Improving fuel economy of bigger and smaller vehicles

[Wozzaaah]

Not according to my mpg meter

It’s wrong then.

 

[mtt.tr]

Wrong, the car uses less fuel if you coast downhill in gear than in neutral.

Depends if it's carb or injection I would have thought

 

[Wozzaaah]

Yes, that’s true. I was assuming (correctly, as it turns out) that his car doesn’t have a carb.

 

[voipio]

My BIL used to drive all over the country delivering cars. One day he drove me down the M6 in my diesel Passat and annoyed the hell out of me because he pulsed the throttle, not losing any speed, but just on for a few seconds, off for a few, on for a few, etc. When I moaned about it, he said he did it to save fuel, and sure enough the trip had gone up from mid 50's for me to mid 60's for him!

The on/off throttle method seems to work on petrol cars as well. Agree that it's annoying to passengers. Never done it consistently enough to get a true measure of the gain, though.

 

[stevethegrease]

Once the car is demisted turn off the aircon.
Re tyre pressures, I think you should run them a bit high , they are only going to go down. In getting on for 50 years of driving I never heard anyone complain that their tyre pressures had gone up!

 

[Aerohead]

Once the car is demisted turn off the aircon.
Re tyre pressures, I think you should run them a bit high , they are only going to go down. In getting on for 50 years of driving I never heard anyone complain that their tyre pressures had gone up!

Steve, spend some time on the autobahn or track and you can see tyre temps soar sometimes!

 

[Gareth J]

It’s wrong then.

Not necessarily. If you firmly believe so, can you explain why the following is incorrect?

Regarding coasting, employed well, it can make a positive difference, even on a newer vehicle.

Yes, modern cars shut off fuel altogether on engine braking but equally, engine braking slows you down. There's a point at which, if the extra momentum gained by coasting is worth more than fuel used to keep the engine idling, you've a net benefit from coasting a bit. Imagine cresting a hill which drops into a valley and crests another hill that's slightly lower than the first crest. If you coast all the way, you can carry enough momentum to crest the second hill at a sensible speed. The whole episode has cost whatever fuel it took you to idle all the way.

The flip side, tou leve it in the topest gear to try to keep revs to a minimum (the fact you do this should hint at the inefficiencies of rotating the engine at a higher speed than needed), you use zero fuel on the downhill but engine braking has reduced the speed you hit the valley bottom. Part way up the other side, you engage throttle to make it over the next crest and use more than the amount of fuel you would've done just idling.

Deciding at what point the net benefit tips one way or the other though is, without lots of testing and more information than is practicable to have and process while driving, rather a seat of the pants guess at best!

To further add to that, I drove a friends car, same as mine, that had had the catalysts removed. There was, maybe, a small difference in power but the engine braking was virtually non existent! We can all surmise why that might be the case - catalysts (and dpfs) are a restriction that has to be overcome. Increase the velocity of the gas being pushed through them, increase the energy required to do so, increase the engine braking.

Catalysts and whatnot are not the only things in a drivetrain that sap a non linear amount of energy related to engine revs.

 

[redlig]

Ware soft sole shoes. You would be suprised how much you can "feel" through the pedals of what the engine is doing AND how much you can lift your foot up on the accelerator with out the speed dropping (lead footed!)

Strange that it may seem but a stronger throttle return spring really does make a difference.

 

[Justme]

Always coast in neutral/tickover, when traveling on any downhill

Uses more fuel.

 

[Farside]

That shouldn't work.

Moving an body through a fluid at a certain speed takes an amount of energy. Applying energy in busts will average out at the same amount as a constant application otherwise the body will accelerate or decelerate.

I once had the dubious privilege of being passenger of a guy who applied throttle up to 70+, let it coast in neutral down to 50-odd then repeat, ad nauseum, all the way along the M8.
Stupid and dangerous, in my opinion.

 

[Edward Teach]

Taught me a valuable lesson.

You were a learner driver,
Need I so more

 

[Edward Teach]

This coasting in neutral,
None of you seem to understand general physics of momentum,
Yes the engine is using fuel on tick over,
But when I am coasting, picking up free speed, I am not using any extra fuel to drive me along the road.
I have been doing this for several years now, more of a game, than to conserve fuel.
See how far you can go before you need to engage the gearbox and drive.
You will be surprised

 

[Justme]

The front brake pads on my BMW bike lasted me 45,000 miles. Just changed them this summer. I rarely use brakes much, but prefer to just use engine braking into bends etc. Mind you, I’m hardly a Bazza Sheena on my bike at the best of times.

I get around 50mpg from it,1100cc.

Also get 50mpg from my Diseasel Mondeo 1600.

Down side is that engines gearboxes & clutches cost a lot to replace when eventually needed compared to brakes that are cheap even if need more often.

 

[Gareth J]

Uses more fuel.

Does it? Why? If you strongly believe that's the case, can you explain why the following is incorrect?

Regarding coasting, employed well, it can make a positive difference, even on a newer vehicle.

Yes, modern cars shut off fuel altogether on engine braking but equally, engine braking slows you down. There's a point at which, if the extra momentum gained by coasting is worth more than fuel used to keep the engine idling, you've a net benefit from coasting a bit. Imagine cresting a hill which drops into a valley and crests another hill that's slightly lower than the first crest. If you coast all the way, you can carry enough momentum to crest the second hill at a sensible speed. The whole episode has cost whatever fuel it took you to idle all the way.

The flip side, tou leve it in the topest gear to try to keep revs to a minimum (the fact you do this should hint at the inefficiencies of rotating the engine at a higher speed than needed), you use zero fuel on the downhill but engine braking has reduced the speed you hit the valley bottom. Part way up the other side, you engage throttle to make it over the next crest and use more than the amount of fuel you would've done just idling.

Deciding at what point the net benefit tips one way or the other though is, without lots of testing and more information than is practicable to have and process while driving, rather a seat of the pants guess at best!

To further add to that, I drove a friends car, same as mine, that had had the catalysts removed. There was, maybe, a small difference in power but the engine braking was virtually non existent! We can all surmise why that might be the case - catalysts (and dpfs) are a restriction that has to be overcome. Increase the velocity of the gas being pushed through them, increase the energy required to do so, increase the engine braking.

Catalysts and whatnot are not the only things in a drivetrain that sap a non linear amount of energy related to engine revs.

"It uses more fuel" is what people are told but show me a real world test. Or, at least, attempt to explain with some logic/physics

 

[James1979]

Regardless of perceived fuel savings it is dangerous to coast as you have no engine braking and if you need to slow you’ll have to rely solely on brakes. If you’re on a decline then you can overheat them and suffer brake fade.

 

[ajlelectronics]

Down side is that engines gearboxes & clutches cost a lot to replace when eventually needed compared to brakes that are cheap even if need more often.

This is a common myth that often gets aired during discussions about using engine braking. It is patent nonsense as there is no additional, let alone excessive load on the drivetrain when on the overrun.

 

[Gareth J]

Regardless of perceived fuel savings it is dangerous to coast as you have no engine braking and if you need to slow you’ll have to rely solely on brakes. If you’re on a decline then you can overheat them and suffer brake fade.

Absolutely agree with that!

 

[ajlelectronics]

If you’re on a decline

I have been on a decline for years. Just call me Marvin.

 

[Edward Teach]

Regardless of perceived fuel savings it is dangerous to coast as you have no engine braking and if you need to slow you’ll have to rely solely on brakes. If you’re on a decline then you can overheat them and suffer brake fade.

Really,
Some people

 

[Justme]

Does it? Why? If you strongly believe that's the case, can you explain why the following is incorrect?






"It uses more fuel" is what people are told but show me a real world test. Or, at least, attempt to explain with some logic/physics

I think your own words do that best.

"a seat of the pants guess at best!"


Most modern high (especially diesel) efficiency cars have very little over run engine braking.

Your going down hill so any engine braking effect is more than covered by the effect of gravity.

The little braking effect there is is helping you maintain control (speed & handling).

You started at the top if the hill at max legal speed so there is no speed increase gain to be had.
You would keep that max speed with or without coasting.
With coasting you will also need to use the brakes.