# The Naked Scientists Forum

### Author Topic: Gas consumption questions  (Read 5832 times)

#### narasimeena

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##### Gas consumption questions
« on: 11/09/2006 14:37:25 »
Hello Naked Scientists!
I enjoy your show and the hosts to a great extent and listen to your Podcast every week.
I have an interesting question for you guys and I hope you can come up with a scientific deduction to this myth.
My colleagues at work claim that a car would consume more gas when it is left in idle position (no A/C running, no one is pushing the gas pedal etc) as opposed to when it is running or in motion.
To simplify this questions lets say if a car is running or in motion  for 4 hours at say 45 miles/hr and another similar car is at a state of rest with its ignition on which would consume more gas (the conditions will be identical to maximum extent possible like make& model, condition etc)? Is there a formula to prove that the one is rest consumes less and the other more gas.
In the past I have seen that I have had little mileage changes when I had let my car run idle while I took short naps during long trips at rest areas along the highway. So my hypothesis is that a car in idle position with its ignition on consumes less gas as opposed to a car in motion. As there is no work done as displacement is 0 and also there is not mass involved while car is at a state of rest. Using these principles I believe my hypothesis is correct.
Gainesville, FL
Go Gators!
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« Last Edit: 28/06/2008 08:25:48 by narasimeena »

#### Karen W.

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##### Re: Gas consumption questions
« Reply #1 on: 11/09/2006 15:36:00 »
Welcome Narasi,
I am sure someone will come to answer your Question soon. I just want to thankyou for listening to the podcasts and make your self comfortable and enjoy the forum.. Pleased you have enjoyed the podcasts!

Karen

#### Carolyn

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##### Re: Gas consumption questions
« Reply #2 on: 11/09/2006 16:08:25 »
Hi Narasi - Welcome.  I've been told the same thing and don't see how this is possible.  Hopefully someone will come along and set us straight.

Carolyn
Eastpoint, FL (originally from Tallahassee, FL)
GO NOLES!!!

#### chris

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##### Re: Gas consumption questions
« Reply #3 on: 11/09/2006 18:47:50 »
When a car moves it is doing work against air resistance.

For this reason you have to keep supplying energy (by burning fuel) to keep the speed constant. If this were not the case then you'd be able to turn of the engine once the car was in motion and it would keep going at the same speed.

When a car is stationary (not moving) it is not doing any work against air-resistance. But there are still energy loses - the engine is driving a gearbox, an alternator, and itself. These processes all consume energy, so the engine burns fuel, albeit at a lesser rate, even when it is just ticking over.

So you're correct. A car in motion at a constant or increasing speed is consuming far more fuel than a car at stand-still, although the stationary car is still consuming fuel (assuming the engine is running!).

Chris

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#### Soul Surfer

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##### Re: Gas consumption questions
« Reply #4 on: 12/09/2006 00:01:10 »
It is reasonably obvious that a car that is travelling uses much more fuel per hour than one that is stationary with its engine runnig at idling speed.  firstly the engine revs  for a travelling car are uasually considerably higher when it is travelling.  Secondly the engine is doing more work.  and finally you have to push the accelerator (gas) pedal down to let more fuel in to achieve this.

A car that is travelling at 30mph and  consuming 30Mpg is using one gallon per hour of fuel. in contrast a car that is just idling will use about one pint per hour.

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#### another_someone

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##### Re: Gas consumption questions
« Reply #5 on: 12/09/2006 01:52:34 »
In general, a car in motion has to do more work than one that is stationary.  There are exceptional situations, such as when a car is coasting downhill, when it may actually use less fuel than when it is stationary (but you make up for it in the fuel needed to travel uphill).

George

#### David_D

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##### Re: Gas consumption questions
« Reply #6 on: 13/09/2006 04:59:17 »
It is not a pure function of work performed; it is a function of tuning.  You could run a car ultra lean at cruise and ultra rich at idle, but no factory would send a car out that way because it would fail emissions.

If you look at a standard fuel map you will find that the injector duty cycle is lowest at idle.  A car at idle has the engine turning at a lower RPM and there are obviously fewer injector cycles. At a given air/fuel ratio the injector on time will be about the same for each cylinder firing regardless of RPM. On most factory cars the AFR is a little leaner at idle than under light cruise.

However, some modified turbo cars are tuned to run very rich at idle because fuel is not well atomized at low airflow, because the cams are not optimized for low RPM and because a richer mixture gives a smoother, cooler idle and provides enough torque to run the accessories.  At slightly higher RPM (under partial throttle at light cruise) those same cars may be set to run leaner.  Again, it depends on how you tune.  Also, when driving, you will have periods of deceleration where the fuel is cut.  So yes, there may be rare cases where a car burns more fuel at idle.  Under any appreciable load you will generally burn much more fuel and fuel pressure will be higher too.

The bottom line is to look at a duty cycle map.  Clearly, if the injector is on for less time and the fuel pressure is lower then you are burning less fuel.

#### moonfire

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##### Re: Gas consumption questions
« Reply #7 on: 13/09/2006 05:33:03 »
quote:
Originally posted by Carolyn

Hi Narasi - Welcome.  I've been told the same thing and don't see how this is possible.  Hopefully someone will come along and set us straight.

Carolyn
Eastpoint, FL (originally from Tallahassee, FL)
GO NOLES!!!

Did you mean those ole moles?  Just kidding![}:)]

"Lo" Loretta

#### moonfire

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##### Re: Gas consumption questions
« Reply #8 on: 13/09/2006 05:34:56 »
Is there a difference with hybrid on electricity?  Or same point on this matter?

"Lo" Loretta

#### daveshorts

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##### Re: Gas consumption questions
« Reply #9 on: 13/09/2006 14:33:30 »
A hybrid works by using a small petrol engine to charge a battery which is then used to move the car around, this means that the engine can run at it's most efficient speed, or not at all. So if the car is in 'idle' the engine is either working quite hard to charge the batteries or if they are full it will be off. The difference with a hybrid is that if the engine is running at idle it is doing something useful, so the petrol isn't just wasted.

#### The Naked Scientists Forum

##### Re: Gas consumption questions
« Reply #9 on: 13/09/2006 14:33:30 »