Is there any point to P&G in a diesel?

[user removed]

In 1970 I read about an Opel Kadett that was hypermiled with pulse and glide from 15 MPH to 45, then engine off coasting. It was in a contest in Popular Science of Popular Mechanics. The rules required an average speed of 26 MPH.

The Wagon was more aero than a sedan. They put radial tires on the car (allowed by the rules), and disconnected the vacuum operated second barrel of the carburetor). A piece of wood under the gas pedal so it would always go the ideal distance.

124 MPG.

Now if you consider the car got about 32-35 MPG at a constant 25 MPH, you can see that there is a benefit above and beyond increasing the engines efficiency.

Most IC gasoline engines average about 18% efficiency, but they can peak at 30-36% when operated at their best BSFC. Diesels also have a sweet spot but it is not as dramatic as a throttled gasoline engine, becasue the diesel never has manifold vacuum to reduce the effective in cylinder compression whe ncombustion occurs.

That being said Mercedes went from a butterfly throttle control to no butterfly in 1982. Proir to 82 they had manifold vacuum, like a gasoline engine. After 1982 the throttle control was only the injection pump.

The net gain in mileage from that change alone was 7%. I worked for a Benz dealer when the change occured in 82. They added a vacuum pump that ran off the timing chain. The throttled engines would not shut off if there was a vacuum leak in the central locking system, until you opened the hood and pushed the fuel shutoff lever on the injection pump. Not something most customers liked to mess with on a $40 k car in 1982.

Pulse and glide will improve you mileage, especially if you glide with engine off.
It takes a certain amount of HP to just spin the engine with no load.

Now a lot of diesel vehicles are underpowered compared to most passenger cars. Plenty of torque at low engine speeds but not a lot of HP for acceleration blasts.

Combine that with transmission gearing that allows lower RPM at highway speeds and the P&G window for better mileage closes considerably.

You may find it effective up to a certain speed but after than the benefits will become insignificant.

Another factor is the aerodynamics of your vehicle. Very high CDs will cost you a lot of your energy in wind resistance, expecially in a large frontal area high CD truck.

Bottom line, I would guess the P&G will probably help up to about 45 MPH. After that the aero drag will probably kill any benefit especially at peak speeds of 60 MPH and above.

regards
Mech

[tasdrouille]

Originally Posted by milesgallon.com

Will advance timing affect the fuel economy?

Slightly, for the better.

[JacobAziza]

Originally Posted by Old Mechanic

Pulse and glide will improve you mileage, especially if you glide with engine off.
It takes a certain amount of HP to just spin the engine with no load.

True, but it also takes a certain HP to accelerate.

Now a lot of diesel vehicles are underpowered compared to most passenger cars. Plenty of torque at low engine speeds but not a lot of HP for acceleration blasts.

The average new sedan has all of 50% MORE HP than my big old 2.5 ton, 3 ton capacity truck!!

Another factor is the aerodynamics of your vehicle. Very high CDs will cost you a lot of your energy in wind resistance, especially in a large frontal area high CD truck.

Actually, the truck seems decently aerodynamic. I notice that on hills where, on my bike (EX250) in a tight tuck under the extended windshield I need to keep on the gas to maintain 50mph, in the truck I accelerate from 45 to 65 while EOC.

Combine that with transmission gearing that allows lower RPM at highway speeds and the P&G window for better mileage closes considerably.

exactly. I want to know by how much. From all the answers I've gotten so far, still seems the window could close completely for some vehicles.

You may find it effective up to a certain speed but after than the benefits will become insignificant.

Bottom line, I would guess the P&G will probably help up to about 45 MPH. After that the aero drag will probably kill any benefit

Well, if your guess is right, then I don't need to bother.
I never drive faster than 45mph, (except on the downhills, EOC)

[Sulfuric]

I learned that I can actually take my key OUT of the ignition, when moving above a certain speed, and not have the steering lock up.

I want to get two models of my car and get them to use the same key, and have both cars driven with one key, and we can just throw it out the window to the other driver when we need to accelerate...

Yeah, that's what I think about when I'm bored.

[Christ]

Originally Posted by Sulfuric

I learned that I can actually take my key OUT of the ignition, when moving above a certain speed, and not have the steering lock up.

I want to get two models of my car and get them to use the same key, and have both cars driven with one key, and we can just throw it out the window to the other driver when we need to accelerate...

Yeah, that's what I think about when I'm bored.

My Father has a Ford key that only works if you put it in the right way. It's a double-cut key, and it's cut for two different vehicles. We don't own the second one anymore, so you have to put the key in the truck the correct way, or it won't turn.

[milesgallon.com]

I tried the key thing a couple of days ago and I was wrong, I can turn off the engine without locking the steering wheel if I don't turn the key all the way to off. Still I could imagine it's easy to turn the key one step too far....

My memories of this is that a couple of times when parking the car I've turned off the engine (full off) before coming to a full stop and had to break when the wheel was locked (click, ooops), luckily always at very low speeds.

My Father has a Ford key that only works if you put it in the right way. It's a double-cut key, and it's cut for two different vehicles. We don't own the second one anymore, so you have to put the key in the truck the correct way, or it won't turn.

That's cool, I have to remember that if I ever have two cars of the same brand. ...but with todays radio locks that would not be so practical....

[JacobAziza]

in college I had a friend who had the same model car as me.
my key could open her car, but not the other way around
that was fun

[milesgallon.com]

Car locks are not good.

I don't know how many times I have heard about someone getting into the car, then thinking that something isn't quite right, then realizing that they are sitting in someone elses car (same model and color), suddenly feeling wery guilty and getting out of that car really quick

[user removed]

My 37 Ford had a steering column lock.

Didn't know there was a car in the US where you could remove the key and not lock the column.

regards
Mech

[Frank Lee]

'59 Bel Air, '60 VW, '66 Corvair, '68 Chev 1/2Ton... Yeah, '68!