MondeoGhiaX Posted February 13, 2011 Report Share Posted February 13, 2011 Hi. Imagine you're cruising along a fast A road or dual carriageway in 5th gear doing, say, 60mph. The road starts to go up hill and the car starts to struggle a bit. You can just about hold 55mph but there's nothing in reserve and putting your foot to the floor makes no difference to your speed. Is it better to drop down a gear and use more revs to maintain speed, or is it more economical to try and stay in as high a gear as you can? Does putting you foot to the floor increase fuel consumption, even if there is no response from the engine? Just something I keep wondering about but have always been afraid to ask... :nowink: Quote Link to comment Share on other sites More sharing options...
Rich Posted February 13, 2011 Report Share Posted February 13, 2011 Interesting question as I have this problem with the mondeo and hills. Most of the time I have to change down a gear to keep it going as it'll get halfway up and gradually start slowing down. I would have thought putting the pedal to the floor will allow more fuel in? I think taking your foot off and letting the car coast in gear, going downhill for instance uses no fuel, could be wrong though but I read that somewhere. Quote Link to comment Share on other sites More sharing options...
MondeoGhiaX Posted February 13, 2011 Author Report Share Posted February 13, 2011 Not just my mondeo then! Quite often I find myself in the same situation, wondering if I can make it to the crest of the hill without changing down (and losing momentum) as the speed starts to drop. I usually can't help but put my foot down more, even though it makes absolutely no difference. I even start to lean forward in my seat as if it will somehow squeeze an extra mph or two out of the car! Quote Link to comment Share on other sites More sharing options...
SMARTLY Posted February 13, 2011 Report Share Posted February 13, 2011 If it starts to struggle, you should drop it down a gear. Quote Link to comment Share on other sites More sharing options...
MondeoGhiaX Posted February 13, 2011 Author Report Share Posted February 13, 2011 On hills I'm familiar with I will often drop down a gear at the foot of the hill in anticipation, but at 60-70mph it feels like you are thrashing the engine. And its surely drinking more petrol than if you can just hold it in 5th. Where I live, there are some pretty severe hills even on the main A30 dual carriageway. I suppose its just frustrating when you can't maintain a decent speed in 5th gear. Quote Link to comment Share on other sites More sharing options...
liner33 Posted February 13, 2011 Report Share Posted February 13, 2011 Dropping it down a gear will use much less fuel than putting your foot down further on the throttle Quote Link to comment Share on other sites More sharing options...
Tango Posted February 13, 2011 Report Share Posted February 13, 2011 Dropping it down a gear will use much less fuel than putting your foot down further on the throttle Thought it was obvious, that's why you've got a gearbox in the first place :nowink: Would you try and pull away from a standstill in too high a gear? Quote Link to comment Share on other sites More sharing options...
MondeoGhiaX Posted February 13, 2011 Author Report Share Posted February 13, 2011 Why is it obvious? Is going along at 40 on a flat road in 3rd gear more economical than in 4th or 5th? Quote Link to comment Share on other sites More sharing options...
Sagitar Posted February 13, 2011 Report Share Posted February 13, 2011 Dropping it down a gear will use much less fuel than putting your foot down further on the throttle Thought it was obvious, that's why you've got a gearbox in the first place :nowink: Would you try and pull away from a standstill in too high a gear? Decisions, decisions. Ah the joy of not having a gearbox . . . . . Quote Link to comment Share on other sites More sharing options...
scorps Posted February 13, 2011 Report Share Posted February 13, 2011 Exactly....get an auto and let the ECU make that decision for you :nowink: Quote Link to comment Share on other sites More sharing options...
Rich Posted February 13, 2011 Report Share Posted February 13, 2011 Why is it obvious? Is going along at 40 on a flat road in 3rd gear more economical than in 4th or 5th? Although I always change down a gear when faced with a hill as I know the car won't cope that is a good point. I regularly do 25-30 in 3rd gear as if I need to pull away quickly I have the power there. In 4th gear that won't happen so have to change down first. Decisions, decisions. Ah the joy of not having a gearbox . . . . . :nowink: Takes the fun out of driving though, unless I really needed an auto I'll be sticking to manuals. Quote Link to comment Share on other sites More sharing options...
scorps Posted February 13, 2011 Report Share Posted February 13, 2011 You get fed up with it when it becomes your job though, i change gear thousands of times a week and it is tedious :nowink: Autos the way forward Quote Link to comment Share on other sites More sharing options...
SMARTLY Posted February 13, 2011 Report Share Posted February 13, 2011 When the motor slows going up a hill you are putting a strain on the engine ( not good ) by putting your foot on the gas more means you will use more fuel for no gain in speed. Quote Link to comment Share on other sites More sharing options...
SHAHZ Posted February 13, 2011 Report Share Posted February 13, 2011 If it starts to struggle, you should drop it down a gear. The correct answer ! :nowink: Quote Link to comment Share on other sites More sharing options...
parthiban Posted February 14, 2011 Report Share Posted February 14, 2011 If you allow the engine to struggle are you doing damage to it? Quote Link to comment Share on other sites More sharing options...
scorps Posted February 14, 2011 Report Share Posted February 14, 2011 You should never labour an engine (put it under strain) it will respond better in the right gear/rev range. Quote Link to comment Share on other sites More sharing options...
Sagitar Posted February 14, 2011 Report Share Posted February 14, 2011 Hi. Is it better to drop down a gear and use more revs to maintain speed, or is it more economical to try and stay in as high a gear as you can? Does putting you foot to the floor increase fuel consumption, even if there is no response from the engine? Those who have warned about the dangers of labouring an engine are quite right and choosing the right gear to keep the car comfortable is vital. The economy question in isolation is an interesting one and there is no simple answer. Several interacting variables are involved and you would need to use a differential equation of some sort to resolve them. If we take just two variables as an example. A slow moving vehicle has a lower instantaneous specific energy usage than a faster moving vehicle (all else being constant), but it will take longer to climb the hill. Its total energy requirement in climbing the hill is the instantaneous energy usage multiplied by the time taken. So the total fuel consumed will depend upon the way in which energy usage varies at different speeds and that will depend upon (among other things) the gear ratio and the throttle setting chosen. There are other variables that come into play; a fast reving engine has greater pumping losses than one reving slowly; the aerodynamic losses increase as the square of the speed; the vehicle gains or loses potential energy as it accelerates or decelerates. Some vehicles that I drove many years ago included a vacuum gauge on the dashboard. If you drove in a way that minimised the vacuum, consumption improved. Hill climbing economically involved learning how to choose the gear ration and throttle opening that kept the car moving comfortably while minimising the vacuum reading. It seemed to me that the process gave a good feel for how things were working and generated a sensitivity to the mechanisms used that a lot of modern drivers never develop. It makes me cringe when I am being driven by someone who obviously has little feel for the right gear ratio. In general I would say that being aware of engine speed, either by its sound or by using the rev counter is the key to economical hill climbing. Choose a gear and a throttle setting that keeps the engine speed in the middle of the rev range with the vehicle moving at an appropriate speed and you wont go far wrong. Quote Link to comment Share on other sites More sharing options...
parthiban Posted February 14, 2011 Report Share Posted February 14, 2011 Wow, do you mind if I ask what you do/did as a profession? Not being rude, but you just seem to know about everything! Back on topic, is that the same logic behind the theory that it is more economical to accelerate hard to your desired speed and then feather the throttle to maintain it, rather than very gradually accelerate to that same speed? Quote Link to comment Share on other sites More sharing options...
Sagitar Posted February 14, 2011 Report Share Posted February 14, 2011 Wow, do you mind if I ask what you do/did as a profession? Not being rude, but you just seem to know about everything! I can assure you that I really don't know about everything. The secret is to try to comment only on the things that I do know. I get a lot of pleasure from trying to answer technical questions in my own area, because it forces me to think and to challenge preconceptions. I find that so many people are prepared to take in things as read and then to repeat them as gospel without really understanding them. There is so much guff on the internet. I earned a living as a professional engineer and as a manager of professional engineers and though I have been retired for a long time now, I still read technical papers and attend lectures. I have been driving since the late 1940s and have seen a lot of motor cars in my time. I find that if you subject technical questions to the basic laws of physics and mathematics it usually reveals good insights . . . . . Quote Link to comment Share on other sites More sharing options...
parthiban Posted February 14, 2011 Report Share Posted February 14, 2011 The secret is to try to comment only on the things that I do know. Wise words indeed Quote Link to comment Share on other sites More sharing options...
adam_r Posted February 14, 2011 Report Share Posted February 14, 2011 Wow, do you mind if I ask what you do/did as a profession? Not being rude, but you just seem to know about everything! Back on topic, is that the same logic behind the theory that it is more economical to accelerate hard to your desired speed and then feather the throttle to maintain it, rather than very gradually accelerate to that same speed? As far as im aware that method of accelerating is just a myth. As an example my old civic had an 'eco' lamp on the dash. there are several reasons for this 'eco' light that was light for just 600rpm throught the rev range. Going back to basics, An Engine produces max torque at 2.5k rpm (red line at 7k). at 2.5k rpm the momentom of the engine is most easily mantained, again meaning that this is the point where the engine is most efficient and uses least fuel per stroke for the power that it provides. If you where able to keep the engine running at this optimum point during driving the fuel efficiency would be very good. stray below this mark and the engine started to 'labour', no engine vibration but it would increase the % load per stroke. Above the 3k margin the eco light goes out and the engine is then turning quicker than necassary and even though the % load has decreased, more fuel is being used and the engine is not working as efficiently. The best way to accelerate to 30mph would be to keep the engine revs between 2.3k and 3k accelerating slowly and smoothly. If you accelerate hard (WOT) your injectors inject slightly too much fuel (rich mixture). which means some fuel going in is not burt efficiently causing a reduction in mpg People should always consider how an engine works for optimal performance in the same way of doing a manual labour task... Put a bag of cement on your back and run 20meters as fast as you can... this will take more effort and energy than just walking the 20m.. but running it would take less time obviously Now consider a moving start, this means you already have the bag of cement moving, requiring less energy to keep you and it moving. now consider your engine now having to pull 1.2tons of metal from a standing start to 30mph as quickly as you can and compare that to slowly acelerating and getting up to speed gently. Enabling your engine to rotate at a comfortable speed and changing speed as little as often is the most economical way of driving. It drives me nuts when people just shove their cars in 5th gear at 30mph just because it 'can'. Far too many people associate low rpm with least fuel used, when infact its better for your engine and fuel economeny to keep the engine turning at a resonable rate (as with my old civic was 2.5k, or my current diesel accord around 1.6k rpm). Quote Link to comment Share on other sites More sharing options...
HighlandPete Posted February 18, 2011 Report Share Posted February 18, 2011 Wow, do you mind if I ask what you do/did as a profession? Not being rude, but you just seem to know about everything! Back on topic, is that the same logic behind the theory that it is more economical to accelerate hard to your desired speed and then feather the throttle to maintain it, rather than very gradually accelerate to that same speed? There is certainly good reason to accelerate quite briskly, using the engine where it is pulling strongly and working most efficiently, which gives the lowest BSFC, (Brake Specific Fuel Consumption), but we don't often get to see a fuel consumption map for our particular engine, to know exactly the best revs and loading to get best fuel consumption. It's a bit of trial and error, learning to get the economy 'sweet spot'. Also brisk acceleration in an appropriate gear, allows us to block change, (skip a gear or two) when up to cruising speed, which can also help economy. HighlandPete Quote Link to comment Share on other sites More sharing options...
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