Wednesday, August 20, 2008

Coasting, Engine Braking, Pulse and Glide

Read a very interesting post at about engine braking vs coasting in neutral.

Apparently when coasting in gear or engine braking (on a standard), the engine cuts off fuel altogether while decelerating until it hits a predetermined limit. When coasting in neutral the engine still uses fuel but at a low rate(idle).

I'll have to experiment with this and compare a downhill glide in neutral vs. say 5th gear.

ModeSustain SpeedFuel Ef.Safety
Coasting in Gear worst best best
Coasting in Neutral good worst good
Coasting Engine Off best best worst

Tuesday, August 19, 2008

33.3 mpg today

Got 33.3 mpg today.
The main changes made today were leaving earlier to avoid congestion and changing my ratio of pulse to glide.

By leaving 15 minutes earlier I was able to avoid some congestion. Will try leaving even earlier tomorrow.

I tried changing my ratio of pulse to glide from 1:1 to 1:4 but only got it up to say 1:2 ot 1:3. This really did not contribute much to overall mpg. Will try again on the way back.

I got stuck behind some slowass this morning ( calling kettle black... I know ) which ruined my opportunity to practice p&g. Worked around it by just going slow ( 30 in fifth ) - ok, so this was probably good in terms of wind resistance but really did not add anything to my mpg.

Thursday, August 14, 2008

Mileage Stats Day 3

I got 33mpg on the same commute today on the way to the train, 31mpg round trip. ( again, car is rated at 20/27 and has AWD )

What was done differently today:

Tried using pulse and glide a few times on somewhat flat areas ( gliding in neutral )
- not sure if the ECU cuts off fuel when gliding in neutral or if you need to down-shift for this to happen.
- not sure what event tells the ECU that it should cut off fuel

Tried making turns more gradually where possible ( to reduce friction at tires )
- wider more gradual turns - ie. moving a little to right side of lane before turning left (when safe to do so)
- going as straight as reasonably possible though S shaped turns.

Tried idle-gliding faster when possible
- I figure idle-gliding slowly should be less efficient then idle-gliding fast since when gliding slower
you spend more time running idle.

I got stuck in traffic and had to crawl for about 2-3 minutes total - with no good opportunity to turn off engine.
Lost an opportunity to glide down hill due to traffic jam.

Was a little sloppy in controling rpm's. Last few days I have been keeping rpm's below 2250rpm at all times, doing 30-40 in 5th etc... I few times today I did rev higher - perhaps 3k. Wondering how optimal 2krpm really is. Will try commute at 3k on day, and 3.5k another day to see if its better/worse

Wednesday, August 13, 2008

Mileage Stats for Day 2

Day 2

Took Backroads ( 10.4 miles )
To Train : 31.5mpg = 0.3301g

This is what was done differently:
Drove in neutral largest hill on my route.
Used Slower acceleration especially in first and second gears.
Short shifted at about 2,250 to 2,500 rpm
Took advantage of slopes with modified pulse and cruise technique.
Tried to avoid stopping at lights by slowing down well in advance

Traffic got stopped on one hill on the way resulting in need to brake and loss of momentum.

Tuesday, August 12, 2008


I have found a new interest, "Hypermiling".

In a nutshell, "Hypermiling" is the art of driving with as little fuel as possible - or attaining the highest possible mpg while driving. You can hypermile with just about any car - you don't need a Prius however it does help if you have a manual transmission aka. stick.

My 08 Subaru Impreza is rated at 20/27mpg, however using simple hypermiling techniques I have been able to get anywhere from 27.5 to 31.5mpg so far on short 10 mile runs in a very hilly area - and the car hasn't even been broken in yet. This car is not an optimal choice for hypermiling. I chose it primarily for its all wheel drive feature. The question remains however, how much mileage can it get given optimal driving techniques ?

Some common techniques which work with this vehicle are:

1. Gentle Acceleration

Accelerating rapidly forces more gas into the engine and as a result it gets burned less efficiently. By accelerating at a slower rate you get better mpg.

2. Driving at low RPM

For each (engine) rotation, the engine sucks in gas (in one cylinder), compresses it(in another cylinder), ignites it(in another cylinder), and dumps the exhaust out ( from another cylinder). So the more rotations per minute, the more gas you are igniting. Lower rpm means less gas burnt. There is of course a limitation. if the rpm gets too low the car will lose power and then stall. The trick here is to find a comfortable place where the engine runs optimally. In short this means driving at the "bottom" of each gear. In my case, doing 40-55mph in 5th, 30 in 4th, and staying in 1 and 2 for as short a duration as feasibly possible.

** Correction - there are cases where the ECU stops injecting fuel into the engine. In cases like this the fuel efficiency would be far better than idling. As I understand this will happen when you coast down hill and engine-brake. not sure what triggers the ECU to do this **

3. Short Shifting

This hinges off driving at low rpm. If the car has enough torque, you can shift up to the next gear early - say at 2,250 to 3000 rpm instead of 4,000 to 5500 rpm. The car has less power in the lower rpm range, and more power in the higher range. Each car has its own unique "curve".

4. Coasting in neutral
Again, this is about reducing rpm. The idea here is to switch to neutral when you see an opportunity to coast down hill. I've read articles that discuss turning off the car while going down hill - while this is more optimal its not safe. If you value your safety and the safety of others over a few mpg then turning the car off while moving is not an option. Coasting lowers the rpm to idle ( 750-1000 rpm in my case ) which is a significant savings in fuel efficiency.

5. Pulse and Glide
As explained in other articles this involves accelerating past the speed you want to travel at, and then cutting off the engine while you drift down slightly below your target speed. The result is roughly double your mpg however this cause wear and tear on your starter and means having to turn off your car. I have applied the same principal to my driving but without having to turn off the car. Here is what I do.

A. When a hill is approaching, gently/gradually accelerate to about 10mph more than the desired speed.
B. If the decline after the crest is going to be large enough to accelerate the car, then lift off the pedal and shift to neutral before hitting the crest allowing the car to slow slightly as it climbs using its existing momentum. Once past the crest gravity will take over and pull it down - ride it out from there.
C. If the decline after the hill is shallow to flat, then continue to gently accelerate until the crest is reached, then switch to neutral and ride it out as far as possible.

The results of doing this will vary given the road's inclines and declines so its hard to estimate how much this actually saves.

Below is a small listing of the results I achieved so far:

Commute to train via Highway = 11.4 miles
Commute to train via back-roads = 10.4 miles

Day 1

To Train: 29.7mpg = 0.3838 gal
Round trip: 27.6mpg = 0.8260 gal

To Train 28.2 mpg. = 0.3638g
Round trip: 27.9mpg = 0.7526g Traffic stalled at one point