Soldato
But you use cruise control and the lie of the terrain with how agressive cruise chases the target speed can greatly effect fuel economy.
What is most likely to cause high fuel consumption?
- Thread starter azmi
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Soldato
So when you press the brake you dont lift off the throttle anyway and get fuel cut.... ?
The difference isnt marked if you brake to a stop, the issue is heavy braking means you have to get to a slower speed to avoid stuff ahead etc so you have to accelerate more, ie from 10mph rather than say 18? I see it all the time with cars ahead of me. Our average speed is no different as we get the places at the same time but they do a lot more braking, over brake and more accelerating.
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You are saying the same thing as me, thats just another scenario. In all examples it means that you are on the gas for a longer time.
No idea what cruise control has to do with that last example by the way, that is completely irrelevant to what i was saying.
Associate
The problem with the "Aggressive braking" answer is that it must depend on the situation.
If you have a limited view ahead and you have to stop, releasing the accelerator alone probably isn't going to slow you down in time (the majority of cases IMO). So if you're using engine braking anyway, what difference is there going to be between braking early for longer or leaving it until the last minute?
The same amount of energy will need to be converted into heat/noise regardless and you wont be accelerating (or maintaining speed for the pedantic of you) anyway so I don't see how it would affect the fuel consumption.
Avoiding stopping by gradually slowing down is preferable of course, if the situation allows. I'm not a physics guru by all means...
Soldato
a) I was stating why braking had those effects, its not the same though, You were citing braking and the main reason, I was citing speed, although i seem to have an appreciation of the magnitude of either on the cars fuel economy.
b) Cruise control has a marked effect on fuel economy when the road isnt flat compare to how either a normal driver with drive. Ie target speed, or a economy driver with a target load, ie they sit at xmpg and let the car slow and speed up with the lie of the land. 50mph versus 70mph cruise can sometimes produce erroneoud results away from the simply aerodynamic massive difference of 50mph and 70mph, of course you have to factor in the time to travel journeys though than compare the drag force you need to overcome but its about 18% more energy use.
Soldato
Exactly, very little difference for a stop. In most situations that will be a red light though so approaching slower with earlier speed retardation (under fuel cut aswell) may mean you dont have to stop.
A good one I avoid is a series of 5-6 cars approaching a roundabout, sit on the tail of the car ahead and you have to stop like all the cars, but if you ease off on the approach very early you can end up getting to the roundabout, zipping around it and joining the accelerating traffic infront that has just left the roundabout without even touching your brakes.
Those two reasons above are the most common reasons for having to stop a car.
Thanks for the lesson in how cruise control maintains a target speed, but this still has absolutely nothing to do with the point being made. Forget cruise control, it is not relevant. The car is more economical at 70mph than it is at 50mph on flat ground and all other conditions being equal. The gearbox is clearly the reason why, whether it is due to it not locking through, or the fact that at 50mph it will use a lower gear at a similar engine speed to 70mph uses in top.
Whatever the mechanical reason (of which i have absolutely no interest, just as i have no interest in fuel consumption in general) the answer is a nonsense one.
Whatever the mechanical reason (of which i have absolutely no interest, just as i have no interest in fuel consumption in general) the answer is a nonsense one.
It is a badly worded, almost trick question.
I had this exact question on my theory, and instinctively picked B, because I assumed it meant over a long normal journey.
I was going through my answers at the end, looked at this question again and realised that it meant literally 'what causes more fuel use', obviously braking of any kind doesn't physically increase the amount of fuel you are using.
The only thing that 'increases fuel consumption' right now is putting your foot on right pedal.
Changed my answer and got 100% for the test.
I had this exact question on my theory, and instinctively picked B, because I assumed it meant over a long normal journey.
I was going through my answers at the end, looked at this question again and realised that it meant literally 'what causes more fuel use', obviously braking of any kind doesn't physically increase the amount of fuel you are using.
The only thing that 'increases fuel consumption' right now is putting your foot on right pedal.
Changed my answer and got 100% for the test.
Soldato
50mph and 70mph may result in the car sitting at the same cruising revs.
Hence engine BSFC is the same. Now if 50mph is using more fuel that may be due to the viscous losses associated with the autobox not being in top where it can mechanically lock either side of the drivetrain and not waste energy heating the transmission fluid up that would account for the difference you are seeing.
My point on cruise control should have started with a question i guess. For your 50mph versus 70mph do you use cruise control? The point im making is the agressiveness of meeting that speed ends up some quite high engine loads/throttle opening that a human driver wouldnt do.
You may need to do the lesson some other time on cruise control Jez as I refered in no way as to how cruise control actually functions... I was making the assumption, as Ive stated, that you use cruise control.
Hence engine BSFC is the same. Now if 50mph is using more fuel that may be due to the viscous losses associated with the autobox not being in top where it can mechanically lock either side of the drivetrain and not waste energy heating the transmission fluid up that would account for the difference you are seeing.
My point on cruise control should have started with a question i guess. For your 50mph versus 70mph do you use cruise control? The point im making is the agressiveness of meeting that speed ends up some quite high engine loads/throttle opening that a human driver wouldnt do.
You may need to do the lesson some other time on cruise control Jez as I refered in no way as to how cruise control actually functions... I was making the assumption, as Ive stated, that you use cruise control.
Soldato
Its not at all.
For most cars out these its simply too much speed is the reason of lower economy. Remember back when economy figures were given at 56mph and 75mph cruise I think they are just trying to make new drivers aware of something like that.
You have a large engined executive saloon with an auto box so its not really reflective of a car a new driver will be using.
I think your just struggling with some sort of document or process that exists somewhere that conflicts with your personal opinion, which is pretty typical isn't it?
You were incorrect in your assumption as i have tried various combinations of everything over the massive mileage i have put on this car. Regardless though, it still has nothing to do with the cruise control as we are comparing the same terrain at 2 different speeds.
The gearbox answer must be the correct one, which leads us back round in a massive circle to the conclusion that the answer isnt correct.
Having said this, its hardly surprising to see an incorrect generalised answer in a theory test.
The gearbox answer must be the correct one, which leads us back round in a massive circle to the conclusion that the answer isnt correct.
Having said this, its hardly surprising to see an incorrect generalised answer in a theory test.
Opinion doesnt come into it, its a fact that it totally depends on the vehicle. The question is a nonsense one which is simply there to subtly tell new drivers that going fast is bad.
I have to agree. Answer B makes sense if you are driving at a speed and encounter traffic, which is what happens in real life. Answer C is only correct if harsh braking is not matched by the harsh acceleration which most people would use if they were rushing around in the car.
Soldato
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B obviously, thats just common sense for theory.
Braking aggressively = braking late.
Good fuel consumption is about planning every move early.
Let me put it this way, you see the lights have changed up ahead. Two scenarios.
1. Getting right up to the lights and braking late, stopping at the lights dead, pulling away from a standstill.
2. Slowing down, coasting up to the lights which you probably won't even need to stop at most of the time if you time it right, meaning less fuel needed in acceleration.
Obviously the second one.
Braking aggressively = braking late.
Good fuel consumption is about planning every move early.
Let me put it this way, you see the lights have changed up ahead. Two scenarios.
1. Getting right up to the lights and braking late, stopping at the lights dead, pulling away from a standstill.
2. Slowing down, coasting up to the lights which you probably won't even need to stop at most of the time if you time it right, meaning less fuel needed in acceleration.
Obviously the second one.
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Associate
Both of those are making assumptions about the situation though. Harsh braking in itself doesn't cause increased fuel consumption, which is what the question is asking.
Smooth driving in the real world does help but emergency stopping wont use any more fuel then deciding to hit the child/pet while slowing more gently.
But the question is clearly misleading! It is supposed to be a test to help teach someone to drive and is not only misleading but factually incorrect!
Associate
Hey, I never said it wasn't misleading! However, it does say agressive braking, not agressive driving.
One thing I have learnt in doing my accountancy exams, the key is in the detail
Soldato
No, its not. You are mis reading the question. It does not mention 'aggressive driving', or 'aggressive acceleration'. Aggressive braking has absolutely no effect on your fuel consumption.
You can gently brake from 60 to 40 or you can aggressively brake from 60 to 40. Either way, it wont change your mpg.
Multiple choice questions like this are always misleading.
I appreciate that Jez, your auto box may make your car drive less efficiently at 50, but if you could force it to stay in top gear, it would be more fuel efficient at 50.
Soldato
To be fair one of those uses a higher portion of engine braking than the other, resulting in the agressive braking car having a lot hotter brakes, that heat energy had to come from somewhere.
That somewhere is the time the steady guy has started braking and the agressive is holding throttle at 60mph.
Pretty negliable though.