Hello all. I wonder if you can tell me if there is a general rule for the best gear/RPM to choose for the best fuel efficiency, or if it all depends on so many variables that there is no general rule.
I've often heard that you should drive in the highest gear possible that does not result in the engine labouring. But what does this actually mean? My car (a 1.6 petrol Focus mkI) pulls fairly smoothly from about 1200rpm, and doesn't 'shudder' under throttle unless you go well below that. Does that mean it's not labouring at those engine speeds? But I've also read (possibly here) that the most efficient engine speed is at peak torque, at least for diesels. I presume that wouldn't apply to a petrol engine where peak torque is at, say, 4000rpm?
Is there a general rule for petrol cars and diesel cars? Does the same rule apply at (a constant) 30mph as at (a constant) 50mph? Or are there too many contingencies for generalization?
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You need engine's power torque curve (against RPM) to explain this :)
Bigger the engine, higher torque it produces. So, you can change gear even at lower RPM.
What you've heard is right, as long as you can move to higher gear, without juddering the car, it is better for fuel economy.
If you want max acceleration, then you need to change gear at toque peak point (often near red line or at 4000-4500 RPM in petrol cars)
Edited by movilogo on 30/03/2008 at 10:24
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There are no hard and fast rules as all cars are different and circumstances change, but a rule of thumb I use which is effective for me is 3rd at 30, 4th at 40, 5th at 50 (in a 5 speed car, in a 6 speed 4th at 30, 5th at 40 6th at 50). This also allows me to keep in control of the car, which is more important to me than top rate fuel economy.
Think of it as pedalling a bike. You choose a gear which allows you to pedal at a rate which enables you to maintain a desired speed without you getting tired. Too low a gear and you will do too many revolutions needing lots of energy most of which is wasted, too high and you will need too much energy to push on the pedals - a car is no different - if you want to maintain 5 mph you probably won't choose 10th gear on a 10 speed bike.
Edited by daveyjp on 30/03/2008 at 18:16
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In general the optimum economy for a given speed on a given gradient will be where the torque demand is maximised or close to that point - in other words the lowest revs at which there is sufficient power being produced to maintain speed, which means the highest usable gear (not necessarily peak torque for the engine).
In practice it doesn't always feel that comfortable to have little or no extra power on hand without changing gear, and the effect of constant high torque on components that are designed for the average user has to be considered. The composite flywheels now common on diesel cars can fail expensively, and intuitively I would expect constant higher torque to increase the likelihood of a problem on a model that is prone to this type of failure.
Edited by Manatee on 30/03/2008 at 21:49
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Thanks for the replies all, that makes sense. I now see why I went wrong about peak torque versus maximum torque demand. On a more practical level, I think I gather from your responses that if the engine 'feels' like it's not being strained (i.e. no 'judder'), then it's probably not being unreasonably strained (though bearing in mind Manatee's point about the long-term wear on engine components)? In other words, if say 1500 revs feels comfortable (traffic conditions etc. permitting), then efficiency is maximized?
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It depends upon how you define efficiency.
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I am defining it terms of fuel efficiency i.e. mpg, though I realize that's not an exhaustive definition. What do you have in mind?
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I was thinking about how much you value your time at. If you do enough miles to be concerned about obtaining maximum "efficiency", you must also spend a significant amount of time behind the wheel, the cost of which should also be included in any calcs you do.
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Fair enough, but my reason for asking was really so that I had some kind of reference point so that I know how to optimize my driving style as and when I wish to. I usually can't resist flooring it in second out of a roundabout onto a dual carriageway (CO2 and £1.05 a litre be damned!), but if cruising at say 50mph, I want to be sure that I'm not inadvertently driving less efficiently in 5th when I should really be in 4th (with greater responsiveness to boot).
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To answer your [thankfully!] more limited question properly, you need to obtain the performance map for your engine. Good luck!
As an example, see;
www.eng-tips.com/viewthread.cfm?qid=144109&page=9
There's a lot going on on this graph, so, bit by bit;
The x axis, reading from left to right is engine speed in rpm
The y axis is in mean effective pressure - which is proportional to torque
The thick blue line represents the engines maximum mean effective pressure at that speed, i.e. wide open throttle, and as such, is just a scaled torque curve for the engine.
The thin blue lines are contours of equal brake specific fuel consumption, in g/kWh - the engine is most efficient near the "centre of the onion"
The thin black curves, with one highlighted in red are curves of constant power. The dots on the red line indicate where there are gears available. In this example, at the road speed where the drag is 30 kW, you could be in one gear running at 2000rpm, and 260g/kWh, or running at 4000 rpm and 320 g/kWh - as these lie on a line of constant power, they are directly proportional to the mass of fuel burnt in a given time.
Now, at any given constant road speed, your car needs a given amount of power to overcome the various forms of drag, so you would need to select the gear that places you closest to the "centre of the onion".
Note that the centre of the onion is just below the engine speed for maximum torque, and that it isn't at low engine loads.
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