the floor you're loosing a whole lot of gas. It's easier to see if your car as a tachymeter.

4. Jan 15, 2004

### chroot

Staff Emeritus
But of course keep in mind that rpms and fuel consumption are not directly related. Your fuel consumption is also dependent upon the position of the accelerator pedal and thus the position of the throttle vane. The engine computer adds the appropriate amount of fuel for the air entering the engine. Turning the engine fast only burns more fuel when the throttle is also open. You can run your engine very fast while still injecting little fuel. The trade off, of course, is that the loss due to friction of pistons against cylinder walls is greater at greater rpms. People use this fact to slow their cars when going down a hill -- downshift, let the engine run faster, and thus experience more friction, and leave your foot off the gas. With the throttle closed, the engine is not burning any fuel at all.

- Warren

5. Jan 16, 2004

### rody084

i have been curious about this for awhile... now is it completely related to the throttle postition?

ex: would running the car with ur foot off the pedal down a hill consume no gas?

6. Jan 16, 2004

### chroot

Staff Emeritus
That is correct. I used that specific example in my last post. A fuel-injected car would not burn any gas at all while cruising down a hill in gear (so that the wheels keep the engine turning).

- Warren

7. Jan 16, 2004

### rody084

so then would it be correct to also say that you are creating no emissions?

so by the friction caused bythe pistons against the cylinders...were u refering to that as the force which is slowing down the car?

8. Jan 16, 2004

### chroot

Staff Emeritus
Yes. You're not burning any fuel.
Among other kinds of friction. Engines, transmissions, air intakes, wheel bearings, and even tires contribute friction losses.

- Warren

9. Jan 18, 2004

### firefly

Hmm. I have trouble with this NO fuel at all bit. It seems to me the car would burn a very small amount of fuel, but still > 0.

Say I am parked, on level ground, car off. I disengage clutch and turn car on. Now the car is at rest, but ON, and quite surely burning fuel at "idle" - rpm are very low, but > 0. If I engage the clutch (NO throttle) the car will stall (even if i'm a clever clever clutcher ). Q1: Isn't this because the engine (being the only power source) "tries" to turn faster but fails because the idle supply is insufficient, or...?

Now, if the car is on, going down a hill, brakes off, throttle closed, clutch engaged but gravity providing most of the energy for "pulling" the car downward and turning the engine over. Q2: Wouldn't the engine STILL consume AT LEAST what fuel it would at idle?

Q3: Is there some mechanism I am not aware of that, once the car is in motion, the computer lowers the "idle" fuel quota to zero?

Q4: Is there a difference in the fuel provision in fuel injected vs carbureted engine?, in so far as this 0-fuel behaviour?

Thanks in advance for your patience. Look forward to clarification, or corrections of my misunderstandings - I am no mechanic!

firefly

10. Jan 18, 2004

### rody084

thats a good point...I think that when you are at idle and the car is not moving..then the ECU is tellin it to inject sum air/fuel to keep it going and not stall otu... but when you are going down a hill, where there is no worry of stalling out...then maybe the ECU isnt injecting any fuel

Carburated engine works by sucking in fuel with the air intake... air intake sucks in fuel instead of throttle position. throttle position only tells it how much air to suck in... so therefore at cruising speed then there would b alot of air coming in, so it owuld b automatically sucking in air.

i kno my explanations arent very good...lol... can someone else confirm what i am saying? because i basically just made this up right now

11. Jan 18, 2004

### chroot

Staff Emeritus
No.
That's a quite different circumstance than rolling down a hill in gear.
No. The engine computer will not be injecting any fuel at all. The engine is already turning at greater than idle speed.
Q3: Is there some mechanism I am not aware of that, once the car is in motion, the computer lowers the "idle" fuel quota to zero?[/b][/quote]
There is no "idle quota." The engine computer simply injects enough fuel to keep the engine turning at least, say, 1000 rpm, in a closed loop. If you rev the engine and then take your foot off the gas, you'll notice the rpm's falling briefly below idle -- undershoot -- and then the engine computer pulling the rpms back to idle. When the wheels are turning the engine faster than idle (by going down a hill in gear), the engine computer does not inject any fuel at all, since it doesn't need to.
Yes. A fuel-injected car can turn off its fuel injectors. A carbureted car, however, is always burning some fuel. The amount of fuel is determined by the air flow through the carburetor. This is what a carburetor does: it mixes a specific volume of fuel with a specific volume of air. You can't turn it off.

- Warren

12. Jan 21, 2004

### rockind78

The amount of gas you burn at any given time depends on a lot of factors, but only (ultimately) one you can control while you are in the cabin (unless you are using a direct link to a fuel injected car with aftermarket software)is the throttle position. GENERALLY speaking, driving faster versus slower DOES burn more gas. Specifically speaking, an engine is most efficient at peak torque, and if you look at most moderately priced cars, the torque band is from usually just off-idle to right around 5500 RPM's (this varies a lot with engine configuration, pushrod versus OHC, camshaft design, etc.) So, the closer you are to the torque peak in a factory built passenger car, the more efficient your engine is. Gearing is a way to keep in the engine in the desired powerband.

As far as driving downhill goes, you WILL be using gas. There is no way around it. Your engine is either running, or it isn't. If it isn't, then energy is drawn from the battery to the starter to turn the motor over and start it. If this process isn't occuring, then you MUST have gasoline being injected into the engine to keep the engine running. If there is no gasoline being injected in, then you would need to restart the car at another time. If you need any proof, pickup some scanning software such as AutoXray or Diacom (domestic cars, I dunno about foreign, but the principle still applies) sometime. They will give you all the info you need when driving down the hill.

13. Jan 21, 2004

### chroot

Staff Emeritus
Wrong. With what part of my previous explanation do you disagree?

- Warren

14. Jan 21, 2004

### rockind78

Basically just the part about the engine not needing any fuel at all. There are a lot of reasons I disagree, but I will just name one for now.

What you are describing is essentially "displacement on demand". A neat concept developed in the mid-late '70's by Cadillac in which the theory was that an engine did not need to use all eight of its cylinders at cruise down the highway. The application failed because the technology wasn't there. The technology is now here, but only possibly in the newest cars(and I am not even sure about that). The theory remains the same: shut off fuel to some of the cylinders to save gas when they aren't needed. Now, if we are gonna discuss this further, I think maybe we should state which automotive computers we are talking about. If you are talking about brand new stuff (2004+), I will concede that it MAY be possible. If so, I am interested in hearing what sensors specifically measure the engine load and turn off the injectors, as well as how its done, because I am always interested in learning more stuff about cars. If we are talking about OBD I, I am standing firm on my position.

Last edited: Jan 21, 2004
15. Jan 21, 2004

### chroot

Staff Emeritus
OBD I is ancient. You are correct that the OBD I computer has a very simple fuel map, and does not have many smarts.

There is no "engine load sensor." It's called a tachometer. When the engine is being turned mechanically by the wheels, as happens when cruising down a hill in gear, the control system does not need to inject any fuel to keep the engine turning. It's simple. The throttle is closed, so the computer will go into closed-loop monitoring of the rpm. As long as the rpm is higher than idle, no fuel will be injected. My car, for example, will run at 2000 rpm idle when warming up, then automatically begin running at 1000 rpm idle when warm. My car, btw, is OBD II.

- Warren

16. Jan 21, 2004

### rockind78

Well, I wasn't aware that any necessary readings measured by the computer were taken from the tach. Unless I am mistaken, the amount of fuel metered by the ECM for the engine is based on MAF, TPS,, CTS, and IAT/MAT readings. In your scenario, the engine would still be pumping air to some degree, despite the closed throttle angle.

The only way I could see to substantiate this claim would be to get MAF and O2 sensor readings during a downhill cruise. In fact, when you examine studies from emmissions testings, there are still emmissions given off, even during dowhill driving.

Last edited: Jan 21, 2004