Is using cruise control more efficient?

One More Toy

Trying to settle a debate here!

MrMusician18

Yes.

tossy

Any rough calculations I’ve ever done on any car I’ve owned showed that CC is slightly less efficient.

Damien360

One More Toy wrote: »

Trying to settle a debate here!

I would argue it is. I use it on motorways all the time and you can spot the people not on cruise as I both gain and loose people on the road in terms of distance. For example, you will see people slow down on hill climbs and speed on downhills. They tend to go faster on the downhill to gain back distance lost and that eats fuel.

Besides, it is much more relaxing except when you get the clown that speeds past and then slows down in front of you and then I pass by still on cruise. And we repeat this for miles and miles....

SweetCaliber

One More Toy wrote: »

Trying to settle a debate here!

As in more fuel efficient? Yes and no.

On long stretches of roads (Motorway) yes, because it will maintain a set speed (Does depend on what speed you are wanting it to maintain though).

However on hills and such it will try maintain the set speed in too high a gear and therefore will be less fuel efficient. You could however reduce the set speed or change down a gear if you reach an incline.

Cee-Jay-Cee

On long open roads my car (2010 Mazda 6 2.0 petrol) is definitely more efficient while using CC even on my 10km commute to work if traffic is light I use it set to 100km/h.

CoBo55

No, because on downhill stretches it doesn't close the throttle completely giving you the benefit of fuel cutoff.

kaahooters

Yes and no, it depends

Interslice

MrMusician18 wrote: »

Yes.

No

beauf

Depends on how intelligent the cruise control is likewise for human driver.

grogi

One More Toy wrote: »

Trying to settle a debate here!

Less efficient than skilled driver. More efficient than oblivious one.

mickdw

It's more efficient than blasting along with a heavy foot and braking hard behind traffic etc but less efficient than an efficiency minded driver.
One reason it's not great is that it doesn't read the road ahead.

grogi

CoBo55 wrote: »

No, because on downhill stretches it doesn't close the throttle completely giving you the benefit of fuel cutoff.

Why wouldn't it?

XsApollo

No it’s not more fuel efficient compared to say if you were hypermiling. Also going up and down hills it’s not more efficient.
Depends on that manufacturers CC system too.
My Audi A6 would brake going down hills to keep the set speed and accelerate hard to get to its speed. CC is inefficient in acceleration too as it feels like it accelerating at 3/4 throttle.

Where as my Saab will roll down hills and back off without trying to slow the car down.

John_Rambo

SweetCaliber wrote: »

As in more fuel efficient? Yes and no.

On long stretches of roads (Motorway) yes, because it will maintain a set speed (Does depend on what speed you are wanting it to maintain though).

However on hills and such it will try maintain the set speed in too high a gear and therefore will be less fuel efficient. You could however reduce the set speed or change down a gear if you reach an incline.

I've always wondered about this. What about peasants like me:) with manual gear boxes? I really feel CC is a fuel saver particularly when you set it at 120kph by satnav which is slightly more accurate than my car speedo and slow overtaking is possible without touching the brakes & gear changes, decelerating and accelerating is avoided.

(*Motorway driving. I don't adhere to this system if someones hooning up behind me as I overtake... I tend to put the foot down and get the overtake done in less time to facilitate the faster driver, always presume there's a sick kid in the hooning car!!)

Less efficient if there are hills.

wonski

Cruise control offer full benefits with auto gear box.

Manual drivers like me from time to time need to intervene.

Most motorways are not that hilly though and you should be fine by just staying in high gear.

Diesels are probably better with no need to change a gear at all on the motorways.

I only use it on motorways. Only because it is petrol manual and basic cc.

Most modern stuff can be used all the time without a hassle.

CoBo55

grogi wrote: »

Why wouldn't it?

I don't know tbh, but looking at the instant fuel consumption shows that it doesn't close the throttle completely. The last car I had had acc with DSG and it only closed the throttle completely when it needed to brake.

grogi

CoBo55 wrote: »

I don't know tbh, but looking at the instant fuel consumption shows that it doesn't close the throttle completely. The last car I had had acc with DSG and it only closed the throttle completely when it needed to brake.

It really is simple: if the car doesn't need to brake to maintain the speed, it does need to put power to keep the speed. When the car goes with the fuel supply cut off, it is breaking with the engine.

colm_mcm

grogi wrote: »

It really is simple: if the car doesn't need to brake to maintain the speed, it does need to put power to keep the speed. When the car goes with the fuel supply cut off, it is breaking with the engine.

This is something people overlook in the coasting out of gear vs engine running with no fuel, there is a braking effect, which means you will use fuel down the road to regain speed.

kirving

I would expect that it does stop injecting fuel off throttle, but certainly odd that it doesn't show 0.

I think basically any car with electronic injection will cut fuel when torque request is 0, and will leave the throttle open to prevent engine braking off throttle.

Possibly that even though the wheels are driving the engine, ACC keeps injecting fuel for some other reason? DPF burn off maybe? High alternator loading possibly? Keep the engine warm instead of pumping cold air?

Or that it always thinks *some* torque is need to keep the engine spinning, but I can't think why.

My E300 will clutch in and let the engine revs go to 0 off throttle, with or without CC active, to save the last few % of friction in the drivetrain. Will also brake on steep hills I think whereas my previous manual car would only come off throttle.

Really annoying that the car has the expensive hardware to brake and steer itself, can park itself, but no ACC.

CoBo55

colm_mcm wrote: »

This is something people overlook in the coasting out of gear vs engine running with no fuel, there is a braking effect, which means you will use fuel down the road to regain speed.

I've never met anyone who coasts out of gear, never ever.

One More Toy

CoBo55 wrote: »

I've never met anyone who coasts out of gear, never ever.

Nice to meet you

CoBo55

grogi wrote: »

It really is simple: if the car doesn't need to brake to maintain the speed, it does need to put power to keep the speed. When the car goes with the fuel supply cut off, it is breaking with the engine.

I don't understand any of your post tbh, are you talking about acc or standard cc?

Zurbaran

Any car I’ve owned no because as mentioned it is not reading ahead, it just wants to stay at the same speed no matter what. My current car with cc on will drop Kph going up a hill then suddenly realise and then the revs will shoot up higher than they would had I been using the throttle myself trying to drive smoothly.

Newer systems seem to be far better though so it’s hard to say one way or the other.

grogi

CoBo55 wrote: »

I don't understand any of your post tbh, are you talking about acc or standard cc?

About physics. It doesn't matter if it is ACC, CC or manual operation.

If the engine enters the cut-off territory, it is slowing down the car. Thus if you put a precondition of maintaining a said speed, the only condition when the cut-off might be applied is when in need of breaking. And that's exactly the behaviour you have been seeing.

grogi

colm_mcm wrote: »

This is something people overlook in the coasting out of gear vs engine running with no fuel, there is a braking effect, which means you will use fuel down the road to regain speed.

The savings are to be made when you need to brake in the end. Engine on idle uses around l/h. So if one costed down in gear with the engine that doesn't use fuel for 3 minutes instead of coasting out of the gear and using the pads only at the end you saved 0.05l of fuel.

What's more - coasting out of the gear is plain stupid from safety point of view.

colm_mcm

grogi wrote: »

The savings are to be made when you need to brake in the end. Engine on idle uses around l/h. So if one costed down in gear with the engine that doesn't use fuel for 3 minutes instead of coasting out of the gear and using the pads only at the end you saved 0.05l of fuel.

What's more - coasting out of the gear is plain stupid from safety point of view.

Oh yeah, I never coast like that myself, but the argument has often come up here about which is more efficient.

grogi

grogi wrote: »

What's more - coasting out of the gear is plain stupid from safety point of view.

I wonder how that is different to my car (auto box)? When in efficient mode when I take foot off the accelerator it disengages the engine to save on the engine slowing it down.

biko

It's less efficient overall.

Odyssey 2005

One More Toy wrote: »

Trying to settle a debate here!

Efficient how, fuel or time ?

Fuel efficient...no
Time efficient yes
Comfort yes

DaveyDave

It would be more efficient if people used the ****ing thing. People really struggle to maintain speed on dual carriageways and motorways then you have to use more fuel to pass them.

Last week a car in the middle lane on the N4 randomly dropped around 10kph, nobody in front of either of us, flat road. I simply cruised past...

grogi

[Deleted User] wrote: »

I wonder how that is different to my car (auto box)? When in efficient mode when I take foot off the accelerator it disengages the engine to save on the engine slowing it down.

In emergency, how much time would it take the autobox to re-engage? How much time a panicked human would need?

grogi

grogi wrote: »

In emergency, how much time would it take the autobox to re-engage? How much time a panicked human would need?

I coast in gear from time to time, while costing in gear or not I’ve yet to ever be in a situation where I needed to rengage in a hurry.

I think any dangers of it are highly over exaggerated.

I do understand it’s slightly less efficient than coasting in gear but it’s often more relaxing in traffic etc to just roll along rather than having to be constantly engaging and disengaging drive also you can coast in gear with much less speed and momentum compared to in gear. For example there is a stretch of motorway I drive with a slight downhill for a long enough time if I knock the car to neutral at 120 it will keep going at 120 for ages but engine braking would slow it (I just do it as a challenge some times on this stretch to see how far I can go out of gear while staying at 120 or higher).

inforfun

Season 4 ep 4 of Top Gear, Clarkson drove an Audi A8 4.2 tdi from London to Edinburgh and back on 1 tank of diesel.
He started on cruise control but took it off as soon as he noticed he was going to be more economical downhill without it.

So, in The Netherlands it probably is more economic but as soon as there is some uphill and downhill to do, it isnt.

inforfun

inforfun wrote: »

Clarkson drove an Audi A8 4.2 tdi from London to Edinburgh and back on 1 tank of diesel.

Top gear is to serious motoring what wrestling is to sport. That may have been a correct observation, but quoting Clarkson doesn't help.

inforfun

It was series 4. It wasnt just ****ing about back then yet.

kirving

grogi wrote: »

If the engine enters the cut-off territory, it is slowing down the car.

If you release the throttle on a fuel injected (electronic throttle) car, will the throttle adjust to minimise the amount of manifold vacuum, and hence minimise slowly of the car?

If it dropped to idle, would that not cause harsh engine braking?

You don't necessarily need to inject fuel to stop the engine braking effect, just open the throttle and let air flow. And I thought any modern ECU would do that.

If the ECU wanted to keep a turbo spinning though, and be ready for the next acceleration even, injecting a small amount of fuel might be necessary?

Actively braking though could warrant fully cutting fuel of course, since the next acceleration event would not be expected immediately.

Or is it simply that the hill alone isn't enough to keep the car going?

grogi wrote: »

Thus if you put a precondition of maintaining a said speed, the only condition when the cut-off might be applied is when in need of breaking.

But the car's speed is being maintained due to it going down hill, why would it need fuel injected still?

grogi wrote: »

And that's exactly the behaviour you have been seeing.

That's true yep. I'm no expert in this, just wondering since it doesn't make sense to me.

grogi

Kevin Irving wrote: »

If you release the throttle on a fuel injected (electronic throttle) car, will the throttle adjust to minimise the amount of manifold vacuum, and hence minimise slowly of the car?

If it dropped to idle, would that not cause harsh engine braking?

That's true, especially on lower gears. Fortunately the engine itself has a bit of inertia (there is a freewheel, isn't there?)

You don't necessarily need to inject fuel to stop the engine braking effect, just open the throttle and let air flow. And I thought any modern ECU would do that.

Piston engine is not a simple rotary device. There are multiple processes happening that require power: while there is nothing that can be done to eliminate the power required to slow down and speed up the pistons, the pumping loses can be reduced. The way to minimise those is to change the valves timing sequence - and keep all the valves closed. Some modern engines do that (exp. EA211 1.4 TSI ACT) - it will effectively shut down two out of four cylinders.

If the ECU wanted to keep a turbo spinning though, and be ready for the next acceleration even, injecting a small amount of fuel might be necessary?

That's correct. But that would also generate torque, that might not be desireable in particular situation. In turbocharged petrol engines this is mitigated by igniting the mixture much later in the cycle, when it doesn't have any room to do work. CR diesel engines might simply inject the fuel late - but I am not sure if the air would be hot enough to start the combustion.

Actively braking though could warrant fully cutting fuel of course, since the next acceleration event would not be expected immediately.

Or is it simply that the hill alone isn't enough to keep the car going?

Exactly that. There is also active gear selected. The lower the gear, the more power is required to keep the engine rotating.

But the car's speed is being maintained due to it going down hill, why would it need fuel injected still?

If the hill is steep enough, yes. The hill will be enough. Not all are - and to maintain the speed, the engine still needs to do some work.

HappyAsLarE

Does cruise control alter the laws of thermodynamics?

Drag increases to the cube of the speed. Cruise control or not your fuel efficiency on a motorway depends on your speed.

saabsaab

I used it on the motorway with a manual Saab diesel and was expecting that it would be more economical but it wasn't in my case - fairly steady driving.

Dravokivich

HappyAsLarE wrote: »

Does cruise control alter the laws of thermodynamics?

Drag increases to the cube of the speed. Cruise control or not your fuel efficiency on a motorway depends on your speed.

I think the question is more about how such a system maintains speed against how a driver maintains speed can affect it.

HappyAsLarE

Dravokivich wrote: »

I think the question is more about how such a system maintains speed against how a driver maintains speed can affect it.

Is it not an obvious answer?

If a driver doesnt use CC and as a result achieves an average speed greater than what CC is set to then there will be a lower fuel efficiency.

colm_mcm

HappyAsLarE wrote: »

Is it not an obvious answer?

If a driver doesnt use CC and as a result achieves an average speed greater than what CC is set to then there will be a lower fuel efficiency.

Well, no - it’s more about how a driver can anticipate hills etc. a cruise control is set up to maintain a set speed, where varying the speed is actually more efficient

HappyAsLarE

colm_mcm wrote: »

Well, no - it’s more about how a driver can anticipate hills etc. a cruise control is set up to maintain a set speed, where varying the speed is actually more efficient

Eh, it’s quite simple. If the driver goes up the hill slower than the cruise control setting then he will save fuel.

grogi

HappyAsLarE wrote: »

Eh, it’s quite simple. If the driver goes up the hill slower than the cruise control setting then he will save fuel.

I think we agree that both would need to achieve same average speed... Otherwise you compare apples to oranges

colm_mcm

HappyAsLarE wrote: »

Eh, it’s quite simple. If the driver goes up the hill slower than the cruise control setting then he will save fuel.

Not necessarily. If they approach the hill faster and use the momentum to climb the hill, they’ll use less fuel and still be going faster.

HappyAsLarE

colm_mcm wrote: »

Not necessarily. If they approach the hill faster and use the momentum to climb the hill, they’ll use less fuel and still be going faster.

Ha good one. So the laws of thermodynamics don’t apply. Cool.

grogi

HappyAsLarE wrote: »

Ha good one. So the laws of thermodynamics don’t apply. Cool.

In an ICE, There isn't a linear relationship between output power and rate of consumption.

colm_mcm

HappyAsLarE wrote: »

Ha good one. So the laws of thermodynamics don’t apply. Cool.

Oh Jesus, a thermodynamics lawyer.

You’re saying that maintaining a completely constant speed (which is what cruise control will aim to do no matter how hard it has to work the car) is more efficient than just averaging that speed?

Hybrid cars are a great comparison to show the limitations of a regular ICE setup when it comes to fuel economy.

HappyAsLarE

grogi wrote: »

In an ICE, There isn't a linear relationship between output power and rate of consumption.

Of course there isn’t. There are too many variables. The wind could change direction.

But at motorway speeds (110-130 km/h) in the same gear, a significant difference in speed (>7-10 km/h) is what will determine your consumption, other variables being equal.