gallons per hour / RPM - no load - Fuelly Forums

Android Users - Coming Soon! - Migrating from aCar 4.8 to 5.0

Reply
 
Thread Tools Display Modes
 
Old 08-15-2007, 09:18 PM   #1
Registered Member
 
Join Date: Jun 2007
Posts: 3
Country: United States
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.
__________________

Hondaaccord98 is offline   Reply With Quote
Old 08-16-2007, 04:35 AM   #2
Registered Member
 
skewbe's Avatar
 
Join Date: Jan 2007
Posts: 771
Country: United States
I did a similiar simple experiment:
http://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.
__________________

__________________
Standard Disclaimer
skewbe is offline   Reply With Quote
Old 08-16-2007, 08:55 AM   #3
Registered Member
 
brucepick's Avatar
 
Join Date: Feb 2007
Posts: 722
Country: United States
Location: Connecticut
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.
__________________
Currently getting +/- 50 mpg in fall weather. EPA is 31/39 so not too shabby. WAI, fuel cutoff switch, full belly pan, smooth wheel covers.

Now driving '97 Civic HX; tires ~ 50 psi. '89 Volvo 240 = semi-retired.
brucepick is offline   Reply With Quote
Old 08-31-2007, 08:52 PM   #4
Registered Member
 
Join Date: Oct 2006
Posts: 303
Country: United States
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
__________________
usedgeo
usedgeo is offline   Reply With Quote
Old 11-07-2007, 01:03 PM   #5
Registered Member
 
1993CivicVX's Avatar
 
Join Date: Aug 2007
Posts: 1,066
Country: United States
Send a message via AIM to 1993CivicVX
Quote:
Originally Posted by Hondaaccord98 View Post
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%.
__________________
three stripes the charm!

Car mods are overrated. Just gotta adjust that nut behind the wheel for best mpg.



Forget about World Peace...Visualize using your turn signal.
1993CivicVX is offline   Reply With Quote
Old 11-19-2007, 07:34 PM   #6
Registered Member
 
Join Date: Jul 2007
Posts: 163
Country: United States
Send a message via Yahoo to 91CavGT
Quote:
Originally Posted by 1993CivicVX View Post
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.
__________________
91CavGT is offline   Reply With Quote
Old 11-19-2007, 09:21 PM   #7
Registered Member
 
VetteOwner's Avatar
 
Join Date: Mar 2007
Posts: 1,546
Country: United States
Quote:
Originally Posted by 91CavGT View Post
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
VetteOwner is offline   Reply With Quote
Old 11-20-2007, 05:42 PM   #8
Registered Member
 
Join Date: Aug 2007
Posts: 313
Country: United States
Using LPH instead of GPH would have given us almost 4 times the detail next time you are in scientific mode...
__________________
2TonJellyBean is offline   Reply With Quote
Old 11-29-2007, 03:57 PM   #9
Registered Member
 
Join Date: Nov 2007
Posts: 125
Country: United States
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.
__________________
2000 Ford F-350 Super Cab Pickup
4x2, 6 speed manual
Regeared to 3.08:1
4 inch suspension slam
Aero mods: "Fastback" fairing and rugged air dam and side skirts
Stock MPG: 19
Summer MPG: 27.0
Winter MPG: 24
Big Dave is offline   Reply With Quote
Old 11-30-2007, 12:49 AM   #10
Registered Member
 
Mighty Mira's Avatar
 
Join Date: Jun 2006
Posts: 315
Country: United States
Quote:
Originally Posted by Hondaaccord98 View Post
I was wondering how much energy the engine expends just keeping itself alive at different RPM's.
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.
Quote:
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.
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.

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

Mighty Mira is offline   Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
drivetrain resistance bagpipe goatee General Fuel Topics 9 09-14-2008 10:04 PM
Fuel Heaters GasSavers_Red General Fuel Topics 31 12-14-2006 12:13 PM
Grill Block 88 CRX UfoTofU Experiments, Modifications and DIY 11 11-11-2006 02:56 PM
Are CRXs easy to break into? GasSavers_Lance General Maintenance and Repair 20 09-20-2006 10:56 PM
BMW C.L.E.V.E.R. Concept MetroMPG General Fuel Topics 4 04-24-2006 11:59 PM

Powered by vBadvanced CMPS v3.2.3


All times are GMT -8. The time now is 03:55 AM.


Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.