Getting to the Trailhead; My MPG experiment !!

[Paradox]

...
I don't know if water injection could help the internal combustion engine. There have been a number of such claims, many of which belong in the snake oil category. I do know that the original Boeing 707 jet engines used water injection during takeoff. Evaporating water absorbed some of the heat and produced a massive expansion to help power the engine--without the water, the engine would have overheated at that power level. (Jet engines are thermal engines and have compression, heating, and expansion phases just like car engines.)....Doug

Water injection was used alot in WWII aircraft to boost the horsepower of an engine at maximum loads. It was selected by the pilot just before take off or at the point of initiating an escape manuver. It absorbed excess heat and used it for expansion (pushing the cylinders), but the effect is limited. At lower throttle levels it could stall an engine at the worst possible moment.

 

[Kevin Rooney]

The modern automobile gasoline engine has an average efficiency of 25-30%, peak ~75%. (Ref http://en.wikipedia.org/wiki/Engine_efficiency.) So there is a bit of room for increased engine efficiency as well as more efficient use of the available mechanical energy.
Doug

Adding to thread drift - isn't the modern engine considered a reciprocating engine? I seem to recall an engine which used a rotary design, invented by someone named Wankel. I believe Mazda used it for awhile, but think it had some problems they couldn't really solve - maybe keeping seals intact? But, I think the rotary design had either a greater efficiency or more HP for the same displacement as a reciprocating engine (maybe more HP within the same displacement is the same thing as higher efficiency).

 

[DougPaul]

Adding to thread drift - isn't the modern engine considered a reciprocating engine? I seem to recall an engine which used a rotary design, invented by someone named Wankel. I believe Mazda used it for awhile, but think it had some problems they couldn't really solve - maybe keeping seals intact? But, I think the rotary design had either a greater efficiency or more HP for the same displacement as a reciprocating engine (maybe more HP within the same displacement is the same thing as higher efficiency).

The Wankel engine had problems with hydrocarbon emisson (which can be cured by an exhaust reactor, but this results in poor efficiency). The seals are also problematical.

Lots more detail at http://en.wikipedia.org/wiki/Wankel_engine.

Doug

 

[Rivet]

more real world testing

"You may have to be a little patient, but driving at 55 mph instead of 65 or 75 will save you money. When we increased the Camry's highway cruising speed from 55 mph to 65, the car's fuel economy dropped from 40 mpg to 35. Speeding up to 75 mph cost the car another 5 mpg. One reason is that aerodynamic drag increases exponentially the faster you drive; it simply takes more fuel to power the car through the air."

http://www.consumerreports.org/cro/...t-the-most-mileage-for-your-fuel-dollars-406/

 

[Kevin Rooney]

Reducing the speed to 55 makes a big difference in gas mileage - I noticed this especially with my Tacoma, which isn't too surprising as when coupled with the cap, it isn't nearly as aerodynamically efficient as a car.

Using cruise control helps increase mileage as well, although there can be situations when it shouldn't be used for this purpose on small, fuel-efficient cars. For example, my wife has a Yaris sedan, and the manual recommends not using the cruise on long, steep hills as it tends to maintain the set speed by engaging the passing gear, wasting gas.

 

[DougPaul]

"One reason is that aerodynamic drag increases exponentially the faster you drive;"

Strictly speaking consumer reports is wrong here. An exponential is a precise mathematical function--the drag increases with the square of the windspeed, not exponentially. (An exponential increase would be much worse than a square increase.)

Of late, common usage of the word "exponential" has been inconsistent with its mathematical meaning. Consumer reports should know better.

Doug

 

[Dugan]

I find I tend to get slightly better mileage when I'm driving, as opposed to using cruise control.

I'm still not understanding an earlier comment (not sure if it was in this thread) about keeping a car in gear versus shifting into neutral for a long downslope. I know I'm not nearly as car smart as some of you, but, I shift into neutral and see the rpm's decrease sharply. If I stay in gear and coast, I don't give the car extra gas, but the rpm's stay up and I don't gain any momentum. Cruise control seems to be the worst - it keeps goosing the acceleration a tiny bit on almost all downslopes.

 

[Tom Rankin]

Reducing the speed to 55 makes a big difference in gas mileage - I noticed this especially with my Tacoma, which isn't too surprising as when coupled with the cap, it isn't nearly as aerodynamically efficient as a car.

Using cruise control helps increase mileage as well, although there can be situations when it shouldn't be used for this purpose on small, fuel-efficient cars. For example, my wife has a Yaris sedan, and the manual recommends not using the cruise on long, steep hills as it tends to maintain the set speed by engaging the passing gear, wasting gas.

Serious thread drift here, but do not use CC when it's raining or snowing. This can lead to quick loss of control of the vehicle...

 

[bikehikeskifish]

I'm still not understanding an earlier comment (not sure if it was in this thread) about keeping a car in gear versus shifting into neutral for a long downslope. I know I'm not nearly as car smart as some of you, but, I shift into neutral and see the rpm's decrease sharply. If I stay in gear and coast, I don't give the car extra gas, but the rpm's stay up and I don't gain any momentum.

FYI - Coasting is illegal in NH. Reading the following carefully

http://www.gencourt.state.nh.us/rsa/html/xxi/265/265-99.htm

    265:99 Coasting Prohibited. –
    I. The driver of any vehicle when traveling upon a downgrade shall not coast with the gears of such vehicle in neutral.
    II. The driver of a truck or bus when traveling upon a downgrade shall not coast with the clutch disengaged.
    III. The fine for a violation of this section shall be $100.

it appears you can "coast" with the clutch pressed in, but the car cannot be in neutral. Unless you are a truck in which case coasting in gear with the clutch depressed is illegal.

Tim

p.s. Don't shoot the messenger

 

[Quietman]

Gee, I coast down Temple mtn. on route 101 every time I drive it. I guess that makes me a hardened criminal.

 

[spencer]

To add to the thread drift, the former Georgia Pacific Mill in Old town Maine (Red Shield) is converting the pulp mill into a combination pulp and hemicellulosic ethanol plant.

More about the Red Shield project
More about the larger efforts

 

[Dugan]

Not to be a scofflaw, but how could someone outside the car know if you're in gear as opposed to neutral?

 

[halia and flammeus]

oops

Coasting illegal??? Yikes. I thought I was doing my part to reduce my carbon footprint!

FWIW, on my first attempt at "hypermiling" - which in my limited knowledge/expertise amounted to lots of illegal activities, like rolling through stop signs and coasting (who knew?) - my mileage went from 44 mpg to over 50 mpg. Jettas roll (a euphemism for "coast") very nicely! Flammeus drives a Toyota Tundra, and he hasn't been able to do the calculation yet - because he still doesn't need to get gas! He used to fill up weekly, but we're into the third week, and he is still rolling.

In terms of paring down fuel usage so that getting to the trail head is less of an issue, we are actually arranging to do a couple of hikes with other people (something we just didn't really consider in the past). Being antisocial is a luxury we can no longer afford !

 

[bikehikeskifish]

A) I didn't make the law
B) I don't enforce the law So I am not sure how anyone could know. Any NH law enforcement types reading that could enlighten us?
C) I suspect (and this is purely conjecture) that the concern about coasting in a passenger vehicle is around stalling said vehicle when putting it back into gear, where stalling could cause a drastic and immediate change in speed which would be a Bad Thing(tm) for traffic. For trucks, the presumption would be that in gear allows for engine braking and helps prevent runaways.
D) I'm not passing judgment on anyone who does coast. Just so you know that technically it is against the law. I heard this many years ago (like 20 or so) and found it odd at the time. A search of the NH statues shows it to still be on the books, near as I can tell.
E) If you own a car with a manual transmission, it appears that you can leave the gearshift in a forward gear and depress the clutch causing nearly the same effect as being in neutral (some portion of the transmission will probably still be spinning while in gear...)

Tim

 

[MichaelJ]

I shift into neutral and see the rpm's decrease sharply. If I stay in gear and coast, I don't give the car extra gas, but the rpm's stay up and I don't gain any momentum.

And indeed, if you keep the car in gear, many newer cars will stop supplying fuel to the pistons so long as your momentum is keeping the engine turning at an adequate speed. If you drop into neutral, sure, the engine speed will drop to a few hundred rpm, but you will definitely be injecting gas to maintain that.

Downhill-neutral-coasters have the potential to burn their brakes, wreck their rotors, or just plain end up out of control. It's illegal in many more states than just NH, widely considered dangerous, and if you don't believe me, ask Click & Clack.

 

[John H Swanson]

I also notice in my auto trans car that going into neutral can kick up the rpm up to a fast idle (1400rpm) and that may be higher rpm than coasting in gear at 900-1100 rpm. This usaully happens below 30mph. So now when I'm approaching a red light and I'm not being followed I coast down to 30 mph and then idle from there.

I have increase mpg from 27.5 to 29.4 with judicious use of the brake, mostly from anticipation of my stop and coasting; moderate acceleration (tach below 2500rpm); and by driving 65mph vs 70-75mph. Of course this gives me a lot more time in the car so I use it to catch up on my sleep

 

[bikehikeskifish]

Is it also true that driving at too slow an RPM can cause incomplete combustion and increased particulates in the exhaust? Someone once told me that was the "down side" of consciously driving in a lower RPM range. A quick google search didn't reveal any clues about this.

Tim

 

[Neil]

I have increase mpg from 27.5 to 29.4 with judicious use of the brake, mostly from anticipation of my stop and coasting; moderate acceleration (tach below 2500rpm); and by driving 65mph vs 70-75mph. Of course this gives me a lot more time in the car so I use it to catch up on my sleep

I never understand people who floor it and then have to break hard for the next red light. I call that spending money on gas in order to wear out your brakes.

More time in the car gives me time to listen to Symphonies, string quartets and entire opuses of groups like King Crimson. I suppose one could always listen to motivational CD's by Tony Robbins.

(A verbal account of the past 5 years of VFTT trip reports might be more inspiring than TR)

 

[DougPaul]

Is it also true that driving at too slow an RPM can cause incomplete combustion and increased particulates in the exhaust? Someone once told me that was the "down side" of consciously driving in a lower RPM range. A quick google search didn't reveal any clues about this.

This might be running in too high a gear for one's speed (ie too low an engine rpm), causing the engine to sound over-stressed and give very little torque. (Any standard-transmission driver will recognize the condition and upshift.) I suppose this could cause pre-ignition or poor ventilation, but I'm just guessing. (I regard this as a dangerous practice because you will have no acceleration if you suddenly need it.)

Running the engine at sub-idle rpms might also not work very well. (My engine bucks if I do it.)

Another possible problem area is during engine braking--the engine will be turning at medium to high rpm with very little gas being fed through it, possibly resulting in poor combustion. One of my previous cars (an early Nisson Maxima sedan, standard trans, fuel injection) would artificially add some throttle under engine braking to improve combustion and reduce the pollution output. (This made engine braking unpredictable and dangerous--thus I defeated the "feature".) I haven't seen this on any other car so the problem may be engine model dependent.

Doug

 

[MattC]

Same principle works if you and your buddy are cycling together. You can alternately draft each other. Reduces the effort considerably.

Or if you're riding with JayH you can just let him go ahead the whole time.

Interesting thread.

http://www.hypermiling.com/

Matt