4

Quote :

"coasting with the engine off."

That just seems like a bad idea on so many levels.

[Edited on July 8, 2008 at 8:16 AM. Reason : .]

7/8/2008 8:13:37 AM

It has to save gas to coast in neutral. If i'm going down a long hill and would normally be at like 3000 rpm, then switching out of gear brings me down to like 200rpm or whatever.

7/8/2008 9:46:46 AM

But in neutral your engine is still on, it's just in neutral.

7/8/2008 9:53:26 AM

yeah but in gear the engine could be going at 3500 RPM whereas in neutral it might only be going a few hundred RPM.

7/8/2008 9:55:32 AM

I'm not disagreeing, I'm just saying that's different from

Quote :

"coasting with the engine off."

7/8/2008 9:59:51 AM

oh yeah sorry i was just saying that in general.

7/8/2008 10:05:19 AM

I started doing this to a degree just to save money, didn't realize there was a whole following.

Got 37 MPG driving to Lake Norman last weekend in a Civic SI, rated 28 highway. Basically just drove 67 in 70 zones, 63 in 65 zones, and 58 in 55 zones the whole way, with cruise control always on. Just used A/C periodically.

Taking the two business 40 options through Greensboro and Winston is less miles and lower speed limits, so that's a double bonus right there.

7/8/2008 10:40:30 AM

Quote :

"It has to save gas to coast in neutral. If i'm going down a long hill and would normally be at like 3000 rpm, then switching out of gear brings me down to like 200rpm or whatever"

Negative. if you are in neutral, you are using gas to turn the engine. With modern EFI, the injectors stop firing when the engine is being driven by the car's inertia (i.e., coasting)...so you are using MORE gas to coast in neutral.

now, some of that is negated by the extra speed you will maintain or build by coasting in neutral. my guess is that if you really wanted to gnat's ass it, there would be certain scenarios where each would be advantageous. in general, though, there's no point in shifting to neutral to coast down hills.

also, my BMW had a fuel mileage guage...it registered noticeably better economy by coasting in gear rather than in neutral (or clutch in).

Quote :

"I took statics as well. But inertia does play a part. There's theoretical, then there's real world factors. "

Quote :

"Stopping distance is based soley on speed and friction coefficient"

there's a lot more than statics at play when it comes to stopping distances. Inertia is dependant on speed, as well (the square of speed, actually)...but yes, mass (the other component of inertia) does play a role in stopping distances.

Quote :

"I followed a hypermiler home from work today. I wanted to kill them. "

Seriously. Fuck these fucking retards. Driving conservatively, accelerating gently, easing off the gas early if you know you're gonna have to stop...those are ok. These more extreme tactics are fucking stupid.

7/12/2008 5:07:42 AM

the extreme stuff is just plain dangerous

7/12/2008 10:25:57 AM

^^I don't do it, and don't doubt you i'm just wondering why that is.

So you're saying running at 3000rpm for 30 seconds uses less gas than running at 300rpm for 30 seconds?

-oh ok that makes sense. During "deceleration" when the throttle is closed, the injectors cut off fuel to the engine? But it still seems weird that you'd be using more gas idling in neutral than running at like 3000 rpm going down a hill.

[Edited on July 12, 2008 at 11:14 AM. Reason : ..]

7/12/2008 11:08:31 AM

ps idle is more like 600-1000 depending on engine size and fuel type

7/12/2008 11:37:38 AM

^^when rolling down a hill in gear, the car's inertia is keeping the rpms at 3000, not fuel being put into the engine

you're making the mistake of thinking that engine speed (rpms) is directly related to the amount of fuel being put into the engine, but (for all practical matters) the only thing that decides how much fuel is being put into the engine is the position of the gas pedal

7/12/2008 11:52:12 AM

It's not too hard to get better gas mileage without doing all these crazy things (drafting behind trucks, turning off your car while it's still moving, etc.).

All's you gotta do is
1) Accelerate gently
2) Don't let your car idle over a minute (instead of waiting in a long drive-through line, just park and go inside. It's faster anyways).
3) Try to stay in your optimal speed range (varies from car to car).

I agree, anyone who takes hypermiling too far is just causing danger for themself and others.
Found this mess here:
http://www.ecojoes.com/how-to-increase-your-car-mpg/

7/12/2008 1:35:45 PM

^^ok that makes sense. I know the throttle has more to do with the amount of gas used than the rpms. Guess i wasn't making the connection.

7/12/2008 4:43:09 PM

take the internal combustion engine class

[Edited on July 12, 2008 at 5:08 PM. Reason : 

7/12/2008 4:54:29 PM

Took the internal combustion class. Learned a lot. Been a mechanic at three different shops. Learned a lot more practical information and how everything comes together in the real world. Been working on cars for more than 20 years, and I NEVER take anything I have to a shop. I don't trust anybody but myself and a select few to work on anything I own.

All that said, there's a lot to argue for some of the hypermiling techniques, as long as you don't pose a hazard to yourself or others on the road.

1. Pulse and glide is no bullshit, especially if you time your pulses and coasting to match terrain, and you accelerate gently. Fuel consumption is a function of a lot more than just rpms or throttle position. As far as the engine goes, much of the load interpolation is done using the MAP (manifold absolute pressure) sensor, and gently accelerating produces much less of a vacuum drop than hard acceleration. It's also not THAT different than maintaining a constant speed. Engine uses a LOT less fuel idling than even maintaining "station-keeping" speeds on the highway. You do end up using less fuel.

2. Driving slower (though keeping the engine/powertrain in a sweet spot) consumes less gas, not just because of rolling resistance or friction, but because of aerodynamic conditions. Aerodynamic drag increases exponentially with road speed, and unless you're driving something really aerodynamically clean, it makes a hell of a difference in fuel consumption.

3. I don't see any harm in coasting on long grades, as long as you keep the engine running and the tranny in neutral. You actually do risk harming the transmission on either automatic or manual transmission cars with the engine stopped and coasting in neutral. In manuals, the countershaft may not spin and sling oil to the upper bearings. In an automatic, the transmission's fluid pump isn't providing vital lubrication to critical areas of the transmission.

So why coast in neutral with the engine running instead of just letting up on the gas (which everybody who's been reading this thread knows by now cuts the injectors out entirely)? Two things: pumping losses and compression braking. Your engine is still pulling a lot of vacuum on the intake side, creating resistance to pumping. It's also still compressing what air charge is making its way into the engine, creating more resistance. I think the small amount of fuel that the engine uses to maintain idle speed in coasting circumstances more than offsets the losses while decelerating in gear and losing vehicle speed at a much faster rate.

7/12/2008 9:54:20 PM

49.1mpg

330 miles of 7 mile trips.

Not bad

7/14/2008 8:52:32 AM

^^ Well said and ^ well done

7/14/2008 10:00:23 AM