Two Letters Re: Extending Your Vehicle's Driving Range, Without Cost

Tuesday, Aug 20, 2013

Hello Mr. Rawles,
Some thoughts on the recent post on Pulse and Glide driving (PGD), taking for granted that safety is always more important than fuel economy, and not considering any survivalist aspects:

I don't doubt the core claim made by Steven B. - that his use of PGD has reduced his fuel consumption. I am, however, skeptical about some of the other statements made in support. I note that while my comments are
based on my experience as an engineer and physicist, I have not done any tests of PGD versus other driving styles. If an actual automotive engineer writes in, please trust them and ignore me!

Disengaging the transmission and letting the engine idle while the car coasts is not necessarily the best advice. The fuel control system in a modern car does "cutoff on overrun." It will not open the fuel injectors
at all if it thinks that the car is "driving the engine". In this case letting the engine idle will increase fuel consumption since the engine must now burn fuel to avoid stalling. Exactly how modern the car must be
varies with manufacturer, but I would expect a 2008 vintage car to have this feature. Your car manufacturer can tell you, or you might be able to find out yourself with one of the myriad diagnostic port readers on
the market.

If you have cutoff on overrun the best thing to do on a downgrade is leave the transmission engaged. The fuel consumption will be reduced as the power from the descent replaces the power from the fuel. On all but
the mildest downgrade the descent will provide more power than necessary to idle the engine, fuel consumption will drop to zero, and you will have to use the brakes slightly to prevent picking up speed.

When using PGD on level ground the utility of cutoff on overrun is less clear. With the transmission engaged during the glide (and your foot off the gas!) the fuel consumption will still be zero, but the car will decelerate faster, making the next pulse come sooner, meaning more pulses and thus higher fuel consumption per trip. This would be interesting to test, if I could spare a few weeks and tanks (fuel here is about $ 9 USD per gallon). When approaching stop signs, lights, etc. you should have the transmission engaged to get the zero fuel consumption. There might be no reason to burn more fuel than at idle, but there is a
reason to burn less!

As you point out in there is a minor safety issue in having the transmission disengaged and thus being unable to quickly accelerate out of danger, though I can count on one finger the number of times I've had
to do this in twenty years of driving.

The advice to go easy on the brake pedal is spot on - brakes "throw away" the car's energy rather than using it to overcome drag or climb hills. In the long term the energy has to come from the fuel, so every
bit of braking is burning a tiny bit of fuel for no reason.

The advice to avoid engine braking is less well founded. Engine braking is exactly what's needed to activate cutoff on overrun (assuming the car can do it). And while it's certainly more stressful than idling it's
still a very small stress compared to acceleration. As Steven states, engine braking is not an effective (i.e. quick) way to slow down, and mild deceleration means mild stress. Compare any car's 0-60 time with
its 60-0 time using purely engine braking - the engine's working far harder during acceleration.

The advice on drag is correct, though the term should be "parasitic" drag. This point is actually the most important, and deserves to be at the top. For a given car with a fixed body shape and fixed accessory load (e.g. air conditioning) the biggest contribution to fuel consumption is drag a.k.a. air resistance. At city speeds it's a major component of the total fuel use. At highway speeds it's overwhelmingly the greatest.

If drag reduction is number one then load reduction is number two, and Steven's advice here is good. Air conditioning ("aircon") is likely the largest load and keeping it off will reduce fuel consumption but if you then open the
windows to keep cool the increased drag may negate the savings. Another interesting thing to test.

Headlights are a much smaller load than aircon, but you just might detect an improvement from keeping them off. Other electrical loads tend to be things you can't usefully and/or safely turn off, like fuel pumps,
power steering pumps, demisters, etc.

Not mentioned is the importance of keeping your tire pressure correct which I would rank number three, though the fuel consumption change between "correct" and "dangerously underinflated" is probably less than 5%.

The use of high octane a.k.a. premium fuel is debatable. On one hand modern engine control systems are smart enough to adjust the fueling and ignition to avoid knocking with whatever fuel they're using (within
reasonable limits!) On the other hand "premium" fuel here does not have the 5% ethanol that the regular does, which could create the appearance that "high octane" provides better range despite octane rating having
nothing to do with it. For what it's worth I follow Steven's advice - burn the cheapest fuel you can find that doesn't knock.

Regarding the core claim: I suspect the reason PGD is giving reduced fuel consumption is nothing to do with pulsing, gliding, idling, braking, or anything like that. I suspect it's simply that PGD results in a lower average speed. If a safe speed is (say) 60 mph, then driving normally you'll likely stick close to that, and maintain an average speed of 60 mph. If you repeatedly pulse to 60 mph then glide to 50 mph your average speed is somewhere around 54 mph.

54 mph is 90% of the non-PGD average speed. Since drag force is roughly proportional to speed squared, the average drag force is now 90% squared i.e. 81%. Since power required is roughly proportional to force
multiplied by speed the average power consumption is down to around 73% (81% force by 90% speed). In the long term power can only come from fuel, so power consumption is fuel consumption. These numbers are not perfect, but I think they're close enough to explain the observed 25% saving. I would be very interested to hear from anyone who has compared driving with PGD, then normal driving at the same average speed, say for a month of each style.

My thanks to Steven for an interesting letter, and to you for SurvivalBlog. - Ross E.

 

James:
Sorry, but I have to take exception with Steven B. and his use of this technique of PGD (Pulse and Glide) on public roads that must be shared with others.
 
I have seen this “technique” in use on the roadways here in Florida and while I never knew it had a name, it doesn’t surprise me. I always have thought of people using this driving “skill set” as the problem drivers or more commonly as “That A**hole”.
 
What you are doing is extremely dangerous to others moving at constant speeds and your sanctimonious technique of slowing down then suddenly speeding up, will inevitable causes someone to have to make an evasive maneuver or slam on their brakes because they were accelerating to get around you and your indecisiveness in not maintaining a more normal and acceptable speed.
 
In short you will be the cause of road rage in others.
 
I would suggest that you may want to rethink this dangerous practice – frankly it is likely to get you pushed into a ditch or shot in the event that you cross paths and upset someone of with a short fuse – especially in a SHTF or bug out situation. - G.W.


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