I travel quite often on fairly long distance bus routes that go on dual carriageways, country lanes etc. Anyway I have noticed that on a lot of these buses, when the speed limit changes from say 40mph to the national speed limit for the road, they seem to kickdown gears briefly to pick up revs and lurch forward before going back up a gear. This is the best example I could find:
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to about 36 seconds.
Anyway my question is, is there any need to do this in a car with a manual gearbox? AFAIK these buses are all automatic transmission. Does it waste a lot of fuel?
enough torque to keep you at 30 is not enough torque to accelerate you. Dropping a gear to get good acceleration is standard practice.
As to whether short bursts of acceleration improve things over slow steady acceleration, well that's a hard one to answer. The energy increase is the same in both cases although the linger you travel at high speed the greater overall losees there are..but so much depends on how the efficiency changes with accelerator input and that's totally different between diesel and petrol and turbo and non turbo cars.
Busses do have a lot of mass so it will take a bit of power to accelerate them up to speed but the clip you linked to is a perfect example of lead footed driving wasting fuel. The driver booted it causing the automatic box to kick down and boost acceleration but only yards later he was forced to brake for the red lights up the road and the fuel he used for that burst of acceleration was wasted.
In general, drivers may be under pressure to keep to schedules and feel they need to boot it in order to meet targets, they may just be drivers that aren't in harmony with their machinery or trying to break the monotony by feeling a bit of G on their journeys. I would rate a PSV driver by their ability to keep it smooth which would, to an extent, mean smooth application of power and little kick-down but to some extent they are at the mercy of the auto-box's brain.
As to whether this is necessary in a manual car, the answer is that it depends:
Big lazy engined car (say 3litre odd+) can pootle around town in 4th and will accelerate smoothly without the need to change down.
Peaky engined car (modern trash!) or one with turbo will need use of the gears to make reasonable progress.
Back to fuel usage, it all comes down to litres per bhp per hour, boot it more and you get disproportionately poorer fuel consumption. My record is half a tank in 25 miles but that is another story.
In the good old days of Routemaster buses and their ilk, the gearbox was a "Pre-Selector" type whereby you use the gears as normal but select the next gear predicted on your experience and the terrain. When the "Pre-Selected" gear is required it's a case of dipping the clutch to engage it, then while it is in that gear selecting the next. Sort of pre-empting the ride and giving a smoother run. Fuel economy wasn't affected..
My dad was a bus driver for 36 years and was always trying to do that if his concentration went while driving the family cars, quite amusing on a mini gearbox ;-)
Some cars had pre-selector boxes as well, the most common being the Wilson gearbox. Technically, it was a gear change pedal, not a clutch pedal, as the Wilson gearbox did not use a clutch, although it was in the same place.
The selector pedal disengage the brake bands. If you used it as a 'clutch' for starting off, this caused excessive wear. The coupling between engine and box was a fluid flywheel, which at low revs gives no drive and progressively takes up the drive as revs increase. You'll see a (somewhat) similar device on many modern cars driving the cooling fan.
Reminds me of my mother complaining that her second husband's Renault 5 turbo was really slow, until I explained she should stay in a lower gear longer and wait for the turbo to kick in.
Yep I noticed that, never had that with any of the drivers I have though :p
I've been reading a bit more about bus gearboxes and I think the gearbox definitely has an effect, from what I understand, there are 3 different auto gearboxes that are most common with buses nowadays, one particular one is called a 'Voith', usually in 3 speed or 4 speed variants. It always seem reluctant to change up or down and usually waits a long time before changing, it is also a noisy bugger...
Another video here:
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- good examples at 45 seconds and 04:32
Interesting, I asked my mum if she has ever had to change down to get better acceleration and she says she hasn't had to do with it the car she know drives - a VW Passat 2.0TDI 135bhp think
In common with car autos, many buses now have a great many more ratios than 3. I'd expect that to be more common on buses which go out of town, so need a higher top speed. Hybrid designs seem to be coming in for town buses - which won't need a so sophisticated gearbox.
I was thinking of petrol turbos which are generally used as high end performance enhancements, you can fall off the boil with not enough gas passing through the exhaust side to power the inlet compressor but changing down will get the revs up, spin up the turbo and boost performance.
Diesels have a flatter power curve to start with (but run out of revs) and are torquey so don't benefit so much from gear changing. A turbo on a diesel is just there to lift that flat but pathetically low power curve into the barely acceptable zone rather than into the performance zone ;-).
Compare 2litre diesel turbo with 135bhp to 2litre petrol turbo where
At approx 36seconds the driver goes from being under 'some' power in I think
4th gear to basically opening the throttle fully where the transmission kicks down into 3rd to give max acceloration. These have a 6pseed box.
To answer your question, best use of fuel with regard to MPG is to accelerate relatively quickly using 2/3-3/4 throttle and change gear at the revs where the engine is generating max torque. In a petrol car this would be ~4000rpm ish. So it would use less fuel to have a quick burst of acceleration rather than staying in a higher gear and being under acceleraton for longer.
Back in the seventies fuel crisis, the physics and maths teachers at my school (this was actually before my time there) carried out tests of different driving styles and worked out that accelerating up to the required speed quickly was more fuel efficient than a sustained low rate of acceleration. Whether modern cars react the same way I wouldn't know.
ISTR Mercedes recommending large throttle openings, but small revs.
My knowledge suggests you want to be on the torque band, and not quite Wide Open Throttle. (WOT tells the engine computer you want everything it can give you, and it's likely to richen the mixture a little).
Don't forget that if you aren't on WOT the peak torque revs will drop a little.
Once you get to cruise you almost certainly want top gear.
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