Tuesday 21 January 2014

What IS a Streetlite Hybrid?

Well, it isn't a "hybrid" Hybrid!
A hybrid electric bus combines a conventional internal combustion engine propulsion system with an electric propulsion system. These type of buses normally use a Diesel-electric powertrain and are also known as hybrid Diesel-electric buses. For pea brains like fbb this means that the diesel engine drives a generator which charges batteries and powers electric motors which drive the wheels.
1 Batteries, 4 Electric Motor, 6 Diesel Engine & Generator
2, 3, 5, 7 and 8 are all sorts of other clever stuff

The diesel engine only runs when power is needed (hence all the other clever stuff) which means that the engine cuts out repeatedly; this makes fbb think that the bus has stalled or broken down! Spooky!

So why buy Hybrids? They cost a goodly chunk more than a bog standard diesel. But because the diesel is "only" generating current it can be smaller; plus, it doesn't run all the time, hence less emissions and less fuel costs. Do the benefits and disbenefits add up? Probably not in pure cash terms BUT the environmental pressures are such that SOMETHING must be done.

Hybrid, Hydrogen, Pure Electric? 

Or Streetlite Micro Hybid?
The technology here is explained by Wrightbus' briefing sheet which fbb will summarise, leaving out some of the technology.
Firstly, the engine drives the wheels via the normal standard Streetlite tasmission. So what's hybrid about it?

A question. What happens when the bus driver pushes the brake pedal? Hopefully the bus stops! But ("O" level physics!) there is lots of energy stored in wheels, engine and all the other whizzy bits and pieces that has to be got rid of. This is "expelled" as heat (brakes get hot) which warms up the atmosphere and that is bad news as well as being, simply, a waste.

The StreetLite Micro Hybrid uses the same engine, transmission and rear axle as the conventional StreetLite, but re-jigs the auxiliary systems to give the vehicle smart control of energy generation and usage on the vehicle.

A standard bus driveline has three important extra bits powered by the engine:
1. Air compressor (for brakes) which runs when the air tank pressure is below a given set point
2. Alternator (to charge battery and power lights etc.) which runs continually with a load proportional to the electrical load on the bus
3. Hydraulic pump for cooling fan (to stop the bus melting) which runs continually with a load proportional to engine speed.

Now in the Micro Hybrid, when the driver presses the brake pedal, some of the waste energy is stored in compressed air tanks and the compressor only runs when really, really needed (which is hardly ever on a busy stop-go city route - lots of brake presses.

Likewise with the alternator, but here the spare energy is stored in the batteries. Again the alternator is hardly ever used on a busy city service. The hydraulic pump becomes an electric pump, again running only when needed.

In this way, the load on the engine is significantly reduced, and the auxiliary systems are powered by waste energy that would otherwise be lost. This has the additional benefit of reducing the load on the retarder and brakes, which will also reduce the load on the cooling pack and extend the life of the friction brakes. The overall efficiency gain when Micro Hybrid is added to a conventional StreetLite is around 9.2% and this additional fuel saving is enough to place the vehicle comfortably below the threshold for Low Carbon Emission Bus Certification.

Or, to put is simply, you don't spend as much on fuel.

Mr Fearnley must like them as he's just ordered 274!
Having taken a look at vehicle lengths and capacity** ...
... fbb understands that each Streetlite comes complete with one of these ...
... to ensure maximum crush loading. Ouch! But, according to Wrightbus, trials on a mock-up at their Balleymena factory have proved entirely successful.
Here, boss William Wright gives a helping hand. Ouch again!

** You're right (Wright?), the numbers don't add up. That's because you can't usually have maximum seats and maximum standees both together; more seats, less standing!

Delivery of the First order for 274 begins later this year. Who will get the first batch?

 Next bus blog : Wednesday 22nd January 


  1. With all that saving in heat, there ought to be some left over to provide the heating for the passenger compartment. I hope that these buses, unlike almost all of the London fleet will have a heating control in the hands of the driver. In London, it's necessary for the depot engineers to do something technical to switch the saloon heaters to Hot or Cold.

    It usually takes them six months to get round to it, by which time, we're in the wrong season!

  2. As with most buses the calculation of total standing capacity is a theoretical figure that is basically taken from taking the weight of a vehicle with a full seated load from the maximum design/authorised weight and then dividing that by the legally defined average weight of a passenger to give the number of standees allowed. In practice this tends to be a bit of a theoretical number though it is nice to know that Wright actually checked they could actually fit them in the vehicle (it is unlikely to happen in the real world as it requires co-operation from everyone to get them to spread themselves around the bus properly).

    There are two types of diesel-electric hybrid: The Series (as used by ADL & Optare in the Solo & Versa) is the one where the engine simply charges the batteries which are then used to power the vehicle, this is best suited to urban work with lots of stopping & starting. The alternative is the Parallel system (as used by Volvo & Optare in the Tempo) which uses a smaller than usual engine to power the vehicle most of the time but also charging a set of batteries which provide additional power at times of extra need and, if the driver is gentle on the accelerator, will normally power the vehicle away from stationary to a certain speed (somewhere around 15mph) where the engine kicks in which gives it the ability to cover more interurban work though still best suited to more urbanised duties. It is the parallel system that has the slightly strange sensation of the engine kicking in & out, particularly on the Volvo where it does so with a noticeable thump. Experience with hybrids is showing that the financial payback is there in theory (the lingering question is over battery life) but the up-front cost is still too high to justify it - if fuel price goes up further the calculation changes.

    As well as the Micro Hybrid Wright are working on a second cheap hybrid option the first vehicles of which should be appearing this year. This is the Flybrid system which uses a flywheel being powered up as part of the breaking system that then releases this power during acceleration. It provides better fuel efficiency than the micro hybrid but less than a full hybrid, however the cost increase for fitting a Flybrid is substantially less than a normal hybrid and not a great increase over the cost of a normal bus.

  3. Slightly disappointing, as one of the gains at the point of use of a "real" hybrid is that they can crawl in traffic purely on the battery, with the engine only kicking in when things speed up. This is a big improvement in pollution terms in a city where such things matter.

    The "flybrid" is an interesting one, as that's the same thing as the Parry People Mover rail vehicle, used on the Stourbridge branch, no?