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Old 03-15-2017, 08:36 AM   #1
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Fuel economy misconceptions

Since the tips sections has a 650 CHARACTER limit, I will be posting my rant here, and linking to it via tips.

Fuel economy misconceptions

I have seen so many "tips" on this website that are misleading and just plain wrong, so I will address several common misconceptions about fuel economy that are being perpetuated by people who don’t know what they are talking about.
If you really want to know how to boost your fuel economy, visit a dedicated hypermiling website such as Ecomodder.com or cleanmpg.com to learn tips and tricks. No, you don’t have to use all the crazy techniques they use, instead, look at their tips, DECIDE WHICH YOU ARE OKAY WITH USING, and then use them. 100+ Hypermiling / ecodriving tips & tactics for better mpg - EcoModder.com

Alrighty, let’s get to mythbusting!

Larger engine not working hard is more efficient than small engine working hard: Nope

If this were true, then things such as cylinder deactivation never would have been conceived, as they would REDUCE efficiency by that “logic”. Engines are efficient at converting FUEL to POWER when they are loaded down. As an example, an idling engine doesn’t use much fuel. Sure, but it also produces VERY LITTLE power. The result is a terrible FUEL to POWER ratio. When you load the engine down by asking it to produce power, it consumes more fuel, sure, but it also produces more power. This time, the FUEL to POWER ratio is much better.
Now, that being said, there are certain ‘islands’ where the engine is the most efficient. Usually this is achieved around 30-40% through the engine’s rpm range at about 80% load. Take note that 80% throttle does not mean 80% load. At these rpms it should take only 40-50% throttle to reach 80% load, with more throttle needed as the rpms increase. For example, the Honda S2000, a 2.0L 4 cylinder screamer capable of almost 9000 rpm reaches optimum bsfc at 3000 rpm between 78-82% load, there is an ‘island’ of excellent efficiency between 2500 and 4000 rpm, also between 78 and 82% load. Look up “brake specific fuel consumption map”. In this case, smaller numbers are better.

Example: You are putting along in traffic, you only need 4 horsepower to maintain your current speed. At this speed, your 2.4 liter engine is idling, using very little fuel, but also producing very little power. At this zone of the bsfc map, your engine may be using as much as 800 grams of fuel to produce 1 hp for 1 hr. If (unrealistic) you were to swap a little 150cc ‘traffic putter’ engine into your car to take the job of the main engine while in these kinds of conditions, you could save fuel. The ‘traffic putter’ engine will need to work much harder to produce this 4 horsepower since it is so much smaller than the main engine. However, since the engine is working hard, it will also be working efficiently. At this zone of the ‘traffic putter’ engine’s bsfc map, it will be down to about 300 grams of fuel to produce 1 hp for 1 hr. So, assuming you are stuck in a 4 horsepower traffic situation for one hour, idling your 2.4L engine would use 3.2 kilograms of fuel. The ‘traffic putter’ engine would only use 1.2 kilograms.

Air filters affect fuel economy: Only with carburetors
For carbureted engines, yes, fuel injected, no. A clogged air filter will affect acceleration, but not fuel economy on fuel injected engines.
https://www.fueleconomy.gov/feg/pdfs...02_26_2009.pdf

DFCO is more fuel efficient than coasting in neutral: It depends on the situation
This is the most common misconception I read about on this site, and the answer to this question is that it depends on the situation.

If you need to slow down faster than coasting in neutral allows for, keep it in gear. Otherwise put it into neutral. Sure, DFCO cuts fuel use to zero, but it costs you your momentum in return. The cost to your momentum far outweighs the zero fuel use. In fact, coasting in neutral allows you to GAIN speed downhill, as opposed to DFCO lowering or maintaining speed. Decide how fast YOU want to go, then decide whether to be in gear or in neutral. I frequently use BOTH methods on my commute, both can help fuel economy, but only when used in the correct situation.

Examples:
You are approaching a stop sign at the bottom of a hill. In this case it would be better to keep it in gear, cut fuel use to zero, and benefit from engine braking. Shifting to neutral in this situation would only increase fuel use because the engine will be idling.

You are on an expressway descending a slight hill, with a hill you need to climb on the other side. In this case, the far more fuel efficient option is to put it into neutral, you will gain speed and momentum instead of wasting it by using fuel cut. If you were to fuel cut, you would need to make that momentum back by ACCELERATING which uses FAR more fuel than an idling engine. The catch to this is that coasting in neutral may allow you to pick up too much speed. Use your judgment and determine the fastest speed you want to go, fuel is cheaper than a ticket.

Approaching stop sign quickly. Keep it in gear, utilize the fuel cut. In this situation, putting it in neutral would not slow you down quickly enough to be the efficient choice, and you will also burn fuel idling the engine.

Approaching stop sign slowly. Put it into neutral. By using neutral, you can coast longer, which translates to less acceleration needed to travel the same distance, thus saving fuel.

Accelerate slowly for best fuel economy: It depends on the situation
If you are in a situation where you might suddenly need to slow down such as a line of intersections threatening to have a red light, then yes, accelerating slowly is probably the best choice. Otherwise, see above, a harder working engine is a more efficient engine. Many times I have accelerated efficiently (quickly) only to have to come to a stop because of a fresh red light. In this case, it would have been more efficient to accelerate slowly because there would have been less fuel and speed wasted because I needed to stop.

A/C above 50 mph: Nope
If I turn my A/C on, I will get low 30s in MPG at most, I cannot say the same for rolling down the windows. Now here is the question you have to ask yourself, are you rolling down the windows ALL THE WAY? If yes, bump that cutoff speed closer to 60 mph. If you only crack them open a few inches like me, feel free to use that below 75 mph without fear of costing more mpg. I will keep my windows fully down only up to 30 mph, at that point I will roll them up until they are only open a few inches. I NEVER use my A/C unless I need to defrost or I have whiny passengers in the car.

A note on A/C use: If you are in a situation (see above) where DFCO is more efficient, then you can turn on your A/C without costing any mpg, until you drop out of DFCO. Using A/C in these situations can help you slow down faster too, keeping your brakes from overheating. If you are driving with load (see below) on a hilly expressway, turning off A/C when climbing hills and turning it back on descending them will help to keep your speed in check and minimize the mpg hit from using A/C.

Cruise control is more efficient: Depends on you, but probably not
The most efficient way of maintaining your speed depends on you, if you are someone who gradually gains speed on the highway, then yes, use cruise control. Otherwise, you should use a technique called driving with load. In this technique, you make slight changes to your throttle position to maintain the same manifold vacuum in your engine. Without an OBD II reader to know the exact load percentage, it is difficult to do this correctly. What this means is that as you climb a hill and lose speed, you will slightly lift off the throttle, as you go back down the hill you will apply even more throttle than when you were cruising on level ground. The catch to this is that you might gain more speed than you are comfortable with, the efficient solution to this is to put it into neutral to slow back down to the desired speed. If you were to use fuel cut, you would be wasting the fuel you burned accelerating down the hill. See above for examples for when to use fuel cut.

Ideal MPG speed is definitely 70-80 mph: Nope
The only reason you are getting reasonable mileage at 80 mph is because you are ONE car length behind the guy in front of you, that’s it. I see it all the time, don’t try to deny it. Speed vs mpg charts for cars generally show best mpg between 30-45 mph. A semi or RV may achieve best mpg at 55 mph, but don’t expect cars to do the same. Below the ideal speed range you are losing mpg due to reduced engine efficiency (see above), above the ideal speed range, you are losing mpg due to rapidly increasing air drag.
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Old 03-19-2017, 03:33 PM   #2
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...Larger engine not working hard is more efficient than small engine working hard: Nope...
Sometimes, I can't leave "well enough" alone.

The folks who believe you'll always get better fuel economy from a larger displacement engine that's "not working as hard" than a smaller displacement engine that's straining, are as wrong as those who believe you'll always get better fuel economy just by getting a smaller displacement engine.

Engine displacement is just one of many factors that affects fuel economy. Even apparently identical engines can vary significantly. Just ask any piston twin pilot, and they'll tell you that brand new engines -- theoretically identical -- will have different fuel flows.

Other factors that influence fuel economy include gearing, engine management computers, regional emission requirements (i.e., pollution control hardware and software), and even subtle things like how well an engines is designed and assembled. In the "old days," there was process called "blueprinting" in which an engine would be hand-assembled from hand-selected parts, with special care taken to ensure that reciprocating and rotating mass was within very fine tolerances, so as to reduce vibration and increase power and fuel economy. Today, it's more common (but not guaranteed) for stock factory engines to rival "blueprinted" specs from a couple of decades ago.

Another big factor that influences fuel economy and emissions is how you use your engine. In other words: Under what conditions are you measuring fuel economy? As a consequence of VW's Dieselgate scandal, governments are wisening up to the useless nature of in-lab-only emissions test. One of the things they discovered is that manufacturers were designing their engines to "shine" -- or "be optimized" -- for laboratory test conditions. What a surprise! [that's feigned shock, and sarcasm, BTW]

In the real world, under real use, these "shine in the lab" engines would not do nearly as well, typically getting notably worse than lab-number fuel economy and considerably worse than lab-level emissions. Well beyond lab-legal levels, in fact. The auto industry is abuzz about the fact that as emissions (and fuel economy) requirements and testing methodologies change to more closely resemble real world use, manufacturers will end up using larger displacement engines that are optimized for the new test conditions.
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Old 03-20-2017, 01:10 PM   #3
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Originally Posted by SteveMak View Post
Sometimes, I can't leave "well enough" alone.
Acknowledged.

Quote:
Originally Posted by SteveMak View Post
The folks who believe you'll always get better fuel economy from a larger displacement engine that's "not working as hard" than a smaller displacement engine that's straining, are as wrong as those who believe you'll always get better fuel economy just by getting a smaller displacement engine.
I'm going to have to say no to that one. Yes, downsizing engines can be overdone, but in general, downsizing engines leads to an engine working harder more of the time, thus being more efficient more of the time. I know some examples of downsizing such as the new F150's 2.7 ecoboost can fail to deliver, but that doesn't mean thinking downsizing will improve fuel economy is as wrong as thinking upsizing will improve fuel economy.

Quote:
Originally Posted by SteveMak View Post
Another big factor that influences fuel economy and emissions is how you use your engine. In other words: Under what conditions are you measuring fuel economy? As a consequence of VW's Dieselgate scandal, governments are wisening up to the useless nature of in-lab-only emissions test. One of the things they discovered is that manufacturers were designing their engines to "shine" -- or "be optimized" -- for laboratory test conditions. What a surprise! [that's feigned shock, and sarcasm, BTW]

In the real world, under real use, these "shine in the lab" engines would not do nearly as well, typically getting notably worse than lab-number fuel economy and considerably worse than lab-level emissions. Well beyond lab-legal levels, in fact. The auto industry is abuzz about the fact that as emissions (and fuel economy) requirements and testing methodologies change to more closely resemble real world use, manufacturers will end up using larger displacement engines that are optimized for the new test conditions.
Um..All of them? Not sure why you are talking about engines designed to meet the test.

Hey, at least you didn't come in here saying that DFCO is god's gift to man, and how dare I suggest that coasting in neutral can be more efficient.. I'll give you credit for that.
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Old 03-26-2017, 07:11 AM   #4
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In my experience. I do think that a CV Transmission changes things. Also my more refined cruise control does also.
In my car. I have found that at or above 45 is my best mileage. At or below 65 is good. The wind makes a definite difference.
I have an engine brake gear. If I cannot approach a stop in neutral. The engine brake is doing a good job of saving fuel. I'm not sure why. It does.
My cruise control. When in hilly driving. Going downhill. My cruise will shift the transmission into 'regular gearing'. That slows the car a little. If that doesn't work. It will shift the car into 'engine brake' gear. That does a much better job of slowing the car. It will shift back to drive when I'm down to where I set the cruise control.
It is interesting, watching the tachometer with this transmission.
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Old 03-26-2017, 07:29 AM   #5
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