A long post on coasting vs rolling

[iariss]

Hi! This is my first post, and it's a big one, sorry! I hope it's not too long and complicated.

Many hypermilers will already know what I've written here but here is a breakdown of one of the biggest factors in fuel efficiency to help any others get their heads around how people get such huge mpgs.

When cruising I can just about squeeze 100mpg(US) out of my 1.4l diesel Yaris (2003) and 77mpg overall. I use drafting*, coasting* and pulse-and-glide methods when it is suitable and safe to do so. But my yaris' official figure for rural mileage is 54mpg. So here's how almost double the efficiency is possible with the help of coasting...

We know that the faster your engine runs, the more fuel it consumes. As an example, my Yaris uses 0.15gal/hour at idle and 0.4gal/hour to hold 2200rpm. That's before the car's even started moving. 2200rpm would be roughly 60mph and so the engine itself is consuming 0.4 x 60 = 24mpg worth of fuel at this speed (2.67 times more than idle). Assuming the official figure is attained at 60mph this means that the engine itself is using up 44% of the fuel!!

We also know that modern engines cut the fuel supply when we lift our feet off the gas. The kinetic energy of the car turns the engine via the transmission so no fuel is needed in the engine. If we coast, the engine will continue consuming fuel at roughly 0.15gph (in my case) which at 60mph is 9mpg. So it seems logical to not depress the clutch and gain – at least momentarily — 9mpg. "But what about engine braking?" I here you say...

So is it better to coast and lose engine braking but use an extra 9mpg or lose the 9mpg and be lumbered with engine braking when we don't want it?
The answer is to depress the clutch and lose the engine braking*. This ALWAYS holds true when you are wanting to maintain anything near a cruising speed on the flat, regardless of car, fuel, transmission, weather, drafting possibility. Even if you're in a hybrid actually. Here's why...

Each time energy is transformed to a different type you lose some. In the engine, lots of energy is wasted transforming the chemical energy to heat energy and then again into kinetic energy. But when the engine is idling here's where this wastage ends. So in my case I lose 9mpg.

If we coast with engine braking, kinetic energy is transferred from the car's mass through the transmission and back to the engine. This means energy from the fuel has been transferred to the car's mass and back again, losing more efficiency. The engine is also being turned over at 2200rpm. So not only do we lose more energy but way more is required (2.67 times as much) to turn over a faster engine. The result is engine braking and a loss of distance/speed which far outweighs the amount you'd save in fuel.

So my tip is to coast on the flat when you don't actively want engine braking. However, if you're going downhill and can maintain a reasonable speed while rolling, then rolling is best option (If it is a steep downhill you should never coast!). Also if you're traveling at in-town speeds, it doesn't hurt to shove it in top gear and let the engine be turned over at or just above idle speed by the wheels. Here the losses are almost negligable.

To see how big a benefit this is get yourself a cheap OBDII device and download a free app for it. Go say a (necessary) ten-mile drive and try the two methods and see the difference. Consumption will seem high on the pulse part of the pulse-and-glide but you will find that your average consumption plummets.

A quick note on accelerating: Only allow the engine to turn at high speed for as little time as possible. This reduces the time your engine spends swallowing 0.4gph (in my case) for its greedy little self. Accelerate at a decent rate of around 50% gas and then make a long slow change into the next gear while your engine momentarily drinks next to nothing. Don't rev the engine high, try to make use of its torque. Use this method also to keep pulses as short as possible when pulsing and gliding.

Hybrids and electrics (regeneration / engine braking): Even with these cars the same holds true; energy is wasted each time it is converted. Don't think for a minute that the car's momentum can more efficiently turn the engine when traveling at high speed than the engine can turn itself when idling at low speed! Also with fully electric cars, it is more efficient to not use the energy from the battery in the first place than to use it to turn the wheels then use those turning wheels to charge the battery again!

My trip to work takes 40 minutes. It takes about 45 minutes when I use this method and use about 25% less fuel. Its a no-brainer!!

* Illegal in some juristictions. Also be aware that constant clutch use when cruising will result in increased clutch wear. When coasting only ever depress the clutch; never actually put it in neutral. This allows faster reaction times in emergency stops etc. or if the engine cuts out it can be restarted by simply lifting your foot.

[Draigflag]

Very interesting, thanks for taking the time to write this, how did you calculate rate of consumption at idle? I have experimented with coasting in neutral in the past as there are some large hills here. After reading this, I'm going to try an entire tankful and see if I get better fuel economy.

[sea_king18]

I've found in my cars (manual transmission) that the drag from engine braking is quite significant and I estimate that I roll about 4X longer out of gear than in it. This would mean that the math would work out like this:

Let's be conservative and say that I only roll three times as far. When rolling, I would cover three units of distance X while the engine consumes however much fuel it needs to idle. To engine brake and take advantage of the fuel cutout, I'd end up driving two of those units in 5th (consuming whatever it consumes in fifth @ 100 km/h) and using zero fuel for the third unit.

So it would look like:
|idle|idle|idle|

vs

|5th |5th |zero|

To cover the same distance, if my consumption at idle is less than 2/3 of the consumption in 5th, I'm further ahead to roll and let the car idle.

In my one car that does have a fuel cutout, it only kicks in when the engine is warm and the car is in one of 3rd, 4th or 5th gears above 1900 rpm. (Below about 1200 rpm it puts the fuel back on to avoid stalling.) 1900 rpm in 3rd gear is 50 km/h. This means that in town, going from stop to stop where I never reach 50, there is no value whatsoever to being in gear. Bump the car up to speed, roll to the next stop sign. It will take the engine longer to warm up this way, so it's less helpful in the winter if I don't plug in the block heater.

If I were driving an automatic, I'd do it differently. I hate putting an auto back in drive at speed. I just let the fuel cutout do its thing and hope for the best. With a manual, it's fairly natural.

Note too - as above, if I'm on an incline and don't want to pick up speed, I'll leave it in gear. No point in being dangerous.

[R.I.D.E.]

At 20 mph my Mirage is getting 120 mpg coasting in neutral with the engine idling.

Engine off it's infinite mpg.

Getting 60 mpg at 60 mph requires something I call pulse-drafting. On our interstate 64 there is a construction zone immediately to the east of where I get on the road. To get an idea of how heavy the traffic can get in the worst case, at the eastbound entrance to the Hampton Roads Bridge Tunnel, during tourist season, a car passes over the same spot in each of the two lanes every two seconds. 94,000 per day, 86,400 seconds per day. That leaves an AVERAGE SEPARATION of about 130 feet at 55 mph (speed limit).

I just get behind a larger truck or bus and let them break the wind for me, coasting at even the slightest downhill section of the road. Here the downhill is never are more than the downside of an average interstate overpass on otherwise perfectly flat terrain.

Since the Mirage weighs only 1960 pounds driving without power assist to the brakes or the electric power steering is no problem, having driven a 1967 383 Formula S Barracuda that had neither power brakes or steering, weighed 2800 pounds with a 700 pound lump of cast iron over the front wheels. Or the 1959 Corvette with no power steering and manual DRUM brakes.

My idling fuel consumption is about .15 gallons per hour (US).

Coasting in neutral at 65 mph shows close to 500 mpg, engine idling. At that speed the engine does not drop to as low an RPM as it does when the car is stationary.

[benlovesgoddess]

Interesting stuff!

[iariss]

Hi Draigflag,
I have a cheap but effective OBD11 device from Amazon, only 15EUR, and a free app, "Oliviadrive".
This tells you the current consumption – even at a standstill – in litres per hour etc. So to find out your engine's consumption alone simply sit at a standstill with your foot on the accelerator at the desired rpm and read what it says.
You can calibrate the app so it's accurate to within a couple of percent I reckon. My entries on here though I take from each time I fill up so they're definitely accurate.
BTW, how do I get my consumption figures in my signature? Can't find it in my profile...

[iariss]

Originally Posted by sea_king18

In my one car that does have a fuel cutout, it only kicks in when the engine is warm and the car is in one of 3rd, 4th or 5th gears above 1900 rpm.

Hmmm that's interesting. It would certainly make it more difficult to work out the difference! But yes coasting is definitely more of a benefit to you in this case.

[iariss]

Originally Posted by R.I.D.E.

Getting 60 mpg at 60 mph requires something I call pulse-drafting.

That should be the new name for it! This is something I do when I'm not in a rush and when I don't spend too much time thinking about clutch wear. Typically a pulse will last 4 seconds or so and a draft 12 on flat ground behind a truck at 55-60mph. I love a nice tailwind too allowing that draft to be stretched out really looooong.

[OliverGT]

Drafting is dangerous and gives hypermilers a bad name, everyone thinks we use drafting.

I for one would not recommend it to anyone and don't believe there are any gains to be had from it.

If you want to use it, fine, but don't mention it in the forums as though it is an accepted hypermiling technique, it's not. In fact there are other forums dedicated to hypermiling that go out of their way to distance themselves from it.

I suggest we do the same.

There are plenty of safe techniques that will result in good MPG gains, lets stick to discussing those. I'm not stopping you using it, but don't bring it to the forums please.

Apologies if this is a little strongly worded, I'm not trying to offend anyone, but let's keep the discussions to safe and legal techniques.

Oliver.

[Draigflag]

Originally Posted by iariss

Hi Draigflag,
I have a cheap but effective OBD11 device from Amazon, only 15EUR, and a free app, "Oliviadrive".
This tells you the current consumption – even at a standstill – in litres per hour etc. So to find out your engine's consumption alone simply sit at a standstill with your foot on the accelerator at the desired rpm and read what it says.
You can calibrate the app so it's accurate to within a couple of percent I reckon. My entries on here though I take from each time I fill up so they're definitely accurate.
BTW, how do I get my consumption figures in my signature? Can't find it in my profile...

Yes both my cars have lots of useful feedback about fuel economy, when in gear coasting, I get 0.00 MPG meaning the cars fuel cut off is working, in neutral, I get 99.9 meaning the car is using a minuet amount to idle.

To add a signature, go onto your cars profile and scroll down. Towards the bottom right, you'll see a link for banners and badges. Copy and paste the code into your signature, which you can edit on the forum by clicking "user CP" above, and then "Edit signature"