gallons per hour / RPM - no load
 

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.

I did a similiar simple experiment:
https://www.gassavers.org/showthread....half#post64054

It's only a rough estimate, but at 50mph half my fuel seems to go to just keeping the engine spinning and keeping the muffler warm.

Given that gasoline engines are typically about %25 efficient (%40 for diesels), it is probably a very optimistic guess. Far more fuel is going into creating waste heat than is indicated.

Very interesting.

Also shows why neutral coast P&G helps in cars with auto tranny.
(for those who don't want to shut off the engine)
Anything you can do to keep the RPMs down is a help.

I don't want to drag us into an off-topic discussion of the value of neutral coast vs. coast in D vs. EOC vs. fuel-cut coast in D. Short summary: the less rpms the better.

Is idle fuel always wasted?
 

I have thought about this quite a bit. I think that part of the fuel that was spent just turning things over begins to do work as the engine is loaded. The efficiency increases with load.

My version of displacement on demand runs like this. I have purchased a 7 hp Briggs and Stratton to possibly use on a pusher wheel. It should push the car at 45 mph. With Somender grooves I might get it down to .6 lb/hp*hr bsfc. That would be about 4 lbs of fuel per hour. So the car should run 1.5 hours at 45 mph or get 67.5 mpg. Basjoos beats this with his car's regular engine while going 65 mph. I can of course beat this a lot using P&G.

Now look at what my car has demonstrated several times. 60 mpg at 60 mph. I estimate the car requires 10-12 hp for that speed. This implies that the bsfc is .5-.6 at that load. 6 lbs/11 hp*hr=.54. I have run through these numbers several times over the last couple months. I think my estimates are reasonable. At 60 mph over half of my fuel is used to turn the big motor but if I substitute a smaller motor the fuel savings are negligible unless I can find a much more efficient small motor. (diesel)

At steady speeds lower than 50 mph I do not gain fuel mileage with my car. The engine load is so low the efficiency becomes poor. Basjoos has a flat mpg number between about 35 and 65 mph, if I recall correctly. These sorts of ratiocinations have lead me to wonder if the fuel that is used to turn the engine in neutral is actually the amount of fuel that is wasted when the engine is loaded. I don't think it is.

Another interesting number is to look at the hypothetical efficiency of the engine assuming it was using only the incremental fuel. That would lead to a calculation of 3 lbs/11 hp*hr=.27 bsfc. Of course this is a phony number but this is just a thought experiment.

These sort of thoughts lead me again and again to the conclusion that a plug in hybrid will be near impossible to beat for most reasonable commuting. The 7 hp engine would just be the mule to demonstrate how much fuel could be displaced by a small electric motor. In that proof I would not count the fuel it was using.

Ernie

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.

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.:thumbup: 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
:rolleyes:

Using LPH instead of GPH would have given us almost 4 times the detail next time you are in scientific mode... ;)

I have wanted to do such a test. I have the other side of the eqaution covered.

Ford F-350s can use their ZF-6 to operate a PTO. I never used mine as such but I have the Auxiliary Idle Controller which can very accurately hold a desired RPM.

People do not realize how big engine friction HP can be. This is why numerically lower gearing can help MPG.

Thanks very much for that. It has confirmed what I thought... that fuel consumed to keep the engine ticking over is at least proportional to the rpm. It's roughly proportional, next time measure in lph to get more resolution.



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. I think you are assuming that you are not stuck with the car's gearing here, that you can use any rpm range to cruise at.

In my car for example, fuel used to keep the engine ticking over would probably rise in the middle somewhere, because my car is geared low and I have a slippery car.

e.g. At low speeds, load is negligible, hence the primary load on the engine in steady state is to keep itself ticking over. At say, 50kph, the load has increased somewhat but through keeping it in top gear, I'm able to keep the engine just ticking over as much as at 20kph. From then to maybe 70-80kph, the fuel burn rate is increasing because the engine is working harder to tick over, but the load hasn't yet increased fast. Above that point, the load is increasing rapidly.

Yep. Basically you are keeping the engine in its most efficient zone, and lowering/eliminating rpm otherwise.