How To Increase Your Gas Mileage

wstar · 04-07-2009, 01:14 AM

Well, if he's right about the fuel getting cut during engine braking, then yes engine braking would save more gas than coasting. However, I find this hard to believe. Cut fuel = no more combustion. The drivetrain would simply be spinning the engine and sucking air through the pistons for compression braking, but without any ignitions happening (which means really they could cut the spark plug current off too). I suppose in theory this sounds possible, but I would think I would hear the difference between an engine that's undergoing combustion and one that isn't, and I would think the transition between the two states would be rather abrupt and noticeable. Sure seems pretty smooth to me.

I'm not saying it isn't true, I'm just saying I find it hard to believe

ChrisSlicks · 04-07-2009, 11:28 AM

The mechanical noise of all the engine parts moving far exceeds any noise the actual ignition makes.

In full lift off the injectors do not fire (there may be no spark either, but I'm not sure). It matters not the gear or the RPM's as long as the engine is above idle speed. As you approach idle the injectors will begin firing again to prevent stall.

You do get great mileage in neutral as well because idle fuel burn is minimal and there are times where that is going to save more fuel than lift off if you can do it for longer. But if you're going down hill you absolutely should be in gear.

monthtrial · 04-08-2009, 12:41 AM

Theory and engine mechanics are one thing... real-life dynamics and physics are another. With an engine stationed @ a laboratory, I can see that an engine will conserve fuel by being in gear, however, not on our highways...

What I'm getting at is that the gear you're in makes a difference! To elaborate, lets assume we're coasting down a hill. Without using the gas pedal whatsoever, first gear will be less efficient to coast in than 2nd gear, than 3rd, gear, etc.... this is due to the ability to gain greater speeds. Even coasting in 6th gear will eventually reach the point where the car will NOT gain more speed. Popping the car in neutral in this case SHOULD result in increased speed (since the engine RPM's don't hold the car at a steady speed) and this should result in more distance covered in less time (speed). This is inherently what fuel mileage equations take into account...

So as I see it, when variables of physics are thrown into the equation, the whole concept of fuel savings needs to be reevaluated.

ChrisSlicks · 04-08-2009, 08:40 AM

Quote:

Originally Posted by monthtrial

Theory and engine mechanics are one thing... real-life dynamics and physics are another. With an engine stationed @ a laboratory, I can see that an engine will conserve fuel by being in gear, however, not on our highways...

What I'm getting at is that the gear you're in makes a difference! To elaborate, lets assume we're coasting down a hill. Without using the gas pedal whatsoever, first gear will be less efficient to coast in than 2nd gear, than 3rd, gear, etc.... this is due to the ability to gain greater speeds. Even coasting in 6th gear will eventually reach the point where the car will NOT gain more speed. Popping the car in neutral in this case SHOULD result in increased speed (since the engine RPM's don't hold the car at a steady speed) and this should result in more distance covered in less time (speed). This is inherently what fuel mileage equations take into account...

So as I see it, when variables of physics are thrown into the equation, the whole concept of fuel savings needs to be reevaluated.

Yes, the compression of the engine will slow you down, and the gear you are in will determine how much it will slow you down. You will have used the same amount of fuel getting to the bottom no matter what the gear (zero as long as the hill is steep enough) but your speed at the bottom will obviously be very different depending on how much engine braking there was. You will go further and faster in neutral which means you could potentially save more fuel that way in certain circumstances as you will have more inertia and hence wont need to re-accelerate up the next hill as much.

dad · 04-08-2009, 12:53 PM

Quote:

Originally Posted by wstar

Well, if he's right about the fuel getting cut during engine braking, then yes engine braking would save more gas than coasting. However, I find this hard to believe. Cut fuel = no more combustion. The drivetrain would simply be spinning the engine and sucking air through the pistons for compression braking, but without any ignitions happening (which means really they could cut the spark plug current off too). I suppose in theory this sounds possible, but I would think I would hear the difference between an engine that's undergoing combustion and one that isn't, and I would think the transition between the two states would be rather abrupt and noticeable. Sure seems pretty smooth to me.

I'm not saying it isn't true, I'm just saying I find it hard to believe

There is a distinct change in pitch between accelerating and coasting.

wstar · 04-08-2009, 01:48 PM

Quote:

Originally Posted by dad

There is a distinct change in pitch between accelerating and coasting.

Agreed. I just find it hard to believe that what my ears are hearing during engine braking is just restricted airflow pumping through the cylinders with no combustion.

dad · 04-08-2009, 01:55 PM

Quote:

Originally Posted by wstar

Agreed. I just find it hard to believe that what my ears are hearing during engine braking is just restricted airflow pumping through the cylinders with no combustion.

It can get mind bogging, and complicated to explain and understand.

ChrisSlicks · 04-08-2009, 02:01 PM

Quote:

Originally Posted by wstar

Agreed. I just find it hard to believe that what my ears are hearing during engine braking is just restricted airflow pumping through the cylinders with no combustion.

Back in the early 80's (before FI was common) they sold mod kits that did this. There was an inline fuel switch, an RPM sensor and a throttle switch. When you lifted off fuel was cut to your carburetor provided you were above the preset stall speed. It's standard practice on modern cars with fuel-injection and manual transmissions.

Automatics don't always implement this as the transmission must be in full lockup mode for it to work.

04-07-2009, 01:14 AM	#1 (permalink)
wstar A True Z Fanatic Join Date: Mar 2009 Location: Houston, TX Posts: 4,024 Drives: too slow Rep Power: 3595	Well, if he's right about the fuel getting cut during engine braking, then yes engine braking would save more gas than coasting. However, I find this hard to believe. Cut fuel = no more combustion. The drivetrain would simply be spinning the engine and sucking air through the pistons for compression braking, but without any ignitions happening (which means really they could cut the spark plug current off too). I suppose in theory this sounds possible, but I would think I would hear the difference between an engine that's undergoing combustion and one that isn't, and I would think the transition between the two states would be rather abrupt and noticeable. Sure seems pretty smooth to me. I'm not saying it isn't true, I'm just saying I find it hard to believe __________________ 7AT Track Car! Journal thread / Car setup details

04-08-2009, 12:41 AM	#3 (permalink)
monthtrial Base Member Join Date: Jan 2009 Location: Fullerton Age: 48 Posts: 46 Drives: '13 370z Rep Power: 17	Theory and engine mechanics are one thing... real-life dynamics and physics are another. With an engine stationed @ a laboratory, I can see that an engine will conserve fuel by being in gear, however, not on our highways... What I'm getting at is that the gear you're in makes a difference! To elaborate, lets assume we're coasting down a hill. Without using the gas pedal whatsoever, first gear will be less efficient to coast in than 2nd gear, than 3rd, gear, etc.... this is due to the ability to gain greater speeds. Even coasting in 6th gear will eventually reach the point where the car will NOT gain more speed. Popping the car in neutral in this case SHOULD result in increased speed (since the engine RPM's don't hold the car at a steady speed) and this should result in more distance covered in less time (speed). This is inherently what fuel mileage equations take into account... So as I see it, when variables of physics are thrown into the equation, the whole concept of fuel savings needs to be reevaluated. Last edited by monthtrial; 04-08-2009 at 01:04 AM.

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04-07-2009, 11:28 AM	#2 (permalink)
ChrisSlicks A True Z Fanatic Join Date: Mar 2009 Location: North East Posts: 6,203 Drives: 09 370Z Sport M6 Rep Power: 654	The mechanical noise of all the engine parts moving far exceeds any noise the actual ignition makes. In full lift off the injectors do not fire (there may be no spark either, but I'm not sure). It matters not the gear or the RPM's as long as the engine is above idle speed. As you approach idle the injectors will begin firing again to prevent stall. You do get great mileage in neutral as well because idle fuel burn is minimal and there are times where that is going to save more fuel than lift off if you can do it for longer. But if you're going down hill you absolutely should be in gear.