Consumption/Efficiency vs speed for buses

[IKBrunel]

Hi folks,

In a local transport committee discussion about 20mph limits, a bus company rep claimed that buses are less efficient at 20mph than 30mph (thereby using more fuel & emitting more C02. At first I thought that didn't sound right, but I'm starting to think it might be true. (I also suspect that commercial concerns about journey timings are the real motive here, but am casting that aside & focusing on the argument put forward).
From what I understand engines have an rpm sweet spot for efficiency, above or below which efficiency is reduced. So I suppose then it comes down to how they're geared. Are buses supplied with different gearboxes for rural/urban/trunk road focused usage or is it one size fits all?

I'm interested in modern bus fleets eg Trent Barton / Nottingham City Transport.

In particular I'm trying to find Consumption/Efficiency vs speed graphs for buses to clarify the issue.

I suspect also the problem with viewing this in terms of relative efficiency of 20mph vs 30mph is that for a conventional (non hybrid engine) in urban stop-start driving whilst there may be repeated section where the bus might reach 30mph they might be relatively short. So it's had to burn additional fuel to accelerate to that speed, energy which is then lost due to braking shortly afterwards. i.e. if a road is changed to 20mph you will get a flatter speed vs distance profile. So I wonder if the amount of fuel consumption due to acceleration is actually the dominant factor rather than the top speed reached in a given section?

cheers, ikb

 

[starrymarkb]

I wonder if Gear Ratios would come into it. It's possible that at 30mph the bus will be running in a higher gear so therefore be more efficient.

 

[IKBrunel]

absolutely, I know that gearboxes are changed on DMUs sometimes if they switch routes etc, to optimise for top speed or acceleration. This was even done sometimes on electric milkfloats sometimes if they changed route! Not sure about buses though?

 

[CatfordCat]

I'd say it's certainly theoretically possible to have buses / coaches set up to have subtly different gear ratios.

In London Transport days, there were different back axle gear ratios for 'central bus', 'country bus' and 'green line' RT (and I think RF) type buses to reflect the different operating characteristics of the routes - central buses required faster acceleration but lower top speeds, green line vice versa, and country bus somewhere between the two.

I think the country RMLs may also have been set up differently to the central bus ones. The RMC / RCL types designed for green line use, and the airport coach (later RMA) class also had performance designed for higher top speeds.

A quick look at the spec for the Enviro 400 suggests there's a choice of either 4 or 6 speed gearbox, no mention of gear ratios.

As for the specifics here - I'm not sure what the optimum cruising speed of a modern bus is. I know that cars generally achieve maximum fuel efficiency somewhere around 55 - 60 mph.

For buses, it's going to be a mix of engine speed, whether they are in free flowing or stop-start traffic, and how much accelerating / braking goes on.

In this case, there's also possibly the argument that if the running time is extended, then more buses (and therefore more emissions) will be needed to offer the same level of service.

 

[PG]

For buses, it's going to be a mix of engine speed, whether they are in free flowing or stop-start traffic, and how much accelerating / braking goes on.

In this case, there's also possibly the argument that if the running time is extended, then more buses (and therefore more emissions) will be needed to offer the same level of service.

These two points are the crux of the matter.
In a 20mph zone it will take the bus longer to travel between 2 stops then it will in a 30mph zone, and it is likely to be in a lower gear so using higher revs to do so thus consuming more fuel.

While the odd 20mph zone is unlikely to affect the PVR (peak vehicle requirement) given enough of them or the length of them as a percentage of the overall route length then it is possible that the PVR would be increased (and thus emissions) in order to provide the same level of service.

 

[DJL]

Would the loading affect the optimum speed or only the amount of fuel consumed at that optimum speed?

 

[Titfield]

I understand that within the range of fuel consumption figures achieved by "local" bus drivers the single biggest determining factor is the driver him/herself.

You may find it useful to speak to someone like Greenroad who have copious amounts of operator data (but may not able to share it with you) and be ablew to give you some generalised guidance.

 

[IKBrunel]

Ah, that's a very good point - that the driving style of the driver could make as much difference to mpg as any losses due to periodic 20mph limits.