“Be kind, for everyone you meet is fighting a hard battle” - Often attributed to Plato but likely from Ian McLaren (pseudonym of Reverend John Watson)

Thursday, June 22, 2006

More on acceleration

I timed some of my snail-like starts, timing how long, on average, it takes me to go from 0 to 10, 10 to 20, etc. and finally from 50 to 55 (the fastest I ever go). I took the average of four times for each interval and plugged them into a spreadsheet. Then I graphed them and found a regression curve that had the best fit.

It turns out that it takes me, on average, about 53 seconds to go from 0 to 55 m.p.h. I'm guessing, from their wailing and gnashing of teeth, and their honking, flashing of lights and gesturing, that this seems kind of slow to many of my passengers and fellow drivers. Ah well, anything for science.

In any event, I took the regression curve and integrated it twice from 0 to 53 seconds and determined that it takes me about 890 meters, or about 2900 feet to accelerate from a stop to 55 m.p.h. I then plugged times into my spreadsheet that fit the same shape of curve but that totalled 9.7 seconds (I was looking for about 10 seconds) to model fast acceleration from a standstill. This isn't flooring it, I determined a long time ago that the Jeep will go from 0 to 60 in about 7.4 seconds minimum.

Utilizing the same mathematics, I determined that it would take about 150 meters or 490 feet to go from 0 to 55 m.p.h. Now, in each case, I have changed the potential energy of the chemical bonds in gasoline into an amount of kinetic energy of the car that is identical for each acceleration regime (a little over 600,000 joules). As I mentioned in an early post in this blog, the energy of burning fuel goes to overcoming the forces working against the motion of the car and to adding kinetic energy to the car.

So I've gone about 2400 feet farther on the same amount of fuel by accelerating slowly. That's about 0.45 miles. Supposing I do the equivalent of this amount of accelerating about 15 times per day, I get something like 6.75 miles extra by accelerating slowly. At 32 miles per gallon on the highway, that's about 0.21 gallons of fuel saved.

I'm saving something like 1.5 gallons per day compared to my old driving methods, so I'm estimating that about 14% of my fuel savings come from leisurely acceleration. The rest come from some combination of lower freeway speeds and extremely conservative energy management, that is, coasting to stops, coasting in neutral when going downhill, turning off the car on long downgrades and at long stops, etc. I'm not sure of the division here, but I'll try to figure it out.


Bill Anderson said...

Depending on the age of your vehicle, putting it in neutral may use more gasoline than leaving it in gear and coasting. This is due to software management choices.

I know that in the Chevy LS1 engine the software knows when you are coasting and if you are in gear or not. It uses this to determine how much fuel is needed to maintain the engine running. With the vehicle in gear the assumption of the programmers was that the motion of the car would be sufficient to maintain engine rotation. As a result they set it to dramatically cut fuel supply.

When you take it out of gear and/or push in the clutch, the EMS will use more fuel than the above scenario to ensure the engine stays running. While I can not say which cars have an EMS so programmed, I am told by those involved that it is pretty much a standard practice.

I'm working on a hardware/software combo for extracting the particulars out of my LS1 and should be able to measure this as well as hards start versus soft start using engine data. Until then I don't know just what the differences (neutral versus gear) are but the LS1 engineers tell me for them it was worth doing.

Just a thought. Do keep up with your experiments, I find them interesting. It will be interesting to compare the data I get from my project with your calculations.

King of the Road said...

Yes, I have heard this and don't know if it's true for my vehicle, or actually, for any vehicle. I do know that the "instant mileage" indicator on the display cannot discriminate between in gear and out of gear coasting.

A few posts back I had one called "Dilemma" I think it was. In it I mentioned that I'm interested in experimentation but loathe to lose tenths of a gallon. This may be one I can try, but I suspect that MANY tanks will be required to determine a trend, as the overall effect will be small and tend to get lost in the noise.

Anyway, thanks for the feedback.

Roland said...

As far as I am aware any vehicle with electronic fuel injection has this feature. My driving instructor taught me that the fuel would be cut off when the car drives the engine, thus saving gas (or in my case back then diesel).

I drive a 2007 Civic with a manual transmission and this technique is my best friend when driving. Even in my father-in-law's 2007-2008 Camry with the slushbox the engine will drag a little (keeping revs to 1100-1300) when coasting in D.

I am quite certain that your engine does the same when decelerating. Not meaning to presume you haven't attempted this, but still being compelled to ask: Have you tried "engine braking", i.e. downshifting to use the mechanical drag of the engine to slow the car?

BTW: I thoroughly enjoy your different take on efficiency and how it relates to improving gas mileage. Keep up the great work!

superchow (also on Ecomodder)