gallons per hour / RPM - no load

[1993CivicVX]

Originally Posted by Hondaaccord98

I was wondering how much energy the engine expends just keeping itself alive at different RPM's. I've got a Scangauge so once when the engine was warm I checked GPH (gallons per hour) at various RPM's with the car in neutral. Actually what I did was gradually increase the RPMs and note when the GPH moved to a higher value. This is an imprecise method because the throttle (without load) can't accurately control RPM but none the less here's what I got. This is with a 98 Honda Accord 4 cylinder vtec 5 speed manual:

GPH RPM
0.3 1070
0.4 1180
0.5 1580
0.6 1880
0.9 2900
1.0 3050
1.2 3400


I made a graph out of the results and made it linear. 70 mph is a common speed for me and the engine is turning 2700 rpm there which uses 0.82 GPH (according to the graph) just keeping itself alive. This car gets around 36 mpg at this speed which is 1.95 GPH.
Amazingly 42% of the gasoline burned at 70mph is just keeping the engine alive ... only 58% is used to motivate the car. I would surmise that at higher speeds the percent to keep the engine alive would be a smaller percentage of total gasoline used and at slower speeds it would be a greater percent.
This is the reason burn and glide gives such an increase in mileage ... during the glide portion, with the engine off, this keeping the engine alive inefficiency is eliminated.

But you are not at full throttle all the time when driving 70mph at 2700RPM. When you go down a hill, or when you let off the accelerator to slow up for a car in front of you, you are then just spinning the engine, and from what I've heard, no fuel is being used because when you are above 1500RPM with the foot off the pedal, you are engine breaking which has the fuel injectors not putting any fuel in your engine! Can someone confirm this? So it's not quite 42% to 58%. It's probably more like 36% to 64%.

[91CavGT]

Originally Posted by 1993CivicVX

But you are not at full throttle all the time when driving 70mph at 2700RPM. When you go down a hill, or when you let off the accelerator to slow up for a car in front of you, you are then just spinning the engine, and from what I've heard, no fuel is being used because when you are above 1500RPM with the foot off the pedal, you are engine breaking which has the fuel injectors not putting any fuel in your engine! Can someone confirm this? So it's not quite 42% to 58%. It's probably more like 36% to 64%.

It varies from vehicle to vehicle. My truck for instance will not shut the injectors off unless the rpms are above 3,000 for a few seconds with the truck in gear and with no throttle input. Once the rpms come down to 2,500 though, the injectors are turned back on, accept while in 5th gear. In 5th gear, it will allow the rpms to drop down to 1,800 rpm before turning the injectors back on, and anything over 2,000 rpm for an extended period with no throttle input will turn the injectors off.

[VetteOwner]

Originally Posted by 91CavGT

It varies from vehicle to vehicle. My truck for instance will not shut the injectors off unless the rpms are above 3,000 for a few seconds with the truck in gear and with no throttle input. Once the rpms come down to 2,500 though, the injectors are turned back on, accept while in 5th gear. In 5th gear, it will allow the rpms to drop down to 1,800 rpm before turning the injectors back on, and anything over 2,000 rpm for an extended period with no throttle input will turn the injectors off.

yea thats what im testing now in my truck neutral cosating everywhere vs. engine braking in 5th.(i cant engien brake in 5th fully otherwise i litterally slow down) so i set the cruise and let it use the least ammount of pedal to keep steady speed down a hill. so far results are leaning towards neutral coasting is better.... cuz that bit of pedal at 2300 rpms at 55 is prolyl more than idle of 900

[Mighty Mira]

Originally Posted by 1993CivicVX

But you are not at full throttle all the time when driving 70mph at 2700RPM. When you go down a hill, or when you let off the accelerator to slow up for a car in front of you, you are then just spinning the engine, and from what I've heard, no fuel is being used because when you are above 1500RPM with the foot off the pedal, you are engine breaking which has the fuel injectors not putting any fuel in your engine! Can someone confirm this? So it's not quite 42% to 58%. It's probably more like 36% to 64%.

Even if you are coasting down a hill in gear, with your car using no fuel, you are still slowing down faster than if your car were in neutral. This is chewing up energy. To understand which alternative will ultimately lower your total fuel consumption, you have to understand energy balance issues, i.e. where the energy is going. Understanding that well requires a good understanding of highschool level physics.

Another way to look at it - say you coast down in neutral versus coasting down with the engine on, in gear. After 10 seconds, you will be at a lower speed when the car is in gear, and you will have to put more energy into the pulse (more acceleration) to get back to the original speed.

Of course, if you have to slow down while you are going down a hill because you will exceed the speed limit and don't know the area well enough to slow down at the peak of the hill (or the hill is just too big), or if there is another reason to stop that you couldn't have anticipated, then that's another issue and engine braking or feathering the engine so that no fuel is used may be the best option. But it's not, as a rule.