More fuel efficient drivers = better drivers?

[neanderthalphysics]

All other things like make and condition of the car, manual/automatic/hybrid, traffic, etc. being equal.

Just got my driving license and since I started paying for my own gas bills now I am starting to read about fuel efficient driving

It occurred to me that many fuel efficient driving techniques, like anticipating traffic & road conditions, driving smoothly, not accelerating/braking excessively, not driving aggressively, etc are what my driving instructor recommended as good practice anyway.

So what do you think?

Also, two other questions I had...
1. What determines fuel consumption, the amount you depress the gas pedal or your engine's RPM? My mechanic said only how much you depress the pedal, but I think it is both, because how much you depress the gas pedal determines how much gas is fed to the combustion chamber per injection, while the RPM determines the number of injections per second. So fuel consumption is really both multiplied together.

2. When you go downhill and use engine braking with a low gear (e.g. 2-3), your foot is off the gas but is fuel consumption still high because of (1) above? The engine RPM is high due to engine braking, and even though your gas pedal is at the idle setting, idle setting x RPM still gives a moderate number.

 

[Borek]

I believe in modern cars with injection (as opposed to a carburetor) gas consumption is cut off completely while engine braking (no injections at all). Not sure about that though.

 

[neanderthalphysics]

I believe in modern cars with injection (as opposed to a carburetor) gas consumption is cut off completely while engine braking (no injections at all). Not sure about that though.

Hi Borek. But if someone steps on the gas while going downhill, the engine is going to rev up even more and be fed more fuel. Also, if the incline changes suddenly from downhill to uphill, surely we would know if our engine was drawing no gas and then suddenly had torque demanded of it - it would stall.

Unless there is a specific feedback sensor (perhaps measuring torque?) which I doubt exists, fuel flow to the injectors while going downhill must not be binary off/on situation but rather one of degree.

I suppose one way to find out is to ask for a volunteer to check my car's exhaust while doing engine braking going downhill...

 

[Borek]

if someone steps on the gas while going downhill

They are no longer engine braking, not sure what your point is.

Modern ICEs are driven by a computer, which measures plenty of parameters and adjusts amount of fuel accordingly, determining whether you are trying to accelerate or brake is one of things these systems do.

 

[neanderthalphysics]

They are no longer engine braking, not sure what your point is.

Modern ICEs are driven by a computer, which measures plenty of parameters and adjusts amount of fuel accordingly, determining whether you are trying to accelerate or brake is one of things these systems do.

First off I am not a mechanic or an automotive engineer so pardon me for my amateur analysis here.

OK so let's assume you are right that the fuel consumption of a car with fuel injectors undergoing engine braking is zero.

If the fuel consumption is zero then there is no flame in the combustion chamber, although they still get hot from the adiabatic compression. Makes sense as the gravitational potential energy is converted to mechanical energy of the pistons, and then thermal energy to heat up the air in the combustion chamber.

Let's say now you depress your gas pedal by 1%. The lower/upper flammability limits of diesel and iso-octane (petrol) 0.6-7.5 and 0.79-5.94 respectively, by % volume of air. Which means for a 1% increase in throttle, you have a jump in fuel consumption from 0% -> 0.6 or 0.79 by % vol of air (at least*). For a 1L engine volume that's about 6-7.9 cc of fuel vapours. I personally find it hard to believe there is a control system in cars with fuel injection systems but without onboard computers that does this fuel control. For a car with an onboard computer I can believe that it is possible.

Also, if this is true, then if we depress the gas pedal very slightly when going downhill, the car should lurch forward (it doesn't, it accelerates steadily), because instead of it eating up torque from the wheels it suddenly produces net torque.

*I presume at just above zero throttle setting, the fuel in the air is at the lower flammability limit + some tolerance. At the max throttle setting, the fuel in the air is at the upper flammability limit - some tolerance. Obviously with corrections for higher ambient temperatures and pressures.

 

[jrmichler]

It occurred to me that many fuel efficient driving techniques, like anticipating traffic & road conditions, driving smoothly, not accelerating/braking excessively, not driving aggressively, etc are what my driving instructor recommended as good practice anyway.

Very true. This is the fundamental basis of fuel efficient driving. If you want to get good at fuel efficient driving, here is a good place to start: https://ecomodder.com/forum/EM-hypermiling-driving-tips-ecodriving.php.

Fuel efficient driving has other advantages. My truck has 150,000 miles on the original brakes, and the brake linings appear to be good for another 100,000 miles.

 

[jack action]

1. What determines fuel consumption, the amount you depress the gas pedal or your engine's RPM?

Of course, both will affect fuel consumption. But if we set a common denominator to compare them, i.e. keeping the engine power output the same in both cases, the engine with a slightly open throttle and a high RPM will consume more fuel than one at a low RPM and wider open throttle.

That is because engine friction (i.e. losses) is proportional to the piston speed which is related to the RPM. A higher RPM will always mean higher losses.

2. When you go downhill and use engine braking with a low gear (e.g. 2-3), your foot is off the gas but is fuel consumption still high because of (1) above?

When you are using engine braking, fuel consumption doesn't really matter as you are ... braking. I'm not sure if EFI engines always cut the fuel completely, but I don't think it matters for your question. What is mainly causing the engine braking force is not the engine friction, but the forced vacuum created by closing the throttle. Because we do not allow air getting in the engine, the vacuum created over the piston during the intake cycle slows it down. The higher the RPM, the stronger the vacuum. Also on the plus side, the lower gear ratio mutiplies the braking torque effect.

Diesel engine do not have a throttle and therefore cannot do engine braking. In big trucks, they add an extra mechanism to achieve the same effect during the power stroke.

As you can see, fuel has little to do with engine braking.

 

[russ_watters]

[thread moved to mechanical engineering]

All other things like make and condition of the car, manual/automatic/hybrid, traffic, etc. being equal.

It occurred to me that many fuel efficient driving techniques, like anticipating traffic & road conditions, driving smoothly, not accelerating/braking excessively, not driving aggressively, etc are what my driving instructor recommended as good practice anyway.

So what do you think?

In general it is true as long as you don't overdo it by driving far below the speed limit, accelerating excessively slowly, etc.

Also, two other questions I had...
1. What determines fuel consumption, the amount you depress the gas pedal or your engine's RPM? My mechanic said only how much you depress the pedal, but I think it is both, because how much you depress the gas pedal determines how much gas is fed to the combustion chamber per injection, while the RPM determines the number of injections per second. So fuel consumption is really both multiplied together.

You are correct. Your mechanic, sadly, wrong.

2. When you go downhill and use engine braking with a low gear (e.g. 2-3), your foot is off the gas but is fuel consumption still high because of (1) above? The engine RPM is high due to engine braking, and even though your gas pedal is at the idle setting, idle setting x RPM still gives a moderate number.

@Borek is correct. The gas pedal isn't directly linked to the injectors and there is no "idle setting". The car's computer will provide as much gas as needed to maintain idle RPM -- which when you are coasting is zero.

Notes:
1. At an actual stop and idle the car also has to disengage the drivetrain.
2. Some modern cars (like mine) will attempt to determine if you are trying to engine-brake on a hill and leave the car in a higher than optimal gear. Mine usually guesses wrong; I prefer accelerating down a hill due to gravity.

Also, if the incline changes suddenly from downhill to uphill, surely we would know if our engine was drawing no gas and then suddenly had torque demanded of it - it would stall.

No, what stalls an engine is not being able to maintain the minimum rpm. When the slope changes the car starts decelerating, but the rpm doesn't instantly drop to zero. The car has plenty of time to sense when the rpm is dropping to idle, disengage the drivetrain and add fuel.

Unless there is a specific feedback sensor (perhaps measuring torque?) which I doubt exists, fuel flow to the injectors while going downhill must not be binary off/on situation but rather one of degree.

The feedback is rpm. The car senses that the rpm is above idle and reduces fuel flow until it gets to idle. When coasting, zero fuel flow is needed because the rpm is above idle.

I suppose one way to find out is to ask for a volunteer to check my car's exhaust while doing engine braking going downhill...

You can buy a cheap bluetooth interface device that plugs into your cars computer and use it to watch the engine's operating telemetry on your phone.

If the fuel consumption is zero then there is no flame in the combustion chamber, although they still get hot from the adiabatic compression. Makes sense as the gravitational potential energy is converted to mechanical energy of the pistons, and then thermal energy to heat up the air in the combustion chamber.

But nowhere near as much as burning gas. In fact, if you coast downhill in winter and have a good coolant monitor (from the bluetooth device...) you will notice the coolant temperature drop.

 

[BWV]

Tailgating semis on the highway to reduce drag is the most fuel-efficient technique for long distance driving. While there are some benefits to following a few car lengths behind, the optimal distance is 2 feet

https://www.treehugger.com/cars/drafting-behind-trucks-does-it-work.html

 

[russ_watters]

The feedback is rpm. The car senses that the rpm is above idle and reduces fuel flow until it gets to idle.

...I once had an older car that had a dead battery, and the car's computer lost its memory of how to idle. It took several days to re-learn, and in the meantime the car stalled frequently. It made it difficult to get the car to the service station to replace the battery -- I had to keep my foot on the gas even at stoplights.

 

[essenmein]

Couple of miss conceptions. ICE engine speed and fuel consumption. Internal combustion engines to varying degrees are tuned resonant systems. That is the inlet length and exhaust length, and valve timing etc play a huge role at how good an engine is at converting fuel to mechanical work at a given rpm.

Contrary to common belief, there is an optimal speed for power production, you loose efficiency operating above or below this speed. Caveat here is parasitic loads, if at a higher speed these parasitic loads exceed significantly your actual load then running slower will be more efficient.

Image below (specific fuel consumption) shows you that optimal speed for this engine from an efficiency stand point is ~3800rpm.

Re total fuel starvation, only diesel can do this without problem, I'm fairly sure gasoline engines need to maintain fuel/air ratios correctly other wise bad things happen*.

* Direct injected gasoline engines may not have this issue, and can likely completely shut off fuel without problem.

 

[essenmein]

Also don't trust your "mpg" gauge etc in your dash to really show you what's going on, my car will indicate 0L/100km if I lift off the gas completely, yet the data from the cars computer on my OBD2 device definitely shows a non zero fuel delivery rate at the injectors.

 

[russ_watters]

Contrary to common belief, there is an optimal speed for power production, you loose efficiency operating above or below this speed. Caveat here is parasitic loads, if at a higher speed these parasitic loads exceed significantly your actual load then running slower will be more efficient.

Image below (specific fuel consumption) shows you that optimal speed for this engine from an efficiency stand point is ~3800rpm.

Can you expand a bit on how this ties into the thread, because I'm not seeing the relevance. I fear that it may mislead one into believing the car should be operated around 3800 rpm* as much as possible for best fuel economy, and that generally isn't the case. For most cars, the extra loss of running in lower gear is much worse than the loss from running at low load and rpm. When cruising the best fuel economy tends to occur at the lowest speed you can comfortably be in your highest gear. It's worth noting that as scaled on that graph the difference between best and worst specific fuel consumption is only 10%.

*And a wide open throttle?

 

[DaveC426913]

Also don't trust your "mpg" gauge etc in your dash to really show you what's going on, my car will indicate 0L/100km if I lift off the gas completely

That may simply be a rounding issue.

A more pedantic statement would be 'don't assume it's more accurate than its sig digs indicate'.
0L can be as much as 0.49L before rounding to one sig dig.

 

[russ_watters]

Also don't trust your "mpg" gauge etc in your dash to really show you what's going on, my car will indicate 0L/100km if I lift off the gas completely, yet the data from the cars computer on my OBD2 device definitely shows a non zero fuel delivery rate at the injectors.

That's surprising. On my last two cars it has shown zero when coasting most of the time. The only caveat is that my cars would not drop the consumption to zero until they are warmed-up. I can't see any other reason not to except perhaps if it is a big engine and they don't want it to be too much of a drag on the car.

 

[essenmein]

That may simply be a rounding issue.

A more pedantic statement would be 'don't assume it's more accurate than its sig digs indicate'.
0L can be as much as 0.49L before rounding to one sig dig.

I don't think so, actual fuel consumption coasting down a hill with foot off the gas on the plugged in OBD2 computer is coming in at ~2l/100km, yet dash reads 0. Its an analog needle, with 2l/100km graduations, as soon as I touch the gas pedal its ~ right, ie even a little gas while coasting will make it read ~correctly. Also, at a stop, you'd expect it to be infinity l/100km, yet as soon as you go below ~10kmhr, it goes to zero. I read somewhere this is legislated in Canada to avoid confusion, but can't find a reference to it.

The new car is DI so I'll have to plug my computer into this and see what the actual fuel delivery is doing and see if it shuts off the injectors while coasting. My buddies TDI Jetta went to 0 on the injectors when coasting.

 

[essenmein]

Can you expand a bit on how this ties into the thread, because I'm not seeing the relevance. I fear that it may mislead one into believing the car should be operated around 3800 rpm* as much as possible for best fuel economy, and that generally isn't the case. For most cars, the extra loss of running in lower gear is much worse than the loss from running at low load and rpm. When cruising the best fuel economy tends to occur at the lowest speed you can comfortably be in your highest gear. It's worth noting that as scaled on that graph the difference between best and worst specific fuel consumption is only 10%.

*And a wide open throttle?

This question:
1. What determines fuel consumption, the amount you depress the gas pedal or your engine's RPM?

That image is a simplification, but it does highlight that engines are quite complicated beasts, and speed and efficiency are linked in interesting ways. I do believe you a right that this was a "WOT" chart.

From what I understand for a NA diesel, since air flow is always "wide open", engine RPM has large impact on combustion efficiency due to tuning of the valve timing and resonances in the inlet/exhaust.

For a gasoline engine where throttle position affects air flow, actual efficiency map will be a bit more complicated* with at minimum a surface with output power being one variable.

The condition I'm talking about, getting much better in modern engines with variable every thing timing, is shown on the upper left of the "current engine" chart, at low speed, as torque exceeds a point, efficiency stops increasing and reduces again, this "hump" gives you multiple operating points at a given power with the same efficiency, at different engine speeds, depending on how much power you need...

What should be really clear though, generally operating an engine at light load regardless of speed is terrible for efficiency. This is the science I claim as the justification for why I PWM my throttle...

*lol

 

[OCR]

. . .during the power stroke.

Close to TDC during the compression stroke. . . .

.

 

[jack action]

As for BSFC maps, this one should be of more help:

As you can see, for any constant power with respect to RPM (red lines), the BSFC tends to increase with RPM (except maybe close to WOT, near the lowest BSFC). Even though you can achieve the lowest BSFC at about 4200 RPM , close to WOT, that correspond to a certain (very high) power output. The '50 kW' line is closer to what needs to be considered for cruisin' speeds.

 

[russ_watters]

The '50 kW' line is closer to what needs to be considered for cruisin' speeds.

Meh, we're way off the bottom: cruising at 30 mpg and 60mph is more like 15 kW.

 

[essenmein]

As for BSFC maps, this one should be of more help:

View attachment 258197​

As you can see, for any constant power with respect to RPM (red lines), the BSFC tends to increase with RPM (except maybe close to WOT, near the lowest BSFC). Even though you can achieve the lowest BSFC at about 4200 RPM , close to WOT, that correspond to a certain (very high) power output. The '50 kW' line is closer to what needs to be considered for cruisin' speeds.

So at anyone power output, there is a single optimum operating speed, if the power demand changes, that optimum operating speed moves up or down. 50kw optimum speed is ~1200rpm, 80kw is 1700, 200kw is around 3700rpm.

Note not sure those power curves are correct? 250Nm @ 4000rpm is only ~100kW (Power (W) = Rpm*Nm/9.5488)

So as your vehicle is driving down the road at constant speed, which with fixed gear box means fixed engine speed. But power requirement is not constant, maybe wind resistance changes, climbing a hill etc, this wants to move the operating speed around to maintain optimal efficiency.

Then as mentioned cruising power is very low 10-15kw average for 100km/hr highway driving, at these low operating powers efficiency is very poor. The 50kW curve (actually closer to 10kW) is basically constant (poor) BSFC from 2000rpm to 5500rpm, but optimal is at the lowest speed this power is achieved with WOT. If you want to operate at cruising speed most efficiently, you need a smaller engine pushed harder, ie the 10kW load line should hit the 230g/kwh blob.

Loading the ICE with a generator under light driving load is something that is done in some hybrid systems if there is room in the battery to try to keep the engine in its most efficient state at a given rpm, which is generally determined by road speed in parallel hybrids.

 

[jack action]

Note not sure those power curves are correct? 250Nm @ 4000rpm is only ~100kW (Power (W) = Rpm*Nm/9.5488)

You are right with that and I'm not sure why it is like that; I just found the image on a search engine. Still, the general shape of the constant power curve stands: As you increase RPM, you must reduce torque equally.

But power requirement is not constant

If you want to operate at cruising speed most efficiently, you need a smaller engine pushed harder, ie the 10kW load line should hit the 230g/kwh blob.

That is not the point. Knowing you have a determined vehicle, going at cruising speed, having a set power requirement at a particular moment, is it better to be at a low or high RPM to get a better fuel consumption? The answer is most likely always that it is better at a low RPM.

Of course, it could be better with a smaller engine at full load. But you are not going to switch engines when you want to accelerate, therefore you are always cruising with a larger-than-needed engine. 10kW is a lawnmower engine: Not too many of those in a car.

BSFC is the fuel consumption PER horsepower. So 100 kW @ 300 g/h/kW requires a LOT more fuel than 10 kW @ 400 g/h/kW. The point is that BSFC map is relatively unimportant compared to the power requirement.

 

[essenmein]

BSFC is the fuel consumption PER horsepower. So 100 kW @ 300 g/h/kW requires a LOT more fuel than 10 kW @ 400 g/h/kW. The point is that BSFC map is relatively unimportant compared to the power requirement.

The units of BSFC is grams/kwhr, ie mass of fuel per unit of energy produced, this is directly an efficiency number, which is why I looked for charts that showed efficiency rather than BSFC, since BSFC requires more arithmetic to see what is going on.

At 100kw you consume more fuel per second, but you are converting that fuel to energy far more efficiently (~2x), and very importantly, are doing so for a 10th of the time to reach your target speed then you would at 10kw.

Simple numerical example:
3000kg vehicle to 50km/hr (13.9m/s) = 285kJ of kinetic energy.
1kwhr = 3.6MJ => 285kJ = 0.08kWhr
At 10kw you need ~0.08kwhrs*400g/kwhr = 32g of fuel.
At optimum engine efficiency you need 0.08kwhrs*230g/kWhr = 18.4g for the same amount of kinetic energy gained by the vehicle.

To me it would seem "accelerating gently" does not offer the highest fuel economy, accelerating at peak engine efficiency, would achieve that.

At the moment I don't see why this would be an incorrect analysis?

For cruising I agree since you are so far out of the optimum operating point all you can do is run as slow as possible.

I was talking about this stuff to some guys we were working with in hybrid applications, and I asked why they don't always aim to run the engine at the optimal thermal efficiency point, to charge the batteries (series hybrid), generate at optimum, then shut the engine down and run on battery, and simply modulate like that, always charging the batteries at the sweep spot.

The answer was that drivers don't want to hear an engine rev to some high rpm randomly while under way, or stopped for that matter, so the "noises" the car makes somewhat have to reflect what the car is doing other wise people really reacted negatively.

 

[russ_watters]

The units of BSFC is grams/kwhr, ie mass of fuel per unit of energy produced, this is directly an efficiency number, which is why I looked for charts that showed efficiency rather than BSFC, since BSFC requires more arithmetic to see what is going on.

At 100kw you consume more fuel per second, but you are converting that fuel to energy far more efficiently (~2x), and very importantly, are doing so for a 10th of the time to reach your target speed then you would at 10kw.

It's more like 1/4th of the time since if you're driving normal or ludicrous cruising speed, the dominant loss is wind resistance, which scales as a square function of speed. (it would be 1/3 if it were all quadratic; 1/4 allows for a linear component, but it is probably worse than that).

At 4x the speed, 10x the power and twice the efficiency, with those numbers you are using 25% more fuel to go the same distance in 1/4 the time, unless you get arrested for reckless driving in which case you have to factor in the jail time and location of the tow yard where your car is, and compare the fuel cost (savings or loss) against the extra distance and fine.

You were kidding when you said you PWM your throttle, right?

Simple numerical example:
3000kg vehicle to 50km/hr (13.9m/s) = 285kJ of kinetic energy.
1kwhr = 3.6MJ => 285kJ = 0.08kWhr
At 10kw you need ~0.08kwhrs*400g/kwhr = 32g of fuel.
At optimum engine efficiency you need 0.08kwhrs*230g/kWhr = 18.4g for the same amount of kinetic energy gained by the vehicle.

To me it would seem "accelerating gently" does not offer the highest fuel economy, accelerating at peak engine efficiency, would achieve that.

At the moment I don't see why this would be an incorrect analysis?

Per the above example, there and my previous post, there are two reasons:
1. Accelerating more slowly means driving more slowly during the acceleration period, reducing losses from wind resistance.
2. When you accelerate quickly you keep the car in a lower gears, and the additional drivetrain losses are enormous. This can be somewhat mitigated with early shifting, keeping the RPM low and torque high, as seen on the BSFC graph.

I was talking about this stuff to some guys we were working with in hybrid applications, and I asked why they don't always aim to run the engine at the optimal thermal efficiency point, to charge the batteries (series hybrid), generate at optimum, then shut the engine down and run on battery, and simply modulate like that, always charging the batteries at the sweep spot.

The answer was that drivers don't want to hear an engine rev to some high rpm randomly while under way, or stopped for that matter, so the "noises" the car makes somewhat have to reflect what the car is doing other wise people really reacted negatively.

Disappointing, but not shocking. At least it is mitigated somewhat by the hybrid engine being much smaller, and therefore running much closer to its peak efficiency in either case than a normal car's.

 

[hmmm27]

You were kidding when you said you PWM your throttle, right?

Actually...

 

[russ_watters]

Note not sure those power curves are correct? 250Nm @ 4000rpm is only ~100kW (Power (W) = Rpm*Nm/9.5488)

You are right with that and I'm not sure why it is like that; I just found the image on a search engine. Still, the general shape of the constant power curve stands: As you increase RPM, you must reduce torque equally.

I think the power is somehow scaled against BSFC. The curves are not as smooth as they should be if it were just torque vs rpm; they sag in the lower-left where the BSFC is lowest.

 

[russ_watters]

Actually...

Yes, the science is sound, I just didn't think anyone actually did it themselves (PWM). Programming a hybrid or cruise control to do it would make some sense, but it would be a pain to try to do it yourself.

 

[essenmein]

Yes, the science is sound, I just didn't think anyone actually did it themselves (PWM). Programming a hybrid or cruise control to do it would make some sense, but it would be a pain to try to do it yourself.

lol, I've been told I have a digital foot, but not for fuel economy reasons...

 

[jrmichler]

To me it would seem "accelerating gently" does not offer the highest fuel economy, accelerating at peak engine efficiency, would achieve that.

This is correct. The optimal acceleration in my truck is about equal to that of most other people on the road. Hard enough to get the engine operating fairly efficiently, but less than full throttle. "Driving like a granny" leaves the transmission in low gear longer, where the gas mileage is really bad. My truck gets much less than 10 MPG idling along in first gear, so I get out of that gear as soon as possible.

For the best gas mileage, the single most important thing is to drive as if your brakes barely work. You should almost never need brakes for a speed limit change, coming up behind a slower vehicle, a slow corner, or for a red light. All of this requires paying full attention to the road and traffic, which makes for safer driving. And it pays off. I paid a total of $310.75 for car insurance last year, which includes comprehensive coverage. Comprehensive includes collision and uninsured (other) drivers.

 

[LawrenceC]

I was a driving instructor for Porsche and BMW car clubs for 12 years. My track car was an older turbocharged Porsche. Most of my events were at Road Atlanta, a fast track where one is mostly at full throttle or threshold braking. I drove my track car to the events. The distance was 236 miles each way. Driving gently getting to the track with the car fully loaded including 5 mounted R compound tires, tool box, floor jack, etc. , I would get between 31 and 32 miles per gallon. The engine was modified to increase its output. On the track my mileage was between 6 and 7 mpg. My trips to Road Atlanta numbered about 60 over the years and fuel mileage was checked each trip. So I conclude that driving gently saves a considerable amount of fuel.

But whether a fuel efficient driver is a better driver is a moot question. A fuel efficient driver not keeping up with traffic is a hazard. A fuel efficient driver not accelerating quickly to merge onto a highway is a hazard. A fuel efficient driver not paying attention to his surroundings is not a better driver. A fuel efficient driver not looking far ahead for potential hazards is not a better driver. Track driving greatly improves awareness, attention, car control, and makes him/her a much better overall driver. While expensive, you would gain much from it.

 

[Baluncore]

You were kidding when you said you PWM your throttle, right?

I have a 1950 Rover P4 “cyclops” here. It has a freewheel clutch designed for Pulse and Glide economy driving. There is a switch on the dash to disable the freewheel clutch when engine braking is needed descending hills. Pulse and Glide is OK when you are on an empty road, but with heavier traffic it is more relaxing for everyone to maintain a steady speed. As the model developed over the next few years, the freewheel clutch was replaced with an overdrive unit.

 

[John Archer]

I think we have different definitions of "better". I'm with LawrenceC Here (Former motorcycle road racing instructor for 5 years here).
If I want to be very fuel efficient, provided the risks are low, I'm going to be cornering at a speed as close to the speed limit as is safe.
I'll be choosing lines that allow for maximum speed based on the limits of the vehicle.

While I can't say how it works for ALL cars, in general, one can often tell if the fuel injection system is giving you a small amount of fuel as you are "engine braking". In a straight line, kill the ignition. if the tone of your exhaust changes...your fuel injection system is giving you fuel while decelerating.

I do NOT recommend this! You will lose power steering until you fire the engine again. And I don't know if they would mess up any other electronics or controls and the newest cars.

I can say that MY vehicle definitely gives the engine at least a minimal amount of fuel while on trailing throttle. ;-)

 

[essenmein]

I think we have different definitions of "better". I'm with LawrenceC Here (Former motorcycle road racing instructor for 5 years here).
If I want to be very fuel efficient, provided the risks are low, I'm going to be cornering at a speed as close to the speed limit as is safe.
I'll be choosing lines that allow for maximum speed based on the limits of the vehicle.

While I can't say how it works for ALL cars, in general, one can often tell if the fuel injection system is giving you a small amount of fuel as you are "engine braking". In a straight line, kill the ignition. if the tone of your exhaust changes...your fuel injection system is giving you fuel while decelerating.

I do NOT recommend this! You will lose power steering until you fire the engine again. And I don't know if they would mess up any other electronics or controls and the newest cars.

I can say that MY vehicle definitely gives the engine at least a minimal amount of fuel while on trailing throttle. ;-)

FYI killing an engine, esp in more modern FWD automatics can be risky, some loose lubrication oil pressure in the transmission with the engine not spinning!

 

[John Archer]

The engine would still be spinning.

 

[essenmein]

The engine would still be spinning.

If its still spinning then you should also still have power steering, depending on how electric this assist is off course, but older hydraulics should keep working.

My comment is really more for engine off coasting, ie transmission in neutral, engine off, at some non zero road speed, aside from the very real danger of loosing power steering and brake assist in this condition, you risk damaging your transmission.