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Old 11-30-2007, 01:50 AM   #11
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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%.
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.
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Old 11-30-2007, 09:21 AM   #12
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Looks like it WILL take that much fuel at those speeds to start producing usable power to the wheels and YES it does help to have gearing which lowers your RPM to minimize these losses. Often however when the engine is really warmed up and running at higher RPMS for a while, these numbers will drop a bit as it can be a function of engine oil temperature. Going to have to run this test on my xB.
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Old 11-30-2007, 09:56 AM   #13
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Originally Posted by skewbe View Post
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.
25% is peak efficiency. That's usually 75-90% demand of peak power at the torque peak or just before it. On the freeway I'd be surprised if you saw anything above 15-18% just cruising.
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Old 12-05-2007, 09:12 AM   #14
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As a general rule, a vehicle will burn a 1/4 tank in 3 hours of idling.

And yes, the higher rpm's is why they started making 6 speed and double o/d trannies.
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Old 12-05-2007, 04:37 PM   #15
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NO WAY!! not at a 0.1gph idle burn rate.

Funny thing happened last night 0 was parked in a friends driveway waiting for her to come home - she didn't - and noticed that the engine was idling nicely at about 650rpm with the heater fan off. I turned the heater up and the RPMs increased and when on the highest fan setting the idle ran up to 1360rpm. It varied with electrical load of the alternator and maybe from the demand for heat for the heater since it was cold outside. I did not expect that RPM to climb up that much however and the fuel rate rose to 0.3gph also.
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Old 12-06-2007, 03:43 PM   #16
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Ok was sitting in a cold car with a warmed up engine after a 1.5 mile trip and decided to take some measurements. Temperature initially at 182 final 195 - lots of grill blocking and heater fan was off during rev testing - radiator fan comes on at 205 degrees. I gave a range of RPM since the gph did not increase until a threshold rpm was reached adn it really was hard to get the engine to slowly increase in rpm and register on the ScangaugeII at each threshold increase in gph.

gph rpm
.1 650-800
.2 800-1300
.3 1310-1842
.4 1850-2340
.5 2350-2710
.6 2720+
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Old 12-06-2007, 07:43 PM   #17
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Originally Posted by 8307c4 View Post
As a general rule, a vehicle will burn a 1/4 tank in 3 hours of idling.
So you're saying 12 hours as a general rule?

According to my SG for our 2 stock vehicles (Accord and Santa Fe), both would take at least 50 hours and one would actually be closer to 60 hours.

I'm not so sure about your general rule.
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Old 12-07-2007, 02:22 PM   #18
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JanGeo, I plotted your points on a graph, and it looks like for rpm>1000, Fuel Consumption Rate (gph) = rpm/5000.

Idle fuel consumption is actually lower than predicted by the rpm>1000 results.

This suggests that engine friction does rise in proportion to engine rpm.

This in turn suggests that the glide interval (in neutral, engine on) of a pulse and glide will be saving a substantial amount of fuel at speeds where the alternative in gear would result in high rpm. (e.g. in my car I'm likely using 1/5 or 1/6 of the fuel I otherwise would at 100kph).
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Old 12-07-2007, 05:24 PM   #19
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I just graphed it in excel and used both high and low rpm points on a double line graph and it is interesting how lines bend at the low rpm and it does look like friction increases at higher rpms in a slightly steeper than linear rate. Be interesting to try the test again after a good highway run - engine usually loosens up a little after a good run. Could use a big lever on the gas cable to better control the throttle more accurately and maybe repeat in LPH instead.
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Old 12-09-2007, 04:10 PM   #20
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Questions: which vehicle did you test the idle rate for? And what size engine? If it's for the xB, do you think a reasonable assumption for my idle rate (660cc engine) would be 0.1gph * .66/1.5 = 0.044gph?
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