How to Become a Hypermiler

Step 6 Pulse and Glide

Assume you want to average 35mph.

Start at 40mph and allow your vehicle to decelerate, in Neutral, to 30mph -- this is called the glide.

Then, accelerate back up to 40mph in the same amount of time that your glide took -- this is called the pulse.

For the more intense, glide with your engine off. This will increase your FE number dramatically while gliding in gear will reduce them as you'll be combating engine braking.

This technique has been proven many times over to be an effective way to increase mileage. But, your mileage will vary based on traffic and other drivers. Other drivers will get quite pissed off at your pulse and glide, so use your best judgment.

Comments

[crispy010]

The reason P&G may get you better mileage at low speeds is because the engine is operating so far below its peak thermal efficiency when powering the car at those speeds. Put another way, the car has too much power. Peak thermal efficiency is, as I think was stated above, around 75-85% of full power. To get maximum mileage while still being able to cruise at highway speeds, all cars should have about 65 hp (in general, it takes 30-50 hp to cruise at 65-70mph). However, turning your car OFF while moving is a BAD idea. When the car is off, your power steering and brake boosters no longer function. This essentially means your car becomes a 3000 lb uncontrolled brick on wheels. Please DO NOT turn your car off while moving. Shifting into neutral will get you most of the efficiency benefits. Additionally, turning your car on repeatedly will wear out your starter motor, battery, and electrical system - they are not designed for that many cycles. Even in hybrids such as the Prius, the gasoline motor is "bump started" instead of being turned over by an electric motor.

[vandal1138]

People also don't realize that the amount of gas utilized in starting the motor is greater than leaving it running for the time being, so unless you are coasting down a LOOOOOOONNG hill, you're wasting gas by shutting off the engine and starting it back up again.

[xaenon]

The rule for carbureted models, if I remember, was 'shut it off if you're going to idle more than about five minutes' - the five-minute figure being based on the idea the engine would use as much fuel at restart as it would 'waste' idling for five minutes. All assuming, of course, that everything was working as it should, which with carburetors, was not all that often the case.

With modern fuel-injected cars (port-injected in particular), the amount of fuel required for restart is far, far less - enough that you'll actually save fuel if you shut it off and restart it when needed, rather than let it idle for TWO minutes.

[V-Man737]

Really old cars use a lot of gas on startup. More recent models (1996+, I think) have really cut back on that. Turning off your engine is quite effective, if it is new.

[Dude567]

Yes but in the first 90s of your engine running 75% of the pollution is emitted because a cold catalytic converter is not very efficient

[ac-dc]

It's not going to be very cold, there aren't long enough periods the engine would be off for it to cool down that much, and if you were implying that it takes 90 seconds to warm up, no it does not, the thin honeycomb structure in any recent vintage cat. converter heats to temperature far faster. For once, greed on the part of manufacturers to reduce the amount of costly precious metal in a cat. resulted in several useful gains.

Modern engines can be designed so that their emissions stay within EPA mandates for pollution, haven't you noticed some street legal cars now do shut off their engine when they are stopped (stop light, etc.)?

[swander]

This is a joke. If you dont have a hybrid or electric car, this pulse and glide is a wash. Take a carbureted car for instance: Every time you mash the pedal, you are squirting up to 80cc of raw fuel into the carb (Holley 850 DP with 50cc pump on rear) This is an extreme example but it is a valid concern. This fuel is way more than is needed to combust so you get the telltale black puff out the rear. Keeping a steady rate will net more mileage than this dangerous illegal practice of turning the motor off or varying your speed 25% on a shared road.

[mijhy]

As a professional driver, i thought i would add my 2cents to this particular step. While they are obvious benefits of "coasting" I want to point out it can also be very dangerous. In a winter climate much of the control you have over your vehicle is from your engine, disconnecting it from your wheels leaves you no chance to correct a slide if you get into one. Yes you could only do it in summer, but once you fall into a familiar practice it would be all to easy to repeat it with out thinking, and then when your in a ditch or upside down you realize that 1% gain in fuel efficacy wasn't worth it. There are some comments about brakes going down hills, well of coarse you have to break, but what if you coast to a stop light, forget your not in gear, and roll back into another car. Every state is different on passenger cars, but it is federal law, that any commercial vehicle never be out of gear for more than a few seconds (time to shift). Again its only my 2 cents, but it is something to keep in mind.

[ac-dc]

while using powered traction to get yourself out of a slide works for professional drivers, the average person on the road tends to overcompensate trying to do so and ends up that much worse out of control than if they only concentrated on steering and braking.

[reddeth]

It takes a lot of power to get up to 40 MPH, even going from 30, this glide on/off idea is not only dangerous, but cannot stand to be truly effective, you're using a lot of an engines power to make it back to 40 MPH, this may be different for electric/hybrid cars running on the electric motors, which in turn use deceleration and braking to generate more power but on a traditional gas powered car this would not be a productive enough maneuver to outweigh the danger you present to other motorists by the constant accelerating/decelerating.

[trebuchet03]

...but cannot stand to be truly effective...

The pulse and glide method has been empirically tested by many people. Remember, while linear energy (momentum) is conserved - power is not. This is where the pulse and glide method shines. As for danger, possibly. I find myself much more alert and in every defensive driving course I have taken - evasive action is done by braking and steering.... Not gas and steer ;) Also take into consideration that you should be giving yourself plenty of space to begin with ;)

Here's Just one, well written example

[Dude567]

Yes, but the laws of the universe require that one day all drivers will have to deal with a tailgating jackass, trying to save a little on has fuel bill. And, personally, an acceleration is physically more difficult than maintaining speed. Ever ridden a bike? Then you know what i'm talking about.

[reddeth]

I still find it questionable, but this isn't a forum for discussion, thank you for the link, thats all I'll say on the subject.

[trebuchet03]

I still find it questionable,

That exactly the attitude I want from people :) And I mean that with no sarcasm whatsoever :D Go out and test for yourself, see what data you end up with :D There are some better explanations out there which I can dig up if you'd like - but all I ask is that you keep an open mind :)

But, as a quite example from my numbers.... I might pulse for 7 seconds while getting 20 or so mpg -- and then I'll glide for at least double that getting ~180-200 mpg (that's engine on, clutch off). If I were cruising for that same 21 second period, I might be getting 31. That's not enough information to put everything together (I'll have to go out to my car and grab my notebook for that), but perhaps that glide number brings it into the realm of feasibility for you ;)

[ac-dc]

The correct answer is that for most vehicles pulse and glide results in worse fuel economy. It only helps with specific vehicles, most of which are hybrids.

The problem is the method was suggested as if it applies to any vehicle (by omission of vehicle parameters necessary to benefit from it) when for "most" people it is worse because most people don't own applicable vehicles.

[Dude567]

Engine on clutch off you are technically getting 0 mpg to be useful that data should be gph crossed with the coasting time and distance moved while coasting. Call me when THAT'S been calculated.

[jimmytvf]

yes, is effective in save money in gas, but it doesn't if you have to buy new brakes. is going to be more expensive the brakes than a few ml. of fuel.

[trebuchet03]

Don't brake during pulse and glide ;) This is a well documented driving technique.

[jimmytvf]

i'm saying that because in downhill the engine make a little inertial brake, and if you turn off the engine, that inertial brake won't work and you have to apply more brakes. in flat surface will work good ;)

[trebuchet03]

It is quite possible to inertial brake with the engine off if you have a manual transmission ;)

Then again - on hills, I'd rather use my brake than use my engine as a brake. Brakes even have the name :) Needlessly having pistons slide in the head to slow down your car - it's much cheaper to replace brakes than rebuild an engine ;)

[Achan20]

lol. can you even drive that car...... because i would...... haha

[Rishnai]

Seems to me that it would be more efficient to smoothly accelerate to cruising speed where you can have a nice low rpm at a steady speed, no revving up and down. Depending on your gearing, that might be 30 or 40, not 35, but it's a big factor.

[slurp812]

I know it seems that constant speed is better, but the motor is more efficient when given around 70% throttle. I was testing this out myself with my scangauge II today. 37.6 mpg (20 mile trip including expressway) is not bad for a car rated 26/30. I actually got better mileage when I got off the freeway, and pulsed/coasted the car...

[trebuchet03]

That's the problem - the "seems like" intuitive approach doesn't work - instrumentation dictates what works. Pulse and Glide is a proven technique, it's not new nor is it novel ;) Intuition is great for many things, but not for determining power/fuel density.

[Rishnai]

I've got an idea: let's test it with a car that automatically computes its milage, and see what's the better approach. Crazy as it sounds, your pulse-and-glide approach might actually be quantitatively superior.

[trebuchet03]

See step 5 - http://www.instructables.com/id/S5EGQZ7F5HVHRXQ/

I, and others, have tested using this instrumentation (provides instant, several averages, etc. consumption feedback and can be calibrated or used as a relative scale comparison) ;)

A more eloquently written (and more encompassing) explanation of P&G (pulse and glide) can be found here and includes empirical data. That sort of detail is beyond the scope of this introductory type of project.

Also, if you check out techniques used by ecomarathoners (whom get crazy 1,000+++ mpg) - this is one of them.

[Rishnai]

Well, I stand corrected. My apologies. And thank you for the link, it was quite interesting.

[Outlander]

This step does work, but at a cost. The cost is that engaging and dis-engaging the clutch on and off, on and off, on and off, over and over will wear out the clutch disc, the slave and/or master cylinder much quicker. Simply costing up to a speed say 65mph, than dirfting without using the clutch will help save on gas. Only put the clutch in if one can coast down a hill, or maintain more than a few seconds of time before one needs to "pulse" again. In an automatic, switching between drive and neutral all the time will destroy the transmission, no question or if's about it. Hope the fuel economy out weights the repair costs, cause on my mustang it sure doesnt. Even though I have gotten 30mpg highway by using the "pulse and glide" method, the repairs are more expensive.

[trebuchet03]

We'll let you know when undue repairs come up ;) As of yet, no problems ;)

I probably should have mentioned that it's highly beneficial (for an driving) to rev match before engaging the clutch disk. I'll add then when I do my next edit :)

In an automatic, switching between drive and neutral all the time will destroy the transmission, no question or if's about it.
I should tell the auto guys that :p They'll probably laugh, but they were told :D

[Outlander]

1995 ford Taurus. Destroyed it when I switched from drive to neutral while driving. At least it was under warranty. ASE certified, have rebuilt GM TH200C's TH200-4R's TH350's(a real POS always drops 3rd gear) throwing the trans out of gear and back into gear will destroy it, at worst you will wear the bands and clutches out early, possibly even blow out a seal from the rapid increase of pressure and drop a gear(200C). Also playing with the overdrive(on off, on off) with kill a trans as well. For an example, abuse the neutral to drive until you wear out the trans, take it apart and see what you find. On a manual, if you have ever had to change a clutch set( raise the car on stands, remove the driveshaft, then the mounting hardware, then the trans, then the clutch, then the flywheel) you will understand the grief and pain of swapping a out a clutch and finding someone to resurface the flywheel. Take it from a mechanic, this method is not all what its cut up to be. Especially if all you have to do is replace a stupid $20 throwout bearing, a real nightmare. Automatics are MUCH easier in terms of maintenance.

[trebuchet03]

a real nightmare.

I actually enjoyed it ;) A few people have asked why I'm doing Engineering rather than being a mechanic... If I got payed to do the work, I wouldn't enjoy it so much ;) I don't have to take it from you, or another mechanic - I know what's involved - and yes, it sucks when you're replacing the stupid parts (RMS in my case). Finding someone to resurface isn't such a big deal -- just make a few phone calls, and drop off/pick up... Done.

Taking apart an automatic is much more fun, in my opinion - but only because there's more mechanisms and bits :p It is for this reason I don't drive an automatic.