Author Topic: 40 MPG in my Civic, w00t!  (Read 9580 times)

Uncephalized

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40 MPG in my Civic, w00t!
« on: June 20, 2012, 12:15:42 PM »
Just wanted to share that I have adopted hypermiling techniques in my 5-spd '98 Civic coupe, and am getting good results! We took a trip down to Tucson from Phoenix last weekend to visit family (a 200+ mile round trip plus some in-town driving). I just filled the tank again yesterday and calculated 39.6 MPG--which I am rounding to 40, because it sounds better. :-) That's mostly freeway. I only started hypermiling halfway through my last tank before that, and brought my city/freeway commute average up from about 28 MPG to about 32 MPG, so I'm expecting my next tank will probably land around 35 MPG or so, since I'll be using the whole tank this way instead of half.

What I'm doing to achieve these results:

On surface streets:
-Accelerating gradually, shifting up at 1800-2300 RPM instead of 3000-4000
-Cruising at or just below the speed limit
-Opening windows instead of air conditioning
-Scanning far ahead to watch for potential need to slow down or stop
-Coasting to a stop in high gear using engine braking only wherever possible
-Turning off the engine if stuck at a red light for more than a few seconds (you can almost always tell how long the light will be by what the cross-street signals are doing)

On the freeway:
-Get behind the biggest truck in sight (maintaining a safe following distance of course!) to draft off it
-Keep windows closed to decrease drag
-Alternate turning the A/C on and off to maintain minimum tolerable comfort level
-If there are no trucks to follow, stay in the slow lane and keep speed relatively low to minimize drag
-Use cruise control wherever possible to avoid creeping up in speed or constantly cycling speed up and down

Side bonuses:
-Driving this way is actually more relaxing than trying to get everywhere faster. Driving behind transport trucks on the freeway is especially stress-free as long as you make sure to maintain a comfortable following distance
-Using less air conditioning is assisting in my quest to become as heat-tolerant as possible, which is making me generally more comfortable in the Phoenix summer (in the same vein we are keeping our house at a balmy 85 degrees all the time)

Increasing my 'stash one commute mile at a time! Can't wait until we move closer to work and I won't need to use my car every day at all. :-)

Jamesqf

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Re: 40 MPG in my Civic, w00t!
« Reply #1 on: June 20, 2012, 01:40:10 PM »
-Use cruise control wherever possible to avoid creeping up in speed or constantly cycling speed up and down

Actually not good, unless you're on a perfectly flat road.  For best economy, you don't want to hold a constant speed, you want to hold a constant engine load, which means you will slow down somewhat going up small hills, then speed up going down.

Another suggestion is to be aware of the road surface.  Often it's better to drive slightly off-centered in the lane, so you'll be on a smoother surface.

Just FYI, my long-term average (110K miles over 8 or 9 years) in my Insight is 71.4 mpg.

Uncephalized

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Re: 40 MPG in my Civic, w00t!
« Reply #2 on: June 20, 2012, 01:50:42 PM »
-Use cruise control wherever possible to avoid creeping up in speed or constantly cycling speed up and down

Actually not good, unless you're on a perfectly flat road.  For best economy, you don't want to hold a constant speed, you want to hold a constant engine load, which means you will slow down somewhat going up small hills, then speed up going down.

Another suggestion is to be aware of the road surface.  Often it's better to drive slightly off-centered in the lane, so you'll be on a smoother surface.

Just FYI, my long-term average (110K miles over 8 or 9 years) in my Insight is 71.4 mpg.
Fair enough, though I do almost all my driving in southern Arizona, which is as close to 100% flat as it gets outside of the Midwest. So cruise is a good idea for me.

Lavender

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Re: 40 MPG in my Civic, w00t!
« Reply #3 on: June 20, 2012, 05:50:39 PM »
Is cruise better than pulse-and-glide, or worse, or the same? I'm semi-hypermiling too, and do see the difference, and it feels awesome.

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Re: 40 MPG in my Civic, w00t!
« Reply #4 on: June 20, 2012, 05:51:47 PM »
Bakari should be along soon...
I am a former teacher who accumulated a bunch of real estate, retired at 29, spent some time traveling the world full time and am now settled with three kids.
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TLV

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Re: 40 MPG in my Civic, w00t!
« Reply #5 on: June 20, 2012, 06:02:04 PM »
Is cruise better than pulse-and-glide, or worse, or the same? I'm semi-hypermiling too, and do see the difference, and it feels awesome.

I'm not an experienced hyper-miler, but here's what I've observed:

Hardcore hypermilers rave about pulse-and-glide, but from what I can tell most of them A. drive hybrids that completely shut off the engine when you glide (or have a custom kill switch like Bakari), and B. P&G at less-than-freeway speeds (ie <50).

On the freeway in an automatic I didn't see an improvement in MPG using P&G (pulse to 70, glide to 60) vs cruising at 70, and it had a big disadvantage in being harder to draft behind big trucks.

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Re: 40 MPG in my Civic, w00t!
« Reply #6 on: June 26, 2012, 07:53:41 PM »
Great job so far!

But you can do more - especially with light timing in city situations and only downshifting, rarely using brakes.

I drove a 5-spd 2000 Civic EX for ~145,000 miles and routinely achieved between 43-45 mpg., or 43.5 for the life of the car. One third of the mileage was in South Carolina, with the rest in DC and Boston. I did not draft, did not PaG, and did not shut off the car on the highway. (Windows up at highway speed!) In other words, no deep hypermiling tricks.

The car is high geared, so keeping down the revs tends to be the modus operandi (though not always). If I were feeling particularly thrifty, I kept shifts to below 1500 rpms - especially around town. And starting downhills with the engine off and selecting second worked wonders.

Your most controllable variable? Speed. Keep it low. In a 55, I go 55. In a 65, I go 59. With a little time and modeling you can build an understanding of how temperature, barometric pressure, and vehicle surface area work against you. If youre determined to use cruise, say, for a long commute, you can use it to your advantage: decrease the speed a touch or two as you approach a hill...Increase it a bit on the downside: don't fight gravity as much, and then make it work for you. Maybe I'm a bit scientific. But that's my nature - and it's second nature, if you'll excuse the pun.

Today's cars, for want of being safer, heavier, and sometimes more distracting, really are pretty economical. My wife practices no efficient methodology other than reasonable speed and makes 44-46mpg in her ride. In mine (no longer a Civic), I can make 53mpg, with effort.

Good hunting: and be safe!

Uncephalized

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Re: 40 MPG in my Civic, w00t!
« Reply #7 on: June 27, 2012, 10:19:06 AM »
I cannot come up with a convincing argument for why it would be more economical to lose speed on an uphill, and increase speed on the downhill while keeping constant throttle, as people are recommending.

My reasoning goes like this: drag increases as the square of air speed (wind resistance) and linearly with ground speed (rolling resistance). Fuel consumption can be described as a graph with engine speed on the horizontal axis, and gas consumed per second per horsepower on the vertical. For any engine there is an optimum range of RPM wherein the least gas is consumed per second for a given amount of power output. In other words, if you are maintaining speed and your power output increases (increased throttle going up a hill or turning into a headwind), this will increase fuel consumption, and vice versa. If you change speed, you are moving to a different section of the consumption curve, and economy will go up or down accordingly. But let's assume that you are operating in a relatively flat section of the curve, in a high gear, so changes in speed of <10mph magnitude don't significantly affect consumption per second per horsepower. What they do affect is drag, which is where we need to investigate.

On a flat road at constant speed, drag is our only consideration. It will be equal to a*v+b*v^2, where a and b are coefficients describing rolling resistance and wind resistance, respectively, and v is vehicle speed (assuming air speed equals ground speed, no wind). A and b are unique to the vehicle and environmental variables.

Now say we encounter a hill. For simplicity, assume that the actual average elevation change is zero, that is, we end up at the same elevation after the hill as we started. Let's try the constant-throttle, slow-down-uphill-then-speed-up-downhill method first. As we hit the hill, vehicle speed decreases and engine output remains the same. We're not going to worry about the consumption effect of RPM right now because of our earlier assumption. The car was previously in dynamic equilibrium where engine output was equal to the force of drag times vehicle speed, P = D*v, or P = (a*v+b*v^2)*v. Now that we're going uphill, we also have to fight gravity, so that becomes P = (a*v+b*v^2)*v+w*dh/dt, where w is vehicle weight, and dh/dt is the climb rate, change in elevation per second. Dh/dt is actually a function of v, equal to c*v, where c is a constant determined by the slope of the hill. So we have P = (a*v+b*v^2)*v+w*c*v = w*c*v+a*v^2+b*v^3 when you distribute the v. Since we're holding power constant, v has to decrease to counteract the added drag of gravity until all the terms on the right side add up to P again. This has the advantage of also decreasing wind and rolling drag, which is "lost" energy. The energy we are "losing" going up the hill will be gained back again when we go down, because it is increasing the vehicle's potential energy, which will be converted back into kinetic energy on the downhill.

Now we hit the top of the hill, and start going down. Dh/dt becomes negative, so at constant power output v must increase so the right side equals P. However, since the drag terms are non-linear, v will not increase above flat-land speed on the downhill by as much as it decreased on the uphill--it will take a smaller increase in v to add up to P since the slope of the drag function is steeper the faster you drive. In other words, the advantage in fuel economy you get from going down the hill is not as great as the disadvantage you got from going uphill--so the uphill and downhill don't quite cancel each other out. We get back the potential energy we "lost" in climbing, but we lose more on average to wind resistance than we gained on the way up. Interestingly, this effect is greater the steeper the hill is, because a steeper hill will cause larger deviations from the average flat-land speed, pushing us into ever-increasing drag slopes on the downhill and ever-decreasing ones on the uphill.

Now consider the other option: hold speed constant, adjusting throttle (and therefore power) as necessary to maintain it. Here we'll have to make the additional assumption that speed (and therefore drag) is great enough, or the hill grade shallow enough, that we will need to apply some positive throttle to maintain speed on the downhill. Otherwise we would be relying on the same natural speed equilibrium mechanism as the first model. But let's see what happens.

This time, when you hit the uphill slope, you increase power to precisely counteract the additional drag effect of gravity. This keeps speed, and therefore rolling and wind resistance, constant. So your power output climbs linearly with hill slope. On the way back down, your power output can decrease below baseline levels by precisely the same amount, because you have held the second- and third-power terms constant, and only changed the linear one. Thus no matter how steep the hill (as long as it is not so steep you need to downshift on the way up, or brake on the way down to maintain speed), you will gain back on your way down precisely what you lost on the way up, because you didn't "throw away" any extra energy fighting the wind--you put it in the gravity bank on the ascent and then withdrew it on the descent, and gravitational potential doesnt charge ATM fees.

Now, we left out the effect of RPM on fuel economy, because that's idiosyncratic to the particular vehicle and speed. Theoretically that could make up the difference on a particular hill at a particular speed, but over a variety of hills and speeds, it should average out and the second method should come out ahead. It will also be less noticeable on shallower hills because the drag function will be close to linear over a smaller speed band.

The other thing to consider IMO is very short hills, where the transitional effects of altering the throttle position might overwhelm the steady-state advantages of maintaining constant speed. I don't know how important this is and it probably varies somewhat from car to car. But for a long, smooth grade the steady-state considerations will totally outweigh any transitories.

Can anyone spot anything I'm missing here? Some X factor I'm not including?


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Re: 40 MPG in my Civic, w00t!
« Reply #8 on: June 27, 2012, 01:26:20 PM »
I think you are misunderstanding what's going on.  You keep a constant throttle going up the hill (assume a perfect hill where you can do this and still maintain a reasonable speed), then you let off the throttle going down, so that gravity restores your speed.  Compare this to a typical mindless driver who keeps a constant speed (say the speed limit): he burns extra fuel keeping his speed up on the uphill, then must brake to keep at the speed limit going down.

GregK

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Re: 40 MPG in my Civic, w00t!
« Reply #9 on: June 27, 2012, 02:17:39 PM »
Hey Uncephalized. Here's my contribution; you're effectively accelerating if you go up a hill at constant speed (a=dh/dt*g). If, on the other hand you decelerate linearly at exactly -a (as just defined) as you climb the hill and accelerate linearly at +a coming down the hill, there is no effective acceleration, and it is as if you maintained constant speed on a flat surface, which is the most fuel efficient way to drive (except for possibly adding pulse and glide, but that complicates things A LOT, so I'm ignoring it hah).

Uncephalized

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Re: 40 MPG in my Civic, w00t!
« Reply #10 on: June 27, 2012, 03:06:32 PM »
I think you are misunderstanding what's going on.  You keep a constant throttle going up the hill (assume a perfect hill where you can do this and still maintain a reasonable speed), then you let off the throttle going down, so that gravity restores your speed.  Compare this to a typical mindless driver who keeps a constant speed (say the speed limit): he burns extra fuel keeping his speed up on the uphill, then must brake to keep at the speed limit going down.
Well, yes, this would naturally increase fuel economy, because you are decreasing your average speed. This is a hybrid of techniques 1 and 2, not the technique that has been recommended to me more than once, which is "hold throttle constant on both uphill and downhill slopes to keep a constant engine load", which is what I am arguing against doing. I also pointed out that this argument only works on hills that are not so steep that you would need to brake on the descent to stay below the speed limit. On most freeways this is reasonable since the grade is usually shallow enough that drag will slow you down even in neutral, with no need to brake. I did post a wall of text so individual little caveats might have been easy to miss. :-)

Hey Uncephalized. Here's my contribution; you're effectively accelerating if you go up a hill at constant speed (a=dh/dt*g). If, on the other hand you decelerate linearly at exactly -a (as just defined) as you climb the hill and accelerate linearly at +a coming down the hill, there is no effective acceleration, and it is as if you maintained constant speed on a flat surface, which is the most fuel efficient way to drive (except for possibly adding pulse and glide, but that complicates things A LOT, so I'm ignoring it hah).
Aha! I was thinking about something wrong. Thank you. On a net-zero-elevation hill under constant power, you won't ever end up going faster than flat-land speed; you'll end up going exactly that fast at the end of the hill, and somewhat slower for the duration of the hill itself. Which neatly explains the advantage; you are going slower the whole time you are on the hill, and therefore experiencing less drag force, and therefore losing less energy.

If you are an extreme hypermiler, though, you should already be traveling at your minimum-tolerable speed, no? Otherwise you are wasting fuel all the time. So we actually have to add a consideration we weren't thinking about before--namely that on public roads, it's not acceptable to go an arbitrarily low speed, because we will be obstructing the road and it will take forever to get anywhere.

That suggests to me that the best strategy is to pick your lowest and highest tolerable speeds, and never use more throttle than required to maintain your minimum, or more braking (including engine braking) than required to maintain your maximum. If you are ever going faster than your minimum speed at a steady state on a flat road, you are wasting gas by increasing drag. If you ever brake before reaching your maximum speed on a downgrade, you are wasting gas by dumping kinetic energy that you will later have to resupply through burning fuel. If you allow yourself to fall below your minimum tolerable speed at any time, you are wasting your time and possibly being a nuisance on the road.

This suggests that optimal strategy is to allow the car to speed and slow naturally on hills, intervening only when necessary to avoid going outside of your speed boundaries for safety. In all cases you only need to remember to only use throttle to stay above your minimum and only brake to stay below your maximum. On flats you should simply be going your minimum speed already and so it would be intolerable to slow down more, hence you maintain constant (minimum) speed uphill and allow free gain of speed up to maximum on the down, then allowing your car to coast back to minimum through drag on the flat before applying throttle again. it would be sweet if you could set a cruise control to do this... hmm... suggests an invention that needs inventing...

Bakari

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Re: 40 MPG in my Civic, w00t!
« Reply #11 on: June 27, 2012, 10:39:12 PM »
Bakari should be along soon...

Sorry, I've been doing overtime with the US Coast Guard, not so much computer time...

Is cruise better than pulse-and-glide, or worse, or the same? I'm semi-hypermiling too, and do see the difference, and it feels awesome.

P&G works without a hybrid (or dedicated cut-off switch - although it sure makes it a lot easier to do engine off) as long as its a manual.  You can save gas with P&G even if you just let it idle while you coast instead of turning the engine off.  It doesn't work so well with automatics.  It also is optimal at lower speeds (depending on the cars aerodynamics).
The reason it works is because internal combustion engines are by nature so ridiculously inefficient that it actually takes more energy just to turn the engine itself than it takes to accelerate the car.  Because of that, any time you can have the engine off (or at least at neutral) more than makes up for having to accelerate back up to speed later.

P&G also ties in really well with this discussion re: hills.

Here's what I do: on the uphill, I maintain my regular, slow, flat land speed (which is the minimum I can practically drive on the highway, usually 45-50) until I am close enough to the top that if I give it 80% throttle, I'll reach 65mph by just after the crest of the hill (the exact point varies with the steepness).  Then, at the top I shift into neutral.  Basically, its regular pulse and glide, but using the hill allows me to a) maximize engine load at low RPM (which is generally the optimal point on the BSFC chart that Uncephalized was talking about) and b) maximize the length of the glide.  Its all about the amount of time you spend with the engine off (or idling at neutral, thereby minimizing RPMs and internal friction)

The discussion on hills started by way of not using cruise control, and one big point that Uncephalized's post missed was that cruise control will frequently cause the car to downshift (in automatics) in order to maintain speed, and also that there are plenty of hills steep enough that one can maintain speed without additional throttle (esp in neutral, so there is no engine braking effect).  On shallower hills, I would agree there is less point, although it still has the effect of enhancing P&G; in fact, I never use P&G unless there is at least a shallow hill to help maximize my glide.
But though I think going faster on the downhill than the up is valid, I agree that there is no particular reason to hold a constant throttle.

Re: not going too fast on the downhill, the majority of modern cars cut out the fuel injectors automatically when at cruising RPM, in gear, with the foot totally off the throttle.  So, if you were coasting down hill, just pop it in gear, don't touch any pedals, and it will scrub off a little speed while dropping your fuel consumption to zero.
If you still are going too fast, you probably were going too fast at the peak of the hill (as MMM once said "if you have to brake, you made a mistake") unless you are descending the rocky mountains or something.

Re: cruise control set to constant engine load instead of constant speed, people have done it, see ecomodder.com

Personally, I find semi-trucks drive too fast to draft behind.  Legally they aren't supposed to go above 55, but they rarely do less than 65.  If they actually drove 55 I might do it more, but I find speed to make the bigger difference in fuel economy.

But your mileage may vary (pun intended!)
The best way to know what works best for the specific engine, gearing, and aero of your car is to get a scanguage or mpguino and actually see how different options affect your mpg in real time.  I believe they cost something like $100 or so, (if you sign up at ecomodder, and then buy it through them, its slightly cheaper than retail), and its super easy to instal - plug in one wire under the dash at your code reader port.
Instant feedback is probably the biggest boost to fuel economy a hypermiler can get, so it eventually pays for itself.
Plus, built in code-reader, so it may save you some mechanic fees someday too.

BTW, I'm up to 38 highway in my truck after having installed my new overdrive tranny! :)

grantmeaname

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Re: 40 MPG in my Civic, w00t!
« Reply #12 on: June 28, 2012, 05:56:15 AM »
To quickly interject something totally unrelated: normally there's some sort of pattern you can repeat involving the ignition when you start the car to get OBD-II codes to display on an odometer or something (on mine you put the key in and turn it to the driving position, then back, 3 times). If your vehicle lacks this functionality, parts places like Autozone or Advance Auto Parts will also let you borrow their code reader for free (along with surprisingly many other tools).

Uncephalized

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Re: 40 MPG in my Civic, w00t!
« Reply #13 on: June 28, 2012, 09:43:43 AM »
@Bakari, I should have listed in assumptions that I was thinking of a manual vehicle, where CC can't shift for you. Thanks for the additional info.

Bakari

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Re: 40 MPG in my Civic, w00t!
« Reply #14 on: June 28, 2012, 10:51:29 PM »
In that case the CC might go wide-open-throttle in attempting to keep the same speed, which often means a sub-optimum air-to-fuel ratio (i.e. overly rich)

Also, I forgot to mention: as slow as practical is not always the most ideal - depending on gearing and aerodynamics, many modern cars have maximum mpgs at 50-60mph.  It might be reasonable to go 45 on a 50mph street, but hypermiling wouldn't call for it in that case.  In which case, if slowing down on an uphill meant avoiding WOT, it could make sense.
But yeah, I think as a general rule, it hold more for automatics than manuals.

Uncephalized

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Re: 40 MPG in my Civic, w00t!
« Reply #15 on: June 29, 2012, 10:29:00 AM »
In that case the CC might go wide-open-throttle in attempting to keep the same speed, which often means a sub-optimum air-to-fuel ratio (i.e. overly rich)

Also, I forgot to mention: as slow as practical is not always the most ideal - depending on gearing and aerodynamics, many modern cars have maximum mpgs at 50-60mph.  It might be reasonable to go 45 on a 50mph street, but hypermiling wouldn't call for it in that case.  In which case, if slowing down on an uphill meant avoiding WOT, it could make sense.
But yeah, I think as a general rule, it hold more for automatics than manuals.
Yep, that pesky real world always has more variables and unknowns than nice clean thought experiments! Thanks for the input Bakari.

chipdude

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Re: 40 MPG in my Civic, w00t!
« Reply #16 on: July 23, 2012, 10:22:05 PM »
Bakari,
What make &model truck do you own.
Manual, 4cyl.?
Cab size.?

Bakari

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Re: 40 MPG in my Civic, w00t!
« Reply #17 on: July 24, 2012, 09:57:31 AM »
1983 Ford F-250, 6.9L IDI V8 diesel, 5500lbs empty, up to 2.5 ton payload.
Regular cab, full bed (8x5)
Manual - came with a 4spd (where first is granny, so really a 3spd), just removed it and installed a 5spd (with overdrive)
I've made some pretty extensive modifications:
http://www.instructables.com/id/Vehicle-efficiency-upgrades/

Original mileage: 15mpg mixed / 18 highway
Current mileage: 30 mixed / 38 highway

I'm coming for your 40mpg, Uncephalized ;)

grantmeaname

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Re: 40 MPG in my Civic, w00t!
« Reply #18 on: July 24, 2012, 10:08:35 AM »
You get 30/38 on a 7L diesel engine? I am ridiculously impressed right now.

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Re: 40 MPG in my Civic, w00t!
« Reply #19 on: July 24, 2012, 01:44:28 PM »
:)
I get 30/38 on a 30-year-old 7L engine.

I.E. pre common rail fuel injection, pre computer control, pre variable valve timing.
No turbo, and run on 100% biodiesel (slightly less energy per gallon than petrol)

And the 30mpg is using it like a real work truck:



reverend

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Re: 40 MPG in my Civic, w00t!
« Reply #20 on: August 02, 2012, 02:03:27 AM »
In that case the CC might go wide-open-throttle in attempting to keep the same speed, which often means a sub-optimum air-to-fuel ratio (i.e. overly rich)

But with the throttle plate more opened, the volumetric efficiency increases...

As for BTUs, here's a good guideline for what different fuels contain;

Regular Diesel Fuel = 128,500 BTUs
Gasoline = 125,071 BTUs
Biodiesel = 118,296 BTUs
Ethanol = 76,000 BTUs