yes, but the question is:

in what circumstances is it more efficient to coast with the car idling in neutral -vs- doing engine braking but without any fuel usage?

(remember: engine braking slows the car, and that inertia will have to be regained, thus using fuel)

I have a feeling that coasting to an anticipated stop is better than using engine braking, but when going down hill, engine braking is the best bet (for many reasons, incl. which it isn't good to ride the brakes downhill).

this is all academic anyway-- we're talking about negligible amounts of fuel usage, but I'm curious as to how the #s work out, if it can be calculated for certain fixed circumstances anyway.

 

Can you elaborate? I may be misreading this. At what point would you be using fuel that you wouldn't use otherwise?

 

Yeah, I was kidding. In the long run, the only time leaving it in gear would get you farther on less fuel is going downhill, where the engine braking would be counteracted by gravity.

 

I think what tritium is saying is that if you engine brake then you will deccelerate faster than if you were to coast in neutral. Hence if you engine brake to a queue that begins moving before you join it then you will have slowed down more than if you had've coasted resulting in more acceleration needed to reach the speed that you would have been doing if you had of coasted.

If this is what you are referring to Tritium then wouldn't placing it in a higher gear achieve the benefit of increased mileage whilst allowing more speed to be carried in case of a restart situation?

 

yes, that's pretty much it. if you have the car in gear, it will slow down that much faster than if you were just coasting in neutral, and to regain your speed back up to the speed limit (ya right), you'd have to give that much more gas. I believe that the gas you saved by engine braking will be lost when you have to accelerate more afterwards.

putting it in a higher gear-- like 6th-- to slow down and save fuel wouldn't work either though, because the engine only cuts fuel when it's above a particular RPM (1500 I believe, according to an earlier post) and is decellerating. put the car in 6th, and it'll drop down to 1500 RPM pretty quickly and thus, be burning fuel again. above 1500 rpm or in a different gear, the engine (and drivetrain) is still decellerating the car more quickly than if you just had it in neutral.

 

let's use 2 examples for analysis, and let's assume no traffic and perfectly level ground:

1) approaching a stale red light (can turn green at any time), going at whatever speed. do you put it in neutral and coast? or keep the foot on the gas and then engine brake when you get closer? or put it in a high gear and sorta engine brake?

2) approaching a stop sign. coast from farther away? or drive up a lil bit closer then engine brake when u get closer? or do some funky shift into a higher gear thing?

my belief: coasting in neutral in both circumstances is probably the best bet, but I'm not certain and am open to opinions. of course, there's so many variables (initial speed, distance from red light or stop sign, etc.) that the calculations can get complicated quickly. but anyway, that's what makes em fun.

 

Hmmmmm, I'm not a fan of coasting (or freewheeling). This kinda takes the car out of the driver's control. You just never known when an emergency acceleration (even to a limited extent) might be required. Having it in the correct gear for the correct speed is my prefered method. For me it's not even an issue of which one (clutch or brakes) is cheaper to replace.
Although the fuel issue (savings) being discussed in this thread is a fun exercise, it has never been a consideration of mine when it comes to car control.

 

^ come to California-- specifically the Bay Area-- and see how much we pay for gas. you'll think twice about the fuel saving issue. West Coast = coast to a stop (for me anywayz). refueling expenses definitely take their cut outta my 401K donations.

 

[QUOTE]Originally posted by tritium_pie
^ come to California-- specifically the Bay Area-- and see how much we pay for gas.

 

[QUOTE]Originally posted by ellisnc