London bus engines - are they turbines?

September 2, 2011 4:17 AM

I've noticed that most newish London busses don't sound like a normal diesel engine: they sound more like some kind of turbine. What engines do they use and why?

Older London busses sound like normal diesels, just the same as any large vehicle or lorry. All the newer ones make a very loud roaring, whining, whirring, whoosing noise, almost as if they had a jet engine or some sort of turbine based engine. Why is that? What kind of engine do they use? Why don't new lorries make the same noise?

posted by richb to science & nature (15 answers total)

Have not been to London in a while to hear what you're describing, but just throwing some ideas out there:

1. An Electric/Diesel hybrid?

2. In my neck of the woods, the local mass transit buses have switched to natural gas to power their engines, and it's a much different noise than the diesel bus engine. Not quite what you describe, but somewhat similar...
posted by kuanes at 4:22 AM on September 2, 2011

TfL are introducing a number of hybrid buses, so it might well have been one of them.

"There are 106 diesel-electric hybrid buses in London, running on routes 16, 24, 76, 94, 129, 139, 141, 149, 243, 276, 328, 360, 371, 482 and E8."
posted by biffa at 4:25 AM on September 2, 2011

@biffa: I saw a 393 today which made the same noise, so it doesn't seem to be that. All the modern looking busses make this noise.
posted by richb at 4:28 AM on September 2, 2011

Guessing it's just the turbine from a turbo-diesel, maybe a different design with a different sound.
posted by 6550 at 4:43 AM on September 2, 2011

The 393 is a Dennis Enviro 200 which apparently uses a Cummins diesel engine. It burbles a bit but doesn't sound too odd to me. Used to go right past my front window...

What does sound odd are some of the double deckers - the 390 for example, if you've heard one of those. Makes a really noisy whooshing sound, I've always wondered why.
posted by ComfySofa at 5:13 AM on September 2, 2011

It was a double decker which was making the turbine noise. Must have not been a 393 then. It was in Kentish Town.
posted by richb at 5:49 AM on September 2, 2011

I think it was a 390 actually -- it must have been up in Tufnell Park. Does anyone know who makes that type and what engine they have?
posted by richb at 5:52 AM on September 2, 2011

Around 1:05 in this clip has the engine noise? is it similar? youtube link
posted by MarvinJ at 6:15 AM on September 2, 2011

God... I AM NOT A BUS NERD! I have just been googling, and I have wondered about this for a while...

They're Volvo B7TLs, apparently, and the whooshy noise is the engine cooling fans. TFL have had a few complaints, but they claim they're not really all that noisy.

Here's an explanation from TFL

I really should do some work now...
posted by ComfySofa at 6:15 AM on September 2, 2011

@MarvinJ: no, it's much louder than that. I think ComfySofa has it -- these busses are Volvo B7TLs: e.g. http://www.flickr.com/photos/31074376@N06/3522829208/

http://en.wikipedia.org/wiki/Volvo_B7TL

So the engine is a normal diesel, but for some reason this bus has really noisy engine cooling fans attached, when all other heavy vehicles seem to get by just fine without needing to sound like a taxiing 747.
posted by richb at 7:06 AM on September 2, 2011

This sort of noise, right?
posted by ComfySofa at 7:08 AM on September 2, 2011

Yes, that's it. It doesn't sound like a diesel engine to me -- the normal "chugging" of the pistons is completely drowned out by the roar of those fans. So much so, that I was starting to think they were some kind of gas turbine engine :-)
posted by richb at 7:28 AM on September 2, 2011

Back to the second half of my question: does anyone know why those bus engines have such a noisy fan? If it was for better fuel efficiency (say), then why don't lorries sound like that? What is it about these busses which prompted that design choice?
posted by richb at 7:31 AM on September 2, 2011

does anyone know why those bus engines have such a noisy fan?

The most likely answer is that it costs money to build a quiet fan, and it costs money to properly redesign a part that's running hot -- so if the original design was running too hot, the cheapest solution would be to smack in some loud, oversized fans.
posted by davejay at 2:30 PM on September 2, 2011

If it's like this, what you are hearing is the turbo and the gears.

You're not hearing the classic diesel noise because they've probably got proper mufflers on them so they don't sound awful.
posted by gjc at 4:49 PM on September 2, 2011